56 SPECIES OF LARGE GENERA Chap. II. 



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 gene- 

 rally to find the manufactory still in action, more espe- 

 cially as we have every reason to believe the process of 

 manufacturing new species to be a slow one. And this 

 certainly is the case, if varieties be looked at as inci- 

 pient 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 intermediate links have not 

 been found between doubtful forms, naturalists are com- 

 pelled 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 



