ted, but are intelligible if all species
first existed as varieties.

As each species tends by its geometrical ratio of reproduction to
increase inordinately in number; and as the modified descendants of each
species will be enabled to increase by so much the more as they become
more diversified in habits and structure, so as to be enabled to seize
on many and widely different places in the economy of nature, there
will be a constant tendency in natural selection to preserve the most
divergent offspring of any one species. Hence during a long-continued
course of modification, the slight differences, characteristic of
varieties of the same species, tend to be augmented into the greater
differences characteristic of species of the same genus. New and
improved varieties will inevitably supplant and exterminate the older,
less improved and intermediate varieties; and thus species are rendered
to a large extent defined and distinct objects. Dominant species
belonging to the larger groups tend to give birth to new and dominant
forms; so that each large group tends to become still larger, and at
the same time more divergent in character. But as all groups cannot thus
succeed in increasing in size, for the world would not hold them, the
more dominant groups beat the less dominant. This tendency in the large
groups to go on increasing in size and diverging in character, together
with the almost inevitable contingency of much extinction, explains the
arrangement of all the forms of life, in groups subordinate to groups,
all within a few great classes, which we now see everywhere around us,
and which has prevailed throughout all time. This grand fact of the
grouping of all organic beings seems to me utterly inexplicable on the
theory of creation.

As natural selection acts solely by accumulating slight, successive,
favourable variations, it can produce no great or sudden modification;
it can act only by very short and slow steps. Hence the canon of "Natura
non facit saltum," which every fresh addition to our knowledge tends to
make more strictly correct, is on this theory simply intelligible. We
can plainly see why nature is prodigal in variety, though niggard in
innovation. But why this should be a law of nature if each species has
been independently created, no man can explain.

Many other facts are, as it seems to me, explicable on this theory. How
strange it is that a bird, under the form of woodpecker, should have
been cre