THE ORIGIN OF SPECIES 159 



vejy generally, but not quite universally, fertile. Nor 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 repro- 

 ductive 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 

 fact, so far as the inquiry into the origin of species goes, 

 is, that there are such things in Nature as groups of animals 

 and of plants, whose members are incapable of fertile 

 union with those of other groups ; and that there are 

 such things as hybrids, which are absolutely sterile when 

 crossed with other hybrids. For if such phenomena as 

 these were exhibited by only two of those assemblages of 

 living objects, to which the name of species (whether it 

 be used in its physiological or in its morphological sense) 

 is given, it would have to be accounted for by any theory 

 of the origin of species, and every theory which could not 

 account for it would be, so far, imperfect. 



Up to this point we have been dealing with matters of fact, 

 and the statements which we have laid before the reader 

 would, to the best of our knowledge, be admitted to contain 

 a fair exposition of what is at present known respecting 

 the essential properties of species, by all who have studied 

 the question. And whatever may be his theoretical views, 

 no naturalist will probably be disposed to demur to the 

 following summary of that exposition : 



Living beings, whether animals or plants, are divisible into 

 multitudes of distinctly definable kinds, which are morpho- 

 logical species. They are also divisible into groups of 

 individuals, which breed freely together, tending to repro- 

 duce their like, and are physiological species. Normally 

 resembling their parents, the offspring of members of these 

 species are still liable to vary, and the variation may be 

 perpetuated by selection, as a race, which race, in many 

 cases, presents all the characteristics of a morphological 

 species. But it is not as yet proved that a race ever 

 exhibits, when crossed with another race of the same species, 



