GENERAL DISCUSSION. 7 



in distant localities it is seen that a certain degree of sameness runs 

 through all of them. Tliere is similarity in all of the higher groups 

 and, a few new ones aside, the genera are either the same or very closely 

 allied ; it is oiily on reaching the species that distinctness is found to 

 be the rule. The bathybial fauna of one locality corresponds as closely 

 with that of each other as it might if one list had been made for all, 

 with allowance for occasional exceptions and for differences among the 

 minor divisions, that is in the species and the varieties. In comparisons 

 of allied species from different and distant localities, it is among the 

 forms for which nature has made the weakest provision in the way of 

 locomotive organs tliat the divergences are greatest ; those species of a 

 genus that are fixed to particular places by reason of inability to travel 

 distances of considerable length are most distinct, while those more able 

 to migrate are less divergent in their specific characters. Migrations, 

 possibly aided by the currents of the depths, widening the areas of dis- 

 tribution of particular abyssal species no doubt occur, but in most cases 

 they are limited in extent or the migrations proceed slowly through 

 long periods of time, since the individuals taken in localities perhaps 

 only a few degrees apart show marked differences as compared with 

 others of the same species and the occurrence of identical species in 

 localities separated by wide stretches of the ocean is really excej^tional. 

 Many so-called identical species from widely separated localities, as from 

 the Atlantic and the Pacific, have been recognized by conservative author- 

 ities, but subsequent comparative studies have led to such different con- 

 clusions and subdivided so many of the species that doubts are raised 

 as to absolute identity in any species said to occur in localities very 

 distant from one another. The idea that the same species might exist 

 in an abyssal depth under the equator and near the Arctic or the Antarctic 

 circle, the tellurian conditions being supposed to be the same in the 

 different localities, is not to be accepted unless applied to particular 

 migrants and limited in time. The existence of separated localities in 

 which all the conditions are identical is only supposable, not probable, 

 and even if they might exist, which is not to be admitted, the same con- 

 ditions affecting different individuals (varieties or species) differently 

 induce, directly through impress and indirectly through the effort occa- 

 sioned by it, different divergent tendencies in variation which preclude 

 the existence of the same variety or species for any considerable length 



