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. From these two great classes of facts,
which are universal and indisputable, there necessarily arises, as
Darwin termed it, the "preservation of favoured races in the struggle
for life," the continuous action of which, under the ever-changing
conditions both of the inorganic and organic universe, necessarily leads
to the formation or development of new species.

But, although this general statement is complete and indisputable, yet
to see its applications under all the complex conditions that actually
occur in nature, it is necessary always to bear in mind the tremendous
power and universality of the agencies at work. We must never for an
instant lose sight of the fact of the enormously rapid increase of all
organisms, which has been illustrated by actual cases, given in our
second chapter, no less than by calculations of the results of unchecked
increase for a few years. Then, never forgetting that the animal and
plant population of any country is, on the whole, stationary, we must be
always trying to realise the ever-recurring destruction of the enormous
annual increase, and asking ourselves what determines, in each
individual case, the death of the many, the survival of the few. We must
think over all the causes of destruction to each organism,--to the seed,
the young shoot, the growing plant, the full-grown tree, or shrub, or
herb, and again the fruit and seed; and among animals, to the egg or
new-born young, to the youthful, and to the adults. Then, we must always
bear in mind that what goes on in the case of the individual or family
group we may observe or think of, goes on also among the millions and
scores of millions of individuals which are comprised in almost every
species; and must get rid of the idea that _chance_ determines which
shall live and which die.