'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 approaching, 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
