130 ESSAYS OF A BIOLOGIST 



bloodedness or efficiency of circulation, but also of 

 various psychical characters. The power of profit- 

 ing by experience becomes greater, as does that of 

 distinguishing between objects; and there is most 

 markedly an increase in the intensity of emotion. It 

 has somehow been of advantage, direct or indirect, to 

 birds to acquire a greater capacity for affection, for 

 jealousy, for joy, for fear, for curiosity. In birds 

 the advance on the intellectual side has been less, on 

 the emotional side greater: so that we can study in 

 them a part of the single stream of life where emo- 

 tion, untrammelled by much reason, has the upper 

 hand. 



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 Darwin, C, 71. "The Descent of Man, and Selection in 



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 Groos, K., '98. "The Play of Animals." New York, 



1898. 

 Howard, E., '20. "Territory in Bird Life." London, 



1920. 

 Hudson, W. H., '12. "The NaturaHst in La Plata" (5th 



Ed.). London, 1912. 

 Huxley, J. S., '14 and '23. (Courtship in Birds) Proc. 



Zool. Soc, 1914, and Proc. Linn. Soc, 1923. 

 Kirkman, F. B. (ed.), '10. "British Bird Book." Lon- 

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 Levick, G. M., '14. "Antarctic Penguins." London, 



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AthencEum. London, 21st April, 1923. 

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 Selous, E., '05. "Bird Life Glimpses." London, 1905. 



