den dropping of a yacht's anchor or the turn of a
screw might cause its untimely death.

Again, in the case of a bird in virtue of its more complex organization,
there is command over a much larger area of environment. It can take
precautions such as the _Medusa_ could not; it has increased facilities
for securing food; its adjustments all round are more complex; and
therefore it ought to be able to maintain its Life for a longer period.
There is still a large area, however, over which it has no control. Its
power of internal change is not complete enough to afford it perfect
correspondence with all external changes, and its tenure of Life is to
that extent insecure. Its correspondence, moreover, is limited even with
regard to those external conditions with which it has been partially
established. Thus a bird in ordinary circumstances has no difficulty in
adapting itself to changes of temperature, but if these are varied
beyond the point at which its capacity of adjustment begins to fail--for
example, during an extreme winter--the organism being unable to meet the
condition must perish. The human organism, on the other hand, can
respond to this external condition, as well as to countless other
vicissitudes under which lower forms would inevitably succumb. Man's
adjustments are to the largest known area of Environment, and hence he
ought to be able furthest to prolong his Life.

It becomes evident, then, that as we ascend in the scale of Life we rise
also in the scale of longevity. The lowest organisms are, as a rule,
shortlived, 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 marvellously 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 long