ll seldom affect it in the embryo. Thus
we can understand the greater size of rudimentary organs in the embryo
relatively to the adjoining parts, and their lesser relative size in the
adult. If, for instance, the digit of an adult animal was used less and
less during many generations, owing to some change of habits, or if an
organ or gland was less and less functionally exercised, we may infer
that it would become reduced in size in the adult descendants of this
animal, but would retain nearly its original standard of development in
the embryo.

There remains, however, this difficulty. After an organ has ceased being
used, and has become in consequence much reduced, how can it be still
further reduced in size until the merest vestige is left; and how can it
be finally quite obliterated? It is scarcely possible that disuse can go
on producing any further effect after the organ has once been rendered
functionless. Some additional explanation is here requisite which I
cannot give. If, for instance, it could be proved that every part of the
organisation tends to vary in a greater degree towards diminution than
toward augmentation of size, then we should be able to understand how an
organ which has become useless would be rendered, independently of the
effects of disuse, rudimentary and would at last be wholly suppressed;
for the variations towards diminished size would no longer be checked by
natural selection. The principle of the economy of growth, explained in
a former chapter, by which the materials forming any part, if not useful
to the possessor, are saved as far as is possible, will perhaps come
into play in rendering a useless part rudimentary. But this principle
will almost necessarily be confined to the earlier stages of the process
of reduction; for we cannot suppose that a minute papilla, for instance,
representing in a male flower the pistil of the female flower, and
formed merely of cellular tissue, could be further reduced or absorbed
for the sake of economising nutriment.

Finally, as rudimentary organs, by whatever steps they may have been
degraded into their present useless condition, are the record of a
former state of things, and have been retained solely through the
power of inheritance--we can understand, on the genealogical view of
classification, how it is that systematists, in placing organisms in
their proper places in the natural system, have often found rudimentary
parts as useful as, or even so