42 SPECIES OF LAKGEK GENERA [CHAP. IL 



species, are altogether excluded from the tables. These facts are 

 of plain signification on the view that species are only strongly- 

 marked and permanent varieties ; for wherever many species of 

 the same genus have been formed, or where, if we may use the 

 expression, the manufactory of species has been active, we ought 

 generally to find the manufactory still in action, more especially 

 as we have every reason to believe the process of manufacturing 

 new species to be a slow one. And this certainly holds true, if 

 varieties be looked at as incipient species ; for my tables clearly 

 show as a general rule that, wherever many species of a genus 

 have been formed, the species of that genus present a number of 

 varieties, that is of incipient species, beyond the average. It is 

 not that all large genera are now varying much, and are thus 

 increasing in the number of their species, or that no small genera 

 are now varying and increasing ; for if this had been so, it would 

 have been fatal to my theory ; inasmuch as geology plainly tells 

 us that small genera have in the lapse of time often increased 

 greatly in size ; and that large genera have often come to their 

 maxima, decline, and disappeared. All that we want to show 

 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 tJie Species included within t/te 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 species 

 greater than the average are now manufacturing, many of the 



