FOOD, HIBERNATION AND MIGRATION II7 



from that for breeding, it is quite possible that they are of the same 

 type. For example, frogs might be guided to the pond by one 

 olfactory stimulus that began to be obvious at the beginning of migra- 

 tion, but, once there, might wait until another olfactory stimulus sets 

 them breeding. After all, a woodpecker seeking a nesting hole must 

 first of all fmd a tree, and before that a wood, very likely seen from so 

 far off that neither tree nor hole would be visible. These are three 

 different stimuli, but all of the same type. Frogs on migration enter 

 the pond at the point where they happen to be when they reach it. 

 Only after they are in the water do they gather at the spawning site. 

 I concluded from this that recognition occurred in that order — first the 

 pond, perceived from afar, then the site, perceived only in the water. 



The "Poikilothermous EfFect" 



It is unfortunate that zoology is burdened with a special term to 

 describe what is commonly included under the simple term "cold- 

 blooded," but there is no other convenient way of describing events 

 in an animal when its blood is not particularly cold, but has merely 

 varied with the temperature of the environment, in contrast with 

 homoiotherms, whose blood keeps at a constant temperature whatever 

 the temperature of its surroundings. 



In this chapter and in the next two, the activities of frogs under 

 different temperature conditions are described. In several places, I 

 have thrown doubt on the high importance attached by most authors 

 to temperature. This is sometimes because analysis of the figures does 

 not support the assumptions, but partly for a different reason, in which 

 the meaning of words plays a part. 



We ourselves are homoiothermous animals. We live, as it were, in 

 a thermostat of our own, adjusted to 37°C, whatever the outside 

 temperature, so long as we are in normal health. Physiological events 

 in our bodies are not at the mercy of fluctuating temperatures, and 

 take place on the whole at constant speeds. In the world of a poikilo- 

 therm, everything goes faster in hot weather. The animals eat more, 

 excrete more, their hearts beat faster, their muscles contract more 

 quickly and so on. There is probably notliing in their bodies that 

 does not act more quickly when the temperature is high. They have 

 a different time scale from ours, for more things happen in an hour 

 on a hot day than on a cold one. It would not do for a poikilotherm 

 to use an ordinary clock. 



