INTRODUCTION XXI 



longer questioning the fact of evolution ; they are busied 

 rather with the attempt to further explore and more perfectly 

 understand the operation of the factors that are at work to 

 produce that development of animals and plants which we 

 call organic evolution. 



But though the fact of organic evolution seems satis- 

 factorily established, we are still far from a satisfactory knowl- 

 edge of the factors which are at work to produce it, and 

 especially are we ignorant of the manner of their operation. 

 For many generations to come there will be in this field 

 abundant opportunity for profitable study. It is not my 

 purpose to enter into much discussion of the more doubtful 

 questions, but rather to give, as briefly as is consistent with 

 clearness, an outline of the apparently well established facts 

 as to the theory and some of its important corollaries. 



By thus avoiding critical discussion as far as possible, I 

 would not create the impression that biologists are entirely 

 agreed upon all points of the theory. There is endless dis- 

 cussion of many phases of the subject. In three cases 

 where there is general difference of opinion upon a funda- 

 mental point I have tried to state the divergent opinions and 

 to show what seems to me to be the safest conclusion, with 

 the reasons for my opinion. Two of these much mooted 

 points are the degree of efficiency of natural selection, and 

 the inheritance of the effects of use and disuse. Another 

 much discussed factor in evolution is sexual selection. This 

 I have treated largely by pictures, showing some of the phe- 

 nomena about the explanation of which there is so much dif- 

 ference of opinion. But however much difference of opinion 

 there may be among biologists in regard to many subsidiary 

 points of the theory, there is substantial agreement upon the 



