CHAPTER I. 

 INTRODUCTION. 



THE adoption of experimental methods of evolutionary study 

 early in the present century was accompanied by sanguine 

 hopes that a general and universally applicable method of evolution 

 might thus he discovered. But two decades of intensive experi- 

 mental work with plants and animals has led to a greater diversity 

 of opinion concerning evolutionary factors than ever before. 

 Wherever a particular species or group of forms has been 

 intensively studied, the results have illuminated a particular field 

 of inquiry concerning evolution. But other results, dealing with 

 organisms having different bionomic relations, have answered other 

 questions and at the same time propounded new series of problems. 

 While there has therefore been a great accumulation of data | 

 concerning the variations of organisms, the inheritance of char- I 

 acters new and old, and the relations and reactions of these / 

 organisms to their varied environments; yet this has led to no/ 

 unitary result which can be universally applied. These experiments^ 

 have rather served to emphasize the manifold character of the) 

 evolutionary process. 



Not only have various groups of organisms contributed their 

 quota to this multifarious result, but different methods of 

 experiment and different fields of observation have no less clearly 

 tended to emphasize particular evolutionary factors. Individual 

 bias has also of course played a part in the interpretation of many 

 results. 



Important and valuable as these experimental studies have 

 been in opening a new era of evolutionary investigation, and 

 leading to very definite conceptions concerning heredity and many 

 aspects of variation and distribution, yet I believe they have been 

 perhaps most generally useful in a direction which has not as yet 

 been recognized. For their very variety, apart from interpretations 

 put upon them, is sufficient to show that in the search for one all- 

 explanatory evolutionary principle, man is following an ignis fatuus. 

 Darwin recognized this even in his day, for while he laid chief stress 

 upon natural selection, he also relied upon the Lamarckian factor, 



