102 THE PRINCIPLES OF HEREDITY 



preceded by reversed selection, or when it brings into activity 

 some dormant trait. 



VI. Among the most considerable and important of the 

 inborn characters possessed by the higher animals are various 

 capabilities of developing in certain fixed directions, that is 

 of making various definite acquirements in response to fixed 

 stimuli usually those resulting from use. Man's body 

 develops mainly in this way and his mind almost entirely so. 



VII. Evolution is adaptive racial change. A theory of 

 evolution seeks to account for such changes. Every theory of 

 evolution is founded on a corresponding theory of heredity. 

 A theory of heredity seeks to account for inborn likenesses 

 and differences between parents and offspring. The only 

 theory of evolution which is in accord with the facts of Nature 

 is the theory of the Natural Selection of favourable variations. 

 The only theory of heredity which is in accord with the 

 doctrine of Natural Selection is the theory that the likenesses 

 between parents and offspring are due to the recapitulation 

 by each of the life-history of the species, and that variations 

 arise " spontaneously." 



VIII. The tendency to vary spontaneously has been 

 evolved by Natural Selection. Like any other character it 

 may be increased or diminished by the same agency, or by 

 reversion to a more or less variable ancestral type. When- 

 ever an hitherto stable environment changes, the tendency is 

 increased. If the stringency of selection is increased, the 

 tendency to vary is exalted by Natural Selection. If the 

 stringency is diminished, the tendency is exalted by reversion. 

 In both cases adaptation to the changing environment is 

 hastened. 



IX. An " acquirement " is a modification of an inborn 

 character due, speaking generally, to the action of the environ- 

 ment on the soma. It is not necessarily a new character. 

 Acquirements, therefore, differ toto ccelo from variations. 



X. Only inborn characters are transmissible to offspring. 



XI. Evolution, therefore, proceeds solely on lines of inborn 

 characters. 



XII. Every individual in his development follows (with 

 variations) in the footsteps of his predecessors. 



XIII. The development of every individual is, therefore, a 

 recapitulation (with inaccuracies, with omissions and addi- 

 tions) of the life-history of his race. 



XIV. Since regressive variations tend to be of greater 

 magnitude and prepotency than progressive variations, the 

 tendency to regression is always greater than the tendency to 



