eed I have observed in parts of South
America) the vegetation: this, again, would largely affect the insects;
and this, as we just have seen in Staffordshire, the insectivorous
birds, and so onwards in ever-increasing circles of complexity."

In the struggle for life, the strongest live; or, in other words, those
best fitted to live in the environment endure. Animals and plants
produce in vast excess of the possibility of life. A destruction of life
is going on to an almost incredible amount. Were this not the case, the
slowest breeders in existence would soon cover the earth so as to occupy
every inch of space. Darwin reckons that the elephant, the slowest
breeder, if allowed to go on unchecked, and to live his allotted term of
years, would in five centuries produce fifteen millions of elephants
from one pair. If every cod's egg had developed into a full-grown fish,
the whole ocean would, ages ago, have been packed with them, like
herrings in a box. In this destruction, the weaker animals and
plants--those least fitted to thrive under the influences around--become
the prey of others better fitted for the struggle, or die of their own
lack of assimilative force. Thus, through untold ages of shifting
outward circumstances, the plastic forms of organic life have been
remoulded. A little obscure plant, the food of an insect, dies out; the
insect itself, deprived of its food, dies out or migrates; the bird
which fed upon it dies out or migrates; the bird of prey follows the
like course. Migration introduces them to an entirely new state of
existence, temperature, food, and antagonists. The migrating animals are
replaced by others, which likewise experience new surroundings; and thus
the extirpation of a single plant may determine a long series of
changes. Instances of this kind are not uncommon. What must these
changes have been throughout the remote ages which have turned sea-shore
mud into uplifted mountain chains, and sunk long-stretching, sunny hills
into the ocean depths!

Darwin constructs his theory of gradual differentiation on the evidence
thus obtained. He takes a given specific animal form, and supposes that,
owing to some external change in a given locality, it takes on some
correspondent variation. But all of the individuals of the species may
not be likewise affected. The circumstances may alter in one place and
not in another. The result will be two varieties of animal. The variety
goes on increasing in dive