74 VERTEBRATE REPRODUCTIVE CYCLES 



the physiology of reproduction. Most of the analyses 

 that have so far been attempted relate only to one factor, 

 that of increasing light. However, there is also some 

 evidence of the manner in which the so-called psycho- 

 logical factors operate, and this also bears on the 

 problem. 



In the first place it is necessary to reconsider the original 

 experiments of Rowan in which he stimulated matura- 

 tion of the gonads of the junco by means of increasing 

 light in autumn. 159-1^2 ^t that time he thought that the 

 light /)er se could not be the critical factor involved, and 

 he suspected that the stimulus to the gonads came from 

 the extra exercise which the birds were able to enjoy. He 

 therefore carried out an investigation without the use of 

 extra light in which, by means of rotating perches, the 

 birds were kept awake and moving for hours after the 

 winter darkness had fallen, and again the result was an 

 increase in testis size over that of the normally sleeping 

 controls.i^i The experiment was repeated by Bisson- 

 nette,3i who used the starling, but neither this nor any 

 other species subsequently examined has responded in 

 like manner. However, although it has been impossible 

 to confirm Rowan's results, it is important to remember, 

 as always in this work, that a factor which is of critical 

 importance for one species may be unimportant to 

 another. Thus the stimulus of spring to a junco may be 

 through its increasing wakefulness (i.e. through the 

 increased activation of its proprioceptors) while to a 

 starling it may be through the increasing light itself. 



All the later attempts to analyse the effect of increasing 

 light have been made on the assumption that it is the light 

 itself which exerts the stimulus. In the first place 

 attempts were made to discover which wavelength is 

 most effective, and again contradictions arose. The 

 starling was found to react most to the longer waves of 

 red light and to be relatively unresponsive to green, blue 

 and ultraviolet,33 while the ferret was found to be most 

 responsive to the short ultraviolet waves, although it also 



