The
researches of Dr. Weismann illustrate this progressive adaptation. The
very young larvae of several species are green or yellowish without any
markings; they then, in subsequent moults, obtain certain markings, some
of which are often lost again before the larva is fully grown. The early
stages of those species which, like elephant hawk-moths (Chaerocampa),
have the anterior segments elongated and retractile, with large eye-like
spots to imitate the head of a vertebrate, are at first like those of
non-retractile species, the anterior segments being as large as the
rest. After the first moult they become smaller, comparatively; but it
is only after the second moult that the ocelli begin to appear, and
these are not fully defined till after the third moult. This progressive
development of the individual--the ontogeny--gives us a clue to the
ancestral development of the whole race--the phylogeny; and we are
enabled to picture to ourselves the very slow and gradual steps by which
the existing perfect adaptation has been brought about. In many larvae
great variability still exists, and in some there are two or more
distinctly-coloured forms--usually a dark and a light or a brown and a
green form.
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