94 COMMUNAL LIFE OF ORGANISMS sect, ii 



undoubtedly represent tolerable uniformity in the demands they make 

 as regards conditions of life, and in so far they are ahke. And among them 

 a severe competition for food must be taking place. In those cases in 

 which certain species readily grow in each other's company and cases 

 of this kind are famihar to florists when, for instance, Isoetes, Lobelia 

 Dortmanna and Litorella lacustris occur together the common demands 

 made as regards external conditions obviously form the bond that unites 

 them. Between such species a competitive struggle must take place. 

 Which of the species shall be represented by the greatest number of 

 individuals certainly often depends upon casual conditions, a slight 

 change in one direction or the other doubtless often playing a decisive 

 role ; but apart from this it appears that morphological and biological 

 features, for example development at a different season, may change the 

 nature of the competition. 



Yet there are in every plant-community numerous species which 

 differ widely in the demands they make for light, heat, nutriment, and 

 so on. Between such species there is less competition the greater the 

 disparity in their wants ; the case is quite conceivable in which the one 

 species should require exactly what the other would avoid ; the two species 

 would then be complementary to one another in their occupation and 

 utihzation of the same soil. 



There are also obvious cases in which different species are of service 

 to each other. The carpet of moss in a pine-forest, for example, protects 

 the soil from desiccation and is thus useful to the pine, yet, on the other 

 hand, it profits from the shade cast by the latter. 



As a rule, a limited number of definite species are the most potent, 

 and, like absolute monarchs, can hold sway over the whole area ; while 

 other species, though possibly present in far greater numbers than these, 

 are subordinate or even dependent on them. This is the case where 

 subordinate species only flourish in the shade or among the fallen frag- 

 ments of dominant species. Such is obviously the relationship between 

 trees and many plants growing on the ground of high forest, such as 

 mosses, fungi and other saprophytes, ferns, Oxalis Acetosella, and their 

 associates.^ In this case, then, there is a commensalism in which individuals 

 feed at the same table but on different fare. An additional factor steps 

 in when species do not absorb their nutriment at the same season of the 

 year. Many spring-plants for instance, Galanthus nivalis, CorydaHs 

 solida and C. cava have withered before the summer-plants commence 

 properly to develop. Certain species of animals are Mkewise confined 

 to certain plant-communities. But one and the same tall plant may, 

 in different places or soils, have different species of lowly plants as com- 

 panions ; the companion-plants of high beech-forest depend, for instance, 

 upon climate and upon the nature of the forest soiP ; Pinus nigra, accord- 

 ing to von Beck, can maintain under it in the different parts of Europe 

 a Pontic, a Central-European, or a Baltic vegetation. 



There are certain points of resemblance between communities of 

 plants and those of human beings or animals ; one of these is the com- 

 petition for food which takes place between similar individuals and 

 causes the weaker to be more or less suppressed. But far greater are 



1 See Hock, 1892, 1893. ' P. E. Miiller, 1887. 



