12 PRIMAR Y FA CTORS OF ORGANIC E VOL UTION. 



the appearance of characters in the embryonic succes- 

 sion, through influences confined to the germ-plasma, 

 Weismann invented a theory which requires the pres- 

 ence of distinct molecular aggregates within it, which 

 represent the potentialities or causes. To these he has 

 given the names of ids, idants, determinants, etc. As 

 Weismann's contribution to evolution has been con- 

 fined to the department of heredity, I will consider it 

 more particularly in the third part of this book, which 

 is devoted to that subject. 



Weismann has, however, subsequently modified 

 his views to a considerable extent. He has always 

 admitted the doctrine of Lamarck to be applicable to 

 the evolution of the types of unicellular organisms. 

 His experiments on the effect of temperature on the 

 production of changes of color in butterflies, showed 

 that such changes were not only effected, but were 

 sometimes inherited. This he endeavors to explain as 

 follows. 1 ' 'Many climatic variations may be due wholly 

 or in part, to the simultaneous variation of correspond- 

 ing determinants in some parts of the soma and in the 

 germ-plasm of the reproductive cells." This is an ad- 

 mission of the doctrine which in 1890 I called Diplo- 

 genesis, 2 and which is adopted in the present work. It 

 appears to have been first propounded by Galton in 

 1875. In the chapter on Heredity I hope to offer some 

 reasons for believing that the suggestion of Galton 

 embraces the true doctrine of heredity. 



From what has preceded, two distinct lines of 

 thought explanatory of the fact of organic evolution 

 may be discerned. In one of these the variations of 

 organisms which constitute progressive and regressive 



1 The Germ Plasm, Contemporary Science Series, 1893, p. 406. 

 ^American Naturalist, Dec., 1889; published in 1890. 



