v GRAY ami HOOKEB ON THE ROCKY MOUNTAIN FLORA. 57 



of the world and an eighth pari of thai of North America. It forma 

 fully an eighth pari of the Atlantic flora, it appears to form between 

 a sixth and a seventh pari of the species in the districl wesl of the Rockj 

 Mountains, and a Btill larger proportiou of the genera. Bere are found 

 most of the Helenioidea and almost all the Ufarfieft, and of the other 

 tribes there is no lack, except <>f Vernoniat 



The Scrophulariacecesxe tar more conspicuous and preponderant on t be 

 western side of the continent, not so much, if at all, in genera, I »ut \ a>lh 



in the number of species. This is mainly owing t<> the wonderful devel- 

 opment of certain genera {Pentstemon^ Mimulus, Castilleia, Orthocarpiis), 

 as has been already stated. 



The Polemoniacea form an even more marked feature, the western 

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

 as many species as the eastern. 



The Hydrcphyllacea arc in nearly a similar case: the Borragim 

 approach to it, and so do the Chenopodiacece. 



The Eriogon&Bj however, claim the flrst rank; considering the num- 

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

 ordinal or subordinal rank is SO completely characteristic of Western 

 North American botany as this. 



Finally, as to the Liliacece (in the extended sense), although the At 

 lantic flora Is rich in them, yet the Pacific region considerably surpi 

 it in the number of genera, and largely surpasses it in the number ol 

 species and in the conspicuousness of the flowers. 



III. 



NORTH AMERICAN TYPES IN sol Til AMERICA. 



The botany of the southern pari 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 

 able that from these plateaux our western regions received the 

 er part of their present forms and types. 

 We may expect soon to know more than we now d<> of the botany ol 

 the cooler parts of .Mexico, ami to have this knowledge in a conveni 

 ently available form. 



It appears, however, that the Texano-Arizonian srfeciesor their repre- 

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

 alpine species and other northern types of the higher mountains 

 soon replaced southward by andine forms. 

 There are clear if not very numerous indications that there bat 

 former time been greater opportunity than now for the . 

 oi' Sortn American species and types into the southern bemispl 

 [)pears that this has talon place mainly along the western 

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

 American phi: ts w Inch have found then 



