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Descent of Man [ 1871]

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Bullseye
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« on: February 09, 2009, 02:36:56 am »

 * Prof. Wyman in Proceedings of the American Academy of Sciences,
vol. iv., 1860, p. 17.
  *(2) Owen, Anatomy of Vertebrates, vol. i., p. 533.
  *(3) Die Grosshirnwindungen des Menschen 1868, s. 95.
  *(4) Anatomy of Vertebrates, vol. ii., p. 553.
  *(5) Proc. Soc. Nat. Hist., Boston, 1863, vol. ix., p. 185.
  *(6) Man's Place in Nature, p. 65.

  Rudiments. This subject, though not intrinsically more important
than the two last, will for several reasons be treated here more
fully.* 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 mammee 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.

  * 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
Shopfungsgeschichte.

  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 fairly be attributed to it, and when the saving to be
effected by the economy of growth would be very small,* are
difficult to understand. The final and complete suppression of a part,
already useless and much reduced in size, in which case neither
compensation or economy can come into play, is perhaps intelligible by
the aid of the hypothesis of pangenesis. But as the whole subject of
rudimentary organs has been discussed and illustrated in my former
works,*(2) I need here say no more on this head.

  * Some good criticisms on this subject have been given by Messrs.
Murie and Mivart, in Transactions, Zoological Society, 1869, vol.
vii., p. 92.
  *(2) Variation of Animals and Plants under Domestication, vol. ii
pp. 317 and 397. See also Origin of Species.(OOS)

  Rudiments of various muscles have been observed in many parts of the
human body;* and not a few muscles, which are regularly present in
some of the lower animals can occasionally be detected in man in a
greatly reduced condition. Every one must have noticed the power which
many animals, especially horses, possess of moving or twitching
their skin; and this is effected by the panniculus carnosus.
Remnants of this muscle in an efficient state are found in various
parts of our bodies; for instance, the muscle on the forehead, by
which the eyebrows are raised. The platysma myoides, which is well
developed on the neck, belongs to this system. Prof. Turner, of
Edinburgh, has occasionally detected, as he informs me, muscular
fasciculi in five different situations, namely in the axillae, near
the scapulae, &c., all of which must be referred to the system of
the panniculus. He has also shewn*(2) that the musculus sternalis or
sternalis brutorum, which is not an extension of the rectus
abdominalis, but is closely allied to the panniculus, occurred in
the proportion of about three per cent. in upward of 600 bodies: he
adds, that this muscle affords "an excellent illustration of the
statement that occasional and rudimentary structures are especially
liable to variation in arrangement."
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