tten,
when we look to the special parts of allied species, instead of to
distinct species, that numerous and wonderfully fine gradations can be
traced, connecting together widely different structures.

Many large groups of facts are intelligible only on the principle that
species have been evolved by very small steps. For instance, the fact
that the species included in the larger genera are more closely related
to each other, and present a greater number of varieties than do the
species in the smaller genera. The former are also grouped in little
clusters, like varieties round species; and they present other analogies
with varieties, as was shown in our second chapter. On this same
principle we can understand how it is that specific characters are more
variable than generic characters; and how the parts which are developed
in an extraordinary degree or manner are more variable than other parts
of the same species. Many analogous facts, all pointing in the same
direction, could be added.

Although very many species have almost certainly been produced by
steps not greater than those separating fine varieties; yet it may
be maintained that some have been developed in a different and abrupt
manner. Such an admission, however, ought not to be made without strong
evidence being assigned. The vague and in some respects false analogies,
as they have been shown to be by Mr. Chauncey Wright, which have been
advanced in favour of this view, such as the sudden crystallisation of
inorganic substances, or the falling of a facetted spheroid from one
facet to another, hardly deserve consideration. One class of facts,
however, namely, the sudden appearance of new and distinct forms of
life in our geological formations supports at first sight the belief in
abrupt development. But the value of this evidence depends entirely on
the perfection of the geological record, in relation to periods remote
in the history of the world. If the record is as fragmentary as many
geologists strenuously assert, there is nothing strange in new forms
appearing as if suddenly developed.

Unless we admit transformations as prodigious as those advocated by Mr.
Mivart, such as the sudden development of the wings of birds or bats, or
the sudden conversion of a Hipparion into a horse, hardly any light
is thrown by the belief in abrupt modifications on the deficiency of
connecting links in our geological formations. But against the belief in
such abrupt changes