8 DISTRIBUTION OF ANIMALS. [pabt i. 



will separate countries the productions of which most widely 

 and radically differ; while the most recently-depressed seas, 

 or the last-formed mountain ranges, will separate countries 

 the productions of which are almost or quite identical. It 

 will be evident, therefore, that the study of the distribution 

 of animals and plants may add greatly to our knowledge 

 of the past history of our globe. It may reveal to us, in a 

 manner which no other evidence can, which are the oldest 

 and most permanent features of the earth's surface, and which 

 the newest. It may indicate the existence of islands or conti- 

 nents now sunk beneath the ocean, and which have left no 

 record of their existence save the animal and vegetable pro- 

 ductions which have migrated to adjacent lands. It thus 

 becomes an important adjunct to geology, which can rarely do 

 more than determine what lands have been raised above the 

 waters, under what conditions and at what period; but can 

 seldom ascertain anything of the position or extent of those 

 which have sunk beneath it. Our present study may often 

 enable us, not only to say where lands must have recently 

 disappeared, but also to form some judgment as to their ex- 

 tent, and the time that has elapsed since their submersion. 



Having thus briefly sketched the nature and objects of the 

 subject we have to study, it will be necessary — before entering 

 on a detailed examination of the zoological features of the 

 different parts of the earth, and of the distribution of the orders, 

 families, and genera of animals — to examine certain preliminary 

 facts and principles essential for our guidance. "We must first 

 inquire what are the powers of multiplication and dispersal of 

 the various groups of animals, and the nature of the barriers 

 that most effectually limit their range. We have then to 

 consider the effects of changes in physical geography and in 

 climate ; to examine the nature and extent of such changes as 

 have been known to occur ; to determine what others are possible 

 or probable ; and to ascertain the various modes in which such 

 changes affect the structure, the distribution, or the very exist- 

 ence of animals. 



