230 ON THE ORIGIN OF SPECIES 



cross, and which rarely produce any offspring, are 

 generally very sterile ; but the parallelism between the 

 difficulty of making a first cross, and the sterility of the 

 hybrids thus produced — two classes of facts which are 

 generally confounded together — is by no means strict. 

 There are many cases, in which two pure species can be 

 united with unusual facility, and produce numerous 

 hybrid -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, ror 

 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 the fertility of pure species. But the degree of 

 fertility is likewise innately variable ; for it is not 

 always the same when the same two species are crossed 

 under the same circumstances, but 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 exactly the 

 same conditions. 



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 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 



