10 dej:p sea fishes. 



what higher, wliich 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 Raite, the Discoboles, the Pedicu- 

 lates, the Zoarcoids, the Brotuloids, the Myxinoids, etc., etc., 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 Oneoeephalus 

 porrectus, a close ally of 0. vcsjjertiUo 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 



