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.
This special modification of form, again, may or may not be accompanied by certain further modifications of structure, according as such rooting parts are to serve, as mere holdfasts, simply for attachment, or (as in most plants) for the absorption of food also.
Another modification is also correlated with these conditions. We have seen[58] that an interchange of gases takes place between each organism and its surrounding medium. But such interchange cannot take place in the subterranean part of the body, and a corresponding difference of structure between such subterranean part and other parts must therefore obtain.
Again, as to colour, we find differences which are evidently related to the different degrees in which different parts of a living body are exposed to the influence of light. Such contrasts notoriously exist, not only between the green parts of plants above the soil and the lighter coloured roots, but between the foliage of a plant which is exposed to sun light and another of the same kind kept in a dark cellar. Many animals which live in permanent darkness are colourless, as, e.g., the Proteus;[59] but yet this is not an invariable rule, some, as the mole, being of a dark colour.
The forms of organisms are evidently often directly related to surrounding influences. A plant or plant-like animal fixed to the soil may be so fixed that light, air, food, friends and enemies can have access equally on all sides or not. Thus, a tree so placed that light and air are excluded on one side, will not grow freely towards that side, but only in directions from whence light and air have access. A coral reef increases much more rapidly towards the open sea (the waves of which bring in food and facilitate gaseous interchange) than towards an adjacent shore.
The mere contiguity of parts will often affect the form of organisms. Thus, in many flowers parts which are adjacent become dwarfed, while others which are freely exposed become fully developed, as we see in the flowers of many Umbelliferæ, or plants of the parsley, fennel, and hemlock order.
The shapes of flowers bear relation (as we shall see later) to their need for attracting insects which by their visits effect the development of seed, and for repelling others the access of which would be hurtful.
The avoidance of enemies may be so effected by an organism that their access may be made impossible save in one direction, the extent of vulnerable surface even in that direction being minimized. We have an example of such a condition in those worms which live in calcareous tubes, and which are some of those called “tubicolous annelids.”[60]
Again, the medium in which an organism lives—whether aërial or aqueous—has an important relation with its form. A delicate seaweed, the beautifully radiating form of which is a just object of admiration as long as it is supported by its denser natural medium (the sea water), collapses into an amorphous mass when withdrawn thence into the thin air. Obviously a much greater rigidity and strength of structure is needed to support an aërial organism than an aquatic one, unless the former can support itself on other solid structures, such as rocks or trees. In the latter case the form attained may be very elongated and slender, as in the many creeping and climbing plants, which are so often furnished with processes for grasping (tendrils) to aid them in their mode of life.