the most stolid animals. Indeed, he
has usually retrograded. The need to fight for food and home has been
the spur that has ever driven man forward to establish the manifold
forms of physical and mental life which make up human existence today.
Like the simple adaptive mechanisms of the plant by which it gets air,
and of the animal by which it overcomes its rivals in battle, the
supremely differentiated functions of thought and human relations are
the outcome of the necessity of the organism to become adapted to
entities in its environment.

B. COMPETITION AND SEGREGATION

1. Plant Migration, Competition, and Segregation[188]

Invasion is the complete or complex process of which migration, ecesis
(the adjustment of a plant to a new home), and competition are the
essential parts. It embraces the whole movement of a plant or group of
plants from one area into another and their colonization in the latter.
From the very nature of migration, invasion is going on at all times and
in all directions.

Effective invasion is predominantly local. It operates in mass only
between bare areas and adjacent communities which contain species
capable of pioneering, or between contiguous communities which offer
somewhat similar conditions or contain species of wide range of
adjustment. Invasion into a remote region rarely has any successional
effect (effect tending to transform the character of a plant community),
as the invaders are too few to make headway against the plants in
possession or against those much nearer a new area. Invasion into a new
area or a plant community begins with migration when this is followed by
ecesis. In new areas, ecesis produces reaction (the effect which a plant
or a community exerts upon its habitat) at once, and this is followed by
aggregation and competition, with increasing reaction. In an area
already occupied by plants, ecesis and competition are concomitant and
quickly produce reactions. Throughout the development migrants are
entering and leaving, and the interactions of the various processes come
to be complex in the highest degree.

Local invasion in force is essentially _continuous_ or _recurrent_.
Between contiguous communities it is _mutual_, unless they are too
dissimilar. The result is a transition area or ecotone which epitomizes
the next stage in development. By far the greater amount of invasion
into existing vegetation is of this sort. The movement into a bare area
is likewise c