144 ESSAYS OF A BIOLOGIST 



either possibility of normal sex-behaviour, although 

 their nervous machinery is untouched. 



There are, further, some facts of observation which, 

 even if they have not yet been fully analysed by ex- 

 periment, still throw light on the matter. Although 

 many of the most familiar birds — fowls, pheasant, 

 peacock, duck, finches, and so forth — have bright- 

 coloured males and drab females, with marked differ- 

 ence of behaviour between the sexes, there are, as 

 we have seen, many others, such as herons, divers, 

 swans, grebes, moorhens, and auks, in which the 

 sexes are alike in plumage and furthermore show what 

 may be called a ' mutual ' courtship in which both 

 male and female play similar roles. In this latter 

 class it seems clear that the secretions of the male 

 and female reproductive organs must be more alike 

 than in the markedly dimorphic species : and this 

 is borne out by some strange facts regarding not 

 merely the courtship but the actions concerned with 

 pairing itself In the crested grebe and the little 

 grebe, for example, close observation has shown that 

 either member of the pair may assume the passive 

 * female ' attitude or the active * male ' attitude in 

 pairing : and in the moorhen we meet with the still 

 more extraordinary phenomenon of double pairing, 

 in which an act of pairing with male and female in 

 normal position is immediately followed by a second 

 act in which the normal position is reversed.^ It 



1 See Selous, '02 j Huxley, '14. 



