240 BIOLOGY: GENERAL AND MEDICAL 



of cells and entire organisms which are capable of taking on a male 

 and female form. But only one half of such a double determinant 

 remains inactive, while the other remains active. The sexual 

 differentiation of the germ cells must thus be due to the presence 

 of Spermatogenetic and Oogenetic double determinants; and even 

 all the secondary sexual characters must be traced to a similar 

 origin in the idioplasm. 



"The assumption of double determinants is also able to throw 

 some light upon certain enigmatical phenomena of heredity 

 exhibited by human beings. It has long been known that hemo- 

 philia (the bleeder's disease) occurs in men only, but is trans- 

 mitted by women. This disease, like a secondary sexual character, 

 is only transmitted to the sex in which it first appeared, for this 

 half of the double determinants of the 'mesoblast germ ' has alone been 

 modified by the disease. 



" It is self-evident from the theory of heredity here propounded 

 that only those characters are transmissible which have been 

 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." 



