90 THE DESCENT OF MAN 



assigned), tlie result would probably be not a mere slight in- 

 dividual difference, but a well-marked and constant modifi- 

 cation, though one of no physiological importance. Changed 

 structures, which are in no way beneficial, cannot be kept 

 uniform through natural selection, though the injurious will 

 be thus eliminated. Uniformity of character would, how- 

 ever, naturally follow from the assumed uniformity of the 

 exciting causes, and likewise from the free intercrossing 

 of many individuals. During successive periods the same 

 organism might, in this manner, acquire successive modifi- 

 cations, which 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 we have seen that as man at 

 the present day is liable, like every other animal, to multi- 

 form individual differences or slight variations, so no doubt 

 were the early progenitors of man; the variations being for- 

 merly induced by the same general causes, and governed by 

 the same general and complex laws as at present. As all 

 animals tend to multiply beyond their means of subsistence, 

 so it must have been with the progenitors of man; and this 

 would inevitably lead to a struggle for existence and to nat- 

 ural-selection. The latter process would be greatly aided by 

 the inherited effects of the increased use of parts, and these 

 two processes would incessantly react on each other. It ap- 

 pears, also, as we shall hereafter see, that various unimpor- 

 tant characters have been acquired by man through sexual 

 selection. An unexplained residuum of change must be left 

 to the assumed uniform action of those unknown agencies 

 which occasionally induce strongly marked and abrupt devi- 

 ations of structure in our domestic productions. 



Judging from the habits of savages and of the greater 



