94 THE ORIGIN OF SPECIES 



which to distinguish species and well-marked varieties; 

 and when intermediate links have not been found be- 

 tween 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 

 sufliccs 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 re- 

 gard to plants, and Westwood in regard to insects, that 

 in large genera the amount of difference between the spe- 

 cies 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 de- 

 liberation, they concur in this view. In this respect, 

 therefore, the species of the larger genera resemble varie- 

 ties 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 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 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 pre- 

 tends 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 



I 



