10 DEEP SEA FISHES. 
what higher, which is suggestive of a possible but comparatively short 
vertical migration in the breeding season, by which deep sea fishes secure a 
slightly greater degree of warmth for their young. These fishes no doubt, 
like their shoal water allies, make journeys in the spawning seasons, to give 
their progeny a warmer temperature or to place the fry in feeding grounds 
especially suited to it. With the immense vertical ranges of many of the 
species in mind, pressure is not to be considered a factor of moment in 
vertical distribution ; temperature is a great deal more effectual. 
The nearness of its locality to the isthmus of Panama is an important 
element in an estimate of the value of this collection, because of the bearing 
upon the question of a sometime passage between the Caribbean and the 
Pacific. In regard to this, however, the testimony of the material is not very 
definite, for there is much less evidence of a connection across the isthmus 
among deep sea fishes than among those of the shoals. In fact the 
abyssal forms that favor the existence of such a thoroughfare are those like 
Centroscyllium, Antimora and others, which swim freely and are not confined 
to the bottom, but are distributed around the continent to both southward 
and northward; while those which may be cited as against the idea, or, 
rather, as disproving the existence of a crossing in times at all recent, are 
everywhere present in such groups as the Raiw, the Discoboles, the Pedicu- 
lates, the Zoarcoids, the Brotuloids, the Myxinoids, etce., ete., of the less 
migratory. The weight of the evidence goes to substantiate the theory of a 
gradual upheaval of the isthmus, permitting a connection in the shoals to a 
much more recent date than in the depths, which would allow the fishes of 
the shoal waters readily to pass across while presenting a barrier to those of 
the deep sea. If the relative measurements of barriers and abysses remained 
the same as at present, and the isthmus underwent a subsidence sufficient to 
admit of the passage of the shoal water species without going low enough to 
affect the species of the depths, conditions similar to those indicated by the 
evidence of the collection would again prevail. Perhaps even a tidal wave 
of extraordinary dimensions might accomplish in a very short time a large 
proportion of that for which we have to account. As noted below there are 
in the material at hand surface species, such, for instance, as Oncocephalus 
porrectus, a close ally of O. vespertilio and others from the Caribbean, evi- 
dently of recent derivation from ancestors common to both the Pacific and 
the Atlantic species. The testimony of these and similar forms is to the effect 
that in comparatively recent times, yet so long ago as to permit of great 
