It is, again, a great advantage to the rabbit and hare to be colored like earth; black or white rabbits are more easy to see, and consequently more likely to be killed.  This, however, does not apply to those which are kept in captivity, and we know that tame rabbits are often black and white.  Again, in the far north, where for months together the ground is covered with snow, the white color, which would be a danger here, becomes an advantage; and many Arctic animals, like the polar bear and polar hare, are white, while others, such as the mountain hare and ptarmigan, change their color, being brown in summer and white in winter.  So are the Arctic fox and the ermine, to whom it is then an advantage to be white, not to avoid danger, but in order that they may be the more easily able to steal unperceived upon their prey.

Many of the cases in which certain insects escape danger by their similarity to plants are well known; the leaf insect and the walking-stick insect are familiar and most remarkable cases.  The larvae of insects afford, also, many interesting examples, and in other respects teach us, indeed, many instructive lessons.  It would be a great mistake to regard them as merely preparatory stages in the development of the perfect insect.  They are much more than this, for external circumstances act on the larvae, as well as on the perfect insect:  both, therefore, are liable to adaptation.  In fact, the modifications which insect larvae undergo may be divided into two kinds—­developmental, or those which tend to approximation to the mature form; and adaptational or adaptive, those which tend to suit them to their own mode of life.

It is a remarkable fact, that the forms of larvae do not depend on those of the mature insect.  In many cases, for instance, very similar larvae produce extremely dissimilar insects.  In other cases, similar, or comparatively similar, perfect insects have very dissimilar larvae.  Indeed, a classification of insects founded on larva would be quite different from that founded on the perfect insects.  The group to which the bees, wasps, and ants belong, for instance, and which, so far as the perfect insects are concerned, form a very natural division, would be divided into two; or rather one portion of them—­namely, the saw-flies—­would be united to the butterflies and moths.  Now, why do the larvae of saw-flies differ from those of their allies, and resemble those of butterflies and moths?  It is because their habits differ from those of ants and bees, and they feed on leaves like ordinary caterpillars.

In some cases the form changes considerably during the larval state.  From this point of view, the transformations of a small beetle, called Sitaris, which have been carefully observed by M. Fabre, are peculiarly interesting.