CYCLES OF REPRODUCTION 5 



Indian elephant which may not do so until after 15 

 years.* Man, as is well known, also takes some 12 to 

 15 years. 



2. SEASONAL CYCLES IN TEMPERATE REGIONS 



After the onset of maturity, seasonal reproductive 

 cycles are evident in most species. These are most clearly 

 seen in the animals living in temperate and sub-arctic 

 regions although, as will be shown, they are also common 

 in the tropics. 



In considering these cycles, it must be stressed again 

 that spring and summer are not the only seasons of 

 reproduction. There is no month of the year in which 

 some species of vertebrate does not regularly breed. 

 Indeed, at least one species braves the polar winter for 

 this purpose. This is the emperor penguin, which has 

 developed the astounding habit of laying, incubating 

 and hatching its eggs in the autumn and winter darkness 

 of the antarctic continent in temperatures ranging from 

 — 25° to —55° C.6^ By the following spring the young 

 have grown into downy chicks, and are ready to take 

 advantage of that season for their growth to adult stature. 



Nearer home, although spring breeding is an almost 

 universal habit among the amphibians and reptiles, 

 autumn is the breeding time of many British fishes, like 

 the bullhead, the salmon and sea trout, and the various 

 species of Coregonus. In winter other fishes spawn includ- 

 ing the halibut, the cod and the fresh-water turbot.ii^ 

 In explanation of these differences in habit it has often 

 been remarked that the breeding season of each species 

 is so timed as to give the maximum advantage to the 

 young during their early period of growth. For most 

 animals this appears to be true, although some cases are 

 not yet adequately explained. 



Clearly, the timing of a reproductive cycle to meet the 

 needs of a given species is something that can be achieved 

 by the operation of natural selection. Even to-day such 



