256 Influence of eyivironment on animals 



III. The Influence of Temperature. 



(a) The influence of temperature upon the density of pelagic 

 organisms and the duration of life. 



It has often been noticed by explorers who have had a chance to 

 compare the faunas in different climates that in polar seas such 

 species as thrive at all in those regions occur, as a rule, in much 

 greater density than they do in the moderate or warmer regions 

 of the ocean. This refers to those members of the fauna which live 

 at or near the surface, since they alone lend themselves to a 

 statistical comparison. In his account of the Valdivia expedition, 

 Chun^ calls especial attention to this quantitative difference in the 

 surface fauna and flora of different regions. "In the icy water of 

 the Antarctic, the temperature of which is below 0° C, we find an 

 astonishingly rich animal and plant life. The same condition with 

 which we are familiar in the Arctic seas is repeated here, namely, that 

 the quantity of plankton material exceeds that of the temperate and 

 Avarm seas." And again, in regard to the pelagic fauna in the region 

 of the Kerguelen Islands, he states: "The ocean is alive with 

 transparent jelly fish, Ctenophores (Bolina and Callianira) and of 

 Siphonophore colonies of the genus Agalma.'"' 



The paradoxical character of this general observation lies in the 

 fact that a low temperature retards development, and hence should 

 be expected to have the opposite effect from that mentioned by 

 Chun. Recent investigations have led to the result that life-pheno- 

 mena are affected by temperature in the same sense as the velocity 

 of chemical reactions. In the case of the latter van't Hoff had 

 shown that a decrease in temperature by 10 degrees reduces their 

 velocity to one half or less, and the same has been found for the 

 influence of temperature on the velocity of physiological processes. 

 Thus Snyder and T. B. Robertson found that the rate of heartbeat in 

 the tortoise and in Daphnia is reduced to about one-half if the 

 temperature is lowered 10° C, and Maxwell, Keith Lucas, and 

 Snyder found the same influence of temperature for the rate >vith 

 which an impulse travels in the nerve. Peter observed that the 

 rate of development in a sea-urchin's egg is reduced to less than one- 

 half if the temperature (within certain limits) is reduced by 10 

 degTees. The same eflect of temperature upon the rate of develop- 

 ment holds for the egg of the frog, as Cohen and Peter calculated 

 from the experiments of 0. Hertwig. Tlie writer found the same 

 temperature-coefficient for the rate of maturation of the e^^ of a 

 mollusc (Lottia). 



'^ Chun, Aus den Tie/en des Weltmeeres, p. 225, Jena, 1903. 



