This conclusion, which follows irresistibly from Wallace's theorem, that in the genesis of Humanity natural selection began to follow a new path, already throws a light of promise over our whole subject, like the rosy dawn of a June morning. But the explanation of the genesis of Humanity is still far from complete. If we compare man with any of the higher mammals, such as dogs and horses and apes, we are struck with several points of difference: _first_, the greater progressiveness of man, the widening of the interval by which one generation may vary from its predecessor; _secondly_, the definite grouping in societies based on more or less permanent family relationships, instead of the indefinite grouping in miscellaneous herds or packs; _thirdly_, the possession of articulate speech; _fourthly_, the enormous increase in the duration of infancy, or the period when parental care is needed. Twenty-four years ago, in a course of lectures given yonder in Holden Chapel, I showed that the circumstance last named is the fundamental one, and the others are derivative. It is the prolonged infancy that has caused the progressiveness and the grouping into definite societies, while the development of language was a consequence of the increasing intelligence and sociality thus caused. In the genesis of Humanity the central fact has been the increased duration of infancy. Now, can we assign for that increased duration an adequate cause? I think we can. The increase of intelligence is itself such a cause. A glance at the animal kingdom shows us no such thing as infancy among the lower orders. It is with warm-blooded birds and mammals that the phenomena of infancy and the correlative parental care really begin. [Illustration] VII _The Chief Cause of Man's lengthened Infancy_ The reason for this is that any creature's ability to perceive and to act depends upon the registration of experiences in his nerve-centres. It is either individual or ancestral experience that is thus registered; or, strictly speaking, it is both. It is of the first importance that this point should be clearly understood, and therefore a few words of elementary explanation will not be superfluous.