ORIGIN OF THE SPECIFIC TYPE 329 



assumes mutation-periods, I believe rightly ; but they are not periods 

 prescribed, so to speak, from within, as those who believe in a 

 ' phyletie force ' must suppose ; they are caused by the influences of the 

 environment which affect the nutritive stream within the germ-plasm, 

 and which, increasing latently, bring about in part mere variability, 

 in part mutations, just as I have indicated in the section on Isolation 

 (vol. ii. p. 280), and indeed, in one of my earliest contributions to the 

 theory of descent '. In that essay I suggested the conclusion that 

 periods of constancy alternate with periods of variability, basing my 

 opinion on general considerations, and on Hilgendorf's study of the 

 Steinheim snail-shells. According to De Vries's (Eaothera experiments 

 we nvA.y assume that periods of increased variability may lead to the 

 marked variations sometimes affecting several characters simultan- 

 eously, and occurring in many ids, which have hitherto been called 

 ' saltatory ' variations, and which we should perhaps do well to call 

 in future, with De Vries, mutations. 



We cannot yet determine liow far the influence of such mutations 

 reaches. I think it is plain that De Yries himself overestimated it, 

 but how many of the species-types which we find to-day depend upon 

 mere mutation can only be decided with any certainty after further 

 investigations. For the present it is well to be clear as to the validity 

 of the general conclusion, that all ' complex,' and especially all 

 ' harmonious,' adaptation, must dej)eud, not upon ' mutation,' but 

 only upon ' variation ' guided by selection. As species are essentially 

 complexes of adaptation, originating from a basis of previous complexes 

 of adaptation, there remains, as far as I can see, only the small field of 

 indifferent characters to be determined by mutations, unless indeed 

 we are to include under the term ' mutation ' all variations which gain 

 stability, but this would be inerely a play upon words. In my opinion 

 there is no definable boundary line betiveen variation and mutation, 

 and the difference betiveen these two phenomena, depends solely on the 

 number — larger or smaller — of ids ivhich have varied in the same 

 direction. 



' Ueler den Einfluss cler Isolirung aufdie Artbildung, Leipzig, 1872, p. 51. 



