VARIATION UNDER NATURE 45 



into two nearly equal masses, the species of the larger genera on 

 one side, and those of the smaller genera on the other side, and it 

 has invariably proved to be the case that a larger proportion of 

 the species on the side of the larger genera presented varieties, 

 than on the side of the smaller genera. Moreover, the species of 

 the large genera which present any varieties, invariably present a 

 larger average number of varieties than do the species of the small 

 genera. Both these results follow when another division is made, 

 and when all the least genera, with from only one to four 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 increas- 

 ing; 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 cer- 

 tainly holds good. 



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 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 dif- 

 ference is one very important criterion in settling whether two 



