The same unusual family conditions prevail with the rhea and the emu, and also among the painted snipes, cassowaries, tinamous, and some of the button-quails.[31] There are probably instances of other birds, but I do not know of details of their habits; Wallace[32] also mentions several species in different parts of the world, among whom all care of the young falls entirely upon the father. In all these bird families exactly opposite conditions prevail to what we are accustomed. It should be specially noted that these unnatural (I use the word simply to mean unusual) mothers are larger and more vividly coloured than the hard-worked fathers; in all such cases polyandry is practised. Why is this? The only attempt at an explanation that I have been able to discover is given by Mr. Pycraft in his fascinating book, _The Courtship of Animals_. He says—[33] “The solution of this problem probably lies with the physiologist. We now know that the problem of sex does not rest merely in the complete development of the primary sexual organs; we know that fertile unions do not depend merely on the act of pairing, but on the functional activity of the ancillary glands. And it may well be that some change in the character of the secretions has not only affected the numerical values of the sexes, but reversed the normal rôle of coloration and behaviour.” Mr. Pycraft does not consider that the polyandrous habits of these birds are due primarily to a preponderance of the females in the species over the males, but holds that this condition must rather be regarded as having arisen from a transference to the females, or development in them, of increased sexual hunger, which intensity of passion would tend to lead to an exhaustion of the males. This is exceedingly interesting. Mr. Pycraft continues— “Neither polygamy nor polyandry among the lower animals, at any rate, has been brought about or is maintained by the excessive death rate due to combats for the possession of mates, but must be explained as demonstrating inherent changes in the germ-plasm, disturbing the relative proportions of the sexes and correlated with a profound transformation, not only in the behaviour of the sexes during the period of reproductive activity, but also in their physical characteristics.”