CHAPTER XVII 



VARIATION : CONTINUOUS AND DISCONTINUOUS. INTER- 

 BREEDING 



Paucity of information on variation in birds. The work of J. A. Allen. 

 Relation of variation to natural selection. Barrington's work. Colour variation. 

 Dimorphism. Mutation. Albinism and Melanism. Swallows. 



BEARING in mind the immense amount of work which 

 has been expended in recording observations on birds, 

 it is astonishing to find how little has been set down 

 with regard to the deeper problems of avian evolution. It 

 might have been expected that the stimulus to patient inquiry- 

 given by Darwin's magnificent work would have borne some 

 fruit among the vast army of those who profess an interest in 

 Ornithology. Yet the most laborious research among the 

 formidable array of books on birds which have been written 

 since the Darwinian theory was given to the world, yields but 

 a miserable reward. While splendid work has been done, and 

 vast sums have been expended, in hunting for new species, 

 there have been few who have availed themselves of this har- 

 vest. Those who are most enthusiastic in this task of heaping 

 up novelties, care least for the problems which these novelties 

 present. This need not surprise us ; but it is certainly strange 

 that the study of Ornithology should have produced no philo- 

 sophic mind bent on making use of such a wealth of material. 

 One might have expected, at least, that some attention 

 would have been paid to the phenomena of variation among 

 birds. Yet less has been brought to light on this subject per- 

 haps than any other. Though more than 19,000 species have 

 been described, and though the descriptions of every species 

 have entailed measurements, nowhere will there be found any- 

 thing more than the bald statement that such measurements as 

 are given are " average " measurements ; and generally even 

 this is not done. The range of variation is nowhere given in 



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