When he set forth, with great daring, to write the "Principles of Biology," Spencer was already at an advantage compared with the accepted writers upon the subject, not merely because of his stupendous intellectual endowment, but also because the idea of the conservation of energy was a permanent guiding factor in all his thought. Thus it was, one supposes, that this bold young amateur, for he was little more, perceived in the light of the evolutionary idea of which he was one of the original promulgators, a simple truth which had been unperceived by all previous writers upon biology, from Aristotle onwards. It is in the last section of his book that Spencer propounds his "law of multiplication," depending upon what he calls the "antagonism between individuation and genesis." As I have observed elsewhere, the word antagonism is perhaps too harsh, and may certainly be misleading, for it may induce us to suppose that there is no possible reconciliation of the claims and demands of the race and the individual, the future and the present. I believe most devoutly that there is such a reconciliation, as indeed Spencer himself pointed out, and a central thesis of this book is indeed that in the right expression of motherhood or foster-motherhood, woman may and increasingly will achieve the highest, happiest, and richest self-development. Thus one may be inclined to abandon the word antagonism, and to say merely that there is a necessary inverse ratio between "individuation" and "genesis," to use the original Spencerian terms. This principle has immense consequences--most notably that as life ascends the birth-rate falls, more of the vital energy being used for the enrichment and development of the individual life, and less for mere physical parenthood. We shall argue that, in the case of mankind, and pre-eminently in the case of woman, this enrichment and development of the individual life is best and most surely attained by parenthood or foster-parenthood, made self-conscious and provident, and magnificently transmuted by its extension and amplification upon the psychical plane in the education of children and, indeed, the care and ennoblement of human life in all its stages. This law of Spencer's has been discussed at length by the present writer in a previous volume,[2] and we may therefore now proceed to its notable illustration in the case of womanhood and the female sex in general, as made by Geddes and Thomson now more than twenty years ago. It is surprising that the distinguished authors do not seem to have recognized that their law is a special case of Spencer's; but one of them granted this relation in a discussion upon the present writer's first eugenic lecture to the Sociological Society.[3]