THE ENVIRONMENT AND REPRODUCTION 21 



indicate that any one species ot bird, or any related 

 group of species, tends to breed at a certain time of the 

 year. In the northern hemisphere this may involve egg- 

 laying in any month from June in the arctic to January 

 on the equator, while in the southern hemisphere the 

 times are from December to June. Thus on the equator 

 there is a marked divergence of habit, with some 

 northern species breeding about January and some 

 southern species doing so about June. This would 

 appear to act as an effective bar to the interbreeding of 

 northern and southern birds, but clearly the equator 

 means nothing to an animal. It is only an imaginary line 

 which offers no obstacle to movement, and innumerable 

 species must have extended their range across it on 

 innumerable occasions. The existence of identical or 

 related species in both hemispheres is clear proof of this. 

 It follows that a bird which extends its breeding range 

 across the equator, and penetrates deep into the opposite 

 hemisphere must suffer a reversal of its breeding season 

 so that a species which typically breeds in May in the 

 north will come to breed in November in the south. 



Of course, as pointed out above, there is evidence that 

 such a reversal does not occur suddenly the moment 

 latitude o° is crossed. Species from the north may extend 

 some distance south of the equator while still retaining 

 their northern habits of breeding, and the case has been 

 quoted of a southern albatross which breeds in Hawaii 

 in the northern hemisphere at the time of the southern 

 spring. 10 



While such changes must often have happened natur- 

 ally during the spread of successful species, they have 

 also happened on many occasions due to man's inter- 

 vention. This has been particularly true during the last 

 hundred years with the settlement of South Africa, 

 Australia and ^[ew Zealand by emigrants from the 

 northern hemisphere, and with the foundation and 

 increasing popularity of zoological gardens. The settlers 

 in the southern hemisphere naturally took with them 



