-known advantages. But, on the
other hand, it brings with it greater danger of serious damage and
devastation wrought by parasites.

The bonds that hold like individuals to a like habitat are, as already
indicated, identical demands as regards existence, and these demands 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 (Ammophila) 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 favored
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 ling
heath--and only thus far does