254 BIOLOGY: GENERAL AND MEDICAL 



controlled i.e., produced by determinants of the germ, and that 

 consequently only those variations are hereditary which result 

 from the modification of several or many determinants in the 

 germ plasm, and not those which have arisen subsequently in con- 

 sequence of some influence exerted upon the cells of the body. In 

 other words, it follows from this theory that somatogenic or 

 acquired characters cannot be transmitted. 



"This, however, does not imply that external influences are 

 incapable of producing hereditary variations; on the contrary, they 

 always give rise to such variations when they are capable of 

 modifying the determinants of the germ plasm. Climatic in- 

 fluences, for example, may well produce permanent variations by 

 slowly causing gradually increasing alterations to occur in the 

 determinants in the course of generations. The primary cause of 

 variation is always the effect of external influences. When 

 deviations only affect the soma, they give rise to temporary, 

 non-hereditary variations; but when they occur in the germ plasm, 

 they are transmitted to the next generation and cause correspond- 

 ing hereditary variations in the body." 



The chief feature of Weismann's theory is thus ex- 

 pressed by Wilson: "It is a reversal of the true point of 

 view to regard inheritance as taking place from the body 

 of the parent to that of the child. The child inherits 

 from the parent germ cell, not from the parent body, 

 and the germ cell owes its characteristics not to the body 

 which bears it, but to its descent from a pre-existing 

 germ cell of the same kind. Thus the body is, as it were, 

 an offshoot from the germ cell. As far as inheritance 

 is concerned, the body is merely the carrier of the germ 

 cells which are held in trust for coming generations." 



It goes without saying that so suggestive and com- 

 plete a theory as that of Weismann must take a strong 

 hold and leave a deep impression upon the thoughtful 

 mind. Whatever may be thought of the biophors, 

 determinants, ids, and idants and some, like Adami, 

 believe that they have demolished this elaborate succes- 

 sion by showing the physical impossibility of a sufficient 

 number of them being packed away in the germ plasm 

 the doctrine of the continuity of the germ plasm re- 

 mains unassailed and forms the foundation of much of 

 the thought of the present day. 



During these lengthy excerpts from the writings upon 

 inheritance the reader cannot but have observed that 



