240 COLLEGE BOTANY 



survival of the fittest. Plants must be fitted to tkeir eii\'ironineiit 

 or perish ; if there should be a pronounced change in the environ- 

 ment, siich as a change in the water supply, it would be necessary 

 for the plants to undergo corresponding chang'es or perish. 

 Therefore, the great changes in the earth, involving elevation or 

 subsidence of large areas, accompanied by changes in climate, 



Fig, 109. — Variation in leaves from a single branch of a mulberry tree. 



had necessitated changes in the forms, of life on the eaxth. 

 Furthermore, plants were in competition with each other ; when 

 a number of plants tr)' to occupy the same area, the weaker in- 

 dividual and species must perish. Therefore, the plants were 

 in a continuous struggle with both their environment and tlieir 

 rivals ; and only the fittest would survive and perpetuate 

 their kind. 



Darwin'.s theories of evolution were met by opposition on 



