124 PROBLEMS OF EVOLUTION 



rest, at safe spots, but they must be able to stand, walk, run in 

 biped fashion. In fact individuals dictate to their offspring what 

 mode of life they shall follow ; choose the environment that is to 

 act upon them, and, each generation making a similar choice, 

 development proceeds cumulatively along certain lines ; only 

 variations adapted to the chosen environment are selected, and in 

 a long series of generations the structures and qualities most in 

 demand are brought to a high pitch of excellence. Two more 

 examples will help to make this clearer. 



Herons Imagine the progenitors of the heron taking to fishing in the 

 heron style. As preliminaries they must have some favourable 

 variations ; a length of leg beyond the normal, a corresponding 

 length of neck this is desirable if not essential and also a beak 

 not entirely of the wrong kind. But they do not walk on stilts 

 like their modern descendants, nor have they the other excel- 

 lencies with which we are familiar. However, by painstaking 

 effort they get over their difficulties and survive in virtue of their 

 piscatorial skill. Moreover, they dictate to their young that they 

 shall be fishermen, and shall fish too in the heron style ; no 

 diving is allowed. A propensity to live on carrion is severely 

 discouraged, though a variety of live food, including lizards, 

 insects and worms, is permitted. Among the young some will 

 be failures qua herons ; will fall short of their parents' almost 

 inadequate development ; their necks and legs will suggest any- 

 thing but fishing in the only style admissible. Nevertheless 

 they will be taken to the water : from the water must come their 

 main food supply. But those that have the heron build, being at 

 the worst not inferior to their parents, will be successes in the 

 line marked out for them, and thus a heron species, afterwards 

 to be dignified as a genus containing many species, will be 

 founded, with Jong legs, long necks, and ferocious bills. 

 Men's One more example may be very briefly given. Let us imagine 

 rs our own supposed ancestors, tree-climbing animals for long ages, 

 at length taking to walking biped-fashion upon the ground, be- 

 cause the change of habit offered better chances of obtaining 

 food. The new gait would require a whole set of adjustments, 

 for an upright posture is by no means so simple a thing as it 



