and Embryological Development. 369 



we find they have a general resemblance, and that the Cys- 

 tideans and Blastoids represent among the fossil Echinoderins 

 the nearest approach we have yet discovered to this imaginary 

 prototype of Echinoderms. 



This may not seem a very satisfactory result to have 

 attained. It certainly has been shown to be an impossibility 

 to trace in the palseontological succession of the Echini any 

 thing like a sequence of genera ; no direct filiation can be 

 shown to exist ; and yet the very existence of persistent types, 

 not only among the Echinoderms but in every group of 

 marine animals, genera which have continued to exist with- 

 out interruption from the earliest epochs at which they occur 

 to the present day, would prove conclusively that at any rate 

 some groups among the marine animals of the present day 

 are the direct descendants of those of the earliest geological 

 periods. When we come to types which have not continued 

 so long but yet have extended through two or three great 

 periods, we must likewise accord to their latest representatives 

 a direct descent from the older. The very fact that the ocean 

 basins date back to the earliest geological periods, and have 

 afforded to the marine animals the conditions most favourable 

 to an unbroken continuity under slightly varying circum- 

 stances, probably accounts for the great range in time during 

 which many genera of Echini have existed. If we examine 

 the interlacing in the succession of the genera characteristic 

 of later geological epochs, we find it an impossibility to deny 

 their continuity from the time of the Lias to the present day. 

 The Gidaris of the Lias and the Rhabdocidaris of the Jura 

 are the ancestors of the Gidaris of today. The Salemce of 

 the Lower Chalk are those of the Salenia; of today. Acro- 

 salenia extends from the Lias to the Lower Cretaceous, with 

 a number of recent genera, which begin at the Eocene. The 

 Pygaster of today dates back to the Lias ; Echinocyamus and 

 Fibularia commence with the Chalk. Pyrina extends from 

 the Lower Jura through the Eocene. The Echinobrissus of 

 today dates back to the Jura. Holaster lived from the 

 Lower Chalk to the Miocene ; and the Hemiaster of today 

 cannot be distinguished from the Hemiaster of the Lower 

 Cretaceous. 



Such descent we can trace, and trace as confidently as we 

 trace a part of the population of North America of today as 

 the descendants of some portion of the population of the 

 beginning of this century. But we can go no further with 

 confidence, and bold indeed would he be who would attempt, 

 even in a single State, to trace the genealogy of the inhabi- 

 tants from those of ten years before. We had better acknow- 



