68 SCIENCE SKETCHES. 
correspond to our closet-formed definition of its 
species must therefore be a “hybrid” with some 
other. There is no evidence worth mentioning, 
known to me, of extensive hybridization in a state 
of nature in any group of fishes. This matter is 
much in need of further study; for what is true of 
the species in one region, in this regard, may not 
be true of others. The species of trout, also, may 
perhaps hybridize, while Salmo salar and the 
species of Oncorhynchus certainly do not. Dr. 
Giinther observes: -— 
“Johnson, a correspondent of Willughby, had already 
expressed his belief that the different salmonoids inter- 
breed ; and this view has since been shared by many who 
have observed these fishes in Nature. Hybrids between 
the sewin (Salmo trutta cambricus) and the river-trout 
(Salmo fario) were numerous in the Rhymney and other 
rivers of South Wales before salmonoids were almost ex- 
terminated by the pollutions allowed to pass into these 
streams, and so variable in their characters that the pas- 
sage from one species to the other could be demonstrated 
in an almost unbroken series, which might induce some 
naturalists to regard both species as identical. Abundant 
evidence of a similar character has accumulated, showing 
the frequent occurrence of hybrids between Salmo fario 
and S. trutta. . . . In some rivers the conditions appear 
to be more favorable to hybridism than in others, in which 
hybrids are of comparatively rare occurrence. Hybrids 
between the salmon and other species are very scarce 
everywhere.” 
The black-spotted trout, forming the sub-genus 
Salar, differ from Salmo salar and Salmo trutta in 
the greater development of the vomerine teeth, 
