VI AUTHORS PREFACE. 



this sense that the contents of these essays are to be looked 

 upon as research. 



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 described in Essay VI, led to what I 

 believe to be a truer knowledge of them. In this way the 

 causes of parthenogenesis, as developed in Essay V, received 

 an important addition in the fact published 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 



