But if
variations useful to any organic being ever do occur, assuredly individuals
thus characterised will have the best chance of being preserved in the
struggle for life; and from the strong principle of inheritance, these will
tend to produce offspring similarly characterised. This principle of
preservation, or the survival of the fittest, I have called natural
selection. It leads to the improvement of each creature in relation to its
organic and inorganic conditions of life; and consequently, in most cases,
to what must be regarded as an advance in organisation. Nevertheless, low
and simple forms will long endure if well fitted for their simple
conditions of life.
Natural selection, on the principle of qualities being inherited at
corresponding ages, can modify the egg, seed, or young as easily as the
adult. Among many animals sexual selection will have given its aid to
ordinary selection by assuring to the most vigorous and best adapted males
the greatest number of offspring. Sexual selection will also give
characters useful to the males alone in their struggles or rivalry with
other males; and these characters will be transmitted to one sex or to both
sexes, according to the form of inheritance which prevails.
Whether natural selection has really thus acted in adapting the various
forms of life to their several conditions and stations, must be judged by
the general tenour and balance of evidence given in the following chapters.
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