368 STRUGGLE BETWEEN PLANT-COMMUNITIES SECT, xvn 



pure associations until above the limit of Laurus canariensis, where it is 

 common. 



One circumstance of significance in determining the distribution of 

 species, is which species is the first to arrive. If the conditions be equally 

 favourable to several species, the result of any struggle between them 

 will depend upon which species succeeds in seizing the ground ; in such 

 a case the first settlers may be able to retain possession (as beati possidentes). 



Possibly in this way we can explain the distribution of phragmiteta, 

 scirpeta, and other associations in European reed-swamps, or the distri- 

 bution of various dwarf-shrubs on dwarf-shrub heaths. 



Sometimes species can avoid competition by developing at a different 

 time or by having their subterranean organs at different depths. Wood- 

 head * names these ' complementary associations ', and describes an 

 English wood, near Huddersfield, where Holcus mollis, Pteris aquilina, 

 and Scilla festalis amicably give rise to a ' complementary society '. 

 Holcus lies nearest the surface, and is succeeded by Pteris, while Scilla 

 lies deepest in the soil ; Scilla appears above ground first and then Pteris. 

 ' Their soil-requirements, their mode of life, their periods of active vege- 

 tative growth, their times of flowering and fruiting, are for the most part 

 different.' 



The results of struggles among plants are therefore : 



i. The arrangement of species in natural communities. 



ii. Unceasing change in the composition of vegetation all the world 

 over. 



iii. Occurrence of rare species. 



iv. Possibly, the origin of new species. 



CHAPTER XCIX. RARE SPECIES 



THE struggle among species finds expression in the occurrence of rare' 

 species, which are so interesting to many botanical collectors. 



A species may be rare in an area for various reasons, including : 



i. Because a suitable habitat is lacking, for instance, rocky soil in' 

 plains. 



ii. Because it is an immigrant that has just reached the district in' 

 question, though it may be becoming more abundant here every year, as 

 is the case with Elodea canadensis. 



iii. Because it is a ' relic-plant *, that is to say, a relic of a former 

 but now suppressed vegetation. 



The extensive migration of plants that took place after the Glacial 

 Epoch, 2 has perhaps left its traces in many supposed relic-plants, which 

 have maintained themselves here and there, but now occur only sporadically 

 in small numbers and are gradually dying out. The localities where they 

 have survived are those agreeing most closely with the conditions that 



Prevailed in the Tundra Epoch, namely, cold, wet low-moors and 

 phagnum-moors. Among such survivals in Denmark and northern 

 Germany various botanists include Cornus suecicus, Rubus Chamae- 



1 Woodhead, 1906. * See Chap. XCVI. 



