302 THE INFLDENCE OF INANIMATE SDEROBNDINGS. 



effects of wind on the migrations of fresh-water animals, there 

 are other cases in which they can be 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 by the wind, bat 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 siich 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 occuning 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 : Tn the first place, the 



Fig. 79.— rf, An Amceba in its plastic state, with small powers of resistance ; b, tlie same 

 encysted, i.e. enclosed in an envelope wMch 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 be capable 

 of enduring the associated desiccation. These conditions are in 

 fact fulfilled, but only with microscopic animals and the eggs of 

 minute Invertebrata. All Infusoria, for instance, have the power 

 of enclosing their soft bodies in a firm envelope, the cyst (see 

 fig. 79) ; this they dp regularly before reproduction or when- 

 ever the external conditions are too unfavourable. In this 

 encysted state they are able to endure desiccation without any 

 injury to their vitality, and, what is more, they can lie dry 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, being 

 extremely light, they are naturally easy to transport, and it will 

 therefore not surprise the reader to hear that Ehrenberg was 

 able to detect, in dust collected in Germany at certain seasons, 



