OG SPECIES OF LARGER GENERA Chap. II. 



aiul disappeared. All that we -want to show is, that, where 

 many species of a genus have been formed, on an average 

 many arc still forming ; and this certainly holds good. 



Many of tJic Sjiccies included iclthln the Larger Genera re- 

 semble Varieties in being very closely, but unequally, related 

 to each other, and in having liestricted Manges. 



There are other relations between the species of large gen- 

 era and their recorded varieties which deserve notice. We 

 have seen that there is no infallible criterion by which to dis- 

 tinguish species and well-marked varieties ; and in those cases 

 in which intermediate links have not been found between 

 doubtful forms, naturalists are compelled to come to a deter- 

 mination by the amount of difference between them, judging by 

 analogy whether or not the amoimt 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 sjiecies is 

 often exceedingly small. I have endeavored to test this nu- 

 merically 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, tlie 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 num- 

 ber of varieties or incipient species greater than the average 

 are now manufacturing, many of the species already manufac- 

 tured 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 large 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 arc 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 certain 

 other species. And what are varieties but groups of forms, 

 imequally related to each other, and clustered round certain 

 forms — that is, round their parent-species ? Undoubtedly there 



