448 KINSHIP AND ADAPTATION 
Darwin defined natural selection as meaning “the preserva- 
tion of favored races in the struggle for existence.”’ He 
accepted Herbert Spencer’s phrase, ‘‘survival of the fittest’”’ 
as a good equivalent. | 
The theory of natural selection assumes, then, a struggle 
for existence among contemporary individuals in a given 
environment—a struggle resulting from vastly more of each 
kind being produced than can possibly survive, and so in- 
tense that even a slightly advantageous peculiarity may be 
enough to secure for its possessor both life and offspring. 
Such favoring peculiarities, according to the theory, are to 
be found in the small differences observable among indi- 
viduals of the same kind living in the same environment. 
Hence we need not suppose them to be acquired, and we 
may safely regard them as hereditary because experience 
shows that peculiarities of this sort are regularly inherited. 
Since these slight departures from the parent form take 
various directions under the same circumstances, they are 
termed fluctuating variations, in distinction from definite 
variations, which are all in the same direction, as must be 
the case with those due to the same influence—like a direct 
effect of the environment—acting upon similarly constituted 
organisms. Darwin did not pretend to explain the origin 
of fluctuating variations further than to regard them as con- 
stitutional peculiarities resulting from the interplay of in- 
ternal forces. These he saw might be so delicately balanced 
as to be readily disturbed by change of environment, and he 
recognized that a change of conditions commonly induces — 
variability, and so gives a wider range of differences for selec- 
tion to work upon, as when a horticulturist desiring to develop 
a new variety subjects his seedlings to unaccustomed condi- 
tions. But in so far as the variations are in different direc- 
tions, they must be regarded as induced by the change rather 
than produced directly by it, and so may be spontaneous 
rather than acquired. 
Darwin’s theory carried to its extreme by his modern 
followers, known as Neo-Darwinians, denies that acquired 
peculiarities are ever fixed by inheritance. Only what was 
inborn in the parent, they say, can be transmitted to the 
