As to figure and structure there is more constancy, and the amount of variation which may in these respects be found between different individuals of the same animal species, is generally but slight. In plants and in plant-like animals much greater differences exist as to external configuration; but even in them the internal structure of each species varies but little. Colour is a character which some readers may be disposed to regard as extremely inconstant. We are familiar with many differently coloured varieties of our cultivated flowers; and white blackbirds, and black leopards are not very uncommon objects. Nevertheless, colour is really a character of much constancy, and is one not only constantly present in different individuals of one kind of plant or animal, but is one constantly present in particular groups of kinds. Thus, for example, all the English plants of the dandelion order which have opposite leaves, have yellow flowers, with the single exception of the eupatory (_Eupatorium cannabinum_), and whole groups of butterflies are respectively characterized as being blue, or white, or yellow. We have seen that the life of every living being is accompanied by, and may be described as, a series of adjustments of action and structure to external conditions which surround it. Accordingly we may expect to find that the sizes, shapes, structures, and colours of living beings bear relations, which are in very many cases obvious, to their external circumstances, as directly favouring their nutrition, reproduction, or preservation from external injury. Every living creature must be either fixed (like a rooted tree), or capable of spontaneously moving, or of being passively drifted from place to place, and must have a structure and figure suitable to one or other of these conditions. Again, every living creature, whether free or fixed, is either a terrestrial, an aquatic, or an aërial organism; and it may be fitted to live in any two, or even in all three of these conditions--as, for example, is the swan. If terrestrial, it may inhabit the surface of the earth only, or it may occasionally or habitually dwell beneath it. The structure, forms, and even colours of organisms are in most cases plainly adapted to their modes of life in the above respects. Thus, any living creature, which is fixed to the surface of the earth, must either adhere to it by having one side or portion of its body spread out and adjusted to irregularities in the supporting surface, or else by sending prolongations of its substance into the substance of the supporting body, as a plant sends its roots into the soil. Such prolongations, moreover, must (in order to hold fast) either sink deeply or else expand, at a slight depth, into a rounded or discoidal mass, or into radiating processes whereby the whole structure may be securely anchored.