But this jump from the "Eocene period" to the present is as nothing compared with the secular acrobatics of some of the fishes and especially of the invertebrates. The living Cestraciont sharks, of which there are four species found in the seas between Japan and Australia, seem to disappear with the Cretaceous, skipping the whole Tertiary Epoch, as do also a tribe of modern barnacles which, as Darwin says, "coat the rocks all over the world in infinite numbers." The Dipnoans or Lung-fishes (having lungs as well as gills, such as the _Ceratodus_ and _Lepidosiren_), which are represented by several living species in Australia and South Africa, are the remains of a tribe found in whole shoals in the Carboniferous, Triassic and Jurassic rocks, but not, so far as I know, in any of the intervening rocks. The living Ceratodus was only discovered in 1870, and was regarded as a marvel of "persistence." On a pinch, as when his native streams dry up, this curious fellow can get along all right without water, breathing air by his lungs like a land animal. If in the meantime he was off on a trip to the moon, he must have "persisted" a few million years without either. But his cousin, the _Polypterus_ of the Upper Nile, has a still more amazing record, for he has actually skipped all the formations from the Devonian down to the modern; while the Limuloids or sea scorpions have jumped from the Carboniferous down. The Mollusks and Brachiopods would afford us examples too numerous to mention. How is it possible that these numerous families disappear suddenly and completely with the Mesozoic or even the "early" Palaeozoic, and are not found in any "later" deposits, though alive now in our modern world? Parts of Europe and America have, we are told, been down under the sea and up again a dozen times since then; why then should we not expect to find abundant remains of these "persistent" types in the Mesozoic and Tertiaries? Surely these feats of time-acrobatics show the folly of arranging contemporaneous, taxonomic groups in single file and giving to each a time value. The Chalk points a similar lesson. It was not till the time of the "Challenger" Expedition that the modern deposits of Globigerina ooze, made up of species identical with those of the Chalk, were known to be now forming over vast areas of the ocean floor. In the words of Huxley, these modern species "bridge over the interval between the present and the Mesozoic periods."[46] As for the silicious sponges found in the Chalk, which were such puzzles for the scientists during the first half of the nineteenth century, because their living forms were unknown, the deep-sea investigations have solved the problem, for in 1877 Sollas demonstrated "the identity of their structure with that of living Hexactinellids, Lithistids, and Monactinellids."[47]