A first book in organic evolution

by D. Kerfoot Shute

The upper Carboniferous rocks represent the later Carboniferous period (Permian Period). The life of this period is transitional between that of the Palæozoic and of the Mesozoic eras. Here we meet with the last of many types which had persisted from Cambrian times, associated with forms which are prophetic of the characteristic types of the Mesozoic era. We also meet with types in the Permian that are peculiar to the period. In this section of the Carboniferous period many of the genera of the fishes are the same, while new ones are introduced. Among the lung-fishes the genus Ceratodus is introduced, a creature very closely allied to the modern lung-fish of Australia. The Amphibia are still represented by the Stegocephala, several of the older genera persisting, while many new forms appear for the first time; several of the latter surpass the earlier Carboniferous genera greatly in size. The transitions from many of the Devonian fishes to the Carboniferous Labyrinthodonts are so gradual that it is sometimes difficult to say whether we are dealing with Fish or Amphibian. In other words, the Devonian fishes are generalized fishes, that is, connecting links between Fish and Amphibian. When the Amphibians finally separated from the fishes they were not the highly specialized forms of more recent times; but they were generalized Amphibians, having some reptilian characteristics. As time passed on and the creatures continued to modify, some varied more and more in the direction of true amphibians, and others more and more in the direction of reptiles, until in the Permian stage early, generalized Reptiles, of lizard-like form, appeared in large numbers. The most interesting of these primitive Reptiles are the Theromorpha (beast forms), which present many remarkable approximations to the structure of the mammals. As yet no snakes or turtles, no alligators or crocodiles had come into existence. In spite of the fact that great advance in animal life was made during Carboniferous times, there was little of the life in those ancient woods that we associate with the forests of the present; they were gloomy wastes of shade, without the presence of bright flowers, no humming of the bees, no song of birds, and few sounds save the gurgling of running streams, the sighing of the wind through the leaves, the splash of waves upon the shore, and the bursting of the thunder clouds. The interest and importance of the Carboniferous Amphibians as the first land backboned animals, is so great as to cause the Carboniferous period to be spoken of as the Age of Amphibians.

Triassic Period. In this period the ferns and horsetails continue under new forms; but the next higher orders of plants, the Cycads and Conifers, now predominate. The Goniatites are replaced by the Molluscan Ceratites, the latter being characteristic of the Triassic period.

One of the most characteristic changes from the Palæozoic to the Mesozoic era consists in the great reduction of the Brachiopods.

The Vertebrata of this period are of extraordinary interest and show great progress. The fishes exhibit the least progress. The Dipnoan Ceratodus is very characteristic, continuing from the Permian. The Crossopterygians have greatly declined. The Ganoidei continue to be the dominant fish-type, and are most like the existing gar-pikes. The Amphibia (Stegocephala) culminate in this period, multiplying and diversifying greatly, and far surpass in size the Carboniferous and Permian genera, and then become extinct.

It is in the Reptilian class that we find the most remarkable changes. The abundance and diversity of the reptiles in this and the two succeeding periods are incomparably greater than those in the Permian age. The Triassic rocks have representatives of almost all the orders of Mesozoic reptiles, though often these are comparatively small and rare forms.

The reptiles with mammalian characters (Theromorpha), which first appeared in the Permian period, culminate in the Triassic, especially in southern Africa, and then become extinct. No birds, or reptiles which can be regarded as the ancestors of birds, have been found in this period. In this period one of the greatest advances in the progress of life is indicated by the first appearance of low mammals,—mammals having very decided reptilian characters, and belonging to the orders of generalized Monotremes and Marsupials (Dromatherium; Microlestes).

Jurassic Period. The Fishes have advanced greatly beyond those of the Triassic rocks. The Sharks have advanced practically to their modern condition, and a new order of Elasmobranchs, the broad and flat Rays, are introduced.

The Chimæroids occur in this period, and were more numerous than in modern times. Dipnoans have become very scarce. The Crossopterygians are greatly reduced. The Ganoidei are still the dominant type; some of these latter approximate the Teleosts so closely that it seems arbitrary to call them Ganoidei. In Europe the Reptiles culminate in this period and show extraordinary development in variety, huge size, number and degree of organization. They were rulers in every department of nature. They were the rulers in the air in place of birds; rulers on the land in place of mammals; and rulers in the sea instead of sharks and whales.

Immense land reptiles (Dinosauria) as large as our largest mammals, and in some cases larger than the elephant, moved sluggishly over the land. Some walked on all fours; others were occasionally or usually bipedal, and walked upright like birds, and had many structural features in common with the latter; some were herbivorous, feeding on plants and even reaching into the branches of trees for their food, others were carnivorous, feeding on their fellow-creatures. Huge reptiles (Ichthyosauria) swam about in the sea in great numbers. Immense bat-like forms (Pterosauria) sailed through the air like birds, being literally flying dragons.

These reptiles, which had branched off from the generalized Amphibians, were themselves very generalized creatures. It is interesting to remember that the Monotreme and Marsupial mammals found in this age were very low reptilian mammals; they were not typical, specialized Monotremes and Marsupials, like the modern creatures, but very generalized forms, being probably connecting links with low generalized Insectivora, which were the first of the true mammals.