1 86 Animal Life and Intelligence. 



feature, and to say that, through- natural selection, those 

 individuals have survived which exhibited predominant 

 strength and vitality for a shortened period, even at the 

 expense of natural decay and death. The increased life- 

 power, not the seeds of decay and death, was that which 

 natural selection picked out for survival, or rather that 

 which elimination allowed to survive. 



In such ways — a short life with heightened activity 

 being of advantage to some forms, a more prolonged 

 existence at a lower level of vitality being essential to 

 others — natural selection may have determined in some 

 degree the relative longevity of different organisms. That 

 it caused the introduction of senility as a preparation for 

 death is a less tenable hypothesis. 



And here we may note, in passing, that in using the 

 phrase, "of advantage to the race or species," we must 

 steadily bear in mind the fact that it is with individuals 

 that the process of elimination deals. In the individual it 

 is that every modification must make good its claim to 

 existence and transmission. Where the principle of asso- 

 ciation for mutual benefit obtains, as in the case of social 

 insects, it is still the individual that must resist elimina- 

 tion. Self-sacrifice, whether conscious or unconscious, 

 must not be carried so far as to lead to the elimination of 

 the self-sacrificing individual, for in this event it cannot 

 but defeat its own ends. Within these limits, self-sacrifice 

 is of advantage, as in the case of parental self-sacrifice, in 

 that it enables certain other individuals to escape elimina- 

 tion. We should endeavour, then, not to use the phrase, 

 " of advantage to the species," vaguely and indefinitely, but 

 to indicate in what particular ways certain individuals are 

 to be so advantaged as to escape the Nemesis of elimination. 



(2) The second point that I mentioned above scarcely 

 needs exemplification. That the advantage which enables 

 an organism to escape elimination must be present and 

 existent, not merely prospective, is obvious. Still, the 

 mistake is sometimes made. I have heard it stated that 

 feathers were evolved for the sake of flight. But clearly, 



