OTHER FACTORS OF EVOLUTION 131 



We must now see how far Weismann is consistent in holding his 

 doctrines, and to what extent the logic of facts may have com- 

 pelled him to make admissions, which practically bring him almost 

 into accord with Spencer and his followers. 



The blastegenic changes with which heritable characters are 

 correlated are, in the opinion of Weismann, due either (i) to 

 artificial or to natural selection, or else (2) to ' spontaneous ' varia- 

 tions of the germ-plasm, from whatsoever cause arising. 



In considering these views we must first come to some under- 

 standing as to the starting points for artificial and natural selection. 

 According to Darwin, ' " selection does nothing without varia- 

 bility, and this depends in some manner on the action of surround- 

 ing circumstances on the organism." On this same subject 

 Weismann says, ^ " The ordinary, never-ceasing, always active 

 germinal selection depends, we must assume, upon intragerminal 

 fluctuations of nuti'ition, or inequalities in the nutritive stream which 

 circulates within the germ-plasm." Such changes, however, he 

 thinks, " have their roots in external influences." He asks, indeed, 

 " how could the germ-plasm be changed except by the operation 

 of external influences, using the words in their widest sense ? " 

 (" Essays," I, p. 424). And in his recent work he says (II, p. 137) 

 when speaking of some of the influences of external conditions : 

 " I call this form of germinal variation ' induced ' germinal selection, 

 and contrast it with ' spontaneous ' selection, which is caused not 

 by extra-germinal influences but by the chances of the intra- 

 germinal nutritive conditions." 3 



Thus it is, in part, these ' induced ' changes in the ids of the 

 germ-plasm that, according to Weismann, produce the ''varia- 

 bility" without which, as Darwin said, "selection does nothing." 

 This direct action of external conditions upon the germ-plasm is, 

 in fact, freely admitted. After speaking of the effects of warmth 

 and cold on the pupae of certain butterflies, Weismann says 

 (II, p. 273) : " It is thus intelligible that somatic variations like 

 the blackening of the wings through warmth appear to be directly 

 inherited and accumulate in the course of generations ; in reality, 

 however, it is not the somatic change itself which is transmitted, 



' " Animals and Plants," I, page 7. 



" " The Evolution Theory," II, p. 266. 



3 "Amphimixis as the means by which a continued new combination of 

 variations is effected," need not be considered here, since this, as Weismann says, 

 is not " the real root of variation " (II, p. 194). 



