REPORT OF THE DIRECTOR I916 121 



and thoroughly on land through glacial periods, mountain folding, 

 etc. than they did in the oceans. 



Nevertheless we find some remarkable cases of persistence on 

 the continents, as that of Ceratodus, ranging from Triassic to 

 recent time, and of Diplomystus, a teleost which arising in Cre- 

 taceous time still exists in the rivers of New South. Wales and of 

 Chili ; of the phyllopod Apus, persisting since Triassic time ; of the 

 pulmonate genera Auricula, Carychium, Lymnaea, Planorbis, 

 Physa, persisting since Jurassic time. 



Among the continental forms again the limnal and fluviatile 

 forms appear to be more persistent than the terrestrial forms. 

 This is indicated by the greater number of persistent limnal snails 

 (as Lymnaea, Planorbis, Physa) ; such very ancient Phyllopods 

 as Estheria and Apus, and the fact that all the relicts (see below) 

 among the ganoids and amphibians (Phanerobranchia and Crypto- 

 branchia) are fresh-water forms. It is possible that this greater 

 persistency is only apparent and principally due to the greater 

 probability of the water organisms to become entombed in the 

 rocks and thus preserved; but it is also to be noted that the limnal 

 and fluviatile faunas are to a much greater percentage composed 

 of the lower classes of animals which are more apt to become per- 

 sistent than the higher terrestrial faunas ; and finally, isolated river 

 systems, like that of the Nile, and land-locked lakes, like those of 

 Mexico, produce conditions clearly more isolated and favorable 

 for the continuation of ancient types than any terrestrial regions, 

 other than islands, can afford. 1 



1 Related to the persistent terrestrial and limnal types are the relicts that 

 survive as the last of their races in slightly changed forms. They have been 

 termed persistent types by several writers. Thus Henn (Amer. Naturalist, 

 vol. 46, 1912, p. 543) has discussed as persistent forms, what we prefer here 

 to call, with the Germans, relicts (or "Relicten"), and found that they owe 

 their presence to their (1) insular, or (2) subterranean habitats, and (3) 

 their insignificant size. All these features tend to shield them from the battle- 

 fields of the struggle for existence. The famous surviving Rhynchocephalous 

 reptile Sphenodon or Hatteria of some islands off New Zealand is a striking 

 example of such a relict. Having no fossil record of it, we can not consider 

 it as a persistent form. Other such curious relicts of antiquity are the Aus- 

 tralian organisms, some of which, as Ceratodus, date back almost unchanged 

 to great antiquity ; the primitive Insectivora, as the shrew, which are pro- 

 tected by their small size, and the few surviving Ganoidea, as Polypterus, etc. 



All these relicts owe their survival principally to their isolation. Suess in 

 the final volume of his Antlitz dcr Erdc has pointed out that there are regions 

 where the terrestrial population has not been exposed for a long time to any 

 physical changes, transgressions and orogenic forces. He calls these asylums 

 and distinguishes four, namely, Laurentia, Ansraraland, Gondwanaland and 

 Antarctica. These have not taken part in upfoldings since the Carboniferous. 

 He shows the biological peculiarities of the asylums, of which it may bemer 

 tioned tba* fill the oldest fish types live in them, as Ceratodus in Australia 



