Part 10
The _Devonian_ period, including that of the Old Red Sandstone, was one of the most significant periods in the earth's history. For it was the time of the establishment of flowering plants upon the earth and of terrestrial backboned animals. One would like to have been the discoverer of the Devonian foot-print of _Thinopus_, the first known Amphibian foot-print--an eloquent vestige of the third great invasion of the dry land. It was probably from a stock of Devonian lung-fishes that the first Amphibians sprang, but it was not till the next period that they came to their own. While they were still feeling their way, there was a remarkable exuberance of shark-like and heavily armoured fishes in the Devonian seas.
EVOLUTION OF LAND ANIMALS
§ 1
Giant Amphibians and Coal-measures
The _Carboniferous_ period was marked by a mild moist climate and a luxuriant vegetation in the swampy low grounds. It was a much less strenuous time than the Devonian period; it was like a very long summer. There were no trees of the type we see now, but there were forests of club-mosses and horsetails which grew to a gigantic size compared with their pigmy representatives of to-day. In these forests the jointed-footed invaders of the dry land ran riot in the form of centipedes, spiders, scorpions, and insects, and on these the primeval Amphibians fed. The appearance of insects made possible a new linkage of far-reaching importance, namely, the cross-fertilisation of flowering plants by their insect visitors, and from this time onwards it may be said that flowers and their visitors have evolved hand in hand. Cross-fertilisation is much surer by insects than by the wind, and cross-fertilisation is more advantageous than self-fertilisation because it promotes both fertility and plasticity. It was probably in this period that _coloured_ flowers--attractive to insect-visitors--began to justify themselves as beauty became useful, and began to relieve the monotonous green of the horsetail and club-moss forests, which covered great tracts of the earth for millions of years. In the Carboniferous forests there were also land-snails, representing one of the minor invasions of the dry land, tending on the whole to check vegetation. They, too, were probably preyed upon by the Amphibians, some of which attained a large size. Each age has had its giants, and those of the Carboniferous were Amphibians called Labyrinthodonts, some of which were almost as big as donkeys. It need hardly be said that it was in this period that most of the Coal-measures were laid down by the immense accumulation of the spores and debris of the club-moss forests. Ages afterwards, it was given to man to tap this great source of energy--traceable back to the sunshine of millions of years ago. Even then it was true that no plant or animal lives or dies to itself!
The Acquisitions of Amphibians.
As Amphibians had their Golden Age in the Carboniferous period we may fitly use this opportunity of indicating the advances in evolution which the emergence of Amphibians implied. (1) In the first place the passage from water to dry land was the beginning of a higher and more promiseful life, taxed no doubt by increased difficulties. The natural question rises why animals should have migrated from water to dry land at all when great difficulties were involved in the transition. The answers must be: (_a_) that local drying up of water-basins or elevations of the land surface often made the old haunts untenable; (_b_) that there may have been great congestion and competition in the old quarters; and (_c_) that there has been an undeniable endeavour after well-being throughout the history of animal life. In the same way with mankind, migrations were prompted by the setting in of prolonged drought, by over-population, and by the spirit of adventure. (2) In Amphibians for the first time the non-digitate paired fins of fishes were replaced by limbs with fingers and toes. This implied an advantageous power of grasping, of holding firm, of putting food into the mouth, of feeling things in three dimensions. (3) We cannot be positive in regard to the soft parts of the ancient Amphibians known only as fossils, but if they were in a general way like the frogs and toads, newts and salamanders of the present day, we may say that they made among other acquisitions the following: true ventral lungs, a three-chambered heart, a movable tongue, a drum to the ear, and lids to the eyes. It is very interesting to find that though the tongue of the tadpole has some muscle-fibres in it, they are not strong enough to effect movement, recalling the tongue of fishes, which has not any muscles at all. Gradually, as the tadpole becomes a frog, the muscle-fibres grow in strength, and make it possible for the full-grown creature to shoot out its tongue upon insects. This is probably a recapitulation of what was accomplished in the course of millennia in the history of the Amphibian race. (4) Another acquisition made by Amphibians was a voice, due, as in ourselves, to the rapid passage of air over taut membranes (vocal cords) stretched in the larynx. It is an interesting fact that for millions of years there was upon the earth no sound of life at all, only the noise of wind and wave, thunder and avalanche. Apart from the instrumental music of some insects, perhaps beginning in the Carboniferous, the first vital sounds were due to Amphibians, and theirs certainly was the first voice--surely one of the great steps in organic evolution.
[Illustration: _Photo: British Museum (Natural History)._
FOSSIL OF A PTERODACTYL OR EXTINCT FLYING DRAGON
The wing is made of a web of skin extended on the enormously elongated outermost finger. The long tail served for balancing and steering. The Pterodactyls varied from the size of sparrows to a wing-span of fifteen feet--the largest flying creatures.]
[Illustration: _From Knipe's "Nebula to Man."_
PARIASAURUS: AN EXTINCT VEGETARIAN TRIASSIC REPTILE
Total length about 9 feet. (Remains found in Cape Colony, South Africa.)]
[Illustration: _From Knipe's "Nebula to Man."_
TRICERATOPS: A HUGE EXTINCT REPTILE
(From remains found in Cretaceous strata of Wyoming, U.S.A.)
This Dinosaur, about the size of a large rhinoceros, had a huge three-horned skull with a remarkable bony collar over the neck. But, as in many other cases, its brain was so small that it could have passed down the spinal canal in which the spinal cord lies. Perhaps this partly accounts for the extinction of giant reptiles.]
[Illustration: _Photo: "Daily Mail."_
THE DUCKMOLE OR DUCK-BILLED PLATYPUS OF AUSTRALIA
The Duckmole or Duck-billed Platypus of Australia is a survivor of the most primitive mammals. It harks back to reptiles, e.g. in being an egg-layer, in having comparatively large eggs, and in being imperfectly warm-blooded. It swims well and feeds on small water-animals. It can also burrow.]
Evolution of the Voice
The first use of the voice was probably that indicated by our frogs and toads--it serves as a sex-call. That is the meaning of the trumpeting with which frogs herald the spring, and it is often only in the males that the voice is well developed. But if we look forward, past Amphibians altogether, we find the voice becoming a maternal call helping to secure the safety of the young--a use very obvious when young birds squat motionless at the sound of the parent's danger-note. Later on, probably, the voice became an infantile call, as when the unhatched crocodile pipes from within the deeply buried egg, signalling to the mother that it is time to be unearthed. Higher still the voice expresses emotion, as in the song of birds, often outside the limits of the breeding time. Later still, particular sounds become words, signifying
## particular things or feelings, such as "food," "danger," "home,"
"anger," and "joy." Finally words become a medium of social intercourse and as symbols help to make it possible for man to reason.
§ 2
The Early Reptiles
In the _Permian_ period reptiles appeared, or perhaps one should say, began to assert themselves. That is to say, there was an emergence of backboned animals which were free from water and relinquished the method of breathing by gills, which Amphibians retained in their young stages at least. The unhatched or unborn reptile breathes by means of a vascular hood spread underneath the egg-shell and absorbing dry air from without. It is an interesting point that this vascular hood, called the allantois, is represented in the Amphibians by an unimportant bladder growing out from the hind end of the food-canal. A great step in evolution was implied in the origin of this ante-natal hood or foetal membrane and another one--of protective significance--called the amnion, which forms a water-bag over the delicate embryo. The step meant total emancipation from the water and from gill-breathing, and the two foetal membranes, the amnion and the allantois, persist not only in all reptiles but in birds and mammals as well. These higher Vertebrates are therefore called Amniota in contrast to the Lower Vertebrates or Anamnia (the Amphibians, Fishes, and primitive types).
It is a suggestive fact that the embryos of all reptiles, birds, and mammals show gill-clefts--_a tell-tale evidence of their distant aquatic ancestry_. But these embryonic gill-clefts are not used for respiration and show no trace of gills except in a few embryonic reptiles and birds where their dwindled vestiges have been recently discovered. As to the gill-clefts, they are of no use in higher Vertebrates except that the first becomes the Eustachian tube leading from the ear-passage to the back of the mouth. The reason why they persist when only one is of any use, and that in a transformed guise, would be difficult to interpret except in terms of the Evolution theory. They illustrate the lingering influence of a long pedigree, the living hand of the past, the tendency that individual development has to recapitulate racial evolution. In a condensed and telescoped manner, of course, for what took the race a million years may be recapitulated by the individual in a week!
In the Permian period the warm moist climate of most of the Carboniferous period was replaced by severe conditions, culminating in an Ice Age which spread from the Southern Hemisphere throughout the world. With this was associated a waning of the Carboniferous flora, and the appearance of a new one, consisting of ferns, conifers, ginkgos, and cycads, which persisted until near the end of the Mesozoic era. The Permian Ice Age lasted for millions of years, and was most severe in the Far South. Of course, it was a very different world then, for North Europe was joined to North America, Africa to South America, and Australia to Asia. It was probably during the Permian Ice Age that many of the insects divided their life-history into two main chapters--the feeding, growing, moulting, immature, larval stages, e.g. caterpillars, and the more ascetic, non-growing, non-moulting, winged phase, adapted for reproduction. Between these there intervened the quiescent, well-protected pupa stage or chrysalis, probably adapted to begin with as a means of surviving the severe winter. For it is easier for an animal to survive when the vital processes are more or less in abeyance.
Disappearance of many Ancient Types
We cannot leave the last period of the Palæozoic era and its prolonged ice age without noticing that it meant the entire cessation of a large number of ancient types, especially among plants and backboneless animals, which now disappear for ever. It is necessary to understand that the animals of ancient days stand in three different relations to those of to-day. (_a_) There are ancient types that have living representatives, sometimes few and sometimes many, sometimes much changed and sometimes but slightly changed. The lamp-shell, _Lingulella_, of the Cambrian and Ordovician period has a very near relative in the _Lingula_ of to-day. There are a few extremely conservative animals. (_b_) There are ancient types which have no living representatives, except in the guise of transformed descendants, as the King-crab (_Limulus_) may be said to be a transformed descendant of the otherwise quite extinct race to which Eurypterids or Sea-scorpions belonged. (_c_) There are altogether extinct types--_lost races_--which have left not a wrack behind. For there is not any representation to-day of such races as Graptolites and Trilobites.
Looking backwards over the many millions of years comprised in the Palæozoic era, what may we emphasise as the most salient features? There was in the _Cambrian_ the establishment of the chief classes of backboneless animals; in the _Ordovician_ the first fishes and perhaps the first terrestrial plants; in the _Silurian_ the emergence of air-breathing Invertebrates and mud-fishes; in the _Devonian_ the appearance of the first Amphibians, from which all higher land animals are descended, and the establishment of a land flora; in the _Carboniferous_ the great Club-moss forests and an exuberance of air-breathing insects and their allies; in the _Permian_ the first reptiles and a new flora.
THE GEOLOGICAL MIDDLE AGES
§ 1
The Mesozoic Era
In a broad way the Mesozoic era corresponds with the Golden Age of reptiles, and with the climax of the Conifer and Cycad flora, which was established in the Permian. But among the Conifers and Cycads our modern flowering plants were beginning to show face tentatively, just like birds and mammals among the great reptiles.
In the _Triassic_ period the exuberance of reptilian life which marked the Permian was continued. Besides Turtles which still persist, there were Ichthyosaurs, Plesiosaurs, Dinosaurs, and Pterosaurs, none of which lasted beyond the Mesozoic era. Of great importance was the rise of the Dinosaurs in the Triassic, for it is highly probable that within the limits of this vigorous and plastic stock--some of them bipeds--we must look for the ancestors of both birds and mammals. Both land and water were dominated by reptiles, some of which attained to gigantic size. Had there been any zoologist in those days, he would have been very sagacious indeed if he had suspected that reptiles did not represent the climax of creation.
The Flying Dragons
The _Jurassic_ period showed a continuance of the reptilian splendour. They radiated in many directions, becoming adapted to many haunts. Thus there were many Fish Lizards paddling in the seas, many types of terrestrial dragons stalking about on land, many swiftly gliding alligator-like forms, and the Flying Dragons which began in the Triassic attained to remarkable success and variety. Their wing was formed by the extension of a great fold of skin on the enormously elongated outermost finger, and they varied from the size of a sparrow to a spread of over five feet. A soldering of the dorsal vertebræ as in our Flying Birds was an adaptation to striking the air with some force, but as there is not more than a slight keel, if any, on the breast-bone, it is unlikely that they could fly far. For we know from our modern birds that the power of flight may be to some extent gauged from the degree of development of the keel, which is simply a great ridge for the better insertion of the muscles of flight. It is absent, of course, in the Running Birds, like the ostrich, and it has degenerated in an interesting way in the burrowing parrot (_Stringops_) and a few other birds that have "gone back."
The First Known Bird
But the Jurassic is particularly memorable because its strata have yielded two fine specimens of the first known bird, _Archæopteryx_. These were entombed in the deposits which formed the fine-grained lithographic stones of Bavaria, and practically every bone in the body is preserved except the breast-bone. Even the feathers have left their marks with distinctness. This oldest known bird--too far advanced to be the first bird--was about the size of a crow and was probably of arboreal habits. Of great interest are its reptilian features, so pronounced that one cannot evade the evolutionist suggestion. It had teeth in both jaws, which no modern bird has; it had a long lizard-like tail, which no modern bird has; it had claws on three fingers, and a sort of half-made wing. That is to say, it does not show, what all modern birds show, a fusion of half the wrist-bones with the whole of the palm-bones, the well-known carpo-metacarpus bone which forms a basis for the longest pinions. In many reptiles, such as Crocodiles, there are peculiar bones running across the abdomen beneath the skin, the so-called "abdominal ribs," and it seems an eloquent detail to find these represented in _Archæopteryx_, the earliest known bird. No modern bird shows any trace of them. [Illustration: SKELETON OF AN EXTINCT FLIGHTLESS TOOTHED BIRD, HESPERORNIS
(_After Marsh._)
The bird was five or six feet high, something like a swimming ostrich, with a very powerful leg but only a vestige of a wing. There were sharp teeth in a groove. The modern divers come nearest to this ancient type.]
[Illustration: SIX STAGES IN THE EVOLUTION OF THE HORSE, SHOWING GRADUAL INCREASE IN SIZE
(_After Lull and Matthew._)
1. Four-toed horse, Eohippus, about one foot high. Lower Eocene, N. America.
2. Another four-toed horse, Orohippus, a little over a foot high. Middle Eocene, N. America.
3. Three-toed horse, Mesohippus, about the size of a sheep. Middle Oligocene, N. America.
4. Three-toed horse, Merychippus, Miocene, N. America. Only one toe reaches the ground on each foot, but the remains of two others are prominent.
5. The first one-toed horse, Pliohippus, about forty inches high at the shoulder. Pliocene, N. America.
6. The modern horse, running on the third digit of each foot.]
There is no warrant for supposing that the flying reptiles or Pterodactyls gave rise to birds, for the two groups are on different lines, and the structure of the wings is entirely different. Thus the long-fingered Pterodactyl wing was a parachute wing, while the secret of the bird's wing has its centre in the feathers. It is highly probable that birds evolved from certain Dinosaurs which had become bipeds, and it is possible that they were for a time swift runners that took "flying jumps" along the ground. Thereafter, perhaps, came a period of arboreal apprenticeship during which there was much gliding from tree to tree before true flight was achieved. It is an interesting fact that the problem of flight has been solved four times among animals--by insects, by Pterodactyls, by birds, and by bats; and that the four solutions are on entirely different lines.
In the _Cretaceous_ period the outstanding events included the waning of giant reptiles, the modernising of the flowering plants, and the multiplication of small mammals. Some of the Permian reptiles, such as the dog-toothed Cynodonts, were extraordinarily mammal-like, and it was probably from among them that definite mammals emerged in the Triassic. Comparatively little is known of the early Triassic mammals save that their back-teeth were marked by numerous tubercles on the crown, but they were gaining strength in the late Triassic when small arboreal insectivores, not very distant from the modern tree-shrews (_Tupaia_), began to branch out in many directions indicative of the great divisions of modern mammals, such as the clawed mammals, hoofed mammals, and the race of monkeys or Primates. In the Upper Cretaceous there was an exuberant "radiation" of mammals, adaptive to the conquest of all sorts of haunts, and this was vigorously continued in Tertiary times.
There is no difficulty in the fact that the earliest remains of definite mammals in the Triassic precede the first-known bird in the Jurassic. For although we usually rank mammals as higher than birds (being mammals ourselves, how could we do otherwise?), there are many ways in which birds are pre-eminent, e.g. in skeleton, musculature, integumentary structures, and respiratory system. The fact is that birds and mammals are on two quite different tacks of evolution, not related to one another, save in having a common ancestry in extinct reptiles. Moreover, there is no reason to believe that the Jurassic _Archæopteryx_ was the first bird in any sense except that it is the first of which we have any record. In any case it is safe to say that birds came to their own before mammals did.
Looking backwards, we may perhaps sum up what is most essential in the Mesozoic era in Professor Schuchert's sentence: "The Mesozoic is the Age of Reptiles, and yet the little mammals and the toothed birds are storing up intelligence and strength to replace the reptiles when the cycads and conifers shall give way to the higher flowering plants."
§ 2
The Cenozoic or Tertiary Era
In the _Eocene_ period there was a replacement of the small-brained archaic mammals by big-brained modernised types, and with this must be associated the covering of the earth with a garment of grass and dry pasture. Marshes were replaced by meadows and browsing by grazing mammals. In the spreading meadows an opportunity was also offered for a richer evolution of insects and birds.
During the _Oligocene_ the elevation of the land continued, the climate became much less moist, and the grazing herds extended their range.
The _Miocene_ was the mammalian Golden Age and there were crowning examples of what Osborn calls "adaptive radiation." That is to say, mammals, like the reptiles before them, conquer every haunt of life. There are flying bats, volplaning parachutists, climbers in trees like sloths and squirrels, quickly moving hoofed mammals, burrowers like the moles, freshwater mammals, like duckmole and beaver, shore-frequenting seals and manatees, and open-sea cetaceans, some of which dive far more than full fathoms five. It is important to realise the perennial tendency of animals to conquer every corner and to fill every niche of opportunity, and to notice that this has been done by successive sets of animals in succeeding ages. _Most notably the mammals repeat all the experiments of reptiles on a higher turn of the spiral._ Thus arises what is called convergence, the superficial resemblance of unrelated types, like whales and fishes, the resemblance being due to the fact that the different types are similarly adapted to similar conditions of life. Professor H. F. Osborn points out that mammals may seek any one of the twelve different habitat-zones, and that in each of these there may be six quite different kinds of food. Living creatures penetrate everywhere like the overflowing waters of a great river in flood.
§ 3
The _Pliocene_ period was a more strenuous time, with less genial climatic conditions, and with more intense competition. Old land bridges were broken and new ones made, and the geographical distribution underwent great changes. Professor R. S. Lull describes the _Pliocene_ as "a period of great unrest." "Many migrations occurred the world over, new competitions arose, and the weaker stocks began to show the effects of the strenuous life. One momentous event seems to have occurred in the Pliocene, and that was the transformation of the precursor of humanity into man--the culmination of the highest line of evolution."
The _Pleistocene_ period was a time of sifting. There was a continued elevation of the continental masses, and Ice Ages set in, relieved by less severe interglacial times when the ice-sheets retreated northwards for a time. Many types, like the mammoth, the woolly rhinoceros, the sabre-toothed tiger, the cave-lion, and the cave-bear, became extinct. Others which formerly had a wide range became restricted to the Far North or were left isolated here and there on the high mountains, like the Snow Mouse, which now occurs on isolated Alpine heights above the snow-line. Perhaps it was during this period that many birds of the Northern Hemisphere learned to evade the winter by the sublime device of migration.
Looking backwards we may quote Professor Schuchert again: