216 FISHES. 



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 indi- 

 cate the degree of af&nity 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, CyprinidcB 

 and Siluridcc, 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 

 origm in the Alpine region, dividing the temperate and tropical 

 parts of Asia ; endowed with a greater capability of accHma- 

 tising 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, 



