U^' THE DlbTKLBLTIO^ OF THE GEAEUA. 365 



ON THE DISTEIBUTION OF THE GENEBxV. 



When dealing with species it was possible to divide the collection 

 approximately into two groups, one of which contained the deep sea forms, 

 the other those from the shoals or near the surface, but such a division of 

 the genera is not practicable, owing to the great majority of the deop sea 

 types themselves being congeneric with species only represented in the 

 upper waters. "While the closest affinities are necessarily to be determined 

 by comparisons of particular species or of varieties of particular species, 

 rather than of genera in their entirety, considerations of the last are not to 

 be ignored as if without bearings of importance. Compared with a o-enus 

 exclusively an inhabitant of the surface waters, another which dwells both 

 near the surface and in the depths is likely to possess the wider distribution 

 horizontally, as the surface genus may have its range limited by peculiarities 

 of food and of temperature, agents which are less variable and less restric- 

 tive in the depths. From this it follows that a genus, or species, of great 

 vertical range may possess a comparatively narrow horizontal range at the 

 surface and a much broader one in the abysses ; this is shown by fishes like 

 Careproctus, Paraliparis, Lycodes, or Merluccius, known in the higher lati- 

 tudes from both surface and bathybial waters, but found only at great 

 depths in the torrid regions. In the case of a genus well established at con- 

 siderable depths there is always a likelihood that its deep sea distribution is 

 greater than its range near the surface. The existence of the wider abyssal 

 ranges and the expectation of their probable determination by future re- 

 search farther toward the poles, closer to the surface as well as in the depths, 

 tend to deprive the deep sea genera of some of their importance in the solu- 

 tion of questions relating to origin and derivation, or to possible migrations 

 through a channel once existing between the Caribbean Sea and the Pacific, 

 or to others through a strait once dividing the Isthmus of Suez. In present 

 knowledge, at the best, it may be said that definite ccnclusions regarding the 

 sources of the Panamic deep sea fauna are not to be drawn from bathybial 

 fishes alone with any great degree of assurance. 



