40 VERTEBRATE REPRODUCTIVE CYCLES 



In all considerations of gonad growth the question 

 of food looms large. It is well known that if insufficient 

 food of the right kind is available animals may survive, 

 perhaps for long periods, but they do not breed. Whether 

 food supply ever acts as a trigger mechanism in deter- 

 mining the time of the breeding season is, however, 

 another question. Baker has suggested that it may do 

 so, and he has pointed for example to the possible effect 

 in an equatorial rain forest of the seasonal production of 

 fruits which so many animals eat.^^ However, there is 

 little trustworthy evidence on this point, and until more 

 observations have been made it is impossible to do more 

 than speculate. 



It appears from what is now known that there are few 

 if any parts of the world which lack some environ- 

 mental variant by which animals and plants can regulate 

 their breeding seasons. The only region that is perhaps 

 completely seasonless is the very deep sea. Light pene- 

 trates into the ocean to a maximum depth of about 

 3,500 feet, and below this all is absolute and perpetual 

 darkness. There are also no temperature changes, and 

 the water remains near to freezing-point. In these regions 

 it seems possible that fishes may solve the problem by 

 eliminating altogether their old habits of periodic breed- 

 ing. Of course, this need only be done by one sex. If the 

 female continued to breed in a cyclic manner the times 

 of ovulation could not remain seasonal and Would there- 

 fore become irregular, but if the male was always capable 

 of shedding spermatozoa the eggs could be fertilized 

 whenever they were laid. Alternatively, the problem 

 could be solved in another way which would provide an 

 explanation for the well-known existence of parasitic 

 males among the deep sea angler fishes. When males and 

 females become fused together in this way the rhythms 

 of tl\eir two systems can be synchronized so that matura- 

 tion in one of them is always accompanied by maturation 

 in the other. Then, however irregular the breeding 

 times, fertilization could always be achieved. 



