f nature--that is to
say, from man downwards--we certainly perceive a sustained but irregular
degradation of organism, with an increasing simplicity both in organism
and faculties.

"This fact should throw light upon the order taken by nature, but it
does not show us why the gradation is so irregular, nor why throughout
its extent we find so many anomalies or digressions which have
apparently no order at all in their manifold varieties.[272] The
explanation of this must be sought for in the infinite diversity of
circumstances under which organisms have been developed. On the one
hand, there is a tendency to a regular progressive development; on the
other, there is a host of widely different surroundings which tend
continually to destroy the regularity of development.

"It is necessary to explain what is meant by such expressions as 'the
effect of its environment upon the form and organization of an animal.'
It must not be supposed that its surroundings directly effect any
modification whatever in the form and organization of an animal.[273]
Great changes in surroundings involve great changes in the wants of
animals, and these changes in their wants involve corresponding changes
in their actions. If these new wants become permanent, or of very long
duration, the animals contract new habits, which last as long as the
wants which gave rise to them.[274] A great change in surroundings, if
it persist for a long time, must plainly, therefore, involve the
contraction of new habits. These new habits in their turn involve a
preference for the employment of such and such an organ over such and
such another organ, and in certain cases the total disuse of an organ
which is no longer wanted. This is perfectly self-evident.[275]

"On the one hand, new wants have rendered a part necessary, which part
has accordingly been created by a succession of efforts: use has kept it
in existence, gradually strengthening and developing it till in the end
it attains a considerable degree of perfection. On the other, new
circumstances having in some cases rendered such or such a part useless,
disuse has led to its gradually ceasing to receive the development which
the other parts attain to; on this it becomes reduced, and in time
disappears.[276]

"Plants have neither actions nor habits properly so called, nevertheless
they change in a changed environment as much as animals do. This is due
to changes in nutrition, absorption and transpiration,