ent. Man and animals operate as
marked barriers to ecesis wherever they alter conditions unfavorably to
invaders or where they turn the scale in competition by cultivating,
grazing, camping, parasitism, etc. The absence of pollinating insects is
sometimes a curious barrier to the complete ecesis of species far out of
their usual habitat or region. Parasitic fungi decrease migration in so
far as they affect seed production. They restrict or prevent ecesis
either by the destruction of invaders or by placing them at a
disadvantage with respect to the occupants.

By the term _reaction_ is understood the effect which a plant or a
community exerts upon its habitat. In connection with succession, the
term is restricted to this special sense alone. It is entirely distinct
from the response of the plant or group, i.e., its adjustment and
adaptation to the habitat. In short, the habitat causes the plant to
function and grow, and the plant then reacts upon the habitat, changing
one or more of its factors in decisive or appreciable degree. The two
processes are mutually complementary and often interact in most complex
fashion.

The reaction of a community is usually more than the sum of the
reactions of the component species and individuals. It is the individual
plant which produces the reaction, though the latter usually becomes
recognizable through the combined action of the group. In most cases the
action of the group accumulates or emphasizes an effect which would
otherwise be insignificant or temporary. A community of trees casts less
shade than the same number of isolated individuals, but the shade is
constant and continuous, and hence controlling. The significance of the
community reaction is especially well shown in the case of leaf mold and
duff. The leaf litter is again only the total of the fallen leaves of
all the individuals but its formation is completely dependent upon the
community. The reaction of plants upon wind-borne sand and silt-laden
waters illustrates the same fact.

2. Migration and Segregation[189]

All prehistoric investigation, as far as it relates to the phenomena of
the animate world, necessarily rests upon the hypothesis of migration.
The distribution of plants, of the lower animals, and of men over the
surface of the earth; the relationships existing between the different
languages, religious conceptions, myths and legends, customs and social
institutions--all these seem in this one assumption