pendent on light _per se_, but that the presence of free oxygen
is necessary; light and oxygen together accomplishing what neither
can do alone: and the inference seems irresistible that the effect
produced is a gradual oxidation of the constituent protoplasm of
these organisms, and that, in this respect, protoplasm, although
living, is not exempt from laws which appear to govern the
relations of light and oxygen to forms of matter less highly
endowed. A force which is indirectly absolutely essential to life
as we know it, and matter in the absence of which life has not yet
been proved to exist, here unite for its destruction."

What is the obvious implication? If oxygen in presence of light destroys
one of these minutest portions of protoplasm, what will be its effect on
a larger portion of protoplasm? It will work an effect on the surface
instead of on the whole mass. Not like the minutest mass made inert all
through, the larger mass will be made inert only on its outside; and,
indeed, the like will happen with the minutest mass if the light or the
oxygen is very small in quantity. Hence there will result an envelope of
changed matter, inclosing and protecting the unchanged protoplasm--there
will result a rudimentary cell-wall.

FOOTNOTES:

[Footnote 41: It is probable that this shortening has resulted not
directly but indirectly, from the selection of individuals which were
noted for tenacity of hold; for the bull-dog's peculiarity in this
respect seems due to relative shortness of the upper jaw, giving the
underhung structure which, involving retreat of the nostrils, enables
the dog to continue breathing while holding.]

[Footnote 42: Though Mr. Darwin approved of this expression and
occasionally employed it, he did not adopt it for general use;
contending, very truly, that the expression Natural Selection is in some
cases more convenient. See _Animals and Plants under Domestication_
(first edition) Vol. i, p. 6; and _Origin of Species_ (sixth edition) p.
49.]

[Footnote 43: It is true that while not deliberately admitted by Mr.
Darwin, these effects are not denied by him. In his _Animals and Plants
under Domestication_ (vol. ii, 281), he refers to certain chapters in
the _Principles of Biology_, in which I have discussed this general
inter-action of the medium and the organism, and ascribed certain most
general traits to it. But though, by his expressions, he implie