CHAPTER LXX 



DISTRIBUTION OF ANIMALS 



ZOOGEOGRAPHY 



The study of the geographical distribution of animals upon the surface 

 of the earth and of the factors which have brought about such distribu- 

 tion forms the subject matter of a field of zoology which has generally 

 been known as zoogeography. It is related on the one hand to paleon- 

 tology, since present-day distribution depends in part upon past dis- 

 tribution; and on the other hand to ecology, in that the environment 

 affects, and in many cases determines, the ability of individual animals 

 to maintain themselves in any given locality. 



569. Present Distribution. — The area occupied by any species of 

 animal is usually termed its range. Generally speaking, throughout 

 that area there must be, to a certain degree at least, similarity of condi- 

 tions in so far as they are determining conditions in the life of the animal. 

 Some animals, which are dependent apparently upon a very particular 

 set of conditions, are confined to a limited range; others, able to adjust 

 themselves to greater differences in conditions, possess a very extended 

 range. Indeed there are a few which are cosmopolitan, being distributed 

 practically throughout the world. 



As a rule a particular species as well as related species occupy ranges 

 which are continuous; but this is not always true, for there are many 

 cases of discontinuous distribution when a given species or genus is repre- 

 sented in small areas far apart. This is rarely true of birds, because of 

 their power of flight, but it is know^n of many mollusks and insects. The 

 genus Peripatus offers an example of a type which has a very wide and at 

 the same time discontinuous distribution (Sec. 307). 



570. Past Distribution. — It is clear from the evidence furnished by 

 the fossil remains of animals that their distribution over the earth has 

 been very different in the past from what it is at the present time. An 

 extreme instance of this fact is the occurrence of remains of animals 

 which are now confined to the tropics as far north as the northern United 

 States and the northern parts of Europe and Asia. Since the animals 

 living on the earth today are descendants of those of the past, the facts 

 of past distribution have a distinct bearing upon that of the present and 

 may be the clue to discontinuous distribution. In order to explain some 

 marked cases of such distribution, especially when it involves a number of 

 different types, the existence of former land masses serving as bridges and 



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