328 COOK 



unit mechanisms of descent. Some regard heredity as a sum- 

 mary of environmental influences, and some as the result of an 

 intracellular mechanism of predetermination, having no relation 

 to the environment. 



The environment does not form organisms, but neither can 

 organisms be thought of correctly without bearing in mind their 

 normal diversities and powers of individual accommodation to 

 different external conditions, powers which are as incompatible 

 with ideas of complete predetermination from within as they are 

 with ideas of direct causation from without. Heredity, as signi- 

 fying the succession of organisms in continuous lines of descent, 

 is an actual fact, though as yet quite unexplained. Heredity, 

 in the sense of a normal uniformity of organisms in species, 

 does not exist. Instead of like producing like, the rule of hered- 

 ity is that unlike produces unlike. To assist in an understand- 

 ing of evolution and of the processes of descent the conception 

 of heredity must be modified, and for some purposes entirely 

 replaced, by a recognition of the facts of heterism, the normal 

 inherent diversity shown by the individuals, castes and sexes of 

 the same species. It is only when the members of a species are 

 compared with the members of other species that they can be 

 said to be alike. Compared with the members of their own 

 species, all organisms are different. 



Heredity and variation are not uncommonly personified as two 

 opposing agents or " forces," the one striving to make organ- 

 isms alike, the other to make them different. The late Pro- 

 fessor Hyatt and others have even gone so far as to definitely 

 locate all the heredity inside the organism and all the variation 

 outside, holding that the organisms would be identical in form 

 and structure were it not for variable external influences. The 

 conception of heredity as an ideal uniformity is more applicable 

 to some species than to others, but is not completely true of any. 

 Experiment has everywhere shown that the members of the 

 species and varieties are alike — as far as they are alike — 

 because they breed together, not because they live in the same 

 environments or because their form is definitely predetermined 

 by an internal mechanism. The network of descent is a part 

 of the mechanism of heredity, quite as truly as any character- 

 unit particles can be. 



