PREFACE. Vi 



the broad facts of the case are of fundamental im- 

 portance; and, so far as these are concerned, I ven- 

 ture to hope that no error has slipped into my 

 statement of them. As for the details, it must be 

 remembered, not only that some omission or mis- 

 take is almost unavoidable, but that new lights 

 come with new methods of investigation ; and that 

 better modes of statement follow upon the improve- 

 ment of our general views introduced by the gradual 

 widening of our knowledge. 



I sincerely hope that such amplifications and 

 rectifications may speedily abound; and that this 

 sketch may be the means of directing the attention of 

 observers in all parts of the world to the crayfishes. 

 Combined efforts will soon furnish the answers to 

 many questions which a single worker can merely 

 state; and, by completing the history of one group 

 of animals, secure the foundation of the whole 

 of biological science. 



In the Appendix, I have added a few rotes re- 

 specting points of detail with which I thought it 



