118 THE INFLUENCE OF INANIMATE SUEEOUNDINGS, 



glacier flea (fig. 29), Degeeria nivalis, two species of Sminthurus, 

 GJiionea araneoides, (fee. — their internal temperature is not that 

 of the ice ; nay, most of these creatures belong to the warm- 

 blooded races, who themselves can supply the deficient warmth. 

 Besides, the temperature of the air at the surface of the ice or 

 snow is by no means always below the freezing-point ; on the 

 contrary, often considei'ably above it. The fact that animals live 

 on or in the ice must not be taken as proving that an active 

 animal life is generally possible at temperatures below zero, or 

 that a rise in the temperature of the air or water from — 10° to 

 — 5° can be of any importance to animal life. A rise of tempera- 

 ture above the freezing-point of the water and of the juices of the 

 body must, on the contrary, have much effect on the animals ; 

 but, as we have seen, it has by no means the same effect on all 

 the animals that live together in one spot and exposed to the 

 same rise in temperature. 



The effect of a rising temperature on animals depends of 

 course on their nature; as this differs, so will they be differently 

 affected. This is familiar to all, and easily observed. Some 

 animals rouse from their winter sleep eai-lier than others ; some 

 at a low temperature remain rigid in morning torpor, while 

 others, roused to sexual activity, display their charms, and are 

 already busy, laying their eggs or bringing forth their living 

 young. In short, from the very fi'eezing-point, a rising tem- 

 perature begins to exert its stimulating influence on the vital 

 functions of every living thing up to the point where these 

 functions are at the highest possible stress under the optimum of 

 temperature, which, as is proved by the above-mentioned facts, 

 is different for every animal. If the heat rises above this 

 optimum, the effects are reversed ; functional activity is more 

 and more reduced, till at last a sleep like condition or heat- 

 coma precedes death, which ensues under too gi-eat heat. 



The optimum of temperature varies not only according to the 

 species, but in every individual, nay, in every organ of every 

 individual. The best, or at any rate the best known, example 

 of tliis is offered by the Infusoria. Eossljach, in the work 

 already referred to, showed that the rhythmical contractions of 

 the contractile vesicle always grew more rapid undei a rising 



