iv AUTHOR'S PREFACE TO FIRST EDITION. 



The fact that they contain the record of research made it 

 impossible to introduce any essential alterations in the trans- 

 lation, even in those points about which my opinion has 

 since changed to some extent. I should to-day express some 

 of the points in Essays I, IV, and V, somewhat differently ; 

 but had I made such alterations, the relation between the 

 essays as a whole would have been rendered less clear, for 

 each of the earlier ones formed the foundation of that which 

 succeeded it. Even certain errors of interpretation are on 

 this account left uncorrected. Thus, for instance, in Essay 

 IV it is assumed that the two polar bodies expelled by 

 sexual eggs are identical ; for at that time there was no 

 reason for doubting that they were physiologically equivalent. 

 The discovery of the numerical law of the polar bodies de- 

 scribed in Essay VI, led to what I beheve to be a truer 

 knowledge of them. In this way the causes of partheno- 

 genesis, as developed in Essay V, received an important ad- 

 dition in the fact pubhshed in Essay VI, that only one polar 

 body is expelled by parthenogenetic eggs. This fact alone 

 explains why sexual eggs cannot as a rule develope without 

 fertilization. 



Hence the reader must not take the individual essays as 

 the full and complete expression of my present opinion ; 

 but they must rather be looked upon as stages in research, 

 as steps towards a more perfect knowledge. 



I must therefore express the hope that the essays may be 

 read in the same order as that in which they appeared, and 

 in which they are arranged in the present volume. The 

 reader will then follow the same road which I traversed in 

 the development of the views here set forth ; and even 

 though he may be now and then led away from the direct 

 route, perhaps such deviations may not be without interest. 



I should wish to express my warm thanks to Mr. Poulton 

 for the great trouble he has taken in editing the translation, 

 which in many places presented exceptional difficulties. The 

 greater part of the text I have looked through in proof, and 

 I believe that it well expresses the sense of the original ; 



