in fact, almost momentarily—as is the case in many fishes, amphibians, reptiles and cephalopods. The light that calls forth the colour does not here act directly, or at any rate principally, on the elements of the skin that produce the colour; but a complicated nervous apparatus exists connecting these and the part that is first stimulated by the light, viz. the nerve-endings in the skin or in the eye. If in the latter case the optic lobes of the brain are artificially destroyed, the capacity for changing colour ceases: it therefore depends on a reflex mechanism, the origin of which, again, cannot be attributed to any cause acting directly, but can only be referred to processes of selection.
In these and similar instances, the dimorphism is not consequent on double sets of primary constituents of which only one or the other can attain to development: it depends on the different susceptibilities of the histo-logical elements which in exquisite combination make up the skin.
Many facts indicate that the differentiation of sex can also within certain limits be regarded from a similar point of view. The primary constituents for the characters of both sexes are included in the same egg, and in many instances it appears that a stimulus decides as to which group of them shall undergo development—whether the male or the female. Unfortunately in only very few cases can we as yet determine the stimulus with certainty, but we are at any rate certain that it is not the same in every instance. Yung's well-known experiments indicate, in the case of tadpoles, that the sex is partly determined by the nature and
