The Evolution of Plants 325 



between species long ago suggested that the one species may 

 have arisen from the other. For example, the common asters, 

 violets, hawtliorns, evening primroses, and willows are highly 

 variable, and it is frequently impossi])le definitely to classify 

 a particular specimen and to say that it belongs to this or 

 that species. If such intermediate forms were rare, they 

 would only suggest the possibility of evolution ; but they are 

 numerous, occurring in hundreds of families throughout the 

 plant kingdom. These intergrades make it impossible for 

 us to think of the plant kingdom as being composed of dis- 

 tinct and unrelated species, and so they must be regarded as 

 evidences of evolution. 



Plant breeding and evolution. If wc study the histories of 

 various cultivate*d plants, — for example, the many varieties 

 of cabbage, tomatoes, corn, wheat, apples, peaches, chrysan- 

 themums, and asters, — we find abundant evidence that 

 they have been derived from wild plants. The plant breeder 

 has simply selected and preserved desirable forms that have 

 arisen by mutation and through hybridization. 



When we realize the extent to which these plants have been 

 modified from the types of their wild ancestors in the com- 

 paratively short time that plants have been cultivated, it is 

 less difficult to understand the great changes in plant life 

 that ma}' have occurred during the long period since plants 

 first grew on the earth. 



Plant breeding furnishes us with experimental evidence that 

 plants may change in various ways, so that not all the plants 

 of succeeding generations arc like their ancestors. Here and 

 there an indivickial arises that is different from its parents, 

 and such new individuals are the beginnings of new races, 

 varieties, and perhaps species. The experience of plant 



