The Presidential Address. 71
rigorous necessity of coping with new conditions is met by a
call upon the variational resources of the species, those
individuals which are best adapted to the new requirements
surviving and leaving offspring which inherit the ad-
vantageous qualities, whilst the less favoured individuals
succumb in the struggle. The process has been happily
termed by Herbert Spencer, "the survival of the fittest."
In this way a species becomes gradually transformed, or,
when occupying an extended area, may give rise to two or
more species; and since Nature's operations are seldom
conducted by cataclysms, the changes in external conditions
come on slowly, the adaptation being perfected from genera-
tion to generation by the continuous selection of the fittest—
by the summing up of all the advantageous variations
through inheritance. Making use of a mechanical simile, we
may say that the individual constituents of the species move
in the direction of least resistance towards the position of
new equilibrium.
The struggle for life is necessarily the more severe the
more nearly alike are the competing organisms, because the
nearer the relationship between the latter the more closely do
they agree in their requirements—the greater is the similarity
in their structure, habits, and constitution. "We may
assume that the modified descendants of any one species will
succeed so much the better as they become more diversified
in structure, and are thus enabled to encroach on places
occupied by other beings."6 Now survival of the fittest
involves the extinction of the unfitted, and as diversity gives
the greater chance of success to the larger number of
organisms, there is a tendency for variational extremes to
survive at the expense of the less divergent varieties—a
tendency for species to break up into heterogeneous forms
through "divergence of character."
Such in broad outline is the theory of Natural Selection
advanced by Darwin to account for the origin of species, and
it has now become a part of the scientific history of our time
that very similar, if not identical, views were put forward by
6 'Origin of Species,' 6th ed., p. 90.