SELECTION 147 



better development of some useful organ, and this 

 in time would become a generic character. Occa- 

 sionally members of more widely separated groups 

 are linked together in the same way ; as parrots and 

 woodpeckers by the feet, or hawks and owls by the 

 beak. But these similar adaptations are always 

 combined with other characters, which shew that 

 the animals are not really related. With genera, 

 however, it is different, for the species are closely 

 related, but have been combined into genera after 

 the species were formed. This explains why genera 

 are often more difficult to discriminate than the 

 species. 



Retrograde Development. Some cases of retro- 

 grade development are undoubtedly due to natural 

 selection. A good example is the loss of the teeth 

 in the upper jaw of the Sperm-whale. This cannot 

 be accounted for by disuse, for its ancestors must 

 have used the teeth of both jaws equally. But we 

 have an explanation in natural selection. These 

 whales feed largely on floating animals which they 

 catch by swimming on the surface of the sea with 

 the mouth open and the lower jaw hanging down. 

 In this position the teeth of the upper jaw would 

 be detrimental, for they would act like the cow- 

 catcher of a railway engine and prevent animals 

 from drifting into the whale's mouth ; while the 

 teeth in the lower jaw are useful for capturing the 

 prey. On the other hand, there are many cases of 

 retrograde development which cannot be accounted 

 for by natural selection. The degeneration in the 

 wings of the flightless duck of the Auckland Islands 



