THE BREEDING BEHAVIOUR OF THE FROG I75 



invalidated the truth of the hypothesis that it was the absence of eggs 

 that was the real stimulus. When watching normal oviposition in tliis 

 species, it was most striking to see the phase of behaviour that usually 

 preceded oviposition repeated at the end of the whole process (which 

 in tliis species goes on for half an hour or more), but with no emergence 

 of eggs, hnmediate dismissal followed as if by magic. I think we 

 have here an example of the too vigorous use of Occam's razor. In 

 other directions, these authors are above criticism of this kind. For 

 example, they admit that the total dismissal stimulus is a complex of a 

 number of separate stimuli, and have not fallen into the error of 

 supposing that, having found one effective sign-stimulus, there camiot 

 be another. 



In fact, it has been found that in general an animal attains the 

 accuracy of reactions it needs by requiring for full release several 

 different sign-stimuh. It is possible to express this mathematically. 

 Let us suppose that a frog needs to recognize its mate with something 

 like complete certainty. Now let us suppose that one sign is the sex 

 croak, but that in a crowded pond, mistakes are so easy that he may 

 be wrong one time in five. Now suppose that the girth of the female 

 is another sign, with a probability of being wrong of one in ten, for all 

 male frogs are not the same size. Next suppose that a third sign is the 

 warning croak, with a chance of being wrong of once in six. Proba- 

 bihties are combined as their products, so that the probability of all 

 these signs being wrong at the same time is only one in 300, and this 

 might be near enough to certainty to serve the purpose. Note that tliis 

 accuracy has been reached by the use of only three very imperfect 

 signs. Of course this is just what we do ourselves — we do not usually 

 recognize other people or things by one infallible sign, but by the 

 combination of a number of signs, none of which may be quite 

 distinctive, but wliich, together, tell us at once what we want to know. 



The American workers clearly distinguished between the unnatural 

 extrusion of eggs that results from injection of hormones in the absence 

 of the male, and natural oviposition. They used hormone injections 

 when they wanted to observe, but found that although eggs were 

 extruded from injected females, there was no normal behaviour in the 

 absence of an excited male. I share their opinion that Waring, Land- 

 grebe and Neill (1941) were not witnessing normal oviposition in R. 

 temporariay but merely extrusion of eggs. 



I think it is, however, now possible to see the part played by the 



