ewer could survive and
the fitter they must be to survive at all. This is Selection. These,
then, are Darwin's four great factors of evolution: Heredity,
Variation, Multiplication, Selection.

From these it results that the animals and plants naturally become
better adapted to the situation in which they are placed. When, as is
constantly happening through the history of the earth, a change occurs
in the physical geography of any region, when a plain is lifted to be
a plateau, or a mountain chain is submerged until it becomes a row of
small islands, this alteration will produce uncommon hardships among
animals, even though they were well fitted to the old conditions. Any
animal or any species of animals which meets such a calamity has
before it only three possible outcomes of the struggle. First it may
be plastic enough and it may vary enough in the right direction to
adjust itself to the changed conditions. In this case it and a favored
few like it will occupy the altered territory. The second possibility
is that it may migrate while the actual change is going on, thus
remaining in the sort of situation suited to it and its kind. The
third possibility is the one which overtakes a great majority of
animals--they die. Even the entire line dies out, and the strata of
the rocks are filled with the bones, shells, and teeth of such as have
met this fate. They have become extinct.

Thus far in this chapter we have been considering the influences under
which it is conceivable that animals should advance. There is no
question whatever that there are too many animals born, nor is there
any possible question that a very large proportion of them must
certainly die. There is equally no doubt that every animal produces
after its own kind, and that its offspring, while they resemble it
closely, still vary a little from it and from each other. This fact is
perfectly plain to the most superficial observer who thinks on the
matter at all. It is not so plain, nor is it easily demonstrated, that
all of these acting together do surely, even if slowly, alter the form
and behavior of the animal world. It is difficult to prove that there
is going on under our eyes a steady and real improvement in the
adaptation of the animals and plants around us to the situation in
which they are placed. As far back as man's memory runs they seem to
have been about what they now are; as far even as man's historical
record runs they seem to have suffered no