Darwin, and After Darwin Volume Ii Part 11

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].

[104] _Ibid._ p. 94.

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 cla.s.sed with specific characters" (or with anything of the nature of specific characters), it would follow that the niata peculiarities _can_ be so cla.s.sed. 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 ill.u.s.tration of how little we are able to judge from the ordinary habits of an animal, on what circ.u.mstances, 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]."

[105] Darwin, _Variation_, &c. vol. i. p. 94.

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 a.s.sumption 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 p.r.o.nounced and deleterious peculiarities can arise and be perpetuated with such constancy and fidelity, much more is this likely to be the case with less p.r.o.nounced 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 ill.u.s.tration. 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 ill.u.s.tration 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 ill.u.s.tration. For the ill.u.s.tration 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].

[106] Should it be objected that useless characters, according to my own view of the Cessation of Selection, ought to disappear, and therefore cannot be constant, the answer is evident. For, by hypothesis, it is only those useless characters which were at one time useful that disappear under this principle.

Selection cannot cease unless it was previously present--i.e.

save in cases where the now useless character was originally due to selection. Hence, in all cases where it was due to any other cause, the useless character will persist at least as long as its originating cause continues to operate. And even after the latter (whatever it may be) has ceased to operate, the useless character will but slowly degenerate, until the eventual failure of heredity causes it to disappear _in toto_--long before which time it may very well have become a genetic, or some higher, character.

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 const.i.tute 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 cla.s.sification 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 ma.s.s of evidence to prove that the a.s.semblage 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 a.s.sume 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 const.i.tutes 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 mult.i.tudes 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, da.s.s der eine langsamer, der andere aber rascher bei dem Ziele ankommt. Anfanglich muht 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 Fussen, die bisher fur unverruckbar gehaltenen Grenzen der gut geglaubten Arten stellen sich als eine der Natur angelegte Zw.a.n.gsjacke heraus, die Uebcrzeugung, da.s.s die Grenzen, welche wir ziehen, eben nur kunstliche sind, gewinnt immer mehr und mehr die Oberhand, und wer nicht gerade zu den hartgesottenen Eigensinnigen gehort, und wer die Wahrheit hoher stellt als das starre Festhalten an seinen fruheren Ansichten, geht schliesslich bewusst oder unbewusst in das Lager derjenigen uber, in welchem auch ich mir ein bescheidenes Platzchen aufgesucht habe."

By these "hard-boiled" botanists he means those who entertain the traditional notion of a species as an a.s.semblage of definite characters, always and everywhere a.s.sociated together. This notion (Artsbestandigkeit) 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, alt.i.tude, 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 cla.s.sified 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 a.s.sist 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 ill.u.s.trations 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 a.s.semblage 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 ent.i.tled 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 Gartner, De Candolle, Nageli, 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 princ.i.p.al 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 circ.u.mstances 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, alt.i.tudes, lat.i.tudes, 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 alt.i.tude, lat.i.tude, 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 lat.i.tude to lat.i.tude, longitude to longitude, alt.i.tude to alt.i.tude, &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 circ.u.mstances), 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 circ.u.mstances, 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 ill.u.s.trations. 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 mult.i.tudes of species belonging to different genera, families, orders, and even cla.s.ses. 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.