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.