502 THE AMEBIC AN NATURALIST [Vol. XLVII 



50 fathoms in the Arctic Ocean; but the great majority 

 of the species occur below 1,000 fathoms, extending 

 downward to 2,900 fathoms. 



Now in forms confined to the deep sea, or to exceed- 

 ingly high latitudes, or subjected to widely varying tem- 

 peratures or salinities, or occurring in highly saline, 

 alkaline or acid water, or under unnatural conditions 

 generally, the geological age of the maximum virility of 

 the genera or families may be guessed by the amount of 

 difference between the chemical and physical surround- 

 ings among which they now live, and the conditions ob- 

 taining slightly below low-tide mark on a tropical coast 

 bathed by ocean water free from any admixture of fresh, 

 and containing the normal proportion, in amount and in 

 kind, of salts. Such forms as Artemia, occurring in salt 

 pans, Xiphosurus, Tachypleus and Carcinoscorpius 

 ("Limulus") occurring in more or less foul and muddy 

 situations, and the Elpidiidae, characteristic of great 

 depths, we therefore suspect of being relics of the earli- 

 est geologic times, representing what were once the domi- 

 nant types in their respective groups, possessed of such 

 vigor and adaptability that, forced by internal specific 

 pressure, due to increase in the number of the individu- 

 als, they were able to accommodate themselves to these 

 conditions. When these types began to wane new and 

 vigorous forms arose in the tropical littoral which extir- 

 pated them from all the more desirable locations, allow- 

 ing them to persist only in such unnatural situations as 

 those into which they had intruded when in the prime 

 of their vigor. 



There is not the slightest reason for supposing that 

 any markedly new animal type ever originated in the 

 deep sea, or under conditions differing much from those 

 found just below the low-water mark. 



If we are ever to discover any recent representatives 

 of such groups as the trilobites, the eurypterids, the Mas- 

 toids or the cystids we shall find them not in the tropical 

 or temperate littoral, but living under some highly ab- 



