On every continent, in proceeding from
north to south, from lowland to upland, etc., we meet with a host of
closely related or representative species; as we likewise do on certain
distinct continents, which we have reason to believe were formerly
connected. But in making these and the following remarks, I am compelled
to allude to subjects hereafter to be discussed. Look at the many outlying
islands round a continent, and see how many of their inhabitants can be
raised only to the rank of doubtful species. So it is if we look to past
times, and compare the species which have just passed away with those still
living within the same areas; or if we compare the fossil species embedded
in the sub-stages of the same geological formation. It is indeed manifest
that multitudes of species are related in the closest manner to other
species that still exist, or have lately existed; and it will hardly be
maintained that such species have been developed in an abrupt or sudden
manner. 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.
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