2IO 



GENERAL PRINCIPLES OF ZOOLOGY. 



B 



between the lower plants and animals. In the lower algcz 

 the cells have power to emerge from their cellulose mem- 

 brane, and to swim about freely (Fig. 

 110), before they encapsule themselves 

 anew. On the other hand, most uni- 

 cellular animals encyst ; they pause in 

 their ordinary functions of life, become 

 spherical, and surround themselves 

 with a firm membrane, in some cases 

 even of cellulose. Since in both cases 

 an alternation between the encapsuled 

 antf the free-living condition occurs, 

 only the longer duration of the one or 

 of the other can lead to a distinction. 

 But herewith occurs the possibility 

 that undifferentiated intermediate forms 

 exist ; their actual existence prevents, 

 even yet, a sharp distinction between 



FIG. no. (Edogonium in 

 spore- formation. A , a 

 piece of the filament of the 

 alga with escaping cell- 

 contents ; Z>, zoospore 

 formed from the contents ; 

 C, zoospore fixed and ger- 

 minating. (After Sachs.) 



the animal and vegetable kingdoms. 



V. GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 



The Different Faunal Regions. Even a superficial 

 knowledge of the mode of distribution of animals shows 

 that the animal fauna in different regions of the earth has 

 an essentially different character. In part this difference 

 of fauna is the immediate result of climatic differences. 

 The polar bear, arctic fo.v, cider-ducks, and many aquatic 

 birds are restricted to the polar zones, because they can- 

 not endure more than a certain degree of warmth ; on the 

 other hand, the larger species of cats, the a/>cs, the /utin- 

 miiig-birds, etc., occur only in tropical or sub-tropical 

 regions, because they are not sufficiently protected against 

 cooler weather. 



Climate not the Only Consideration. If climate were 

 the sole factor determining distribution, the faunal char- 

 acter of two lands which had similar climatic conditions 

 would be essentially the same ; conversely, the separate 



