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
undoubtedly represent tolerable uniformity in the demands they make as
regards conditions of life, and in so far they are alike. And among them
a severe competition for food must be taking place. In those cases in
which certain species readily grow in each other's company--and cases of
this kind are familiar to florists--when, for instance, Isoetes, Lobelia
Dortmanna, and Litorella lacustris occur together--the common demands
made as regards external conditions obviously form the bond that unites
them. Between such species a competitive struggle must take place. Which
of the species shall be represented by the greatest number of
individuals certainly often depends upon casual conditions, a slight
change in one direction or the other doubtless often playing a decisive
role; but apart from this it appears that morphological and biological
features, for example, development at a different season, may change the
nature of the competition.

Yet there are in every plant-community numerous species which _differ
widely_ in the demands they make for light, heat, nutriment, and so on.
Between such species there is less competition, the greater the
disparity in their wants; the case is quite conceivable in which the
_one species should require exactly what the other would avoid_; the two
species would then be complementary to one another in their occupation
and utilization of the