. Animal life as affected by the natural conditions of existence. Animal ecology. 302 THE INFLUENCE OF INANIMATE SUEUOtNDINGS. effects of wind on the migrations of fresh-water animals, there are other cases in which they can he recognised with the greatest ease. We know that our atmosphere is densely full of the desiccated germs of minute organisms which are most easily raised and borne liy the wind, hut which fall to the ground as soon as the air is still again. We have learned from the highly important experiments made by Tyndall on lower organisms and their distribution, that the only unfai
. Animal life as affected by the natural conditions of existence. Animal ecology. 302 THE INFLUENCE OF INANIMATE SUEUOtNDINGS. effects of wind on the migrations of fresh-water animals, there are other cases in which they can he recognised with the greatest ease. We know that our atmosphere is densely full of the desiccated germs of minute organisms which are most easily raised and borne liy the wind, hut which fall to the ground as soon as the air is still again. We have learned from the highly important experiments made by Tyndall on lower organisms and their distribution, that the only unfailing method of freeing the air of such microscopic elements is absolute stillness. Thus, if it were possible to trace with any certainty this sediment of the atmosphere, so to speak, we should be in a position to deter- mine the direction which the different animals occurring in it must have taken through the air. But two conditions must be fulfilled in order that the distri- bution of animals may thus be effected : In the first place, the. Fig. 79,—(7, An Anireba in its plastic state, with small powrrs of rosistanco ; h, the same encysted, enclosed in an envelope which protects it against injurious influences. force of the air in motion must suffice to raise the organisms high up ; and, secondly, the organisms themselves must lie capable of enduiing the associated desiccation. These conditions are in fact fulfilled, but only with microscopic animals and the eggs of Invertebrata. All Infusoria, for instance, have the power of enclosing their soft bodies in a firm envelope, the cyst (see fig. 79); this they do i-egularly before reproduction or when- ever the external conditions are too In this encysted state they are able to endure desiccation without any injury to their vitality, and, what is more, they can lie diy for years—how long is not known—and then, after tens or perhaps even thousands of years, revive to a new life. In this state, bein
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