September 3, 



1914] 



NATURE 



ironnient, and are therefore somatogenic, at any 

 1 .0 in the first instance, though there is reason 

 [i believe that some of them may find expression in 

 the germ-cells in the formation of organ-forming 

 substances, and possibly in other ways. Blastogenic 

 characters which actually originate in the germ-cells 

 appear to be of quite secondary importance. 



We still have to consider the question. How is it 

 that organic evolution has led to the formation of 

 those more or less well-marked groups of organisms 

 which we call species? We have to note in the first 

 place that there is no unanimity of opinion amongst 

 biologists as to what a species is. Lamarck insisted 

 that nature recognises no such things as species, and 

 a great many people at the present day are, I think, 

 still of the same opinion. In practice, however, every 

 naturalist knows that there are natural groups to 

 which the vast majority of individuals can be assigned 

 without any serious difficulty. Charles Darwin main- 

 tained that such groups arose, under the influence of 

 natural selection, through gradual divergent evolution 

 and the extinction of intermediate forms. To-day we 

 are told by de Vries that species originate as muta- 

 tions which propagate themselves without alteration 

 for a longer or shorter period, and by Lotsy that 

 species originate by crossing of more or less distinct 

 forms, though this latter theory leaves quite unsolved 

 the problem of where the original forms that crossed 

 with one another came from. 



I think a little reflection will convince us that the 

 origin of species is a different problem from that of 

 the cause of progressive evolution. We can scarcely 

 doubt, however, that Darwin was right in attributing 

 prime importance to divergent evolution and the dis- 

 appearance of connecting links. It is obvious that 

 this process must give rise to more or less sharply 

 separated groups of individuals to which the term 

 species may be applied, and that the differences be- 

 tween these species must be attributed ultimately to 

 differences in the response of the organism to differing 

 conditions of the environment. It mav be urged that 

 inasmuch as different species are often found living 

 side by side under identical conditions the differences 

 between them cannot have arisen in this way, but 

 we may be quite certain that if we knew enough of 

 their past history we should find that their ancestors 

 had not always lived under identical conditions. 



The case of flightless birds on oceanic islands is 

 particularly instructive in this connection. The only 

 satisfactor}- wav of explaining the existence of such 

 birds is by supposing that their ancestors had well- 

 developed wings, by the aid of which they made their 

 way to the islands from some continental area. The 

 conditions of the new environment led to the gradual 

 disuse and consequent degeneration of the wings 

 until they either became useless for flight or, in the 

 case of the moas, completely disappeared. It would 

 be absurd to maintain that any of the existing flight- 

 less birds are specifically identical with the ancestral 

 flying forms from which they are descended, and it 

 would, it appears to me, be equally absurd to suppose 

 that the flightless species arose by mutation or by 

 crossing, the same result being produced over and 

 over again on different islands and in different groups 

 of birds. This is clearly a case where the environ- 

 ment has determined the direction of evolution. 



In such cases there is not the slightest ground for 

 believing that crossing has had anything whatever to 

 do with the origin of the different groups to which the 

 term species is applied ; indeed, the study of island 

 faunas in general indicates very clearly that the pre- 

 vention of crossing, by isolation, has been one of the 

 chief factors in the divergence of lines of descent and 

 the consequent multiplication of species, and Romanes 



NO. 2340, VOL. 94] 



clearly showed that even within the same geographical 

 area an identical result may be produced by mutual 

 sterility, which is the cause, rather than the result, 

 of specific distinction. 



Species, then, may clearly arise by divergent evolu- 

 tion under changing conditions of the environment, 

 and may become separated from one another by the 

 extinction of intermediate forms. The environmental 

 stimuli (including, of course, the body as part of its 

 own environment) may, however, act in two different 

 ways : (i) Upon the body itself, at any stage of its 

 development, tending to cause adaptation by individual 

 selection of the most appropriate response ; and (2) 

 upon the germ-plasm, causing mutations or sudden 

 changes, sports, in fact, which appear to have no 

 direct relation whatever to the well-being of the 

 organism in which they apf>ear, but to be purely 

 accidental. Such mutations are, of course, inherited, 

 and, inasmuch as the great majority of specific char- 

 acters appear to have no adaptive significance, it 

 seems likely that mutation has had a great deal to 

 do with the origin of species, though it may have 

 had very little to do with progressive evolution. 



Similarly with regard to hybridisation, we know 

 that vast numbers of distinct forms, that breed true, 

 may be produced in this way, but they are simply 

 due to recombinations of mutational characters in the 

 process of amphimixis, and have ver}- little bearing 

 upon the problem of evolution. If we like to call 

 the new groups of indrviduals that originate thus 

 "species," well and good, but it only means that we 

 give that name, as a matter of convenience, to any 

 group of closely related individuals which are dis- 

 tinguished by recognisable characters from the indi- 

 viduals of all other groups, and which hand on those 

 characters to their descendants so long as the con- 

 ditions remain the same. This, perhaps, is what we 

 should do, and just as we have learnt to regard indi- 

 viduals as the temporary offspring of a continuous 

 stream of germ-plasm, so we must regard species as 

 the somewhat more permanent but nevertheless tem- 

 porary offshoots of a continuous line of progressive 

 evolution. Individuals are to species what the germ- 

 plasm is to individuals. One sf>ecies does not arise 

 from another species, but from certain individuals in 

 that species, and when all the individuals become so 

 specialised as to lose their power of adaptation, then 

 changes in the environment may result in the extinc- 

 tion of that line of descent. 



It is scarcely necessary to point out that no explana- 

 tion that we are able to give regarding the causes 

 of either phvlogenetic or ontogenetic evolution can be 

 complete and exhaustive. Science can never hope to 

 get to the bottom of things in any department of 

 knowledge; there is always something remaining be- 

 vond our reach. If we are asked why an organism 

 chooses the most appropriate response to any par- 

 ticular stimulus, we may suggest that this is the 

 response that relieves it from further stimulation, but 

 we cannot say how it learns to choose that response 

 at once in preference to all others. If we are asked 

 to account for some particular mutation, we may say 

 that it is due to some modification in the constitu- 

 tion or distribution of the chromosomes in the germ- 

 cells, but even if we knew exactly what that modifica- 

 tion was, and could express it in chemical terms, we 

 could not really say why it produces its particular 

 result and no other, any more than the chemist can 

 sav whv the combination of two gases that he calls 

 oxvgen and hydrogen gives rise to a liquid that he 

 calls water. 



There is one group of ontogenetic phenomena in 

 particular that seems to defy all attempts at mechan- 

 istic interpretation. I refer to the phenomena of 



