456 THE GENERAL PRINCIPLES OF ZOOLOGY 



they have become decidedly objectionable. Let us consider, 

 then, some of the factors which affect the geographical dis- 

 tribution of animals. 



Many of the smaller animals, such as Protozoa, are capable 

 of passing into resting stages by inclosing themselves in cysts 

 capable of resisting desiccation ; they may thus be blown about by 

 the winds and so obtain a universal distribution. Many small 

 animals, such as worms, Crustacea, and Mollusca, may be carried 

 about from place to place in the mud which adheres to the feet 

 of wading birds. Animals of sufficient size and capable of free 

 locomotion may migrate from place to place as the climatic con- 

 ditions change or the necessity for more abundant food dictates. 

 But the extent of such migration varies with the different ani- 

 mal types as hindrances to further advance arise. New natural 

 enemies may be met in the new legions entered and so further 

 progress arrested. The desert is an impassable barrier to the 

 majority of animals; broad rivers are a barrier to such animals 

 as the monkeys, which cannot swim; lakes and inland seas are 

 usually obstructions to all but aerial types, and the same is true 

 of lofty mountains. Hut what is to-day a barrier may not have 

 been so in the past, and vice versa. Thus in the Pleistocene 

 period the British Islands were a part of the continent of Europe, 

 hence it is not surprising that the animals comprising the fauna 

 of Great Britain are found likewise on the adjacent continent. 

 The Isthmus of Panama was until recent times submerged 

 beneath the ocean, and North and South America were separated 

 from one another; the marine fauna on the two sides of the 

 isthmus is identical, while the two continents have few animals 

 in common. As animals have migrated so as to cover greater 

 and greater areas, it was very natural that they should be ren- 

 dered entirely extinct in some regions while still abundant in 

 others; thus arose discontinuity in distribution, and it readily 

 follows that the greater the discontinuity the longer must have 

 been the period during which the group in question has occupied 

 the earth. Thus we see that the Dipnoi must be a group of 

 great antiquity. 



Finally, the faunae of certain islands which have never been 

 connected with the continents is interesting, such as the islands 

 of volcanic origin in the midst of the Pacific. Here ocean cur- 



