552 General Botany 



questionably true that most of the plants that start life in nature 

 die before reaching maturity ; but there are differences of opinion 

 as to whether or not the plants that do survive can through re- 

 peated selections in nature develop into new species. Man can 

 pick out new forms that originate among the plants that he culti- 

 vates and by breeding from them secure new varieties, and it is 

 believed by some that in nature certain advantageous mutations 

 are selected or preserved in a similar way. 



The agencies that are supposed to do the selecting in nature 

 are the factors of the environment. Almost any environmental 

 factor may become a limiting factor for the growth of some par- 

 ticular variety of plant. If mutants occur which are not limited 

 in the same way or to the same degree, such mutants survive. 

 During geological time the great changes in the elevation of conti- 

 nents, in connections between continents and islands, in the 

 climate, and in the habitats available have been major factors 

 in determining the changes in the kinds of plants that survived. 



Summary. In this final chapter an attempt is made to define 

 evolution, to show the sources from which the proofs of evolution 

 are obtained, and to distinguish between the fact of evolution 

 and the tentative explanations which have been offered to ac- 

 count for evolution. Botanists are now generally agreed (i) that 

 variations are the possible sources of evolution, (2) that those vari- 

 ations which are inherited, particularly mutants, are the only 

 ones which lead to new varieties and species, and (3) that from 

 among these mutants some survive and some perish, according as 

 they fit into or fail to fit into the environment. 



