2. Which of them are to be considered to be _autochthont_ species, that is, forms which came in the tertiary epoch or later into existence within the area to which they are still limited, or from which they have since spread? 3. Which are the forms which must be considered to be _immigrants_ from some other region? The mode of division of the earth’s surface into zoological regions or areas now generally adopted, is that proposed by Mr. Sclater, which recommends itself as most nearly agreeing with the geographical divisions. These regions are as follows:-- I. PALÆOGÆA. 1. The _Palæarctic_ region; including Europe, temperate Asia, and North Africa. 2. The _Ethiopian_ region; including Africa, south of the Sahara, Madagascar, and the Mascarene Islands; also Southern Arabia. 3. The _Indian_ region; including India south of the Himalayas, to Southern China, Borneo, and Java. 4. The _Australian_ region; including Australia, the Pacific Islands, Celebes, and Lombock. II. NEOGÆA. 5. The _Nearctic_ region; including North America to Northern Mexico. 6. The _Neotropical_ region; including South America, the West Indies, and Southern Mexico. Comparatively few classes and orders of animals have been carefully studied with regard to their geographical distribution, but the majority of those which have been examined show that the difference of latitude is accompanied by a greater dissimilarity of indigenous species than that of longitude, and that a main division into an old world and new world fauna is untenable. More especially the Freshwater fishes, with which we are here solely concerned, have been spread in _circumpolar_ zones, and in a but limited degree from north to south. No family, much less a genus, ranges from the north to the south, whilst a number of families and genera make the entire circuit, and some species more than half of the circuit round the globe within the zone to which they belong. Not even the Cyprinoids and Siluroids, which are most characteristic of the freshwater fauna of our period, are an exception to this. Temperature and climate, indeed, are the principal factors by which the character of the freshwater fauna is determined; they form the barriers which interfere with the unlimited dispersal of an ichthyic type, much more than mountain ranges, deserts, or oceans. Hence the tropical zone is an impassable barrier to the northern Freshwater fish in its progress towards the south; where a similarly temperate climate obtains in the southern hemisphere, fish-forms appear _analogous_ to those of the north, but _genetically_ and _structurally distinct_.