pon the capsules of poppies, for the shades of
color of human eyes, for the number of spines on the backs of shrimps, and
for the number of days that caterpillars feed before they turn into pupae.

To summarize the foregoing facts, we have learned that variation is
universal throughout the living world, and that the primary factors
causing organic difference--the counterparts of human ingenuity in the
case of dead mechanisms--are the natural influences of the environment, of
organic physiological activity, and of congenital inheritance. These
factors are accorded different values in the evolution of new species, as
we may see more clearly at a later juncture, but the essential point here
is that they are not unreal, although they may not as yet be described by
science in final analytical terms.

* * * * *

We come now to the second element of the whole process of evolution,
namely, what we may call overproduction or excessive multiplication. Like
variation and so many other phenomena of nature, this is so real and
natural that it escapes our attention until science places it before us in
a new light. The normal rate of reproduction _in all species of animals_
is such that if it were unchecked, any kind of organism would cumber the
earth or fill the sea in a relatively short time. That this is universally
true is apparent from any illustration that might be selected. Let us take
the case of a plant that lives for a single year, and that produces two
seeds before it withers and dies; let us suppose that each of these seeds
produces an adult plant which in its turn lives one year and forms two
seeds. If this process should continue without any interference, the
twentieth generation after as many years would consist of more than one
million descendants of the original two-seeded annual plant, provided only
that each individual of the intervening years should live a normal life
and should multiply at the natural rate. But such a result as this is
rendered impossible by the very nature which makes annual plants multiply
in the way they do. Let us take the case of a pair of birds which produce
four young in each of four seasons. Few would be prepared for the figures
enumerating the offspring of a single pair of birds at the end of fifteen
years, if again all individuals lived complete and normal lives: at the
end of the time specified there would be more than two thousand millions
of descendants