14 KENDALL: NEW ENGLAND SALMONS. 



series with very small scales; the rainbow (Salmo shasta) series with somewhat larger 

 scales; and the steelhead {Salmo gairdnerii) series with still larger scales. The latter com- 

 prises the form previously known as Salmo irideus, which is typified by the marine and 

 coastwise form. The rainbow-trout series comprises the mountain trouts of California 

 of the Sacramento and San Joaquin basins, excepting perhaps the Mount Whitney 

 region. It is possible that the red-throat-trout series permits of a subdivision, the finer- 

 scaled form comprising Salmo lewisii, virginalis, stomias, pleuriticus, etc., and a coarser- 

 scaled form, Salmo clarkii, etc. 



In endeavoring strictly to adhere to the aforementioned rules and practices, one 

 difficulty encountered is in ascertaining wherein certain forms constantly differ and 

 wherein they intergrade. It has long been the custom, and owing to insufficient material 

 necessarily so, to stereotype certain external characters. Naturally the most conspicuous 

 of these were made prominent in description. Some of these characters may show con- 

 stant differences or they may be comprised in those mentioned by GUnther, which 

 disappear with the removal of the cause, whatever that may be. Herein Ues another 

 difficulty: the difficulty of ascertaining whether or not certain characters wiU disappear 

 under changed conditions leads to different conclusions according to the decision regard- 

 ing them. Besides being one of convenience, the term 'species' has been said to be an 

 expression of ignorance; and it may be added that inabihty to distinguish species is 

 also due to ignorance. It is believed that the evolution of a species would progress 

 in an unchanged environment by interbreeding, thus transmitting with certain super- 

 specific characters, progressively developing new combinations of characters derived 

 from a large number of individual variations. Isolation according to its degree restricts 

 the transmission of individual characters to a smaller number of individuals, but by 

 selection emphasizes some one or more of these characters, either singly or in com- 

 bination. 



Among the salmonoid species are numerous geographical groups which exhibit various 

 combinations of characters, but apparently all infringe upon each other to such an 

 extent that it seems impossible, in some instances, to distinguish species. They seem to 

 be so interrelated, though in some respects different, that certain authorities do not 

 admit that these geographically distinct groups are distinct species. It would seem that 

 the significance of these groups cannot be understood until the reason of their existence 

 is at least better understood. The reason can hardly be found by studying the struc- 

 tures of the fish alone. It must be sought by considering geological history, not only in 

 respect to phylogeny but in its relation to its probable, or at least possible, past environ- 

 ment and distribution. 



Origin of Species of Salmonid^. 



The ancestral salmonids were marine forms which gradually acquired an anadromous 

 habit, and some of them, later, a permanent fresh-water abode. They had invaded every 

 accessible region suitable to their existence, which their present distribution and the 



