24 TEXTBOOK OF ZOOLOGY 



8. Ecology is a study of the relation of the organism to its en- 

 vironment. Many adjustments in structure and function have been 

 made by animals to bring them into harmony with the conditions 

 of the environment. Such conditions as the relation of the organ- 

 ism to the medium in which it lives to temperature, to light, to 

 food, to competition, to enemies, to mating, and many other fac- 

 tors, all become a part of an ecological study. This study usually 

 draws somewhat upon a knowledge of all branches of biology. This 

 branch of the field has become prominent in comparatively recent 

 3"ears. 



9. Zoogeography or geographical distribution of animals is con- 

 cerned with the extent of the regions over Avhich species are dis- 

 tributed and the association of species in individual regions. In 

 some respects this field is closely related to ecology. It is con- 

 cerned with the regions in which species exist and with the factors 

 affecting their distribution. The regional distribution of an animal 

 group is limited in part by the extent and relations of favorable 

 environmental conditions, but no species occupies all of the regions 

 where environment would permit. The point of origin of the group 

 may be cut off from other favorable regions by unsurmountable 

 obstacles. Conditions which prevent dispersal of animals from one 

 area to another are known as barriers. Oceans, mountains, forests, 

 deserts and land are all barriers to different types of animals. Even 

 a slight difference in the salinity or acidity of the water becomes a 

 barrier to many aquatic animals. The failure of a species to occupy 

 a suitable region usually means that it has been unable to reach 

 that region, perhaps because of the topography of the region, its 

 geological history, or the remoteness of the place of origin of the 

 species. The English sparrow, which originated in Europe, was not 

 found in America until after it was introduced by man, and in 

 relatively few years it became a dominant bird. 



10. Paleozoology is a study of the animals of the past as they are 

 presented by their fossil remains. Parts of many of the ancient 

 animals are embedded and preserved in the sedimentary rocks. 

 The relative age of the fossils is determined from the depth of 

 the rock strata in which they are found. Many of the probable 

 lines of descent of animals have been discovered by studies of the 

 fossils. Much concerning the facts and the fate of extinct species 

 has been learned through this field of study. Paleozoology is ordi- 



