II THE ORIGIN OF SPECIES 49 



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 mon- 

 grel offspring, are very generally, but not quite universally, 

 fertile. Nor is this nearly general and perfect fertility sur- 

 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 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 argu- 

 ments, 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, the members of which are in- 

 capable of fertile union with those of other groups ; 

 and that there are such things as hybrids, whicli 

 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. 



