This,
however, is a portion of the very large subject of organic colour and
marking, which will be fully discussed and illustrated in a separate
chapter.
In this way every possible modification of an animal or plant, whether
in colour, form, structure, or habits, which would be serviceable to it
or to its progeny at any period of its existence, may be readily brought
about. There are some curious organs which are used only once in a
creature's life, but which are yet essential to its existence, and thus
have very much the appearance of design by an intelligent designer. Such
are, the great jaws possessed by some insects, used exclusively for
opening the cocoon, and the hard tip to the beak of unhatched birds used
for breaking the eggshell. The increase in thickness or hardness of the
cocoons or the eggs being useful for protection against enemies or to
avoid accidents, it is probable that the change has been very gradual,
because it would be constantly checked by the necessity for a
corresponding change in the young insects or birds enabling them to
overcome the additional obstacle of a tougher cocoon or a harder
eggshell.
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