v Natural Selection. 61 



organisms propagate after their kind and likeness, 

 yet the resemblance of offspring to parent is not 

 rigidly close, but admits of slight variations. The 

 conditions and causes of these variations of the off- 

 spring from the parent form constitute a most 

 obscure physiological question ; but it appears certain 

 that such variation is promoted by slight changes in 

 the conditions of life. 



Darwin's entire theory is based on these two 

 obvious facts : the high rate of natural increase 

 throughout the entire organic creation, necessitating 

 the destruction of the greater part of the germs 

 which are produced; and the tendency of the off- 

 spring to differ, however slightly, from the parent 

 form. The high rate of increase, leading to the 

 destruction of the vast majority of germs, causes a 

 "struggle for life"; and this prevails in vegetable 

 species as much as in animal ones : thickly -sown 

 seedlings, whereof the weaker perish because they 

 are deprived of sunshine by the stronger, are en- 

 gaged in the struggle for life, no less than animals 

 which fight with each other for food. And from the 

 fact of variability, it follows that some individuals 

 must have points of superiority strength to obtain 

 food, or fleetness to escape enemies, or any of the 

 innumerable points which may give to one some 

 advantage over another ; those which have any such 

 advantage will, on the average and on the whole, be 

 the most likely to succeed and to survive, and to 

 leave offspring; which, on the average and on the 



