﻿GATES— PAIRS OF SPECIES 



Peak, Chehalis County, Washington, 3500 ft. elevation, July 24, 1897 ;« 

 G. E. CoghiU 151, Pecos River, T.R., New Mexico, August 5, 1898;^ Fendler 12, 

 Xcw Mexico, 1847. 2 " 



It seems obvious that in studying the species of such a genus as 

 Actaea the variations which lead from one species to another have, 

 at least in many cases, not been gradual or continuous, but definite 

 and in certain well defined directions. It seems clear that the 

 nature of these variations has been determined by the internal 

 structure of the germ plasm, the environment acting for the most 

 part merely as a releasing stimulus. 



Spiraea tomentosa L. and S. alba DuRoi 

 Spiraea alba DuRoi 25 (fig. n) is better known under the name 

 of the European species S. salicifolia L. It has a diagonal distri- 

 bution across North America from North Carolina, New York, and 

 Ontario to Saskatchewan, Iowa, and Missouri, and is also found in 

 Siberia. The members of this pair of species are evidently much 

 less closely related than in the other pairs I have mentioned. 

 Fossil predecessors of 5. tomentosa (fig. 10) show that the tomentose 

 group of species has long been separated from the non-tomentose 

 group. In other words, the characteristic tomentum on the 

 ventral leaf surfaces appeared in these forms long ago, and heredity 

 has handed it down since that time. It is only the accident of dis- 

 tribution, therefore, that makes S. tomentosa and 5. alba a pair. 

 Indeed, we could better consider the group a trio, for in its more 

 eastern range in the Atlantic states and eastern Canada S. tomentosa 

 is paired with 5. latifolia (Ait.) Borkh. (fig. 12). The latter species 

 is still frequently known under the name S. salicifolia. Certain 

 more recent segregates from S. salicifolia, such as S. corymbosa Rat. 

 and S. virginiana Britton, which are more restricted in their dis- 

 tribution, also occupy portions of the range of S. tomentosa. 



