THE ENVIRONMENT AND REPRODUCTION 23 



early southern spring, the animals arriving at their 

 destination in October. In the spring conditions they 

 shortly bred again although it was only six months since 

 their last breeding season in the north. Following this, 

 they became adapted to the southern seasons, and bred 

 at yearly intervals in the southern spring. 



Such examples could be multiplied, and it is also 

 interesting to notice that a similar reversal is shown by 

 animals which normally breed in seasons other than the 

 spring. This is true, for instance, for autumn-breeding 

 sheep. Late in 1932, 21 pregnant ewes were transported 

 from England to Cape Province, South Africa, and they 

 lambed after arrival in January 1933. When the southern 

 autumn started in March they again came into breed- 

 ing condition, and they were mated in May. There- 

 after they bred only in the southern autumn. ^^o 



Following his surveys, Marshall has concluded that 

 the sexual periodicity of mammals in temperate regions 

 is governed by two main factors. The first is an internal 

 gonadal rhythm which is characteristic of the species, 

 while the second is that external environmental factor 

 which determines the precise time of breeding. Normally 

 these two forces act together, but when an animal is 

 transferred to the other hemisphere a conflict arises 

 between them. If the internal rhythm is unusually 

 powerful this may cause some disorganization during 

 the first year, but thereafter the animal becomes adjusted 

 to the new seasons. This shows clearly that the influence 

 of the environment is stronger than that of the internal 

 rhythm. 



In the birds, reptiles, amphibians, and fishes the same 

 general conclusion seems to apply. Many common 

 spring-breeding English songbirds have been taken to 

 Australia and New Zealand, and nowadays they all 

 breed in the spring of their new home. Lizards, frogs, 

 and fishes are also known to have experienced the same 

 change. 



