nce in arithmetical progression. In the preface
to the second edition of his _Essay on the Principle of Population_
Malthus acknowledged his indebtedness to "Hume, Wallace, Dr. Adam Smith
and Dr. Price." Adam Smith no doubt anticipated and perhaps suggested to
Malthus his thesis in such passages in the _Wealth of Nations_ as,
"Every species of animals naturally multiplies in proportion to the
means of their subsistence," "The demand for men necessarily regulates
the production of men." These statements of the relation of population
to food supply, however, are incidental to Smith's general theories of
economics; the contribution of Malthus lay in taking this principle out
of its limited context, giving it the character of scientific
generalization, and applying it to current theories and programs of
social reform.

The debt of biology to Malthus is acknowledged both by Darwin and by
Wallace. Fifteen months after Darwin had commenced his inquiry a chance
reading of Malthus' _Essay on the Principle of Population_ gave him the
clue to the explanation of the origin of species through the struggle
for existence. During an attack of intermittent fever Wallace recalled
Malthus' theory which he had read twelve years before and in it found
the solution of the problem of biological evolution.

Although the phrase "the struggle for existence" was actually used by
Malthus: Darwin, Wallace, and their followers first gave it a general
application to all forms of life. Darwin in his _The Origin of Species_,
published in 1859, analyzed with a wealth of detail the struggle for
existence, the nature and forms of competition, natural selection, the
survival of the fittest, the segregation and consequent specialization
of species.

Biological research in recent years has directed attention away from the
theory of evolution to field study of plant and animal communities.
Warming, Adams, Wheeler, and others have described, in their plant and
animal ecologies, the processes of competition and segregation by which
communities are formed. Clements in two studies, _Plant Succession_ and
_Plant Indicators_, has described in detail the life-histories of some
of these communities. His analysis of the succession of plant
communities within the same geographical area and of the relations of
competitive co-operation of the different species of which these
communities are composed might well serve as a model for similar studies
in human ecology.