HEREDITY AND ADAPTATION. l6l 



highly important evolutionary function. Heredity. Just 

 as we were able to trace Adaptation back to nutrition, we 

 can also show that Heredity is a necessary phenomenon 

 of reproduction ; and this is equally true of both kinds 

 of Heredity — of conservative, as well as of progressive 

 Heredity. As I have also fully explained these highly 

 important Laws of Heredity, which maintain constant 

 reciprocal relations with the Laws of Adaptation, in my 

 "History of Creation," vol. i. Chapter VIII. p. 175, we 

 wiU not stop to examine them here. (See also Generelle 

 Morphologic, vol. ii. pp. 170-191.) 



Division of labour, or differentiation, which has but 

 recently begun to be correctly valued, forms a sixth 

 evolutionary function of especial importance. We have 

 already seen that division of labour is the strongest impulse 

 towards progressive evolution, not only in civic and social 

 life, but also in the social cell-confederacy of every many- 

 celled organism. A glance at any community or state 

 organization shows that the first condition of all higher 

 development and civilization, is, on the one hand, the divi- 

 sion of the various duties among the various classes of the 

 citizens ; and, on the other hand, the co-operation of these 

 single individuals for the common jDurposes of the state. 

 This is exactly the case also in every many-celled organism. 

 Every multicellular individual in the plant or animal 

 kingdom is more perfectly developed, and ranks higher in 

 proportion as the division of labour among its constituent 

 cells, the differentiation of its cell-individuals, is more 

 perfect. Therefore in the various classes of organisms we 

 find this differentiation, sometimes in a more, sometimes in 

 a less perfect condition. T'le simplest form of division of 



