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THE AMEBIC AX NATUBALIST [Vol. XLII 



Dall says: 14 "As in mammals and birds so in Pectens 

 the same species in the northern part of its range is larger 

 than in the south unless its habitat is distinctly trop- 

 ical." 15 So too the slight excess of temperature, 3° 

 within the Mediterranean over that of the Atlantic in 

 corresponding latitudes may help to cause the dwarf 

 fauna within that basin. 



Mobius mentions 1 ''' that the same mollusks living on the 

 coast of Greenland and in the Baltic Sea are in the former 

 very large and in the latter small and thin-shelled; this 

 variation he attributes to the constant temperature in the 

 former case and the very great extremes in the latter. 



The dwarf faunas of the Black and Caspian Seas are 

 doubtless partly due, according to Forbes, 17 to the great 

 extremes in temperature which they experience between 

 winter and summer. 



5. Change due to Extremes in Depth of Water.— For 

 each organism there are certain limits of depth of water 

 in which it best flourishes ; outside of these in either direc- 

 tion there naturally results a tendency towards pauperiza- 



(a) Very Shalloiv Pools.— Semper 18 took specimens of 

 Limnaa stagnalis, hatched from the same mass of eggs, 

 and placed them in aquaria containing different volumes 

 of water. "All the animals were under equally favorable 

 conditions" (as to food, temperature, gases, etc.) "irre- 

 spective only of the volume of water which fell to each 

 animal's share; this varied at most between 100 and 2,000 

 c.c. ' ' The result showed that ' ' the smaller the volume of 

 water which fell to the share of each animal, the shorter 

 the shell remained. ' ' The number of whorls was the same, 

 four, but the average length of the shell in the 100 c.c of 

 water was j-ineh, while in the 2,000 c.c. volume it was 



