INTRODUCTION 7 



information at hand and the complexity of the ques- 

 tion, increasing as soon as it bore upon living things, 

 were not by any means the most formidable obstacles : 

 the new theory encountered on its path powerful prej- 

 udices and deeply rooted convictions, not to mention 

 man's intellectual inertia. 



The upholders of religious traditions and the recog- 

 nised scientific authorities joined forces against it, for 

 it overthrew all the idols which mankind had been 

 taught to worship for several centuries. 



More than anything else did the study of life fos- 

 ter the growth of the evolutionary idea proper, the 

 idea, namely, of a processus whose various stages are 

 not only linked by a bond of causality, but constitute 

 an uninterrupted and unreversible sequence, in which 

 a step backward or the exact recurrence of whatever 

 has become the past, is impossible. 



This is what we mean when we speak of the evolu- 

 tion of a living thing in the course of its embryonic 

 development (it was to this development that the term 

 was first applied) ; this is what we mean when we 

 speak of the history of all living things taken in its 

 entirety. 



Finally, the idea of evolution leads to the idea that 

 all organic forms are descended from other organic 

 forms, the more complex forms evolving out of simpler 

 ones; thus we go back to the beginning of the history 

 of the organic world, as far back as the origin of 



