EXTERNAL FACTORS ON THE SPAWN DATE 121 



reproduction in female amphibia: "The general impression is that a 

 complex pattern of external stimuli is necessary to induce clasping and 

 ovulation. . . ." How complex tliis pattern is will be shown in this 

 chapter. 



It is perhaps worth noting, before leaving the endocrinological 

 aspects, that almost all the experiments of the endocrinologists have 

 been concerned wdth the effects of temperature. It seems doubtful if 

 much progress will be made until more attention is paid to the factors 

 that field observations have shown to be important. Otherwise, much 

 time can be spent in a search for a link between the enviroimient and 

 the gonads in places where it does not exist. 



If Beach and Smith are right, as I think they are, we may perhaps 

 suppose that the gonads and level of the hormones are under the rough 

 control of a seasonal influence, which may be mainly temperature. 

 Nothing more accurate than this is needed to bring the animals into 

 breeding condition at about the right time, and indeed, nothing more 

 can be expected of this factor, which is a very rough guide to date. It 

 is not nowadays considered that temperature is a likely regulator of 

 breeding seasons, although before the work of Rowan in about 1926 

 (Rowan, 1938), it was the normal thing to attribute the onset of breed- 

 ing in an animal to the rising temperature. The reason for this belief in 

 the importance of temperature, which is still widely held, is obscure. 



In the climate of the British Isles, the lowest temperature may occur 

 in any of four months, and the highest in another four. The middle 

 ranges characteristic of the spring may occur in any month of the 

 whole year. An animal using temperature as a guide might therefore 

 start to breed at almost any date. It is of course true, that in parts of 

 the country where frogs breed in March or later, the temperature is 

 tending to rise, so that warm days and sequences of rising temperatures 

 often occur when the frogs breed. But this association is also character- 

 istic of any event in the spring. For example, the Chancellor of the 

 Exchequer emerges from No. 11 Downing Street to read his budget 

 on some date in April. Investigation would imdoubtedly show that he 

 tends to emerge on a day when the temperature is higher than it had 

 been lately, and after a sequence of rising temperatures. The longer 

 the investigation, the more certain would the conclusions become, 

 because they are in fact true. But the Chancellor is as unmoved by 

 temperature as he is by many other things. 



There is no great difficulty in making allowances for such trends by 

 9— (T.914) 



