PURE LINES 285 



taneously. And thus the normal variability of the 

 population as a whole is brought about by the com- 

 bination of these two separate factors. 



This statement applies to the case of an organism 

 in which self-fertilization is the general rule, so that in 

 this way the separate lines are kept distinct. Where 

 cross-fertilization takes place between the members of 

 different pure lines the case becomes enormously com- 

 plicated, and this is much the most frequent instance 

 which we have actually to deal with. It has been 

 suggested that the members of different lines when 

 crossed together may display Mendelian phenomena, 

 but the existence of so large a proportion of acquired 

 variability renders the problem of analyzing the result 

 almost insuperable. We have seen, however, that the 

 numerical results obtained by the biometricians do 

 not appear to be inconsistent with the existence of 

 Mendelian inheritance in populations. 



We find, then, that the questions of inheritance of 

 acquired characters and of evolution by the aid of 

 continuous genetic variations are not yet absolutely 

 settled. But the evidence is certainly such that for all 

 practical purposes the former factor at least may be 

 disregarded. Meanwhile the number of cases in 

 which discontinuity of inheritance can be shown to 

 hold good is constantly increasing, and the analysis of 

 some cases of supposed continuous variation into dis- 

 continuous Mendelian factors has already been made. 

 It may be safely concluded that a very large part, if 

 not the whole, of evolution has taken place by the 

 discontinuous method. 



