November 24, 1905.] 



SCIENCE. 



669 



reader of ' The Evolution Theory ' will note 

 many minor changes from the positions taken 

 earlier by Weismann in ' The Germ Plasm,' 

 yet the substance of his theory of evolution 

 remains unchanged. 



The fundamental idea in ' The Germ 

 Plasm ' Avas the mutual independence of soma 

 and germ, that is, of the body exclusive of the 

 reproductive cells on one hand, and the re- 

 productive cells on the other hand. Each, it 

 was maintained, might be modified without 

 modification of the other. This idea, at the 

 time a novel one, has been shown by subse- 

 quent investigations to be substantially cor- 

 rect. It is the great merit of Weismann to 

 have inspired those investigations. Through 

 experimental studies, in which American zool- 

 ogists have borne an honorable part, the effects 

 of various external agencies upon the soma 

 have been carefully analyzed. What effect, 

 if any, these external agencies have upon the 

 germ plasm is less clear. The opponents of 

 Weismann, in common with Darwin, have at 

 times maintained that induced modifications 

 of the soma were handed on directly to the germ 

 plasm and thus became hereditary. Weis- 

 mann has always denied any such modification 

 of the germ plasm through the soma, but con- 

 cedes a modification of the germ plasm parallel 

 with that which is directly induced by the 

 environment in the soma. The germ plasm, 

 however, in his opinion, is less sensitive than 

 the soma to environmental changes, and so 

 responds only to continuous influences, not to 

 those which last for a single generation only. 

 In this way Weismann seeks to find a basis 

 for the innumerable and often marvelously 

 perfect adaptations of organisms to their en- 

 vironment. 



Weismann insists upon the germinal origin 

 of variations which are heritable, but concedes 

 that germinal variation may be given a par- 

 ticular direction by the envirormient. These 

 variations may at first be too slight to have 

 selectional value, but by the persistent action 

 of the environment will be increased until 

 selectional value is attained. Further, they 

 will make their appearance not in an occa- 

 sional individual merely, as we should expect 

 if they are due to chance, but in so much of 



the race as is subjected to the continuous 

 influence of the same environment. In taking 

 this position Weismann attaches less impor- 

 tance than formerly to natural selection, 

 adopts a different conception of the origin of 

 variations which are heritable, and accounts 

 more fully for adaptations. 



Weismann shows his open-mindedness and 

 breadth of view in adopting from his oppo- 

 nents an idea upon which paleontologists have 

 laid stress, that when an organism has once 

 begun to vary in a given direction, there is 

 an inherent internal tendency for it to go on 

 varying in that same direction, this tendency 

 being quite independent of the environment 

 and due to a struggle of the determinants of 

 the germ plasm among themselves, a process 

 which Weismann calls germinal selection. To 

 this position he has been led by two considera- 

 tions : (1) that there occur among organisms 

 adaptations too subtle and complicated to 

 have selectional value in the struggle for 

 existence, and (2) that many organisms are 

 over-adapted, that is, have progressed beyond 

 what is advantageous in a particular sort of 

 adaptive variation, as, for example, the extinct 

 Irish elk with his tree-like antlers. 



' The Evolution Theory,' while containing 

 a full exposition of Weismann's own viev^s, 

 includes much else. It contains an accurate 

 and interesting historical account of the de- 

 velopment of the evolutionary idea from its 

 origin in the speculations of Greek philoso- 

 phers to its culmination in Darwin's ' Origin 

 of Species.' A very full account is given of 

 Darwin's views and of the lines of evidence 

 on which those views were based. The muta- 

 tion theory of de Vries is critically examined, 

 though it finds little favor in Weismann's eyes. 

 Like Darwin, he considers sport variation 

 (mutation) of small consequence in the pro- 

 duction of species, believing its effects to be 

 local and temporary, resulting in the produc- 

 tion of small and peculiar groups within 

 species, but not of the broader species groups 

 themselves. In breadth of scope and fullness 

 of treatment Weismann's book surpasses all 

 other works on the same subject ; it will doubt- 

 less long remain the authoritative statement 

 of Darwinism. 



