HISTOEY OF CRUSTACEA. CHAP. I. 



marvellous cripplings, and so they were supposed to 

 have fallen by their own fault, like Adam, from their 

 previous state of perfection. 



That a great part of the larger and smaller groups 

 into which this class is divided, might be regarded as 

 satisfactorily established, was a further advantage not 

 to be undervalued; whilst in two other classes with 

 which I was familiar, namely, the Annelida and Aca- 

 lephse, all the attempted arrangements could only be 

 considered preliminary revisions. These undisplace- 

 able groups, like the sharply marked forms of the hard, 

 many-jointed dermal framework, were not only import- 

 ant as safe starting points and supports, but were also 

 of the highest value as inflexible barriers in a problem 

 in which, from its very nature, fancy must freely unfold 

 her wings. 



When I thus began to study our Crustacea more 

 closely from this new stand-point of the Darwinian 

 theory, when I attempted to bring their arrangements 

 into the form of a genealogical tree, and to form some 

 idea of the probable structure of their ancestors, I 

 speedily saw (as indeed I expected) that it would require 

 years of preliminary work before the essential problem 

 could be seriously handled. The extant systematic 

 works generally laid more weight upon the characters 

 separating the genera, families and orders, than upon 

 those which unite the members of each group, and con- 

 sequently often furnished but little employable material. 

 But above all things a thorough knowledge of develop- 

 ment was indispensable, and every one knows how im- 



