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? The 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 light. 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 born of the same parent, are 
ever identically alike. Each tends to vary, and some- 
times the variation is very great. 
2. Survival 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 flourishes when it is in 
equilibrium with its surroundings, and is most secure 
