Porchinsky, one of a scientific party engaged in exploring the Caucasus, also witnessed a nearly complete phenomenon of assimilative colouration. The southern limit of the region explored was the steppe of Erivan, a plain covered with sand, with some patches of variously coloured clays appearing in the low hills. A remarkable feature of the animal inhabitants of the steppe, insects and reptiles, and especially of the Lizards, is the most perfect agreement of their colouration with that of the steppe. The same thing was also observed in the steppe of Elizabethpol.[1] This is a similar observation to that made by Canon Tristram in North Africa, and induces the same comment. Dr. A. Leith Adams remarks:—"The colour of the plumage of many desert-loving birds, like the denizens of arctic regions, assimilates to that of surrounding objects, and, moreover, as has been truly said, we also find the bleaching influence of the desert, and the dry and cloudless climate imparting their hues to the Egyptian monuments. So much is the latter the case that the eye fails at first to receive an impression of their immense antiquity, owing to the absence of the grey colouring and weather stains which give so venerable an aspect to those of Northern Europe. There is thus a stamp imprinted on all the animate and inanimate objects, in accordance with their haunts, as, for example, the desert Chats and other birds are much paler in colouring than those which frequent the cultivated districts on the river's banks."[2]
If this appears to be evident on the surface of the earth, the same phenomena seem to exist in the abyssal depths of the ocean. From recent deep-sea researches we know that the floor of the ocean is probably a vast undulating plain of mud; and, to quote both Sir John Murray and Mr. Hickson, of all the deep-sea deposits, the so-called "red mud" has by far the widest distribution. According to the testimony of the late Prof. Wyville Thomson and his colleagues in the 'Challenger' Expedition, this red clay is the residuum left after the calcareous matter of the Globigerinæ ooze has been dissolved away; and Sir John Murray is of opinion that "probably the majority of deep-sea species live by eating the surface-layers of the mud, clay, or ooze at the bottom, and by catching or picking up the
