154 EVOLUTION 



medicinal leech). Similarly, the struggle be- 

 tween species of the same genus will generally 

 be more severe than between the species of 

 distinct genera. This is illustrated by the 

 inevitable replacement of the black rat by 

 the brown, or of the large cockroach by the 

 small. The structure of every organism 

 is related to that of all others with which 

 it competes, from which it escapes, or on 

 which it preys; witness alike the teeth 

 and talons of the tiger, or the legs and 

 claws of the parasite clinging to his hair. 

 The albumen of a seedling favours its strug- 

 gle with plants already growing around it. 

 Darwin goes on to speak of two "canine 

 animals" struggling with each other in a 

 time of dearth; of mistletoe versus mistletoe 

 on the same branch; of mistletoe versus other 

 fruit-bearing plants; of a plant on the edge 

 of the desert in days of drought; and then 

 says, "In these several senses, which pass 

 into each other, I use, for convenience' sake, 

 the general term of Struggle for Existence." 

 NATURAL SELECTION. How will this 

 struggle for existence act in regard to varia- 

 tion? Can the principle of selection, so po- 

 tent in the hands of man, apply under 

 Nature? Most efficiently; for, when we bear 

 in mind the constant occurrence of variation, 

 with the strength of the hereditary tendency, 



