appendix: recent research ioi 



by maturation to the stimulus of natural or artificial 

 increases in daylength. 



However, the importance of increasing daylength 

 in inducing sexual maturity in the birds of temperate 

 regions has recently been questioned. Jenner and 

 Engels^^^ and Kirkpatrick and Leopold ^^^ have suggested 

 that the observed stimulus actually arises from the shorter 

 dark period. They have shown that, without increasing 

 the number of hours of light per day, a stimulus to 

 gonad maturation can be obtained by splitting the long 

 dark night period into two shorter periods. However, 

 Farner et al,^^^ Hammond^^^and MarshalF^^ have all 

 expressed doubt that the dark period has any active role 

 to play. Farner^o' has suggested that during the light 

 period there may be a very rapid synthesis of some 

 stimulatory substance which decays only slowly during 

 the dark period. His data suggest that only about one 

 minute in light is necessary for the synthesis of this sub- 

 stance to equilibrium, while several hours of darkness 

 are needed for its decay to minimum concentration. Thus 

 splitting the long dark period by only a short period of 

 light may induce a long-acting stimulus. 



It is evident that these modern results do not radically 

 alter the general theory of the environment control of 

 vertebrate reproductive cycles. It is unfortunate that no 

 proper analysis has yet been attempted of the reproductive 

 cycles of animals that breed at times other than the spring 

 or that are nocturnal or subterranean, and not nearly 

 enough work has yet been done on tropical species. 

 Until these deficiencies have been made good our know- 

 ledge of this whole subject must continue to be very one- 

 sided. 



3. the role of the hypothalamus 



As already indicated, whatever the nature of the 

 environmental factors that influence or control the 

 breeding cycles of vertebrates, their influence must first 



