Evolution and Adaptation

From the angular yellow seeds:—

From the round green seeds:—

From the angular green seeds:—

Thus there were 9 different kinds of seeds produced. There had been separated out at this time 38 individuals like the parent seed plant, AB, and 30 like the parent pollen plant, ab. Since these had come from similar seeds of the preceding generation they may be looked upon as pure at this time. The forms Ab and aB are also constant forms which do not subsequently vary. The remainder are still mixed or hybrid in character. By successive self-fertilizations it is possible gradually to separate out from these the pure types of which they are compounded.

Without going into further detail it may be stated that the offspring of the parent hybrids, having two pairs of differentiating characters, are represented by the series:—

This series is really a combination of the two series:—

Mendel even went farther, and used two parent varieties having three differentiating characters, as follows:—

The results, as may be imagined, were quite complex, but can be expressed by combining these series:—

In regard to the two latter experiments, in which two and three characters respectively were used, it is interesting to point out that the form of the hybrid more nearly approaches “to that one of the parental plants which possesses the greatest number of dominant characters.” If, for instance, the seed plant has short stem, terminal white flowers, and simply inflated pods; the pollen plant, on the other hand, a long stem, violet-red flowers distributed along the stem, and constricted pods,—then the hybrid resembles the seed parent only in the form of the pod; in its other characters it agrees with the pollen plant. From this we may conclude that, if two varieties differing in a large number of characters are crossed, the hybrid might get some of its dominant characters from one parent, and other dominant characters from the other parent, so that, unless the individual characters themselves were studied, it might appear that the hybrids are intermediate between the two parents, while in reality they are only combinations of the dominant characters of the two forms. But even this is not the whole question.

Mendel points out that, from knowing the characters of the two parent forms (or varieties), one could not prophesy what the hybrid would be like without making the actual trial. Which of the characters of the two parent forms will be the dominant ones, and which recessive, can only be determined by experiment. Moreover, the hybrid characters are something peculiar to the hybrid itself, and to itself alone, and not simply the combination of the characters of the two forms. Thus in one case a hybrid from a tall and a short variety of pea was even taller than the taller parent variety. Bateson lays much emphasis on this point, believing it to be an important consideration in all questions relating to hybridization and inheritance.

The theoretical interpretation that Mendel has put upon his results is so extremely simple that there can be little doubt that he has hit on the real explanation. The results can be accounted for if we suppose that the hybrid produces egg-cells and pollen-cells, each of which is the bearer of only one of the alternative characters, dominant or recessive as the case may be. If this is the case, and if on an average there are the same number of egg-cells and pollen-cells, having one or the other of these kinds of characters, then on a random assortment meeting of egg-cells and pollen-cells, Mendel’s law would follow. For, 25 per cent of dominant pollen grains would meet with 25 per cent dominant egg-cells; 25 per cent recessive pollen grains would meet with 25 per cent recessive egg-cells; while the remaining 50 per cent of each kind would meet each other. Or, as Mendel showed by the following scheme:—

Or more simply by this scheme:—

Mendel’s results have received confirmation by a number of more recent workers, and while in some cases the results appear to be complicated by other factors, yet there can remain little doubt that Mendel has discovered one of the fundamental laws of heredity.

It has been found that there are some cases in which the sort of inheritance postulated by Mendel’s law does not seem to hold, and, in fact, Mendel himself spoke of such cases. He found that some kinds of hybrids do not break up in later generations into the parent forms. He also points out that in cases of discontinuity the variations in each character must be separately regarded. In most experiments in crossing, forms are chosen which differ from each other in a multitude of characters, some of which are continuous and others discontinuous, some capable of blending with their contraries while others are not. The observer in attempting to discover any regularity is confused by the complications thus introduced. Mendel’s law could only appear in such cases by the use of an overwhelming number of examples which are beyond the possibilities of experiment.^[25]

Let us now examine the bearing of these discoveries on the questions of variation which were raised in the preceding pages. It should be pointed out, however, that it would be premature to do more than indicate, in the most general way, the application of these conclusions. The chief value of Mendel’s results lies in their relation to the theory of inheritance rather than to that of evolution.

In the first place, Mendel’s results indicate that we cannot make any such sharp distinction as Darwin does between the results of inheritance of discontinuous and of continuous variations. As Mendel’s results show, it is the separate characters that must be considered in each case, and not simply the sum total of characters.

The more general objection that Darwin has made may appear to hold, nevertheless. He thinks that the evolution of animals and plants cannot rest primarily on the appearance of discontinuous variations, because they occur rarely and would be swamped by intercrossing. If Mendel’s law applies to such cases, that is, if a cross were made between such a sport and the original form, the hybrid in this case, if self-fertilized, would begin to split up into the two original forms. But, on the other hand, it could very rarely happen that the hybrid did fertilize its own eggs, and, unless this occurred, the hybrid, by crossing with the parent forms in each generation, would soon lose all its characters inherited from its “sport” ancestor. Unless, therefore, other individuals gave rise to sports at the same time, there would be little chance of producing new species in this way. We see then that discontinuity in itself, unless it involved infertility with the parent species, of which there is no evidence, cannot be made the basis for a theory of evolution, any more than can individual differences, for the swamping effect of intercrossing would in both cases soon obliterate the new form. If, however, a species begins to give rise to a large number of individuals of the same kind through a process of discontinuous variation, then it may happen that a new form may establish itself, either because it is adapted to live under conditions somewhat different from the parent form, so that the dangers of intercrossing are lessened, or because the new form may absorb the old one. It is also clear, from what has gone before, that the new form can only cease to be fertile with the parent form, or with its sister forms, after it has undergone such a number of changes that it is no longer able to combine the differences in a new individual. This result will depend both on the kinds of the new characters, as well as the amounts of their difference. This brings us to a consideration of the results of De Vries, who has studied the first steps in the formation of new species in the “mutations” of the evening primrose.

The Mutation Theory of De Vries

De Vries defines the mutation theory as the conception that “the characters of the organism are made up of elements (‘Einheiten’) that are sharply separated from each other. These elements can be combined in groups, and in related species the same combinations of elements recur. Transitional forms like those that are so common in the external features of animals and plants do not exist between the elements themselves, any more than they do between the elements of the chemist.”

This principle leads, De Vries says, in the domain of the descent theory to the conception that species have arisen from each other, not continuously, but by steps. Each new step results from a new combination as compared with the old one, and the new forms are thereby completely and sharply separated from the species from which they have come. The new species is all at once there; it has arisen from the parent form without visible preparation and without transitional steps.

The mutation theory stands in sharp contrast to the selection theory. The latter uses as its starting-point the common form of variability known as individual or fluctuating variation; but according to the mutation theory there are two kinds of variation that are entirely different from each other. “The fluctuating variation can, as I hope to show, not overstep the bounds of the species, even after the most prolonged selection,—much less can this kind of variation lead to the production of new, constant characters.” Each peculiarity of the organism has arisen from a preceding one, not through the common form of variation, but through a sudden change that may be quite small but is perfectly definite. This kind of variability that produces new species, De Vries calls mutability; the change itself he calls a mutation. The best-known examples of mutations are those which Darwin called “single variations” or “sports.”

De Vries recognizes the following kinds of variation:—

First, the polymorphic forms of the systematists. The ordinary groups which, following Linnæus, we call species, are according to De Vries collective groups, which are the outcome of mutations. Many such Linnæan species include small series of related forms, and sometimes even large numbers of such forms. These are as distinctly and completely separated from each other as are the best species. Generally these small groups are called varieties, or subspecies,—varieties when they are separated by a single striking character, subspecies when they differ in the totality of their characters, in the so-called habitus.

These groups have already been recognized by some investigators as elementary species, and have been given corresponding binary names. Thus there are recognized two hundred elementary species of the form formerly called Draba verna.

When brought under cultivation these elementary species are constant in character and transmit their peculiarities truly. They are not local races in the sense that they are the outcome in each generation of special external conditions. Many other Linnæan species are in this respect like Draba verna, and most varieties, De Vries thinks, are really elementary species.

Second, the polymorphism due to intercrossing is the outcome of different combinations of hereditary qualities. There are here, De Vries says, two important classes of facts to be kept strictly apart,—scientific experiment, and the results of the gardener and of the cultivator. The experimenter chooses for crossing, species as little variable as possible; the gardener and cultivator on the other hand prefer to cross forms of which one at least is variable, because the variations may be transmitted to the hybrid, and in this way a new form be produced.

New elementary characters arise in experiments in crossing only through variability, not through crossing itself.

Third, variability in the ordinary sense, that is, individual variability, includes those differences between the individual organs that follow Quetelet’s theory of chance. This kind of variability is characterized by its presence at all times, in all groups of individuals.

De Vries recalls Galton’s apt comparison between variability and a polyhedron which can roll from one face to another. When it comes to rest on any particular face, it is in stable equilibrium. Small vibrations or disturbances may make it oscillate, but it returns always to the same face. These oscillations are like the fluctuating variations. A greater disturbance may cause the polyhedron to roll over on to a new face, where it comes to rest again, only showing the ever present fluctuations around its new centre. The new position corresponds to a mutation. It may appear from our familiarity with the great changes that we associate with the idea of discontinuous variability, that a mutation must also involve a considerable change. Such, however, De Vries says, is not the case. In fact, numerous mutations are smaller than the extremes of fluctuating variation. For example, the different elementary species of Draba verna are less different from each other than the forms of leaves on a tree. The essential differences between the two kinds of variation is that the mutation is constant, while the continuous variation fluctuates back and forth.

The following example is given by De Vries to illustrate the general point of view in regard to varieties and species. The species Oxalis corniculata is a “collective” species that lives in New Zealand. It has been described as having seven well-characterized varieties which do not live together or have intermediate forms. If we knew only this group, there would be no question that there are seven good species. But in other countries intermediate forms exist, which exactly bridge over the differences between the seven New Zealand forms. For this reason all the forms have been united in a single species.

Another example is that of the fern, Lomaria procera, from New Zealand, Australia, South Africa, and South America. If the forms from only one country be considered, they appear to be different species; but if all the forms from the different parts of the world be taken into account, they constitute a connected group, and are united into one large species.

It will be seen, therefore, that the limits of a collective species are determined solely by the deficiencies in the genealogical tree of the elementary species. If all the elementary species in one country were destroyed, then the forms living in other countries that had been previously held together because of those which have now been destroyed, would, after the destruction, become true species. In other words: “The Linnæan species are formed by the disappearance of other elementary species, which at first connected all forms. This mode of origin is a purely historical process, and can never become the subject of experimental investigation.” Spencer’s famous expression, the “survival of the fittest,” is incomplete, and should read the “survival of the fittest species.” It is, therefore, not the study of Linnæan species that has a physiological interest, but it is the study of the elementary species of which the Linnæan species are made up, that furnishes the all-important problem for experimental study.

De Vries gives a critical analysis of a number of cases in which new races have been formed under domestication. He shows very convincingly that, whenever the result has been the outcome of the selection of fluctuating variations, the product that is formed can only be kept to its highest point of development by the most rigid and ever watchful care. If selection ceases for only a few generations, the new form sinks back at once to its original level. Many of our cultivated plants have really arisen, not by selection of this sort, but by mutations; and there are a number of recorded cases where the first and sudden appearance of a new form has been observed. In such cases as these there is no need for selection, for if left to themselves there is no return to the original form. If, however, after a new mutation has appeared in this way, we subject its fluctuating variations to selection, we can keep the new form up to its most extreme limit, but can do nothing more.

Another means, frequently employed, by which new varieties have been formed is by bringing together different elementary species under cultivation. For instance, there are a large number of wild elementary species of apples, and De Vries believes that our different races of apples owe their origin in part to these different wild forms. Crossing, cultivation, and selection have done the rest.

De Vries points out some of the inconsistencies of those who have attempted to discriminate between varieties and species. The only rule that can be adhered to is that a variety differs from a species to which it belongs in only one or in a few characters. Most so-called varieties in nature are really elementary species, which differ from their nearest relatives, not in one character only, but in nearly all their characters. There is no ground, De Vries states, for believing them to be varieties. If it is found inconvenient to rank them under the names of the old Linnæan species, it will be better, perhaps, to treat them as subspecies, but De Vries prefers to call them elementary species.

In regard to the distribution of species in nature, it may be generally stated that the larger the geographical domain so much the larger is the number of elementary species. They are found to be heaped up in the centre of their area of distribution, but are more scattered at the periphery.

In any one locality each Linnæan species has as a rule only one or a few elementary species. The larger the area the more numerous the forms. From France alone Jordan had brought together in his garden 50 elementary species of Draba verna. From England, Italy, and Austria there could be added 150 more. This polymorphism is, De Vries thinks, a general phenomenon, although the number of forms is seldom so great as in this case.

Amongst animals this great variety of forms is not often met with, yet amongst the mammalia and birds of North America there are many cases of local forms or races, some of which at least are probably mutations. This can only be proven, however, by actually transferring the forms to new localities in order to find out if they retain their original characters, or become changed into another form. It seems not improbable that many of the forms are not the outcome of the external conditions under which the animal now lives, but would perpetuate themselves in a new environment.

From the evidence that his results have given, De Vries believes it is probable that mutation has occurred in all directions. In the same way that Darwin supposed that individual or fluctuating variations are scattering, so also De Vries believes that the new forms that arise through mutation are scattering. On this point it seems to me that De Vries may be too much prejudiced by his results with the evening primrose. If, as he supposes, many forms, generally ranked as varieties, are really elementary species, it seems more probable that the mutation of a form may often be limited to the production of one or of only a very few new forms. The single variations, or sports, point even more strongly in favor of this interpretation. Moreover, the general problem of evolution from a purely theoretical point of view is very much simplified, if we assume that the kinds of mutating forms may often be very limited, and that mutations may often continue to occur in a direct line. On this last point, De Vries argues that the evidence from paleontology cannot be trusted, for all that we can conclude from fossil remains is that certain mutations have dominated, and have been sufficiently abundant to leave a record. In other words, the conditions may have been such that only certain forms could find a foothold.

De Vries asks whether there are for each species periods of mutation when many and great changes take place, and periods when relatively little change occurs. The evidence upon which to form an opinion is scanty, but De Vries is inclined to think that such periods do occur. It is at least certain from our experience that there are long periods when we do not see new forms arising, while at other times, although we know very few of them, epidemics of change may take place. The mutative period which De Vries found in the evening primrose is the best-known example of such a period of active mutation. Equally important for the descent theory is the idea that the same mutation may appear time after time. There is good evidence to show that this really occurs, and in consequence the chances for the perpetuation of such a form are greatly increased. Delbœuf, who advocated this idea of the repeated reappearance of a new form, has also attempted to show that if this occurs the new form may become established without selection of any kind taking place,—the time required depending upon the frequency with which the new form appears. This law of Delbœuf, De Vries believes, is correct from the point of view of the mutation theory. It explains, in a very simple way, the existence of numerous species-characters that are entirely useless, such, for instance, as exist between the different elementary species of Draba verna. “According to the selection theory only useful characters can survive; according to the mutation theory, useless characters also may survive, and even those that may be hurtful to a small degree.”

We may now proceed to examine the evidence from which De Vries has been led to the general conclusions given in the preceding pages. De Vries found at Hilversam, near Amsterdam, a locality where a number of plants of the evening primrose, Œnothera lamarckiana, grow in large numbers. This plant is an American form that has been imported into Europe. It often escapes from cultivation, as is the case at Hilversam, where for ten years it had been growing wild. Its rapid increase in numbers in the course of a few years may be one of the causes that has led to the appearance of a mutation period. The escaped plants showed fluctuating variations in nearly all of their organs. They also had produced a number of abnormal forms. Some of the plants came to maturity in one year, others in two, or in rare cases in three, years.

A year after the first finding of these plants De Vries observed two well-characterized forms, which he at once recognized as new elementary species. One of these was O. brevistylis, which occurred only as female plants. The other new species was a smooth-leafed form with a more beautiful foliage than O. lamarckiana. This is O. lævifola. It was found that both of these new forms bred true from self-fertilized seeds. At first only a few specimens were found, each form in a particular part of the field, which looks as though each might have come from the seeds of a single plant.

These two new forms, as well as the common O. lamarckiana, were collected, and from these plants there have arisen the three groups or families of elementary species that De Vries has studied. In his garden other new forms also arose from those that had been brought under cultivation. The largest group and the most important one is that from the original O. lamarckiana form. The accompanying table shows the mutations that arose between 1887 and 1899 from these plants. The seeds were selected in each case from self-fertilized plants of the lamarckiana form, so that the new plants appearing in each horizontal line are the descendants in each generation of lamarckiana parents. It will be observed that the species, O. oblongata, appeared again and again in considerable numbers, and the same is true for several of the other forms also. Only the two species, O. gigas and O. scintillans, appeared very rarely.

Thus De Vries had, in his seven generations, about fifty thousand plants, and about eight hundred of these were mutations. When the flowers of the new forms were artificially fertilized with pollen from the flowers on the same plant, or of the same kind of plant, they gave rise to forms like themselves, thus showing that they are true elementary species.^[26] It is also a point of some interest to observe that all these forms differed from each other in a large number of particulars.

Only one form, O. scintillans, that appeared eight times, is not constant as are the other species. When self-fertilized its seeds produce always three other forms, O. scintillans, O. oblongata, and O. lamarckiana. It differs in this respect from all the other elementary species, which mutate not more than once in ten thousand individuals.

From the seeds of one of the new forms, O. lævifolia, collected in the field, plants were reared, some of which were O. lamarckiana and others O. lævifolia. They were allowed to grow together, and their descendants gave rise to the same forms found in the lamarckiana family, described above, namely, O. lata, elliptica, nannella, rubrinervis, and also two new species, O. spatulata and leptocarpa.

In the lata family, only female flowers are produced, and, therefore, in order to obtain seeds they were fertilized with pollen from other species. Here also appeared some of the new species, already mentioned, namely, albida, nannella, lata, oblongata, rubrinervis, and also two new species, elliptica and subovata.

De Vries also watched the field from which the original forms were obtained, and found there many of the new species that appeared under cultivation. These were found, however, only as weak young plants that rarely flowered. Five of the new forms were seen either in the Hilversam field, or else raised from seeds that had been collected there. These facts show that the new species are not due to cultivation, and that they arise year after year from the seeds of the parent form, O. lamarckiana.

Conclusions

From the evidence given in the preceding pages it appears that the line between fluctuating variations and mutations may be sharply drawn. If we assume that mutations have furnished the material for the process of evolution, the whole problem appears in a different light from that in which it was placed by Darwin when he assumed that the fluctuating variations are the kind which give the material for evolution.

From the point of view of the mutation theory, species are no longer looked upon as having been slowly built up through the selection of individual variations, but the elementary species, at least, appear at a single advance, and fully formed. This need not necessarily mean that great changes have suddenly taken place, and in this respect the mutation theory is in accord with Darwin’s view that extreme forms that rarely appear, “sports,” have not furnished the material for the process of evolution.

As De Vries has pointed out, each mutation may be different from the parent form in only a slight degree for each point, although all the points may be different. The most unique feature of these mutations is the constancy with which the new form is inherited. It is this fact, not previously fully appreciated, that De Vries’s work has brought prominently into the foreground. There is another point of great interest in this connection. Many of the groups that Darwin recognized as varieties correspond to the elementary species of De Vries. These varieties, Darwin thought, are the first stages in the formations of species, and, in fact, cannot be separated from species in most cases. The main difference between the selection theory and the mutation theory is that the one supposes these varieties to arise through selection of individual variations, the other supposes that they have arisen spontaneously and at once from the original form. The development of these varieties into new species is again supposed, on the Darwinian theory, to be the result of further selection, on the mutation theory, the result of the appearance of new mutations.

In consequence of this difference in the two theories, it will not be difficult to show that the mutation theory escapes some of the gravest difficulties that the Darwinian theory has encountered. Some of the advantages of the mutation theory may be briefly mentioned here.

1. Since the mutations appear fully formed from the beginning, there is no difficulty in accounting for the incipient stages in the development of an organ, and since the organ may persist, even when it has no value to the race, it may become further developed by later mutations and may come to have finally an important relation to the life of the individual.

2. The new mutations may appear in large numbers, and of the different kinds those will persist that can get a foothold. On account of the large number of times that the same mutations appear, the danger of becoming swamped through crossing with the original form will be lessened in proportion to the number of new individuals that arise.

3. If the time of reaching maturity in the new form is different from that in the parent forms, then the new species will be kept from crossing with the parent form, and since this new character will be present from the beginning, the new form will have much better chances of surviving than if a difference in time of reaching maturity had to be gradually acquired.

4. The new species that appear may be in some cases already adapted to live, in a different environment from that occupied by the parent form; and if so, it will be isolated from the beginning, which will be an advantage in avoiding the bad effects of intercrossing.

5. It is well known that the differences between related species consists largely in differences of unimportant organs, and this is in harmony with the mutation theory, but one of the real difficulties of the selection theory.

6. Useless or even slightly injurious characters may appear as mutations, and if they do not seriously affect the perpetuation of the race, they may persist.

In Chapters X and XI, an attempt will be made to point out in detail the advantages which the mutation theory has over the Darwinian theory.