54 8PSGIE8 OF LARGER GENERA. 



genera and their recorded varieties which deserve notice. 

 We have seen that there is no infallible criterion by which 

 to distinguish species and well-marked varieties; and when 

 intermediate links have not been found between doubtful 

 forms, naturalists are compelled to come to a determination 

 by the amount of difference between them, judging by anal- 

 ogy whether or not the amount suffices to raise one or both to 

 the rank of species. Hence the amount of difference is one 

 very important criterion in settling whether two forms 

 should be ranked as species or varieties. Now Fries has 

 remarked 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 numerically by averages, and, as far 

 as my imperfect results go, they confirm the view. I have 

 also consulted some sagacious and experienced 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 manu- 

 facturing, many of the species already manufactured still 

 to a certain extent resemble varieties, fot 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. 

 Undoubtedly there is one most important point of differ- 

 ence between varieties and species, namely, that the 

 amount of difference between varieties, when compared 

 with each other or with their parent 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 



