The binary or dichotomous system, although it brings out the fundamental differences of the respective regions, is an inconvenient one in its application, and rather increases than obviates the difficulty as to equality or inequality of regions; for although _a_, _b_, _c_, and _d_, may be areas of unequal zoological rank, _a_ being the most important, and _d_ the least, yet this inequality will probably be still greater if we first divide them into _a_, on one side, and _b_, _c_, and _d_, on the other, and then, by another division, make _b_, an area of the second, and _c_, and _d_, of the third rank only. Coming to the second objection, the often incompatible distribution of different groups of animals, affords ground for opposition to any proposed scheme of zoological regions. There is first the radical difference between land and sea animals; the most complete barriers to the dispersal of the one, sometimes offering the greatest facilities for the emigration of the other, and _vice versa_. A large number of marine animals, however, frequent shallow water only; and these, keeping near the coasts, will agree generally in their distribution with those inhabiting the land. But among land animals themselves there are very great differences of distribution, due to certain specialities {56}in their organization or mode of life. These act mainly in two ways,--1stly, 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 locomotion 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 country, 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 evidently be impossible