The Origin of Vertebrates

by Walter Holbrook Gaskell

Fig. 4.—Plan of Geological Strata. (From Lankester.)

Again, another point of great interest is seen here, and that is that these Labyrinthodonts, as Huxley has pointed out, possess characters which bring them more closely than the amphibians of the present day into connection with the fishes; and further, the fish-like characters they possessed are those of the Ganoids, the Marsipobranchs, the Dipnoans, and the Elasmobranchs, rather than of the Teleosteans.

Now, it is a striking fact that the ancient fishes at the time when the amphibians appeared had not reached the teleostean stage. The ganoids and elasmobranchs swarmed in the waters of the Devonian and Carboniferous times. Dipnoans and marsipobranchs were there, too, in all probability, but teleosteans do not appear until the Mesozoic period. The very kinds of fish, then, which swarmed in the seas at that time, and were the predominant race before the Carboniferous epoch, are those to which the amphibians at their first appearance show the closest affinity. Here, again, the same law appears; from the predominant race at the time, the next higher race arose, and arose by a most striking modification, which was the consequence of altering the medium in which it lived. By coming out of the water and living on the land, or, rather, being able to live partly on land and partly in the water, by the acquisition of air-breathing respiratory organs or lungs in addition to, and instead of, water-breathing organs or gills, the amphibian not only arose from the fish, but made an entirely new departure in the sequence of progressive forms.

This was a most momentous step in the history of evolution—one fraught with mighty consequences and full of most important suggestions.

From this time onwards the struggle for existence by which upward progress ensued took place on the land, not in the sea, and, as has been pointed out, led to the evolution of reptiles from amphibians, birds and quadrupedal mammals from reptiles, and man from quadrupeds. In the sea the fishes were left to multiply and struggle among themselves, their only opponents being the giant cephalopods, which themselves had been evolved from a continual succession of the Mollusca. For this reason the struggle for existence between the fishes and the higher race evolved from them did not take place until some members of that higher race took again to the water, and so competed with the fish-tribe in their own element.

Another most important conclusion to be derived from the uprising of the Amphibia is that at that time there was no race of animals living on the land which had a chance against them. No race of land-living animals had been evolved whose organization enabled them to compete with and overcome these intruders from the sea in the struggle for existence. For this reason that the whole land was their own, and no serious competition could arise from their congeners, the fish, they took possession of it, and increased mightily in size; losing more and more the habit of going into the water, becoming more and more truly terrestrial animals. Henceforth, then, in trying to find out the sequence of evolution, we must leave the land and examine the nature of the animals living in the sea; the air-breathing animals which lived on the land in the Upper Silurian and Devonian times cannot have reached a stage of organization comparable with that of the fishes, seeing how easily the amphibians became dominant.

We arrive, then, at the conclusion that the ancestors of the fishes must have lived in the sea, and applying still the same principles that have held good up to this time, the ancestors of the fishes must have arisen from some member of the race predominant at the time when they first appeared, and also the earliest fishes must have much more closely resembled the ancestral form than those found in later times or at the present day.

What, then, is the record of the rocks at the time of the first appearance of fish-like forms? What kind of fishes were they, and what was the predominant race at the time?

We have now reached the Upper Silurian and Lower Devonian times, and most instructive and suggestive is the revelation of the rocks. Here, when the first vertebrates appeared, the sea was peopled with corals, brachiopods, early forms of cephalopods, and other invertebrates; but, above all, with the great tribe of trilobites (Fig. [6]) and their successors. From the trilobites arose, as evidenced by their larval form, the king-crab group, called the Xiphosura (Fig. [5]). Closely connected with them, and forming intermediate stages between trilobites and king-crabs, numerous forms have been discovered, known as Belinurus, Prestwichia, Hemiaspis, Bunodes, etc. (Fig. [5] and Fig. [12]). From them also arose the most striking group of animals which existed at this period—the giant sea-scorpions, or Gigantostraca. This group was closely associated with the king-crabs, and the two groups together are classified under the title Merostomata.