practice.  It is however possible to deduce from very simple considerations the probabilities as to the respective effects of adelphic and father-daughter unions.  In the first place, as has been already pointed out, the father-daughter union implies only one family of in-and-in-bred children; in the case of brother and sister marriage, on the other hand, this state of things may go on indefinitely.  If this is not enough to turn the scale against adelphic unions there is the further fact that, taking the descendants of the first pair of intermarrying descendants of common parents, whose tendency to disease or deformity is we will suppose x^1 on both sides, and assuming that this tendency increases in a simple ratio, the offspring have the same tendencies to the second power of x.  If their children marry each other the measure of degeneracy in the third generation is x^4.  Suppose now a father and mother with index of degeneracy each x^1; a daughter of this union will have as her index x^2; if the daughter bears children to the father, their index will be not x^4, but x^3, if the simple law which I have assumed for the purposes of argument holds good.

It is therefore clear that the offspring of adelphic unions, so far from being at an advantage compared with the offspring of father-daughter unions, are at a disadvantage in the proportion of 4 to 3.  In the third place, in father-daughter unions the male is physically as well as sexually mature.  In adelphic unions both parties are probably immature.  Consequently from this point of view also the advantage is with the supposed injurious type of union.  Now if the father-daughter union was less harmful than the brother-sister union, a fortiori are uncle-niece and similar unions less harmful.  Yet Morgan supposes them to have been prohibited in favour of brother and sister unions.

Mr Morgan’s reformation therefore turns out to have been no reformation at all, but a retrograde step.  Assuming however that the facts were as he supposed them to be, and that the reformation was a real one, it is by no means clear how he supposes it to have been brought about.  It was, as we have seen, an unconscious[150] reformation; it is not supposed therefore that the primeval savage detected more pronounced signs of degeneracy in the offspring of one class of union and by the force of public opinion caused such unions to fall into disrepute and ultimately into desuetude.  So far as can be seen the method which Mr Morgan had in his mind was this:  certain unions resulted in offspring less able to maintain the struggle for existence, and these families consequently tended to die out.  Other unions—­those of sisters and brothers—­on the other hand produced more vigorous children, and tended to perpetuate themselves.  Whereas originally there was no tendency either one way or the other, some families developed from unknown causes, which, whatever they were, were neither moral nor utilitarian, the practice of brother and sister marriage.  This diathesis followed the ordinary laws of descent, and eventually those families which were fortunate enough to be affected in that way exterminated their rivals.