there is a kind of equilibrium between
the forces of destruction and of reproduction. If a species is nearly
exterminated by its enemies, those enemies lose their food-supply and
perish themselves. In some sheltered spot the survivors of the victims
remain and increase till they begin to send out colonies again. In some
species, such as the mice in La Plata, and the beasts and birds which
devour them, there is an alternation of increase and decrease, to be
accounted for in this way. But permanent disturbances of equilibrium
sometimes occur. The rabbit in Australia, having found a virgin soil,
multiplied for some time almost up to the limit of its natural fertility
and is firmly established on that continent. The brown rat (some say)
has exterminated our black rat and the Maori rat in New Zealand. The
microbe of the terrible disease which the crews of Columbus brought back
to Europe, after causing a devastating epidemic at the end of the
fifteenth century, established a kind of _modus vivendi_ with its hosts,
and has remained as a permanent scourge in Europe. Other microbes, like
those of cholera and plague, emigrate from the lands where they are
endemic, like a horde of Tartars, and after slaying all who are
susceptible disappear from inanition. The draining of the fens has
driven the anopheles mosquito from England, and our countrymen no longer
suffer from 'ague.' Cleanlier habits are banishing the louse and its
accompaniment typhus fever.

Fertility and care for offspring seem as a rule to vary inversely. The
latter is the path of biological progress, and is characteristic of all
viviparous animals. That any degree of parental attention is
incompatible with the immense fecundity of the lower organisms needs no
demonstration. Such fertility is not necessary to keep up the numbers of
the higher species, which find abundant food in the swarming progeny of
the lower types, and are not themselves exposed to wholesale slaughter.
Speaking of fishes, Sutherland says:

Of species that exhibit no sort of parental care, the
average of forty-nine gives 1,040,000 eggs to a female each
year; while among those which make nests or any apology for
nests the number is only about 10,000. Among those which
have any protective tricks, such as carrying the eggs in
pouches or attached to the body, or in the mouth, the
average number is under 1000; while among those whose care
takes the form of uteri