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We have now passed in review, in more or less detail, the main facts on
which the theory of "the origin of species by means of natural
selection" is founded. In future chapters we shall have to deal mainly
with the application of the theory to explain the varied and complex
phenomena presented by the organic world; and, also, to discuss some of
the theories put forth by modern writers, either as being more
fundamental than that of Darwin or as supplementary to it. Before doing
this, however, it will be well briefly to summarise the facts and
arguments already set forth, because it is only by a clear comprehension
of these that the full importance of the theory can be appreciated and
its further applications understood.
The theory itself is exceedingly simple, and the facts on which it
rests--though excessively numerous individually, and coextensive with
the entire organic world--yet come under a few simple and easily
understood classes. These facts are,--first, the enormous powers of
increase in geometrical progression possessed by all organisms, and the
inevitable struggle for existence among them; and, in the second place,
the occurrence of much individual variation combined with the hereditary
transmission of such variations.
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