THE EXTINCTION OF SPECIES. 899 



27. Missouri Flora. Eegion of the Missouri Eiver and of the high land between the Rocky 



Mountains and the Cascade Mountains. 



28. Flora of the Pacific Slope. Coast region west of the Cascade Mountains, California. 



29. Texas Flora. Arizona, Texas, North Mexico. 



30. Mexican Flora. Mexico as far as Nicaragua. 



31. Antilles Flora. The Antilles and the Bahamas, southern extremity of Florida. 



32. Brazilian Flora. Bounded to the west bv the Andes from the Gulf of Guayaquil to Tucuman, 



extending southward to 30° S. Lat. 



33. South American Flora. Coast land west of the Andes and the part of South America be- 



tween 30" and 50° S. Lat. 



34. Magellan Flora. The extremity of S. America from 50° S. Lat., excluding the high mountains. 



35. Antarctic Flora. Antarctic Islands, high mountains of the southern extremity of South 



America. 



In this outline only a passing reference has been made to the Floras which 

 at present occupy the higher regions of mountain chains, and which are restricted 

 to comparatively small areas. Yet there is often far more difference between these 

 and the Floras of the lower parts of the mountains and of the adjacent plains than 

 there is between Floras which exist side by side in the lowlands, and are usually 

 scattered over wide areas. Thus, for example, the Flora of the high mountains 

 of Central Europe, commonly known as the Alpine Flora, differs so utterly from 

 the Baltic Flora developed to the north of the Alps, and from the Mediterranean 

 Flora flourishing to the south on the shores of the Mediterranean Sea, that it could 

 not be classed either with the one or with the other. The same phenomenon is 

 observed in other regions where lofty mountains occur, and, probably in addition 

 to the thirty-five Floras above mentioned, as many more high-mountain Floras 

 might be distinguished. The importance of these mountain Floras in relation to 

 the formation of the Floras of the plains and to the general history of the plant- 

 world will be set forth in the next section. 



5 THE EXTINCTION OF SPECIES. 



In the portion of this work which dealt with the origin of species, the view was 

 taken that the new species which make their appearance in the course of ages are 

 the products of cross-breeding between previously existent species. Valuable 

 support is given to this opinion by facts relating to such genera as are repre- 

 sented in one district by a large number of species, and in another by a single 

 species only. In many cases the wealth of forms comprised by a genus is pro- 

 digious. It is no easy matter to bring the numerous species of such a genus under 

 review in a single series, because they are connected not only lineally, but also 

 collaterally amongst themselves in all sorts of ways by intermediate forms. Within 

 these generic spheres new forms continue to spring up in our time, and these are 

 proved to be the results of crossings. For example, the multiplicity of forms 

 included in the Bramble-genus (Rubus) is extremely great in Central Europe. 



