Oct. 25, 1888] 



NATURE 



617 



basis of this concession, or on the ground of this assumption, 

 the theory of natural selection is not shown to be primarily 

 a theory of the origin of species. It follows, indeed, from the 

 assumption— is, in fact, part and parcel of the assumption — that 

 all species have been originated by natural selection ; but why ? 

 Only because natural selection has originated those particular 

 adaptive features in virtue of which species exist as species. It 

 is only in virtue of having created these features that natural 

 selection has created the species presenting them — just as it has 

 created genera, families, orders, &c, in virtue of other adaptive 

 fe Uures extending through progressively wider areas of taxonomic 

 division. Everywhere and equally this principle has been pri- 

 marily engaged in the evolution of adaptations, and if one re-.uk 

 of its work has been that of enabling the systematist to trace 

 lines of genetic descent under his divisions of species, genera, 

 and the rest, such a result is but .--econdary or incidental. A 

 wing, for example, is an adaptive structure which is formed on 

 at least four completely different plans in different classes of the 

 animal kingdom ; and it is the function of natural selection as 

 a theory to explain all this variety of adaptive structure, 

 with its infinite number of subordinate variations through 

 the different forms in each class, whether "species" or 

 otherwise. Now, I say that such a theory is first of all a 

 theory of the evolution of adaptations, even though it be 

 conceded that all species exist in virtue of differing from one 

 another in respect of adaptations, and hence that the theory 

 becomes also a theory of the evolution of species, as it is also a 

 theory of the evolution of genera, families, &c. Take a parallel 

 instance. If a man were to define the nebular theory as a theory 

 of the origin of Saturn's rings, an astronomer would tell him 

 that his definition is much too limited. The theory is, indeed, a 

 theory of the origin of Saturn's rings ; but it is so because it is a 

 theory of the origin of the entire solar system, of which Saturn's 

 rings constitute a part. Similarly, the theory of natural selec- 

 tion is a theory of the entire system of organic Nature in re-pect 

 of adaptations, whether these are distinctive of particular species 

 only, or likewise common to any number of species. In short, 

 it is "primarily" a theory of adaptations wherever these occur, 

 and only becomes " also " or " incidentally " a theory of species 

 in cases where adaptations happen to be restricted in their 

 occurrence to organic types of a certain order of taxonomic 

 division. 



This, I think, is enough to justify my definition in a formal 01 

 logical sense. But as Mr. Huxley's criticism involves certain 

 questions of a material or biological kind, I should like to take 

 this opportunity of considering what he has said upon them. 

 Therefore I will now pass on to the second head of my answer. 



II. Hitherto, for the sake of argument, I have conceded that, 

 in the words of my critic, " it is a necessary consequence of the 

 theory of selection that every species must have some one or 

 more structural or functional peculiarities " of an adaptive kind. 

 But now I will endeavour to show that this statement does not 

 "follow as a necessary consequence" from "the theory of 

 selection." 



Be it observed, the question which I am about to consider is 

 not whether "every species which exists, exists in virtue of 

 adaptation " common to its genus, family, order, class, or sub- 

 kingdom. The question is whether every species which exists, 

 exists in virtue of some advantageous "peculiarity " or adaptive 

 advantage not shared by its nearest allies. In other words, we 

 are not disputing whether it is a necessary consequence of Mr. 

 Darwin's theory that all "species" must present " adaptations." 

 This, of course, I fully admit. But what we are disputing is, 

 whether it is a necessary comequence of Mr. Darwin's theory 

 that every species must present at least one adaptive character 

 (or combination of adaptive characters) peculiar to itself alone. 

 Now, such being the question, let us consider Mr. Huxley's 

 treatment of it. 



Most obviously "it follows " from the theory of selection that 

 "every variety which is selected into a species is favoured and 

 preserved in consequence of being, in some one or more respects, 

 better adapted to its surroundings than its rivals." This, in fact, 

 is no more than a re-statement of the theory itself. But it does 

 not follow that " every species which exists, exists in virtue of 

 adaptation" peculiar to that species; i.e. that every species 

 which exists, exists in virtue of having been "selected." This 

 may or may not be true as a matter of fact : as a matter of 

 logic, the inference is not deducible from the selection theory. 

 Every variety which is selected into a species must, indeed, pre- 

 sent some such peculiar advantage ; but this is by no means 



equivalent to saying, "in other words," that every variety 

 which becomes a species must do so. For the latter statement 

 imports a completely new assumption — namely, that every 

 variety which becomes a species must do so because it has been 

 selected into a species. In short, what we are here told is, that 

 if we believe the selection principle to have given origin to some 

 species, we must further believe, "as a necessary consequence," 

 that it has given origin to all species. 



Not to perceive a consequence so neces-ary is said to betray a 

 fundamental misunderstanding of the first principles of Mr. 

 Darwin's theory. Perhaps, therefore, it is worth while to consider 

 the matter from another and less formal point of view. 



It surely is no essential part of Mr. Darwin's theory to deny 

 that isolation (in all its kinds) may lead to the survival of new 

 varieties, and so, in some cases, to the origin of new species 

 which need not necessarily present any change in the adaptive 

 characters respectively inherited from their parent stocks. Under 

 isolation, and the consequent absence of what Prof. Weismam; 

 has called panmixia, there is much rea-on to believe that new 

 "structural or functional peculiarities " may arise (whether by 

 direct action of changed conditions, by independent variation in 

 the absence of panmixia, or by both these principles combined) 

 which are without any adaptive significance ; and I cannot see 

 why it should be held to constitute any essential part of Mr. 

 Darwin's theory to deny that such is the case. No one, I sup- 

 pose, will venture to express a doubt that there are named 

 species, both of plants and animals, which have been formed 

 under isolation, and which experiments — such as those recently 

 made with our severally-isolated forms of British trout — would 

 prove to be but "local varieties," capable of being changed one 

 into another by mere change of habitat, without any question of 

 " selection " being so much as possible. Here it is the direct 

 action of changed conditions which induces modifications of 

 type sufficiently pronounced to take rank as distinct species in 

 the eyes of a systematist ; and the only difference between such 

 a case and one where the modifications are due to independent 

 variation is that in the former case their non-adaptive character 

 admits of being proved by experiment. According to the 

 general theory of evolution, there is no distinction to be drawn 

 between a local variety and a new species, save as regards the 

 extent to which modification may have proceeded. If, there- 

 fore, as in the case of the trout, mere change of habitat from one 

 district of Great Britain to another (apart from any "selection") 

 is able to induce modifications sufficient in amount to have been 

 ranked as species by expert ichthyologists, much more may this 

 frequently be the case under geographical isolation in larger 

 areas, with exposure to different climates, and subject to the 

 superadded influence of independent variation. 



I have good reason to be well aware that great differences of 

 opinion are entertained by different naturalists touching the 

 degree of importance which should be assigned to isolation as a 

 factor of organic evolution ; and in one of the very last issues of 

 Nature, Mr. Wallace presents with great lucidity the view 

 that isolation alone can never originate a new species by inde- 

 pendent variation without the unavoidable intervention of 

 natural selection, seeing that " at each step of the divergence " 

 there must be " necessarily selection of the fit " from the less fit 

 (September 20, p. 491). I will not wait to show that, if in an 

 isolated section of a species no new peculiarities should be re- 

 quired to render its constituent individuals more " fit," selection 

 need not necessarily effect any change with regard to adaptive 

 characters ; nor need I remark that even when selection is 

 enabled to effect such a change under such circumstances, it 

 does so because it is assisted by isolation, thus becoming, not 

 the cause, but a eon-cause of "the origin of species." A great 

 ileal could be said on both these points ; but, for the sake of 

 brevity, I will take rav stand on the bare fact that, according to 

 the general theory of evolution, a local variety is what Mr. 

 Darwin calls "an incipient species"; and, on the ground of 

 this fact, I ask where the line is to be drawn between varieties 

 and species in respect of adaptive characters? If no answer can 

 be given, we must take it from Mr. Huxley, as "a necessary 

 consequence of the theory of selection," that every variety 

 " which exists, exists in virtue of adaptation." Thus, to take 

 but two illustrations from among several that might be drawn 

 from the trout just alluded to, when two lots of " Lochlevens" 

 were placed in two separate ponds within a very short distance 

 of each other, and exposed, as far as could be ascertained, to 

 parallel conditions of life, remarkable — but in no conceivable 

 respect adaptive — differences in coloration were developed be- 



