4o8 GENERAL PRINCIPLES 



as adaptations. It is a phenomenon not confined to plants. 

 The pecuharities of parasitic animals or of sedentary animals 

 are also adaptations. In fact, the sum total of characteristics 

 of living things is an adaptation to conditions. At various 

 points in this book the idea has been expressed that an organ- 

 ism is what it is because of the conditions under which it lives. 

 For example, one often hears stated that foliage is green because 

 that color is least irritating to the human eye. The same idea 

 is expressed in other forms, but the position or point of view is 

 entirely false. The color of vegetation is part of the environ- 

 ment, and natural selection would result in the development of 

 eyes which were adapted to that color. 



830. There are many special types of adaptations which 

 are of interest because they give us an insight into the method 

 of operation of natural selection. We will call attention to a 

 few of these here. 



831. The dispersal larvae so often found among marine ani- 

 mals may be regarded as an adaptation by which the species 

 make use of oceanic currents to secure transportation from 

 place to place. The various devices employed by plants to 

 secure the scattering of their seeds fall in the same category. 

 On the other hand a free-swimming larva would be fatal to 

 many fresh-water animals because the larvae would be carried 

 to the sea and perish in the salt water. The eggs of very 

 many marine fishes are light and float freely in the water, but 

 the eggs of fresh-water fishes are either attached by means of 

 adhesive secretions or else are so heavy that they lie on the 

 bottom among the pebbles which protect them from the cur- 

 rent. Many fresh-water fishes make nests by excavating the 

 bottom and the eggs are often covered by a layer of pebbles. 

 Some marine fishes also attach their eggs or construct nests, but 

 the habit is not general. 



832. The eggs of the decapod Crustaceae are usually attached 

 to the abdomen of the female, but in the marine forms the 



