I showed that if the view for which I was contending was taken, many facts which, though familiar, were still without explanation or connection with our other ideas, would remain no longer isolated, but be seen at once as joined with the mainland of our most assured convictions. Among the things thus brought more comfortably home to us was the principle underlying longevity. It became apparent why some living beings should live longer than others, and how any race must be treated whose longevity it is desired to increase. Hitherto we had known that an elephant was a long-lived animal and a fly short-lived, but we could give no reason why the one should live longer than the other; that is to say, it did not follow in immediate coherence with, or as intimately associated with, any familiar principle that an animal which is late in the full development of its reproductive system will tend to live longer than one which reproduces early. If the theory of “Life and Habit” be admitted, the fact of a slow-growing animal being in general longer lived than a quick developer is seen to be connected with, and to follow as a matter of course from, the fact of our being able to remember anything at all, and all the well-known traits of memory, as observed where we can best take note of them, are perceived to be reproduced with singular fidelity in the development of an animal from its embryonic stages to maturity. Take this view, and the very general sterility of hybrids from being a _crux_ of the theory of descent becomes a stronghold of defence. It appears as part of the same story as the benefit derived from judicious, and the mischief from injudicious, crossing; and this, in its turn, is seen as part of the same story, as the good we get from change of air and scene when we are overworked. I will not amplify; but reversion to long-lost, or feral, characteristics, the phenomena of old age, the fact of the reproductive system being generally the last to arrive at maturity—few further developments occurring in any organism after this has been attained—the sterility of many animals in confinement, the development in both males and females under certain circumstances of the characteristics of the opposite sex, the latency of memory, the unconsciousness with which we grow, and indeed perform all familiar actions, these points, though hitherto, most of them, so apparently inexplicable that no one even attempted to explain them, became at once intelligible, if the contentions of “Life and Habit” were admitted.