CRUSTACEA OF THE PAST 257 



source, when it is available, is the most important of 

 all, since the order of succession of the various types 

 is given by that of the rock strata in which they 

 occur, and we can be quite certain that we are 

 dealing, if not with the actual ancestors, at least 

 with the forerunners of existing species. The " im- 

 perfection of the geological record," however, is so 

 great that the organisms preserved in the fossil state 

 represent only an insignificant part of the whole 

 number of organisms that have lived on the globe 

 since life began ; and it is not surprising, therefore, 

 that in many groups the study of fossils has 

 hitherto afforded little help towards the working out 

 of their genealogical history. Thus, among Crus- 

 tacea there are many important groups such as the 

 Copepoda, which are entirely unknown as fossils, 

 their small and delicate bodies being ill adapted for 

 preservation, although there is every reason to 

 suppose that they are a very primitive and very 

 ancient group. In many fossil Crustacea only the 

 hard shell or carapace has been preserved, the 

 appendages being lost or represented only by in- 

 decipherable fragments, and in some cases it is 

 hardly possible to guess at the affinities of the 

 animals. Further, several important groups are 

 already represented in some of the oldest of the 

 fossil-bearing rocks at present known, and the 

 differentiation of these groups must have taken place 

 in the dark ages before the record of the fossils 

 17 



