SUMMARY 513 



divided, a writer does but half his duty by stating his doctrine, 

 if he does not also examine, and to the best of his ability judge, 

 those of other thinkers." 1 However completely I may have 

 failed in all else, I am not without hope that I have demonstrated 

 at least that much evidence, hitherto neglected, particularly that 

 furnished by human diseases, is, if used in accordance with the 

 methods employed in the other sciences, valuable to the student 

 of heredity and evolution in his strivings to create a coherent 

 body of thought and knowledge, the various parts of which shall 

 interlock as smoothly and consistently as those of any other 

 deductive science. 



836. We have journeyed in a wide and in parts untravelled 

 region of thought, and have ambitiously endeavoured to solve 

 the greater problems of heredity. If in any instance we have 

 succeeded, then in each such case a law of nature has been formu- 

 lated which, as far as I am able to judge, is as nearly fundamental 

 as can be discovered in the present state of our knowledge. Of 

 these laws, if any of them are real laws, the most important from 

 the scientific standpoint, because the most basic, is the generaliza- 

 tion that the vast majority of variations are under the immediate 

 control of natural selection, and therefore are spontaneous in the 

 sense that they arise independently of the direct action of the 

 environment, and that on these spontaneous variations only is 

 evolution built ; for they only are products of the normal growth 

 and change of the germ-plasm, all others being results of injury 

 to it. If this be a truth, the close and continuous adaptation of 

 persisting species to their environments necessarily follows. On 

 the other hand, the conclusion that the " acquirements " of the 

 individual are as much a part of his inheritance as his " inborn " 

 traits is, from the practical standpoint, the most important of all. 

 If this be a truth, we have a valid explanation 'of the principal 

 line of evolution of the higher animals, and a potent means of 

 influencing our own species for good and evil. 



837. The matters we have dealt with have been complex and 

 difficult, and I fear it is more than probable that the reader has 

 detected numerous errors and inadequacies of fact and reasoning. 

 For them I ask no sort of indulgence more especially as I 

 have often challenged him to indicate material misstatements 

 and omissions, or, given the facts as true and comprehensive, to 

 indicate how any conclusions other than those reached are con- 

 ceivable. But I have the right to claim, either that I shall be 



1 Op. cit., ii. vii. i. 

 33 



