THE ENVIRONMENT AND REPRODUCTION 33 



if not all vertebrates possess an inherent reproductive 

 rhythm, which is possibly of a more or less vague kind, 

 and which in normal circumstances is rendered more 

 precise in its time of action by the effects of the seasonal 

 variations of the environment. This internal rhythm 

 may be likened to that of an alarm clock which through- 

 out the day and night is a moderate or even a poor 

 timekeeper, but which is automatically corrected each 

 day by the rising of the sun so that the sounding of the 

 alarm is always precisely timed. 



It is now known that the strength of this internal 

 rhythm is not always constant, but that it may vary with 

 age and season. As an example it can be mentioned that 

 the gonads of the golden crowned sparrow are resistant 

 to the stimulus of an experimentally increasing day- 

 length until after the month of November. 1*1 This 

 conclusion is of importance because it suggests an ex- 

 planation for the peculiar problem of those migrating 

 birds which breed in one hemisphere but not in the 

 other. The common British swallow is a case in point. 

 When the northern summer is over it migrates south- 

 wards across the equator into the South African spring. 

 If the bird was at that time sensitive to the influence of 

 increasing daylength it would surely come into breeding 

 condition and nest again. That it does not do so must 

 be regarded as proof that it is then in a resistant state 

 similar to that of the golden crowned sparrow. The 

 gonads of such migratory birds actually begin to enlarge 

 early in the southern autumn as the observations of 

 Rowan and Batrawi have shown. ^^''' This cannot be in 

 any way related to increasing light, and it must be 

 presumed to be due to the action of the internal rhythm. 

 The birds then move north, and, as they penetrate into 

 the northern hemisphere, it appears that the increasing 

 daylength exerts its influence to bring them into full 

 breeding condition. 



