386 EVOLUTION AND GENETICS 



duction must result in a struggle for existence in which survival 

 is determined by the inherent characteristics of individuals; 

 those possessing useful variations will survive and those with 

 harmful variations or merely without the useful characters will 

 perish. The surviving individuals alone will perpetuate the 

 species and so their characters will become characters of the spe- 

 cies. Darwin added the belief that this process, through a suc- 

 cession of generations, would result in progressive development 

 of a character. To this simple foundation may be added other 

 principles which exert a similar selective action upon species, but 

 in the beginning it was literally a theory of the "survival of the 

 fittest." 



Underlying Principles. Variation. The importance of varia- 

 tion was impressed upon Darwin's mind especially during the 

 voyage of the Beagle. He noticed as he travelled not only that 

 species varied in a given region but that as he passed from one 

 limit of their range to another their general characteristics also 

 varied according to geographical distribution. These facts showed 

 him that species, far from being rigidly fixed entities, showed a 

 tendency toward intergradation and developed the idea of change- 

 ability which was necessary to any thought of evolution. 



In the classification of variations we have noted that some are 

 a part of individual life, both as process and result. These have 

 been called modifications and acquired characters, and are as 

 aptly characterized by one term as by the other. Better still they 

 may properly be looked upon as individual adaptations. Such 

 characters appear in every individual in response to definite con- 

 ditions of existence. Many examples are familiar to everyone, 

 including such common things as tanning of the skin, calluses, 

 muscular development, tolerance for poisons and the like. Mutila- 

 tions have also commonl}^ been included here, but for reasons to 

 be considered later they cannot properly be considered as charac- 

 ters of the organism. The possibility of modifications affecting 

 the heritage of the species has been one of the most bitterly 

 contested points in evolutionary theory. 



In contrast to this kind of variation are two which are not 

 evident as processes within the individual but only as fully devel- 

 oped characters. To the extent that we are able to apply the 

 idea of heritage and response to characters of the organism we 

 must look upon them as a product of inherited powers responding 



