ding inorganic agencies to which two plants or two animals
are exposed, is followed by some unlikeness in the changes wrought on
them; then it follows that these several agencies taken separately, work
changes in both of them. Hence we must infer that organisms have certain
structural characters in common, which are consequent on the action of
the medium in which they exist: using the word medium in a comprehensive
sense, as including all physical forces falling upon them as well as
matters bathing them. And we may conclude that from the primary
characters thus produced there must result secondary characters.

Before going on to observe those general traits of organisms due to the
general action of the inorganic environment upon them, I feel tempted to
enlarge on the effects produced by each of the several matters and
forces constituting the environment. I should like to do this not only
to give a clear preliminary conception of the ways in which all
organisms are affected by these universally-present agents, but also to
show that, in the first place, these agents modify inorganic bodies as
well as organic bodies, and that, in the second place, the organic are
far more modifiable by them than the inorganic. But to avoid undue
suspension of the argument, I content myself with saying that when the
respective effects of gravitation, heat, light, &c., are studied, as
well as the respective effects, physical and chemical, of the matters
forming the media, water and air, it will be found that while more or
less operative on all bodies, each modifies organic bodies to an extent
immensely greater than the extent to which it modifies inorganic bodies.

* * * * *

Here, not discriminating among the special effects which these various
forces and matters in the environment produce on both classes of bodies,
let us consider their combined effects, and ask--What is the most
general trait of such effects?

Obviously the most general trait is the greater amount of change wrought
on the outer surface than on the inner mass. In so far as the matters of
which the medium is composed come into play, the unavoidable implication
is that they act more on the parts directly exposed to them than on the
parts sheltered from them. And in so far as the forces pervading the
medium come into play, it is manifest that, excluding gravity, which
affects outer and inner parts indiscriminately, the outer parts have to
bea