Man, even in the lower stages of development, possesses a faculty which, for want of a better name, I shall call Number Sense. This faculty permits him to recognize that something has changed in a small collection when, without his direct knowledge, an object has been removed from or added to the collection. Number sense should not be confused with counting, which is probably of a much later vintage, and involves, as we shall see, a rather intricate mental process. Counting, so far as we know, is an attribute exclusively human, whereas some brute species seem to possess a rudimentary sense akin to our own. At least, such is the opinion of competent observers of animal behavior, and the theory is supported by a weighty mass of evidence, Many birds, for instance possess such a number sense. If a nest contains four eggs, one can safely be taken; but when two are removed, the bird generally deserts. In some unaccountable way the bird can distinguish two from three. But this faculty is by no means confined to birds. In fact, the most striking instance we know is that of the insect called the "solitary wasp." The mother wasp lays her eggs in individual cells and provides each egg with a number of live caterpillars (毛虫) on which the young feed when hatched. Now, the number of victims is remarkably constant for a given species of wasp. Some species provide 5, others, 12, others again as high as 24 caterpillars per cell. But most remarkable is the case of a variety in which the male is much smaller than the female. In some mysterious way the mother knows whether the egg will produce a male or a female grub (幼虫) and apportions the quantity of food accordingly; she does not change the species or size of the prey, but if the egg is male, she supplies it with five victims; if female, with ten. The regularity in the action of the wasp and the fact that this action is connected with a fundamental function in the life of the insect make this last case less convincing than the one which follows. Here the action of the bird seems to border on the conscious. A squire was determined to shoot a crow which made its nest in the watchtower of his estate. Repeatedly he tried to surprise the bird, but in vain: at the approach of man the crow would leave its nest. From a distant tree it would watchfully wait until the man had left the tower and then return to its nest. One day the squire hit upon a trick: two men entered the tower, one remained within, the other came out and went on. But the bird was not deceived: it kept away until the man within came out. The experiment was repeated on the succeeding days with two, three, then four men, yet without success. Finally, five men were sent: as before, all entered the tower, and one remained while the other four came out and went away. Here the crow lost count. Unable to distinguish between four and five, it promptly returned to its nest. Counting is different from number sense in that counting is ______.