Chapter VIII

CHARACTERS AS ADAPTIVE AND SPECIFIC (_continued_).

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 _resume_ of all that might be said on these interesting topics.

I. _Climate._

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]."

[107] _Variation_, &c. vol. i. p. 340.

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]."

[108] _Variation_, &c. vol. ii. p. 271.

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. Anfaenglich mueht 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 Fuessen, die bisher fuer unverrueckbar 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 kuenstliche sind, gewinnt immer mehr und
mehr die Oberhand, und wer nicht gerade zu den hartgesottenen
Eigensinnigen gehoert, und wer die Wahrheit hoeher stellt als das
starre Festhalten an seinen frueheren Ansichten, geht schliesslich
bewusst oder unbewusst in das Lager derjenigen ueber, in welchem
auch ich mir ein bescheidenes Plaetzchen 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 (Artsbestaendigkeit) 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 Gaertner, De Candolle, Naegeli, 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].

[109] Since the above paragraphs have been in type, the Rev. G.
Henslow has published his Linnaean Society papers which are
mentioned in the introductory chapter, and which deal in more
detail with this subject, especially as regards the facies of
desert floras.

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].

[110] _Trans. Entom. Soc._ 1889, part i. p. 79 _et seq._

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]."

[111] _Variation_, &c. vol. i. p. 40.

[112] _Variation_, &c. vol. i. p. 40.

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]."

[113] _Variation_, &c. vol. i. p. 120.

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.

II. _Food._

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].

[114] See especially, Koch, _Die Raupen und Schmetterling der
Wetterau_, and _Die Schmetterling des Suedwestlichen
Deutschlands_, whose very remarkable results of numerous and
varied experiments are epitomized by Eimer, _Organic
Evolution_, Eng. Trans. pp. 147-153; also Poulton, _Trans.
Entom. Soc._ 1893.

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]."

[115] Mivart, _On Truth_, p. 378.

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].

[116] Cockerell, _Nature_, vol. xli. p. 393.

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.

III. _Sexual Selection._

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]."

[117] _Darwinism_, pp.[typo: period missing in scan] 296-7: italics
mine.

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.

IV. _Isolation._

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 Delboeuf'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].

[118] _Nature_, vol. xxxiii. p. 100.

[119] _Divergent Evolution through Cumulative Segregation_, Linn.
Journ. Zoology, vol. xx. p. 215.

[120] _Habit and Intelligence_, p. 241.

[121] Allusion may here again be made to the case of the niata
cattle. For here is a case where a very extreme variety is
certainly not unstable, nor produced in varying proportions
from the parent form. Moreover, as we have seen in the
preceding chapter, this almost monstrous variety most probably
originated as an individual sport--being afterwards maintained
and multiplied for a time by artificial selection. Now,
whether or not this was the case, we can very well see that it
may have been. Hence it will serve to illustrate another
possibility touching the origin and maintenance of useless
specific characters. For what is to prevent an individual
congenital variation of any kind (provided it be not harmful)
from perpetuating itself as a "varietal," and eventually,
should offspring become sufficiently numerous, a "specific
character"? There is nothing to prevent this, save panmixia,
or the presence of free intercrossing. But, as we shall see in
the next division of this treatise, there are in nature many
forms of isolation. Hence, as often as a small number of
individuals may have experienced isolation in any of its
forms, opportunity for perpetuation will have been given to
any congenital variations which may happen to arise. Should
any of these be pronounced variations, it would afterwards be
ranked as a specific character. I do not myself think that
this is the way in which indifferent specific characters
_usually_ originate. On the contrary, I believe that their
origin is most frequently due to the influence of isolation on
the average characters of the whole population, as briefly
stated in the text. But here it seems worth while to notice
this possibility of their occasionally arising as merely
individual variations, afterwards perpetuated by any of the
numerous isolating conditions which occur in nature. For, if
this can be the case with a varietal form so extreme as to
border on the monstrous, much more can it be so with such
minute differences as frequently go to constitute specific
distinctions. It is the business of species-makers to search
out such distinctions, no matter how trivial, and to record
them as "specific characters." Consequently, wherever in
nature a congenital variation happens to arise, and to be
perpetuated by the force of heredity alone under any of the
numerous forms of isolation which occur in nature, there will
be a case analogous to that of the niata cattle.

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.

V. _Laws of Growth._

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_.