No one
supposes that the stripes on the whelp of a lion, or the spots on the young
blackbird, are of any use to these animals.
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. In how important a manner this
has acted, has recently been well shown by Sir J. Lubbock in his remarks on
the close similarity of the larvae of some insects belonging to very
different orders, and on the dissimilarity of the larvae of other insects
within the same order, according to their habits of life. Owing to such
adaptations the similarity of the larvae of allied animals is sometimes
greatly obscured; especially when there is a division of labour during the
different stages of development, as when the same larva has during one
stage to search for food, and during another stage has to search for a
place of attachment. Cases can even be given of the larvae of allied
species, or groups of species, differing more from each other than do the
adults. 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 a crustacean: but a glance at the larva shows
this in an unmistakable manner.
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