not very improbable that in short-tailed monkeys, the projecting part
of the tail, being functionally useless, should after many generations
have become rudimentary and distorted, from being continually rubbed
and chafed. We see the projecting part in this condition in the Macacus
brunneus, and absolutely aborted in the M. ecaudatus and in several of
the higher apes. Finally, then, as far as we can judge, the tail has
disappeared in man and the anthropomorphous apes, owing to the terminal
portion having been injured by friction during a long lapse of time;
the basal and embedded portion having been reduced and modified, so as
to become suitable to the erect or semi-erect position.
I have now endeavoured to shew that some of the most distinctive
characters of man have in all probability been acquired, either
directly, or more commonly indirectly, through natural selection. We
should bear in mind that modifications in structure or constitution
which do not serve to adapt an organism to its habits of life, to the
food which it consumes, or passively to the surrounding conditions,
cannot have been thus acquired. We must not, however, be too confident
in deciding what modifications are of service to each being: we should
remember how little we know about the use of many parts, or what
changes in the blood or tissues may serve to fit an organism for a new
climate or new kinds of food. Nor must we forget the principle of
correlation, by which, as Isidore Geoffroy has shewn in the case of
man, many strange deviations of structure are tied together.
Independently of correlation, a change in one part often leads, through
the increased or decreased use of other parts, to other changes of a
quite unexpected nature. It is also well to reflect on such facts, as
the wonderful growth of galls on plants caused by the poison of an
insect, and on the remarkable changes of colour in the plumage of
parrots when fed on certain fishes, or inoculated with the poison of
toads (95. The Variation of Animals and Plants under Domestication,
vol. ii. pp. 280, 282.); for we can thus see that the fluids of the
system, if altered for some special purpose, might induce other
changes. We should especially bear in mind that modifications acquired
and continually used during past ages for some useful purpose, would
probably become firmly fixed, and might be long inherited.
Thus a large yet undefined extension may safely be given to the direct
and indirect results of natural selection; but I now admit, after
reading the essay by Nageli on plants, and the remarks by various
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