502 



GENERAL CONSIDERATIONS 



In the northern part of this country the monarch butterfly migrates 

 southward each fall and northward in the spring. Fish find in continuous 

 bodies of water little to impede their movement and also make extensive 

 migrations. Some fish live in the sea and migrate up rivers to lay their 

 eggs; examples are the salmons, in the case of which the temperature of 

 the water is probably a directing factor. Other fish, such as the eels, 

 which live in rivers, migrate to the sea to spawn. European eels migrate 

 to a region in the Atlantic Ocean southeast of the Bermudas. The 

 adults of both the salmons and the eels die after spawning, so an individual 

 makes but one round trip in its lifetime, but this may cover several 



Snow £/" fee 



- SnovyLine 

 Mosses &" Lichens 

 low Herbaceous Vegefation 

 Tree Line 



Fig. 309. — Diagram to show the correspondence between the vertical life zones met 

 in ascending a tropical mountain with a permanent ice cap and the horizontal zones encoun- 

 tered in traveling from the base of the mountain to the pole. 



thousands of miles. Other fish living in rivers migrate to the headwaters 

 to spawn. Birds, with the ease of locomotion they possess, have the 

 longest migration routes known. Among mammals may be noted the 

 irregular migrations of the Scandinavian lemmings from the mountains to 

 the lowlands near the coast, migrations which have been recorded since 

 the beginning of historic times. Another example is the American bison, 

 which formerly migrated regularly in the spring from the winter pasturage 

 in western Texas and New Mexico to the summer pasturage and breeding 

 grounds in the Dakotas and Montana, returning southward in the fall. 

 574. Altitude. — Altitude has a marked effect on animal distribution 

 through the varying climatic conditions produced. Even in the tropics 

 high mountains may exhibit vertically a series of climates corresponding 

 to those found in passing from the tropics to the poles (Fig. 309). At the 

 base of such mountains is a tropical climate with only the seasonal 



