II. ADAPTATIONS IX ANIMALS AND PLANTS, 

 WITH EXAMPLES 



Adaptation 



IT is common knowledge that animals and plants respond 

 to their environment, that is, to the physical influences 

 exercised over them, and this gives them the power to 

 continue to exist. Many of these responses are purely 

 physiological and temporary, but some of them result in per- 

 manent structural change. Romanes states,^ in regard to 

 plants, "that it may fairly be doubted whether there is any 

 one species of plant whose distribution exposes it to any 

 considerable diff'erences in its external conditions of life, which 

 does not present more or less considerable difl^erences as to 

 its characters in different parts of its range." This is due 

 largely to the effects of climate, the chemical and mechanical 

 nature of the soil, temperature, intensity and duration of light, 

 moisture, and the presence of certain salts in the air, as well 

 as more unknown causes. Similarly, as a necessary part in 

 the maintenance of existence, every animal adjusts itself to 

 the surroundings. It must adapt itself to the food it finds, 

 the air it breathes, as well as the climatic conditions. In 

 consequence of these adjustments, modification with descent 

 causes them to be transmitted from generation to generation. 

 Of these vital adaptive characters, natural selection is pre- 

 sumed to be the important factor in their preservation. Those 

 characters of a supposed non-essential type, such as some 

 color markings on butterfly wings, due to variation, are supposed 

 to be maintained by heredity and aided by isolation. By 

 isolation, Romanes says, is meant "the prevention of inter- 

 crossing between a separated section of a species or kind and 

 the rest of that species or kind, whether such a separation 



1" Darwin and After Darwin," Part II, p. 206. 

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