RESEMBLE VARIETIES 73 



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 in- 

 termediate 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 be- 

 tween the species is often exceedingly small. I have en- 

 deavoured 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, 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 sec- 

 tions, 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 



