498 ESSENTIALS OF ZOOLOGTY 



tions has even given man the advantage of being able to make use 

 of other animals for his own needs. The use of fire, tools, weapons, 

 clothing, shelter, and storage of food is all contemporary and 

 corollary to the development of the human brain. As development 

 along this direction progressed, the power of speech and later the 

 ability to write have become exclusively human traits. Through the 

 development of intelligence the human kind has been able to control 

 certain forces in nature to his own advantage. This superiority of 

 brain and intelligence is man's direction of specialization, and it 

 distinguishes him from the other animals. 



The molluscan, echinoderm, and arthropod fossils give similar 

 stories among invertebrates. The numerous fossils of fish. Amphibia, 

 Reptilia, particularly dinosaurs, and birds with the famous Arche- 

 opteryx, all have their testimony to contribute. 



Distributional Evidence. — The paleontological distribution of ani- 

 mals is considered vertically while geographic distribution is in 

 horizontal plane. It is impossible to separate these two completely. 

 The study of geographical distribution gives essentially a cross 

 section of the vertical distribution, thus giving a sort of ''still pic- 

 ture" of the complex developmental relations of animal groups at 

 one moment in geologic time. In studying this subject, it is neces- 

 sary to have in mind two fundamental conceptions. The first is that 

 the ancestors of related genera first appeared or originated in a 

 locality which is designated as the common center of origin. A sec- 

 ond conception is that as the ancestral form became established and 

 multiplied, migration in search of food and more suitable conditions 

 occurred. Barriers, many of w^hich were geographic, determined 

 the direction and extent of this migration. Large bodies of water 

 blocked the passage of terrestrial animals, as of course land was a 

 barrier to aquatic animals. High mountains or deserts were barriers 

 to all animals unable to withstand low temperatures and altitudes 

 on the one hand, or high temperature and dryness on the other. 

 These forces, and others, are believed to account for the natural 

 distribution of animals. There are the cases of the camel group, 

 originating in North America, migrating to South America and Asia 

 by the land connections of the Eocene to Pliocene epoch, and the 

 tapirs, which are represented by distinct species in two widely sepa- 

 rated regions, South Asia territory, and the Central America-South 

 American territory. Here again paleontological data show that in 



