AUTHOR'S PREFACE TO SECOND 

 VOLUME 



The four essays which constitute this Second Volume sup- 

 plement those of the First, by bringing forward new facts to 

 support the earlier ideas, and by improving and completing the 

 latter on the basis of the most recent discoveries. 



The first two essays afford further support to the arguments 

 in favour of the non-transmission of acquired characters, inas- 

 much as they attempt to prove that these arguments hold in 

 certain cases which at first sight appear to refute them. The 

 diminution of parts which are no longer used has been ex- 

 plained in an earlier essay as the result of the cessation of 

 natural selection, i. e. of panmixia. The conception that every 

 part of the organism is maintained at the level it has reached 

 only by means of the continued activity of natural selection, 

 and that any intermission of this activity leads to a gradual 

 diminution, has passed through many minds. Darwin himself 

 appears to have held this idea, and Romanes, and especially 

 Seidlitz, have more or lea^s clearly expressed it. But the 

 thought first attained its full significance when we arrived 

 at the definite conclusion that the Lamarckian principle of modi- 

 fication had no real existence, because acquired characters, and 

 hence the decrease which organs suffer by disuse, are not 

 inherited. Thus every explanation of the existence of disused 

 parts in a rudimentary state fails, except panmixia ; and the 

 conclusion was unavoidable that the countless characters which 

 enter into our conception of a species can only be maintained 

 at their present level by the ceaseless activity of natural 

 selection. 



