Moving subterranean organisms, inasmuch as they must penetrate through a dense and highly-resisting substance, must evidently either have forms which offer little resistance—reducing friction to a minimum—or must be provided with special means of penetrating such substance. Evidently the least resisting form is presented by a body much elongated, rounded, and more or less attenuated at the advancing end, which end has to effect the requisite penetration. This is the form of the earth-worm—a form which is approximated to by a variety of creatures which have not the least affinity of nature with it, but only more or less resemble it as regards its dwelling-place and mode of locomotion.
Such, for example, are the curious serpents called Typhlops,[79] and such are the legless lizards[80] (Anguis), and such, again, are the simpler vermiform animals allied to frogs, called Cæciliæ.[81]
In order to burrow quickly and easily by means of processes of the body, it is evidently a necessary condition that the earth should be rapidly removed by the powerful action of parts situated towards the body’s anterior end. The similarity of effect of similar conditions in creatures which are most widely divergent in nature is exemplified by the mole and the mole-cricket, which are each provided with a strong and broadened-out pair of anterior digging-limbs.
Living creatures may be sustained in the air for a longer or shorter time at one or another stage of their existence. The reproductive particles of the lowest forms of animals and plants are so excessively minute that they float in the air with the greatest ease, without needing any complication of structure—their spheroidal form harmonizing with the equal action upon them of influences on all sides of them. Reproductive parts which, though less minute than these, are still very small, may also be diffused by floating in the atmosphere. Such are the pollen grains of those trees which are fertilized merely by the action of the winds, such as the hazel, poplar, birch, and of lowly plants, as the grasses. It is by the wind that the pollen grains of these plants are accidentally brought into contact with the appropriate surfaces for their reception. Conspicuous in the spring of the year are the clouds of yellow dust, pollen grains, given off by fir trees, which are plants also wind-fertilized. But here we find a slight complication; for to facilitate the dispersion of such particles the outer coat of each of their pollen grains is produced into a short wing-like process on each side, and these processes help at once to sustain it in the air, and to aid its propulsion by offering more surface to the force of the aërial currents.
Very much more conspicuous are the wing-like expansions of many seeds—such, for example, as those of the maple. These expansions serve to diffuse the seeds which bear them, as do also the delicate cottony filaments which surround the seeds of a variety of plants of widely different natures and affinities, as some kinds of spider float through the air by the aid of the delicate filaments which they send forth to serve as an aërial float. Familiar to every one is the delicate little parachute-like structure of radiating filaments on the seeds of such plants as the dandelion—which seeds most children have at some time helped to diffuse by blowing.
Aërial progress by actual effort is effected by a limited group of organisms, and only in certain cases (bats, birds, and insects) does it take the form of true flight in creatures now existing. In other creatures, such as so-called flying fishes, squirrels, opossums, and the little flying dragon, the more or less prolonged aërial sustentation is effected by expansions of skin, which act as parachutes in ways be later described in detail.
True flight seems to need a definite mechanism of one kind—namely, a mechanism which shall give rapid and reiterated blows to the air from a point towards the dorsal side, and head end of the body, by structures of considerable superficial extent, and capable of rapid and delicate inclinations of surface. Such structures must be light and therefore delicate, and yet possess very considerable strength to resist the strain of the body’s prolonged sustentation, and to effect its occasionally very rapid progress, as in the swift and in dragon-flies. These conditions which we find fulfilled in all existing flying organisms were also fulfilled organisms which have for ages passed away from the surface of this by planet, such as the extinct flying reptiles called Pterosauria or Pterodactyles.[82]
In all such rapidly flying creatures the form of the body is necessarily modified so as to throw the centre of gravity where it may be best sustained. It is this which packs what are practically a bird’s teeth in its belly, and thickens so greatly the muscles on its breast which are formed in such a way as to serve both the usual purposes of breast-muscles, and also that which is effected in most cases by muscles of the back, which in birds are very greatly diminished in volume and extent.
But there are living creatures which have relations with two media; which, though they are aquatic, yet by the help of the air rise and float, so as to be partly bathed in the atmosphere; while others carry down a portion of that atmosphere below the surface of water, so as to be sub-aqueously aërial. Examples of the last-mentioned condition are afforded by such spiders as have the habit of enclosing a bubble of air within the meshes of their self-woven network, and going down with it, being thus able there to maintain themselves as in a diving-bell. The reverse condition obtains in such plants as Valisneria,[83] which secrete air within expanded bladder-like receptacles, and, thus aided, rise to the surface and float. Another example is that of certain polyp animals, such as the Portuguese man of war, which also rise and swim upon the surface of the sea by the aid of floats in the form of bladders, which are also filled with air by means of their own life processes. The same also is the case in many seaweeds.
Thus, these multitudinous forms of living creatures, both animals and plants, are reducible to certain categories in harmony with their modes of life, and the relations existing between them and all surrounding influences. We may see that, without compliance with certain of such laws, their existence would be impossible, and we see that there is a general correspondence between their shape and structure on the one hand, and their environment (that is, the totality of all surrounding agencies and influences) on the other. Are we to consider that such influences are the causes of their form and structure? Obviously the biological facts before us, as yet, are insufficient to enable us to give a satisfactory answer to this question. It will for the present be enough to bear in mind that by some writers the environment is deemed the one and sufficient cause of all the characters of living creatures. But as yet we have not even seen what is the environment. Evidently physical influences—the earth, sea, or air, light, heat, and motion—do not exhaust it. One important factor would be omitted if we neglected to note the share taken in the environment of each living creature by a multitude of other living creatures which are in various ways related to it. This question must occupy us later.