752 GEOLOGICAL SUCCESSION OF THE ECHINI. 



genera of the Crag. Coelopleurus and Temnechinus, no longer found in the 

 European seas, are still extant in the West Indies. 



We cannot fail to be struck with the persistence of many of the types 

 of the chalk up to the present time, either as identical or most closely allied 

 genera. This was of course known from the time Lyell first called attention 

 to the similarity of the fossils of the tertiary deposits with the present fauna; 

 it has recently been brought up still more forcibly after several most marked 

 Cretaceous genera, Salenia. Hemipedina, Phymosoma, Echinothuria. and Ponr- 

 talesia. were dredged from great depths in the West Indies, showing a much 

 more intimate generic; connection between the present time and the chalk 

 than had been admitted. 



We can, in fact, from the time of the Cretaceous period, draw up a table of 

 genera, starting from the Desmosticha characteristic of that period, and end- 

 ing in the Clypeastroids now found in our seas, which would answer all the 

 requisites of a genealogical tree ; hut we cannot in like manner do the same 

 for the Petalostieha except in a most general manner, and we find, as has 

 been mentioned above, that the Petalostieha have existed long before the 

 genera from which we might derive them have come to light; at least, as 

 far as we know the genetic connection of living forms from their embryo- 

 logical development. The question of time, which has always been invoked 

 as necessary to produce the great changes noticed between fossils occurring 

 in successive formations, is not such an important one. as far as Invertebrates 

 are concerned, as we imagine. We have two classes of phenomena which 

 must have occurred side by side, genera retaining their generic features un- 

 changed from the time of the Jura to the present day (the persistent types), 

 and genera suddenly introduced, but differing from those already existing, 

 except in the single instance of the appearance of the Dysasteridae, to a 

 far less degree than the stages now known, from embryologies! data, to 

 belong to one and the same species; these successive stages of growth 

 follow each other in rapid succession, and pass through phases which appear 

 more or less like sudden transitions from those immediately preceding them. 

 The analogy between the sudden appearance of genera in their geological 

 succession, and the sudden transitions observed in the growth of an indi- 

 vidual during its different stages, is remarkable. Somewhat similar views 

 regarding the nature of the changes which have brought about the sudden 

 appearance of types have already been suggested by Xageli, Kolliker, and 

 specialty by Heer. 



