230 Zoological Society : — 



the young and mature fish of one species may be apparently greater 

 than between individuals of the same age but of two distinct species — 

 the laws according to which the changes in the external form proceed 

 from the young to the mature age appearing to be the same in the 

 different species, as far as our present experience goes. It has been 

 observed, in allied species of insects, that, whilst the perfect animals 

 are so completely alike as to be scarcely distinguishable, their larvae 

 are very different in their external characters, and even in their habits. 

 This is not the case with the Charrs : the young individuals of two 

 species differ as much from each other as the old ones. But in order 

 to find out the distinctive characters of two species, it is always 

 necessary to compare specimens of the same age. This can be ascer- 

 tained by the examination of the generative organs, by the develop- 

 ment of the jaws, and finally by comparison of a series of examples 

 from the same locality, assisted by actual observation or information 

 from persons who have been for years acquainted with the Charrs 

 of a certain locality, and know to what size they attain there. 



Among mammals and birds, difference in the size of full-grown 

 animals is admitted as a specific character, whilst ichthyologists have 

 scarcely ever used it as a distinction between closely allied species, 

 because numerous fishes continue to grow for an almost indefinite 

 period after they have attained to maturity. However, if we should 

 be able to ascertain for a series of fishes the age or the size at which 

 they^?'*^ attain to maturity, the differences observed might be of as 

 great value for the distinction of the species of fishes as in the higher 

 classes of vertebrate animals. I have been induced to make these 

 remarks by the fact (to which we shall recur in the progress of this 

 paper) that the Salmo alpinus of Scotland attains maturity at a size 

 inferior to that of an immature Swedish Salmo alpinus. Now, if such 

 a diffei'ence in the size should be considered as a specific character at 

 a future time, the Scotch and Swedish fishes would be separated. 



The specimens which I have examined since the publication of the 

 first paper are the following ; they have been deposited in the Col- 

 lection of the British Museum : — 



a. Salmo Salvelinus (L.), Nilss. 

 Diagnosis, taken from a male specimen from the Lake of Wettern, 

 1 7 inches long. — Body slightly compressed and elongate, its greatest 

 depth being contained five times and a half in the total length (to 

 the end of the middle caudal rays). The length of the head exceeds 

 the height of the body, being contained four times and a half in the 

 total ; it is rather more than one-half of the distance between the 

 snout and the vertical from the origin of the dorsal fin. The maxil- 

 lary extends beyond the orbit in the adult fish. Eye rather small, 

 its diameter being less than one-half of the iuterorbital space. The 

 length of the pectoral fin of the mature fish is equal to, or less than, 

 one-half of the distance of its base from the root of the ventral. 

 Dorsal rays fourteen* ; the length of its longest ray is much less 

 than that of the pectoral, and not much more than one-half of the 



* Including the rudimentary rays iu front of the fin. 



