Chap. IX.] OF FIRST CROSSES AND OF HYBRIDS. 13 



offspring, yet these hybrids are remarkably sterile. 

 On the other hand, there are species which can be 

 crossed very rarely, or with extreme difficulty, but 

 the hybrids, when at last produced, are very fertile. 

 Even within the limits of the same genus, for instance 

 in Dianthus, these two opposite cases occur. 



The fertility, both of first crosses and of hybrids, is 

 more easily affected by unfavourable conditions, than 

 is that of pure species. But the fertility of first crosses 

 is likewise innately variable ; for it is not always the 

 same in degree when the same two species are crossed 

 under the same circumstances ; it depends in part upon 

 the constitution of the individuals which happen to 

 have been chosen for the experiment. So it is with 

 hybrids, for their degree of fertility is often found to 

 differ greatly in the several individuals raised from 

 seed out of the same capsule and exposed to the same 

 conditions. 



By the term systematic affinity is meant, the general 

 resemblance between species in structure and constitu- 

 tion. Now the fertility of first crosses, and of the 

 hybrids produced from them, is largely governed by 

 their systematic affinity. 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 multitude 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, 



