22 ESSENTIALS 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 organism 

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

 competition, to enemies, to mating, and many other factors, all be- 

 come a part of an ecological study. 



9. Zoogeography or geographical distribution of animals is con- 

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

 tributed and the association of species in individual regions. It is 

 concerned 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 fa- 

 vorable 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 unsur- 

 mountable obstacles. Conditions which prevent dispersal of animals 

 from one area to another are known as harriers. Oceans, mountains, 

 forests, deserts and land are all barriers to different types of ani- 

 mals. The Starling, which originated in Europe, was not found in 

 America until after it was introduced by man, and in recent years 

 it is becoming 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 ani- 

 mals 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 de- 

 scent 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. 



Classification of the Animal Kingdom 



Few people realize the number of different kinds of existing ani- 

 mals and their variation in size, structure, and habits of life. The 

 estimated number of kinds is all the way from 1,000,000 to 10,000,- 

 000. To date, approximate!}^ 840,000 species have been named and 

 described. 



The entire kingdom is divided into two subkingdoms: Protozoa, 

 or all single-celled animals, and Metazoa, the many-celled animals. 



