During certain periods of the Tertiary epoch it is true that there was more similarity between Europe and North America than there is at present. It would have been quite necessary to unite both into a Holarctic area, such as is now insisted upon by many; but the reasons for this union would then have been stronger. The fact is, however, that the closer resemblances were due to the larger number of families of animals which existed then than now; these have decayed away from both continents, and allowed the unlikenesses between the mammalian fauna of both to become evident. But the likenesses which still survive have led many to associate the two regions closely together. So far as the history of a genus or family or larger division {84} can be traced, it results as a conclusion that from a given area of origin the group in question migrated in all directions where possible to a varying degree; it then died out in intervening tracts, or was left only in a certain part of its former and more extensive area of range. ZOOLOGICAL REGIONS.--Seeing that each species of animal has its own definite range, it is clear that the earth's surface can be apportioned into divisions which are characterised by their animal inhabitants. We shall divide the earth into realms, which are the largest divisions; then into regions; and finally into subregions. It must be borne in mind that the various groups of the animal kingdom are of different ages, geologically speaking, and have therefore had less or more time, as the case may be, to settle down into their present distribution, and that different animals differ greatly in their rate of multiplication, their power of migration, and their susceptibility to the effectiveness of various natural and other barriers to distribution. It is not, therefore, possible to divide the world into realms and regions which shall express the facts of distribution of the entire animal kingdom. Such divisions, which are common in text-books of zoology having but a small section devoted to zoogeography, are at best mere approximations and averages; no good is gained by taking such a comprehensive view of the matter, as the essential object of subdividing the earth's surface is thereby lost sight of. The zoogeographical division of the earth which will be adopted here is that originally recommended by Dr. Blanford, and now accepted by a number of authorities. There are three "realms," to which a fourth may perhaps be added--though on negative grounds, and merely for the purpose of emphasising the parts of the world to which mammals have not gained access. The realms are again divisible into regions, at least in the case of one of them, and the regions may be again separated into more or less distinct subregions or provinces. The three primary divisions or realms which contain mammals are the Notogaean, including Australia and certain islands to the north of it; the Neogaean, or the South American continent and