ognomy can very easily occur together in the same natural
community. But beyond this, as already indicated, species differing
widely, not only in physiognomy but also in their whole economy, may be
associated. We may therefore expect to find both great variety of form
and complexity of interrelations among the species composing a natural
community; as an example we may cite the richest of all types of
communities--the tropical rain-forest. It may also be noted that the
physiognomy of a community is not necessarily the same at all times of
the year, the distinction sometimes being caused by a rotation of
species.

The different communities, it need hardly be stated, are scarcely ever
sharply marked off from one another. Just as soil, moisture, and other
external conditions are connected by the most gradual transitions, so
likewise are the plant-communities, especially in cultivated lands. In
addition, the same species often occur in several widely different
communities; for example, Linnaea borealis grows not only in coniferous
forests, but also in birch woods, and even high above the tree limit on
the mountains of Norway and on the fell-fields of Greenland. It appears
that different combinations of external factors can replace one another
and bring into existence approximately the same community, or at least
can satisfy equally well one and the same species, and that, for
instance, a moist climate often completely replaces the forest shade of
dry climates.

The term "community" implies a diversity but at the same time a certain
organized uniformity in the units. The units are the many individual
plants that occur in every community, whether this be a beech forest, a
meadow, or a heath. Uniformity is established when certain atmospheric,
terrestrial, and other factors are co-operative, and appears either
because a certain defined economy makes its impress on the community as
a whole, or because a number of different growth-forms are combined to
form a single aggregate which has a definite and constant guise.

The analysis of a plant-community usually reveals one or more of the
kinds of symbiosis as illustrated by parasites, saprophytes, epiphytes,
and the like. There is scarce a forest or a bushland where examples of
these forms of symbiosis are lacking; if, for instance, we investigate
the tropical rain-forest we are certain to find in it all conceivable
kinds of symbiosis. But the majority of individuals of a plant-c