their nests, for we can not take one out without causing in them a disquiet that becomes greater if we remove more. But they manifest a like distress when their eggs are only disarranged. Is this because the geometrical arrangement of the eggs is changed? Five eggs or four make a symmetrical arrangement as the bird disposes of them. But if some are taken out, and three, or two, or one are left, the disposition is very perceptibly changed. When the little birds have been hatched, the differences in their size, liveliness, figure, and voice, give the mother a means of distinguishing them individually. And even the eggs are perhaps not so indistinguishable to her as to us; for sexual and maternal instinct conveys special faculties in these matters. The domestic fowl seems to be less intelligent in such things than sparrows and other wild birds, but this is because domestication has modified her instincts.
Cats certainly know how many kittens they have, but they seem less affected by the loss of one of them according as more are left. If the mother loses one of four or five, she seeks for it a little while with considerable anxiety, and then becomes reconciled to the loss. If only one is left, she becomes greatly troubled, and, if that is taken from her, her distress appears extreme. This may be because of the pain she suffers in her teats when the milk ceases to be removed. When the young have become weaned, she can witness their disappearance with apparent indifference.
Dogs have been observed on various occasions to exhibit primary numerical perceptions in the concrete. When there are a number of them in a house, they quickly remark the absence of one of their companions. But they are rarely troubled by it, and make no effort to find the missing one, and they are still more ready to take notice of the absence of one of the members of their master's family. These traits are more easily explained by the clear knowledge which dogs have of individualities than by ascribing to them notions of unities as forming parts of numbers. The unequal degrees of attachment which they show for the different members of the family, and for the different persons who live in the house or visit it, proves that they make great differences between them. The idea of difference between several persons involves and supports a notion of their number. But it may lie there if wrapped up in their total perception.
Hounds pursuing a hare are troubled for an instant if they raise another one, and will sometimes stop, as if they were uncertain which one they ought to follow. Good dogs will not allow themselves to be diverted from the scent of the first animal, which they have already tired. When the setter pauses before a flock of partridges, the movements of his head and eyes follow the birds that stray to the right or left. If the flock is large, he can not