VII. Geographical Distribution of Animals. 75 



On a closer inquiry into these causeSj it is found that large regions 

 are generally separated from one another by natural barriers of different 

 kinds — large seas, high mountains^ extensive wildernesses — difficult to 

 surmount. That each region retains its own peculiar fauna, is primarily 

 to be ascribed to the fact that the animals have lived for a long time 

 as a relatively circumscribed group, and during this separation have 

 been modified in o n e direction, whilst their relations elsewhere have 

 developed in others. The differentiation of two regions, which were 

 originally continuous and in conformity, but now are separated, has 

 been explained as follows : on the one hand a number of peculiar 

 forms has originated in each ; on the other, some of the original forms 

 have survived in one region and become extinct in the other. In this 

 way it is easy to understand, for example, the great faunistic 

 differences of the Neotropical and Ethiopian regions, which present 

 similar physical conditions, but are divided by extensive seas, so that 

 for immensely long periods of time they have probably had no 

 connection. If some of the regions in sharp zoo-geographical contrast 

 are less clearly separated geographically {e.g., India and Australia) 

 it is probably due to their having been more definitely divided at an 

 earlier period. On the other hand, the circumstance that the 

 Palaearctic and the Nearctic regions, for instance, which are now 

 absolutely distinct, exhibit in many points a considerable similarity — - 

 a number of Mammals is common to both regions, and other types are 

 represented in both by nearly allied species — is plainly explicable, on 

 the ground that in early times, and even relatively late, these regions 

 were more closely connected than they are now. The diversities and 

 similarities of the zoo-geographical regions may consequently be 

 regarded to a great extent, as the result of alterations in 

 the conditions of the earth's surface. 



Regions similar to these may also be established for marine 

 animals, especially for littoral forms. They do not, of course, 

 coincide with the terrestrial divisions ; the fauna of the east coast of 

 South America, for instance, belongs to one region, that of the west 

 coast to another, and so on. The abysmal fauna, on the other 

 hand, has, for the most part, the same characters in all seas. Many 

 abysmal species have the widest geographical distribution ; this is 

 quite intelligible since the physical conditions, the temperature, etc., 

 are relatively uniform, and there seem to be no impassible barriers. 

 This holds also for the pelagic fauna, which, throughout the warm 

 zones, exhibits a very uniform character though in the cold seas, 

 both north and south, it is of a somewhat different type : since other 

 natural barriers are wanting, temperature is here of paramount 

 importance. 



