CHAP, xvii COMPETITION AMONGST SPECIES ON SOIL 71 



in which there flourish on the same soil plants coming from the most 

 diverse soils. But if we neglect the garden, only very few (mainly indi- 

 genous) plants will emerge as victors from the ensuing struggle. Plants 

 are evidently, in general, tolerably impartial as regards soil, if we except 

 certain chemical and physical extremes (abundance of common salt, 

 of lime, or of water), so long as they have no competitors ; only some few 

 plants may perhaps be regarded as obligatory in the one or the other 

 respect ; well-nigh all are facultative, and their occurrence depends upon 

 competitors. If these be present, the one drives back the others, and 

 the victorious species is the one that can best utilize the given combina- 

 tion of soil, light, climate, and so forth. For instance, according to 

 Fliche, the Scots pine (Pinus sylvestris) over the whole Champagne is 

 confined to calcareous soil and is wanting on non-calcareous soil ; the 

 reason for this is that the Scots pine is an introduced plant, to which 

 the climate, without being actually hostile, is yet not favourable ; on 

 non-calcareous soil, upon which it thrives admirably elsewhere, it is 

 here suppressed by other species, and only on calcareous soil does it 

 become dominant, even then without developing really well. We should 

 therefore err were we to describe it as being calciphilous ; like many 

 other forest-trees it will grow on the most diverse soils, and in Denmark 

 it is most frequent on sandy soil. When in Denmark we find the oak 

 growing sometimes on moist compact soil, and sometimes on dry poor 

 soil, the reason for this is not that it prefers these soils, but that it is 

 expelled from others by the beech. Similar competitors are ling (Calluna), 

 and many other species, such as Anthemis Cotula and A. arvensis, Carlina 

 vulgaris and C. acaulis, Prunella vulgaris and P. grandiflora, Veronica 

 Teucrium and V. Chamaedrys. 1 In the Alps, according to Nageli 2 

 Rhododendron ferrugineum and R. hirsutum, as well as Achillea moschata 

 and A. atrata (silicicolous and calcicolous plants) struggle against one 

 another. P. E. Miiller 3 has brought forward several examples of forest- 

 trees in mountains driving each other back in the same manner ; lofty 

 forests of silver-fir, for example, are sharply delimited from lofty forests 

 of another species without it being a question of inability to thrive at 

 the boundaries. Moreover, Bonnier 4 and others came to the conclusion 

 that species restricted to calcareous soil in one district may be calciphobous 

 in another, and indifferent to soil in a third. In the middle of its distri- 

 butional area a species often makes no selection as to soil, but outside 

 this central position it is forced by other species to exercise a choice. 5 



As noteworthy examples of plants being able to flourish luxuriantly 

 in countries other than their own homes, we may cite : Erigeron canadensis, 

 Galinsoga parviflora, from tropical Peru ; Oenothera biennis, and other 

 American weeds that are now common in Central Europe ; Impatiens 

 parviflora and Elodea canadensis may also be mentioned. On the other 

 hand, Salsola Kali, a common littoral European plant, has become a 

 most pestilent weed in the cornfields of North America ; in places it 

 takes nearly complete possession of the soil. 6 



1 After Pietsch, according to Ludwig, 1895, P- 121. * Nageli, 1872. 



1 P. E. Miiller, 1871, 1887. * Bonnier, 1879. ' See Section XVII. 



6 Among more recent literature on this subject readers should consult the works 

 of Cowles, 1901 ; Saint-Lager, 1895 ; Schimper, 1898; Gillot, 1894; Gain, 1895; 

 Ernst, 1967. The older literature is to be found cited in Engler, 1899, on pp. 164-6. 



