F. W. Head ley 357 



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. 



" 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 essen- 

 tial, 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 excellencies 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 

 neck and legs will suggest anything 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 long legs, long 

 necks, and ferocious bills. 



" One more example may be very briefly given. Let us 

 imagine our own supposed ancestors, tree-climbing animals for 

 long ages, at length taking to walking biped-fashion upon the 

 ground, because 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 seems. It requires certain favourable congenital 

 variations, among others a certain hardness of the soles of the 

 feet, or a tendency to harden under certain conditions. Other- 

 wise lameness would ensue ; disease or capture by enemies 



