*Chromatic Regulations in Algae.*--The phenomena of osmotic pressure and its regulation may be said to be the preliminaries of metabolism proper, conditions necessary for it to take place. Now there is another branch of such preliminaries to metabolism, in which the most interesting regulation phenomena have been lately discovered. It is well known that what is called assimilation in plants, that is, the formation of organic compounds out of carbon dioxide (CO_2) and water, occurs only in the light by means of certain pigments. This pigment is in all higher plants and in many others the green chlorophyll, but it may be different in certain species of algae, and can generally be said[99] to be of the colour complementary to the colour of those rays which especially are to be absorbed and to be used for assimilation. But here we have “adaptedness,” not adaptation. It was in some species of primitive algae, the *Oscillariae*, that Gaidukow[100] found a very interesting instance of an active regulation in the formation of pigments. These algae always assume a colour which corresponds to the accidental colour of the rays of the medium and is complementary to it; they become green in red light, yellow in blue light, and so on--that is, they always actively take that sort of colouring which is the most suitable to the actual case.[101] There indeed occurs a sort of complementary photography in these algae; but, though adaptive, it could hardly be said to exceed the limits of “primary phenomena.” [99] See Stahl, *Naturw. Wochenschrift*, N. F. 5, 1906, No. 19. [100] *Arch. Anat. Phys.*, Phys. Abt. Suppl., 1902. [101] The adaptive phenomena discovered by Gaidukow depend upon a real alteration in the formation of pigments. In the (primary) chromatic adaptation of pupae of Lepidoptera with respect to the colour of the ground they live upon, we only have the variable effects of pre-established chromatophores (Poulton, *Phil. Trans. London*, 178 B, 1888; Merrifield, *Trans. Ent. Soc. London*, 1898). The same holds for chromatic adaptations in crabs (Gamble and Keeble, *Quart. Journ. Micr. Sci.* 43, 1900; Minkiewicz, *Arch. Zool. exp. et gén.* sér. 4, 7, notes, 1907).