154 
THE DESCENT OF MAN. 
Part I. 
naturally follow from the assumed uniformity of the 
exciting causes, and likewise from the free intercros- 
sing of many individuals. The same organism might 
acquire in this manner during successive periods suc- 
cessive modifications, and these would be transmitted 
in a nearly uniform state as long as the exciting causes 
remained the same and there was free intercrossing. 
With respect to the exciting causes we can only say, as 
when speaking of so-called spontaneous variations, that 
they relate much more closely to the constitution of the 
varying organism, than to the nature of the conditions 
to which it has been subjected. 
Conclusion . — In this chapter w r e have seen that as man 
at the present day is liable, like every other animal, to 
multiform individual differences or slight variations, so 
no doubt v T ere the early progenitors of man ; the varia- 
tions being then as now induced by the same general 
causes, and governed by the same general and complex 
laws. As all animals tend to multiply beyond their 
means of subsistence, so it must have been with the 
progenitors of man ; and this will inevitably have led 
to a struggle for existence and to natural selection. 
This latter process will have been greatly aided by 
the inherited effects of the increased use of parts; 
these two processes incessantly reacting on each other. 
It appears, also, as we shall hereafter see, that various 
unimportant characters have been acquired by man 
through sexual selection. An unexplained residuum 
of change, perhaps a large one, must be left to the 
assumed uniform action of those unknown agencies, 
which occasionally induce strongly-marked and abrupt 
deviations of structure in our domestic productions. 
J udging from the habits of savages and of the greater 
number of the Quadrumana, primeval men, and even 
