110 THE HARMONIES OF NATURE. 



exist in moist earth, others lead a parasitic life, not only in the 

 stomach of frogs and earth-worms but even in the body of other 

 Infusoria. 



Of all animals they enjoy the widest range of habitation, and 

 several of them may well be called cosmopolites, as they have 

 been found in the Arctic and Antarctic seas, on the coasts, and far 

 away on the ocean, on high mountains, and deep in the mines 

 of Freiberg in Saxony. 



Their reproduction is effected sometimes by eggs or by bud- 

 ding, but generally by spontaneous division, either longitudi- 

 nally or in a transverse direction, and as this operation is 

 capable of being repeated every six or eight hours, we can 

 easily comprehend how the surface of clear stagnant waters may 

 in a short time be covered with a green mass consisting of 

 billions of Infusoria. When we consider that these minute 

 forms of life most probably witnessed the dawn of animal crea- 

 tion, our imagination can form no idea of the number of gene- 

 rations that must have succeeded each other from their first 

 appearance on earth to the present day ; but as only a few of 

 these are enclosed in a solid shell, their geological importance 

 is far inferior to that of the calcareous Foraminifera or of the 

 flint-cased Diatomacese and Desmidiacese. 



One of the most wonderful passages in the life history of the 

 Infusoria is the encysting process which at certain times they 

 undergo, and which serves to preserve them under circumstances 

 which do not permit the continuance of their ordinary vital 

 activity. Previously to the formation of 1?he cyst, the move- 

 ments of the animal diminish in vigour and gradually cease 

 altogether ; its form becomes more rounded ; its oral aperture 

 closes, and its cilia are either lost or retracted. The surface of 

 the body then exudes a gelatinous excretion which hardens 

 around it so as to form a complete coffin-like case in which the 

 torpid animal remains embedded, until the fostering influences 

 of warmth and humidity again recall it to an active life. 



Incalculable numbers of these encysted Infusoria are con- 

 stantly wafted about in the atmosphere or carried by the winds 

 from land to land ; and thus we can readily comprehend how 

 pure watery infusions, when exposed to the air, soon begin to 

 swarm with a little world of the minutest animals and plants. 



The ubiquity and perfectly astounding numbers of the 



