126 ELEMENTARY ZOOLOGY. 



Some are highly adjusted to light; others less so. Some 

 are peculiarly adapted to high, and some to low, tempera- 

 tures. Some are able to capture their food actively, 

 others must take merely such as is brought by the cur- 

 rents in which they lie. There is thus a specialization in 

 adjustment, just as there is in other respects. 



Again, environmental conditions are continually chang- 

 ing. Food, temperature, moisture, and all the rest are 

 variable. Animals must be able to adjust themselves 

 not merely to the forces themselves, but to the changes 

 as well, or they must be prepared to take the consequences. 

 The changes may be daily or annual, may be irregular 

 and sudden, or may be gradual and extending through 

 long ages; but if there are changes in these real condi- 

 tions of life, all animals must meet them in some way. 

 When conditions change, the organism must either change 

 sufficiently to suit them, or migrate and thus regain its 

 accustomed conditions elsewhere, or die. 



140. A Classification of Adaptations. In a broad 

 sense all the structures, powers, instincts, and habits 

 which any animal has are adaptations to some external 

 conditions which has power to influence its life. For 

 example, the digestive tract is an adjustment very 

 varied in different organisms to the existence and 

 nature of food and to the services to which it may be 

 put; the nervous system and the special senses are ad- 

 justments to light, to chemical conditions, to contacts, 

 and other external stimuli; the lungs and gills are adap- 

 tations to the nature and need of oxygen ; the organs of 

 locomotion are adjusted to the pull of gravity and to the 

 resistance of the medium in which the animal lives. 



In addition to these fundamental adjustments there 



