366 Survey of Fishing- Grounds, West Coast of Ireland, 1890—1891. 



Now, if the occurrence of northern littoral forms in the deep-water of the 

 southern region were the only instance of a cliange of vertical habitat in relation 

 to latitude, it might readily be assumed that the temperature was the consideration 

 which drove the northern forms into deeper water, since, as is well known, the 

 abysmal regions are practically unaffected by the surface temperature. Hence 

 we might conclude that such-and-such a species was intolerant of great heat, and 

 varied its vertical habitat accordingly. 



But, although there may be a measure of truth in this theory, the occurrence 

 of southern littoral forms only in the deeper parts of more northern seas, prevents 

 us from accepting it as an entirely satisfactory explanation, and suggests the idea 

 that it may be an equability of temperature throughout the year which is the 

 desideratum, rather than any particular degree of heat or cold. 



There is, however, an interpretation which appears to us infinitely more 

 probable than any that can be deduced from conditions of temperature alone, and 

 that is, that the struggle for existence drives the feeblest forms to the least 

 desirable localities. The principle can, of course, be only very broadly 

 interpreted, since not all the northern forms are feeble, nor do all the deep-sea 

 fishes show signs of being descended from ancestors which were feebler than those 

 which maintained their position nearer the shore. 



It is, indeed, not necessary to assume that the bathybial forms of the present 

 day have descended from fish which differed to any appreciable extent from their 

 more successful competitors, since we know that in an aquarium there will always 

 be found, among a number of fish of one species, which are to all appearance 

 equally capable of taking care of themselves, some which manage to get most of 

 the food, and some which fail to obtain their share, and so fail to grow at the 

 same rate as the others. The reason, no doubt, lies in some congenital defect 

 (whether mental or physical) which is not apparent to our intelligence ; and we 

 may assume that, in order to maintain an existence at all, such individuals 

 migrated into regions of less competition, and (the process of evolution conforming 

 to the changed conditions of environment) begat a race of descendants which 

 ultimately came to exhibit characters of a degenerate order, if judged by the 

 requirements of littoral life. 



We do not, of course, claim originality for this proposition, which will be 

 recognised as practically a recapitulation of Moseley's theory of the evolution of 

 the bathybial fish of the present day, but we suggest that the resemblance of the 

 Arctic to the bathybial fish-fauna is explicable on the ground that it has been 

 brought about by precisely the same cause, viz. a migration to a more barren 

 region. The resemblance disappears when we come to characters which have a 

 distinct relation to bathybial requirements, such as the reduction of the eyes and 

 the development of luminous, and of the presumably tactile, organs. 



