Evolution of Plants 549 



small steps. There has been an occasional addition of a new 

 tissue or organ to the life cycle, or the replacement of one struc- 

 ture by another. 



The life histories of related groups are similar in essentials and 

 differ only in details. This repetition of the stages in the life 

 cycles of the plants of diff'erent groups, when viewed along with 

 other facts of evolution, indicate that the plants with the more 

 complex life histories have evolved from those with less complex 

 life histories. Increase in complexity is one of the general tend- 

 encies of evolution. The order in which we should arrange 

 plants on the basis of the geological record is the same as the 

 order suggested by their life histories and structures. 



Intergrading species. All who have attempted to classify 

 plants — that is, to determine the species to which individual 

 specimens belong — have been impressed by the intergrading 

 of related species. The existence of individuals intermediate 

 between species long ago suggested that one species may have 

 arisen from another. For example, the common asters, violets, 

 hawthorns, evening primroses, and willows are highly variable ; 

 and in any of these genera it is frequently impossible definitely 

 to classify a particular specimen and to say that it belongs to 

 this or that species. If forms intermediate between species 

 were rare, they would only suggest the possibility of evolution ; 

 but they are numerous, occurring in hundreds of genera through- 

 out the plant kingdom. These intergrades make it impossible 

 for us to think of the plant kingdom as being made up of distinct 

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

 of evolution. 



Plant breeding and evolution. Our cultivated plants are the 

 modified descendants of wild species. Many of them, perhaps 

 most of them, were brought into cultivation by wild tribes of 

 men long before the dawn of written history. In many instances 

 the plants have been so greatly modified that it is difficult or 

 impossible to trace their origin to any known wild species. Thus 



