PKEFACE. VU 



the broad facts of tlie case are of fundamental im- 

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

 ture to tope tliat no error lias slipped iato my 

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

 remembered, not only that some omission or mis- 

 take is almost tmavoidable, but that ne"W lights 

 come with new methods of investigation ; and that 

 better modes of statement foUow 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 ia aU parts of the world to the crayfishes. 

 Combined efforts wiU 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 uotes re- 

 specting points of detail with which I thought it 



