Some Salient Points in the Science of the Earth

by Sir John William Dawson

[74] Among these are living species of ferns, one of them our common "Sensitive Fern," of Eastern America, two species of Hazel still extant, and Sequoias or giant pines, like those now surviving in California.

Take from the western Mesozoic a contrasting yet illustrative fact. In the lowest Cretaceous rocks of Queen Charlotte's Island, Mr. Richardson and Dr. G. M. Dawson find Ammonites and allied Cephalopods similar in many respects to those discovered farther south by the California Survey, and Mr. Whiteaves finds that some of them are apparently not distinct from species described by the Palæontologists of the Geological Survey of British India. On both sides of the Pacific these shells lie entombed in solid rock, and the Pacific rolls between, as of yore. Yet these species, genera, and even families are all extinct why, no man can tell, while land plants that must have come in while the survivors of these Cephalopods still lived, reach down to the present. How mysterious is all this, and how strongly does it show the independence in some sense of merely physical agencies on the part of the manifestations of life!

We have naturally been occupied hitherto with the lower tribes of animals and with plant life, because these are predominant in the early ages of the earth. Let us turn now to the history of vertebrate or back-boned animals, which presents some peculiarities special to itself. Many years ago Pander[75] described and figured from the Cambro-Silurian of Russia, a number of minute teeth, some conical and some comb-like, which he referred to fishes, and to that low form of the fish type represented by the modern lampreys. Much doubt was thrown on this determination, more especially as the teeth seemed to be composed not of bone earth, but of carbonate of lime, and it was suggested that they may have belonged to marine worms, or to the lingual ribbons of Gastropod mollusks. Some confirmatory evidence seems to have been supplied by the discovery of great numbers of similar forms in the shales of the coal formation of Ohio, by the late Dr. Newberry. I have had an opportunity to examine these, and find that they consist of calcium phosphate,[76] or bone earth, and that their microscopic structure is not dissimilar from that of the teeth of some of the smaller sharks (Diplodus) found with them. I have therefore been inclined to believe that there may have already been, even in the Cambrian or Lower Silurian seas, true fishes, related partly to the lampreys and partly to sharks; so that the history of the back-boned animals may have gone nearly as far back as that of their humbler relations. This conjecture has recently received further support from the discovery in rocks of Lower Silurian age, in Colorado of a veritable bone bed, rich in fragmentary remains of fishes. They are unfortunately so comminuted as to resemble the débris of the food of some larger animal; but in so far as I can judge from specimens kindly given to me,[77] they resemble the bony coverings of some of the familiar fishes of the Devonian. Thus they would indicate, with Pander's and Rohan's specimens, already two distinct types of fishes as existing almost as early as the higher invertebrates of the sea.

[75] More recently Rohan has described conical teeth (St. Petersburg Academy, 1889), but I have not seen his paper.

[76] Analysis of Dr. B. J. Harrington.

[77] By Mr. F. D. Adams and Dr. Walcott.

In the Silurian (Upper Silurian of Murchison) we have undoubted evidence of the same kind, on both sides of the Atlantic, in teeth and spines of sharks, and the plates which protected the heads and bodies of the plate-covered fishes (Placo-ganoids). But it is in the Devonian that these types appear to culminate, and we have added to them that remarkable type of "lung fish," as the Germans call them, represented in our modern world only by the curious and exceptional Burramunda of Australia, and the mud fishes of Africa and South America,[78] creatures which show, as do some of the mailed fishes, or ganoids, of equally great age, the intermediate stages between a swimming bladder and a lung, and thus approach nearer to the air-breathing animals than any other fishes.

[78] Ceratodus, Lipidosiren, Protopterus.

Many years ago, in "Acadian Geology," I referred to the probability that the mailed and lung fishes of the Devonian and Carboniferous possessed air bladders so constructed as to enable them to breathe air, as is the case with their modern representatives. In the modern species this, no doubt, enables them to haunt badly aërated waters, in swamps and sluggish streams, and in some cases even to survive when the water in which they live is dried up. In the Carboniferous and Devonian it may have served a similar purpose, fitting them to inhabit the lagoons and creeks of the coal swamps, the water of which must often have been badly aërated. It makes against this that some sharks followed them into these waters, and the modern sharks have no swim-bladders. Possibly, however, the sharks habitually haunted the open sea, and only made occasional raids on the dangerous waters tenanted by the ganoids. It is also true that only certain genera of sharks are found to be represented in the carbonaceous shales, and they may have differed in this respect from the ordinary forms of the order. It has been suggested that only a small change would be necessary to enable some of these lung fishes to become Batrachians, and no doubt this is the nearest approach of the fish to the reptile; but we have not yet found connecting links sufficient to bridge over the whole distance.