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Descent of Man [ 1871]

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Author Topic: Descent of Man [ 1871]  (Read 2932 times)
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« Reply #180 on: February 10, 2009, 01:24:30 pm »

* See the fine plates in A. Smith's Zoology of South Africa, and Dr.
Gray's Gleanings from the Menagerie of Knowsley.

  The same conclusion may perhaps be extended to the tiger, one of the
most beautiful animals in the world, the sexes of which cannot be
distinguished by colour, even by the dealers in wild beasts. Mr.
Wallace believes* that the striped coat of the tiger "so assimilates
with the vertical stems of the bamboo, as to assist greatly in
concealing him from his approaching prey." But this view does not
appear to me satisfactory. We have some slight evidence that his
beauty may be due to sexual selection, for in two species of Felis the
analogous marks and colours are rather brighter in the male than in
the female. The zebra is conspicuously striped, and stripes cannot
afford any protection in the open plains of South Africa. Burchell*(2)
in describing a herd says, "their sleek ribs glistened in the sun, and
the brightness and regularity of their striped coats presented a
picture of extraordinary beauty, in which probably they are not
surpassed by any other quadruped." But as throughout the whole group
of the Equidae the sexes are identical in colour, we have here no
evidence of sexual selection. Nevertheless he who attributes the white
and dark vertical stripes on the flanks of various antelopes to this
process, will probably extend the same view to the royal tiger and
beautiful zebra.

  * Westminster Review, July 1, 1867, p. 5.
  *(2) Travels in South Africa, 1824, vol. ii., p. 315.

  We have seen in a former chapter that when young animals belonging
to any class follow nearly the same habits of life as their parents,
and yet are coloured in a different manner, it may be inferred that
they have retained the colouring of some ancient and extinct
progenitor. In the family of pigs, and in the tapirs, the young are
marked with longitudinal stripes, and thus differ from all the
existing adult species in these two groups. With many kinds of deer
the young are marked with elegant white spots, of which their
parents exhibit not a trace. A graduated series can be followed from
the axis deer, both sexes of which at all ages and during all
seasons are beautifully spotted (the male being rather more strongly
coloured than the female), to species in which neither the old nor the
young are spotted. I will specify some of the steps in this series.
The Manchurian deer (Cervus mantchuricus) is spotted during the
whole year, but, as I have seen in the Zoological Gardens, the spots
are much plainer during the summer, when the general colour of the
coat is lighter, than during the winter, when the general colour is
darker and the horns are fully developed. In the hog-deer (Hyelaphus
porcinus) the spots are extremely conspicuous during the summer when
the coat is reddish-brown, but quite disappear during the winter
when the coat is brown.* In both these species the young are
spotted. In the Virginian deer the young are likewise spotted, and
about five per cent of the adult animals living in Judge Caton's park,
as I am informed by him, temporarily exhibit at the period when the
red summer coat is being replaced by the bluish winter coat, a row
of spots on each flank, which are always the same in number, though
very variable in distinctness. From this condition there is but a very
small step to the complete absence of spots in the adults at all
seasons; and, lastly, to their absence at all ages and seasons, as
occurs with certain species. From the existence of this perfect
series, and more especially from the fawns of so many species being
spotted, we may conclude that the now living members of the deer
family are the descendants of some ancient species which, like the
axis deer, was spotted at all ages and seasons. A still more ancient
progenitor probably somewhat resembled the Hyomoschus aquaticus- for
this animal is spotted, and the hornless males have large exserted
canine teeth, of which some few true deer still retain rudiments.
Hyomoschus, also, offers one of those interesting cases of a form
linking together two groups, for it is intermediate in certain
osteological characters between the pachyderms and ruminants, which
were formerly thought to be quite distinct.*(2)

  * Dr. Gray, Gleanings from the Menagerie of Knowsley, p. 64. Mr.
Blyth, in speaking (Land and Water, 1869, p. 42) of the hog-deer of
Ceylon, says it is more brightly spotted with white than the common
hog-deer, at the season when it renews its horns.
  *(2) Falconer and Cautley, Proc. Geolog. Soc., 1843; and
Falconer's Pal. Memoirs, vol. i., p. 196.

  A curious difficulty here arises. If we admit that coloured spots
and stripes were first acquired as ornaments, how comes it that so
many existing deer, the descendants of an aboriginally spotted animal,
and all the species of pigs and tapirs, the descendants of an
aboriginally striped animal, have lost in their adult state their
former ornaments? I cannot satisfactorily answer this question. We may
feel almost sure that the spots and stripes disappeared at or near
maturity in the progenitors of our existing species, so that they were
still retained by the young; and owing to the law of inheritance at
corresponding ages, were transmitted to the young of all succeeding
generations. It may have been a great advantage to the lion and
puma, from the open nature of their usual haunts, to have lost their
stripes, and to have been thus rendered less conspicuous to their
prey; and if the successive variations, by which this end was
gained, occurred rather late in life, the young would have retained
their stripes, as is now the case. As to deer, pigs, and tapirs, Fritz
Muller has suggested to me that these animals, by the removal of their
spots or stripes through natural selection, would have been less
easily seen by their enemies; and that they would have especially
required this protection, as soon as the Carnivora increased in size
and number during the tertiary periods. This may be the true
explanation, but it is rather strange that the young should not have
been thus protected, and still more so that the adults of some species
should have retained their spots, either partially or completely,
during part of the year. We know that, when the domestic ass varies
and becomes reddish-brown, grey, or black, the stripes on the
shoulders and even on the spine frequently disappear, though we cannot
explain the cause. Very few horses, except dun-coloured kinds, have
stripes on any part of their bodies, yet we have good reason to
believe that the aboriginal horse was striped on the legs and spine,
and probably on the shoulders.* Hence the disappearance of the spots
and stripes in our adult existing deer, pigs, and tapirs, may be due
to a change in the general colour of their coats; but whether this
change was effected through sexual or natural selection, or was due to
the direct action of the conditions of life, or to some other
unknown cause, it is impossible to decide. An observation made by
Mr. Sclater well illustrates our ignorance of the laws which
regulate the appearance and disappearance of stripes; the species of
Asinus which inhabit the Asiatic continent are destitute of stripes,
not having even the cross shoulder-stripe, whilst those which
inhabit Africa are conspicuously striped, with the partial exception
of A. taeniopus, which has only the cross shoulder-stripe and
generally some faint bars on the legs; and this species inhabits the
almost intermediate region of Upper Egypt and Abyssinia.*(2)
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