The Descent of Man

Domestication,' vol. ii. pp. 320-335.) Professor
Schaaffhausen first drew attention to the relation
apparently existing between a muscular
frame and the strongly-pronounced supraorbital
ridges, which are so characteristic of the
lower races of man.
Besides the variations which can be grouped
with more or less probability under the foregoing
heads, there is a large class of variations
which may be provisionally called spontaneous,
for to our ignorance they appear to arise
without any exciting cause. It can, however, be
shewn that such variations, whether consisting
of slight individual differences, or of stronglymarked
and abrupt deviations of structure,
depend much more on the constitution of the
organism than on the nature of the conditions
to which it has been subjected. (56. This whole
subject has been discussed in chap. xxiii. vol. ii.
of my 'Variation of Animals and Plants under
Domestication.')