h
a relation, than we have to believe that the same bones in the hand of
a man, wing of a bat, and fin of a porpoise, are related to similar
conditions of life. No one will suppose that the stripes on the whelp
of a lion, or the spots on the young blackbird, are of any use to these
animals, or are related to the conditions to which they are exposed.

The case, however, is different when an animal during any part of its
embryonic career is active, and has to provide for itself. The period of
activity may come on earlier or later in life; but whenever it comes on,
the adaptation of the larva to its conditions of life is just as perfect
and as beautiful as in the adult animal. From such special adaptations,
the similarity of the larvae or active embryos of allied animals is
sometimes much obscured; and cases could be given of the larvae of two
species, or of two groups of species, differing quite as much, or
even more, from each other than do their adult parents. In most cases,
however, the larvae, though active, still obey more or less closely the
law of common embryonic resemblance. Cirripedes afford a good instance
of this: even the illustrious Cuvier did not perceive that a barnacle
was, as it certainly is, a crustacean; but a glance at the larva shows
this to be the case in an unmistakeable manner. So again the two main
divisions of cirripedes, the pedunculated and sessile, which differ
widely in external appearance, have larvae in all their several stages
barely distinguishable.

The embryo in the course of development generally rises in organisation:
I use this expression, though I am aware that it is hardly possible to
define clearly what is meant by the organisation being higher or lower.
But no one probably will dispute that the butterfly is higher than the
caterpillar. In some cases, however, the mature animal is generally
considered as lower in the scale than the larva, as with certain
parasitic crustaceans. To refer once again to cirripedes: the larvae in
the first stage have three pairs of legs, a very simple single eye, and
a probosciformed mouth, with which they feed largely, for they increase
much in size. In the second stage, answering to the chrysalis stage of
butterflies, they have six pairs of beautifully constructed natatory
legs, a pair of magnificent compound eyes, and extremely complex
antennae; but they have a closed and imperfect mouth, and cannot feed:
their function at this stage is, to search