Yl PREFACE. 



and the most difficult problems of zoology; and, 

 indeed, of biological science in general. 



It is for this reason that I have termed the book 

 an " Introduction to Zoology." For, whoever will 

 follow its pages, crayfish in hand, and will try to 

 verify for himself the statements which it contains, 

 will find himself brought face to face with all the 

 great zoological questions which excite so lively an 

 interest at the present day ; he will understand the 

 method by which alone we can hope to attain to 

 satisfactory answers of these questions; and, finally, 

 he will appreciate the justice of Diderot's remark, 

 " II faut tre profond dans Part ou dans la science 

 pour en bien posseder les elements." 



And these benefits will accrue to the student 

 whatever shortcomings and errors in the work itself 

 may be made apparent by the process of verification. 

 " Common and lowly as most may think the cray- 

 fish," well says Eoesel von Eosenhof, "it is yet so 

 full of wonders that the greatest naturalist may be 

 puzzled to give a clear account of it." But only 



