III.] LIFE AND DEATH, 1 39 



own will, his own pleasure or interest, — proceeding forwards, 

 to the right or left, or even backwards, with longer or shorter 

 pauses, and starting at any particular time,— it is obvious that 

 the route taken lies in the man himself and is determined by 

 his own peculiar temperament. His judgment, experience, 

 and inclination will influence his course at each turn of his 

 journey, as new circumstances arise. He will turn aside from 

 a mountain which he considers too lofty to be climbed ; he 

 will incline to the right, if this direction appears to afford a 

 better passage over a swollen stream ; he will rest when he 

 reaches a pleasant halting-place, and will hurry on when he 

 knows that enemies beset him. And in spite of the perfectly 

 free choice open to him, the course he takes is in fact decided 

 by both the place and time of his starting and by circumstances 

 which — always occurring at every part of the journey — impel 

 him one way or the other; and if all the factors could be 

 ascertained in the minutest detail, his course could be pre- 

 dicted from the beginning. 



Such a traveller represents a species, and his route corre- 

 sponds with the changes which are induced in it by natural 

 selection. The changes are determined by the physical nature 

 of the species, and by the conditions of life by which it is 

 surrounded at any given time. A number of different changes 

 may occur at every point, but only that one will actually 

 develope which is the most useful, under existing external 

 conditions. The species will remain unaltered as long as it is 

 in perfect equilibrium with its surroundings, and as soon as 

 this equilibrium is disturbed it will commence to change. It 

 may also happen that, in spite of all the pressure of competing 

 species, no further change occurs because no one of the innu- 

 merable very slight changes, which are alone possible at any 

 one time, can help in the struggle ; just as the traveller who is 

 followed by an overpowering enemy is compelled to succumb 

 when he has been driven down to the sea. A boat alone could 

 save him, without it he must perish ; and so it sometimes 

 happens that a species can only be saved from destruction 

 by changes of a conspicuous kind, and these it is unable to 

 produce. 



And just as the traveller, in the course of his life, can wander 

 an unlimited distance from his starting-point, and may take 



