^0.11 GRAY AND HOOKER ON THE ROCKY MOUNTAIN FLORA. 57 



of tbe world and an eighth part of that of Xorth America. It forms 

 fully an eighth part of the Atlantic flora. It appears to form between 

 a sixth and a seventh part of the species in the district west of the Rocky 

 Mountains, and a still larger proportion of the genera. Here are found 

 most of the Heleiiioidem and almost all the Madiecc, an<l of the otiier 

 tribes there is no lack, except of Vernoniacece. 



The ScroijhulariaceccnTe far more conspicuous and preponderant on the 

 western side of the continent, not so much, if at all, in genera, but vastly 

 in the number of species. This is mainly owing to the Avonderfnl de\'el- 

 opnient of certain genera {Pentstenion, Mimulus, Gastilleia, OrtJiocarpiis), 

 as has been already stated. 



The Polemoniaceoi form an even more marked feature, the western 

 flora having more genera, indeed five times as many forms and five times 

 as many si)ecies as the eastern. 



The nydropliyllacew are in nearly a similar case; the Borraginacece 

 ai^proach to it, and so do the Clienopodiacece. 



The Eriogonccc, however, claim' the first rank ; considering the num- 

 ber of the species and the distribution of the group, no other group of 

 ordinal or subordinal rank is so completely characteristic of Western 

 isTorth American botany as this. 



Finallj', as to the Liliacece (in the extended sense), although the At- 

 lantic flora is rich in them, yet the Pacific region considerably surpasses 

 it in the number of genera, and largely sur^Dasses it in the number of 

 species and in the conspicuousness of the flowers. 



III. 



NORTH AMERICAN TYPES IN SOUTH AMERICA. 



The botany of the southern part of the eastern great plain of the 

 interior arid district, and of Southern California, merges in that of the 

 Texano-Arizonian belt, and .this into that of the Mexican plateau. It is 

 probable that from these plateaux our western regions received the 

 greater part of their i)resent forms and types. 



We may expect soon to know more than we now do of the botany of 

 the cooler jiarts of Mexico, and to have this knowledge in a conveni- 

 ently available form. 



It appears, however, that the Texano-Arizonian species or their repre- 

 sentatives do not prevail far down into Mexico, and that the arctic- 

 alpine species and other northern types of the higher mountains arc 

 soon replaced southward by audine forms. 



There are clear if not verj' numerous indications that there has at 

 some former time been greater opportunity than now for the extension 

 of Xorth American species and types into the southern hemisphere. 

 And it ai)i)ears that this has taken place mainly along the western side 

 of the American continent, on which the mountains abut on the coast — 

 that is. as respects American plants which have found tlieir way to 



