Volume Ii Part 18 (1/2)
Now, with regard to this pa.s.sage it is to be observed, as already remarked, that it refers to the formation of final judgements touching _particular cases_: there is nothing to show that the writer is contemplating _general principles_, or advocating on deductive grounds the dogma that specific characters must be necessarily and universally adaptive characters. Therefore, what he here says is neither more nor less than I have said. For I have always held that it would be ”rather rash” to conclude that any given cases of apparent inutility are certainly cases of real inutility, _merely on the ground that utility is not perceived_. But this is clearly quite a distinct matter from resisting the _a priori_ generalization that all cases of apparent inutility must certainly be cases of real utility. And, I maintain, in every part of his writings, without any exception, where Darwin alludes to this matter of general principle, it is in terms which directly contradict the deduction in question. As the whole of this Appendix has been directed to proving that such is the case, it will now, I think, be sufficient to supply but one further quotation, in order to show that the above ”latest expression of opinion,” far from indicating that in his later years Darwin ”inclined” to Mr. Wallace's views upon this matter, is quite compatible with a distinct ”expression of opinion” to the contrary, in a letter written less than six years before his death.
”In my opinion _the greatest error which I have committed_, has been not allowing sufficient weight to the direct action of the environment, i.e. food, climate, &c., _independently of natural selection_. Modifications thus caused, _which are neither of advantage nor disadvantage to the modified organisms_, would be especially favoured, as I can now see chiefly through your observations, _by isolation in a small area, where only a few individuals lived under nearly uniform conditions_[154].”
[154] _Life and Letters_, vol. iii. p. 158.
I will now proceed to quote further pa.s.sages from Darwin's works, which appear to have escaped the notice of Mr. Wallace, inasmuch as they admit of no doubt regarding the allusions being to _specific_ characters.
”_We may easily err in attributing importance to characters, and in believing that they have been developed through natural selection._ We must by no means overlook the effects of the definite action of changed conditions of life,--of so-called spontaneous variations, which seem to depend in a quite subordinate degree on the nature of the conditions,--of the tendency to reversion to long-lost characters,--of the complex laws of growth, such as of correlation[155], compensation, of pressure of one part on another, &c., and finally of s.e.xual selection, by which characters of use to one s.e.x are often gained and then transmitted more or less perfectly to the other s.e.x, though of no use to this s.e.x. But structures thus indirectly gained, _although at first of no advantage to a species_, may subsequently have been taken advantage of by its modified descendants, under new conditions of life and newly acquired habits[156].”
[155] It must be observed that Darwin uses this word, not as Mr.
Wallace always uses it (viz. as if correlation can only be with regard to adaptive characters), but in the wider sense that any change in one part of an organism--whether or not it happens to be an adaptive change--is apt to induce changes in other parts.
[156] _Origin of Species_, pp. 157-8.
It appeared--and still appears--to me, that where so many causes are expressly a.s.signed as producing useless _specific_ characters, and that some of them (such as climatic influences and independent variability) must be highly general in their action, I was justified in representing it as Darwin's opinion that ”a large proportional number of specific characters” are useless to the _species_ presenting them, although afterwards they may sometimes become of use to genera, families, &c.
Moreover, this pa.s.sage goes on to point out that specific characters which at first sight appear to be obviously useful, are sometimes found by fuller knowledge to be really useless--a consideration which is the exact inverse of the argument from ignorance as used by Mr. Wallace, and serves still further to show that in Darwin's opinion utility is by no means an invariable, still less a ”necessary,” mark of specific character. The following are some of the instances which he gives.
”The sutures in the skulls of young mammals have been advanced as a beautiful adaptation for aiding parturition, and no doubt they may facilitate, or be indispensable for this act; but as sutures occur in the skulls of young birds and reptiles, which have only to escape from a broken egg, we may infer that this structure has _arisen from the laws of growth_, and has been taken advantage of in the parturition of the higher animals[157].”
”The naked skin on the head of a vulture is generally considered as a direct adaptation for wallowing in putridity; and so it may be, _or it may possibly be due to the direct action of the putrid matter_; but we should be very cautious in drawing any such inference [i.e. as to utility] when we see the skin on the head of the clean-feeding male Turkey is likewise naked[158].”
[157] _Ibid._
[158] _Origin of Species_, pp. 157-8.
Similarly, in the _Descent of Man_ it is said:--
”Variations of the same _general_ nature have _often been taken advantage of_ and acc.u.mulated through s.e.xual selection in relation to the propagation of the species, and through natural selection in relation to the general purposes of life. Hence, _secondary s.e.xual characters, when equally transmitted to both s.e.xes, can be distinguished from ordinary specific characters, only by the light of a.n.a.logy_. The modifications acquired through s.e.xual selection are often so strongly p.r.o.nounced that the two s.e.xes have frequently been ranked as distinct species, or even as distinct genera[159].”
[159] _Descent of Man_, p. 615.
As Mr. Wallace does not recognize s.e.xual selection, he incurs the burden of proving utility (in the life-preserving sense) in all these ”frequently” occurring cases where there are such ”strongly p.r.o.nounced modifications,” and we have already seen in the text his manner of dealing with this burden. But the point here is, that whether or not we accept the theory of s.e.xual selection, we must accept it as Darwin's opinion--first, that in their beginnings, as _specific_ characters, these s.e.xual modifications were often of a merely ”_general nature_” (or without reference to utility even in the life-embellis.h.i.+ng sense), and only _afterwards_ ”have often been taken advantage of and acc.u.mulated through _s.e.xual_ selection”: and, secondly, that ”we know they have been acquired in some instances _at the cost not only of inconvenience, but of exposure to actual dangers_[160].”
[160] _Ibid._
We may now pa.s.s on to some further, and even stronger, expressions of opinion with regard to the frequent inutility of _specific_ characters.
”I have made these remarks only to show that, if we are unable to account for the characteristic differences of our several domestic breeds, which nevertheless are generally admitted to have arisen through ordinary generation from one or a few parent stocks, we ought not to lay too much stress on our ignorance of the precise cause [i.e. whether natural selection or some other cause] of the slight a.n.a.logous differences between true _species_.... I fully admit that _many_ structures are now of no use to their possessors, and may never have been of any use to their progenitors; but this does not prove that they were formed solely for beauty or variety.
No doubt the definite action of changed conditions, and the various causes of modification, lately specified, have all produced an effect, _probably a great effect, independently of any advantage thus gained_.... It is scarcely possible to decide how much allowance ought to be made for such causes of change, as the definite action of external conditions, so-called spontaneous variations, and the complex laws of growth; but, _with these important exceptions_, we may conclude that the structure of every living creature either now is, or formerly was, of some direct or indirect use to its possessor[161].”
[161] _Descent of Man_, pp. 159-60.
Here again, if we remember how ”important” these ”exceptions” are, I cannot understand any one doubting Darwin's opinion to have been that a large proportional number of specific characters are useless. For that it is ”species” which he here has mainly in his mind is evident from what he says when again alluding to the subject in his ”Summary of the Chapter”--namely, ”In _many_ other cases [i.e. in cases where natural selection has not been concerned] modifications are probably the direct result of the laws of variation or of growth, independently of any good having been thus gained.” Now, not only do these ”laws” apply as much to species as they do to genera; ”but,” the pa.s.sage goes on to say, ”even such structures have often, we may feel a.s.sured, been subsequently taken advantage of, and still further modified, for the good of _species_ under new conditions of life.” Obviously, therefore, the inutility in such cases is taken to have been prior to any utility subsequently acquired; and genera are not historically prior to the species in which they originate.
Here is another quotation:--
”Thus, as I am inclined to believe, morphological differences, which we consider as important--such as the arrangement of the leaves, the divisions of the flower or of the ovarium, the position of the ovules, &c.--_first_ appeared in _many_ cases as _fluctuating variations_, which sooner or later became constant through the nature of the organism and of the surrounding conditions, as well as through the intercrossing of distinct individuals, _but not through natural selection_; for as these morphological characters do not affect the welfare of the _species_, any slight deviations in them could not have been governed or acc.u.mulated through this latter agency. It is a strange result which we thus arrive at, namely, that characters of slight vital importance to the _species_, are the most important to the systematist; but, as we shall hereafter see when we treat of the genetic principle of cla.s.sification, this is by no means so paradoxical as it may at first appear[162].”
[162] _Descent of Man_, p. 176.