Volume Ii Part 11 (1/2)
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 establis.h.i.+ng 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._