56 DISTKIBUTION OF ANBIALS. [part i. 



in their organization or mode of life. These act mainly in 

 two ways, — Istly, by affecting the facilities with which they 

 can be dispersed, either volnntarily or involnntarily ; — 2ndly, 

 by the conditions wliich enable them to multiply and establish 

 themselves in certain areas and not in otliers. When both 

 these means of diffusion are at a maximum, the dispersal of 

 a group becomes universal, and ceases to have much interest 

 for us. This is the case with certain groups of fungi and 

 lichens, as well as with some of the lower animals; and in 

 a less degree, as has been sliown by Mr. Darwin, with many 

 fresh-water plants and animals. At the other extreme we may 

 place certain arboreal vertebrata such as sloths and lemurs, 

 whicli have no means of passing such barriers as narrow 

 straits or moderately high mountains, and whose survival in 

 any new country they might reach, would be dependent on the 

 presence of suitable forests and the absence of dangerous enemies. 

 Almost equally, or perhaps even more restricted, are the means 

 of permanent diffusion of terrestrial molluscs ; since these are 

 without any but very rare and accidental means of being safely 

 transported across the sea ; their individual powers of locomo- 

 tion are highly restricted ; they are especially subject to the 

 attacks of enemies; and they often depend not only on a 

 peculiar vegetation, but on the geological character of the coun- 

 try, their abundance being almost in direct proportion to the 

 presence of some form of calcareous roclcs. Between these 

 extremes we find animals possessed of an infinite gradation of 

 powers to disperse and to maintain themselves ; and it will evi- 

 dently be impossible that the limits which best define the 

 distribution of one group, should be equally true for all others. 

 Which class of Animals is of most importance in determining 

 Zoological Regions. — To decide this question we have to con- 

 sider wliich groups of animals are best adapted to exhibit, 

 by their existing distribution, the past changes and present 

 ph}'sical condition of the earth's surface; and at the same 

 time, by the abundance of their remains in the various tertiary 

 formations will best enable us to trace out the more recent 

 of the series of changes, both of the earth's surface and 



