THE BASIS OF ADAPTATION 257 



cold, how hot, how wet or dry, or how rocky, some organisms can 

 be found cHnghig precariously to life, unless some deleterious 

 substance or influence prevents for a time. 



Barriers. When any of the factors mentioned at the beginning 

 of the chapter make it necessary or desirable for animals to seek 

 new homes, migration is apt to be one of the chief means of adjust- 

 ment. It may be accomplished through the animal's own powers 

 of locomotion with considerable rapidity, but in some parts of the 

 world there are insuperable barriers. The oceans are impassable 

 to terrestrial species, high mountain ranges are often so, and 

 climatic conditions in some regions may hinder the passage of 

 animals not adapted to them. In addition new contacts with 

 other species may hinder migration. 



Mountain ranges are effective barriers when parallel to the 

 equator. The effect of a few thousand feet of altitude on such 

 a range is equivalent to a change in latitude of many degrees. 

 "This is notably true," says Lull, "of the great Himalayan Range 

 in northern India, which rears its mighty summits far beyond the 

 limits of perpetual snow. On the south we have the hot, moist 

 plains of India, with a very distinct tropical fauna which in many 

 respects resembles that of Africa. North of the barrier, conditions 

 of climate, both in temperature and degree of moisture, are 

 entirely changed, and with them appear animals, with some 

 notable exceptions, of a totally different sort, more nearly com- 

 parable to those of Europe." 



The difference between these ranges and mountains that run 

 north and south, like the Sierra Nevada and Rocky Mountains, 

 is easily understood. The temperature fluctuations, and resulting 

 climatic conditions, at an altitude of eight to ten thousand feet 

 in the latitude of central Colorado are much like those at normal 

 levels far north of the United States. Consequently there is a 

 zone of similar climatic conditions extending gradually upward 

 from north to south in these mountain ranges, and migration may 

 be accomplished along this zone without sudden change. The 

 northern hemisphere contains several species of insects which 

 illustrate the condition admirably. Two of these, related to the 

 butterflies, are found in northern Europe, Asia, and North Americ?^ 

 In the central part of Canada they range southward nearly to th<» 

 international boundary, but in the mountains of the east and west 

 they extend far to the south. One has been taken at an altitude 



