448 KINSHIP AND ADAPTATI()N 



Darwin defined natural selection as meanin;;; "tiie 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 chfferences 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 hereditarj^ 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 similarlj' 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 sulijects his seedlings to unaccustomed condi- 

 tions. But in so far as the variations are in different direc- 

 tions, they must lie 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 (>xtreme li>- his modern 

 followers, known as Neo-Darwinians, denies that acquired 

 peculiarities are ever fixed l)y inheritance. Only what was 

 inborn in the parent, they say, can be transmitted to the 



