CHAP. XXVI COMMENSALISM 93 



are satisfied in their precise habitat to such an extent that the species 

 can maintain itself here against rivals. Natural unmixed associations 

 of forest-trees are the result of struggles with other species. But there 

 are differences as regards the ease with which a community can arise and 

 establish itself. Some species are more social than others, that is to say, 

 better fitted to form communities. The causes for this are biological, 

 in that some species, like Phragmites, Scirpus lacustris, Psamma (Ammo- 

 phila) arenaria, Tussilago Farfara, and Asperula odorata, multiply very 

 readily by means of stolons ; or others, such as Cirsium arvense, and 

 Sonchus arvensis, produce buds from their roots ; or yet others produce 

 numerous seeds which are easily dispersed and may remain for a long 

 time capable of germinating, as is the case with Calluna, Picea excelsa, 

 and Pinus ; or still other species, such as beech and spruce, have the 

 power of enduring shade or even suppressing other species by the shade 

 they cast.^ A number of species, such as Pteris aquilina, Acorus Calamus, 

 Lemna minor, and Hypnum Schreberi, which are social, and likewise 

 very widely distributed, multiply nearly exclusively by vegetative means, 

 rarely or never producing fruit. On the contrary, certain species, for 

 example many orchids and Umbelliferae, nearly always grow singly. 



In the case of many species certain geological conditions have favoured 

 their grouping together into pure communities. The forests of northern 

 Europe are composed of few species, and are not mixed in the same sense 

 as are those in the tropics, or even those in Austria and other southern 

 parts of Europe : the cause for this may be that the soil is geologically 

 very recent, inasmuch as the time that has elapsed since the Glacial 

 Epoch swept it clear has been too short to permit the immigration of 

 many competitive species.- 



UNLIKE COMMENSALS 



The case of a community consisting of individuals belonging to one 

 species is, strictly speaking, scarcely ever met with ; but the dominant 

 individuals of a community may belong to a single species, as in the case 

 of a beech-forest, spruce-forest, or hng-heath and only thus far does 

 the case proceed. In general, many species grow side by side, and many 

 different growth-forms and types of symbiosis, in the extended sense, 

 are found collected in a community. For even when one species occupies 

 an area as completely as the nature of the soil will permit, other species 

 can find room and can grow between its individuals ; in fact, if the soil 

 is to be completely covered the vegetation must necessarily always be 

 heterogeneous. The greatest aggregate of existence arises where the 

 greatest diversity prevails.^ The kind of communal life resulting will 

 depend upon the nature of the demands made by the species in regard 

 to conditions of life. As in human communities so in this case, the struggle 

 between the like is the most severe, that is, between the species making 

 more or less the same demands and wanting the same dishes from the 

 common table. In a tropical mixed forest there are hundreds of species 

 of trees growing together in such profuse variety that the eye can scarce 

 see at one time two individuals of the same species,^ yet all of them 



' See Chap. XCVII. * See Warming, 18996. ' See Darwin, 1859. 



* Sec Warming, 1892, 18996. 



