598 Species According to tJie Theory of Mutation. 



I du not propose to elaborate tliis theme further ; it 

 has often been dealt with and especially in great detail 

 b\' FocKE, who has presented it in a masterly way in his 

 textbook on plant hybrids. The main conclusion, how- 

 ever, is that the majority of authors agree that systematic 

 and sexual affinity, if properly understood, are essentially 

 j^arallel ; indeed, that they are really no more than two 

 manifestations of one and the same thing, but that we 

 have not yet succeeded in explaining the apparent ex- 

 ceptions to this parallel. 



For our purpose, however, the important question is, 

 whether the diagnoses of species and varieties w^ill grad- 

 ually come to be based on elementary characters as units, 

 and whether sexual relationship will come to be judged 

 by the number of differentiating units. Gartner has 

 already pointed out that those genera in which the largest 

 numbers of hybrids have been produced are exactly those 

 in which the number of very closely related species is 

 the greatest (loc. cit., p. 168). Nageli has elaborated 

 this idea and Sachs has followed him in his Lclirbuch 

 (ler Botanik. Abbado, Hurst, Gillot and many others 

 have also subscribed to this view. 



The opinion expressed by these writers on the paral- 

 lel between systematic and sexual affinity, may be sum- 

 marized in the following thesis, viz., that the fertility 

 of crosses and of the hybrids resulting from them, di- 

 minishes, on the average, as the number of points of 

 difference (that is to say, that of the elementary char- 

 acters, which constitute the differences) increases. But 

 many more experiments are necessary before this sug- 

 gestion can be examined critically or be regarded as 

 resting on a sure foundation of experimental facts. 



