REPTILES AND AMPHIBIANS 1 37 



the beginning of the organ of flight, and it was steadily- 

 improved. In this case the feathers afforded an easy 

 basis for the making of organs of aerial movement. In 

 the flying squirrel it was lateral folds of the skin ; also 

 in the flying reptile, in which they were further supported 

 by projecting ribs ; and in the flying fish the already 

 flattened breast-fins were converted into organs of flight. 

 The development of wings in the insects was very 

 different. In their case a small projection developed 

 at each side from the central body of the coat, and grew 

 into two horizontal plates ; with the central part of the 

 body these formed a sort of shield that carried them 

 through the air, like a parachute, when they leaped. 

 The insects were originally jumpers, as the lowest 

 species still are. But there are higher species, such as 

 the grasshoppers, that only use their wings for leaping, 

 so that we can well imagine that the insects that could 

 save themselves by the longest leaps away from their 

 enemy were those whose lateral plates were most exten- 

 sively developed. This use of the plates was fastened 

 on by natural selection for further development, and it 

 at last produced a joint by means of which the plates 

 could be worked backwards and forwards by muscles. 

 Thus the wings were made, and were modified in each 

 species in harmony with its vital conditions.^ 



Let us return from the past to the present. 



The actions of reptiles and amphibians are mainly 



^ Other experts think that the wings of insects have been formed 

 from what are called their trachea-gills. These are small articulated 

 plates in the abdomen of May-fly larvae that live in water, and serve for 

 breathmg. But the wings are situated right in the middle of the 

 insect's body, and so the theory given in the text is more probable. 



