356 ACTION OF NATURAL SELECTION 







Italy and Greece. Though these forms can be more or 

 less transformed into each other, by suitable exposure 

 of the pupse to warmth or cold, yet Weismann found 

 that from German pupae he could never obtain butter- 

 flies so dark as the darkest forms of the southern 

 variety, whilst from Neapolitan pupae he could never 

 get them so light as the ordinary German variety.* 

 It seemed to him, therefore, " that the two varieties 

 may have originated owing to a gradual cumulative in- 

 fluence of the climate, the slight effects of one summer 

 or winter having been transmitted and added to from 

 generation to generation." Weismann explains this 

 case of apparent transmission of acquired characters by 

 supposing that the temperature slightly affects the de- 

 terminants of the wing scales contained in the germ- 

 plasm, as well as more markedly influencing the deter- 

 minants of the rudimentary wings in the chrysalis. 

 Moreover he suggests that " in many other animals and 

 plants influences of temperature and environment may 

 very possibly produce permanent hereditary variations 

 in a similar manner." 



This suggestion of Weismann's contains in it, it 

 seems to me, the germ of an idea which further obser- 

 vation and experiment may prove to be of fundamental 

 importance in evolution. The idea itself is no new 

 one, and has probably occurred independently to many 

 writers. As far as I am aware, it was first suggested 

 by Galton,t when propounding the theory of heredity 

 to which that of Weismann bears so striking a resem- 



* Germ-Plasm," p. 399. 



fProc. Roy. Soc., xx. p. 394, 1872; Contemporary Review, 

 December, 1875; Journ. Anthropol. Inst., 1875, p. 346. 



