xii.] THE ORIGIN OF SPECIES. 275 



is no more reason to think that species have been specially endowed 

 with various degrees of sterility to prevent them crossing and breeding 

 in Nature, than to think that trees have been specially endowed with 

 various and somewhat analogous degrees of difficulty in being grafted 

 together, in order to prevent them becoming inarched in our forests. 



"The sterility of first crosses between pure species, which have 

 their reproductive systems perfect, seems to depend on several circum- 

 stances ; in some cases largely on the early death of the embryo. The 

 sterility of hybrids which have their reproductive systems imperfect, 

 and which have had this system and their whole organization dis- 

 turbed by being compounded of two distinct 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 the offspring ; and that slight changes in the conditions of life are 

 apparently 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 

 correspond, though due to distinct causes ; for both depend on the 

 amount of difference of some kind between the species which are 

 crossed. JSTor is it surprising that the facility of effecting a first cross, 

 the fertility of hybrids produced from it, and the capacity of being 

 grafted together — 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 systematic affinity attempts to express all kinds of 

 resemblance 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 universally, fertile. ISTor is this nearly 

 general and perfect fertility surprising, 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 domestication by the selection of mere 

 external differences, and not of differences in the reproductive system. 

 In all other respects, excluding fertility, there is a close general 

 resemblance between hybrids and mongrels." — Pp. 276-8. 



We fully agree with the general tenor of this weighty 

 passage; but forcible as are these arguments, and little as 

 the value of fertility or infertility as a test of species may 

 be, it must not be forgotten that the really important 



