PURE LINES 321 



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. Recent discoveries by Winkler and Baur 

 regarding the nature of so-called ' graft-hybrids ' go 

 far to prove that acquired characters cannot be in- 

 herited in plants. The classical example of a ' graft- 

 hybrid ' is Cytisus Adami, which was produced in 1825 

 by grafting Cytisus purpureus on the laburnum. This 

 plant, which is more or less intermediate between the 

 two parent species, has been reproduced by further 

 grafting, but its seeds always give rise to plants which 

 are indistinguishable from the laburnum. Adventitious 



21 



