last, will for several reasons be treated here more fully. (22. I had
written a rough copy of this chapter before reading a valuable paper,
Caratteri rudimentali in ordine all origine dell uomo ( Annuario
della Soc. d. Naturalisti, Modena, 1867, p. 81), by G. Canestrini, to
which paper I am considerably indebted. Haeckel has given admirable
discussions on this whole subject, under the title of Dysteleology, in
his Generelle Morphologie and Schopfungsgeschichte. ) Not one of the
higher animals can be named which does not bear some part in a
rudimentary condition; and man forms no exception to the rule.
Rudimentary organs must be distinguished from those that are nascent;
though in some cases the distinction is not easy. The former are either
absolutely useless, such as the mammae of male quadrupeds, or the
incisor teeth of ruminants which never cut through the gums; or they
are of such slight service to their present possessors, that we can
hardly suppose that they were developed under the conditions which now
exist. Organs in this latter state are not strictly rudimentary, but
they are tending in this direction. Nascent organs, on the other hand,
though not fully developed, are of high service to their possessors,
and are capable of further development. Rudimentary organs are
eminently variable; and this is partly intelligible, as they are
useless, or nearly useless, and consequently are no longer subjected to
natural selection. They often become wholly suppressed. When this
occurs, they are nevertheless liable to occasional reappearance through
reversion a circumstance well worthy of attention.
The chief agents in causing organs to become rudimentary seem to have
been disuse at that period of life when the organ is chiefly used (and
this is generally during maturity), and also inheritance at a
corresponding period of life. The term disuse does not relate merely
to the lessened action of muscles, but includes a diminished flow of
blood to a part or organ, from being subjected to fewer alternations of
pressure, or from becoming in any way less habitually active.
Rudiments, however, may occur in one sex of those parts which are
normally present in the other sex; and such rudiments, as we shall
hereafter see, have often originated in a way distinct from those here
referred to. In some cases, organs have been reduced by means of
natural selection, from having become injurious to the species under
changed habits of life. The process of reduction is probably often
aided through the two principles of compensation and economy of growth;
but the later stages of reduction, after disuse has done all that can
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