Volume Ii Part 13 (1/2)
CHAPTER IX.
CHARACTERS AS ADAPTIVE AND SPECIFIC (_continued_).
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 a.s.sumed, 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 const.i.tute 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 const.i.tution of a species. But it is likewise not too much to say that, practically, no one of our systematic naturalists has. .h.i.therto 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 const.i.tuted, whether the peculiar characters on which his diagnoses have been founded are, or are not, hereditary. Doubtless the labour of const.i.tuting (or, still more, of _re_-const.i.tuting) 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 part.i.tioning of species as we have now made them--has no reference to the criterion of heredity. Our system of distinguis.h.i.+ng 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 _qua_ 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 const.i.tuted 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 a.s.sociation on the question to which the present chapters are devoted. But the debate ended as I had antic.i.p.ated 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.”
[122] It is almost needless to say that by a definition as ”logical”
is meant one which, while including all the differentiae of the thing defined, excludes any qualities which that thing may share in common with any other thing. But by definitions as ”logically possible” I mean the number of separate definitions which admit of being correctly given of the same thing from different points of view. Thus, for instance, in the present case, since the above has been in type the late M.
Quatref.a.ges' posthumous work on _Darwin et ses Precurseurs Francais_ has been published, and gives a long list of definitions of the term ”species” which from time to time have been enunciated by as many naturalists of the highest standing as such (pp. 186-187). But while none of these twenty or more definitions is logical in the sense just defined, they all present one or other of the differentiae given by those in the text.
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 a.s.sociation, 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 a.s.semblage of individuals which have become somewhat modified in structure, form, and const.i.tution, _so as to adapt them to slightly different conditions of life_; which can be differentiated from allied a.s.semblages; 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].”
[123] Darwinism, p. 167.
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 const.i.tuted 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 const.i.tuting 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 const.i.tuted for us by their makers.