yd OECOLOGICAL FACTORS AND THEIR ACTION sect, i 



many species in Norway reach their extreme stations in altitude and 

 latitude on calcareous soil. Eugeogenous and dysgeogenous kinds of 

 rock may bear the same flora. It is thus that the distribution of beech 

 in Southern France must be explained. In Denmark it passes for a 

 calciphilous plant ; yet in Mediterranean countries, according to Flahault/ 

 only upon siliceous soil does it form extensive forests, while on dry, warm, 

 calcareous soil it is sporadic, having been expelled by Quercus sessiliflora, 

 excepting in cool valleys with north and east exposure. 



As supporting Thurmann's theory we may name Contejean, who 

 however subsequently adopted the rival theory ; and approaching of 

 nearly similar views are A. P. and Alph. de Candolle, Celakovsky, Krasan, 

 Hoffmann, Kerner, H. von Post, Blytt, P. E. Miiller, Negri, and others.^ 

 Yet Thurmann's theory certainly cannot explain all cases. 



In both theories there is some truth ; both chemical and physical 

 relations operate ; the actual truth seems to be that in some (few) cases, 

 where the soil is specially rich in a chemical substance, it is the chemical 

 characters of the soil, but in other (far more frequent) cases it is the 

 physical characters that are of greatest import. When we consider 

 a country like Denmark or the North German Plain, with a soil produced 

 by corrosion and commixture of multifarious kinds of rock, yet scarcely- 

 possessed of any marked chemical characters, then the chemical signifi- 

 cance of soil becomes evident in the halophytic vegetation on the coast, 

 but probably there alone, whilst everywhere else conditions as regards 

 moisture play the leading part.^ Temperature, illumination, air, atmo- 

 spheric precipitations and humidity, chemical nature of the soil, may all 

 be completely alike, and yet is the vegetation different. One solitary 

 factor is different the amount of water in the soil and this it is that is 

 decisive.^ When we further consider that the most important characters 

 6f soil (temperature, aeration, amount of water, evaporation) are mainly 

 dependent on its structure, it then appears that the physical characters 

 of soil are the weightiest, especially because they react upon the amount 

 of water. Chemical differences are always accompanied by physical 

 ones, and chemical characters seem to be capable of replacement by 

 physical ones, but it would appear that physical attributes are in the 

 last instance most frequently decisive. Yet it must be remarked that 

 the amount of nutriment in soil is likewise of grave import a principle 

 of which Grabner and A. Nilsson ^ are advocates. But even the amount 

 of nutriment depends upon the physical features of soil, that is, upon 

 its water-capacity and absorbent faculty. 



Competition among species as a factor of distribution of plants. 



Darwin and Nageli^ directed attention to one factor affecting the distribu- 

 tion of species and the production of plant-communities the competition 

 among species which has not always been taken into consideration, 

 but which must not be overlooked. How trivial a part may be played 

 by chemical distinctions in soil is shown, for instance, by a botanic garden 



^ Flahault, 1893. 



The most recent literature is cited by Woodhead, 1906. 



' Consult pp. 45-8, dealing with ground- water. * Warming, 1894. 



^ Grabner, 1898, 1901 ; A. Nilsson, 1902 b. * NageU, 1865, 1872. 



