sonal selection favours or rejects these, or,--if it be a question
of organs which have become useless,--it does not come into play at
all, and allows the descending variation free course.

It is obvious that even the problem of _coadaptation in sterile
animals_ can thus be satisfactorily explained. If the determinants are
oscillating upwards and downwards in continual fluctuation, and
varying more pronouncedly now in one direction now in the other,
useful variations of every determinant will continually present
themselves anew, and may, in the course of generations, be combined
with one another in various ways. But there is one character of the
determinants that greatly facilitates this complex process of
selection, that, after a certain limit has been reached, they go on
varying in the same direction. From this it follows that development
along a path once struck out may proceed without the continual
intervention of personal selection. This factor only operates, so to
speak, at the beginning, when it selects the determinants which are
varying in the right direction, and again at the end, when it is
necessary to put a check upon further variation. In addition to this,
enormously long periods have been available for all these adaptations,
as the very gradual transition stages between females and workers in
many species plainly show, and thus this process of transformation
loses the marvellous and mysterious character that seemed at the first
glance to invest it, and takes rank, without any straining, among the
other processes of selection. It seems to me that, from the facts that
sterile animal forms can adapt themselves to new vital functions,
their superfluous parts degenerate, and the parts more used adapt
themselves in an ascending direction, those less used in a descending
direction, we must draw the conclusion that harmonious adaptation here
comes about _without the cooeperation of the Lamarckian principle_.
This conclusion once established, however, we have no reason to refer
the thousands of cases of harmonious adaptation, which occur in
exactly the same way among other animals or plants, to a principle,
the _active intervention of which in the transformation of species is
nowhere proved. We do not require it to explain the facts, and
therefore we must not assume it._

The fact of coadaptation, which was supposed to furnish the strongest
argument against the principle of selection, in reality yields the
cleare