146 THE COURTSHIP OF ANIMALS 



this view appears, it ought, it may be argued, to receive 

 support from nestling plumages. Young gulls, for 

 example, should occasionally revert from the mottled 

 to the earlier striped livery. But we have no evidence 

 of this ; and it does not follow that this sequence of 

 events should occur. The conditions of control are 

 different. 



What exactly are the factors which govern the evolu- 

 tion of resplendent plumage is not known. But they 

 would seem to be more complex than was supposed. 

 That the primary sexual glands play an important part, 

 through the juices or " hormones " which they liberate, 

 there can be no doubt but these are only partial factors. 

 The " hormones " of the pituitary and thyroid glands are 

 also necessary contributors, controUing as they do both 

 fertility and the more superficial characters, such as colour 

 and ornament. Evidence, indeed, is slowly accumulating 

 to show that the problem of the behaviour of animals' 

 during the period of sexual activity, as well as the 

 peculiarities of structure and coloration which they 

 develop at this time, are all largely governed by the action 

 of these secretions. 



These, in their turn, are undoubtedly inhibited, 01 

 increased, by the control of the nervous system, though 

 this control is of course involuntary. This much seems 

 clear from the fact that birds will display when under 

 the excitement of fear, though the character of that 

 display is never the same as that in moments of sexual 

 exaltation. If the nervous system, through the eye, by 

 " suggestion," played no part, there could be no use for 

 display, but it is equally certain that for the realization 

 of the sexual activities a number of other factors have 

 to contribute. 



V 



