SALMONID^. 59 



much in different specimens of the same species that it would be very unsafe to 

 base any specific differences upon such a slender substrata of fact. 



7. The form of the caudal fin in specimens of a given size, age, and sexual 

 development. Here again the locality which the fish inhabited must be taken 

 into consideration, for it has a great bearing upon the formation of the fins, being 

 required of a much greater strength in rapid streams than in such as are sluggish. 

 Age is no criterion as to size : and sexual development depends upon some, at 

 present, unknown factor, for it seems that temporary sterility is not absent 

 among the salmon. 



8. A great development of the pectoral fins when constant in individuals from the 

 same locality. Here sex has an undoubted influence on the size of this fin, it being 

 frequently most developed in the males. Variations in the size of fins do not show 

 sufficient differences on which to constitute species, while in the shape, more 

 especially of the caudal, we find them frequently varying with age, being more or 

 less emarginate in the young compared to what it becomes in the semi -adult and 

 adult stages. 



9. The size of the scales. Here the number of rows existing in a line from the 

 back of the adipose dorsal fin to the lateral-line certainly affords one of the 

 most constant characters, but the number along the lateral-line is much more 

 deceptive. 



Having enumerated the characters stated to be most constant among the two 

 first sub-generic groups in the genus Salmo, I arrive at the conclusion that most 

 of them are fallacious, and that too great a stress having been placed upon 

 them, many errors have resulted. The consequence has been that the number of 

 species has been unduly augmented, and local races having been accorded generic 

 rank, intermediate forms have not been searched for, but new ones constantly 

 hunted up. Thus the synonymy will be a cause of endless confusion to future 

 ichthyologists. Besides this every little variety of form, colour or structure, has 

 been accounted for by terming such hybrids. 



The proportions of different parts of the body vary with age among the 

 Salmonidce as they do in other fishes, and even more considerably in some 

 anadromous forms which have been subjected to unnatural retention in fresh 

 waters, insufficiency of food or sickness from any cause. While the head of the 

 male is usually longer than that of the female, although in barren males the 

 disproportion is not very considerable. 



A few remarks are here necessary upon hybridism among the Salmonidce, for 

 from our earliest authors on ichthyology down to the present period the existence 

 of such has been remarked upon, and of late years artificial propagation has 

 clearly proved that such may occur. Experiments in order to test this question 

 were instituted by Professor Rasch in 1867, and he found that the ova of the sea 

 and river trout are developed regularly, whatever forms were the parent ones, and 

 that the offspring were fertile. That of the ova of the charr fertilized by the milt 

 of the trout, thirty to forty per cent, were developed, but that many young fish 

 perished after being hatched. Trout ova, fertilized by the milt of the charr, only 

 gave ten per cent, of young, many of which were misshapen. Salmon ova 

 fertilized with trout milt yielded forty per cent, of young fish, but more if the 

 milt of the charr were employed. The ova of a hybrid between a trout and a 

 charr could not be fertilized by means of a trout milt. 



The foregoing facts are suggestive that a very close relationship exists between 

 the anadromous marine trout and the non-migratoiy form inhabiting fresh waters. 

 Thus we are informed that the anadromous sewin, or anadromous trout of Welsh 

 rivers, very commonly interbreeds with the brook trout, hybrids being the result. 



In the British Museum may be seen a beautiful series of these fishes presented 

 by the Rev. A. Morgan from the Rhymney, a few of which have been retained in 

 fresh water after the normal period for their descent to the sea had arrived. Some 

 have elongated bodies, others comparatively long pectoral fins ; and, again, hybrids 

 are said to be present between the sewin and the brook trout, a conclusion 

 seemingly arrived at because their teeth are more developed than in the sea-going 

 form, while their pectoral iins and colour approximate to those of the non-migratory 



