Let me summarise now what we have learnt in this and the preceding chapter, so as to establish the lessons that seem to me may be taken from these pre-human parents. The diversity in the expression of the parental instincts must first be grasped. There is no fixed order, nor does there seem to be any continuity of development in this matter of care for the young. We have to give up quite the evolutionary idea of a certain and uninterrupted progress. Throughout our inquiry we have been met with surprises. These things baffle our attempts to find an explanation. What is it that decides and develops the strong instinct of parenthood? A parent in a species that is lower in the scale will often have more parental feeling than a parent in a higher species. Why, for instance, is the stickleback such a devoted father; more self-sacrificing than any other fish-father? and why is the stickleback mother without regard for her children? Why among the dung-beetles is the same parental sacrifice shown by both parents? Again, why is a nursery made in some cases and not in others? why are the young guarded sometimes by the mother and sometimes by the father? We may say that all this wide diversity in habits has arisen through adaptation; the circumstances that have conditioned the life of the species have been different, and this has necessarily caused variety in their behaviour. This is, of course, true, but does it really teach us very much? No sooner do we begin to apply our reasons to any particular case of family behaviour than we find ourselves at a loss. Our reasoning suddenly breaks down, either because our knowledge is incomplete, or because one set of facts we possess seem to be contradicted by other facts of which we are equally sure. Let us at once acknowledge our ignorance; there is much that cannot be explained. If, however, we speculate at all on the matter, certain general ideas may be suggested. We are led to the view that when the father undertakes the care of the young, this reversal in the family duties must be primarily due to some failure on the part of the mother in performing the work in the nursery and home which customarily is hers. It is as if the father steps into her place in order that the species may escape the nemesis of elimination. The facts we have learnt are of no little importance. They tend to minimise, in the beginning of the family at least, the importance of the mother in relation to the young as compared with the importance of the father. It is this that I wish to establish.