598 Species According to the Theory of Mutation. 
I do not propose to elaborate this theme further ; it 
has often been dealt with and especially in great detail 
by 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 
parallel ; 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 will 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 (he. cit., p. 168). NAGELI has elaborated 
this idea and SACHS has followed him in his Lchrbuch 
der 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. 
