_physiological adaptation_ is an acquired power
to survive and develop under abnormal conditions, which is not accompanied
by any visible change in the characteristic structure of the species.

Both of these types of adjustment may be either hereditary (or
evolutionary), or spontaneous in their origin and development. Changes
which are evolutionary are fixed by heredity and become definite habits of
growth in the species. Their origin may be explained in either one of two
ways; namely, the so-called "increase by use," and "the survival of the
fittest." The hypothesis of "increase by use," as an explanation of
adaptations, is based upon the well-known observation that, in animals,
muscles and other organs increase in volume as they are extensively used;
and the assumption of the application of this principle to the phenomenon
of adaptation supposes that the modification of any given structure or
composition is the result of the hereditary accumulations of increased size
resulting from use, or of atrophy from disuse. The "survival of the
fittest" theory supposes that individuals of a species differ from each
other by spontaneous variations, and that in the competitive struggle for
existence those forms which are best adapted to the environmental
conditions survive while the others perish. The contrast between these two
views is that the first holds that adaptation proceeds by development, and
the second that it proceeds by variation and elimination; the first
presupposes the existence in the organism of a mechanism for response to
changing conditions, and the second assumes that there are chance
variations followed by the death through competition of the forms which are
not able to meet the needs of the environment.

Confusion arises whenever an attempt is made to apply either of these
theories to all kinds of adaptations. The idea of increase by use can be
applied with some satisfaction to certain morphological adaptations in
animal structure; and to such phenomena as the increase in strength of the
branches of fruit trees, either with or without corresponding increase in
size, as the load of fruit increases. But it certainly cannot apply to
color change in surface pigmentation of either animals or plants, which is
one of the most common forms of adaptation. Furthermore, it is difficult
to conceive the general application of this idea to alterations of habits
of growth of plants, since a plant cannot have any such thing