THE EVOLUTION OF SPECIES 203 



year. What, then, are the causes or checks which operate 

 to prevent the spreading of a race of plants in proportion 

 to its output of new individuals ? Tha answer is the 

 struggle for existence, resulting in the survival of the fittest, 

 or, as Darwin called it, Natural Selection. One plant has 

 to compete with another for room, food, and Ught. In 

 this struggle the strongest win and the weaker go to the 

 wall. The competition of plants for the right to live is 

 as keen and remorseless as among animals, and most of 

 all between members of the same species. Different 

 species make different demands upon the soil and the 

 sunlight. Plants belonging to the same species have 

 similar needs ; between these the struggle for existence 

 is the fiercest. 



If, then, one individual happens to possess a character 

 which the others have not, or possesses a character in a 

 higher degree than its neighbours, and if this character 

 puts its possessor at an advantage in the struggle for 

 existence, this plant is more likely to survive than its 

 competitors. Further, if this character, making, as it 

 does, for usefulness and efficiency, can be transmitted to 

 its descendants, the latter will form a new species, marked 

 off from its predecessors by this advantageous character, 

 and superior, by the possession of this character, to the 

 race from which it sprang. 



This, according to Darwin, explains the origin of new 

 species, which depends, therefore, upon the truth of the 

 following statements : 



1. Variation. — Though offspring are like their parents, 

 no two individuals, though bom of the same parent, are 

 ever identically alike. Each tends to vary, and some- 

 times the variation is very great. 



2. Sxirvival of the Fittest. — In the struggle for existence 

 the fittest survive, and the weak are ousted by the 

 strong. 



3. Adaptation to Environment. — The land-surface upon 

 which plants grow has been in ages past subject to many 

 changes — changes of climate, changes of soil, changes of 

 physical conditions. Plants which did not vary so as to 

 develop characters enabling them to adapt themselves 

 to these changes inevitably died out ; those that did 

 survived and flourished. A race flovirishes when it is in 

 equilibrium with its surroimdings, and is most secure 



