276 P^'of. Guthrie^ LL.B. — On the Subjective Causes of [April 2o, 



recognises is that of hereditary influence, but he does not seem to 

 deny the possibility of the existence of other subjective tendencies. 



My object in the present paper is to enquire how far the geogra- 

 phical distribution of plants and other organisms on the earth's surface 

 throws light on this question. With respect to the distribution of 

 plants the phenomena to be accounted for are as follows : 



(1.) Large numbers of identical and of closely allied genera and 

 species of plants often extend over continuous areas possessing more 

 or less uniform physical conditions. 



These areas are called Floral Areas. Schouw divides the surface 

 of the earth into twenty-five Floral areas, defining a Floral area 

 as being a region in which : 



I. At least half the known species are endemic. 



II. A fourth part of the genera are endemic or nearly so. 



III. The oiders are either peculiar to the region or have their 

 maximum there. 



(2.) Identical and closely allied genera and species occur in widely 

 separated areas having similar physical conditions. Thus : 



I. Two-thirds of the Antarctic genera and at least thirty species 



are common to that region and Europe. 



II. Between forty and fifty flowering species of Tierra del 

 Fuego are European. 



III. Half the genera of extra-tropical South America are 

 European. 



IV. Half the genera of New Zealand are common to Europe. 



V. Several so-called European species are found on the Austra- 



lian Alps. 



(3.) Regions very similar in climate and other physical conditions 

 have very different Floras : Thus : 



I. North America and Europe. 



II. Mexico, South Africa and Australia. 



III. Tropical Africa, Tropical America and the Moluccas. 

 There are two theories to account for these facts : 



(«.) The theory of local origin and migration. 



(Jb.) The theory of simultaneous evolutionary development. 



These two theories are not absolutely exclusive of each other, but 

 they depend on somewhat different views as to the origin of species. 



There are however certain common grounds of agreement which it 

 will be expedient first to mention. 



