chap. II. Species of Larger Genera variable. 4S 



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, \\.q 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 arc 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 certainly 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 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, 



