The geographical distribution of animals, with a study of the relations ...

56 DISTRIBUTION OF ANIMALS. [part i.in their organization or mode of life. These act mainly intwo ways,— lstly, by affecting the facilities with which ;— theycan be dispersed, either voluntarily or involuntarily 2ndly,by the conditions which enable them to multiply and establishthemselves in certain areas and not in others. When boththese means of diffusion are at a maximum, the dispersal ofa group becomes universal, and ceases to have much interestfor us. This is the case with certain groups of fungi andlichens, as well as with some of the lower animals ; and ina less degree, as has been shown by Mr. Darwin, with manyfresh-water plants and animals.At the other extreme we mayplace certain arboreal vertebrata such as sloths and lemurs,which have no means of passing such barriers as narrowstraits or moderately high mountains, and whose survival inany new country they might reach, would be dependent on thepresence of suitable forests and the absence of dangerous enemies.Almost equally, or perhaps even more restricted, are the meansof permanent diffusion of terrestrial molluscs; since these arewithout any but very rare and accidental means of being safelytransported across the sea ; their individual powers of locomotionare highly restricted ; they are especially subject to theattacks of enemies; and they often depend not only on apeculiar vegetation, but on the geological character of the country,their abundance being almost in direct proportion to thepresence of some form of calcaxeous rocks. Between theseextremes we find animals possessed of an infinite gradation ofpowers to disperse and to maintain themselves ;and it will evidentlybe impossible that the limits which best define thedistribution of one group, should be equally true for all others.Which class of Animals is of most importance, in determiningZoological Regions.—To decide this question we have to considerwhich groups of animals are best adapted to exhibit,by their existing distribution, the past changes and presentphysical condition of the earth's surface; and at the sametime, by the abundance of their remains inthe various tertiaryformations will best enable us to trace out the more recentof the series of changes, both of the earth's surface and