6 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



required for changes in the means of dispersal through organic evolu- 

 tion. We know that many places now occupied by land were formerly 

 covered by seas, that rivers had other courses in former times, that 

 high mountains may be raised anew while others are eroded away, 

 that areas formerly well watered may now be desert, and that ice 

 sheets extended over regions previously inhabited, which were re- 

 populated after the withdrawal of the ice. It is highly probable that 

 land connections formerly existed between certain regions now sepa- 

 rated, such as North Africa and southern Europe, and North America 

 and Eurasia {via Alaska). Through changes of this nature the ranges 

 of related animals, formerly continuous, may be separated, and regions 

 may be united whose faunae were only distantly allied. The older a 

 natural division of the animal kingdom is, the more such changes of 

 barriers will have occurred during its history, and the more opportu- 

 nities for dispersal will have been available to its members. The 

 systematic relations are the primary factors, antedating the changes 

 in distribution. 



Historical zoogeography in this way attempts to work out the 

 development in geologic time of present-day distribution by studying 

 the homologies of animal distribution. For such studies the starting 

 points may be the systematic groups of related animals. The subject 

 matter will then consist of such problems as the restriction of groups 

 like the penguins, hummingbirds, monotremes, lemurs, or armadillos 

 to specific areas; the absence of otherwise widely distributed forms 

 from certain particular areas (as bears in Africa south of the Sahara 

 or of placental mammals in Australia) ; and the presence of related 

 forms in widely separated regions, such as the tapirs in tropical 

 America and in Malaysia, peripatus in New Zealand, Cape Colony, 

 and South America, and the horseshoe crabs on the east coast of North 

 America and in the Moluccas. On the other hand, the geographic unit 

 may be taken as the starting point, and the fauna of a given region 

 may be analyzed by studying the distributions of the subordinate 

 faunae of diverse origin, represented in the area. In Celebes, for ex- 

 ample, Asiatic and Australian elements are intermingled. 3 Four dis- 

 tinct immigrations can be distinguished which entered Celebes at 

 successive periods over four distinct land connections. These highways 

 of immigration were: (1) wa Java; (2) w'aFlores; (3) via the Philip- 

 pines; and (4) via the Moluccas. 



The ecological viewpoint, as contrasted with the historical, regards 

 the analogies between animal communities in similar habitats. Ecol- 

 ogy is the science of the relation of organisms to their surroundings, 

 living as well as non-living; it is the science of the "domestic econ- 



