Chap. II.] SPECIES OF LARGER GENERA. 71 



is, that, where many species of a genus have been 

 formed, on an average many are still forming ; and this 

 certainly holds good. 



Many of the Species included within the Larger Genera 

 resemble Varieties in being very closely, but unequally, 

 related to each other, and in having restricted ranges. 



There are other relations between the species of 

 large 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 analogy 

 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 endeavoured 

 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 

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