56 DISTRIBUTION OF ANIMALS. [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 voluntarily or involuntarily ;—2ndly, 
by the conditions which enable them to multiply and establish 
themselves in certain areas and not in others. 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 shown 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, 
which 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 rocks. 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 which groups of animals are best adapted to exhibit, 
by their existing distribution, the past changes and present 
physical 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 
