Evolution of Plants 551 



extensive field experimentation are going on. The experimental 

 study of evolution is a comparatively new field, and the accumu- 

 lated data are not yet sufficiently numerous to do more than 

 suggest some of the factors which cause evolution to occur. 

 Among these are variation, heredity, and natural selection. 



Variation. Mutants seem to be the chief sources of new varieties 

 of plants. These have been discussed in Chapter XXX. There 

 is another type of variation which is also important in evolution, 

 and that is the variations that result from hybridization. Mu- 

 tants result apparently from some change in the germ cells, due 

 to unknown causes. Hybrid variants are due to new combina- 

 tions of characters, derived in part from the pollen parent and 

 in part from the ovule parent. Among evening primroses, oaks, 

 and haw^thorns, hybrid variants are very common. Mutants 

 are known among evening primroses, sunflowers, grasses, hemp, 

 flax, and many other wild and cultivated species. 



Heredity. The tendency of heredity is to make the offspring 

 like the parent. When a mutation has occurred, heredity be- 

 comes an important factor in evolution, since only through 

 heredity can the new variety be maintained. In hybrid vari- 

 ants heredity can maintain a new variety only when both the 

 pollen and ovule parents have the same constitution. 



Natural selection. Most plants produce offspring by the 

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

 only a smafl part of the offsprmg to live. It is said that those 

 plants survive that are more vigorous, that are better adjusted 

 to their environments, or that happen to start in favorable 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 un- 



