THE STABILITY OF SPECIES 



IF two crows be taken to an ornithologist and he 

 be told that one of them was caught in the 

 Himalayas while the other was captured in 

 Madras, he will not be able to tell which in- 

 dividual came from which area : in other words, the 

 crows of Madras resemble those of the Himalayas. 

 This, of course, is no unusual phenomenon. The same 

 may be said of the myna, the king-crow, and a great 

 many other birds and beasts. Yet the phenomenon is 

 a remarkable one if we take into account the facts of 

 variation. 



If several hundred thousand crows be collected and 

 carefully examined, it will be found that no two of them 

 resemble one another in all respects. This being so, 

 we should expect the crows of Madras to differ from 

 those of the Himalayas, since the two environments 

 are so dissimilar. We may say with tolerable certainty 

 that no intercrossing takes place between the crows of 

 the two localities : for these birds are stay-at-home 

 creatures, and do not wander far afield. In this case, 

 therefore, it is not intercrossing that has prevented the 

 origin of local races. 



A consideration of the main causes which conduce 

 to the stability of species may not be devoid of in- 

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