46 THE ORIGIN OF SPECIES 



forms should be ranked as species or varieties. Now Fries has re- 

 marked in regard to plants, and Westwood in regard to insects, 

 that in large genera the amount of difference between the species 

 is often exceedingly small. I have endeavored to test this numeri- 

 cally by averages, and, as far as my imperfect results go, they con- 

 firm the view. I have also consulted some sagacious and experi- 

 enced observers, and, after deliberation, they concur in this view. 

 In this respect, therefore, the species of the larger genera resemble 

 varieties, more than do the species of the smaller genera. Or the 

 case may be put in another way, and it may be said, that in the 

 larger genera, in which a number of varieties or incipient species 

 greater than the average are now manufacturing, many of the 

 species already manufactured still to a certain extent resemble 

 varieties, for they differ from each other by less than the usual 

 amount of difference. 



Moreover, the species of the larger genera are related to each 

 other, in the same manner as the varieties of any one species are 

 related to each other. No naturalist pretends that all the species 

 of a genus are equally distinct from each other; they may generally 

 be divided into sub-genera, or sections, or lesser groups. As Fries 

 has well remarked, little groups of species are generally clustered 

 like satellites around other species. And what are varieties but 

 groups of forms, unequally related to each other, and clustered 

 round certain forms — that is, round their parent species? Un- 

 doubtedly there is one most important point of difference between 

 varieties and species, namely, that the amount of difference be- 

 tween varieties, when compared with each other or with their par- 

 ent species, is much less than that between the species of the same 

 genus. But when we come to discuss the principle, as I call it, of 

 divergence of character, we shall see how this may be explained, 

 and how the lesser differences between varieties tend to increase 

 into the greater differences between species. 



There is one other point which is worth notice. Varieties gen- 

 erally have much restricted ranges. This statement is indeed 

 scarcely more than a truism, for, if a variety were found to have a 

 wider range than that of its supposed parent species, their denomi- 

 nations would be reversed. But there is reason to believe that the 

 species which are very closely allied to other species, and in so far 

 resemble varieties, often have much restricted ranges. For instance, 

 Mr. H. C. Watson has marked for me in the well-sifted London 

 Catalogue of Plants (4th edition) sixty- three plants which are 

 therein ranked as species, but which he considers as so closely 

 allied to other species as to be of doubtful value: these sixty-three 



