8 DISTRIBUTION OF ANIMALS. [PART 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. 
