ADAPTATIONS IN ANIMALS 439 



very certain that mutations resulting in new and more 

 favorable adaptations or in the increased value of already 

 existing ones, are of the highest importance in perpetua- 

 tion of the race. 



Adjustment Adaptations in Animals. — In a broad 

 sense every adaptation which enables an organism to 

 maintain itself, beget young, and in some instances to 

 care for them, is in the nature of an adjustment to the 

 environment. The gills and fins of aquatic animals, the 

 lungs and legs of land dwellers, the fur, shells, spines, 

 scales, and other protective structures, digging, climbing, 

 and flying organs, and innumerable other adaptations are 

 in every instance of service to the species in adjusting it 

 to the surrounding conditions. In many regions an ani- 

 mal's surroundings are subject to marked changes with 

 periods of high and low temperature alternating. In 

 such a location many animals undergo a winter or sum- 

 mer sleep (Chap. XXII). Where the conditions are less 

 severe a considerable proportion of these same species, as 

 well as numerous others, are active throughout the year. 

 At the same time this last named class usually displays 

 different degrees of activity in response to day and night 

 conditions. The greater number of animals are active 

 during the daytime, and sleep or remain comparatively 

 quiet at night; other species reverse the process. In brief, 

 every species displays a characteristic series of activities 

 in response to heat, light, cold or moisture, or to the 

 nature of its food supply, and is continually adjusting 

 itself to the changing conditions of its environment. 



Plant Adjustments. — Not only are plants and ani- 

 mals adapted to live in a definite territory, but also, 

 within this range, they are continually undergoing minor 

 changes which serve to adjust them to shifting condi- 

 tions. Most plants, in moderate light, expose to the 

 fullest possible extent their leaves and flowers to the 

 sunlight, and some even shift their positions through- 

 out the day as the direction of the light changes. On 



