4 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



The data accumulated by faunal zoogeography have now to be 

 sifted and ordered. The natural laws which lie hidden must be estab- 

 lished, and their interrelations studied. Building material is necessary 

 to erect a science, but a heap of bricks is no structure, and an ac- 

 quaintance with unrelated basic facts is no science. Only the hod- 

 carriers of science could be content with the mere accumulation of 

 facts. Faunal zoogeography is, therefore, also concerned with the 

 classification of the facts of distribution, which may proceed in various 

 directions. 



Comparative zoogeography attempts the classification of animal 

 distributions according to their resemblances. This comparison may be 

 made from different viewpoints. Homologies, or inherited resemblances, 

 and analogies, or acquired resemblances, are distinguished in compara- 

 tive anatomy, and a similar distinction applies to animal distributions. 

 When different faunal lists are compared with reference to the natural 

 relations of their components, faunae may be distinguished whose 

 distribution does not agree with the present geographic divisions of 

 the earth. The animals of North Africa, such as the snails, insects, 

 birds, reptiles, and amphibians, are much more closely related to the 

 animals of the corresponding groups in southern Europe than to those 

 of Africa south of the Sahara. The fauna of southern Asia is more 

 closely related to that of trans-Saharan Africa than it is to the fauna 

 of Asia north of the Himalayas. Many groups of North American 

 mammals and birds are more remote from the corresponding groups 

 in Central and South America than from those in Europe and northern 

 Asia. The significant faunal boundary between the animal life of 

 North America and that of South America lies somewhere in Mexico 

 and not at the dramatic Isthmus of Panama. The homologies among 

 such comparable faunae are based upon the blood relationship of their 

 components, and on a common evolution in time and space. The larger 

 faunae of this kind characterize the faunal regions and their sub- 

 divisions, within which the animal inhabitants are homologously com- 

 parable. For example, the representatives of natural groups in South 

 America such as the iguanid lizards, the ovenbird family, and the 

 rodents, are interrelated and of common origin whether they inhabit 

 the forests, the prairies, or the mountains. 



On the other hand, ecological communities of animals may be 

 recognized which resemble each other superficially in correspondence 

 with resemblances between their environments. These are analogous, 

 instead of homologous. For example, the inhabitants of the rain-forests 

 of the various tropical countries, South America, Africa, and Malaysia, 

 exhibit a whole series of evident resemblances, among which adapta- 



