32 VERTEBRATE REPRODUCTIVE CYCLES 



weeks they all did so, including those which had lambed 

 only ten weeks previously. 



It can therefore be concluded that, at least in some 

 autumn-breeding species, the seasonal reduction in 

 daylength is the critical factor determining the time of 

 onset of the breeding season. 



7. CONCLUSIONS REGARDING THE ACTION OF LIGHT 



Some points of primary importance stand out from the 

 above data. In the first place, it is evident that, depending 

 on the species, animals may come into breeding con- 

 dition as a reaction either to the increasing light of spring 

 or to the decreasing light of autumn. This shows clearly 

 that what an animal appreciates and responds to is not 

 the light per se but the change in the daylength. 



In the second place it is evident that, if reproduc- 

 tive cycles can be regulated by either increases or 

 decreases in daylength, it is probable that these methods 

 of control are merely superficial. ^^ Thus there appears to 

 be no theoretical objection to an animal, because of a 

 peculiar mode of life, adopting any suitable external 

 variant to condition its breeding cycle. There seems to be 

 no doubt that this has in fact happened, and other factors 

 which are already known, or thought, to influence 

 breeding in some species are described on p. 36. 



In the third place evidence has been mentioned that 

 suggests the existence of an internal physiological rhythm 

 with an approximate periodicity of one year. Although it 

 has been shown that this could not by itself maintain 

 indefinitely the regular periodicity of breeding seasons, 

 there is no doubt of its existence. It can be demonstrated 

 by experiments such as those with ferrets in which the 

 imposition of almost total darkness in winter and early 

 spring does not infallibly prevent the maturation of the 

 gonads, 110 and of those with minnows in which a similar 

 treatment did no more than delay gonad growth.*^ 



^he theory has therefore been put forward*^ that some 



