Two Primitive Vertebrates, Palæospondylus (enlarged) and Pterichthys (reduced),
(After Woodward, with some modifications.)
The plate-bearing ganoids of the Silurian and Devonian, at one time supposed to be allied to Crustaceans, but whose dignity as "Forerunners of the back-boned animals" is now generally admitted,[79] are clearly true fishes, and of somewhat high rank, their strange bony armour being evidently a special protection against the attacks of contemporary sharks and gigantic crustaceans; and if we may judge by the Colorado specimens, their existence dates back almost to the close of the Cambrian, and they were probably contemporary with small sharks; while as early as the Silurian and Devonian, if we regard the scaly ganoids as a distinct type, we have already four types of fishes, and these akin to those which in modern time we must regard as the highest of their class.
[79] A. Smith Woodward, "Natural Science," 1892, and Annals and Maga. Nat. Hist., October, 1890. This able naturalist, in introducing his subject, remarks, from the point of view of an evolutionist:—"Whether some form of 'worm' gave origin to the forerunners of the great back-boned race, or whether a primeval relative of the King-crab turned upside down and rearranged limbs and head these are questions still admitting of endless discussion, no doubt fruitless in their main object, but desirable from the new lines of investigation they continually suggest."
One very little fish of the Devonian, of which specimens have been kindly sent me by a friend in Scotland,[80] the Palæospondylus of Traquair, may raise still higher hopes for the early vertebrates. It is a little creature, an inch to two inches in length, destitute or nearly destitute of bony covering, having a head which suggests the presence of external gills, large eyes, and even elongated nasal bones,[81] a long vertebral column composed of separate bony rings, more than fifty in number, with possible indications of ribs in front and distinct neural and haemal processes behind. One cannot look at it without the suggestion occurring of some of the smaller snake-like Batrachians of the Carboniferous and Permian; and I should not be surprised if it should come to be regarded either as a forerunner of the Batrachians or as a primitive tadpole.
[80] James Reed, Esq., of Allan House, Blairgowrie.
[81] I am aware that Woodward regards these parts differently.
However this may be, the upper part of the Devonian, though rich in fishes and plants, has afforded no higher vertebrates than its lower parts, and in the lowest Carboniferous beds we suddenly find ourselves in the presence of Batrachians with well-developed limbs and characters which ally them to the Lizards. True lizard-like reptiles appear in the Permian, and then we enter on that marvellous reign of reptiles, in which this class assumed so many great and remarkable forms, and asserted itself in a manner of which the now degraded reptilian class can afford no conception.
The mammals and birds make their first appearance quietly in small and humble forms in the reign of reptiles, in which there was little place left for them by the latter; but the mammals burst upon us in all their number and magnitude in the Eocene and Miocene, in which quadrupedal mammalian life may be said to have culminated in grandeur, variety, and geographical distribution; far excelling in these respects the time in which we live.
The development in time of the back-boned animals thus stands in some degree by itself; but it illustrates the same laws of early generalised types, and sudden and wide introduction of new forms, which we have seen in the case of the invertebrates and the plants.