24 THE ZOOLOGIST. 



* Mammals,' p. 475). The immense number of eggs laid by some 

 fish, and the amazingly rapid increase of some lowly animals, are 

 well-known facts. Each species has its own place in nature, and 

 produces sufficient offspring to keep that place filled. But how 

 this is regulated is another matter. We are sure that individuals 

 are quite unconscious and regardless of the requirements of their 

 species. Probably the food-supply itself is the chief factor, 

 increasing fertility in times of plenty, and checking it in times 

 of scarcity. 



With birds is it not mainly the food-supply which confines 

 the breeding to a certain season ? Can it be supposed that our 

 insectivorous summer visitants usually nest only once in the 

 season because they feel that the time for migration is approach- 

 ing, and a second nest is therefore useless ? I understand Mr. 

 Davies to suggest this. These birds leave us partly because the 

 supply of insect-food is running short, and partly because a 

 mighty impulse drives them to go. But they cannot be conscious 

 weeks beforehand that the time for their departure is drawing 

 near. If Finches as a rule go in for a second family, I would 

 suggest two possible reasons, though I do so with diffidence, for 

 I feel that I have not sufficient data as evidence for them. (1) Do 

 not our resident Finches as a rule begin to nest earlier than the 

 migratory Warblers, and so get the start of them ? (2) If the 

 particular food needed for feeding young birds is decreasing, the 

 parent Finches can provide their own sustenance in the form of 

 seeds, and so they will not need to draw upon the insect-food to 

 such an extent as Warblers. Moreover, young Finches soon 

 become capable of digesting seed. Nature as a whole keeps 

 those numbers under control. 



I take the rules which Mr. Davies gives to amount to this : — 

 Every individual does what it can to produce offspring, and to 

 increase the number of its species. We can only suppose that it 

 is quite unconscious of what it is doing. 



Now, as to the number of eggs laid by Finches and Warblers. 

 Mr. Davies gives five as the average clutch ; and then proceeds to 

 show why this is the only suitable number. I cannot agree with 

 him that a hen of small size could not well lay more than five. As he 

 himself states, Tits may lay very many more. It seems probable, 

 however, that the number may be limited by the catering powers 



