THE MIGRATIONS OF MAMMALS. 239 



Cousin Man has been good enough to sow the nutritive grain for 

 him this year also. Or, about the same time, a band of long-tailed 

 monkeys, under not less excellent leadership, approaches the edge of 

 the forest in order not to miss the right moment for a profitable, 

 and, as far as possible, undisturbed ravaging of the fields. When 

 the golden orange glows among the dark foliage in South American 

 plantations, the capuchin monkeys make their appearance, often 

 from a great distance, to share the fruit with the owner. Other 

 plant-eaters too are led by the hope of gaining an easier livelihood 

 into regions and districts which they usually avoid; insectivores 

 periodically follow the insects when they are for the time abundant 

 at this place or that, and large beasts of prey keep in the wake of 

 herbivorous mammals, especially of the herds belonging to man. The 

 lion journeys from place to place, following the wandering herds- 

 men on the steppes of Africa; Russian wolves followed close on 

 the retreat of Napoleon's defeated army, pursuing the unfortunate 

 fugitives as far as the middle of Germany. Otters undertake 

 land journeys to get from one river-basin to another; lynxes and 

 wolves in winter often traverse very wide stretches of country. 

 Such journeys bring about a change of residence, but they do not 

 constitute a migration in the true sense of the word. It is only 

 exceptionally, too, that they are undertaken from real necessity, 

 which we must look upon as the cause of all true migrations; in 

 most cases they are undertaken simply to gratify a passing desire. 



Quite otherwise is it with those mammals which, every year 

 about the same time, leave their habitat for some other region often 

 far distant, from which at a definite time they will return to their 

 former abode. These migrate; for they do not seize a chance oppor- 

 tunity, but obey, consciously or unconsciously, a compelling necessity. 



The fundamental cause of all true migration among mammals, 

 is some very distinct and decided seasonal change. In countries of 

 everlasting spring true migrations do not take place, for want is 

 never imminent. Summer must contrast with winter, whether the 

 latter bring frost and snow, or heat and drought; scarcity must 

 alternate with superfluity before the sluggish mammal makes up its 

 mind to migrate. 



