154 EVOLUTION 



require the same food, and are exposed to 

 the same dangers. In the case of varieties of 

 the same species, the struggle will generally 

 be almost equally severe, and we sometimes 

 see the contest soon decided (as in the case of 

 varieties of wheat or of sweet pea, of the 

 mountain sheep or of the medicinal leech). 

 Similarly, the struggle between species of the 

 same genus will generally be more severe 

 than between the species of distinct genera, 

 this is illustrated by the inevitable replace- 

 ment 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; wit- 

 ness 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 struggle 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." 



