in little groups round other species in which respects they resemble
varieties. These are strange relations on the view of each species
having been independently created, 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.
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