With birds and many other animals, it seems to follow from the
comparison of allied species that circular spots are often generated by
the breaking up and contraction of stripes. In the Tragopan pheasant
faint white lines in the female represent the beautiful white spots in
the male (49. Jerdon, Birds of India, vol. iii. p. 517.); and
something of the same kind may be observed in the two sexes of the
Argus pheasant. However this may be, appearances strongly favour the
belief that on the one hand, a dark spot is often formed by the
colouring matter being drawn towards a central point from a surrounding
zone, which latter is thus rendered lighter; and, on the other hand,
that a white spot is often formed by the colour being driven away from
a central point, so that it accumulates in a surrounding darker zone.
In either case an ocellus is the result. The colouring matter seems to
be a nearly constant quantity, but is redistributed, either
centripetally or centrifugally. The feathers of the common guinea-fowl
offer a good instance of white spots surrounded by darker zones; and
wherever the white spots are large and stand near each other, the
surrounding dark zones become confluent. In the same wing-feather of
the Argus pheasant dark spots may be seen surrounded by a pale zone,
and white spots by a dark zone. Thus the formation of an ocellus in its
most elementary state appears to be a simple affair. By what further
steps the more complex ocelli, which are surrounded by many successive
zones of colour, have been generated, I will not pretend to say. But
the zoned feathers of the mongrels from differently coloured fowls, and
the extraordinary variability of the ocelli on many Lepidoptera, lead
us to conclude that their formation is not a complex process, but
depends on some slight and graduated change in the nature of the
adjoining tissues.
GRADATION OF SECONDARY SEXUAL CHARACTERS.
[Fig. 54. Feather of Peacock, about two-thirds of natural size, drawn
by Mr. Ford. The transparent zone is represented by the outermost white
zone, confined to the upper end of the disc.]
Cases of gradation are important, as shewing us that highly complex
ornaments may be acquired by small successive steps. In order to
discover the actual steps by which the male of any existing bird has
acquired his magnificent colours or other ornaments, we ought to behold
the long line of his extinct progenitors; but this is obviously
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