1 88 Animal Life and Intelligence. 



selection is constantly on the watch to select any modifica- 

 tion, however slight, which is of advantage to the species. 

 And it is true that elimination is ceaselessly operative. 

 But it is equally certain that the advantage must be of 

 sufficient value to decide the question whether its possessor 

 should be eliminated or should escape elimination. If it 

 does not reach this value, Natural Selection, watch she 

 never so carefully, can make no use of it. Elimination 

 need not, however, be to the death ; exclusion from any 

 share in continuing the species is sufficient. To breed or 

 not to breed, that is the question. Any advantage affect- 

 ing this essential life-function will at once catch the eye of 

 a vigilant natural selection. But it must be of sufficient 

 magnitude for the machinery of natural selection to deal 

 with. That machinery is the elimination of a certain 

 proportion of the individuals which are born. Which shall 

 be eliminated, and which shall survive, depends entirely on 

 the way in which the individuals themselves come out in 

 life's competitive examination. The manner in which that 

 examination is conducted is often rude and coarse, too rough- 

 and-ready to weigh minute and infinitesimal advantages. 



What must be the value of a favourable or advantageous 

 modification to decide the question of elimination, to make 

 it an available advantage, must remain a matter of conjec- 

 ture. It will vary with the nature and the pressure of the 

 eliminative process. And perhaps it is scarcely too much 

 to say that, at present, we have not observational grounds 

 on which to base a reliable estimate in a single instance. 

 We must not let our conviction of its truth and justice 

 blind us to the fact that natural selection is a logical 

 inference rather than a matter of direct observation. A 

 hundred are born, and two survive ; the ninety-eight are 

 eliminated in the struggle for existence ; we may therefore 

 infer that the two escaped elimination in virtue of their 

 possession of certain advantageous characters. There is 

 no flaw in the logic that has thus convinced the world that 

 natural selection is a factor in evolution. But by what 

 percentage of elimination-marks the second of the two 



