rieties than do the species of the
small genera. Both these 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 {56} 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 {57} 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 confirm the view. I
have also con