452 LOCAL SUCCESSION OF SIMILAR TYPES. 



and water, and brought new lands into existence in geological time. 

 For they have so changed the distribution of life on the earth as to 

 entomb and fossilise successively thirty or more distinct faunas in 

 the sediments laid down upon the same area of sea -bed. Pro- 

 fessor Huxley at first, perhaps, showed excessive caution in the 

 statement " There may, or there may not, have been a progressive 

 development of animal and vegetable life ; but the palaeontological 

 evidence before us does not justify the assertion that any proof of such 

 progressive change, if it ever occurred, exists;" 1 but subsequently,' 

 in elucidating the evolution of the horse 2 and other animals, demon- 

 strated that the discoveries of paleontologists have justified an aban- 

 donment of this negative attitude. 



Ancient Types of Echinoderms now Living. Cidaris is the 

 oldest living genus reaching back to the Trias. At the present day 

 its. habitat is littoral. Among genera which survive from the Jurassic 

 rocks are Echinobrissus, which is littoral, and Hemipedina and 

 Pygaster, which are found iu deeper water. The genera which live 

 on from the chalk number 13. Of these 8 are now found in the 

 littoral zone including Leiocidaris, Echinus, Echinocyamus, Fibu- 

 laria, Rhynchopygus, Nucleolites, Hemiaster. The range of some of 

 these genera in depth is less restricted. Thus the laminarian zone 

 includes Salenia, Cottaldia, Echinus, Echinocyamus, Fibidaria, Cono- 

 clypeus, Catopygus, Hemiaster, and Periaster ; while the deep sea 

 yields Salenia, Echinus, Echinocyamus, Fibidaria, and Hemiaster, 

 which are all Cretaceous genera. It is highly probable that the con- 

 ditions of depth in which Echinoderms live have changed with geolo- 

 gical time, for at the present day we might quote as representatives 

 of littoral conditions most of the Cidaridae, Diademadae, Triplechinidae, 

 Temnopleuridae, Nucleolitida?,, Echinonse, and Palseostominse. Lami- 

 narian conditions are represented by the Salenidae, Galeritidse, and 

 many of the Fibularina and Nucleolitidse. 3 The families of the Abys- 

 sal Ocean are the Ananchytidae and Echinothuridse. Therefore, in- 

 ferences as to the conditions of life of an Echinoderm fauna, like that 

 of the Upper Chalk, need to be very diffident. 



Local Persistence of Types. One of the most interesting pro- 

 blems, and at the same time one of the most difficult of explanation, 

 is the persistence in the same locality of types which have undergone 

 remarkable change ; for it seems to be associated with absence of com- 

 petition from more highly organised races. This succession is most 

 striking among the mammalia of the more recent deposits as compared 

 with living groups. Thus in South America the Edentates reach their 

 maximum development, and it is among the muds of the pampas that 

 we find the gigantic Megatherium and Mylodon, Megalonyx and Sceli- 

 dotherium, representing the living sloths. In the same way the living 



1 Catalogue of a collection of fossils in the Museum of Practical Geology, with 

 an explanatory introduction by Thomas H. Huxley and Robert Etheridge, 

 1865. 



2 Address Geol. Soc. 1870. 



3 Neumayr : " Neuen Jahrbuch fur Min." 1882. 



