Geographical Distribution. 177 



throwing the light of past history on the puzzle of 

 existing relations, and by analysing the various limits 

 of range, and the various modes of dispersal, which 

 hinder or help the diffusion of organisms. 



Since the publication of Wallace's book there have 

 been many detailed studies of particular groups, e.g. of 

 fishes by Giinther; many detailed studies of particular 

 regions, e.g. of the Philippines by Semper; and many 

 criticisms relating both to the regions and the factors 

 recognized by Wallace. 



The study of the geographical distribution of plants 

 began with Humboldt (1805), who not only described 

 the peculiar " Physiognomik " of various ph to . Geo _ 

 regions, but sought an explanation of the graphical 

 peculiarities by reference to climate and soil 

 two undoubted factors which botanists have never 

 ignored and have often exaggerated. 



At the meeting of the British Association at Cam- 

 bridge in 1845 Forbes directed attention to the impor- 

 tance of past geological changes, insulations, changes 

 of level, &c., in relation to the distribution of plants. 



Another important factor was indicated by Unger in 

 1852, who was the first to connect the present distribu- 

 tion of plants with that of previous ages as disclosed in 

 the rock record. In 1855 Alph. de Candolle expounded 

 the same idea, as Engler has also done in more recent 

 years with conspicuous success. 



As in regard to animals, so with plants, numerous 

 suggestions have been made as to the mapping out of 

 the earth, the various "systems", as they are called, 

 differing from one another in emphasizing different 

 factors. 



Humboldt classified according to geographical zones 

 and sea-level, and Meyen followed him in this simple 

 method. 



Schouw (1823) introduced a new idea of taking statis- 

 tics as to the relative predominance of particular types 

 in different areas, distinguishing the Cinchona realm, 

 the Magnolia realm, and so on to the number of twenty- 

 five, many of which have been confirmed as natural by 

 subsequent workers. 



(M623) M 



