Nature of Heredity and Problem of Mechamsm 309 



limitation. Thus in the glossary of Heredity and Enriron- 

 ment in the Development of Man by K. (i. Conkhn, we road 

 that heredity may be defined as "the appearance in the oil- 

 spring of characters whose dilFerential causes are found in 

 the germ cells'';^ 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. Tliis seems to 

 be a case of trying to escape througli definition the difficulties 

 encountered by a theor} ; in other words, to circumscribe the 

 theory for the purpose of excluding from its scope phenom- 

 ena w^hich can not be made to fit in with it. It is surprising 

 that so careful a reasoner 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 tliat 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 

 where germ-cells do not occur, yet take the ground tliat 

 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. Artliur 

 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 tliat each life begins in a 

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

 icizes the sentence: "In asexual reproduction the resem- 



