The Problem of Adaptation 17 



the animal ; but if it is true that the same change takes 

 place in some of the lizards that live in the tropics, where 

 the prevailing color is always green, it would appear that 

 the result may have no direct relation with the surroundings. 

 It has been shown in a number of well-authenticated cases 

 that the pupae of certain butterflies vary in color within 

 certain limits in response to the color of the background. 

 When the caterpillar fixes itself to some surface, and there 

 throws off the outer skin, and acquires a new one, the color 

 of the latter is influenced by the background. The result is a 

 better protection to the pupa. The change is not brought 

 about through the ocelli or eyes, but through the general sur- 

 face of the skin, for the same change takes place when the 

 eyes have been previously covered with a dark pigment. 



The growth of plants toward the light may be looked 

 upon as an adaptive process, since only in the light can they 

 find the conditions necessary for their life. The extraor- 

 dinary elongation of shoots and young plants when grown 

 in the dark may also be considered an adaptation for finding 

 the light, since in this way a plant, deeply embedded in the 

 ground, may ultimately reach the surface. Thus while the 

 actual process of elongation in the dark is not in itself of any 

 use, yet under the ordinary conditions of its life, this response 

 may be of great benefit to the plant. 



The closing together of the leaves of some plants has been 

 supposed to protect them from too rapid radiation of heat, 

 and incidentally this purpose may be fulfilled; but since 

 some tropical plants also close their leaves during the night, 

 it can hardly be maintained that the closing has been 

 acquired for this purpose. It has been suggested that the 

 opening of certain flowers under certain conditions of light 

 is connected with the visits of insects that bring about cross- 

 fertilization. 



The preceding examples will suffice to give a general 

 idea of what is meant by adaptation in organisms. That 



