What we may expect a true theory to do is to enable us to comprehend and
follow out in some detail those changes in the form, structure, and
relations of animals and plants which are effected in short periods of
time, geologically speaking, and which are now going on around us. We
may expect it to explain satisfactorily most of the lesser and
superficial differences which distinguish one species from another. We
may expect it to throw light on the mutual relations of the animals and
plants which live together in any one country, and to give some rational
account of the phenomena presented 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.
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