312 THE SURVIVAL OF THE UNLIKE. [XVIII. 



exception of some arctic and sub -arctic species, the 

 plants of North America are singularly distinct from 

 the European plants, although much like them. There 

 are few species which are common to both continents. 

 Most of the plants which were once thought to be 

 the same in both continents are now separated by 

 botanists, and I am convinced that this separation 

 should proceed to nearly, if not quite all, the remain- 

 ing supposed identical species of the temperate lati- 

 tudes. The more closely we study these species the 

 greater the differences of habit and distribution ap- 

 pear to be. All this proves that, while the European 

 and North American floras had a common origin in 

 circumpolar regions, the present floras of the two con- 

 tinents have diverged, until nearly or quite all the 

 specific types in the central and southern areas are 

 dissimilar. This dissimilarity has been brought about 

 by the action of environments — largely of climate — 

 in the two continents. In other words, the habitual 

 dissimilarity of the floras proves that the climatic 

 environments are so different that identical species 

 rarely thrive in both. And this fact lends plausibility 

 to the statement that horticultural varieties, which 

 differ from species only in degree and not in kind, 

 must constantly tend to diverge in the two countries. 

 The dissimilarity of European and American con- 

 generic species is well illustrated in some of our fruits. 

 Thus our cultivated raspberries, blackberries, goose- 

 berries and grapes are American species, and the 

 profitable cultivation of these kinds of fruits did not 

 begin until we gave up our endeavors to grow the 

 European species. The case of the red raspberries 

 is particularly instructive, because the European and 



