and structure, so as to be able 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 of characteristic of varieties of
the same species, tend to be augmented into the greater differences
characteristic of the 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 within each class 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 go on 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 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 has prevailed throughout
all time. This grand fact of the grouping of all organic beings under
what is called the Natural System, is 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 modifications;
it can act only by short and slow steps. Hence, the canon of "Nature
makes no leaps," which every fresh addition to our knowledge tends to
confirm, is on this theory intelligible. We can see why throughout
nature the same general end is gained by an almost infinite diversity of
means, for every peculiarity when once acquired is long inherited, and
structures already modified in many different ways have to be adapted
for the same general purpose. We can, in short, 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 a woodpecker, should prey
on insects on the ground; that upland geese which rarely or never swim,
wo