Nature of Here/lift/ find Problem of Mechanism 309 



limitation. Thus in the glossary of Heredity and Environ- 

 ment in the Development of Man by E. G. Coiiklin, we read 

 that heredity may be defined as ''the appearance in the off- 

 spring of characters whose differentia] causes are found in 

 the germ cells"; 1 and since nowhere in the volume does 

 Professor Conklin even mention any but germ-cell repro- 

 duction, we are obliged to assume that for him heredity is a 

 phenomenon of germ-cell reproduction alone. This seems to 

 be a case of trying to escape through definition the difficulties 

 encountered by a theory; in other words, to circumscribe the 

 theory for the purpose of excluding from its scope phenom- 

 ena which can not be made to fit in with it. It is surprising 

 that so careful a rcasoner and observer as Conklin should 

 have fallen into this pit. A definition of heredity that would 

 exclude from its. operation the growth of a tiger lily from 

 a bulb, of a sponge from a gemmule, and of an ascidiozooid 

 from an ascidian bud, is so obviously forced that it ought 

 to raise a suspicion that consciously or otherwise it is framed 

 with some other motive in view than that of telling what 

 heredity is; and it is unbelievable that such a definition can 

 gain general and permanent approval. 



While not many authorities are so definitely extreme as 

 this, a large majority of the recent books in which heredity 

 occupies a prominent place tend to lead the reader thus to 

 restrict his conception of heredity. Another class of writers, 

 while tacitly allowing that heredity manifests itself in cases 

 when- grrm-cells do not occur, yet take the ground that 

 sexless propagation is very exceptional and does not need 

 to be taken particularly into account in elaborating theories 

 about heredity. Thus in so excellent a book as J. Arthur 

 Thomson's Heredity we are told that "the exceptions are 

 trivial compared with the vast majority of living creatures 

 in regard to which it is certain that each life begins in a 

 fertilized egg-cell." And on a later page this author ital- 

 ici/es the sentence: "In asexual reproduction the resem- 



