138 LIFE AND DEATH. 



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 occur- 

 ring 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 predicted from the beginning. 



Such a traveller represents a species, and his route corresponds 

 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 surround- 

 ings, 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 innumerable 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 the most 

 tortuous and winding route, so the structure of the original 

 organism has undergone manifold changes during its terrestrial 

 life. And just as the traveller at first doubts whether he will ever 

 get beyond the immediate neighbourhood of his starting-point, 

 and yet after some years finds himself very far removed from it 

 so the insignificant changes which distinguish the first set of 

 generations of an organism lead on through innumerable other 

 sets, to forms which seem totally different from the first, but which 

 have descended from them by the most gradual transition. All 

 this is so obvious that there is hardly any need of a metaphor to 

 explain it, and yet it is frequently misunderstood, as shown by the 

 assertion that natural selection can create nothing new : the fact 

 being that it so adds up and combines the insignificant small de- 



