70 THE DESCENT OF MAN. 



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 present day is liable, like every other animal, to mul- 

 tiform individual differences or slight variations, so no 

 doubt were the early 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 pro- 

 genitors of man; and this would inevitably lead to a strug- 

 gle for existence and to natural selection. The latter pro- 

 cess would be greatly aided by the inherited effects of the 

 increased use of parts, and these two processes would in- 

 cessantly 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 unex- 

 plained residuum of change must be left to the assumed 

 uniform action of those unknown agencies, which occasion- 

 ally induce strongly marked and abrupt deviations of struc- 

 ture in our domestic productions. 



Judging from the habits of savages and of the greater 

 number of the Quadrumana, primeval men, and even 

 their ape-like progenitors, probably lived in society. 

 With strictly social animals, natural selection sometimes 

 acts on the individual, through the preservation of vari- 

 ations which are beneficial to the community. A com- 

 munity which includes a large number of well-endowed 

 individuals increases in number, and is victorious over 

 other less favored ones ; even although each separate 

 member gains no advantage over the others of the same 

 community. Associated insects have thus acquired many 

 remarkable structures, which are of little or no service 

 to the individual, such as the pollen-collecting appa- 

 ratus, or the sting of the worker-bee, or the great jaws of 

 soldier-ants. With the higher social animals, I am not 

 aware that any structure has been modified solely for the 

 good of the community, though some are of secondary 



