132 



SKXl'AL DI.MOIM'IIISM 



dapping were performed under the influence of a sexual 

 frenzy in which the bird would be regardless of pain or 

 injury. 



TrochUidce. The Humming-birds, Trochilidse, with the 

 CypselidcX' or Swifts, are now regarded by many ornithologists 

 as allied to the Pici or Woodpeckers, not as formerly classified, 

 to the Passeres like the Hirundinidce or Swallows. It is un- 

 necessary for the purpose of the present work to consider 

 the structual features on which this change of classification is 

 based. We have to consider only the sexual dimorphism 

 which is developed to such a high degree in the Trochilida*. 

 In these birds the beauty for which they are renowned is 

 chiefly possessed by the males, and consists in various special 

 adornments of plumage, such as feathers of extraordinary 

 length or shape, and tufts of feathers of the most intensely 

 brilliant metallic colours. 



Belt, in his Naturalist in Nicaragua, gives evidence of the 

 special display of plumage by male humming-birds while on 

 the wing before the females, and says that two males of 

 Florisuga mellivora, after such a display of the expanded 

 white tail, engaged in a fight. The important point for my 

 argument is that in the case of the humming-birds, as in so 

 many others, the special enlarged tufts of feathers are not 

 mere passive ornaments, but are definitely and specially 

 erected, and the irritation thus caused accounts for their 

 increased growth. 



Although many interesting points about humming-birds 

 are mentioned and discussed by Darwin, it is curious that 

 he does not mention the habits of the sexes in incubation 

 and care of the young. In one place he states that they are, 

 in the opinion of Mr. Salvin, polygamous. Other naturalists 

 whose works I have consulted are similarly silent concerning 

 the incubation. It was, therefore, with much interest that I 

 read a statement concerning this question in an article in a 



