ECOLOGY AND ZOOGEOGRAPHY 



649 



been found only in the Garden of the Gods 

 in Colorado. 



Continuous and 

 discontinuous distribution 



Certain species or genera are distributed 

 continuously over an area, and this appears 

 to be the general rule; but others are dis- 

 continuous. For example, two species of 

 alligators are known; one lives in the south- 

 eastern United States and the other in 

 China. Tapirs occur in tropical America 

 and in Malaya on the other side of the 

 world. Various physical and biological fac- 

 tors have been suggested to account for their 

 present discontinuous distribution. 



Changes in range 



The extinction of a species in areas be- 

 tween present habitats seems probable, since 

 in a number of cases fossil remains have 

 been discovered in such areas. The study 

 of fossil animals has shown how greatly the 

 ranges of various groups have changed in 

 the course of time. Dinosaurs, for example, 

 once roamed over North America, but have 

 disappeared not only from this continent but 

 from the world of living creatures. The horse 

 tribe was once native to North America, but 

 no horses existed here when Columbus ar- 

 rived. Modern horses were reintroduced by 

 the Spaniards. Many species, such as the 

 great auk and passenger pigeon, have be- 

 come extinct within recent years, and the 

 distributions of other species are known to 

 have changed within historic times, or are 

 changing now. 



METHODS OF DISTRIBUTION 



Dispersion 



Every kind of animal produces a greater 

 number of offspring than can be supported 

 in its particular habitat. This law of over- 



production was particularly emphasized by 

 Malthus (1766-1834) in his Essay on Popu- 

 lation and led both Darwin and Wallace to 

 formulate their theory of natural selection 

 to account for organic evolution. Since par- 

 ents and all their offspring cannot occupy 

 the same area, there ensues a struggle for 

 existence, as a result of which some indi- 

 viduals must migrate or perish. Animals tend 

 to migrate from the region of their birth, 

 and this normal migration accounts in a 

 large part for the distribution of many spe- 

 cies on the earth today. Even attached 

 aquatic animals migrate; the larvae of 

 sponges and the planulae of coelentrates 

 swim about; and young clams may be carried 

 by fish for long distances. 



The range is usually enlarged by the ani- 

 mal invading new territory around the 

 edges, but it is, of course, limited by its 

 required environment; for example, a tree 

 frog is not fitted for life on a grassy plain, 

 and its range must end where the trees give 

 way to grass. Certain types of migration are 

 periodic and appear to have little influence 

 on the breeding range; for example, the 

 migration of birds, bison, and salmon. 

 Sporadic migrations, such as those of the 

 lemming and Rocky Mountain locust, usu- 

 ally come to naught, but they might be re- 

 sponsible for discontinuous distribution if 

 the migrating hordes should encounter a 

 new area with a satisfactory environment. 

 The forced transportation of animals by the 

 wind or by currents of water plays a minor 

 role in distribution, Man has introduced 

 many species, either purposely or acciden- 

 tally, into new regions, but this has had 

 relatively little effect on geographic distribu- 

 tion as a whole. 



Barriers and highways 



Animals are more or less confined to cer- 

 tain habitats by barriers; they are prevented 

 from entering new regions by mountains or 

 lakes, by lack of suitable food, by interfer- 



