The larvæ of insects are generally regarded as being nothing more than immature states—as stages in the development of the egg into the imago; and this might more especially appear to be the case with those insects in which the larvæ offer a general resemblance in form and structure (excepting of course so far as relates to the wings) to the perfect insect. Nevertheless we see that this would be a very incomplete view of the case. The larva and pupa undergo changes which have no relation to the form which the insect will ultimately assume. With a general tendency to this goal, as regards size and the development of the wings, there are coincident other changes having reference only to existing wants and condition. Nor is there in this, I think, anything which need surprise us. External circumstances act on the insect in its preparatory states, as well as in its perfect condition. Those who believe that animals are susceptible of great, though gradual, change through the influence of external conditions, whether acting, as Mr. Darwin has suggested, through natural selection, or in any other manner, will see no reason why these changes should be confined to the mature animal. And it is evident that creatures which, like the majority of insects, live during the successive periods of their existence in very different circumstances, may undergo considerable changes in their larval organization, in consequence of forces acting on them while in that condition; not, indeed, without affecting, but certainly without affecting to any corresponding extent, their ultimate form. I conclude, therefore, that the form of the larva in insects, whenever it departs from the hexapod _Campodea_ type, has been modified by the conditions under which it lives. The external forces acting upon it are different from those which affect the mature form; and thus changes are produced in the young which have reference to its immediate wants, rather than to its final form. And, lastly, as a consequence, that metamorphoses may be divided into two kinds, developmental and adaptional or adaptive. CHAPTER III. _ON THE NATURE OF METAMORPHOSES._