for longer periods than in other
cases. Again, in the working together of those many actions, internal
and external, which determine the lives or deaths of organisms, we see
nothing to which the words fitness and unfitness are applicable in the
physical sense. If a key fits a lock, or a glove a hand, the relation of
the things to one another is presentable to the perceptions. No approach
to fitness of this kind is made by an organism which continues to live
under certain conditions. Neither the organic structures themselves, nor
their individual movements, nor those combined movements of certain
among them which constitute conduct, are related in any analogous way to
the things and actions in the environment. Evidently the word fittest,
as thus used, is a figure of speech; suggesting the fact that amid
surrounding actions, an organism characterized by the word has either a
greater ability than others of its kind to maintain the equilibrium of
its vital activities, or else has so much greater a power of
multiplication that though not longer lived than they, it continues to
live in posterity more persistently. And indeed, as we here see, the
word fittest has to cover cases in which there may be less ability than
usual to survive individually, but in which the defect is more than made
good by higher degrees of fertility.

I have elaborated this criticism with the intention of emphasizing the
need for studying the changes which have gone on, and are ever going
on, in organic bodies, from an exclusively physical point of view. On
contemplating the facts from this point of view, we become aware that,
besides those special effects of the co-operating forces which eventuate
in the longer survival of one individual than of others, and in the
consequent increase through generations, of some trait which furthered
its survival, many other effects are being wrought on each and all of
the individuals. Bodies of every class and quality, inorganic as well as
organic, are from instant to instant subject to the influences in their
environments; are from instant to instant being changed by these in ways
that are mostly inconspicuous; and are in course of time changed by them
in conspicuous ways. Living things in common with dead things, are, I
say, being thus perpetually acted upon and modified; and the changes
hence resulting, constitute an all-important part of those undergone in
the course of organic evolution. I do not mean to imply th