DR. GUNTHER ON THE STUDY OF FISHES. 387 



to make the destroyer, with its victim, rise with increasing rapidity 

 toward the surface, which they reach dead or in a dying condition." 



It was also shown that, as the same species and genera are found in 

 the most distant parts of the globe, the deep-sea fishes are not limited 

 in their range, and consequently, as is admitted on other grounds, that 

 the physical conditions of the ocean-depths must be much alike all the 

 world over. That the deep-sea fishes are not of a peculiar order, how- 

 ever peculiarly organized, but are for the most part modified forms of 

 surface-types, was another conclusion arrived at from the scattered 

 evidence available before dredging at great depths was systematically 

 practiced, and a conclusion that has since proved to be right. Never- 

 theless, it still remained to ascertain more precisely the bathymetrical 

 horizons in which the different kinds lived, and this has been to some 

 extent attained by observations made during the voyage of the Chal- 

 lenger ; but these can not be received without further critical examina- 

 tion, for, unfortunately, no precaution seems to have been taken to 

 keep the mouth of the dredge closed, and therefore it is probable, if 

 not in some cases certain, that fishes were occasionally entrapped while 

 the machine was passing through the surface-water. On the other 

 hand, the majority of the examples taken in the dredge offer literally 

 internal evidence that they were inhabitants of the abysses, being so 

 organized as to be unable to live near the surface, and consequently 

 that they were captured at the greatest depth to which the dredge 

 reached, or nearly so. 



The physical conditions of the deep sea, affecting the organization 

 and distribution of these fishes, are thus formulated by Dr. Giinther : 



1. " Absence of Sunlight. Probably the rays of the sun do not 

 penetrate to, and certainly do not extend beyond, a depth of two hun- 

 dred fathoms ; therefore, we may consider this to be the depth where 

 the deep-sea fauna commences. Absence of light is, of necessity, ac- 

 companied by modifications of the organs of vision, and by simplifica- 

 tion of colors. 



2. " Phosphorescence. The absence of sunlight is in some measure 

 compensated. for by the presence of phosphorescent light, produced by 

 many marine animals, and also by numerous deep-sea fishes. 



3. "Depression and Equality of the Temperature. At a 

 depth of five hundred fathoms the temperature of the water is already 

 as low as 40 Fahr., and perfectly independent of the temperature of 

 the surface-water ; and from the greatest depths upward to about 

 one thousand fathoms the temperature is uniformly but a few degrees 

 above the freezing-point. Temperature, therefore, ceases to offer an 

 obstacle to the unlimited dispersal of deep-sea fishes. 



4. " The Increased Pressure by the Water. The pressure of 

 the atmosphere, on the level of the sea, amounts to fifteen pounds per 

 square inch of surface on the body of an animal ; but the pressure 

 amounts to a ton weight for every one thousand fathoms of depth. 



