148 THE DESCENT OF MAN. [Part I. 



of many individuals. The same organism might acquire 

 in this manner during successive periods successive modi- 

 fications, and these would be transmitted in a nearly uni- 

 form 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 

 multiform individual differences or slight variations, so no 

 doubt were the early progenitors of man ; the variations 

 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 sub- 

 sistence, 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 incessant- 

 ly reacting on each other. It appears, also, as we shall 

 hereafter see, that various itnimportant characters have 

 been acquired by man through sexual selection. An unex- 

 plained residuum of change, perhaps a large one, must be 

 left to the assumed uniform action of those unknown as;en- 

 cies, which occasionally induce strongly-marked and ab- 

 rupt deviations of structure in our domestic productions. 



Judging from the habits of savages and of the greater 

 number of the Quadrumana, primeval men, and even the 

 ape-like progenitors of man, probably lived in society. 

 With strictly social animals, natural selection sometimes 

 acts indirectly on the individual, through the preservation 



