Chap. I. INTRODUCTORY. 3' 



oneself at a moderately certain independent judg- 

 ment upon a matter affecting the highest questions 

 so deej)ly, even this alone could not but be esteemed 

 a great gain. 



Having determined to make the attempt, I had in 

 the first place to decide upon some particular class. 

 The choice was necessarily limited to those the chief 

 forms of which were easily to be obtained alive in some 

 abundance. The Crabs and Macrurous Crustacea, the 

 Stomapoda, the Diastylidse, the Amphipoda and Iso- 

 poda, the Ostracoda and Daphnidae, the Copepoda and 

 Parasita, the Cirripedes and Khizocephala of our coast, 

 representing the class of Crustacea with the deficiency 

 only of the Phyllopoda and Xiphosura, furnished a long 

 and varied, and at the same time intimately connected 

 series, such as was at my command in no other class. 

 But even independently of this circumstance the selec- 

 tion of the Crustacea could hardly have been doubtful. 

 Nowhere else, as has already been indicated by various 

 writers, is the temptation stronger to give to the expres- 

 sions " relationship, production from a common funda- 

 mental form," and the like, more than a mere figurative 

 signification, than in the case of the lower Crustacea. 

 Among the parasitic Crustacea, especially, everybody 

 has long been accustomed to speak, in a manner scarcely 

 admitting of a figurative meaning, of their arrest of 

 development by parasitism, as if the transformation of 

 species were a matter of course. It would certainly 

 never appear to any one to be a pastime worthy of the 

 Deity, to amuse himself with the contrivance of these 



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