46 SUMMARY. [Chap. IX. 



should so generally have become more or less modified, 

 leading to their mutual infertility, we do not know ; but 

 it seems to stand in some close relation to species having 

 been exposed for long periods of time to nearly uniform 

 conditions of life. 



It is not surprising that the difficulty in crossing any 

 two species, and the sterility of their hybrid offspring, 

 should in most cases correspond, even if due to distinct 

 causes : for both depend on the amount of difference 

 between the species which are crossed. Nor is it 

 surprising that the facility of effecting a first cross, and 

 the fertility of the hybrids thus produced, and the 

 capacity of being grafted together — though this latter 

 capacity evidently depends on widely different circum- 

 stances — should all run, to a certain extent, parallel 

 with the systematic affinity of the forms subjected to 

 experiment ; for systematic affinity includes resem- 

 blances of all kinds. 



First crosses between forms known to be varieties, or 

 sufficiently alike to be considered as varieties, and their 

 mongrel offspring, are very generally, but not, as is so 

 often stated, invariably fertile. Nor is this almost 

 universal and perfect fertility surprising, when it is 

 remembered how liable we are to argue in a circle with 

 respect to varieties in a state of nature ; and when we 

 remember that the greater number of varieties have 

 been produced under domestication by the selection of 

 mere external differences, and that they have not been 

 long exposed to uniform conditions of life. It should 

 also be especially kept in mind, that long-continued 

 domestication tends to eliminate sterility, and is there- 

 fore little likely to induce this same quality. Indepen- 

 dently of the question of fertility, in all other respects 



