144 ESSAYS OF A BIOLOGIST 



actual as behaviour it is necessary for the nervous 

 system to be activated by the secretion of one or 

 other of the reproductive organs. Castrated animals 

 fail to realize either possibility of normal sex-be- 

 haviour, although their nervous machinery is un- 

 touched. 



There are, further, some facts of observation 

 which, even if they have not yet been fully analysed 

 by experiment, still throw light on the matter. Al- 

 though many of the most familiar birds — fowls, 

 pheasant, peacock, duck, finches, and so forth — have 

 bright-coloured males and drab females, with marked 

 difference 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 fe- 

 male 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 extraor- 

 dinary phenomenon of double pairing, in which an 

 act of pairing with male and female in normal posi- 



