we rise
also in the scale of longevity. The lowest organisms are, as a rule,
short-lived, and the rate of mortality diminishes more or less regularly
as we ascend in the animal scale. So extraordinary indeed is the
mortality among lowly-organized forms that in most cases a compensation
is actually provided, nature endowing them with a marvelously increased
fertility in order to guard against absolute extinction. Almost all
lower forms are furnished not only with great reproductive powers, but
with different methods of propagation, by which, in various
circumstances, and in an incredibly short time, the species can be
indefinitely multiplied. Ehrenberg found that by the repeated
subdivisions of a single _Paramecium_, no fewer than 268,000,000
similar organisms might be produced in one month. This power steadily
decreases as we rise higher in the scale, until forms are reached in
which one, two, or at most three, come into being at a birth. It
decreases, however, because it is no longer needed. These forms have a
much longer lease of Life. And it may be taken as a rule, although it
has exceptions, that complexity in animal organisms is always associated
with longevity.

It may be objected that these illustrations are taken merely from morbid
conditions. But whether the Life be cut short by accident or by disease
the principle is the same. All dissolution is brought about practically
in the same way. A certain condition in the Environment fails to be met
by a corresponding condition in the organism, and this is death. And
conversely the more an organism in virtue of its complexity can adapt
itself to all the parts of its Environment, the longer it will live. "It
is manifest _a priori_," says Mr. Herbert Spencer, "that since changes
in the physical state of the environment, as also those mechanical
actions and those variations of available food which occur in it, are
liable to stop the processes going on in the organism; and since the
adaptive changes in the organism have the effects of directly or
indirectly counterbalancing these changes in the environment, it follows
that the life of the organism will be short or long, low or high,
according to the extent to which changes in the environment are met by
corresponding changes in the organism. Allowing a margin for
perturbations, the life will continue only while the correspondence
continues; the completeness of the life will be proportionate to the
completeness of the corr