330 ZOOLOOT. 



and heat (maximum) are unfavorable, and tend to cause 

 death. Tlie maximum for the European crayfish is 23° C, 

 for a phyllopod {Oheirocephalus braueri) 19° C, and for 

 the eggs of trout and salmon 6° to 7° C. (42° F.); beyond 

 these temperatures they die. 



Infusoria begin to reproduce at a temperature of from 

 4° to 7° C, but tlieir optimum is about 25° C. The extreme 

 limits of the optimum for the cockroach {Periplcawta 

 orientalis) is from 5° to 6° to 41° to 42° 0. The pond-snail 

 {LimncBU stagnalis) begins to feed and grow at 12° C, but 

 its optimum of growth is 25° C. (76° F.). Wlien LimnEeiB 

 are transported into a cold country where the temj^erature 

 does not reach the optimum, sexual maturity is reached 

 before the animal has attained its full growth, and there is 

 thus formed a dwarf race by simjale change of climate. 

 Hence it is for this reason, says Cuenot, that alpine and 

 arctic species are of very small size compared with those 

 of low lands in the temperate zone. 



When the temperature goes above the optimum, the 

 activity of animals decreases; hence the tanrec of Mada- 

 gascar, snakes, frogs, and lizards in Brazil, and in Northern 

 Africa around the Mediterranean Sea a great nunrber of 

 land-snails, spiders, insects, etc., fall into a torpid existence 

 (called aestivation) during the hot season, just as certain 

 of our northern animals hibernate during severe cold in 

 winter. Trout migrate in hot weather from the heated 

 surface of lakes to deep, cool water. In Europe they are 

 known to quit the streams wdiere the water is beyond 

 15° C, and ascend into cool and shady upper brooks. 



Sudden changes cause death, but if the change of tem- 

 perature is slow and gradual the animal may become 

 adapted to or acclimatized in a temperature relatively high. 

 Thus by warming the water and having it well aerated 

 Kegnard accustomed a goldfish to 39° C, its optimum in 

 England being 20° C. 



By thus raising the optimum, Dallingei' produced prac- 

 tically anew temperature-variety of infusorian(IIeteromita). 



