Chap. VIII. SUMMARY. 277 



to depend on several circumstances ; in some cases 

 largely on the early death of the embryo. The sterility 

 of hybrids, which have their reproductive systems im- 

 perfect, and which have had this system and their whole 

 organisation disturbed by being compounded of two dis- 

 tinct species, seems closely allied to that sterility which 

 so frequently affects pure species, when their natural 

 conditions of life have been disturbed. This view is 

 supported by a parallelism of another kind ; — namely, 

 that the crossing of forms only slightly different is 

 favourable to the vigour and fertility of their offspring ; 

 and that slight changes in the conditions of life are ap- 

 parently favourable to the vigour and fertility of all 

 organic beings. It is not surprising that the degree of 

 difficulty in uniting two species, and the degree of 

 sterility of their hybrid-offspring should generally cor- 

 respond, though due to distinct causes ; for both depend 

 on the amount of difference of some kind between the 

 species which are crossed. Nor is it surprising that 

 the facility of effecting a first cross, the fertility of the 

 hybrids produced, and the capacity of being grafted to- 

 gether — though this latter capacity evidently depends 

 on widely different circumstances — should all run, to a 

 certain extent, parallel with the systematic affinity of 

 the forms which are subjected to experiment ; for sys- 

 tematic affinity attempts to express all kinds of resem- 

 blance between all species. 



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 quite uni- 

 versally, fertile. Nor is this nearly general and perfect 

 fertility sui*prising, when we remember 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 domesti- 



