CHAP. XIII SOUTHWARD MIGRATIONS 247 



North America from the south. Thus a South American fauna 

 entered the United States in the Upper Miocene.^ 



Plants so typical of the Cape flora as Calodcndrnm capcnse, 

 Thunb., and orchids of the genus Disa, occur on the plateaux of 

 Equatorial Africa associated with the Mediterranean and north 

 temperate species. This certainly suggests that in Africa 

 plants have been introduced into the equatorial regions from 

 the south, as well as from the north. That the rule that plants 

 and animals have usually travelled from north to south is not 

 so universal as to prohibit this, is shown by the fauna of the 

 Andes. The theory of a migration from north to south during 

 a cooler period has been applied there, but H. W. Bates, in 

 discussing the results obtained by Mr. Whymper's collection, 

 pointed out that the facts give no support to it. He says : 

 " It seems to me a fair deduction from the facts here set forth, 

 that no distinct traces of a migration during the life-time of 

 existing species, from north to south, or vice versa, along the 

 Andes, have as yet been discovered, or are now likely to be 

 discovered." " On the contrary, he claims that there are no 

 traces in the Andes of Ecuador of temperate forms of either 

 beetles or butterflies, and the fauna consists of modified repre- 

 sentatives of the genera of the neighbouring lowlands. 



Mountain floras, therefore, are of two types — those which 

 have been left as relics of an older or foreign flora, and those 

 which have developed i?i situ by the adaptation of local, low- 

 land species. In British East Africa there are representatives 

 of both types. Many of the mountains are not sufficiently 

 high to reach the levels at which alone the temperate plants 

 can live. In such cases they have been killed off, and the 

 summits are occupied by dwarfed, or otherwise modified, re- 

 presentatives of the species of the adjoining lowlands. The 

 occurrence, however, of species of such genera as Clematis, 

 Polygala, etc., on the summits of the Taita Mountains, shows 

 that in some localities representatives of both groups occur 

 together. 



^ W. B. Scott, " The Mammalia of the Deep River Beds," Trans. Amer. Phil. Soc. 

 vol. xvii. (1894), p. 62. Many similar cases could be quoted. A book has, indeed, 

 recently been issued by C. Di.xon, The Migration of British Birds (1895), which main- 

 tains, as the fundamental law of distribution, that species never migrate to the south. 



2 E. Whymper, Travels amongst the Great Andes of the Equator, vol. ii.. Supple- 

 mentary Appendix (1891), p. 4. 



