40 GEOGRAPHICAL DISTKIBUTIOST. 



fallacy most ornithologists are now agreed. It is a well-ascer- 

 tained fact that the vast majority of birds are migrants to a greater 

 or less degree, and that non-migration with this class of animals 

 is much more of an exception than the rule. Yet, by reason of 

 their peculiar covering, birds generally, as compared with other 

 vertebrates, are but slightly afiected by extremes of either heat or 

 cold, and indeed, as far as we are capable of judging, by most 

 climatic influences, provided only that their food-supply is not 

 ailected thereby. The condor in its aerial flight within a few 

 minutes of time accommodates itself to the most varying climatic 

 conditions, the change from the freezing cold of the mountain 

 heights to the scorching heat of the tropical lowland plains seem- 

 ingly having no effect upon the vigour of the bird. There can be 

 but little doubt, as has been insisted upon by Professor Newton, 

 that a deficiency in the food-supply — ^the necessity for searching 

 for new food — is the most obvious cause or impulse promoting 

 bird migration. Migrations of a somewhat similar character, in- 

 disputably governed, at least in part, by considerations connected 

 with the food-supply, but also in greater part by conditions of 

 climate, manifest themselves among several other classes of ani- 

 mals. Thus, in India, the monkeys habitually ascend the Himalaya 

 Mountains in summer to elevations of ten or twelve thousand feet, 

 and again descend in winter. Semnopithecus schistaceus has been 

 observed at a height of eleven thousand feet, leaping in fir-trees 

 laden with snow wreaths 1 Wolves in severely cold weather descend 

 from the mountain-slopes to the lowlands, and bears not infrequently 

 migrate in great numbers to escape the rigours of an extreme winter. 

 The migratory instincts of the northern hares and sqiiirrels, and 

 more particularly of the Norway rat and lemming, which in severe 

 winters move in amazing numbers in direct lines over lake, river, 

 and mountain, overcoming all obstacles that might be placed in 

 their path, are well known. The Kamtchatka rats, under the 

 pressure of numbers, are stated by Pennant to travel westward for 

 a distance of eight hundred mUes or more. Similar instances of 

 the force of migration are presented by the hoofed animals. The 

 vast herds of moving buffalo were until recently familiar sights to 

 the traveller on the American plains ; in South Africa countless 

 numbers of antelope, impelled by the necessities of food-supply, 

 pour down upon the more favoured districts lying without the 



