OTHER FACTORS OF EVOLUTION 119 



correlations' at the present stage of evolution takes place by 

 heredity, even though the usual irritations are partly or entirely 

 wanting. If, therefore, we were at liberty to assume the inherit- 

 ance of acquired characters (as to which there is so much dispute) 

 the explanation of the existence of secondary sexual characters, 

 and their strict limitations to a particular sex and a particular 

 period of life, and often to a particular season, would present less 

 difficulties than those which beset the explanations of Dai-win 

 and Weismann.' 



Believing as he does in the inheritance of acquired characters 

 the explanation of Secondary Sexual Characters adopted by 

 Cunningham is this : — " that the direct effects of regularly recurrent 

 stimulations are sooner or later developed by heredity, but only in 

 association with the physiological conditions under which they were 

 originally produced, and that this is the explanation of the 

 limitation of particular modiiications, not merely to particular 

 species or kinships, but to particular periods in the life of the 

 individual, to a particular sex, and even to a particular season 

 of the year in that sex." He contends, indeed, that the whole series 

 of multitudinous facts agrees remarkably with the hypothesis that 

 secondary sexual characters are due to the inheritance of acquired 

 characters. It is clear that until this latter doctrine is established 

 — a time which we may hope is not far distant — Cunningham's 

 view concerning secondary sexual characters will stand little 

 chance of being accepted. 



The evidence in regard to this subject of the Inheritance 

 of Acquired Characters must now be briefly considered, as it is 

 the all-important question in relation to Use and Disuse, and 

 the Direct Influence of External Conditions. If the effects of 

 these influences are coniined to individuals and not transmitted 

 to their progeny, neither of them could be regarded as a factor 

 of Evolution ; while, on the other hand, if the effects acquired 

 in either of these ways are capable of being transmitted to 

 offspring it is clear that we should have in each of them a factor 

 of Evolution of great importance. 



Doubts on this subject were first started by Weismann in 1883 



» Cunningham says (loc. cit. p. 40), " To my thinking the suppression of male 

 characters in consequence of castration is in itself sufficient to disprove the 

 theory of the absolute continuity of the germ-plasm, and its absolute indepen- 

 dence of the somatic cells." 



