ORGANIC EVOLUTION 49 



other pair of wings has become more or less 

 degenerated. In the whole order of flies the back 

 pair of wings is represented by a couple of insig- 

 nificant knobs. In the Strepsiptera, a sub-order of 

 beetles, the front-wings are similarly reduced, 

 being mere twisted filaments. Man}^ parasites, 

 such as fleas and ticks, whose mode of life renders 

 organs of aerial locomotion unnecessary, are en- 

 tirely wingless. The insects of small isolated 

 islands are also largely without wings, the propor- 

 tion of wingless species being much larger than 

 among insects inhabiting continents. This is due 

 to their greater hability on small land masses of 

 being carried out to sea and drowned, owing to 

 the feebleness and uncertainty of insect flight. 

 On the island of Madeira, out of the 550 species 

 found there, 220 species no longer have the power 

 of flight. 



Air-breathing animals — amphibians, reptiles, 

 birds, and mammals — have normally a pair of 

 lungs — a right one and a left one. But in 

 snakes and snake-like lizards, where the body is 

 very slender and elongated, only one lung, some- 

 times the right one, and sometimes the left, is fully 

 developed. The right ovary is likewise aborted in 

 all birds, the left one yielding all the eggs. The 

 swifts and frigate birds live almost their whole 

 lives long on the wing, and the legs of these birds 

 have grown so short and weak and rudimentary, as 

 a result of their constant life in the air, that they 

 can scarcely walk. The chimney swift is said 

 never to alight anywhere except on the sooty inner 



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