and arteries running in loops, like those in a fish which has to
breathe the air dissolved in water, by the aid of well-developed
branchi .
Disuse, aided sometimes by natural selection, will often tend to reduce
an organ, when it has become useless by changed habits or under changed
conditions
of life; and we can clearly understand on this view the meaning of
rudimentary organs. But disuse and selection will generally act on each
creature, when it has come to maturity and has to play its full part in
the struggle for existence, and will thus have little power of acting
on an organ during early life; hence the organ will not be much reduced
or rendered rudimentary at this early age. The calf, for instance, has
inherited teeth, which never cut through the gums of the upper jaw,
from an early progenitor having well-developed teeth; and we may
believe, that the teeth in the mature animal were reduced, during
successive generations, by disuse or by the tongue and palate having
been fitted by natural selection to browse without their aid; whereas
in the calf, the teeth have been left untouched by selection or disuse,
and on the principle of inheritance at corresponding ages have been
inherited from a remote period to the present day. On the view of each
organic being and each separate organ having been specially created,
how utterly inexplicable it is that parts, like the teeth in the
embryonic calf or like the shrivelled wings under the soldered
wing-covers of some beetles, should thus so frequently bear the plain
stamp of inutility! Nature may be said to have taken pains to reveal,
by rudimentary organs and by homologous structures, her scheme of
modification, which it seems that we wilfully will not understand.
I have now recapitulated the chief facts and considerations which have
thoroughly convinced me that species have changed, and are still slowly
changing by the preservation and accumulation of successive slight
favourable variations. Why, it may be asked, have all the most eminent
living naturalists and geologists rejected this view of the mutability
of species? It cannot be
asserted that organic beings in a state of nature are subject to no
variation; it cannot be proved that the amount of variation in the
course of long ages is a limited quantity; no clear distinction has
been, or can be, drawn between species and well-marked varieties. It
cannot be maintained that species when intercrossed are invariably
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