Part 8

These plants of the coal period were remarkable, generalized types, connecting classes now widely separated. During this period the insects increase in variety and numbers along with the advancing vegetation. It is interesting to find that as the highest flowering plants are not yet in existence, so those highest orders of insects, the flower-loving and honey-loving ones, such as bees, butterflies and moths, are not yet in existence. Corals and Echinoderms, Molluscs and Crustaceans, etc., continue through the ages with ever shifting forms. Brachiopods have greatly diminished. Foraminifera for the first time assume considerable importance in the earth’s economy.

The Molluscan Goniatites continue through this age with changing forms and advancing organization, and are very numerous. The Trilobites, which are characteristic of Palæozoic times, continue under new forms through this age, and then perish. In this age are introduced for the first time typical Crustaceans of the long-tailed kind, such as shrimps.

Devonian fishes still prevail. The Elasmobranchii are numerous and varied, and some of them are highly specialized. _Pleuracanthus_ is a remarkable shark which has many features in common with the lung-fishes (Dipnoi), such as the character of the pectoral fins, the shape of the tail, the bones which form a roof for the skull, while the skin is naked.

The Dipnoi continue, though in diminished numbers.

The Crossopterygians are much less abundant. The Ganoidei increase in numbers and varieties.

The fact of greatest interest concerning the Carboniferous period is the _expansion of the true land-breathing, backboned animals, such as the_ =Amphibians=. _Their expansion marks a distinct step forward in the scale of life._

The Carboniferous Amphibia all belong to the extinct order of _Labyrinthodonts_ (_Stegocephala_), in which the skull is well covered with a roof of sculptured bones. The Amphibia of the lower Carboniferous rocks are of small or moderate size, not exceeding eight feet in length and mostly much smaller. A great number of these Amphibians are known, most of them like the Salamanders in shape, but some are snake-like in form, being long and slender. An example of the Stegocephala is _Archegosaurus_.

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.

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.

=Tertiary Period.= In reaching Tertiary time we enter upon the threshold of our modern world. The reptiles have dwindled to a few low forms, such as alligators and crocodiles, to the small lizards, to turtles and tortoises, and to those low-grade reptiles, the snakes, which now for the first time are the most numerous of the class. In these ages, as now, the Teleost fishes vastly predominate, and the Ganoid and Crossopterygian fishes are nearly extinct. In Tertiary times, as in the present, all the reptilian birds had disappeared, and only typical birds remain. In other words, the bird-class had now separated from the reptilian class, and the connecting links became extinct.

In this age for the first time the highest flowering plants are abundant, and now for the first time also the highest orders of insects are abundant, such as bees, ants, butterflies, etc. On account of the greater warmth and moisture in the Tertiary period, insect and plant life were fuller then than now. That the climate was warmer in those times is shown by the following facts: In America, during the early Tertiary period, figs, evergreens and palms grew in Dakota, showing a temperature there at that time equal to the temperature in Florida at present. In the middle Tertiary period trees like the Redwood of California, and Magnolias, were abundant in Greenland.

Although _Reptilian Mammals_ (_Multituberculata_) lived in the preceding era, true or typical Mammals first came to view in the earliest Tertiary times. They soon succeeded the vanished giant reptiles as the rulers of the world; so that this and the succeeding (Quaternary) period are known as the =Age of Mammals=. At this period, for the first time, appear generalized _primitive flesh-eating mammals_ (_Creodonta_), and _primitive grass-eating ones_ (_Condylarthra_ and _Amblypoda_); also _primitive Primates_ (_primitive Lemuroids_, and, later, _primitive types of Monkeys_, e. g., _Anaptomorphus_).

The evolution of mammals, compared with that of other animal groups, has been so rapid that each stage of even the Eocene has its own mammalian fauna, differing from those of the succeeding and preceding stages. All through the Tertiary period, as through all the preceding ages, the species that are advancing in life are undergoing greater and greater specialization. Animals at first closely related finally become more and more separated from one another. This is well illustrated by a study of the hoofed animals. In the earliest Tertiary period all these animals seem to unite into one branch but now they consist of many widely separated sub-branches. As we trace the branch down to the earliest Tertiary period, we find that even in early Eocene times it divides into the even-toed and odd-toed ungulates. In the later Miocene times, each of these again separates,—the former, even-toed ones, into the hog and hippopotamus families with four toes, and the ruminant family with two toes; the latter, odd-toed ones, into the elephant family with five toes, the rhinoceros and tapir families with three toes, and the horse family with one toe.

It will be exceedingly instructive to trace briefly the progressive specialization of the horse family through the Tertiary period. A wonderful series representing this family has been found in the American Tertiaries. First of all, the =Eohippus= is found in the earliest Eocene rocks. This little animal was about the size of a fox and had four perfect toes (hoofed) and a fifth, much smaller, imperfect one (splint) on the fore-feet; the hind-feet had three perfect hoofed toes. Later on, in the middle Eocene rocks, was found the =Orohippus= (Fig. 14), with three toes behind (b), and four in front (a)—the fifth imperfect toe (splint) being lost. This animal was also about the size of a fox. Still later on were found, in the Miocene rocks, the =Mesohippus= (c, d) and the =Miohippus= (e, f), with three toes behind (d, f) and three toes and a splint in front (c, e). The splint in the Miohippus is much smaller than in the Mesohippus, and also in the former the two side toes have become smaller and farther removed from the ground. Both of these animals were about the size of sheep. Further on, in the early Pliocene rocks, appears the =Protohippus= (g, h), with three toes on all the feet, but the middle toe considerably larger and longer than the side toes. This animal was about the size of an ass. Then, finally, came the =Pliohippus= (k, l) and =Equus= (m, n) in the latest Pliocene rocks. Here the middle toe is greatly enlarged and the side toes are reduced to useless splints. Thus by degrees, through slight modifications not only of the feet, but also of the skull, teeth, brain, etc., and by adaptations from age to age, was formed the modern horse. In like manner, by gradual changes and adaptation to surroundings, all the early generalized mammals pass into the higher specialized ones.

=Quaternary Period.= The mammals of this age differ from those of the Tertiary period and from living species. Never before this period, and never since, have mammals been so large. They culminate in this period and then decline. The great mastodon, the great cave-bear, and saber-toothed tiger, the Irish elk, and gigantic sloths and armadillos lived in this age.

Most interesting of all, the remains of _man_ now first appear associated with these extinct animals. Man must have been at first an apparently insignificant creature among the mighty mammals that surrounded him, and must for a time have contended more or less doubtfully with them for mastery. But as he increased in numbers and intelligence he became more and more the ruler of the brute creation; so that this age in which he rules may fittingly be spoken of as the Psychozoic Era, or =Age of Man=.

Fig. 15 is introduced to show how easy may be the transitions from one order of animals to another. Two long, slender Lizards are shown and also a Snake. Bipes has four limbs, but they are very small and weak. Cheirotes has lost, probably through disuse, the hind-limbs and the front ones are small and weak, almost useless. The snake has no limbs at all. The two lizards are quite snake-like in external forms. Just as changing environment, use and disuse, etc., can effect transitions in external forms by slight gradations, so also may the whole internal structures undergo marked alterations by slight modifications through long ages.

The brief references we have made to the changing life-forms during the geologic ages, should be very instructive even to a reader not familiar with zoölogy. They teach the important lesson that life, in the main, has ever advanced to higher and higher levels. At one time in the history of the earth no animals were in existence higher than the Invertebrates; these lived through the ages with ever-changing forms. Later, in the course of geologic history, some of the primitive Invertebrates evolved into the lowest Vertebrates, and Fishes finally appeared, at first very primitive and generalized. For long ages there were no creatures on the globe higher than these. Then as the æons slowly rolled on some of the early fishes evolved finally into a higher group of animals, the Amphibians, and these for long ages were the highest creatures in existence. As time slowly passed, some of the primitive amphibians evolved into primitive Reptiles, and these creatures for long ages were the highest in existence and monarchs of the world. Some of the primitive reptiles branched off in one direction leading to modern Birds, and some advanced in another direction that led through primitive mammals to man and other modern mammals. And now in modern times man is monarch among the animals.

We may now begin to realize that truly with the changing and growing continents during the geologic ages, many life-forms have varied and advanced to higher levels, culminating in the final appearance of _man_ as well as other living creatures.

SECTION V.

NATURAL SELECTION.

NATURAL SELECTION.