Chap. IX.J OF FIRST CROSSES AND OF flYBRlDS. 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 condi- 

 tions. 



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 hy- 

 brids 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 syste- 

 matic 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 dis- 

 tinct species which unite with the utmost facility. In 

 the same family there may be a genus, as Dianthus, in 



