AUTHORS PREFACE TO SECOND VOLUME. v 



have left it unchanged in the essays of the First Volume, not 

 only for this reason, but chiefly because it is, I think, of great 

 interest to trace the development of a scientific truth. Hypo- 

 theses, even when not absolutely right, may be of value in 

 advancing our knowledge, if only they are relatively right, 

 i. e. when they correspond with the state of existing knowledge. 

 They are like the feelers which the short-sighted snail stretches 

 forth on its darkened path, testing this way and that, and with- 

 drawing them and altering its route as soon as they come across 

 any obstacle ; just as an unyielding fact may show that we are 

 on a wrong road. 



Rome was not built in a day, and no scientific truth is at once 

 revealed without a prolonged previous history made up of 

 mingled truth and error. The last word has not yet been 

 spoken on the subject dealt with in these essays ; but if we 

 remember the complete obscurity which, only ten years ago, 

 surrounded ever3?thing which is now clearly revealed in the 

 final essay, we shall not be able to refrain from an inward 

 feeling of satisfaction. Of course, much remains to be done in 

 this department of biology ; but a firm foundation has been laid 

 on which much may be erected. 



AUGUST WEISMANN. 

 Freiburg i. B. : 



20 August, 1 891. 



