G. GENERAL NATURAL HISTORY AND ZOOLOGY. 215 



very few living representatives of what was originally the 

 prevailing character of the marine fauna of the silurian and 

 other epochs; and while now they occur only in the very 

 deepest water, they were formerly found crowded in the shal- 

 lower seas. The inquiry, therefore, suggested itself to the 

 professor as to the reason of this difference, and he makes the 

 suggestion that in the progress of the earth's growth we may 

 look to such displacement of conditions favorable to main- 

 taining certain low types as may recall most fully the adap- 

 tation to former ages, and that the deeper waters of the pres- 

 ent constitution of our globe possibly approximate the con- 

 ditions of animal life in the shallow seas of former ages as 

 nearly as any thing can, in the present order of things on the 

 earth. The depth of the ocean alone, he thinks, can place 

 animals under the high pressure which the heavier atmos- 

 phere of the earlier period afforded. But as such pressure 

 can not be a favorable condition for the development of life, 

 it is to be expected that the lower forms only will occur in 

 the deep seas. 



Other causes acting in the same direction are the decrease 

 of light in the greater depths, the smaller amount of free ox- 

 ygen, the reduced amount and smaller variety of nutritive 

 substances, etc. He does not think, however, that facts war- 

 rant the conclusion that any of the animals now living are 

 lineal descendants of those of the earlier ages, nor that we 

 may justly assume that the cretaceous formation is still ex- 

 tant, notwithstanding the similarity of forms. It would be 

 just as true to nature to say that the tertiary period is ex- 

 hibited in the tropics on account of the similarity of the mio- 

 cene mammalia and those of the torrid zone. Letter to Prof . 

 Peirce. 



PROFESSOR AGASSIZ'S PROPHECIES. 



The prophetic announcements of Professor Agassiz in re- 

 gard to the discoveries he intended to make during his pro- 

 posed deep-sea dredgings in the southern waters continue to 

 be realized, as we learn from a letter to Professor Peirce, 

 dated at Rio on the 12 th of February last. The weather had 

 not been favorable for dredging for some time ; but, a suita- 

 ble occasion presenting itself, the work was prosecuted for 

 one day, with very interesting results. 



