358 Appendix B 



would follow in due course. Now among the offspring of a pair 

 who succeeded in this new mode of life, some would have 

 feet of the right sort, together with the other characteristics 

 required, and would survive. Others would be ill-adapted for 

 an upright posture and the associated habits. Nevertheless 

 they would have to follow their parents' mode of life. The 

 species, at the time of which we are thinking, has long advanced 

 beyond the stage at which the young are flung upon the world 

 directly they are born. They cannot, therefore, revert to the 

 trees because walking is painful, and, at last, impossible for 

 them. Their parents choose for them their line of life, thus 

 deciding with certain limits the line of development of the 

 species. 



*' Not only parental care but also the gregarious habit, so 

 common among animals, helps, so to speak, to give the species 

 a continuous policy and so to promote evolution. Here, too, a 

 few examples may make clear what is meant. 



" Among a herd of primitive ruminants some individual bulls 

 may have had, where the horns now are, an exceptional thick- 

 ness of bone and over it a certain epidermal callosity, not 

 sufficient without special treatment to enable them to drive 

 rivals from the field, but sufficient to make them enjoy sparring, 

 so that the parts would get hardened and enlarged during their 

 advance to maturity. Those of inferior natural endowments 

 would improve much less quickly or break down altogether. 

 Thus congenital hardness of head, increased by practice in butting, 

 would become a character having selection-value, and bulls that 

 were not richly endowed in this respect would leave no offspring 

 behind them. Among animals living in herds there are special 

 facilities for sexual selection by battle. All the males must 

 fight or efface themselves : there is no standing out. And thus 

 if they are to survive, males must vary in a certain direction, 

 viz., towards hardness of head and weapons for butting. Hence 

 by gradual accumulation will arise horns or branching antlers. 



" Many species of birds owe their success in the world to 

 their sociability. Rooks {Corvus frugilegus) in their crowded 

 rookeries, or as they fly in large flocks, are able to beat off 



