240 LAY SERMONS, ESSAYS, AND REVIEWS. [xn. 



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 pro 

 bably 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 morphological 

 species. They are also divisible into groups of individuals, which 

 breed freely together, tending to reproduce 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 morpho 

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

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

 those phenomena of hybridization which are exhibited by many 

 species when crossed with other species. On the other hand, not 

 only is it not proved that all species give rise to hybrids infertile 

 inter se, but there is much reason to believe that, in crossing, 

 species exhibit every gradation from perfect sterility to perfect 

 fertility. 



Such are the most essential characteristics of species. Even 

 were man not one of them a member of the same system and 



