458 Evolution and Adaptation 



have begun to get a different conception of how evolution has 

 come about. It may be well, therefore, to go once more over 

 the main points in regard to the different kinds of variation. 



While it has been found that no two individuals of a 

 species are exactly alike, yet, taken as a group, the variations 

 appear as though they followed the law of chance. The 

 descendants of the group show the same differences. Thus 

 the group as a whole appears constant, while the individuals 

 fluctuate continually in all directions. This is what we 

 understand by fluctuating variation. If the external condi- 

 tions are changed, a new " mode " may appear, but the change 

 is generally only a temporary one, and lasts only as long as 

 the new conditions remain. Thus, while the direct influence 

 of the environment may show for a time, the result is tran- 

 sient. Even if it were permanent, there is no evidence that 

 the adaptation of organisms could be accounted for in this 

 way unless the response were useful.. It appears that this 

 sometimes really occurs, especially in responses to tempera- 

 ture, to moisture, to the amount of salts in solution, to 

 poisonous substances, etc. In this way, one kind of adapta- 

 tion is brought about, but there is no evidence that the great 

 number of structural adaptations have thus arisen. 



The Lamarckian principle of the inheritance of acquired 

 characters has also been supposed by many writers to be an 

 important source of adaptive variation. An examination of 

 this theory is not found to inspire confidence. We have, there- 

 fore, eliminated this hypothesis on the ground that it lacks 

 evidence in its favor, and also because it appears improbable 

 that in this way many of the adaptations in organisms could 

 have been acquired. 



Finally, there is the group of discontinuous variations. Of 

 these there may be several kinds, and there is some evidence 

 showing that there are such. For the present we may in- 

 clude all the different sorts under the term mutation, mean- 

 ing that the new character or group of characters suddenly 



