320 The Realm of Nature CHAP. 



of these characterise the subarctic zone and the higher slopes 

 of mountains. 



409. Animals and their Life Conditions. The life 

 conditions of some of the marine animals of most import- 

 ance from the physiographical standpoint, have already 

 been touched on ( 273, 279). Amongst all animals the 

 struggle for life is harder, or at least more apparent, than 

 with plants, the stronger hunting down and devouring the 

 weaker. Animals in their native haunts should therefore 

 be inconspicuous if they are not to attract the attention of 

 their enemies, or to arouse the suspicion of their prey. 

 Almost all fishes, and many caterpillars, rapidly assume the 

 colour of their surroundings. The hare and ptarmigan, 

 living amongst the brown heather of northern hillsides in 

 summer, are brown in fur or plumage, but in winter, when 

 the land is white with snow, their colour also changes to 

 white, and they remain inconspicuous in their new sur- 

 roundings. This periodical adaptation to environment, 

 which is common in Arctic animals, is one of the causes 

 which has led to the preservation of the race. Some 

 insects are so like withered leaves or twigs that even an 

 experienced eye is often deceived by them. Strange 

 resemblances have also been traced out between entirely 

 different species of animals ; and since the similarity is 

 always brought about by the weaker or inferior type assum- 

 ing the appearance of the stronger or superior, almost as if 

 of purpose to impose on enemies, it is called mimicry. 



410. Faunal Realms. Animals exhibit more marked 

 peculiarities of distribution than do plants. {.Similar forms 

 are usually, though by no means always, found in like con- 

 ditions. ) The fauna, or collection of animals, of each one of 

 the northern continents bears a close resemblance to that of 

 the others ; while the fauna of the three southern continents 

 are similar in a much less degree, and, as a rule, totally 

 unlike that of any of the northern. The most generally 

 accepted division of the Earth into realms occupied by 

 different faunae is that suggested by Dr. Sclater, shown 

 in Plate XIX. The names adopted for these divisions 

 or realms are the Palcearctic or Old Northern, the 



