n 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 species of the smaller genera.
Or the case may be put in another way, and it may be said, that in
the larger genera, in which a number of varieties or incipient species
greater than the average are now manufacturing, many of the species
already manufactured still to a certain extent resemble varieties, for
they differ from each other by a less than usual amount of difference.

Moreover, the species of the large genera are related to each other, in
the same manner as the varieties of any one species are related to
each other. No naturalist pretends that all the species of a genus are
equally distinct from each other; they may generally be divided into
sub-genera, or sections, or lesser groups. As Fries has well remarked,
little groups of species are generally clustered like satellites around
certain other species. And what are varieties but groups of forms,
unequally related to each other, and clustered round certain forms--that
is, round their parent-species? Undoubtedly there is one most important
point of difference between varieties and species; namely, that the
amount of difference between varieties, when compared with each other or
with their parent-species, is much less than that between the species of
the same genus. But when we come to discuss the principle, as I call it,
of Divergence of Character, we shall see how this may be explained, and
how the lesser differences between varieties will tend to increase into
the greater differences between species.

There is one other point which seems to me worth notice. V