From the foregoing examples it may, I think, be taken as established that among the reptiles there are many exemplary fathers. If the question is asked as to why in some species the care of the young is undertaken by the father and in others by the mother, I can only answer that I do not know. It would seem almost that at this early stage of life Nature was making experiments as to which was the better parent. I would suggest that possibly such a reversal of the family duties was started by chance, possibly by the loss of the mother, or even by a specially energetic father, and on being found successful the arrangement was continued and became fixed as a habit. I have not sufficient knowledge to know if this is possible. At any rate, it appears to be plain that, where for any reason the family duties are neglected by the mother, and where the maintenance of the species demands protection being given to the young, the father steps in to take the place of the mother; and by his care and devotion he becomes a truly constituent part—a working member—of the family group. I would ask you to keep this fixed in your attention, as I shall have to refer again and again to this fact that is here suggested. What obtains among reptiles with regard to the father’s care for the young is even more frequent among fish-fathers. The common stickleback of our ponds and streams affords an admirable illustration of intelligent and devoted fatherhood. In this species the rôle of the two sexes is completely reversed; when once the eggs are deposited by the mother, the whole task of guarding them is undertaken by the father. His labours begin with the construction of a nest. This is formed of bits of weed, of fibre and dirt, collected with much care, the whole being held together by a cement produced by the clever father out of a secretion from his kidneys. Having prepared the nursery, the stickleback sets out to find a wife heavy with eggs. His love choice apparently is decided by the capability of his spouse for her maternal function. By means of much persuasion and passionate courtship he woos her and induces her to deposit her burden of eggs in his nest.