Chap. YIIL LAWS OF STEKILITY. 257 
By the term systematic affinity is meant, the resem¬ 
blance between species in structure and in constitution, 
more especially in the structure of parts which are of 
high physiological importance and which differ little in 
the allied species. Now the fertility of first crosses 
between species, and of the hybrids produced from 
them, is largely governed by their systematic af¬ 
finity. This is clearly shown by hybrids never having 
been raised between species ranked by systematists in 
distinct families ; and on the other hand, by very closely 
allied species generally uniting with facility. But the 
correspondence between systematic affinity and the 
facility of crossing is by no means strict. A mul¬ 
titude of cases could be given of very closely allied 
species which will not unite, or only with extreme 
difficulty; and on the other hand of very distinct 
species which unite with the utmost facility. In the 
same family there may be a genus, as Dianthus, in 
which very many species can most readily be crossed ; 
and another genus, as Silene, in which the most 
persevering efforts have failed to produce between 
extremely close species a single hybrid. Even within 
the limits of the same genus, we meet with this same 
difference ; for instance, the many species of Nicotiana 
have been more largely crossed than the species of 
almost any other genus; but Gartner found that N. 
acuminata, which is not a particularly distinct species, 
obstinately failed to fertilise, or to be fertilised by, no 
less than eight other species of Nicotiana. Very many 
analogous facts could be given. 
No one has been able to point out what kind, or what 
amount, of difference in any recognisable character is 
sufficient to prevent two species crossing. It can be 
shown that plants most widely different in habit and 
general appearance, and having strongly marked differ- 
