results follow when another division is made, and when all the smallest
genera, with from only one to four species, are absolutely 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 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 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 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 always confirm the view. I have also
consulted some sagacious and most experienced observers, and, after
deliberation, they concur in this view. In this respect, therefore, the
species of the larger genera resemble varieties, more than do the
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