reased 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 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 consulted some sagacious and 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 the usual amount of difference.

Moreover, the species of the larger 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
other species. And what are varieties but groups of forms, unequally
related to each other, and c