SPECIES OF SALMONIDiE. 11 



the typical salmonoid forms, having their center of abundance and perfection in size in 

 the arctic and sub-arctic regions. Southward in all regions Salvelinus gradually gives 

 way to Salmo. 



The Pacific is pecuhar in its genus Oncorhynchus which comprises several species. In 

 the far north, comprised in a narrow irregular zone, Salvelinus is associated with no other 

 member of the salmon family, although its geographical range encircles the northern 

 hemisphere. There it is essentially a marine fish, occasionally for food, or periodically 

 for procreation, entering fresh water. On each coast of both the Atlantic and Pacific 

 oceans, there are certain extents of southward projections of the anadromous-Salvelinus 

 zones. But these marine forms gradually disappear, becoming almost or quite exclu- 

 sively fresh-water inhabitants at the southern terminus of each range. Interdigitating 

 with the southward extensions of the Salvelinus zone are northward extensions of the 

 species of marine anadromous and fresh-water Salmo which has its center of abundance 

 in a more southern zone, the northern geographical limits of which are fully as irregular 

 in extent as is the southern extent of the Salvelinus zone. 



Species of Salmonid^. 



The genus Salmo comprises numerous, more or less vaUd, taxonomic species which 

 appear to be most abundant in Europe and western North America. The Manual of 

 the Vertebrate Animals of the Northeastern United States by David Starr Jordan (1929, 

 p. 50-51) transferred all the species formerly included in the genus, excepting Salmo 

 salar, to the genus Trutta. In fact, in North America, no species of Salmo naturally 

 occurs east of the western mountains, except on the Atlantic coast and in its coastwise 

 waters. Disregarding the chars, the eastern Atlantic has an anadromous salmon, which 

 is also present in the western Atlantic, one or more anadromous trouts, and predomi- 

 nantly fresh-water forms. Besides the salmon, the western Atlantic has only the so-caUed 

 'landlocked' sahnon. 



Pacific North America has its sea trouts and many nominal species of predominantly 

 fresh-water trout. The Asiatic-Pacific region has a limited number of known trouts, 

 anadromous in the north and permanently fresh-water residents in the south. Mention 

 of the Pacific salmons has already been made. Of these there are five well-differentiated 

 species belonging to a very definitely determined genus {Oncorhynchus) which occurs on 

 both the North American, and Asiatic coasts. 



Jordan (1905a, p. 293-294) says, 'The word "species" then, is simply a term of con- 

 venience, including such members of a group similar to each other as are tangibly 

 different from others, and are not known to be connected with these by intermediate 

 forms. Such connecting links we may suppose to have existed in all cases. We are only 

 sure that they do not now exist in our collections, so far as these have been carefully 

 studied. 



'When two or more species of any genus now inhabit the same waters, they are usually 

 species whose differentiation is of long standing, — species, therefore, which can be 



