vi 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, 
“Tl faut étre profond dans V’art ou dans la science 
pour en bien posséder les éléments.” 
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 Roesel von Rosenhof, “it is yet so 
full of wonders that the greatest naturalist may be 
puzzled to give a clear account of it.” But only 
