8 DEEP SEA FISHES. 
of time in localities far from one another without repeated migrations 
from one to the other. The possibilities of exact coincidence in the 
lines of variation of the same species in two localities or of convergent 
lines bringing different species to coincide are too remote to be con- 
sidered. In the case of an identical species discovered in widely sepa- 
rated localities it is safest to consider it to be in the line of its migrations, 
either upon its travels or a comparatively recent migrant at the time 
of its capture. An assertion of the existence of a non-migratory species in 
localities isolated by distance or physical barriers is not to be received 
without serious question. 
Given similarity in isolation, differences in degrees of plasticity not 
being taken into consideration, specific differentiation would probably be 
less active or less rapid in the low temperatures of the depths than among 
fishes near the surface, and in consequence it might be expected that con- 
ditions on the sea bottom would favor the existence of persistent types, 
that is of some of the fossil forms commonly designated as extinct types. 
That no such forms have yet been discovered among the deep sea fishes 
is in all probability due to the fact that the fossil forms were not them- 
selves deep sea fishes. Excessive amounts of lime in bones or armatures 
and great firmness and strength in skeletal structure characteristic of 
the fossils do not obtain in bathybial species of the present, and there 
are no reasons to suppose the earlier inhabitants of the abysses differed 
from them in these respects. Though the extinct forms may not have 
been of the depths it may be that, owing to retardation in the rates of 
differentiation by deep sea conditions, there is a likelihood that relatives 
exist in the abysses in possession of closer affinities than nearer the sur- 
face, such kindred being offshoots from the stem that produced the extinct 
forms rather than direct descendants of the latter. 
The decrease in the amount of heat, of light, and of oxygen, and the 
increase in the pressure encountered by the fishes on their way down to 
the great depths tend to reduce the activity and to prolong the lives of the 
individuals. From such conditions it is to be expected that as they descend 
beyond the influence of the sun and the seasons the deep sea fishes do 
not mature their eggs or develop their young so rapidly as their ancestors 
of the shoals or the surface were accustomed to do, that the periods inter- 
vening between the spawning times are lengthened and that the species 
gradually depart from the yearly recurrent extrusion period and become 
