Many years ago[21] I drew attention to the fact that the different stages in the life-history of insects, especially of Lepidoptera, might be variable in quite different degrees. Thus, for instance, the caterpillar might be very variable, and yet the butterfly which arises from it might be extremely constant. I concluded from this--what probably no one now will dispute--that the various stages may vary phyletically independently of one another, that, for instance, the caterpillar may adapt itself to a new manner of life, a new food-plant, a new means of defence, while the butterfly, unaffected by this, goes on quietly as it was before. Every new adaptation necessarily implies variability, and so the stage which is in process of transformation must have its period of variability, which gradually returns again to greater constancy, and this the more completely the longer the series of generations through which the weeding out of the less well-adapted has endured. [21] _Studien zur Descendenztheorie_, Leipzig, 1876. But it is not only the individual stages of development that may be unequally variable; the same is true of the characters of a species which occur simultaneously. The most striking example of this known to me is the leaf-butterfly, which I have already mentioned many times in the course of these lectures--the Indian _Kallima paralecta_. In this species the brown and red upper surface is almost alike in colour and marking in all individuals, but the under surface, the colour and marking of which is so deceptively mimetic of a leaf, is variable to such a degree that it is difficult, among a large number of specimens, to find even a few which are as like one another as are the members of species in which the under side is constant. It need not be urged that this is due to the complexity of the marking on the under side. In many of our indigenous butterflies the under side is just as complex in coloration and marking, and nevertheless it is very constant, being almost identical in all individuals, as for instance in _Vanessa cardui_. In _Kallima_ the great variability of the under surface certainly depends not merely on the fact that the mimetic character has been only recently acquired (phyletically speaking), but chiefly on the fact that the dead leaves to which they approximate are themselves very diverse in appearance, for many are dry, others moist and covered with mould, and that the adaptations have therefore gone in different directions, and as yet, at least, have neither combined to form a single constant type, nor diverged to form two or three distinct types. The various 'leaf-pictures' seem equally effective in concealing the insects from their enemies, and thus there is still a continual crossing and mingling of the different essays at leaf-picturing.