Nor should it be forgotten, 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.