82 GENETICS 



characters in amphimixis and hybridization will re- 

 ceive further attention in a later chapter. 



The fact that successive parthenogenetic genera- 

 tions, in which amphimixis does not of course occur, 

 may show a larger degree of variability than sexually 

 produced generations, indicates that amphimixis in 

 itself is by no means sufficient to account for all kinds 

 of variations. 



The abrupt way, for instance, in which mutations 

 appear in apparent independence of external influences 

 suggests that there may be some internal factor, as 

 yet unknow^n, acting directly through the germplasm, 

 regardless of external causes. 



The assumption of an unknown factor does not 

 necessarily imply a return to "vitalism," which is so 

 elusive of experimental test and hence so unsatisfac- 

 tory to the scientific mind, nor does it admit, simply 

 because this factor is at present an unknown quantity, 

 that it is consequently doomed to remain so. 



It is easily conceivable that the external factors 

 acting upon the germplasm may be grouped into two 

 alternative classes : first, external factors that act upon 

 the somatoplasm and through the agency of the 

 somatoplasm affect the germplasm ; and second, those 

 that act directly upon the germplasm without neces- 

 sarily at the same time influencing the somatoplasm. 



The first category, that of somatic modifications 

 which leave their impress upon the germplasm, in- 

 cludes true acquired characters according to our 

 definition, while the second, which includes cases of 

 the direct influence of external stimuli upon the 



