Book - Vertebrate Zoology (1928) 36

From Embryology

Vertebrate Zoology G. R. De Beer (1928)

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PART IV EVOLUTIONARY MORPHOLOGY

Chapter XXXVI The Bearing of Physical and Climatic Factors on Chordates

To understand their evolution and life it is essential to consider animals in relation to their environment. During the time since chordate animals first appeared, the environment has changed very considerably at one time or another. Of the most primitive forms there is no record preserved, for the simple reason that these animals did not possess structures capable of preservation by fossilisation. The earliest known vertebrates are from the Silurian period, and they were fish. The earth was at this time covered with shallow seas containing coral-reefs which are indicative of a mild climate. In the ensuing Devonian period, shallow lagoons and enclosed basins were in abundance, and the land which had emerged enjoyed desert conditions with little rainfall. It is towards the end of this period that the first land-vertebrates (Stegocephalian amphibia) appeared. The next or Carboniferous period was one of tropical climates, during which luxuriant forests covered the land. The trees had no rings of growth, which fact proves that there were no seasons. True reptiles first appeared here. In the late Carboniferous and Permian period the climate became colder as the continents rose and mountain chains were formed, resulting in an ice-age or glacial period. In the following Trias, warm conditions returned, without seasonal variation. The earliest known mammals belong to this period. Warm conditions persisted throughout the Jurassic period, in which the first birds are found, but this period is pre-eminently the " age of reptiles," not only on account of the number of different types which flourished, but also because of the gigantic size to which many of them grew.


In the Cretaceous, cold conditions returned with seasonal variations. Mountain-building and glaciation occurred in some parts of the earth, the temperature of which was now considerably reduced. At this time and perhaps for this reason the majority of the reptiles which had hitherto been so successful, went extinct and were superseded by mammals as the dominant animals. After this time, hot conditions set in again for the main part of the Tertiary era, gradually diminishing towards its close when a fresh bout of mountain-building erected the Alps. Then followed the great Ice- Age. Mammals continued evolving during this period, towards the end of which man appeared.


The most important changes in the environment as far as the vertebrates were concerned were the drying-up of the lagoons and estuaries in the Devonian, and the variations of temperature.


It is a characteristic feature of desiccated areas that the water expanses which they possess shrink to ponds, and the oxygen-content of the water decreases owing to the quantities of decomposing organic matter with which the ponds become filled. Under such circumstances it is obvious that fish which are provided with means of supplementing their branchial respiration would have a much greater chance of surviving, and the first step in this direction was the habit of taking air into the pharynx when at the surface. At the present day, inhabitants of such waters show diverse adaptations, but by far the most important of these from the present point of view are the Dipnoi, with their lungs. There is little doubt that the ancestors of the Tetrapods encountered and mastered conditions of desiccation in fresh water, in the same way as the modern Dipnoi. There is the further danger that under these circumstances the water may dry up altogether, as it does in the case of the swamps in which Protopterus lives, and then the possession of a means for pulmonary respiration is the only condition for survival.


Temperature may vary in several different ways, either in space or in time, or in both. So the tropics and the temperate and polar regions differ in temperature, as do day and night or summer and winter.


Homothermous animals are largely independent of temperature variation in the outer environment since they live in a constant internal environment of their own. However, the outer environmental temperature has a bearing on their size. This follows readily from a consideration of the ratios of surface to volume at different sizes. The surface increases as the square, but the volume increases as the cube of the linear dimensions, so that there is relatively more surface in small animals than in large ones. The importance of this for homothermous animals is that the amount of internal heat produced (by metabolism) and lost (by radiation) varies relatively with the surface. So, of two dogs weighing 20 and 3 J kg. respectively, the former will have a surface of 7,500 sq. cm., the latter 2,423 sq. cm. For every kg. of dog, there is in the large dog 375 sq. cm., and in the small one 757 sq. cm. of surface, and the amount of heat given off from the dogs per kg. is twice as high in the case of the small dog as in the case of the large one.


Small homothermous animals therefore radiate relatively more heat from their surface than large animals, and this heat-loss has to be compensated by relatively more active metabolism and intake of food. In spite of the fact that mammals and birds grind their food up small (in the mouth in mammals : in birds, in the gizzard) so that the processes of digestion are accelerated, a stage of smallness is reached when the animals have to spend all their time feeding. Shrews and humming-birds are of about this size. If they were smaller than this they would need to consume quantities of food which they would not have time to eat. Especially true is this of regions in which because of seasonal variation the days are short for a period in each year. The ratio of surface to volume therefore establishes a minimum limit of size for homothermous animals in a given outer environmental temperature.


In cold climates, such as prevail in polar regions, homothermous animals tend to be large. They profit by their relatively small surface from which they lose heat, and also by the fact that they do not require to spend all their daylight eating as they would if their surface/volume ratios were large and they were small in size. On the other hand, tropical homothermous animals can afford to be small and to have large surface/volume ratios. The intensity of heat radiation is less than in polar regions because of the higher temperature of the air, and small size enables them to get rid of their heat. Also, there is ample food, and daylight to eat it in, to make up for the heat lost. Homothermous animals as small as humming- birds could not live in really cold climates.


It is worth noticing that fat, which is a poor conductor of heat, forms a layer underlying the skin in the animals inhabiting polar regions (seal, penguin), and so assists in minimising the amount of heat lost by radiation. When, on the other hand, fat is stored by homothermous animals living in hot climates, it is not distributed under the skin all over the body, where it would interfere with heat- radiation, but it is localised and forms humps as in the camel or the zebu.


Whereas homothermous animals tend to be large in polar regions and small in the tropics, poikilothermous animals show precisely the opposite tendency, and for the same reasons. The reptile depends on the outer environment for its heat. In cold climates, when it is not hibernating, it is to its advantage to absorb as much as possible of what heat there is. This is assisted by a large surface/ volume ratio, and consequently a small size. Effectively, it is found that the fish, amphibia, and reptiles inhabiting cold climates are smaller than their relatives living under warmer conditions. For in tropical climates, these animals can afford to be large. The giant frogs, turtles, lizards, snakes, and crocodiles of the tropics illustrate this point well. The huge size of the reptiles in the Jurassic period must have been made possible by the hot conditions which prevailed then.


It is further to be noticed that in tropical regions, the poikilothermous animals can compete successfully with the homothermous ; whereas in polar regions, the homothermous animals dominate over the poikilothermous by reason of their constant internal temperature. It follows that if a region of high temperature, populated by poikilothermous and homothermous animals, were to undergo a reduction of temperature (as by greater elevation of the land above sea-level or the approach of an ice-age), the homothermous animals would survive, whereas the poikilothermous forms would be very likely to go extinct, especially if they were of large size. This may be what happened at the cold end of the warm secondary era (Trias to Cretaceous inclusive, the " age of reptiles "), when the reptiles all but went extinct, and were only survived by the present-day forms, which furnish a miserable sample of former richness of the reptilian fauna. At the same time, the homothermous birds and mammals survived, the latter to become the dominant animals.


It is seen, therefore, that certain of the greatest episodes in the history of the vertebrates, such as the evolution of the amphibia, may have been largely conditioned by climatic changes in the earth's crust. Other episodes were probably related to adaptations to more fixed climatic conditions. Of these, two only will be mentioned here. The first concerns the evolution of the early fish. The original ancestors of the chordates must have been marine forms, but there are certain considerations which suggest that the evolution of the early chordates took place in fresh or estuarine water. The typical chordate method of locomotion by undulations of the body from side to side may be regarded as an adaptation to life in rivers in which there is a more or less constant flow of water in a certain direction.


The other episode concerns the evolution of man, part of whose ancestral history is related to the habit of living in trees. It is common for arboreal animals to retain unspecialised limbs, and to acquire the capacity of opposing one or more digits to the others, and so be able to grasp branches firmly. At the same time the sense of smell becomes less important, while that of sight becomes dominant, leading to binocular and stereoscopic vision, and the capacity to estimate distance. This is of importance to an arboreal animal in estimating the strength of its leaps from branch to branch. The neurological changes which accompany these anatomical ones are the subordination of the olfactory cortex of the cerebral hemisphere (hippocampus) and the elevation of the non-olfactory cortex or neopallium to a dominant position. In other words, arboreal life favoured the development and evolution of the brain, which is the organ which most distinguishes the Primates, and especially man, from the remainder.


In previous paragraphs it was shown how the minimum limit of size of homothermous animals was determined, and it was found to be affected by the climatic temperature. The minimum size of poikilothermous Tetrapods has no relation to temperature, but is determined by the capacity of the muscles to actuate the skeleton and move the animal about.


The maximum size of land-vertebrates is limited by the ratio between the weight of the body and the supporting strength of the legs. The weight varies with the volume which is proportional to the cube of the linear dimensions of the animal. But the strength of the legs is measured by the cross-sectional area, which is proportional to the square only of the linear dimensions. The larger the animal is, therefore, the relatively heavier will the load be which the legs have to carry. If the length of a rabbit is 10 times more than that of a mouse, the weight which the rabbit's legs carry is iooo times greater than that which the legs of the mouse support. Against this, the cross-sectional area of the rabbit's leg is ioo times that of the leg of the mouse. The result is that the weight per square millimetre on the legs of the rabbit is 10 times more than that on the legs of the mouse. As the strength of the skeletal material (bone) cannot be increased, a stage is reached at which the legs can no longer safely carry the weight of the body, or they must be so large as to be almost immovable. Already in the elephants they are like pillars, and these animals are near the maximum size for land- vertebrates. For aquatic forms, the conditions are of course different, since by Archimedes' principle the buoyancy of the water reduces the relative weight of the animal, which is usually not borne on the limbs at all. So the whales and sharks can reach sizes which are impossible for land forms. For this reason, it is likely that the largest of the Dinosaurs were more or less aquatic.



Literature

Brooks, C. E. P. Climate through the Ages. Benn, London, 1926.

d'Arcy Thompson. Growth and Form. Cambridge University Press, 1917.

Haldane, J. B. S. Possible Worlds. Chatto and Windus, London, 1927.

Hesse, R. Tiergeographie auf okologischer Grundlage. Fischer, Jena, 1924.

Przibram, H. Form und Formel im Tierreiche. Deuticke, Leipzig and Wien, 1922.



Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
Vertebrate Zoology 1928: PART I 1. The Vertebrate Type as contrasted with the Invertebrate | 2. Amphioxus, a primitive Chordate | 3. Petromyzon, a Chordate with a skull, heart, and kidney | 4. Scyllium, a Chordate with jaws, stomach, and fins | 5. Gadus, a Chordate with bone | 6. Ceratodus, a Chordate with a lung | 7. Triton, a Chordate with 5-toed limbs | 8. Lacerta, a Chordate living entirely on land | 9. Columba, a Chordate with wings | 10. Lepus, a warm-blooded, viviparous Chordate PART II 11. The development of Amphioxus | 12. The development of Rana (the Frog) | 13. The development of Gallus (the Chick) | 14. The development of Lepus (the Rabbit) PART III 15. The Blastopore | 16. The Embryonic Membranes | 17. The Skin and its derivatives | 18. The Teeth | 19. The Coelom and Mesoderm | 20. The Skull | 21. The Vertebral Column, Ribs, and Sternum | 22. Fins and Limbs | 23. The Tail | 24. The Vascular System | 25. The Respiratory system | 26. The Alimentary system | 27. The Excretory and Reproductive systems | 28. The Head and Neck | 29. The functional divisions of the Nervous system | 30. The Brain and comparative Behaviour | 31. The Autonomic Nervous system | 32. The Sense-organs | 33. The Ductless glands | 34. Regulatory mechanisms | 35. Blood-relationships among the Chordates PART IV 36. The bearing of Physical and Climatic factors on Chordates | 37. The origin of Chordates, and their radiation as aquatic animals | 38. The evolution of the Amphibia : the first land-Chordates | 39. The evolution of the Reptiles | 40. The evolution of the Birds | 41. The evolution of the Mammalia | 42. The evolution of the Primates and Man | 43. Conclusions | Figures | Historic Embryology



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