20. _Anguilla._ The distribution, no less than the mode of propagation, and the habits generally, of the so-called Freshwater-eels still present us with many difficult problems. As far as we know at present their birthplace seems to be the coast in the immediate neighbourhood of the mouths of rivers. They are much more frequently found in fresh water than in brackish water, but the distribution of some species proves that they at times migrate by sea as well as by land and river. Thus _Anguilla mauritiana_ is found in almost all the fresh and brackish waters of the islands of the Tropical Indian Ocean and Western Pacific, from the Comoros to the South Sea; _Anguilla vulgaris_ is spread over temperate Europe (exclusive of the system of the Danube, the Black and Caspian Seas), in the Mediterranean district (including the Nile and rivers of Syria), and on the Atlantic coast of North America; _Anguilla bostoniensis_, in Eastern North America, China, and Japan; _Anguilla lati__rostris_, in Temperate Europe, the whole Mediterranean district, the West Indies, China, and New Zealand. The other more local species are found, in addition to localities already mentioned, on the East Coast of Africa, South Africa, on the continent of India, various East Indian Islands, Australia, Tasmania, Auckland Islands; but none have ever been found in South America, the West Coast of North America, and the West Coast of Africa: surely one of the most striking instances of irregular geographical distribution. 21. Numerous _Syngnathidæ_ have established themselves in the Northern Zone as well as in the Equatorial, in the vegetation which flourishes in brackish water. This list could be considerably increased if an enumeration of species, especially of certain localities, were attempted; but this is more a subject of local interest, and would carry us beyond the scope of a general account of the distribution of Fishes. [Illustration: Fig. 105.--_Mugil octo-radiatus._] [Illustration: Fig. 106.--_Mugil auratus._] [Illustration: Fig. 107.--_Mugil septentrionalis._ Heads of Grey Mullets, fishes of Brackish water.] CHAPTER XIX. THE DISTRIBUTION OF MARINE FISHES. Marine fishes fall, with regard to their mode of life and distribution, into three distinct categories:--