longevity. Death being brought about by
the failure of an organism to adjust itself to some change in the
Environment, it follows that those organisms which are able to adjust
themselves most readily and successfully will live the longest. They
will continue time after time to effect the appropriate adjustment, and
their power of doing so will be exactly proportionate to their
complexity--that is, to the amount of Environment they can control with
their correspondences. There are, for example, in the Environment of
every animal certain things which are directly or indirectly dangerous
to Life. If its equipment of correspondences is not complete enough to
enable it to avoid these dangers in all possible circumstances, it must
sooner or later succumb. The organism then with the most perfect set of
correspondences, that is, the highest and most complex organism, has an
obvious advantage over less complex forms. It can adjust itself more
perfectly and frequently. But this is just the biological way of saying
that it can live the longest. And hence the relation between complexity
and longevity may be expressed thus--the most complex organisms are the
longest lived.

To state and illustrate the proposition conversely may make the point
still further clear. The less highly organized an animal is, the less
will be its chance of remaining in lengthened correspondence with its
Environment. At some time or other in its career circumstances are sure
to occur to which the comparatively immobile organism finds itself
structurally unable to respond. Thus a _Medusa_ tossed ashore by a wave,
finds itself so out of correspondence with its new surroundings that its
life must pay the forfeit. Had it been able by internal change to adapt
itself to external change--to correspond sufficiently with the new
environment, as for example to crawl, as an eel would have done, back
into that environment with which it had completer correspondence--its
life might have been spared. But had this happened it would continue to
live henceforth only so long as it could continue in correspondence with
all the circumstances in which it might find itself. Even if, however,
it became complex enough to resist the ordinary and direct dangers of
its environment, it might still be out of correspondence with others. A
naturalist for instance, might take advantage of its want of
correspondence with particular sights and sounds to capture it for his
cabinet, or the sud