GENERAL DISCUSSION. i 
in distant localities it is seen that a certain degree of sameness runs 
through all of them. There 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 only 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 that 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 exceptional. 
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 
