592 General and Applied Biology 



to ascertain and explain the various reasons for the particular distribu- 

 tion of various organisms. The eflfects of various environmental factors 

 on the morphologic, physiologic, and developmental characters of animals 

 also may be observed. Such a study of the interrelationships between 

 organisms and their various environmental factors is known as the science 

 of ecology. It will be observed also that a similar environment in two 

 different and widely separated places does not necessarily contain similar 

 animals. (3) To see that in the past the boundaries of sea and land have 

 changed repeatedly, in some instances erecting natural barriers, in others 

 providing favorable highways for dispersal. It will be seen also that two 

 widely separated present-day types may have had a common ancestor 

 in the past. The fossils of extinct North American camels were the an- 

 cestors of the present Old World camel and the llamas of South America. 



II. TYPES OF GEOGRAPHIC DISTRIBUTION IN SPACE 



There are two general types of geographic distribution of animals in 

 space. The first is the lateral or longitudinal distribution throughout 

 the various geographic regions of the world (Fig. 310). This type of 



(AUSTRALIAN 



«-v ^ 



Fig. 310. — Geographic regions of the world. 



distribution is limited to the spread of animals over the face of the earth 

 in the various directions of the compass. The second is vertical distribu- 

 tion of animals throughout the various altitudes (Fig. 311). This type 

 of distribution emphasizes the differences in animal distribution on 



