12 VERTEBRATE REPRODUCTIVE CYCLES 



birds nor those of the southern birds change sharply at 

 the equatorial line. Sometimes they stop short of the 

 line and sometimes they overlap it. An example is 

 provided by the Laysan albatross which, when it breeds 

 north of the equator in Hawaii, lays its eggs at the time 

 of the southern spring like the rest of its kind in that 

 hemisphere, i** 



Finally, it must be pointed out that while much, if not 

 most, egg laying occurs while the days are lengthening, 

 many species do not follow this rule. It is evident 

 as Baker concludes, ^^ that neither a very long day, nor 

 yet a rapidly increasing day, is necessary for active 

 reproduction in birds, though, generally speaking, 

 ovulation is rare all over the world when the day length 

 is less than ii hours. Further discussion of these points 

 is reserved for the next chapter. 



4. THE EFFECTS OF DOMESTICATION 



So far the evidence put forward has stressed the 

 common rule that reproduction in wild animals is a 

 seasonal phenomenon. However, it is common knowledge 

 that domestic animals frequently depart from this rule 

 by breeding at almost any season of the year. This is 

 particularly obvious in the case of man himself, the 

 animal that invented domestication. The breeding of 

 man is continuous at all times of the year, although 

 some have searched for and have claimed to have 

 detected traces of an old seasonal cycle in statistical 

 analyses of monthly birth records.®^ It has often been 

 suggested that primitive man did experience an annual 

 breeding season in spring, and there are old reports, 

 now largely discredited, that the Eskimo still experiences 

 one to-day. 



Whether early man was a seasonally breeding animal 

 or not, his newly acquired habits of living in caves and 

 houses, of using fire for added warmth and light, and 

 of storing food against a famine, have protected him 



