GROUPS OF INCOMPLETE SPECIES. i8i 



the right bank, but this is removed during winter, 

 and the swollen waters of the river made all the usual 

 fords impassable for the present. 



Many forms of Escallonia were abundant along the 

 stream. A few species only of this genus are culti- 

 vated in English gardens, but in their native home, 

 the middle and lower slopes of the Andes, they 

 exhibit a surprising variety of form while preserving 

 a general similarity of aspect. They are all ever- 

 green shrubs, some rising to the stature of small trees, 

 with undivided, thick, usually glossy leaves, and white, 

 red, or purplish flowers. Although forty-three dif- 

 ferent species have been described from Chili alone, 

 it is easy to find specimens not exactly agreeing with 

 any of them, and to light upon intermediate forms 

 that seem to connect what appeared to be quite 

 distinct species. They afford an example of a fact 

 which I believe must be distinctly recognized by 

 writers on systematic botany — that in the various 

 regions of the earth there are some groups of vege- 

 table forms in which the processes by what we call 

 species are segregated are yet incomplete ; and amid 

 the throng of closely allied forms, the suppression of 

 those least adapted to the conditions of life has not 

 advanced far enough to differentiate those which can 

 be defined and marked by a specific name. 



To the believer in evolution, it must be evident 

 that at some period in the history of each generic 

 group there must have occurred an interval during 

 which species, as we understand them, did not yet 

 exist ; and perhaps the real difficulty is to explain 

 why such instances are not more frequent than they 



