What, then, has the alternative hypothesis of natural selection to advance in explanation of these facts? On this hypothesis instinctive acts have biological value in such degree that they have become congenital through the preservation of adaptive variations. But if this be so, why does not the chick respond instinctively to the sight of that which is so essential to its existence as water to drink? In reply to this question it may be suggested that, under natural conditions, the hen teaches all her chickens to peck at the water, and thus shields them from the eliminating influence which gives rise to natural selection, in the absence of which the habit of drinking in response to the sight of water, though acquired by each succeeding generation of birds, has not become instinctive and congenital. Or, to put the matter from a slightly different point of view, the maternal instincts of the hen protect her chicks from any elimination in this respect; and in the absence of such elimination the habit has not been inherited as instinct. But though the hen can lead her young to peck at the water, she cannot teach them how to perform the complex movements of the mouth, throat, and head in actual drinking. In this matter, therefore, her own instinctive procedure does not shield them from the incidence of that elimination which leads to survival under natural selection. Those chicks would be eliminated which, on pecking the water, failed to respond to the stimulus by the complex behaviour involved in drinking, leaving those to survive in which the response had been congenitally established. Thus it would seem that, when natural selection is excluded, the habit has not become congenitally linked with a visual stimulus; but when natural selection is in operation, the response has been thus linked with the stimulus of water in the bill. Whence we may infer that the co-operation of natural selection is an essential factor in the evolution of instinctive behaviour.