8 sAiMoyukm. 



two forms distinguished by peculiarities of organization such as can- 

 not be conceived to be the effects of an external or internal cause, 

 disappearing with the disappearance of that cause, and which forms 

 have been propagated and are being propagated uniformly through 

 all the generations within the limits of our observation, and jftre yet 

 most probably to be propagated during the existence of mankind, he 

 is obliged to describe these two forms as distinct, and they will 

 commonly be called species. 



We have mentioned above that many points in the life-history of 

 the Salmonoids still remain very obscure : — 



1. Johnson, a correspondent of Willughby (Hist. Pise. p. 194), had 

 already expressed his belief that the different Salmonoids interbreed ; 

 and this \-iew has since been shared by many who have observed these 

 fishes in nature. Yet. no instance had been clearly made out, until 

 we were enabled, through the liberality of the Rev. Augustus Morgan, 

 to convince ourselves of the existence of a hybrid between the Sewin 

 (S. cambricus) and the River Trout {S. fario). These hybrids are so 

 numerous in the Rhymney and other rivers of South Wales, and so 

 variable in their characters, that the passage from one species to the 

 other may be demonstrated in an almost unbroken series; wbich 

 might induce some naturalists to regard both species as identical. 

 We have evidence sufficient, but of a less convincing character, of 

 hybridism between S. trutta and S. fario. In some rivers the con- 

 ditions appear to be more favourable to hybridism than in others, in 

 which hybrids are of comparatively rare occurrence. We have never 

 seen individuals which could with any certainty be regarded as 

 hybrids between the Salmon and some other species ; and although 

 there are not a few examples in the Collection of the British Museum 

 which indicate some affinity to the Salmon, they differ so much 

 among themselves that they are evidently of different origin, and 

 some of them may even eventually prove to be distinct species. The 

 hybrids are sexually as much developed as the pure breed, but nothing 

 whatever is known of their further propagation and progeny. 



2. Siebold has shown that some individuals of every species are 

 not sexually developed, and that such individuals differ also exter- 

 nally from those normally developed. However, he appears to have 

 gone too far when he stated that this rotate of sterility extends over 

 the whole period of existence of such individuals, and that therefore 

 the external peculiarities also remain permanent throughout life. 

 Widegren has shown that this sterility is merely a temporary imma- 

 turity, and that a part of the individuals arrive at a full sexual 

 development at a later or much later period than others. To this 

 we may add that many Salmonoids cease to propagate their species 

 after a certain age, and that all so-called overgrown individuals (that 

 is, specimens much exceeding tlie usual size of the species) are barren. 

 Externally they retain the normal specific characters. 



The Salmon offers a most remarkable instance of irregularity as 

 regards the age at which the individuals arrive at maturity. Shaw 

 has demonstrated, in the most conclusive manner, that those small 

 Salmonoids generally called Parr are the offspring of the Salmon, 



