This progressive development of the individual--the ontogeny--gives us a clue to the ancestral development of the whole race--the phylogeny; and we are enabled to picture to ourselves the very slow and gradual steps by which the existing perfect adaptation has been brought about.

In many larvae great variability still exists, and in some there are two or more distinctly-coloured forms--usually a dark and a light or a brown and a green form.

The larva of the humming-bird hawk-moth (Macroglossa stellatarum) varies in this manner, and Dr.Weismann raised five varieties from a batch of eggs from one moth.

It feeds on species of bedstraw (Galium verum and G.mollugo), and as the green forms are less abundant than the brown, it has probably undergone some recent change of food-plant or of habits which renders brown the more protective colour.
_Special Protective Colouring of Butterflies._ We will now consider a few cases of special protective colouring in the perfect butterfly or moth.

Mr.Mansel Weale states that in South Africa there is a great prevalence of white and silvery foliage or bark, sometimes of dazzling brilliancy, and that many insects and their larvae have brilliant silvery tints which are protective, among them being three species of butterflies whose undersides are silvery, and which are thus effectually protected when at rest.[73] A common African butterfly (Aterica meleagris) always settles on the ground with closed wings, which so closely resemble the soil of the district that it can with difficulty be seen, and the colour varies with the soil in different localities.