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.