56 
SPECIES OF LAEGE 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 
