I do not claim that this theory in complete—it is impossible to understand the process completely in the present state of knowledge—but I maintain that it is the only theory which affords any explanation of the remarkable facts concerning the influence of the hormones from the reproductive organs on the development of secondary sexual characters, while at the same time explaining the adaptive relation of these characters or organs to the sexual habits of the various species. On the mutation hypothesis, adaptation is purely accidental. T. H. Morgan considers that the appearance of two slightly different shades of eye colour in male and female in a culture of a fruit-fly in a bottle is sufficient to settle the whole problem of sexual dimorphism, and to supersede Darwin's complicated theory of sexual selection. The possibility of a Lamarckian explanation he does not even mention. He would doubtless assume that the antlers of stags arose as a mutation, without explaining how they came to be affected by the testicular hormone, and that when they arose the stags found them convenient as fighting weapons. But the complicated adaptive relations are not to be disposed of by the simple word mutation. The males have sexual instincts, themselves dependent on the testicular hormone, which develop sexual jealousy and rivalry, and the Ruminants fight by butting with their heads because they have no incisor teeth in the upper jaw, or tusks, which are used in fighting in other species. Doubtless, mutations have occurred in antlers as in other characters; in fact all hereditary characters are subject to mutation. This in the most probable explanation, not only of the occasional occurrence of hornless individual stags, but of the differences between the antlers of different species, for there is no reason to believe that the special character of the antler in each species is adapted to a special mode of fighting in each species.
The different structure of the horns of the Bovine and Ovine Ruminants is, in my view, the result of a different mode of fighting. If we suppose that the fighting was slower and less fierce in the Bovidae, so that the skin over the exostosis was subject to friction but not lacerated, the result would be a thickening of the horny layer of the epidermis as we find it, and the fact that the skin and periosteum are not destroyed explains why the horns are not shed but permanent.
There is a tendency among Mendelians and mutationists to overestimate the importance of experiments in comparison with reasoning, either inductive or deductive. Bateson, however, has admitted that Mendelian experiments and observations on mutation have not solved the problem of adaptation. It seems to be demanded, nevertheless, that characters must be produced experimentally and then inherited before the hereditary influence of external stimuli can be accepted. Kammerer's experiments in this direction have been sceptically criticised, and it must be granted that the evidence he has published is not sufficient to produce complete conviction. But experiments of this kind are from the nature of the case difficult if not impossible. There is, however, another method—namely, to take a character which is certainly to some extent hereditary, and then to ascertain by experiment if it is 'acquired.' If it be proved that a hereditary character was originally somatogenic, it follows that somatogenic characters in time become hereditary. This is the reasoning I have used in reference to my experiments on the production of pigment on the lower sides of Flat-fishes, and I obtained similar evidence with regard to the excessive growth of the tail feathers in the Japanese Tosa-fowls, [Footnote: 'Observations and Experiments on Japanese Long-tailed Fowls,' Proc. Zool. Soc., 1903.] which is a modification of a secondary sexual character. In these fowls the feathers of the tail in the hens are only slightly lengthened.
I learned from Mr. John Sparks, who himself brought specimens of the breed from Japan, that the Japanese not only keep the birds separately on high perches in special cages, but pull the tail feathers gently every morning in order to cause them to grow longer. One question which I had to investigate on my specimens, hatched from eggs obtained from Mr. Sparks, was the relation of the growth of the feathers to the moult which occurs in ordinary birds. My experiment consisted in keeping two cocks, A and B, the first of which was left to itself, while in the second the feathers were gently pulled by stroking between the finger and thumb from the base outwards. The feathers in the tail were seven pairs of rectrices, two rows of tail coverts, anterior and posterior, four or five pairs in each row, a number of transition feathers: all these were steel-blue, almost black; in front of them on the saddle were a number of reddish yellow, very slender saddle hackles.
In September 1901, when the birds ware just over three months old, the adult feathers of the tail were all growing. The growing condition can be distinguished by the presence of a horny tubular sheath extending up the base of the feather for about one inch. When growth ceases this sheath is shed. In cock A growth continued till the end of the following March, when the longest feathers, the central rectrices, 2 feet 4-1/2 inches long. One of the feathers—namely, one of the anterior tail coverts—was accidentally pulled out on 11th February 1902, when it was 15-1/4 inches long and had nearly ceased to grow and formed its quill, and it immediately began to grow again and continued to grow till the following September, when it was accidentally broken off at the base: it was then 18 inches (44.5 cm.) long.
The effect of stroking in cock B was to pull out from time to time one of the growing feathers. Of the original feathers, one, the left central posterior covert, continued to grow till 13th July 1902, when it was 2 feet 9-1/2 inches long without the part contained in the follicle. All the feathers pulled out immediately commenced to grow again, except the last two pulled out 27th May and 13th July, which did not grow again till the following moulting season, in September.
The first right central rectrix in cock B was accidentally pulled out on 13th April 1902, when it was 2 feet 9-7/8 inches long. Its successor began to grow immediately, and in course of time pieces of it were broken off accidentally without injury to the base in the socket, which continued to grow until 16th June 1905, when it torn out of its socket. The total length of the feather with the pieces previously broken off, which were measured and preserved, was 11 feet 5-1/2 inches. It therefore continued to grow without interruption for three years and two months at an average rate of 3.6 inches per month.
In cock A only four of the short outer rectrices were moulted in the beginning of September 1902: the longer feathers—namely, central rectrices and tail coverts—which ceased to grow naturally in the spring of 1902, were not moulted till the beginning of October. This shows the great importance of pulling out the feathers as soon as they show signs of ceasing to grow, in order to obtain the abnormally long feathers. The central rectrices continued to grow till the beginning of September 1903, when that of the left side was 3 feet 6 inches long, that of the right about an inch shorter. The coverts had ceased to grow of their own accord some time before this, and the central ones of the posterior row were about 3 feet long.
As it seemed possible that there was some natural congenital difference in growth of feathers between cocks A and B, I commenced early in March 1903 to pull and stroke the feathers of the left side only in cock A, leaving those of the right side untouched. On 30th July on the left side the central rectrix and the first and second posterior coverts were still growing, on the right side the central rectrix was also growing, but the first and second posterior coverts had ceased growth and formed their quills. The first posterior covert on the left or pulled side was 3 inches longer than that of the right. The second posterior covert on the left side was still longer. The first and second posterior coverts of left side did not cease growth till 26th August. On 2nd September the left central rectrix was almost at the end of its growth, the right had ceased to grow a little before. The left was about an inch longer than the right. Thus both in length in duration of growth the feathers of the pulled side were longer than those of the right, and this was the result of treatment continued only six months, and commenced some months after the feathers had begun to grow. I have no doubt, however, that the pulling out of the feather as soon as it shows signs of forming quill, so that its successor at once grows again, is even more important in producing the great length of feather than the stroking of the feather itself.
In this case, then there is no doubt (a) that the long-tailed birds are artificially treated with the utmost care and ingenuity by the Japanese, who produced them; (b) that the mechanical stimulus in my experiments did cause the feathers to grow for a longer period and attain greater length; (c) that the tendency to longer growth is, even when no treatment is applied, distinctly inherited. It is a legitimate and logical conclusion that the inherited tendency is the result of the artificial treatment. No other breed of fowls shows such excessive growth of tail feathers. It may be admitted that individuals differ considerably in their congenital tendency to greater growth, i.e. greater length of the tail feathers, but according to my views this is not contradictory to the main conclusion, for every hereditary character shows individual variation.
It may be pointed out here that on the Lamarckian theory the conception of adaptations is not teleological: they do not exist for a certain purpose, but are the result of external stimulations arising from the actions and habits of the organism. The latter conception is the more general, for cases of somatic sexual characters exist which cannot be said to have a use or function. For example, the comb and wattles of Gallus are sexually dimorphic, being in the original species larger in the cock than in the hen. There is no convincing evidence that these appendages are either for use or ornament. They are, in fact, a disadvantage to the bird, being used by his adversary to take hold of when he strikes. The first thing that happens when cocks fight is the bleeding and laceration of the comb, as they peck at each other's heads. This laceration of the skin is, in my view, the primary cause of the evolution of these structures, leading to hypertrophy. But in this, as in other cases, the hereditary result is regular, constant, and symmetrical, while the immediate effect on the individual is doubtless irregular.
CHAPTER V
Mammalian Sexual Characters
Evidence Opposed To The Hormone Theory
Perhaps the most remarkable of all somatic sexual characters are those which are almost universal in the whole class of Mammalia, the mammary glands in the female, the scrotum in the male. We have considered the evidence concerning the relation of the development and functional action of the milk glands to hormones arising in the ovary or uterus, now we have to consider the origin of the glands and of their peculiar physiology in evolution. The obvious explanation from the Lamarckian point of view, and in my opinion the true one, is that they owed their origin at the beginning to the same stimulation which is applied to them now in every female mammal that bears young. There is, as we have seen, a difficulty in explaining how the occurrence of parturition causes the secretion of milk to begin, but it is certain that the secretion soon stops if the milk is not drawn from the glands by the sucking action of the offspring, or the artificial imitation of that action. A cow that is not milked or milked incompletely ceases to give milk. When the stimulus ceases, lactation ceases. The pressure of the secretion in the alveoli causes the cells to cease to secrete, much in the same way that pressure in the ureters injures the secretory action of the renal epithelium. In the earliest Mammals we may suppose that the young were born in a well-developed condition, for at first the supply of milk would not have been enough to sustain them for a long time as their only food. We must also suppose that the mother began to cherish the young, keeping them in contact with her abdomen. Then being hungry they began to suck at her hair or fur. The actual development of the milk glands in Marsupials has been described by Bresslau [Footnote: Stuttgart, 1901.] and by O'Donoghue. [Footnote: Q.J.M.S., lvii., 1911-12.] The rudiment of the teat is a depression or invagination of the epidermis from the bottom of which six stout hairs arise. The follicles of these hairs extend down into the derma, and from the upper end of the follicle, i.e. near the aperture of the invagination, a long cellular outgrowth extends down into the derma, branches at its end, and becomes hollow. These branches are the tubules of the future milk gland. Another outgrowth from the follicle forms a sebaceous gland. Later on the hairs and the sebaceous glands entirely disappear, and the milk gland alone is left with its tubules and ducts opening into the cavity of the teat. This is clear evidence that the milk gland was evolved in connexion with hairs, and was an enlargement of glands opening into the hair follicle, but it is difficult to understand why a sebaceous gland is developed and afterwards disappears. This would seem to indicate that the milk gland was not a hypertrophied sebaceous gland, but a distinct outgrowth, which however had nothing to do with sweat glands.
That the intra-uterine gestation, or its cessation, were not originally necessary to determine the functional periodicity of the milk glands is proved by their presence in the Monotremes, which are oviparous. It is evident from the conditions in these mammals that both hair and milk glands were evolved before the placenta.
It may also be pointed out here that, according to the evidence of Steinach, in the milk glands at least among somatic sexual characters there is no difference between the male and female in the heredity of the organs. The zygote therefore, whether the sex of it is determined as male or female, has the same factor for the development of milk glands. On the chromosome theory as formulated by Morgan this factor must be in the somatic chromosomes and not in the sex-chromosomes, and must be present in every zygote. All the cells of the body, assuming that somatic segregation does not occur, must possess the same chromosomes as the zygote from which it developed, and whether the sex chromosomes are XX or XY or X, there must be at any rate one chromosome bearing the factor for milk glands. The functional development of these depends normally, according to the evidence hitherto discovered, on the presence or absence of hormones from the ovary or from the uterus.
If we attribute, as in my opinion we must, the primary origin of the milk glands in evolution to the mechanical stimulus of sucking, we may attempt to reconstruct the stages of the evolution of the present relation of the glands to the other organs and processes of reproduction. In the earliest stage represented by the Monotremata or Prototheria, there was no intra-uterine development. We must suppose that in the beginning the sucking stimulus caused both growth and secretion, for at first there was nothing but sebaceous or sweat glands, and although a mutation might be supposed to have produced larger glands, no mutation could explain the influence of hormones on the growth and function of such glands. Then heredity of the effect of stimulus took place to some slight degree, and this would occur, according to my theory, only in the presence of the hormone from the ovary in the same condition as that in which the modification was first caused. This would be of course after ovulation, and after hatching of the eggs. In the next stage, if we adopt the modern view that Marsupials are descended from Placental Mammals, the eggs would be retained for increasing periods in the uteri, and would be born in a well-developed condition, since lactation would demand active sucking effort on the part of the young. The early Placentalia would inherit from the Monotreme-like ancestors the development of the milk glands after ovulation, although no sucking was taking place while the young were inside the uterus. It seems probable that the relation between parturition and actual milk secretion originated with the sucking stimulus of the young after birth.
There is good evidence that the secretion of milk may continue almost indefinitely under the stimulus of sucking or milking. Neither menstruation nor gestation put an end to it. Cows may continue to give milk until the next parturition, and if castrated during lactation will continue to yield milk for years. Women also may continue to produce milk as long as the child is allowed to suck, and this has been in some cases two or three years or even more. Moreover, lactation may be induced by the repeated act of sucking without any gestation. This has happened in mares, virgin bitches, mules, virgin women, and in one woman lactation continued uninterruptedly for forty-seven years, to her eighty-first year, long after the ovary had ceased to be functional. Lactation has also been induced in male animals, e.g. in a bull, a male goat, male sheep, and in men. [Footnote: Knott, 'Abnormal Lactation,' American Medicine, vol. ii (new series), 1907.] We may conclude, therefore, that the secretion of milk normally begins by heredity after parturition, and this, in accordance with what we have learned about hormones in connexion with the reproductive system, is probably the consequence of the withdrawal of the hormone absorbed from the foetus. I do not think it is necessary to suppose, as do Lane-Claypon and Starling, that the hormone physiologically inhibits the dissimilative process and augments the assimilative, and that the withdrawal of the hormone at parturition therefore causes the dissimilative process, i.e. secretion of milk. My conclusion is that the process of secretion set up by the mechanical stimulus of sucking is inherited as it was acquired, so that it only begins to take place in the individual in the absence of the hormone from the foetus, which was absent when the process was acquired. The growth of the gland during gestation would then be due to the postponement of the process of secretion in consequence of the presence of the foetal hormone, and in this way this hormone has become in the course of evolution at once the stimulus to growth and the cause of the inhibition of secretion.
This interpretation does not, however, agree with the case of Dasyurus. If the foetal hormone is absorbed from the pouch, as I have suggested, in order to explain the persistence of the corpora lutea during lactation, then the secretion of milk after parturition ought not to take place. But in this case the sucking stimulus has been applied to the glands after a very short gestation, while the hormone from the foetus is being absorbed in the pouch, and therefore the hereditary correlation between secretion and absence of foetal hormone may be assumed to have been lost in the course of evolution.
We have next to consider the question of the evolution of the corpora lutea. If these bodies are formed only in Mammals which have uterine gestation, and not in Prototheria, they cannot be the only essential source of the hormone which stimulates the development of the milk glands, since the latter develop in Prototheria. Again it is difficult, it might be said impossible, to believe that an accidental mutation gave rise to corpora lutea the secretion of which caused uterine gestation and ultimately the formation of the placenta. It seems more probable that the retention of the originally yolked ova within the oviduct, however this retention arose, was the essential cause of the formation of the placenta and all the changes which the uterus undergoes in gestation. The absorption of nutriment from the walls of the uterus, and the chemical and mechanical stimulation of those walls, might well be the cause of the diversion of nutrition from the ovary, leading gradually to the decline of the process of secretion of yolk in the ova.
The conceptions and the mode of reasoning of the physiologist are very different from those of the evolutionist. The former concludes from certain experiments that a given organ of internal secretion has a certain function. The corpora lutea, for example, according to one theory are ductless glands, the function of whose secretion is to establish ova in the uterus and promote their development. Another function suggested for the secretion of the corpora lutea is to prevent further ovulation during pregnancy. The evolutionist, on the other hand, asks what was the origin of this corpora lutea, why should the ruptured ovarian follicles after the escape of the ova in Mammals undergo a progressive development and persist during the greater part of the whole of pregnancy? It seems obvious that the corpora lutea in evolution were a consequence of intra-uterine gestation, for they occur only in association with this condition, and it is impossible to suppose that a mutation could arise accidentally by which the ruptured follicles should produce a secretion which would cause the fertilised ova to develop within the oviducts. The developing ovum within the uterus may, however, reasonably be supposed to give off something which is absorbed into the maternal blood, and this something would be of the same nature as that which was given off by the ovum while still within the ovarian follicle. The presence of this hormone might cause the follicular cells to behave as though the ovum was still present in the follicle, so that they would persist and not die and be absorbed. But this leaves the question, what is lutein and why is it secreted? Lutein is a colouring matter sometimes found in blood-clots, and probably derived from haemoglobin. In the corpus luteum the lutein is contained in the cells, not in a blood-clot.
Chemical investigation shows that the lutein of the corpus luteum is almost if not quite identical with the colouring matter of the yolk in birds and reptiles. Escher [Footnote: Ztschr. f. Physiol. Chem., 83 (1912).] found that the lutein of the corpus luteum had the formula C{40}H{56} and was apparently identical with the carotin of the carrot, while the lutein of egg-yolk was C{40}H{56}O{2} and more soluble in alcohol, less soluble in petroleum ether, than that of the corpus luteum. The difference, if it exists, is very slight, and it is evident that one compound could easily be converted into the other. Moreover, the hypertrophied follicular cells which constitute the corpus luteum secrete fat which is seen in them in globules. The similarity of their contents therefore to yolk is very remarkable, and it may be suggested that the hormones absorbed from the ovum or embryo in the uterus acts upon the follicular cells in such a way as to cause them to secrete substances which in the ancestor were passed on to the ovum and formed the yolk. It may be urged that this idea is contradictory to the previous suggestion that the absorption of nourishment by the intra-uterine embryo was the cause of the gradual decline of the process of yolk-secretion by the ova in the ovary, but it is not really so. Originally in the reptilian ancestor, or in the Monotreme, the ovum in the follicle secreted yellow-coloured yolk. The materials for this, at any rate, passed through the follicle cells, and it is probable that these cells were not entirely passive, but actively secretory in the process. Substances diffusing from the ovum would be present in the follicle cells during this process, and probably act as a stimulus. The same substances diffusing from the ovum during its development in the uterus would continue to stimulate the follicle cells, and thus explain not merely their persistence, but their secretory activity. The ovum being no longer present in the ovary, the secretions would remain in the follicular cells, and the corpus luteum would be explained.
If this theory is sound, it would follow that corpora lutea are not formed in cases where the ova are not retained in the oviduct during their development. The essential process in the development of these structures is the hypertrophy and, in some cases at least, multiplication of the follicular cells in the ruptured follicle. I have already mentioned that this process does not occur in Teleosteans whose ovaries were studied by me. These were species of Teleosteans in which fertilisation is external. Marshall, in his Physiology of Reproduction, [Footnote: London, 1910, p. 151.] quotes a number of authors who have published observations on the changes occurring in the ruptured follicle in the lower Vertebrata, and also in the Monotremes. According to Sandes, [Footnote: 'The Corpus Luteum of Dasyurus,' Proc. Lin. Soc., New South Wales, 1903.] in the latter there is a pronounced hypertrophy of the follicular epithelium after ovulation, but no ingrowth of connective tissue or blood-vessels from the follicular wall. Marshall himself examined sections of the corpus luteum of Ornithorhynchus and saw much hypertrophied and apparently fully developed luteal cells, but no trace of any ingrowth from the wall of the follicle. This fact would appear to be quite inconsistent with the theory above proposed, but it must be remembered that the ovum of Monotremes is known to remain for a short period in the oviduct, or in other words to pass through it very slowly, and to absorb fluid from its walls, as shown by the considerable increase in size which the ovarian ovum undergoes before it is laid. It would be interesting to know how long the rudimentary corpus luteum persists in Ornithorhynchus: the period, according to my views, should be very short. It is remarkable that in the results quoted by Marshall a well-developed corpus luteum was found and exclusively found in the lower Vertebrates which are viviparous. For example, among fishes in the Elasmobranchs Myliobatis and Spinax; in Teleosteans, in Zoarces; in Reptiles, in Anguis and Seps. Bühler on the other hand, confirmed my own negative result with regard to oviparous Teleosteans, and also found no hypertrophy of the follicle in Cyclostomes which are also oviparous. In the viviparous forms mentioned there is yolk in the ovum which is retained in oviduct or ovary, but additional nutriment is also absorbed from the uterine or ovarian walls. In these cases there is no placenta and generally no adhesion of ovum or embryo to walls of oviduct or ovary. These facts alone would be sufficient to disprove the theory that the corpora lutea are organs producing a secretion whose function is to cause the attachment of the embryo to the uterine mucosa. It is also, in my opinion, unreasonable to suppose that the rudimentary corpora lutea of lower viviparous Vertebrates arose as a mutation the result of which was to cause internal development of the ovum. Habits might easily bring about retention of the fertilised ova for gradually increasing periods, [Footnote: According to Geddes and Thomson (Evolution of Sex, 1889), the common grass-snake has been induced under artificial conditions to bring forth its young alive.] and the correlation between the retained developing ova and the hypertrophy of the ruptured follicles is comprehensible on my theory of the influence of substances absorbed by the walls of oviduct or ovary from the developing ovum.
The case of Dasyurus, however, seems inconsistent with this argument, for, as previously mentioned, Sandes found that in this Marsupial the corpora lutea persisted during the greater part of the period of lactation, which continues for four months after parturition. During the whole of this time there are no embryos in the uteri, and therefore it might be urged absorption of hormones from the embryos cannot be the cause of the persistence of corpora lutea in pregnancy. But it seems to me that a complete answer to this objection is supplied by the peculiar relations of the embryos to the pouch in Dasyurus and other Marsupials. The skin of the pouch while the embryos are in it is very soft, congested, and glandular; at the same time the embryos when transferred to the pouch at parturition are very small, immature, and have a soft delicate skin. The relation of embryos to pouch in Dasyurus, therefore, is closely similar to that of embryos to uterus after the first few days of pregnancy in the Eutheria. It is true there is no placenta, but the mouths of the embryos are in very close contact with the teats, and both the skin of the embryos and that of the pouch are soft and moist. If any special substances are given off by the embryos in the uterus in ordinary gestation, the same substances would continue to be given off by the embryos in the marsupial pouch, and these must be absorbed by the skin of the pouch. In this way it seems to me we have a logical explanation of the fact that the corpora lutea in the Marsupial are not absorbed at parturition as in Eutheria. As Sandes says the 'greater part of the period of lactation,' it would appear that absorption of the corpora lutea takes place when the young Dasyurus have grown to some size, become covered with hair, and are able to leave the teats or even the pouch at will. Under these conditions it is obvious that diffusion of chemical substances from the young through the walls of the pouch would come to an end. It would be interesting in this connexion to know more of the relation of egg and embryo to the pouch and to the corpora lutea in Echidna. In Ornithorhynchus the eggs are hatched in a nest and there is no pouch.
On this view that the corpora lutea are the result, not the cause, of intra-uterine gestation, it would no longer be possible to maintain the theory that the corpus luteum in the human species is the cause by its internal secretion of the phenomenon of menstruation. This was the theory of Born and Fränkel. [Footnote: See Biedl, Internal Secretory Organs (Eng. trans.), 1912, p. 404.] Biedl's conclusion is that the periodic development and disintegration of the uterine mucous membrane in the menstrual cycle is due to the hormone of the interstitial cells of the ovary. Leopold and Ravana found that ovulation as a rule coincides with menstruation, but may take place at any time. Here, again, the problem must be considered from the point of view of evolution. It can scarcely be doubted that the thickening and growth of the mucous membrane in the menstrual cycle is of the same nature as that which takes place in pregnancy. When the ovum or ova are not fertilised the development comes to an end after a certain time, differing in different species of Mammals, and the membrane sloughs, returns to its original, state, and then begins the same process of development again.
Menstruation, then, must be interpreted as an abortive parturition, both in woman and lower Mammals, though in the latter it is not usually accompanied by hemorrhage, and is called pro-oestrus. The question then to be considered is, what determines parturition and menstruation? The presence of the fertilised ovum must have been the original cause of the hypertrophy of the uterine mucous membrane, and in its congenital or hereditary development the chemical substances diffusing from the ova in the uterus or even in the Fallopian tube may well be the stimulus starting the hypertrophy. But what determines the end of the pregnancy? Is it merely the increasing distension of the uterus by the developing foetus? This could scarcely be the case in the Marsupials in which the foetus when born is quite minute. Nor can we attribute parturition to renewed ovulation, for this occurs in Dasyurus only once a year. All we can suggest at present is that a certain periodic development takes place by heredity in presence of the hormones exuded by the fertilised ovum and the embryo developed from it. When the ovum or ova, not being fertilised, die the period of development is (usually) shortened and pro-oestrus or menstruation occurs. In the dog, however, the period of the oestrus cycle is about the as that of gestation—namely, six months.
The so-called descent of the testicles occurs exclusively in Mammals, in which with a few important exceptions it is universal. This is a very remarkable case of the change of position of an organ in the course of development. The original position of the testis on either side is quite similar to that of the same organ in birds or reptiles. The genital ridge runs along the inner edge of the mesonephros, with which the testicular tubules become connected. The testis, with the mesonephros, forming the epididymis, closely attached to it, projects into the coelom, and without losing its connexion with the peritoneum changes its position gradually during development, passing backwards and downwards until it comes to lie over the wall of the abdomen just in front of the pubic symphysis of the pelvic girdle. There the abdominal wall on either side of the middle line becomes thin and distended to form a pouch, the scrotal sac, into which the testis passes, still remaining attached to the peritoneum which lines the pouch, while the distal end of the vas deferens retains its original connexion with the urethra. The movement of the testis can thus be accurately described as a transposition or dislocation.
Various causes have been suggested for the formation of the scrotum, but no one has ever been able to suggest a use for it. It has always been quite impossible to bring it within the scope of the theory of natural selection. The evolution of it can only be explained either on the theory of mutation or some Lamarckian hypothesis. The process of dislocation of the testis does not conform to the conception of mutation, nor agree with other cases of that phenomenon. A mutation is a change of structure affecting more or less the whole soma, but showing itself especially in some particular organ or structure. But I know of no mutation occurring under observation which consisted, not in a change of structure or function, but merely in a change of position of an organ from one part of the body to another, and moreover a change which takes place by a continuous process in the course of development. If the testes were developed from the beginning in a different part of the abdomen, there might be some reason in calling the change a mutation. Moreover, if it is a mutation, why has it never occurred in any other class of Vertebrates except Mammals?
In 1903 Dr. W. Woodland published [Footnote: Proc. Zool. Soc., 1903, Part 1.] a Lamarckian theory of this mammalian feature, the probability of which it seems to me has been increased rather than decreased by the progress of research concerning heredity and evolution since that date. Dr. Woodland correlated the dislocation of the testes with the special mechanical features of the mode of locomotion in Mammalia. His words are: 'The theory here advocated is to the effect that the descent of the testes in the Mammalia has been produced by the action of mechanical strains causing rupture of the mesorchial attachments, such strains being due to the inertia of the organs reacting to the impulsiveness involved in the activity of the animals composing the group.' The 'impulsiveness' is the galloping or leaping movement which is characteristic of most Mammals when moving at their utmost speed, as seen, for example, in horses, deer, antelopes, dogs, wolves, and other Ungulata and Carnivora. It is obvious that when the body is descending to the ground after being hurled upwards and forwards, the abdominal organs have acquired a rapid movement downwards and forwards; when the body reaches the ground its movement is stopped suddenly, while the abdominal organs continue to move. The testes therefore are violently jerked downwards away from their attachments and at the same time forward. The check to the forward movement, however, is momentary, while the body is immediately thrown again upwards and forwards, which by the law of inertia means that the testes are thrown still more downwards and backwards. There is no reason to suppose, as Dr. Woodland suggests, that any rupture of the mesorchium was the usual result of these strains, but a constant pull or tension was caused in the direction in which the testes actually move during development. On this theory we have to consider (1) how such strains could cause a shifting of the peritoneal attachment, (2) why the testes should be supposed to be particularly affected more than other abdominal organs. The answer to the first question is that the strains would cause a growth of the connecting membrane (mesorchium) at the posterior end, accompanied by an absorption of it at the anterior end. The answer to the second question is that the testes are at once the most compact and heaviest organs in the abdomen, and at the same time the most loosely attached. The latter statement does not apply to the mesonephros or epididymis which has moved with the testis, but the latter cannot function without the former, and it may be supposed that the close attachment of the epididymis to the testis had come about in the early Mammalia before the change of position was evolved.
It is evident that the violent shocks of the galloping or leaping movement do not occur in Birds, Reptiles, or Amphibia. Ostriches run very fast and do not fly, but their progression is a stride with each foot alternately, not a gallop. The Anura among the Amphibia are saltatory, but their leaps are usually single, or repeated only a few times, not sustained gallops. The exceptions among the Mammalia still more tend to prove the close correspondence between the 'impulsive' mode of progression and the dislocation of the male gonads. In the Monotremata there is no scrotum, the testes are in a position similar to that which obtains in Reptiles, and they are the only Mammals in which these organs are anterior to the kidneys. In locomotion they are sluggish, there is no running or galloping among them. Ornithorhynchus is aquatic in its habits, and Echidna is nocturnal and moves very slowly. In Marsupials the scrotum is in front of the penis, but really in the same position as in other Mammals—that is, in front of the ventral part of the pelvic girdle. It is the penis which is different, as the skin around the organ has not united in a ventral suture below it, while the organ itself has not grown forward adnate to the abdominal skin as in most other Mammals. The scrotum is always anterior to the origin of the penis, although in the Eutheria apparently behind that organ. The larger Marsupials like the kangaroos are eminently saltatory, and the others are active in locomotion. The aquatic Mammals Sirenia and Cetacea have no scrotum, the testes being abdominal. It is unnecessary to inquire whether this is the original position, or whether they are descended from ancestors which had a scrotum: in either case the position of the testes corresponds to the absence of what Dr. Woodland calls impulsiveness in progression. The Fissipedia offer an instructive example, for while the Otariidae have the hind feet turned forward and can move on land somewhat like ordinary Mammals, the Phocidae cannot move their hind legs independently or turn them forward, and can only drag themselves about on land for short distances. In the former the testes are situated in a well-defined scrotum, in the latter these organs are abdominal. The Phocidae are probably descended from Mammals of the terrestrial type with a scrotum, which has disappeared in the course of evolution. Perhaps the most curious exception is that of the elephants, in which the testes are abdominal. Here, in consequence of their structure and massive shape, locomotion in usually a walk, and though they run occasionally the gait is a trot, not a sustained gallop, and leaping is out of the question. Sloths which hang from branches upside down have abdominal testes, but even here they are in a posterior position, between, the rectum and the bladder, so there has apparently been a degree of dislocation, probably inherited from ancestors with more terrestrial habits.
The fact that the ovaries do not occupy normally a position similar to that of the testes is in accordance with the theory, for they are very much smaller than the testes; and yet they have undergone some change of position, for they are posterior to the kidneys.
The facts agree with the hormone theory, for it is to be noted that although the development of the scrotum is confined to the males, the 'descent' or dislocation takes place in the foetus, and not at the period of puberty. This is in accordance with the fact that the mechanical conditions to which the change is attributed are not related to sexual habits, but to the general habits of life which begin soon after birth. The development, therefore, may be considered to be related to the presence of a hormone derived from the normal testis, but not to a special quantity or quality of hormone associated with maturity or the functional activity of the organ. In Rodents, however, there is a difference in the organs, not only at maturity, but in every rutting season, at any rate in Muridae such as rats and others. In the rutting season the testes become much larger and descend into the scrotal sacs, at other times of the year being apparently more or less abdominal. In rabbits and hares, which have a much more impulsive progression, the organs seem to be always in the scrotal sacs.
It might be thought that in this case, although the hormone theory of heredity might be applied, there was no reason to suppose that a hormone derived from the testis in the individual development was necessary in order that the hereditary change should take place. If the individual was male and therefore had a testis, this organ would by heredity go through the process of dislocation. But there is the curious fact that when the descent is not normal and complete, in what is called cryptorchidism, the organs are always sterile. The retention of the testes within the abdomen may be regarded as a case of arrested development, like many other abnormalities, but this does not explain why the retained testes should always be sterile, without spermatogenesis. If the inherited or congenital process of dislocation requires the presence of hormones produced by a normal testis, then we can understand why a defective testis does not descend completely, because it does not produce the hormone which is necessary to stimulate the hereditary mechanism to complete dislocation. It is often stated that in cryptorchidic individuals the sexual instincts and somatic sexual characters are well developed, which would appear contradictory to the above explanation, but according to Ancel and Bouin such individuals in the case of the pig show considerable differences in the secondary signs of sex and in the external genital organs, presenting variations which lie between the normal and the castrated animal.
We have here, then, in the position of the testes in Mammalia a condition which is not in the slightest degree 'adaptive' in the ordinary sense— that is, fulfilling any special function or utility. The condition must be regarded as distinctly disadvantageous, since the organs are more exposed to injury, and the abdominal wall is weakened, as we know from the risk of scrotal hernia in man. But from the Lamarckian point of view the facts support the conclusion that the condition is the effect of certain mechanical strains, and is of somatic origin, while the correlations here reviewed are entirely unexplained by any theory of mutation or blastogenic origin.
OPPOSING EVIDENCE
We have now to review certain cases which seem to support conclusions contrary to those which we have maintained in the preceding pages, and to consider the evidence which has been published in support of other theories. It must be admitted that the occurrence of male secondary characteristics on one side of the body, and female on the other, is in consistent with the view that the development of such characters is due to the stimulus of a hormone, since the idea of a hormone means something which diffuses by way of the blood-vessels, lymph-vessels, and interstices of the tissues, throughout the body, and the hormone theory of secondary sexual characters assumes that these characters are potentially present by heredity in both sexes. The occurrence of male somatic characters on one side or in some part of the body and female on the other, usually associated with the corresponding gonads, has been termed gynandromorphism, and has long been known in insects. Cases of this condition have been observed, though much more rarely, in Vertebrates. I am not aware of any authentic instances in Mammals, and the supposition that in stags reduction or abnormality of one antler may be the result of removal or injury to the testis of one side, or the opposite, have been completely disproved by experiments in which unilateral castration has been carried out without any effect on the antlers at all. In birds, however, a few cases have been recorded by competent observers with a definiteness of detail which leaves no possibility of doubt. One of the more recent of these is that of a pheasant of the white-ringed Formosan variety, P. torquatus, of the Chinese pheasant. [Footnote: C. J. Bond, 'Unilateral Development of Secondary Male Characters in a Pheasant,' Journ. of Genetics, vol. iii., 1914.] On the left side this bird shows the plumage, colour, and the spur of the male; on the right leg there is no spur except the small rudiment normally occurring in the hen. The difference in plumage between the two sides, however, is not complete. The white collar is strictly limited to the left side, but the iridescent blue green of head and neck is present on both sides, though more marked on the left. Only a few male feathers appear in the wing coverts of the left side. The breast feathers are rufous, especially on the left side. The tail coverts show marked male characters, more especially on the left side. In the tail, however, the barred character of the male is not present on one side, absent on the other, but in most of the feathers is confined to one, the outer side of each feather. With regard to the gonads, in this bird a single organ was found on the left side, i.e. in the position of the ovary in normal females, and there was no trace of a gonad on the right side. The organ present was small, 3/4 inch long by 1/2 inch broad, and microscopic sections showed in one part actively growing areas of tubular gland structure in some of which bodies like spermatozoa could be detected, while in another were fibrous tissue with degenerating cysts. The latter appear to have been degenerating egg follicles. The author concludes that the organ was originally a functional ovary, and that the ovarian portion had atrophied while a male portion had become functionally active.
Another case in birds was described by Poll [Footnote: B.B. Ges. Naturf. Freunde, Berlin, 1909.] and is mentioned by Doncaster. [Footnote: Determination of Sex, Cambridge, 1914.] It is that of a Bullfinch which had the male and female plumage sharply separated on the two sides of the body. The right side of the ventral surface was red like a normal male, the left side grey like a normal female. In this case there was a testis on the right side, on the left an ovary as in normal females.
A third case in birds, somewhat different from the two first mentioned, is that of a domestic fowl described by Shattock and Seligmann. [Footnote: Trans. Pathol. Soc. (London), vol. 57, Part i., 1906.] It was a bird of the Leghorn breed, two years old, and had the fully developed comb and wattles of the cock. Each leg bore a thick blunt spur, nearly an inch in length, but in the Leghorn breed spurs are by no means uncommon in hens of mature age, before they have ceased to lay eggs. In plumage the characters were mainly female. The colour being white could not show sexual differences, the neck hackles were but moderately developed, saddle hackles practically absent, the tail resembled that of the hen. There was a fully developed oviduct on the left side, on the right another less than half the full length. There was also a vas deferens on each side. There was a gonad on each side, that of the right about one-fourth the size of that on the left. In microscopic structure the right gonad resembled a testis consisting entirely of tubuli lined by an epithelium consisting of a single layer of cells. In one part of this organ the tubules were larger than elsewhere, and one of them exhibited spermatogenesis in progress. The left and larger gonad had a quite similar structure, but at its lower end were found two ova enclosed within a follicular epithelium.
With regard to the last case it is to be remarked that though the gonad on the right side was entirely male, there was no unilateral development of male characters. With regard to the other two cases it must be pointed out (1) that the difference between the two somatic sex-characters on the two sides is chiefly a difference of colour, except the difference in the spurs in Bond's pheasant; (2) that the evidence already cited shows that in fowls castration does not prevent the development of the colour and form of the male plumage, nor of the spurs: that in drakes, although castration does not seem to have been carried out on young specimens before the male plumage was developed, when performed on the mature bird it prevents the eclipse, and does not cause the male to resemble the hen. Castration, then, tends to prove that in Birds the development of the male characters is not so closely dependent on the stimulation of testicular hormone as in Mammals. The characters must therefore be developed by heredity in the soma, which implies that the soma must itself be differentiated in the two sexes. The development must therefore be more in the nature of gametic coupling. It does not follow that the primary sex-character or the somatic characters are exclusive in either sex. We may suppose that the zygote contains both sexes, one or other of which is dominant, and that dominance of one primary sex involves dominance of the corresponding sexual characters. This does not, however, agree with the result of removal of the ovaries in ducks, for this causes the characters of the male to appear, so that the dominance of the female is not a permanent condition of the soma but is dependent on the ovarian hormone.
In the hermaphrodite individuals mentioned above the difference of dominance is on two sides of the body instead of two different individuals. It may also be remarked here that while it is very difficult to believe that spurs were not due in evolution to the mechanical stimulation of striking with the legs in combat, and while specially enlarged feathers are erected in display, we cannot at present attribute the varied and brilliant colour of male birds to the direct influence of external stimuli.
In Lepidoptera among insects the evidence concerning castration tends to prove that hormones from the gonads play no part at all in the development of somatic sexual characters. Kellog, an American zoologist, in 1905 [Footnote: Journ. Exper. Zool. (Baltimore), vol. i., 1905.] described experiments in which he destroyed by means of a hot needle the gonads in silkworm caterpillars (Bombyx mori), and found no difference in the sexual characters of the moths reared from such caterpillars. Oudemans had previously obtained the same result in the Gipsy Moth, Limantria dispar. Meisenheimer [Footnote: Experimentelle Studien zur Soma- und Geschlechtedifferenzierung. Jena, 1909.] made more extensive experiments on castration of caterpillars in the last-mentioned species, in which the male is dark in colour and has much-feathered antennae, while the female is very pale and has antennae only slightly feathered. In the moths developed from the castrated larvae there was no alteration in the male characters, and in the females the only difference was that some of them were slightly darker than the normal. Meisenheimer and Kopee after him claim to have grafted ovaries into males and testes into females, with the result that the transplanted organs remained alive and grew, and in some cases at least became connected with the genital ducts. Even in these cases the moth when developed showed the original characters of the sex to which belonged the caterpillar from which it came, although it was carrying a gonad of the opposite sex. It will be seen that these results are the direct opposite of those obtained by Steinach on Mammals. We have no evidence that the darker colour of the normal male in this case is adaptive, or due to external stimuli, but the feathering of the antennae is generally believed to constitute a greater development of the olfactory sense organs, and is therefore adaptive, enabling the male to find the female. This is therefore the kind of organ which would be expected to be affected by hormones from the generative organs. It is stated that the sexual instincts were also unaltered, a male containing ovaries instead of testes readily copulating with a normal female.
These results, almost incredible as they appear, are in harmony with the relatively frequent occurrence of gynandromorphism in insects.[Footnote: See Doncaster, Determination of Sex (Camb. Univ. Press, 1914), chap. ix.] One of the most remarkable cases of this is that of an ant (Myrmica scabrinodis) the left half of which is male, the right half not merely female, but worker—that is, sterile female, without wing. Cases in Lepidoptera, e.g. Amphidasys betularia, have frequently been recorded. Presumably not only the antennae and markings, but also the genital appendages and the gonads themselves, are male and female on the two sides. On the view that both sexes and the somatic sex-characters of both sexes are present in each zygote, and that the actual sex is due to dominance, we must conclude that the male primary and secondary characters are dominant on one side, and the female on the other, and it is evident that hormones diffusing throughout the body cannot determine the development of somatic sexual characters here. Various attempts have been made to explain gynandromorphism in insects in accordance with the chromosome theory of sex-determination. These are discussed by Doncaster in the volume already cited, but from the point of view of the present work the important question is that concerning the somatic sex-characters. According to Doncaster it has been found that in some Lepidoptera the different sex-chromosomes occur in the female, not in the male as in other insects. Half the eggs, therefore, contain an X chromosome, and half a Y, while all the sperms contain an X chromosome. Doncaster has seen in Abraxas grossulariata ova with two nuclei both undergoing maturation. If one of these in reduction expelled a Y chromosome, the other an X, then one would retain an X and the other a Y. Each was fertilised by a sperm, one becoming therefore XX or male and the other XY or female. It may be supposed that as there was only the cytoplasm of one ovum, each nucleus would determine the characters of half the individual developed. The question remains, therefore, where are the factors of the somatic sex-characters? One suggestion which might be made is that the female characters are present in the Y, in this case female producing chromosome, or, if the female characters are merely negative, that the male characters are in the X chromosome, but only show themselves in the homozygous condition, thus:—
FEMALE x MALE
XY XX
| \/ |
| /\ |
XX YX
MALE FEMALE
The male characters in the male, XX, would appear because present in two chromosomes, but would be recessive in the female because present only in one chromosome. The validity of this scheme, however, is disproved by the fact that males can transmit the female characters of their race, as in the case mentioned by Doncaster where a male Nyssia zonaria when crossed transmits the wingless character of its own female.
Another, perhaps better, suggestion is that the somatic characters of both sexes are present in each. Then as each somatic cell is descended without segregation from the fertilised ovum, we may suppose that the presence of the sex-chromosomes in the somatic cells themselves in some way determines whether male or female characters shall develop, without the aid of any hormones from the gonads. This theory would be quite compatible with the belief that adaptive somatic sex-characters may be due to external stimulation, for supposing that the hypertrophy or modification is conveyed to the determinants in the gametocytes, and was confined to one sex, e.g. the male, then these determinants would be modified in association with the sex-chromosomes of that sex, and thus though after reduction and fertilisation they would be present in the female zygote also, they would not develop in that sex. Thus supposing M to represent a modification acquired in the male and m the absence of the modification, such as the feathered antenna of a moth, and the sex-chromosomes to be X and Y, then we should have in the gametocytes—
Male Female
MM mm
XX XY
Gametes MX, MX: mXmY
Zygotes MmXX male, MmXY female,
and the character M would only appear in the male because it only develops in association with XX in the somatic cells descended from the male zygote. This would be the result in the first generation in which a somatic modification affected the factors in the chromosomes. In the next generation m in the male would be affected, and the male for the sake of simplicity might be supposed to become MMXX. When the female gametes segregated, some would always be mY, and some zygotes therefore MXmY. Others might be MMXY. On this theory, therefore, there would always be some females heterozygous for the male character.
Geoffrey Smith, one of the many promising young scientific investigators whose careers were cut short in the War, maintained views concerning somatic sex-characters different from that which explains their development as due to a hormone from the testis or ovary. Nussbaum in 1905 [Footnote: 'Ergebuisse der Anat. und Entwicklungsgesch.,' Bd. xv.; Pflügers Archiv, Bd. cxxvi, 1909.] had recorded experiments on Rana fusca (which is identical with the British species commonly called R. temporaria) which appeared to prove that in the male frog after castration the annual development of the thumb-pad and the muscles of the fore-leg does not take place, and if these organs have begun to enlarge before castration they atrophy again. When pieces of testis were introduced into the dorsal lymph-sac of a castrated frog the thumb-pads and muscles developed as in a normal frog. Geoffrey Smith and Edgar Schuster [Footnote: Quart Journ. Mic. Sci., lvii, 1911-12.] investigated the subject again with results contrary to those of Nussbaum.
Smith and Schuster begin by describing the normal cycle of changes in the testes on the one hand and the thumb-pad on the other. After the discharge of the spermatozoa in March or April the testes are at their smallest size. From this time onwards till August they steadily increase in size, attaining their maximum at the beginning of September. From then till the breeding season no increase in size or alteration of cellular structure occurs, the testes apparently remaining in a state of complete inactivity during this period. With regard to internal development, after the discharge of spermatozoa in the breeding season the spermatogonia divide and proliferate, forming groups of cells known as spermatocysts. In June and July spermatogenesis is active, and from August to October the formation of ripe spermatozoa is completed.
The corresponding changes in the thumb-pads are as follows. Immediately after the breeding season the horny epidermis of the pad with its deeply pigmented papillae is cast off, and the thumb remains comparatively smooth from April or May until August or September. When the large papillae are shed, smaller papillae remain beneath, and are gradually obliterated by the epidermis growing up between them. The epidermis is therefore growing while the spermatogenesis is taking place. In August and September the epidermic papillae begin to be obvious, and from this time till February a continuous increase in the papillae and their pigmentation occur. Geoffrey Smith argues that the development of this somatic character occurs while the testes are inactive and unchanged. Considering that the testes throughout the winter months are crammed with spermatozoa, which must require some nourishment, and which may be giving off a hormone all the time, the argument has very little weight. Smith and Schuster found that ovariotomy, with or without subsequent implantation of testes or injection of testis extract, had no effect in causing the thumb of the female to assume any male characters.
Castration during the breeding season causes the external pigmented layer with its papillae to be cast off very soon—that is to say, it has the same effect as the normal discharge of the spermatozoa. Smith and Schuster found that castration at other seasons caused the pad to remain in the condition in which it was at the time, that there was no reduction or absorption as Nussbaum and Meisenheimer found, and that allo-transplantation of testes—that is, the introduction of testes from other frogs either into the dorsal lymph-sacs or into the abdominal cavity—or the injection of testis extract, had no effect in causing growth or development of the thumb-pad.
There seems to be one defect in the papers of both Nussbaum and Smith and Schuster—namely, that neither of them mentions or apparently appreciates the fact that the thumb-pads, apart from the dermal glands, consist of horny epidermis developed from the living epidermis beneath. The horny layer is not shown clearly in the figures of Smith and Schuster. It seems impossible that the horny layer or its papillae could atrophy in consequence of castration, or be absorbed. The horny part of the frog's thumb-pad is comparable with the horny sheath of the horns in the mammalian Prong-buck (Antilocapra) which are shed after the breeding season and annually redeveloped. Meisenheimer claims that he produced development of papillae on the thumb-pad, not only by implantation of pieces of testis, but also by implantation of pieces of ovary. This seems so very improbable that it suggests a doubt whether the same investigator was not mistaken with regard to the results of his experiments in transplanting gonads in Moths.
Smith and Schuster conclude that the normal development of the thumb-pad depends on the presence of normal testes, but that there is no sufficient evidence that the effect is due to a hormone derived from the testis. It is equally probable, according to Smith, that the testicular cells take up some substance or substances from the blood, thus altering the composition of the latter and perhaps stimulating the production of these substances in some other organ of the body. These substances may be provisionally called sexual formative substances. Smith's theory therefore is that the action of the testes in metabolism is rather to take something from the blood than to add something to it, and that it is this subtractive effect which influences the development of somatic sexual organs.
Geoffrey Smith in fact, in the paper above considered, attempts to apply to the frog the views he put forward [Footnote: Fauna und Flora des Golfes van Neapel, 29 Monographie Rhizocephala.] in relation to the effect of the parasite Sacculina on the sexual organs of crabs. The species in which he made the most complete investigation of the influence of the parasite was Inachus scorpio (or dorsettensis). Figures showing the changes in the abdomen produced by the presence of Sacculina are given in Doncaster's Determination of Sex, Pl. xv. Sacculina is one of the Cirripedia, and therefore allied to the Barnacles. It penetrates into the crab in its larval stage, and passes entirely into the crab's body, where it develops a system of branching root-like processes. When mature the body of the Sacculina containing its generative organs forms a projection at the base of the abdomen of the crab on its ventral surface, and after this is formed the crab does not moult. Crabs so affected do not show the usual somatic sexual characters, and at one time it was supposed that only females were attacked. It is now known that both sexes of the host may be infected by the parasite, but the presence of the latter causes suppression of the somatic sex-differences. The entry of the parasite is effected when the crab is young and small, before the somatic sex-characters are fully developed. The gonads are not actually penetrated, at least in some cases, by the fibrous processes of the parasite, but nevertheless they are atrophied and almost disappear. In Inachus the abdomen of the normal male is very narrow and has no appendages except two pairs of copulatory styles. The abdomen of the female is very broad, and has four pairs of biramous appendages covered with hairs, the normal function of which is to carry the eggs. The effect of the parasite in the male is that the abdomen is broader, the copulatory styles reduced, and biramous hairy appendages are developed similar to those of the female, but smaller. In the female the abdomen remains broad, but the appendages are much smaller than in the normal female, about equal in size to those of the 'sacculinised' male. Smith interpreted the alteration in the male as a development of female secondary characters, but it is obvious from the condition in Macrura or tailed Decapods, like the lobster or crayfish, that the abdomen or tail of the male originally carried appendages similar to those of the female, and that the male character is a loss of these appendages. The absence of the male character therefore necessarily involves a development of these appendages, and there is not much more reason for saying that the male under the influence of the parasite develops female characters, than for saying that the male character is absent. There is no evidence in the facts concerning parasitic castration for Geoffrey Smith's conclusion that the female characters are latent in the male, but the male characters not latent in the female: both return to a condition in which they resemble each other, and the primitive form from which they were differentiated.
By his studies of parasitic castration Geoffrey Smith was led to formulate a theory for the explanation of somatic sex-characters different from that of hormones. He found that in the normal female crab the blood contained fatty substances which were absorbed by the ovaries for the production of the yolk of the ova. When Sacculina is present these substances are absorbed by the parasite; the ovary is deprived of them, and therefore atrophies. In the male the parasite requires similar substances, and its demand on the blood of the host stimulates the secretion of such substances, so that the whole metabolism is altered and assimilated to that of the female. It is this physiological change which causes the development of female secondary characters. He describes this change as the production of a hermaphrodite sexual formative substance, on the ground that in at least one case eggs were found in the testis of a male Inachus which had been the host of a Sacculina, but had recovered. It must however be noted that the Sacculina itself is hermaphrodite, with ovaries much larger than the testes. It is possible that while the parasite prevents the development of testis or ovary in the host, it gives up to the body of the host a hormone from its own ovaries which tends to develop the female secondary characters: for the parasite is itself a Crustacean, and therefore the hormone from its ovaries would not be of too different a nature to act upon the tissues of the host.
The observation of Geoffrey Smith that eggs may occur in the testis of a crab after recovery from the parasite appears of more importance than his peculiar theoretical suggestions, for it tends to show that sex is not always unalterably fixed at fertilisation. In this case the influence of a parasite predominantly female would seem to be the real cause of the development of eggs in the testis of the host. Geoffrey Smith does not discuss the origin of the somatic sexual characters in evolution, or attempt to show how his theories of sexual formative substance, and of the influence of the gonads by subtraction rather than addition, would bear upon the problem.
CHAPTER VI
Origin Of Non-Sexual Characters: The Phenomena Of Mutation
According to the theory here advocated, modifications produced by external stimuli in the soma will also be inherited in some slight degree in each generation when they have no relation to sex or reproduction. In this case the habits and the stimuli which they involve will be common to both sexes, and the hormones given off by the hypertrophied tissues will act upon the corresponding determinants in the gametocytes. The modifications thus produced will therefore be related to habits, and the theory will include all adaptations of structure to function, but other characters may also be included which are the result of stimuli and yet have no function or utility.
The majority of evolutionists in recent years have taught that influences exerted through the soma have no effect on the determinants in the chromosomes of the gametes, that all hereditary variations are gametogenic and none somatogenic. Mendelians believe that evolution has been due to the appearance of characters or factors of the same kind as those which distinguish varieties in cultivated organisms, and which are the subject of their experiments, but they have found a difficulty, as already mentioned in Chapter II, in forming any idea of the origin of a new dominant character. A recessive character is the absence of some positive character, and if in the cell-divisions of gametogenesis the factor for the positive character passes wholly into one cell, the other will be without it, will not 'carry' that factor. If such a gamete is fertilised by a normal gamete the organism developed from the zygote will be heterozygous, and segregation will take place in its gametes between the chromosome carrying the factor and the other without it, so that there will now be many gametes destitute of the factor in question. When two such gametes unite in fertilisation the resulting organism will be a homozygous recessive, and the corresponding character will be absent. In this way we can conceive the origin of albino individuals from a coloured race, supposing the colour was due to a single factor.
In Bateson's opinion the origin of a new dominant is a much more difficult problem. In 1913 he discussed the question in his Silliman Lectures. [Footnote: Problems of Genetics, Oxford Univ. Press, 1913.] He considers the difficulty is equally hopeless whether we imagine the dominants to be due to some change internal to the organism or to the assumption of something from without. Accounts of the origin of new dominants under observation in plants usually prove to be open to the suspicion that the plant was introduced by some accident, or that it arose from a previous cross, or that it was due to the meeting of complementary factors. In medical literature, however, there are numerous records of the spontaneous origin of various abnormalities which behave as dominants, such as brachydactyly, and Bateson considers the authenticity of some of these to be beyond doubt. He concludes that it is impossible in the present state of knowledge to offer any explanation of the origin of dominant characters. In a note, however, he suggests the possibility that there are no such things as new dominants. Factors have been discovered which simply inhibit or prevent the development of other characters. For example, the white of the plumage in the White Leghorn fowl is due to an inhibiting factor which prevents the development of the colour factor which is also present. Withdraw the dominant inhibiting factor, and the colour shows itself. This is shown by crossing the dominant white with a recessive white, when some birds of the F(2) generation are coloured.[Footnote: Bateson, Principles of Heredity, p. 104.] Similarly, brachydactyly in man may be due to the loss of an inhibiting factor which prevents it appearing in normal persons. It is evident, however, that it is difficult to apply this suggestion to all cases. For example, the White Leghorn fowl must have descended from a coloured form, probably from the wild species Gallus bankiva. If Bateson's suggestion were valid we should have to suppose that the loss of the factor for colour caused the dominant white to appear, and then when this is withdrawn colour appears again, so that the colour factors and the inhibiting factors must lie over one another in a kind of stratified alternation. And then how should we account for the recessive white?
In his Presidential Address to the meeting of the British Association in Australia, 1914, Bateson explains his suggestion somewhat more fully with a command of language which is scarcely less remarkable than the subject matter. The more true-breeding forms are studied the more difficult it is to understand how they can vary, how a variation can arise. When two forms of Antirrhinum are crossed there is in the second generation such a profusion of different combinations of the factors in the two grandparents, that Lotsy has suggested that all variations may be due to crossing. Bateson does not agree with this. He believes that genetic factors are not permanent and indestructible, but may undergo quantitative disintegration or fractionation, producing subtraction or reduction stages, as in the Picotee Sweet Pea, or the Dutch Rabbit. Also variation may take place by loss of factors as in the origin of the white Sweet Pea from the coloured. But regarding a factor as something which, although it may be divided, neither grows nor dwindles, neither develops nor decays, the Mendelian cannot conceive its beginning any more than we can conceive the creation of something out of nothing. Bateson asks us to consider therefore whether all the divers types of life may not have been produced by the gradual unpacking of an original complexity in the primordial, probably unicellular forms, from which existing species and varieties have descended. Such a suggestion in the present writer's opinion is in one sense a truism and in another an absurdity. That the potentiality of all the characters of all the forms that have existed, pterodactyls, dinosaurs, butterflies, birds, etc. etc., including the characters of all the varieties of the human race and of human individuals, must have been present in the primordial ancestral protoplasm, is a truism, for if the possibility of such evolution did not exist, evolution would not have taken place. But that every distinct hereditary character of man was actually present as a Mendelian factor in the ancestral Amoeba, and that man is merely a group of the whole complex of characters allowed to produce real effects by the removal of a host of inhibiting factors, is incredible. The truth is that biological processes are not within our powers of conception as those of physics and chemistry are, and Bateson's hypothesis is nothing but the old theory of preformation in ontogeny. Just as the old embryologists conceived the adult individual to be contained with all its organs to the most minute details within the protoplasm of the fertilised ovum or one of the gametes, so the modern Mendelian, because he is unable to conceive or to obtain the evidence of the gradual development of a hereditary factor, conceives all the hereditary factors of the whole animal kingdom packed in infinite complexity within the protoplasm of the primordial living cells. That man is complex and Amoeba simple is merely a delusion; the truth according to Mendelism is that man is merely a fragment of the complexity of the original Amoeba.
Mendelism studies especially the heredity of characters, and only incidentally deals with recorded instances of the appearance of new forms, such as the origin of a salmon-coloured variety of Primula from a crimson variety. The occurrence of new characters, or mutations as they are called, has been specially studied by other investigators, and I propose briefly to consider the two most important examples of such research, namely, that by Professor T. H. Morgan, which deals with the American fruit-fly Drosophla, and the other which concerns the mutations of the genus of plants OEnothera, exemplified by our well-known Evening Primrose.