nted by their distribution in different
parts of the world. And, lastly, we may expect it to explain many
difficulties and to harmonise many incongruities in the excessively
complex affinities and relations of living things. All this the
Darwinian theory undoubtedly does. It shows us how, by means of some of
the most universal and ever-acting laws in nature, new species are
necessarily produced, while the old species become extinct; and it
enables us to understand how the continuous action of these laws during
the long periods with which geology makes us acquainted is calculated to
bring about those greater differences presented by the distinct genera,
families, and orders into which all living things are classified by
naturalists. The differences which these present are all of the same
_nature_ as those presented by the species of many large genera, but
much greater in _amount_; and they can all be explained by the action of
the same general laws and by the extinction of a larger or smaller
number of intermediate species. Whether the distinctions between the
higher groups termed Classes and Sub-kingdoms may be accounted for in
the same way is a much more difficult question. The differences which
separate the mammals, birds, reptiles, and fishes from each other,
though vast, yet seem of the same nature as those which distinguish a
mouse from an elephant or a swallow from a goose. But the vertebrate
animals, the mollusca, and the insects, are so radically distinct in
their whole organisation and in the very plan of their structure, that
objectors may not unreasonably doubt whether they can all have been
derived from a common ancestor by means of the very same laws as have
sufficed for the differentiation of the various species of birds or of
reptiles.

_The Change of Opinion effected by Darwin_.

The point I wish especially to urge is this. Before Darwin's work
appeared, the great majority of naturalists, and almost without
exception the whole literary and scientific world, held firmly to the
belief that _species_ were realities, and had not been derived from
other species by any process accessible to us; the different species of
crow and of violet they are now, and to have originated by some totally
unknown process so far removed from ordinary reproduction that it was
usually spoken of as "special creation." There was, then, no question of
the origin of families, orders, and classes, because the very first step
of a