Chap. I. INTRODUCTORY. 5 



perfect is our present knowledge of this subject. The 

 existing deficiencies were the more difficult to supply, 

 because, as Van Beneden remarks with regard to the 

 Decapoda, from the often incredible difference in the 

 development of the most nearly allied forms, these 

 must be separately studied — usually family by family, 

 and frequently genus by genus — ^nay, sometimes, as in the 

 case oiFeneus, even species by species ; and because these 

 investigations, in themselves troublesome and tedious, 

 often depend for their success upon a lucky chance. 



But although the satisfactory completion of the 

 " Genealogical tree of the Crustacea " appeared to be 

 an undertaking for which the strength and life of an in- 

 dividual would hardly suffice, even under more favour- 

 able circumstances than could be presented by a distant 

 island, far removed from the great market of scientific 

 life, far from libraries and museums — nevertheless its 

 practicability became daily less doubtful in my eyes, and 

 fresh observations daily made me more favourably in- 

 clined towards the Darwinian theory. 



In determining to state the arguments which I de- 

 rived from the consideration of our Crustacea in favour 

 of Darwin's views, and which (together with more general 

 considerations and observations in other departments), 

 essentially aided in making the correctness of those views 

 seem more and more palpable to me, I am chiefly influ- 

 enced by an expression of Darwin's : " Whoever," says 

 he (' Origin of Species,' p. 482), " is led to believe 

 that species are mutable, will do a good service 

 by conscientiously expressing his conviction." To the 



