Seasonal periodicity is one of the most striking phenomena in the physiology of reproduction. This annual rhythm is most marked in wild animals but is found more or less subdued in domestic species. Thus the wild rabbit, the vole and the wild fowl exhibit a distinct breeding season. The domestic rabbit, while capable of breeding throughout the year, under the best conditions, shows a slight seasonal variation in reproductive activity. Similarly, the domestic fowl passes through an annual cycle of egg production which can be correlated with the seasons (Whetham, 1933).
The results of recent experiments suggest that an important part of the mechanism which controls this seasonal cycle is variation in the daily exposure to light. Voles and ferrets have been brought into full oestrum during their normal anoestrous periods by increasing their daily exposure to light (Baker and Ranson, 1932; Bissonnette, 1932). Likewise...
