n
displayed in the homologous organs, to whatever purpose applied, of the
different species of a class; or to the homologous parts constructed on the
same pattern in each individual animal and plant.

On the principle of successive slight variations, not necessarily or
generally supervening at a very early period of life, and being inherited
at a corresponding period, we can understand the great leading facts in
Embryology; namely, the resemblance in an individual embryo of the
homologous parts, which when matured will become widely different from each
other in structure and function; and the resemblance in different species
of a class of the homologous parts or {458} organs, though fitted in the
adult members for purposes as different as possible. Larvae are active
embryos, which have become specially modified in relation to their habits
of life, through the principle of modifications being inherited at
corresponding ages. On this same principle--and bearing in mind, that when
organs are reduced in size, either from disuse or selection, it will
generally be at that period of life when the being has to provide for its
own wants, and bearing in mind how strong is the principle of
inheritance--the occurrence of rudimentary organs and their final abortion,
present to us no inexplicable difficulties; on the contrary, their presence
might have been even anticipated. The importance of embryological
characters and of rudimentary organs in classification is intelligible, on
the view that an arrangement is only so far natural as it is genealogical.

Finally, the several classes of facts which have been considered in this
chapter, seem to me to proclaim so plainly, that the innumerable species,
genera, and families of organic beings, with which this world is peopled,
have all descended, each within its own class or group, from common
parents, and have all been modified in the course of descent, that I should
without hesitation adopt this view, even if it were unsupported by other
facts or arguments.

* * * * *

{459}

CHAPTER XIV.

RECAPITULATION AND CONCLUSION.

Recapitulation of the difficulties on the theory of Natural
Selection--Recapitulation of the general and special circumstances in
its favour--Causes of the general belief in the immutability of
species--How far the theory of natural selection may be
extended--Effects of its adoption on the study of Natural