of the second generation differ more or less from one another. This
is especially noticeable in domesticated plants and animals, but no
less true of wild forms. If the parent is not exactly like the other
members of the species, some of its descendants will inherit its
peculiarities enhanced, others diminished.

3. Every species tends to increase in geometrical progression. But
most species actually increase in number very slowly, if at all. Now
and then some insect or weed escapes from its enemies, comes under
favorable food conditions, and multiplies with such rapidity that it
threatens to ravage the country. But as it multiplies it furnishes
an abundance of food for the enemies which devour it, or of food and
place for the parasites in and upon it; and they increase with at
least equal rapidity. Hence while the vanguard increases
prodigiously in numbers, because it has outrun these enemies, the
rear is continually slaughtered. And thus these plagues seem in
successive generations to march across the continent.

And yet even they give but a faint idea of the reproductive powers
of plants and animals. The female fish produces often many
thousands, sometimes hundreds of thousands of eggs. Insects
generally from a hundred to a thousand. Even birds, slowly as they
increase, produce in a lifetime probably at least from twelve to
twenty eggs. Now let us suppose that all these eggs developed, and
all the birds lived out their normal period of life, and reproduced
at the same rate. After not many centuries there would not be
standing room on the globe for the descendants of a single pair.

Again, of the one hundred eggs of an insect let us suppose that only
sixty develop into the first larval, caterpillar, stage. Of these