468 The Descent of Man. Part II. 



that the males alone have been modified. Even in the ano- 

 malous cases of the Heliothrix and Mergus, it is probable that 

 originally both adult sexes were furnished — the one species 

 with a much elongated tail, and the other with a much elon- 

 gated crest — these characters having since been partially lost by 

 the adult males from some unexplained cause, and transmitted 

 in their diminished state to their male offspring alone, when 

 arrived at the corresponding age of maturity. The belief that 

 in the present class the male alone has been modified, as far as 

 the differences between the male and the female together with 

 her young are concerned, is strongly supported by some re- 

 markable facts recorded by Mr. Blyth, 5 with respect to closely- 

 allied species which represent each other in distinct countries. 

 For with several of these representative species the adult males 

 have undergone a certain amount of change and can be dis- 

 tinguished; the females and the young from the distinct countries 

 being indistinguishable, and therefore absolutely unchanged. 

 This is the case with certain Indian chats (Thamnobia), with 

 certain honey-suckers (Nectarinia), shrikes (Tephrodornis), cer- 

 tain kingfishers (Tanysiptera), Kalij pheasants (Gallophasis), 

 and tree-partridges (Arboricola). 



In some analogous cases, namely with birds having a different 

 summer and winter plumage, but with the two sexes nearly alike, 

 certain closely-allied species can easily be distinguished in their 

 summer or nuptial plumage, yet are indistinguishable in their 

 winter as well as in their immature plumage. This is the case 

 with some of the closely-allied Indian wag-tails or Motacillse. 

 Mr. Swinhoe 6 informs me that three species of Ardeola, a genus 

 of herons, which represent one another on separate continents, 

 are "most strikingly different" when ornamented with their 

 summer plumes, but are hardly, if at all, distinguishable during 

 the winter. The young also of these three species in their 

 immature plumage closely resemble the adults in their winter 

 dress. This case is all the more interesting, because with two 

 other species of Ardeola both sexes retain, during the winter 

 and summer, nearly the same plumage as that possessed by the 

 three first species during the winter and in their immature 

 state; and this plumage, which is common to several distinct 



5 See his admirable paper in the several distinct races, solely by 



' Journal of the Asiatic Soc. of comparing the adult males. 

 Bengal,' vol. xix. 1850, p. 223 ; see 6 See also Mr. Swinhoe, in ' Ibis,' 



also Jerdon, ' Birds of India,' vol. i. July 1863, p. 131 ; and a previous 



introduction, p. xxix. T n regard to paper, with an extract from a nota 



Tanysiptera, Prof. Schlegel told Mr. by Mr. Blyth, in ' Ibis,' Jan. 18C1, 



Blyth that he could distinguish p. 25. 



