366 A HISTORY OF FISHES 



been equally abortive, and it seems certain that in a large 

 number of cases the individual judgment of the scientific mind 

 will always enter into the matter. This is inevitable, not only 

 in the matter of species, but also in the definition of larger units, 

 such as genera and families. In the main there may be said to 

 be general agreement on these points, but it sometimes happens 

 that a group of individuals regarded by one worker as a true 

 species will be looked upon by another as a mere race or variety 

 of another species ; similarly, two groups regarded as species of 

 a single genus by one, will be placed in distinct genera by 

 another. It is the practice of most systematists to unite two 

 species, formerly considered to be distinct, if intermediate 

 connecting links should subsequently come to light, and the 

 definition of a species given by the late Professor Dendy is 

 quite in accordance with the view that the destruction of the 

 connecting branches has played a large part in the separation 

 of those groups of individuals to which the term is applied. 

 7 "A species," he writes, "is a group of individuals that closely 

 resemble one another owing to their descent from common 

 ancestors, which has become more or less sharply separated 

 from all other co-existing species by the disappearance of 

 intermediate forms." 



It is one of the fundamental characteristics of living organisms 

 that no two are ever exactly alike, and it follows that even 

 within the limits of a species, and quite apart from differences 

 due to age, sex, and so on, there will be a greater or lesser 

 degree of individual variation. Ignorance of the wide range 

 of variation exhibited by some species often leads systematists 

 to describe as distinct species what are in reality nothing more 

 than extreme variations of a single form, and it is only when a 

 more complete series of specimens is studied and intermediate 

 forms come to light that such errors can be rectified. In the 

 case of the European Trout, to quote a characteristic example, 

 specific names have been given to the Phinock or Eastern 

 Sea Trout {Salmo albus), the Sewen or Western Sea Trout 

 {S. cambricus), the Great Lake Trout (S.ferox), the Loch Leven 

 Trout (S. levenensis) , the Brook Trout {S. fario) , the Gillaroo of 

 Ireland {S. stomachius), and the Welsh Black-finned Trout 

 {S. nigripinnis) , among others, but in spite of the diflferences in 

 size, form, colour, number of caecal appendages to the stomach, 

 nature of the vomerine teeth, etc., a complete series of transi- 

 tional forms has now been traced between the Brook Trout 

 and the anadromous Sea Trout, and most modern authorities 



