52 Tfie Descent of Man. Pakv I 



have had its efficient cause. If these causes, whatever they 

 may be, were to act more uniformly and energetically during a 

 lengthened period (and against this no reason can be assigned), 

 the result would probably be not a mere slight individual 

 difference, but a well-marked and constant modification, though 

 one of no physiological importance. Changed structures, which 

 »re in no way beneficial, cannot be kept uniform through natural 

 selection, though the injurious will be thus eliminated. Uni- 

 formity of character would, however, 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 modifications, 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 spon- 

 taneous 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 

 prehent day is -liable, like every other animal, to multiform 

 individual differences or slight variations, so no doubt were the 

 Curly progenitors of man ; the variations being formerly 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 load to a 

 struggle for existence and to natural 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 appears, also, as we shall hereafter see, 

 that various unimportant characters have been acquired by man 

 through sexual selection. An unexplained residuum of change 

 must be left to the assumed ujiiferm"action of~Ehose unknown 

 agencies, which occasionally induce strongly marked and abrupt 

 deviations of structure in our domestic productions. 



Judging from the habits of savages and of the greater numbet 

 of the Quadrumana, primeval men, and even their ape-hks 

 progenitors, probably lived in society. With strictly social 

 animals, natural selection sometimes acts on the individual, 

 through the preservation of variations which are beneficial to 

 the community. A community which includes a large numbei 

 of well-endowed individuals increases in number, and is victo- 

 rious over other less favoured ones ; even although each separate 



