ointed 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--t