PREFACE. vil 
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 notes re- 
specting points of detail with which I thought it 
