PREFACE. 



In presenting in so small a volume a subject of 

 such broad scope, and of such great importance as 

 must pertain to a theory of development and hered- 

 ity,.! have been actuated by several reasons. The 

 theory is based upon the broadest generalisations of 

 natural science, and accordingly it rests upon facts 

 which may be grouped together in great classes. It 

 is necessary to mention only one fact of each class 

 in order to suggest a host of similar familiar facts. 

 As the facts and generalisations are so well known, 

 I have thought it sufficient to give them briefly and 

 only a few examples in each class. At this time, 

 after the critical and minute investigation to which 

 biological phenomena have been subjected for the 

 last thirty years, it is not to be expected that we 

 should suddenly discover some new and recondite 

 fact, far removed from ordinary observation, which 

 should revolutionise all our views in regard to biolog- 

 ical phenomena. In announcing, therefore, a theory 

 of development and heredity, I do not pretend to 

 have made any such discovery. But I believe that. 



