52 ON THE ORIGIN OF SPECIES 



the manufactory still in action, more especially as we 

 have every reason to believe the process of manufactur- 

 ing new species to be a slow one. And this certainly 

 is the case, 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, declined, 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 holds good. 



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 in those cases in which inter- 

 mediate 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 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 

 3pecies 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 generm 



