Correlation 



habits of the White-rumped Swallow ( Tachycineta 

 leiicorrhoa) given by Mr W. H. Hudson on p. 32 

 of Argentine Ornithology. He says that no 

 matter how many nesting sites are available, 

 there is always much fighting amongst these 

 birds for the best places. " Most vindictively," 

 he writes, " do the little things clutch each other, 

 and fall to the earth twenty times an hour, where 

 they often remain struggling for a long time, 

 heedless of the screams of alarm their fellows set 

 up above them ; for often, while they thus lie on 

 the ground punishing each other, they fall an 

 easy prey to some wily pussy who has made her- 

 self acquainted with their habits." 



We have already emphasised the importance 

 to many species of possessing the power of resist- 

 ing the effects of damp. In the case of some 

 organisms favourable variations in this direction 

 may possess a greater survival value than those 

 in the shape of greater speed or physical strength. 



Now, if there be any correlation between the 

 power of resisting damp and the colour an animal 

 bears, it is quite probable that animals of this 

 colour, whether or no it be conspicuous, are likely 

 to survive in preference to those who are more 

 protectively coloured. There is some evidence 

 that in certain cases, at any rate, resistance to 

 climate is correlated with colour peculiarities. 

 For example, some fanciers assert that yellow- 

 legged poultry resist cold and damp better than 



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