360 Introduction to Botany. 



are certain to be some characteristics common to all which 

 afford satisfactory evidence of the relationship. It is clear 

 to every one that the different kinds of roses are nearly 

 related ; and when one compares the flowers and fruits of 

 cherries and plums, the evidences of consanguinity are very 

 striking, but the points of similarity between roses and 

 cherries, for example, are fewer, and for their detection 

 require one to be somewhat familiar with the proper lines 

 of evidence. 



231. Cause of Variation. — If it is true that forms of 

 plants, as we now find them, have descended from fewer 

 and simpler primitive types, as appears to be the case from 

 structural evidence and geological record, then an inherent 

 capacity to vary under changed environment, or under 

 other conditions which we do not understand, must be one 

 of the fundamental causes of the evolution of diverse forms 

 from one or few generalized types ; and an intercrossing 

 between forms which have persisted in definite lines of 

 variation must have contributed to the same result. Gaps 

 occur in the lineage because those forms whose variations 

 are best adapted to the environment have, in the competi- 

 tion for room and food, crowded out other forms less favor- 

 ably adapted. It is these gaps which enable us to classify 

 plants into more or less circumscribed groups. We can- 

 not, of course, follow plants in a state of nature from gen- 

 eration to generation, and in this way make positive record 

 of their parentage and relationships. We have to take 

 them as we find them, and classify them according to the 

 circumstantial evidence of their similarities and dissimi- 

 larities. 



232. Grouping into Orders, Genera, etc. — In classifying 

 plants, we say that those plants belong to the same species 

 which are so nearly alike as evidently to have sprung from 



