As I cannot find that this remarkable skull has been figured before, I have had the accompanying woodcut made in order to compare it with the skull of a Charsley Forest ox; and a glance is sufficient to show what "a wonderfully different appearance" it presents.
Now the important points in the present connexion with regard to this peculiar race of cattle are the following.
Their origin is not known; but it must have been subsequent to the year 1552, when cattle were first introduced to America from Europe, and it is known that such cattle have been in existence for at least a century. The breed is very true, and a niata bull and cow invariably produce niata calves. A niata bull crossed with a common cow, and the reverse cross, yield offspring having an intermediate character, but with the niata peculiarities highly conspicuous[104].
Here, then, we have unquestionable evidence of a whole congeries of very distinctive characters, so unlike anything that occurs in any other cattle, that, had they been found in a state of nature, they would have been regarded as a distinct species. And the highly peculiar characters which they present conform to all "the most essential features of specific characters," as these are stated by Mr. Wallace in his objection to the case of the pig's appendages. That is to say, "they are symmetrical, they are inherited, and they are constant." In point of fact, they are always "constant," both as to occurrence and symmetry, while they are so completely "inherited" that not only does "a niata bull and cow invariably produce niata calves"; but even when crossed with other cattle the result is a hybrid, "with the niata character strongly displayed."
Hence, if we were to follow Mr. Wallace's criteria of specific characters, which show that the pig's appendages "cannot be classed with specific characters" (or with anything of the nature of specific characters), it would follow that the niata peculiarities can be so classed. This, therefore, is a case where he will find all the reasons which in other cases he takes to justify him in falling back upon the argument from ignorance. The cattle are half wild, he may urge; and so the three-fold constancy of their peculiar characters may very well be due, either directly or indirectly, to natural selection—i.e. they may either be of some hidden use themselves, or correlated with some other modifications that are of use: it is, he may say, as in such cases he often does say, for us to disprove both these possibilities.
Well, here we have one of those rare cases where historical information, or other accidents, admit of our discharging this burden of proving a negative. Darwin's further description shows that this customary refuge in the argument from ignorance is most effectually closed. For—
"When the pasture is tolerably long, these cattle feed as well as common cattle with their tongue and palate; but during the great droughts, when so many animals perish on the Pampas, the niata breed lies under a great disadvantage, and would, if not attended to, become extinct; for the common cattle, like horses, are able to keep alive by browsing with their lips on the twigs of trees and on reeds; this the niatas cannot so well do, as their lips do not join, and hence they are found to perish before the common cattle. This strikes me as a good illustration of how little we are able to judge from the ordinary habits of an animal, on what circumstances, occurring only at long intervals of time, its rarity or extinction may depend. It shows us, also, how natural selection would have determined the rejection of the niata modification, had it arisen in a state of nature[105]."
Hence, it is plainly impossible to attribute this modification to natural selection, either as acting directly on the modified parts themselves, or indirectly through correlation of growth. And as the modification is of specific magnitude on the one hand, while it presents all "the most essential features of specific characters" on the other, I do not see any means whereby Mr. Wallace can meet it on his a priori principles. It would be useless to answer that these characters, although conforming to all his tests of specific characters, differ in respect of being deleterious, and would therefore lead to extermination were the animals in a wholly wild state; because, considered as an argument, this would involve the assumption that, apart from natural selection, only deleterious characters can arise under nature—i. e. that merely "indifferent" characters can never do so, which would be absurd. Indeed, I have chosen this case of the niata cattle expressly because their strongly marked peculiarities are deleterious, and therefore exclude Mr. Wallace's appeal to the argument from ignorance of a possible utility. But if even these pronounced and deleterious peculiarities can arise and be perpetuated with such constancy and fidelity, much more is this likely to be the case with less pronounced and merely neutral peculiarities.
It may, however, be further objected that these cattle are not improbably the result of artificial selection. It may be suggested that the semi-monstrous breed originated in a single congenital variation, or "sport," which was isolated and multiplied as a curiosity by the early settlers. But even if such be the explanation of this particular case, the fact would not weaken our illustration. On the contrary, it would strengthen our general argument, by showing an additional means whereby indifferent specific characters can arise and become fixed in a state of nature. As it seems to me extremely probable that the niata cattle did originate in a congenital monstrosity, which was then isolated and multiplied by human agency (as is known to have been the case with the "ancon sheep"), I will explain why this tends to strengthen our general argument.
It is certain that if these animals were ever subject to artificial isolation for the purpose of establishing their breed, the process must have ceased a long time ago, seeing that there is no memory or tradition of its occurrence. Now this proves that, however the breed may have originated, it has been able to maintain its many and highly peculiar characters for a number of generations without the help of selection, either natural or artificial. This is the first point to be clear upon. Be its origin what it may, we know that this breed has proved capable of perpetuating itself with uniform "constancy" for a number of generations after the artificial selection has ceased—supposing such a process ever to have occurred. And this certain fact that artificial selection, even if it was originally needed to establish the type, has not been needed to perpetuate the type, is a full answer to the supposed objection. For, in view of this fact, it is immaterial what the origin of the niata breed may have been. In the present connexion, the importance of this breed consists in its proving the subsequent "stability" of an almost monstrous form, continued through a long series of generations by the force of heredity alone, without the aid of any form of selection.
The next point is, that not only is a seeming objection to the illustration thus removed, but that, if we do entertain the question of origin, and if we do suppose the origin of these cattle to have been in a congenital "sport," afterwards multiplied by artificial isolation, we actually strengthen our general argument by increasing the importance of this particular illustration. For the illustration then becomes available to show how indifferent specific characters may sometimes originate in merely individual sports, which, if not immediately extinguished by free intercrossing, will perpetuate themselves by the unaided force of heredity. But this is a point to which we shall recur in the ensuing chapter.
In conclusion, it is worth while to remark, with regard to Mr. Wallace's argument from constancy, that, as a matter of fact, utility does not seem to present any greater power in securing "stability of characters" than any other cause of like constancy. Thus, for instance, whatever the causes may have been which have produced and perpetuated the niata breed of cattle, they have certainly produced a wonderful "stability" of a great modification in a wonderfully short time. And the same has to be said of the ducks in St. James' Park, as well as sundry other cases. On the other hand, when, as in the case of numberless natural species, modification has been undoubtedly produced by natural selection, although the modification must have had a very much longer time in which to have been fixed by heredity, it is often far from being stable—notwithstanding that Mr. Wallace regards stability as a criterion of specific characters. Indeed—and this is more suggestive still—there even seems to be a kind of inverse proportion between the utility and the stability of a specific character. The explanation appears to be (Origin of Species, pp. 120-2), that the more a specific character has been forced on by natural selection on account of its utility, the less time will it have had to become well fixed by heredity before attaining a full development. Moreover, as Darwin adds, in cases where the modification has not only been thus "comparatively recent," but also "extraordinarily great," the probability is that the parts so modified must have been very variable in the first instance, and so are all the more difficult to render constant by heredity. Thus we see that utility is no better—even if it be so good—a cause of stability in specific characters, as are the unknown causes of stability in many varietal characters[106].
Let us now proceed to indicate some of the causes, other than natural selection, which may be regarded as adequate to induce such changes in organic types as are taken by systematists to constitute diagnostic distinctions between species and species. We will first consider causes external to organisms, and will then go on to consider those which occur within the organisms themselves: following, in fact, the classification which Darwin has himself laid down. For he constantly speaks of such causes as arising on the one hand, from "changed conditions of life" and, on the other hand, from "the nature of the organism"—that is, from internal processes leading to "variations which seem to us in our ignorance to arise spontaneously."
In neither case will it be practicable to give more than a brief résumé of all that might be said on these interesting topics.
There is an overwhelming mass of evidence to prove that the assemblage of external conditions of life conveniently summarized in the word Climate, exercise a potent, an uniform, and a permanent influence on specific characters.
With regard to plants, Darwin adduces a number of facts to show the effects of climate on wheat, cabbages, and other vegetables. Here, for example, is what he says with regard to maize imported from America to Germany:—
"During the first year the plants were twelve feet high, and a few seeds were perfected; the lower seeds in the ear kept true to their proper form, but the upper seeds became slightly changed. In the second generation the plants were from nine to ten feet high, and ripened their seed better; the depression on the outer side of the seed had almost disappeared, and the original beautiful white colour had become duskier. Some of the seeds had even become yellow, and in their now rounded form they approached the common European maize. In the third generation nearly all resemblance to the original and very distinct American parent-form was lost[107]."
As these "highly remarkable" changes were effected in but three generations, it is obvious that they cannot have been dependent on selection of any kind. The same remark applies to trees. Thus,—
"Mr. Meehan has compared twenty-nine kinds of American trees with their nearest European allies, all grown in close proximity and under as nearly as possible the same conditions. In the American species he finds, with the rarest exceptions, that the leaves fall earlier in the season, and assume before their fall a brighter tint; that they are less deeply toothed or serrated; that the buds are smaller; that the trees are more diffuse in growth and have fewer branchlets; and, lastly, that the seeds are smaller—all in comparison with the corresponding European species. Now, considering that these corresponding trees belong to several distinct orders, and that they are adapted to widely different stations, it can hardly be supposed that their differences are of any special service to them in the New and Old worlds; and, if so, such differences cannot have been gained through natural selection, and must be attributed to the long continued action of a different climate[108]."
These cases, however, I quote mainly in order to show Darwin's opinion upon the matter, with reference to the absence of natural selection. For, where the vegetable kingdom is concerned, the fact of climatic variation is so general, and in its relation to diagnostic work so important, that it constitutes one of the chief difficulties against which species-makers have to contend. And the more carefully the subject is examined the greater does the difficulty become. But, as to this and other general facts, it will be best to allow a recognized authority to speak; and therefore I will give a few extracts from Kerner's work on Gute und schlechte Arten.
He begins by showing that geographical (or it may be topographical) varieties of species are often so divergent, that without a knowledge of intermediate forms there could be no question as to their being good species. As a result of his own researches on the subject, he can scarcely find language strong enough to express his estimate of the extent and the generality of this source of error. In different parts of Europe, or even in different parts of the Alps, he has found these climatic varieties in such multitudes and in such high degrees both of constancy and divergence, that, after detailing his results, he finishes his essay with the following remarkable conclusions:—
"Die Wissenchaft geht aber ihren Entwicklungsgang im grossen Ganzen gerade so, wie die Erkenntniss bei jedem einzelnen Naturforscher. Fast jeder Botaniker muss seinen Entwicklungsgang durchmachen und gelangt endlich mehr oder weniger nahe zu demselben Ziele. Die Ungleichheit besteht nur darin, dass der eine langsamer, der andere aber rascher bei dem Ziele ankommt. Anfänglich müht sich jeder ab, die Formen in hergebrachter Weise zu gliedern und die 'guten Arten' herauszulesen. Mit der Erweiterung des Gesichtskreises und mit der Vermehrung der Anschauungen aber schwindet auch immer mehr der Boden unter den Füssen, die bisher für unverrückbar gehaltenen Grenzen der gut geglaubten Arten stellen sich als eine der Natur angelegte Zwangsjacke heraus, die Uebcrzeugung, dass die Grenzen, welche wir ziehen, eben nur künstliche sind, gewinnt immer mehr und mehr die Oberhand, und wer nicht gerade zu den hartgesottenen Eigensinnigen gehört, und wer die Wahrheit höher stellt als das starre Festhalten an seinen früheren Ansichten, geht schliesslich bewusst oder unbewusst in das Lager derjenigen über, in welchem auch ich mir ein bescheidenes Plätzchen aufgesucht habe."
By these "hard-boiled" botanists he means those who entertain the traditional notion of a species as an assemblage of definite characters, always and everywhere associated together. This notion (Artsbeständigkeit) must be entirely abandoned. Summarizing Kerner's facts for their general results we find that his extensive investigations have proved that in his numberless kinds of European plants the following relations frequently obtain. Supposing that there are two or more allied species, A and B, then A' and B' may be taken to represent their respective types as found in some particular area. It does not signify whether A' and B' are geographically remote from, or close to, A and B; the point is that, whether in respect of temperature, altitude, moisture, character of soil, &c., there is some difference in the conditions of life experienced by the plants growing at the different places. Now, in numberless plants it is found that the typical or constant peculiarities of A' differ more from those of A than they do from those of B; while, conversely, the characters of A' may bear more resemblance to those of B' than they do to those of A—on account of such characters being due to the same external causes in both cases. The consequence is that A' might more correctly be classified with B', or vice versa. Another consequence is that whether A and B, or A' and B', be recorded as the "good species" usually depends upon which has happened to have been first described.
Such a mere abstract of Kerner's general results, however, can give no adequate idea of their cogency: for this arises from the number of species in which specific characters are thus found to change, and even to interchange, with different conditions of life. Thus he gives an amusing parable of an ardent young botanist, Simplicius, who starts on a tour in the Tyrol with the works of the most authoritative systematists to assist him in his study of the flora. The result is that Simplicius becomes so hopelessly bewildered in his attempts at squaring their diagnostic descriptions with the facts of nature, that he can only exclaim in despair—"Sonderbare Flora, diese tirolische, in welcher so viele characteristische Pflanzen nur schlechte Arten, oder gar noch schlechter als schlechte Arten, sind." Now, in giving illustrations of this young man's troubles, Kerner fills five or six pages with little else than rows of specific names.
Upon the whole, Kerner concludes that the more the subject is studied, the more convinced must the student become that all distinction between species as "good" and "bad" vanishes. In other words, the more that our knowledge of species and of their diagnostic characters increases, the more do we find that "bad species" multiply at the expense of "good species"; so that eventually we must relinquish the idea of "good species" altogether. Or, conversely stated, we must agree to regard as equally "good species" any and every assemblage of individuals which present the same peculiarities: provided that these peculiarities do not rise to a generic value, they equally deserve to be regarded as "specific characters," no matter how trivial, or how local, they may be. In fact, he goes so far as to say that when, as a result of experiments in transplantation from one set of physical conditions to another, seedlings are found to present any considerable and constant change in their specific characters, these seedlings are no less entitled to be regarded as a "good species" than are the plants from which they have been derived. Probably few systematists will consent to go quite so far as this; but the fact that Kerner has been led deliberately to propound such a statement as a result of his wide observations and experiments is about as good evidence as possible on the points with which we are here concerned. For even Simplicius would hardly be quite so simple as to suppose that each one of all the characters which he observes in his "remarkable flora," so largely composed of "bad or even worse than bad species," is of utilitarian significance.
Be it noted, however, that I am not now expressing my own opinion. There are weighty reasons against thus identifying climatic variations with good species—reasons which will be dealt with in the next chapter. Kerner does not seem to appreciate the weight of these reasons, and therefore I do not call him as a witness to the subject as a whole; but only to that part of it which has to do with the great and general importance of climatic variability in relation to diagnostic work. And thus far his testimony is fully corroborated by every other botanist who has ever attended to the subject. Therefore it does not seem worth while to quote further authorities in substantiation of this point, such as Gärtner, De Candolle, Nägeli, Peter, Jordan, &c. For nowadays no one will dispute the high generality and the frequently great extent of climatic variation where the vegetable kingdom is concerned. Indeed, it may fairly be doubted whether there is any one species of plant, whose distribution exposes it to any considerable differences in its external conditions of life, which does not present more or less considerable differences as to its characters in different parts of its range. The principal causes of such climatic variation appear to be the chemical, and, still more, the mechanical nature of soil; temperature; intensity and diurnal duration of light in spring and summer; moisture; presence of certain salts in the air and soil of marine plants, or of plants growing near mineral springs; and sundry other circumstances of a more or less unknown character.
Before closing these remarks on climatic variation in the vegetable kingdom, prominent attention must be directed to a fact of broad generality and, in relation to our present subject, of considerable importance. This is that the same external causes very frequently produce the same effects in the way of specific change throughout large numbers of unrelated species—i.e. species belonging to different genera, families, and orders. Moreover, throughout all these unrelated species, we can frequently trace a uniform correlation between the degrees of change and the degrees to which they have been subjected to the causes in question.
As examples, all botanists who have attended to the subject are struck by the similarity of variation presented by different species growing on the same soils, altitudes, latitudes, longitudes, and so forth. Plants growing on chalky soils, when compared with those growing on richer soils, are often more thickly covered with down, which is usually of a white or grey colour. Their leaves are frequently of a bluish-green tint, more deeply cut, and less veined, while their flowers tend to be larger and of a lighter tint. There are similarly constant differences in other respects in varieties growing on sundry other kinds of soils. Sea-salt has the general effect, on many different kinds of plants, of producing moist fleshy leaves, and red tints. Experiments in transplantation have shown that these changes may be induced artificially; so there can be no doubt as to its being this that and the other set of external conditions which produces them in nature. Again, dampness causes leaves to become smoother, greener, less cut, and the flowers to become darker; while dryness tends to produce opposite effects. I need not go on to specify the particular results on all kinds of plants of altitude, latitude, longitude, and so forth. For we are concerned only with the fact that these two correlations may be regarded as general laws appertaining to the vegetable kingdom—namely, (A) that the same external causes produce similar varietal effects in numerous unallied species of plants; and, (B) that the more these species are exposed to such causes the greater is the amount of varietal effect produced—so that, for instance, on travelling from latitude to latitude, longitude to longitude, altitude to altitude, &c., we may see greater and greater degrees of such definite and more or less common varietal changes affecting the unallied species in question. Now these general laws are of importance for us, because they prove unequivocally that it is the direct action of external conditions of life which produce climatic variations of specific types. And, taken in connexion with the results of experiments in transplantation (which in a single generation may yield variations similar to those found in nature under similar circumstances), these general laws still further indicate that climatic variations are "indifferent" variations. In other words, we find that changes of specific characters are of widespread occurrence in the vegetable kingdom, that they are constantly and even proportionally related to definite external circumstances, but yet that, in as far as they are climatic, they cannot be attributed to the agency of natural selection[109].
Turning next to animals, it may first be observed that climatic conditions do not appear to exercise an influence either so general or so considerable as in the case of plants. Nevertheless, although these influences are relatively more effective in the vegetable kingdom than they are in the animal, absolutely considered they are of high generality and great importance even in the latter. But as this fact is so well recognized by all zoologists, it will be needless to give more than a very few illustrations. Indeed, throughout this discussion on climatic influences my aim is merely to give the general reader some idea of their importance in regard to systematic natural history; and, therefore, such particular cases as are mentioned are selected only as samples of whole groups of cases more or less similar.
With regard to animals, then, we may best begin by noticing that, just as in the case of plants, there is good evidence of the same external causes producing the same effects in multitudes of species belonging to different genera, families, orders, and even classes. Moreover, we are not without similarly good evidence of degrees of specific change taking place in correlation with degrees of climatic change, so that we may frequently trace a gradual progress of the former as we advance, say, from one part of a large continent to another. Instances of these correlations are not indeed so numerous in the animal kingdom as they are in the vegetable. Nevertheless they are amply sufficient for our present purposes.
For example, Mr. Allen has studied in detail changes of size and colour among birds and mammals on the American continent; and he finds a wonderfully close sliding scale of both, corresponding stage by stage with gradual changes of climate. Very reasonably he attributes this to the direct influence of climatic conditions, without reference to natural selection—as does also Mr. Gould with reference to similar facts which he has observed among the birds of Australia. Against this view Mr. Wallace urges, "that the effects are due to the greater or less need of protection." But it is difficult to believe that such can be the case where so innumerable a multitude of widely different species are concerned—presenting so many diverse habits, as well as so many distinct habitats. Moreover, the explanation seems incompatible with the graduated nature of the change, and also with the fact that not only colouration but size, is implicated.
We meet with analogous facts in butterflies. Thus Lycaena agestis not only presents seasonal variations, (A) and (B); but while (A) and (B) are respectively the winter and summer forms in Germany, (B) and (C) are the corresponding forms in Italy. Therefore, (B) is in Germany the summer form, and in Italy the winter form—the German winter form (A) being absent in Italy, while the Italian summer form (C) is absent in Germany. Probably these facts are due to differences of temperature in the two countries, for experiments have shown that when pupae of sundry species of moths and butterflies are exposed to different degrees of temperature, the most wonderful changes of colour may result in the insects which emerge. The remarkable experiments of Dorfmeister and Weismann in relation to this subject are well known. More recently Mr. Merrifield has added to their facts, and concludes that the action of cold upon the pupae—and also, apparently, upon the larvae—has a tendency to produce dark hues in the perfect insect[110].
But, passing now from such facts of climatic variations over wide areas to similar facts within small areas, in an important Memoir on the Cave Fauna of North America, published a few years ago by the American Academy of Sciences, it is stated:—
"As regards change of colour, we do not recall an exception to the general rule that all cave animals are either colourless or nearly white, or, as in the case of Arachnida and Insects, much paler than their out-of-door relatives."
Now, when we remember that these cave faunas comprise representatives of nearly all classes of the animal kingdom, it becomes difficult, if not impossible, to imagine that so universal a discharge of colouring can be due to natural selection. It must be admitted that the only way in which natural selection could act in this case would be indirectly through the principle of correlation. There being no light in the caves, it can be of no advantage to the animals concerned that they should lose their colour for the sake of protection, or for any other reason of a similarly direct kind. Therefore, if the loss of colour is to be ascribed to natural selection, this can only be done by supposing that natural selection has here acted indirectly through the principle of correlation. There is evidence to show that elsewhere modification or loss of colour is in some cases brought about by natural selection, on account of the original colour being correlated with certain physiological characters (such as liability to particular diseases, &c.); so that when natural selection operates directly upon these physiological characters, it thereby also operates indirectly upon the correlated colours. But to suppose that this can be the explanation of the uniform diminution of colour in all inhabitants of dark caves would be manifestly absurd. If there were only one class of animals in these caves, such as Insects, it might be possible to surmise that their change of colour is due to natural selection acting directly upon their physiological constitutions, and so indirectly upon their colours. But it would be absurd to suppose that such can be the explanation of the facts, when these extend in so similar a manner over so many scores of species belonging to such different types of animal life.
With more plausibility it might be held that the universal discharge of colour in these cave-faunas is due, not to the presence, but to the absence of selection—i. e. to the cessation of selection, or panmixia. But against this—at all events as a full or general explanation—lie the following facts. First, in the case of Proteus—which has often been kept for the purposes of exhibition &c., in tanks—the skin becomes dark when the animal is removed from the cave and kept in the light. Secondly, deep-sea faunas, though as much exposed as the cave-faunas, to the condition of darkness, are not by any means invariably colourless. On the contrary, they frequently present brilliant colouration. Thus it is evident that if panmixia be suggested in explanation of the discharge of colouring in cave-faunas, the continuance of colour in deep-sea faunas appears to show the explanation insufficient. Thirdly, according to my view of the action of panmixia as previously explained, no total discharge of colouration is likely to be caused by such action alone. At most the bleaching as a result of the mere withdrawal of selection would proceed only to some comparatively small extent. Fourthly, Mr. Packard in the elaborate Memoir on Cave Fauna, already alluded to, states that in some of the cases the phenomena of bleaching appear to have been induced within very recent times—if not, indeed, within the limits of a single generation. Should the evidence in support of this opinion prove trustworthy, of course in itself it disposes of any suggestion either of the presence or the absence of natural selection as concerned in the process.
Nevertheless, I myself think it inevitable that to some extent the cessation of selection must have helped in discharging the colour of cave faunas; although for the reasons now given it appears to me that the main causes of change must have been of that direct order which we understand by the term climatic.
As regards dogs, the Rev. E. Everest found it impossible to breed Scotch setters in India true to their type. Even in the second generation no single young dog resembled its parents either in form or shape. "Their nostrils were more contracted, their noses more pointed, their size inferior, and their limbs more slender[111]." Similarly on the coast of New Guinea, Bosman says that imported breeds of dogs "alter strangely; their ears grow long and stiff like those of foxes, to which colour they also incline ... and in three or four broods their barking turns into a howl[112]."
Darwin gives numerous facts showing the effects of climate on horses, cattle, and sheep, in altering, more or less considerably, the characters of their ancestral stocks. He also gives the following remarkable case with regard to the rabbit. Early in the fifteenth century a common rabbit and her young ones were turned out on the island of Porto Santo, near Madeira. The feral progeny now differ in many respects from their parent stock. They are only about one-third of the weight, present many differences in the relative sizes of different parts, and have greatly changed in colour. In particular, the black on the upper surface of the tail and tips of the ears, which is so constant in all other wild rabbits of the world as to be given in most works as a specific character, has entirely disappeared. Again, "the throat and certain parts of the under surface, instead of being pure white, are generally grey or leaden colour," while the upper surface of the whole body is redder than in the common rabbit. Now, what answer have our opponents to make to such a case as this? Presumably they will answer that the case simply proves the action of natural selection during the best part of 400 years on an isolated section of a species. Although we cannot say of what use all these changes have been to the rabbits presenting them, nevertheless we must believe that they have been produced by natural selection, and therefore must present some hidden use to the isolated colony of rabbits thus peculiarly situated. Four centuries is long enough to admit of natural selection effecting all these changes in the case of so rapidly breeding an animal as the rabbit, and therefore it is needless to look further for any explanation of the facts. Such, I say, is presumably the answer that would be given by the upholders of natural selection as the only possible cause of specific change. But now, in this particular case it so happens that the answer admits of being conclusively negatived, by showing that the great assumption on which it reposes is demonstrably false. For Darwin examined two living specimens of these rabbits which had recently been sent from Porto Santo to the Zoological Gardens, and found them coloured as just described. Four years afterwards the dead body of one of them was sent to him, and then he found that the following changes had taken place. "The ears were plainly edged, and the upper surface of the tail was covered with blackish-grey fur, and the whole body was much less red; so that under the English climate this individual rabbit has recovered the proper colour of its fur in rather less than four years!"
Mr. Darwin adds:—
"If the history of these Porto Santo rabbits had not been known, most naturalists, on observing their much reduced size, their colour, reddish above and grey beneath, their tails and ears not tipped with black, would have ranked them as a distinct species. They would have been strongly confirmed in this view by seeing them alive in the Zoological Gardens, and hearing that they refused to couple with other rabbits. Yet this rabbit, which there can be little doubt would thus have been ranked as a distinct species, as certainly originated since the year 1420[113]."
Moreover, it certainly originated as a direct result of climatic influences, independent of natural selection; seeing that, as soon as individual members of this apparently new species were restored to their original climate, they recovered their original colouration.
As previously remarked, it is, from the nature of the case, an exceedingly difficult thing to prove in any given instance that natural selection has not been the cause of specific change, and so finally to disprove the assumption that it must have been. Here, however, on account of historical information, we have a crucial test of the validity of this assumption, just as we had in the case of the niata cattle; and, just as in their case, the result is definitely and conclusively to overturn the assumption. If these changes in the Porto Santo rabbits had been due to the gradual influence of natural selection guided by inscrutable utility, it is simply impossible that the same individual animals, in the course of their own individual life-times, should revert to the specific characters of their ancestral stock on being returned to the conditions of their ancestral climate. Therefore, unless any naturalist is prepared to contradict Darwin's statement that the changes in question amount to changes of specific magnitude, he can find no escape from the conclusion that distinctions of specific importance may be brought about by changes of habitat alone, without reference to utility, and therefore independently of natural selection.
Although, as yet, little is definitely known on the subject, there can be no doubt that in the case of many animals differences of food induce differences of colour within the life-time of individuals, and therefore independently of natural selection.
Thus, sundry definite varieties of the butterfly Euprepia caja can be reared according to the different nourishment which is supplied to the caterpillar; and other butterflies are also known on whose colouring and markings the food of the caterpillar has great influence[114].
Again, I may mention the remarkable case communicated to Darwin by Moritz Wagner, of a species of Saturnia, some pupae of which were transported from Texas to Switzerland in 1870. The moths which emerged in the following year were like the normal type in Texas. Their young were supplied with leaves of Juglans regia, instead of their natural food, J. nigra; and the moths into which these caterpillars changed were so different from their parents, both in form and colour, "that they were reckoned by entomologists as a distinct species[115]."
With regard to mollusks, M. Costa tells us that English oysters, when turned down in the Mediterranean, "rapidly became like the true Mediterranean oyster, altered their manner of growth, and formed prominent diverging rays." This is most probably due to some change of food. So likewise may be the even more remarkable case of Helix nemoralis, which was introduced from Europe to Virginia a few years ago. Under the new conditions it varied to such an extent that up to last year no less than 125 varieties had been discovered. Of these 67, or more than half, are new—that is, unknown in the native continent of the species[116].
In the case of Birds, the Brazilian parrot Chrysotis festiva changes the green in its feathers to red or yellow, if fed on the fat of certain fishes; and the Indian Lori has its splendid colouring preserved by a peculiar kind of food (Wallace). The Bullfinch is well known to turn black when fed on hemp seeds, and the Canary to become red when fed on cayenne pepper (Darwin). Starting from these facts, Dr. Sauermann has recently investigated the subject experimentally; and finds that not only finches, but likewise other birds, such as fowls, and pigeons, are subject to similar variations of colour when fed on cayenne pepper; but in all cases the effect is produced only if the pepper is given to the young birds before their first moult. Moreover, he finds that a moist atmosphere facilitates the change of colour, and that the ruddy hue is discharged under the influence either of sunlight or of cold. Lastly, he has observed that sundry other materials such as glycerine and aniline dyes, produce the same results; so there can be no doubt that organic compounds probably occur in nature which are capable of directly affecting the colours of plumage when eaten by birds. Therefore the presence of such materials in the food-stuffs of birds occupying different areas may very well in many cases determine differences of colouration, which are constant or stable so long as the conditions of their production are maintained.
Passing on now to causes of specific change which are internal, or comprised within the organisms themselves, we may first consider the case of Sexual Selection.
Mr. Wallace rejects the theory of sexual selection in toto, and therefore nothing that can be said under this head would be held by him to be relevant. Many naturalists, however, believe that Darwin was right in the large generalization which he published under this title; and in so far as any one holds that sexual selection is a true cause of specific modification, he is obliged to believe that innumerable specific characters—especially in birds and mammals—have been produced without reference to utility (other, of course, than utility for sexual purposes), and therefore without reference to natural selection. This is so obvious that I need not pause to dilate upon it. One remark, however, may be useful. Mr. Wallace is able to make a much more effective use of his argument from "necessary instability" when he brings it against the Darwinian doctrine of sexual selection, than he does when he brings it against the equally Darwinian doctrine of specific characters in general not being all necessarily due to natural selection. In the latter case, it will be remembered, he is easily met by showing that the causes of specific change other than natural selection, such as food, climate, &c., may be quite as general, persistent, and uniform, as natural selection itself; and therefore in this connexion Mr. Wallace's argument falls to the ground. But the argument is much more formidable as he brings it to bear against the theory of sexual selection. Here he asks, What is there to guarantee the uniformity and the constancy of feminine taste with regard to small matters of embellishment through thousands of generations, and among animals living on extensive areas? And, as we have seen in Part 1, it is not easy to supply an answer. Therefore this argument from the "necessary instability of character" is of immeasurably greater force as thus applied against Darwin's doctrine of sexual selection, than it is when brought against his doctrine that all specific characters need not necessarily be due to natural selection. Therefore, also, if any one feels disposed to attach the smallest degree of value to this argument in the latter case, consistency will require him to allow that in the former case it is simply overwhelming, or in itself destructive of the whole theory of sexual selection. And, conversely, if his belief in the theory of sexual selection can survive collision with this objection from instability, he ought not to feel any tremor of contact when the objection is brought to bear against his scepticism regarding the alleged utility of all specific characters. For assuredly no specific character which is apparent to our eyes can be supposed to be so refined and complex (and therefore so presumably inconstant and unstable), as are those minute changes of cerebral structure on which a psychological preference for all the refined shadings and many pigments of a complicated pattern must be held ultimately to depend. For this reason, then, as well as for those previously adduced, if any one agrees with Darwin in holding to the theory of sexual selection notwithstanding this objection from the necessary instability of unuseful embellishments, a fortiori he ought to disregard the objection altogether in its relation to useless specific characters of other kinds.
But quite apart from this consideration, which Mr. Wallace and his followers may very properly say does not apply to them, let us see what they themselves have made of the facts of secondary sexual characters—which, of course, are for the most part specific characters—in relation to the doctrine of utility.
Mr. Wallace himself, in his last work, quotes approvingly a letter which he received in 1869 from the Rev. O Pickard-Cambridge, as follows:—
"I myself doubt that particular application of the Darwinian theory which attributes male peculiarities of form, structure, colour, and ornament to female appetency or predilection. There is, it seems to me, undoubtedly something in the male organization of a special and sexual nature, which, of its own vital force, develops the remarkable male peculiarities so commonly seen, and of no imaginable use to that sex. In as far as these peculiarities show a great vital power, they point out to us the finest and strongest individuals of the sex, and show us which of them would most certainly appropriate to themselves the best and greatest number of females, and leave behind them the strongest and greatest number of progeny. And here would come in, as it appears to me, the proper application of Darwin's theory of Natural Selection; for the possessors of greatest vital power being those most frequently produced and reproduced, the external signs of it would go on developing in an ever increasing exaggeration, only to be checked where it became really detrimental in some respect or other to the individual[117]."
Here then the idea is, as more fully expressed by Mr. Wallace in the context, that all the innumerable, frequently considerable, and generally elaborate "peculiarities of form, structure, colour, and ornament," which Darwin attributed to sexual selection, are really due to "the laws of growth." Diverse, definite, and constant though these specific peculiarities be, they are all but the accidental or adventitious accompaniments of "vigour," or "vital power," due to natural selection. Now, without waiting to dispute this view, which has already been dealt with in the chapter on Sexual Selection in Part I, it necessarily follows that "a large proportional number of specific characters," which, while presenting "no imaginable use," are very much less remarkable, less considerable, less elaborate, &c., must likewise be due to this "correlation with vital power." But if the principle of correlation is to be extended in this vague and general manner, it appears to me that the difference between Mr. Wallace and myself, with respect to the principle of utility, is abolished. For of course no one will dispute that the prime condition to the occurrence of "specific characters," whether useful or useless, is the existence of some form which has been denominated a "species" to present them; and this is merely another way of saying that such characters cannot arise except in correlation with a general fitness due to natural selection. Or, to put the case in Mr. Wallace's own words—"This development [of useless specific characters] will necessarily proceed by the agency of natural selection [as a necessary condition] and the general laws which determine the production of colour and of ornamental appendages." The case, therefore, is just the same as if one were to say, for example, that all the ailments of animals and plants proceed from correlation with life (as a necessary condition), "and the general laws which determine the production" of ill-health, or of specific disease. In short, the word "correlation" is here used in a totally different sense from that in which it is used by Darwin, and in which it is elsewhere used by Wallace for the purpose of sustaining his doctrine of specific characters as necessarily useful. To say that a useless character A is correlated with a useful one B, is a very different thing from saying that A is "correlated with vital power," or with the general conditions to the existence of the species to which it belongs. So far as the present discussion is concerned, no exception need be taken to the latter statement. For it simply surrenders the doctrine against which I am contending.
It is the opinion of many naturalists who are well entitled to have an opinion upon the subject, that, in the words of Mr. Dixon, "Isolation can preserve a non-beneficial as effectually as natural selection can preserve a beneficial variation[118]." The ground on which this doctrine rests is thus clearly set forth by Mr. Gulick:—"The fundamental cause of this seems to lie in the fact that no two portions of a species possess exactly the same average characters; and, therefore, that the initial differences are for ever reacting on the environment and on each other in such a way as to ensure increasing divergence in each generation, as long as the individuals of the two groups are kept from intergenerating[119]." In other words, as soon as a portion of a species is separated from the rest of that species, so that breeding between the two portions is no longer possible, the general average of characters in the separated portion not being in all respects precisely the same as it is in the other portion, the result of in-breeding among all individuals of the separated portion will eventually be different from that which obtains in the other portion; so that, after a number of generations, the separated portion may become a distinct species from the effect of isolation alone. Even without the aid of isolation, any original difference of average characters may become, as it were, magnified in successive generations, provided that the divergence is not harmful to the individuals presenting it, and that it occurs in a sufficient proportional number of individuals not to be immediately swamped by intercrossing. For, as Mr. Murphy has pointed out, in accordance with Delbœuf's law, "if, in any species, a number of individuals, bearing a ratio not infinitely small to the entire number of births, are in every generation born with a particular variation which is neither beneficial nor injurious, and if it be not counteracted by reversion, then the proportion of the new variety to the original form will increase till it approaches indefinitely near to equality[120]." Now even Mr. Wallace himself allows that this must be the case; and thinks that in these considerations we may find an explanation of the existence of certain definite varieties, such as the melanic form of the jaguar, the brindled or ring-eyed guillemot, &c. But, on the other hand, he thinks that such varieties must always be unstable, and continually produced in varying proportions from the parent forms. We need not, however, wait to dispute this arbitrary assumption, because we can see that it fails, even as an assumption, in all cases where the superadded influence of isolation is concerned. Here there is nothing to intercept the original tendency to divergent evolution, which arises directly out of the initially different average of qualities presented by the isolated section of the species, as compared with the rest of that species[121].
As we shall have to consider the important principle of isolation more fully on a subsequent occasion, I need not deal with it in the present connexion, further than to remark that in this principle we have what appears to me a full and adequate condition to the rise and continuance of specific characters which need not necessarily be adaptive characters. And, when we come to consider the facts of isolation more closely, we shall find superabundant evidence of this having actually been the case.
Under this general term Darwin included the operation of all unknown causes internal to organisms leading to modifications of form or structure—such modifications, therefore, appearing to arise, as he says "spontaneously," or without reference to utility. That he attributed no small importance to the operation of these principles is evident from the last edition of the Origin of Species. But as these "laws of growth" refer to causes confessedly unknown, I will not occupy space by discussing this division of our subject—further than to observe that, as we shall subsequently see, many of the facts which fall under it are so irreconcilably adverse to the Wallacean doctrine of specific characters as universally adaptive, that in the face of them Mr. Wallace himself appears at times to abandon his doctrine in toto.
It must have appeared strange that hitherto I should have failed to distinguish between "true species" and merely "climatic varieties." But it will conduce to clearness of discussion if we consider our subject point by point. Therefore, having now given a fair statement of the facts of climatic variation, I propose to deal with their theoretical implications—especially as regards the distinction which naturalists are in the habit of drawing between them and so-called true species.
First of all, then, what is this distinction? Take, for example, the case of the Porto Santo rabbits. To almost every naturalist who reads what has been said touching these animals, it will have appeared that the connexion in which they are adduced is wholly irrelevant to the question in debate. For, it will be said that the very fact of the seemingly specific differentiation of these animals having proved to be illusory when some of them were restored to their ancestral conditions, is proof that their peculiar characters are not specific characters; but only what Mr. Wallace would term "individual characters," or variations that are not inherited. And the same remark applies to all the other cases which have been adduced to show the generality and extent of climatic variation, both in other animals and also in plants. Why, then, it will be asked, commit the absurdity of adducing such cases in the present discussion? Is it not self-evident that however general, or however considerable, such merely individual, or non-heritable, variations may be, they cannot possibly have ever had anything to do with the origin of species? Therefore, is it not simply preposterous to so much as mention them in relation to the question touching the utility of specific characters?
Well, whether or not it is absurd and preposterous to consider climatic variations in connexion with the origin of species, will depend, and depend exclusively, on what it is that we are to understand by a species. Hitherto I have assumed, for the sake of argument, that we all know what is meant by a species. But the time has now come for showing that such is far from being the case. And as it would be clearly absurd and preposterous to conclude anything with regard to specific characters before agreeing upon what we mean by a character as specific, I will begin by giving all the logically possible definitions of a species.
1. A group of individuals descended by way of natural generation from an originally and specially created type.
This definition may be taken as virtually obsolete.
2. A group of individuals which, while fully fertile inter se, are sterile with all other individuals—or, at any rate, do not generate fully fertile hybrids.
This purely physiological definition is not nowadays entertained by any naturalist. Even though the physiological distinction be allowed to count for something in otherwise doubtful cases, no systematist would constitute a species on such grounds alone. Therefore we need not concern ourselves with this definition, further than to observe that it is often taken as more or less supplementary to each of the following definitions.
3. A group of individuals which, however many characters they share with other individuals, agree in presenting one or more characters of a peculiar kind, with some certain degree of distinctness.
In this we have the definition which is practically followed by all naturalists at the present time. But, as we shall presently see more fully, it is an extremely lax definition. For it is impossible to determine, by any fixed and general rule, what degree of distinctness on the part of peculiar characters is to be taken as a uniform standard of specific separation. So long as naturalists believed in special creation, they could feel that by following this definition (3) they were at any rate doing their best to tabulate very real distinctions in nature—viz. between types as originally produced by a supernatural cause, and as subsequently more or less modified (i.e. within the limits imposed by the test of cross-fertility) by natural causes. But evolutionists are unable to hold any belief in such real distinctions, being confessedly aware that all distinctions between species and varieties are purely artificial. So to speak, they well know that it is they themselves who create species, by determining round what degrees of differentiation their diagnostic boundaries shall be drawn. And, seeing that these degrees of differentiation so frequently shade into one another by indistinguishable stages (or, rather, that they always do so, unless intermediate varieties have perished), modern naturalists are well awake to the impossibility of securing any approach to a uniform standard of specific distinction. On this account many of them feel a pressing need for some firmer definition of a species than this one—which, in point of fact, scarcely deserves to be regarded as a definition at all, seeing that it does not formulate any definite criterion of specific distinctness, but leaves every man to follow his own standards of discrimination. Now, as far as I can see, there are only two definitions of a species which will yield to evolutionists the steady and uniform criterion required. These two definitions are as follows.
4. A group of individuals which, however many characters they share with other individuals, agree in presenting one or more characters of a peculiar and hereditary kind, with some certain degree of distinctness.
It will be observed that this definition is exactly the same as the last one, save in the addition of the words "and hereditary." But, it is needless to say, the addition of these words is of the highest importance, inasmuch as it supplies exactly that objective and rigid criterion of specific distinctness which the preceding definition lacks. It immediately gets rid of the otherwise hopeless wrangling over species as "good" and "bad," or "true" and "climatic," of which (as we have seen) Kerner's essay is such a remarkable outcome. Therefore evolutionists have more and more grown to lay stress on the hereditary character of such peculiarities as they select for diagnostic features of specific distinctness. Indeed it is not too much to say that, at the present time, evolutionists in general recognize this character as, theoretically, indispensable to the constitution of a species. But it is likewise not too much to say that, practically, no one of our systematic naturalists has hitherto concerned himself with this matter. At all events, I do not know of any who has ever taken the trouble to ascertain by experiment, with regard to any of the species which he has constituted, whether the peculiar characters on which his diagnoses have been founded are, or are not, hereditary. Doubtless the labour of constituting (or, still more, of re-constituting) species on such a basis of experimental inquiry would be insuperable; while, even if it could be accomplished, would prove undesirable, on account of the chaos it would produce in our specific nomenclature. But, all the same, we must remember that this nomenclature as we now have it—and, therefore, the partitioning of species as we have now made them—has no reference to the criterion of heredity. Our system of distinguishing between species and varieties is not based upon the definition which we are now considering, but upon that which we last considered—frequently coupled, to some undefinable extent, with No. 2.
5. There is, however, yet another and closer definition, which may be suggested by the ultra-Darwinian school, who maintain the doctrine of natural selection as the only possible cause of the origin of species, namely:—
A group of individuals which, however many characters they share with other individuals, agree in presenting one or more characters of a peculiar, hereditary, and adaptive kind, with some certain degree of distinctness.
Of course this definition rests upon the dogma of utility as a necessary attribute of characters quâ specific—i.e. the dogma against which the whole of the present discussion is directed. Therefore all I need say with reference to it is, that at any rate it cannot be adduced in any argument where the validity of its basal dogma is in question. For it would be a mere begging of this question to argue that every species must present at least one peculiar and adaptive character, because, according to definition, unless an organic type does present at least one such character, it is not a specific type. Moreover, and quite apart from this, it is to be hoped that naturalists as a body will never consent to base their diagnostic work on what at best must always be a highly speculative extension of the Darwinian theory. While, lastly, if they were to do so with any sort of consistency, the precise adaptation which each peculiar character subserves, and which because of this adaptation is constituted a character of specific distinction, would have to be determined by actual observation. For no criterion of specific distinction could be more vague and mischievous than this one, if it were to be applied on grounds of mere inference that such and such a character, because seemingly constant, must "necessarily" be either useful, vestigial, or correlated.
Such then, as far as I can see, are all the definitions of a species that are logically possible[122]. Which of them is chosen by those who maintain the necessary usefulness of all specific characters? Observe, it is for those who maintain this doctrine to choose their definition: it is not for me to do so. My contention is, that the term does not admit of any definition sufficiently close and constant to serve as a basis for the doctrine in question—and this for the simple reason that species-makers have never agreed among themselves upon any criterion of specific distinction. My opponents, on the other hand, are clearly bound to take an opposite view, because, unless they suppose that there is some such definition of a species, they would be self-convicted of the absurdity of maintaining a great generalization on a confessedly untenable basis. For example, a few years ago I was allowed to raise a debate in the Biological Section of the British Association on the question to which the present chapters are devoted. But the debate ended as I had anticipated that it must end. No one of the naturalists present could give even the vaguest definition of what was meant by a species—or, consequently, of a character as specific. On this account the debate ended in as complete a destruction as was possible of the doctrine that all the distinctive characters of every species must necessarily be useful, vestigial, or correlated. For it became unquestionable that the same generalization admitted of being made, with the same degree of effect, touching all the distinctive characters of every "snark."
Probably, however, it will be thought unfair to have thus sprung a difficult question of definition in oral debate. Therefore I allude to this fiasco at the British Association, merely for the purpose of emphasizing the necessity of agreeing upon some definition of a species, before we can conclude anything with regard to the generalization of specific characters as necessarily due to natural selection. But when a naturalist has had full time to consider this fundamental matter of definition, and to decide on what his own shall be, he cannot complain of unfairness on the part of any one else who holds him to what he thus says he means by a species. Now Mr. Wallace, in his last work, has given a matured statement of what it is that he means by a species. This, therefore, I will take as the avowed basis of his doctrine touching the necessary origin and maintenance of all specific characters by natural selection. His definition is as follows:—
"An assemblage of individuals which have become somewhat modified in structure, form, and constitution, so as to adapt them to slightly different conditions of life; which can be differentiated from allied assemblages; which reproduce their like; which usually breed together; and, perhaps, when crossed with their near allies, always produce offspring which are more or less sterile inter se[123]."
From this definition the portion which I have italicized must be omitted in the present discussion, for the reasons already given while considering definition No. 5. What remains is a combination of Nos. 2 and 4. According to Mr. Wallace, therefore, our criterion of a species is to be the heredity of peculiar characters, combined, perhaps, with a more or less exclusive fertility of the component individuals inter se. This is the basis on which his generalization of the utility of specific characters as necessary and universal is reared. Here, then, we have something definite to go upon, at all events as far as Mr. Wallace is concerned. Let us see how far such a basis of definition is competent to sustain his generalization.
First of all it must be remarked that, as species have actually been constituted by systematists, the test of exclusive fertility does not apply. For my own part I think this is to be regretted, because I believe that such is the only natural—and therefore the only firm—basis on which specific distinctions can be reared. But, as previously observed, this is not the view which has been taken by our species-makers. At most they regard the physiological criterion as but lending some additional weight to their judgement upon morphological features, in cases where it is doubtful whether the latter alone are of sufficient distinctness to justify a recognition of specific value. Or, conversely, if the morphological features are clearly sufficient to justify such a recognition, yet if it happens to be known that there is full fertility between the form presenting them and other forms which do not, then the latter fact will usually prevent naturalists from constituting the well differentiated form a species on grounds of its morphological features alone—as, for instance, in the case of our domesticated varieties. In short, the physiological criterion has not been employed with sufficient closeness to admit of its being now comprised within any practical definition of the term "species"—if by this term we are to understand, not what any one may think species ought to be, but what species actually are, as they have been constituted for us by their makers.
From all this it follows that the definition of the term "species" on which Mr. Wallace relies for his deduction with respect to specific characters, is the definition No. 4. In other words, omitting his petitio principii and his allusion to the test of fertility, the great criterion in his view is the criterion of Heredity. And in this all other evolutionists, of whatever school, will doubtless agree with him. They will recognize that it is really the distinguishing test between "climatic varieties" and "true species," so that however widely or however constantly the former may diverge from one another in regard to their peculiar characters, they are not to be classed among the latter unless their peculiar characters are likewise hereditary characters.
Now, if we are all agreed so far, the only question that remains is whether or not this criterion of Heredity is capable of supplying a basis for the generalization, that all characters which have been ranked as of specific value must necessarily be regarded as presenting also an adaptive, or life-serving, value? I will now endeavour to show that there are certain very good reasons for answering this question in the negative.