﻿BOTANICAL GAZETTE 



occupy much the same habitat, it can scarcely be supposed that 

 the tomentum is a character which determines survival, although 

 of course it is conceivable that a change in its physiology renders 

 necessary this extra protection. Possibly experiments in removing 

 the tomentum from young leaves, if it could be done without in- 

 jury, might answer this question. 



Before leaving this genus I wish to point out a condition in 

 another species of Spiraea which can only be supposed to have 

 originated suddenly through a mutation. It is very difficult to 

 conceive a gradual and continuous transition from the foliage of 

 such species of Spiraea as we have been considering to that 

 of S. millejolia Torr., now known as Chamaebatiaria millefolium 

 (Torr.) Maxim., which occurs from Idaho to Arizona and southern 

 California. In this species and the related C. glutinosa described 

 by Rydberg 29 from Nevada, the leaves are pinna tely divided and the 

 primary divisions are again divided, as in many ferns. Various 

 other features separate this genus from Spiraea proper, but the 

 hnely bipinnate type of leaf must have been derived from leaves 

 which were nearly entire, and it is easiest to conceive this as 

 having occurred in a few well marked steps. Complete continuity 

 in such a process is Out of the question. 



Summary and conclusions 



In this paper, which is an attempt to apply the concepts of 

 mutation to the practical discrimination of species and the under- 

 standing of their relationships, I have selected for consideration 

 several pairs of species and their relatives. It is found that these 

 pairs bear very different relationships to each other, both as regards 

 their characters and their distribution. They may occupy the 

 same territory or adjacent areas, they may overlap, or be widely 

 separated. Again, one species may be a giant of the other, or may 

 differ by a few sharp differences which have probably originated 

 as units, or may show differences which cannot be externally ana- 

 lyzed in this way. 



Thus, Spiranthes cermia is a tetraploid giant of 5. gracilis or 

 a related species. Maianthemum dilatatum is perhaps a cell giant 



