256 HYBRIDISM. Chap. VIII. 



of incipient fertilisation. From this extreme degree 

 of sterility we have self-fertilised hybrids producing a 

 greater and greater number of seeds up to perfect fer- 

 tility. 



Hybrids from two species which are very difficult to 

 cross, and which rarely produce any offspring, are gene- 

 rally very sterile ; but the parallelism between the diffi- 

 culty 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, 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 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 t<.rm 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 



