The increasing blackness of _Polyommatus phlæas_, which we have already discussed, must also be regarded as a primary reaction, but not so the variations--often misinterpreted--of those species of _Artemia_ which live in the brine-pools of the Crimea, in regard to which Schmankewitsch showed that, when the amount of salt in the water is diminished, they undergo certain changes which bring them nearer to the fresh-water form _Branchipus_, while when the salt is increased in amount they vary in the contrary direction. Probably these are adaptations to the periodically changing salinity of their habitat. There can be no doubt of this in the case of the caterpillars of different families, in regard to which Poulton showed that in their early youth they possess the power of adapting themselves exactly to the colour of their chance surroundings. It is obvious that the protection which the caterpillar would gain from being coloured _approximately_ like its surroundings would be insufficient, for instance because the surroundings may be very diverse, since the species lives upon different, variously coloured plants and plant-parts. Thus a facultative adaptation arose. Selection gave rise to an extraordinarily specialized susceptibility on the part of the different cell elements of the skin to differences of light, and the result of this is that the skin of the caterpillar invariably takes on the colouring which is reflected upon it in the first few days of its life from the plants and plant-parts by which it is surrounded. Thus the caterpillars of one of the Geometridæ, _Amphidasis betularia_, take on the colours of the twig between and upon which they sit, and they can be made black, brown, white, or light green quite independently of their food, according to the colour of the twigs (or paper) among which they are reared.