4 PRINCIPLES OF ANIMAL BIOLOGY 



Zoogeography. — The goographical distribution of animals, or zoogeog- 

 raphy, is in some respects closely related to ecology. It deals with 

 distribution, as does ecology, but in a much wider sense. It is concerned 

 with regions, rather than kinds of environment. The regional distri- 

 bution of a species is limited by the distribution of environments suitable 

 for it, but no species of animal occupies all the regions of the world where 

 the environment is right. Kangaroos, to cite a striking example, are 

 found only in the Australian region, although many other places on the 

 earth have a temperature, humidity, and other features very similar to 

 those of Australia. Presumably kangaroos could live in most of these 

 similar regions if introduced into them, but they are not there. On the 

 contrary, rabbits were entirely wanting in Australia until taken there 

 by man, since which time they have become, like the English sparrow 

 when introduced into America, almost a scourge. The failure of a species 

 to occupy a region in which the conditions of life are suitable for it usually 

 means that it has been unable to reach that region, and this inability 

 may have been due to the present topography of the region, to its geo- 

 logical history, or to the place of origin of the species. Thus, while 

 zoogeography involves suitabihty of environment (ecology), it is also 

 concerned with historical factors. 



Paleontology. — In paleontology the zoologist studies the animals of 

 the past. Parts of these ancient animals are preserved as fossils in the 

 sedimentary rocks. The relative age of the fossils is determined from 

 the relative depth of the rock strata in which they occur. The vast 

 majority of animals, perhaps all of them, living on the earth in these early 

 times were of different species from those alive today. Though there 

 are numerous similarities, only a very few fossil animals were practically 

 identical with living ones and the similarity of even these few must be 

 stated with some reserve. Moreover, the deeper fossils are quite differ- 

 ent from those in the upper layers. Past ages on earth were thus marked 

 by very different groups of living beings. All of the problems which exist 

 for animals today presumably existed for those of the past. Those ani- 

 mals had structure and functions, they were related to the environment, 

 and they were distributed over the earth in various ways. 



There is thus a zoology of the past just as there is a zoology of the 

 present. The paleontologist is limited, however, in his study of ancient 

 animals to what can be discovered from their remains. He is unable to 

 experiment upon his subjects, nor can he'rear them in his laboratory. It 

 thus happens that paleontology is mostly concerned with structure and 

 classification of fossil animals, and with their distribution in space and in 

 time. Other things can be inferred concerning them such as their mode of 

 life, the reasons for their migrations, or their relation to the environment, 

 and such inferences are of much or little value according to the nature of 



