viii PREFACE 



one hand we may attempt to deal with theories 

 in close connection with observed facts, although 

 the body of facts which can be gathered in support 

 of a theory is limited and the capacity of the 

 human mind for their interpretation is often less 

 than we might wish. It is possible to draw thor- 

 oughly sound conclusions from apparently suffi- 

 cient evidence in many cases, only to find them 

 worthless when extended ever so slightly beyond 

 the field in which they were formulated. On the 

 other hand one may admit the futility of present 

 attempts at detailed correlation of our enormous 

 store of observations and resort to logical funda- 

 mentals as a possible source of useful working 

 hypotheses. The former method must be followed; 

 without it the entire fabric of evolutionary thought 

 must remain forever without adequate scientific 

 foundations. But if the foundations are to be 

 sound when they are finally completed, our efforts 

 must be properly directed; this they have not 

 always been, and it is the purpose of this work to 

 inquire into the trend of evolutionary thought 

 and to seek the proper background for future study. 

 It has long been evident that we are making 

 little progress in our absolute knowledge of evolu- 

 tion, and since our experimental methods have 

 undergone very little change they seem to be the 

 most promising field for correction. Experimental 

 evolution has been so much concerned with en- 



