An introduction to the study of fishes

by Albert C. L. G. Günther

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.

The similarity which obtains in fishes at somewhat distant points of the same degree of longitude, rarely extends far, and is due to the natural tendency of every animal to spread as far as physical conditions will permit. Between two regions situated north and south of each other there is always a debateable border ground, in parts of which sometimes the fishes of the one, sometimes those of the other, predominate, and which is, in fact, a band of demarcation. Within this band the regions overlap each other; therefore, their border lines are rarely identical, and should be determined by the northern and southernmost extent of the most characteristic types of each region. Thus, for instance, in China, a broad band intervenes between temperate and tropical Asia, in which these two faunæ mix, and the actual northern border line of the tropical fauna is north of the southern border line of temperate Asia.

It is the aim of every philosophical classification to indicate the degree of affinity which obtains between the various divisions; but the mode of division into six equivalent regions, as given above, does not fulfil this aim with regard to Freshwater fishes, the distribution of which allows of further generalisation and subdivision. The two families, Cyprinidæ and Siluridæ, of which the former yields a contingent of one-third, and the latter of one-fourth of all the freshwater species known of our period, afford most valuable guidance for the valuation of the degrees of affinity between the various divisions. The Cyprinoids may be assumed to have taken their origin in the Alpine region, dividing the temperate and tropical parts of Asia; endowed with a greater capability of acclimatising themselves in a temperate as well as tropical climate than any other family of freshwater fishes, they spread north and south as well as east and west; in the preglacial epoch they reached North America, but they have not had time to penetrate into South America, Australia, or the islands of the Pacific. The Siluroids, principally fishes of the sluggish waters of the plains, and well adapted for surviving changes of the water in which they live, for living in mud or sea-water, flourish most in the tropical climate, in which this type evidently had its origin. They came into existence after the Cyprinoids, fossil remains being known only from tertiary deposits in India, none from Europe. They rapidly spread over the areas of land within the tropical zone, reaching northern Australia from India, and one species even immigrated into the Sandwich Islands, probably from South America. The Coral Islands of the Pacific still remain untenanted by them. Their progress into temperate regions was evidently slow, only very few species penetrating into the temperate parts of Asia and Europe; and the North American species, although more numerous, showing no great variety of structure, all belonging to the same group (Amiurina). Towards the south their progress was still slower, Tasmania, New Zealand, and Patagonia being without representatives, whilst the streams of the Andes of Chili are inhabited by a few dwarfed forms identical with such as are characteristic of similar localities in the more northern and warmer parts of the South American continent.