72 General Part. 



them in shape and colour and are thereby protected from the attacks 

 of Birds. Not a few moths (Olearwings) of which specimens live 

 in England are, in external appearance, extraordinarily similar to 

 Hymenoptera, which are protected by the possession of stings. 



7. The Power of Resisting Unfavourable 

 Conditions. 



Most cold-blooded animals can remain alive in spite of a consider- 

 able decrease of temperature, so long as the fluids in the body do 

 not freeze ; and in many the water in the tissues does not freeze even 

 if the temperature of the body sinks several degrees below zero. Some 

 animals are so constituted that they revive after being frozen through 

 and through ; although as a rule, the injury occasioned is so great that 

 death ensues. In a dry state, some animals can withstand a very 

 great fall of temperature, even to many degrees below zero; for 

 instance, small dried Nematodes have been exposed to a temperature 

 of — 19°, and have revived afterwards. Not a few forms are still 

 active at freezing point; others, however, are exhausted and 

 stupified, and many, long before it has sunk to this. The warm- 

 blooded Vertebrates, also, whose temperature under normal conditions 

 is fairly constant, may endure a considerable decrease of heat, but die 

 J-ong before freezing point is reached : Rabbits, whose normal heat is 

 31° or 32° C, die if it sinks to 15°. Hibernation forms an exception, 

 for here the temperature falls to within a few degrees of freezing, 

 without danger to life. 



Very often the outer layers of tie body are of sucli a nature that they serve 

 to protect the inner part against cold : fur, in many Mammals ; panniculus 

 adiposus (blubber) of Seals and Whales. To escape the winter cold, many 

 animals betake themselves underground (Earthworms), or, if aquatic, to the 

 warmer waters below the sui-face or to the mud at the bottom. 



Warm-blooded animals suffer more from a rise in the tempera- 

 ture than from a fall. As soon as there is a rise of a few 

 degrees above the normal they die. Protoplasm coagulates at 

 40° — 50° C, and therefore they cannot endure a higher temperature 

 than this. Possibly dried animals alone form an exception. 



When small pools dry up, their inhabitants seem to disappear ; 

 but when they fill again with water, the same animals usually re- 

 appear very soon. This depends, principally, upon the fact that many 

 eggs have a hard covering which resists desiccation. Many Protozoa 

 also can secrete a similar capsule ; more rarely the creature can 

 endure a real drying-up, a withdrawal of much water from the 

 tissues. This has been demonstrated for some forms ; a Nematode 

 {Tylenchus tritici) may, after remaining for a long time in a com- 

 pletely desiccated and shrivelled state, revive, when it is put in 

 water again, absorbing it into the tissues; this is true also of 

 many other small Nematodes, and for Eotifers and Tardigrada, 



