INTRODUCIION 



of adapting their ideas and opinions to the facts. Such 

 argument leads naturally, even without any bad intention, 

 away from the consideration of the opposing facts back to 

 its starting-point — revolving in a circle and seeing nothing 

 which lies without the circle. This method is most surpris- 

 ing in those who, like A. Wagner, with his Load of Migra- 

 tion, oppose the whole of Darwinism in order to put in its 

 place an explanation of evolution which can only find its place 

 in Darwinism itself as a subordinate of that theory. Isolation 

 in space, regarded by AVagner as alone regulating the origin 

 of species, favours and promotes the separation of the gradu- 

 ally produced diverse forms into species ; but apart from the 

 truth that it does not alone explain the separation, its 

 influence cannot possibly be put in the place of that of 

 the principle of utility. 



The effective influence is not necessarily single — many 

 can be effective together. 



On this standpoint I have placed myself in my inquiries 

 from the first, and their course and their results have fully 

 justified me. 



On account of the relations which they bear to others, I 

 have repeated some of the facts already published by me 

 which serve as the foundation of my theory. The last sec- 

 tions of this book are specially devoted to such facts. 



It happens that my views on the question before us do not 

 agree with those of some of my valued friends and honoured 

 teachers, and if I am to uphold my own convictions I must 

 endeavour to refute theirs. 



As this opposition is little pleasing to myself, I am con- 

 vinced that my frank advocacy of scientific opinions will 

 neither by those who are aware of this personal relationship, 

 nor by those who are immediately affected, be stigmatised 

 as unfriendly and ungrateful. 



