A first book in organic evolution

by D. Kerfoot Shute

These early Monotremes and Marsupials were quite small animals, varying in size from a mole to a rabbit. They were insignificant creatures among the mighty giant reptiles, but they carried in their warm blood the promise of future mammalian supremacy.

Birds appear for the first time in this period, but very different from modern birds. Not only did many of the reptiles of this time have bird-like characters, but all the birds of this period were distinctly reptilian birds (Archæopteryx). Birds came off from primitive reptiles as branches from a stem (see Diagram of Development). It was only with the passing epochs that these early birds changed more and more from their reptilian characters and assumed more and more the features of modern birds. Many fossil connecting links between reptiles and birds have been discovered, and afford most useful illustrations of those changes in creatures that we call Evolution.

Cretaceous Period. In the Cretaceous period the aspect of plant life has changed greatly. Now, for the first time, we meet with ordinary hard-wood trees, such as beech, oaks, hickory, maples, poplar, etc., but of very different genera and species from those existing to-day.

In this period, for the first time, we find the highest order of Cephalopods, viz., the naked ones, allied to cuttlefishes and squids. They are known as the Belemnites. Sometimes the fossil ink-bags of these creatures are found so that they can be drawn in their own fossil ink.

The Ammonites proper, which were introduced in the Triassic period and culminated in Jurassic time, become extinct at the close of the Cretaceous.

The Vertebrates form the most characteristic features of the Cretaceous fauna. A revolution has occurred among the Fishes. Sharks of modern type are numerous. Crossopterygians and Ganoidei are rare. There has been an immense expansion of Teleosts or Bony Fishes, which now become the dominant fishes. Most of the Cretaceous Teleosts belong to modern families and even genera.

The Reptiles, in this period, continue to be the dominant types of the land, air, and sea, and it is difficult to decide whether the Cretaceous or the Jurassic is to be regarded as the culminating period of Reptilian history. The flying Reptiles (Pterosauria) of this period are remarkable for their great size, far exceeding, in this respect, those of the Jurassic time. The land Reptiles (Dinosauria) are in greater numbers than in the preceding period. The sea Reptiles are less numerous than in the Jurassic, but are of greatly increased size.

Cretaceous Birds are much more numerous and advanced than the Jurassic ones.

The Mammals of the Cretaceous are much more abundant and varied than those of Jurassic times, but they are nearly all of very small size, and continue to play a very modest rôle. The lower Cretaceous mammals differ but little from those of the Jurassic, except for the larger number of genera. The mammals of the latest Cretaceous time are much more numerous and diversified than those of the early Cretaceous. They also show affinities with the mammals of the next succeeding or Tertiary period. The birds and mammals, though they have varied with the passing time and changing surroundings like all other living creatures, are very different from those around us now. They have thrown off some of their reptilian characters, yet they are still distinctly and pronouncedly reptilian.

The Triassic, Jurassic and Cretaceous periods together constitute the Mesozoic Era, or Age of Reptiles; for never before and never in succeeding ages were reptiles so huge, and varied, and masters in all the realms of nature.