re stages, under such new and greatly changed habits of life, would
commonly be found unoccupied or ill-occupied by other organisms. In this
case the gradual acquirement at an earlier and earlier age of the adult
structure would be favoured by natural selection; and all traces of
former metamorphoses would finally be lost.

If, on the other hand, it profited the young of an animal to follow
habits of life slightly different from those of the parent-form, and
consequently to be constructed on a slightly different plan, or if
it profited a larva already different from its parent to change still
further, then, on the principle of inheritance at corresponding ages,
the young or the larvae might be rendered by natural selection more and
more different from their parents to any conceivable extent. Differences
in the larva might, also, become correlated with successive stages of
its development; so that the larva, in the first stage, might come to
differ greatly from the larva in the second stage, as is the case with
many animals. The adult might also become fitted for sites or habits, in
which organs of locomotion or of the senses, etc., would be useless; and
in this case the metamorphosis would be retrograde.

From the remarks just made we can see how by changes of structure in
the young, in conformity with changed habits of life, together with
inheritance at corresponding ages, animals might come to pass through
stages of development, perfectly distinct from the primordial condition
of their adult progenitors. Most of our best authorities are now
convinced that the various larval and pupal stages of insects have thus
been acquired through adaptation, and not through inheritance from some
ancient form. The curious case of Sitaris--a beetle which passes through
certain unusual stages of development--will illustrate how this might
occur. The first larval form is described by M. Fabre, as an active,
minute insect, furnished with six legs, two long antennae, and four
eyes. These larvae are hatched in the nests of bees; and when the male
bees emerge from their burrows, in the spring, which they do before
the females, the larvae spring on them, and afterwards crawl on to the
females while paired with the males. As soon as the female bee deposits
her eggs on the surface of the honey stored in the cells, the larvae of
the Sitaris leap on the eggs and devour them. Afterwards they undergo
a complete change; their eyes disappear; their