stion to their full dimensions, which, of itself, seems to me quite
adequate to produce the results observed. Recurring to the evidence,
adduced in Chapter III, of the constant variation occurring in all parts
of the organism, while selection is constantly acting on these
variations in eliminating all that fall below the best working standard,
and preserving only those that are fully up to it; and, remembering
further, that, of the whole number of the increase produced annually,
only a small percentage of the best adapted can be preserved, we shall
see that every useful organ will be kept up nearly to its higher limit
of size and efficiency. Now Mr. Galton has proved experimentally that,
when any part has thus been increased (or diminished) by selection,
there is in the offspring a strong tendency to revert to a mean or
average size, which tends to check further increase. And this mean
appears to be, not the mean of the actual existing individuals but a
lower mean, or that from which they had been recently raised by
selection.[199] He calls this the law of "Regression towards
Mediocrity," and it has been proved by experiments with vegetables and
by observations on mankind. This regression, in every generation, takes
place even when both parents have been selected for their high
development of the organ in question; but when there is no such
selection, and crosses are allowed among individuals of every grade of
development, the deterioration will be very rapid; and after a time not
only will the average size of the part be greatly reduced, but the
instances of full development will become very rare. Thus what Weismann
terms "panmixia," or free intercrossing, will co-operate with Galton's
law of "regression towards mediocrity," and the result will be that,
whenever selection ceases to act on any part or organ which has
heretofore been kept up to a maximum of size and efficiency, the organ
in question will rapidly decrease till it reaches a mean value
considerably below the mean of the progeny that has usually been
produced each year, and very greatly below the mean of that portion
which has survived annually; and this will take place by the general law
of heredity, and quite irrespective of any _use_ or _disuse_ of the part
in question. Now, no observations have been adduced by Mr. Spencer or
others, showing that the average amount of change supposed to be due to
_disuse_ is greater than that due to the law of regression to