464 THE DISTRIBUTION OF ANIMALS 



of Entomology has sent biologists to the land from which it came to dis- 

 cover what factors kept it in check there. In some cases they have in- 

 troduced a natural parasite which has brought it under control. 



Land bridges, both past and present, which connect some of the 

 continents are used by animals as migration routes. The Isthmus of 

 Panama allows tropical forms from South America to move into Central 

 America and Mexico ; many monkeys have taken this route. It is known 

 from geological studies that Alaska was connected with Siberia rather 

 recently when the northern climate was much warmer than it is today. 

 At that time there was an extensive migration of both animals and plants 

 between the Old and the New World. As a result, the species and 

 genera are very similar in eastern Asia and parts of North America, 

 particularly the southern Alleghenies and the east coast. The rise of 

 the Rocky Mountains and the development of great western deserts have 

 changed life in the west far more than in the eastern part of the United 

 States. 



Migration of Animals 



Sporadic Migration. These are the most spectacular migrations and 

 create the most public interest. The snowy owl in North America mi- 

 grates south in great numbers whenever their natural food, the lem- 

 mings, become scarce in the arctic regions. Their appearance in the 

 United States always creates excitement. These owls have developed 

 the habit of feeding during daylight hours on the arctic tundra where 

 daylight lasts for months during the summer and where there are few 

 other animals to molest them. When they come south, they continue 

 feeding during daylight ; but they are very conspicuous and are slaugh- 

 tered by the hundreds by hunters and predatory animals. 



In Scandanavia the lemmings gradually increase in numbers until 

 they overpopulate the countryside. Many of them then start a mass mi- 

 gration. On this migration they swim rivers and cross mountains. 

 When they come to the ocean they plunge in and swim until they drown. 

 Some have referred to this as a mass suicide which they undergo as a 

 means of keeping their population under control, but it seems much more 

 likely that they set out in search of new places to feed and reproduce. 

 When they come to the ocean they attempt to swim it as they have swum 

 the rivers and perish in the attempt. According to a local legend, they 

 are headed for the lost continent of Atlantis where they supposedly went 

 in their migrations in the past. 



Annual Migrations. Annual migrations are of many kinds and oc- 

 cur in a great variety of animals. In mountainous regions many animals 

 move down into the valleys in the winter. We have already learned how 



