THE SALMONID^:. 233 



it difficult to distinguish varieties from species, and refers 

 to the well-known changes which are produced by cultiva- 

 tion and domestication. He instances the selection man 

 makes in order to produce new breeds and varieties, and con- 

 siders that such selection often takes place in nature in the 

 struggle for life which all plants and animals must undergo. 



Professor Huxley advocates the hypothesis which sup- 

 poses the species living at any time to be the result of the 

 gradual modification of pre-existing species. Day (" British 

 and Irish Salmonidae ") says : " We must have a perma- 

 nence of variation from the original form to indicate a 

 distinct species." He goes on to say, " that owing to too 

 great importance having been given to inconstant variation, 

 the number of species among this family has been unduly 

 augmented, and varieties have been accorded specific rank, 

 while every little variety of form, colour, or structure has 

 also been reckoned as possibly demonstrating hybridity." 



The author of the article " Species," in the " Encyclo- 

 paedia Britannica," says : " The rash generalisation, that 

 distinct species are to be recognised by their incapacity 

 for the production of fertile hybrids, has been overthrown, 

 while closer study has cleared away the notion of the 

 equal definitiveness of specific forms. 



" The want of any absolute standard of specific difference 

 is largely made up by practical experience and common 

 sense, and the evolutionary systematists are less in danger 

 than were their predecessors of either exaggerating or un- 

 derstating the importance of mere varieties." 



The Salmonidce are described by Dr. Gunther as fol- 

 lows : " Body generally covered with scales, head naked, 

 barbels none. ' Margin of the upper jaw formed by the 

 intermaxillaries mesially, and by the maxillaries laterally ; 

 belly rounded; a small 'adipose fin behind the dorsal; 

 pyloric appendages generally numerous, rarely absent ; air- 

 bladder large, simple ; pseudo branchiae present. The ova 

 fall into the cavity of the abdomen before exclusion. 



Salmo. Body covered with small scales ; cleft of the 

 mouth wide, the maxillary extending below or beyond the 

 eye ; dentition well developed ; conical teeth in the jaw- 



