8 VERTEBRATE REPRODUCTIVE CYCLES 



tuatara lizard of New Zealand. so The eggs are deposited 

 in November, in the southern spring, but the young 

 tuataras do not emerge until the December of the 

 following year after an interval of 13 months. Among 

 amphibians it is also known that salamanders commonly 

 practise a form of delayed fertilization similar to that of 

 bats. Copulation takes place during the summer, and the 

 spermatozoa are then stored by the female until they are 

 required for the fertilization of the eggs in the following 

 spring. 



3. THE EFFECTS OF LATITUDE 



It has been suspected for a long time that the further 

 north one goes from the north temperate regions, or the 

 further south from the south temperate regions, the 

 later in the year is the onset of the breeding season in 

 any particular species or genus. Conversely, the further 

 one goes towards the equator the earlier does breeding 

 commence. There is evidence that this affects all classes 

 of vertebrates, but the only detailed investigation so far 

 made refers to the birds. This was the work of Baker^i 

 who set out to discover how much earlier the breeding 

 seasons become as one goes away from the pole, and also 

 how far towards the equator this tendency is maintained. 

 A further question that he considered was the manner in 

 which breeding seasons cross over from one hemisphere 

 to the other. 



This study had to be restricted to a consideration of 

 egg seasons, the months when living eggs (fresh or 

 incubated) are to be found, since most information is 

 available on this point. In the first place an attempt was 

 made to discover whether for each latitude there are 

 particular times of the year in which birds breed, and 

 the evidence is summarized in Fig. i. From this it can 

 be seen that in 60° - 70" N. June is the optimum month. 

 From 3o''-6o° N. it is May, but in 20^-30° N. the egg 

 season spreads out. Just north of the equator all that 



