The Evolution of Plants 327 



Natural selection. Most plants produce offspring by the 

 hundreds, thousands, or even millions, and there is room for 

 only a small part of the offspring to live. It is said that those 

 plants survive that are more vigorous, that are better ad- 

 justed to their environments, or that happen to start in favor- 

 able places; the weak and the unfortunate perish. Certain 

 variations, or mutations, may fit plants the better to survive, 

 and the persistence of the forms showing these changes may 

 lead to the formation of new varieties and species. The 

 wholesale destruction of individual plants in nature, with the 

 survival of a few, is called Natural Selection, and it has been 

 thought to resemble in some respects the selection made by 

 the plant breeder. It is unquestionably true that most of 

 the plants that start life in nature die before reaching ma- 

 turity ; but there are great 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 in the plants that he 

 cultivates and by breeding from them secure new varieties, 

 but it is believed by some that in nature the variations 

 would be lost by interbreeding with the parent forms. 



Changes in environment. Another factor in Natural 

 Selection is the fact that climates and environments are ever 

 changing. We know, for example, that for a long period 

 of time tropical plants grew in polar regions, for the fossils 

 of tropical plants are now found there in the rocks. At a 

 later period the climate of the earth was much colder than 

 it is now, and all of northern North America was covered 

 with ice that reached as far south as the Ohio and Missouri 

 rivers. As climates change, plants either become adjusted 

 to the changes or perish. These changes in climate afford 



