Book - Vertebrate Zoology (1928) 40

From Embryology

Vertebrate Zoology G. R. De Beer (1928)

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Chapter XL The Evolution Of The Birds

The birds present so many similarities to the reptiles that they have been classified together with them in the group Sauropsida. The resemblances extend to the following features. The heart and arteries of the bird are the same as those of the crocodile with the exception of the left systemic arch, which in birds is abolished. The perivisceral coelomic cavity of birds is divided up into pulmo-hepatic recesses and pleural cavities, by means of the pulmo-hepatic ligaments and oblique septa ; this arrangement is also present in the crocodile. The lung of birds gives rise to a number of diverticula or air-sacs which ramify about inside the body ; small air-sacs are formed by the lung of the chamasleon. With regard to the nervous system, the brain of birds is an elaboration of the grade of structure shown by the brain of crocodiles, and its distinctive feature is that the corpus striatum has been especially developed while the cerebral cortex remains small and thin. The cerebellum of birds presents many resemblances to that of the Pterosaurs, which can be explained as due to the action of similar modes of life working on related materials. The early stages of development, amnion arid allantois, are very similar. Coming to the skeleton, the single occipital condyle, the inter- orbital septum, the limb girdles, the hollow nature of several of the bones and the mesotarsal articulation of the feet, are all characters which appear in some or most of the reptiles of the Sauropsidan branch. In addition, the Jurassic fossil Archaeopteryx had gastralia, a prelachrymal and remnants of two temporal vacuities, a long tail with several separate caudal vertebrae, and a lower jaw perforated by a foramen as in crocodiles. It is possible that Archaeopteryx had cartilaginous uncinate processes on its ribs.

Nothing can be regarded as more certain than that the birds were evolved from reptiles of the Sauropsidan branch, and the only point left to consider in this connexion, is which. Birds share with : — Crocodiles : the structure of the heart and arteries, the arrangement of the ccelom, the large corpus striatum, the foramen in the mandible (of Archaeopteryx) ; and according to the precipitation blood-tests a high degree of blood-relationship ; Dinosaurs : the prelachrymal fossa, the mesotarsal articulation, the hollow bones, and the bipedal mode of progression ; Pterosaurs : the structure of the cerebellum ; Rhynchocephalia : the uncinate processes of the ribs ; with all the above-mentioned reptiles, the two temporal vacuities. Now, the ancestors of all these reptiles were probably closely allied to the Pseudosuchia, of which Euparkeria is an example from the Triassic ; and it is very probable that these forms were the ancestors of the birds also.

The characteristic of birds is their peculiar method of flight, and most if not all of their modifications are adaptations to life in the air, foremost among which are the feathers.

Feathers are one of the most marvellous cases of adaptation known in the animal kingdom. Firmly anchored by the rachis or quill to the skeleton of the forelimb, the vane of the feather gives perfect air-resistance at the downstroke coupled with lack of resistance in the upstroke. The feather is proverbially light, and the structure of the hooks or hamuli and barbules enables the feather to be repaired if damaged, by the bird itself, by simply drawing the vane through the beak. The result of this process is to rehook the hamuli on to the barbules should the barbs have become torn apart.

Other feathers are smaller and serve not for flight but as a non-conducting layer for heat, and enable the bird to maintain a constant and high internal temperature. Birds are homothermous. This condition with its consequent increased efficiency of muscular and nervous processes is necessitated by the great exertion required to maintain the weight of the body in the air during flight.

This was accomplished with the aid of the feathers by the perfection of the vascular system (abolition of the left systemic arch) and the development of the air-sacs. The latter ensure a complete " blow- through " of the lungs and avoid the inefficiency due to the presence of stagnant residual air in blind-ended sac-shaped lungs. Further, the air-sacs assist in regulation of the internal temperature by varying the amount of heat lost.

The characteristic development of the keel (carina) on the sternum in flying birds is a direct adaptation to the need for a firm origin for the large muscles concerned with flight.

As to the evolution of the method of flight itself, there is not yet any certainty, and several theories have been propounded.

The avine method of flight differs from that of all other forms in that no use is made of stretched membranes. From a study of the feathers and scales on the leg of the ostrich, there is reason to believe that feathers were evolved in connexion with scales, but not from them. Be that as it may, the development of feathers must have resulted in an increase in the surface of the animal without any material increase in its weight. It is easy to see how this would have benefited the ancestral birds if they inhabited trees and were in the habit of leaping or parachuting from one branch to another. At the same time it must be remembered that the structure of the pelvic girdle and limbs of the bird are adapted to a cursorial mode of life with a bipedal method of walking or running. The biological success of birds is probably due in no small measure to the fact that the adaptations for flight in no way interfere with or involve the hind pair of limbs. The ancestors of the birds must therefore have been terrestrial and cursorial in habits before becoming arboreal.

The early birds like Archaeopteryx (Jurassic) had a long tail, jaws garnished with teeth, wings with three well-formed ringers ending in claws, and a more or less flat sternum. At the present day, birds have no teeth, and their tails are shortened up, the fused vertebrae forming the pygostyle.

Fig. 178. — A few examples of different types of birds (not drawn to scale). a, Archaeopteryx ; b, bird of prey ; c, swift ; (both b and c are examples of the Neognathas snowing highly perfected aerial adaptation) ; d, ostrich (Palaeognathae, flightless) ; e, dodo (one of the Neognathae which lost the power of flight) ; /, penguin (aquatic adaptation), (a after Pycraft.)

Living birds are divided into two groups : Palaeognathae, and Neognathae, according to the structure of the palate. In the former (which include what used to be called the " flightless birds " or Ratites, plus the Tinamus), the prevomer is large and touches the pterygoids ; whereas in the latter (the so-called Carinates minus the Tinamus) the prevomer is small and does not touch the pterygoids. In Tinamus and Neognathae the sternum bears a large keel or carina, on to which the pectoral muscles (which are used in flight) are attached, and the well-developed clavicles fuse together at their base to form the " merrythought " or furcula. The carpals, metacarpals, and phalanges are much reduced and fused together, but in the young Hoatzin (South American) among others, the hand is well developed and represented by three digits ending in claws. With the help of these claws the young Hoatzin clambers about on trees in a manner suggestive of the presumed habits of its ancestors. The barbs of the flight-feathers are connected to one another by the hooks on the barbules, and most of them are active flyers. All make nests of one kind or another and incubate the eggs, except the cuckoo which is parasitic in that it lays its eggs in the nests of other birds for them to incubate, and the Megapodes which lay their eggs in decaying vegetable matter, relying on the heat engendered by the latter to incubate them.

A point of no small importance in these birds is the high development of courtship-activities and structures, which play a large part in knitting parents together into a family. Normally it is the male which is the active partner in courtship and possesses well-developed " secondary sexual " or courtship characters. In some, such as the Great Crested Grebe, males and females are similar in appearance and in the ardour of their behaviour. In others, of which the Phalarope is an example, the female is the more brilliant and active, and the male incubates the eggs, and in fact does almost everything except lay them.

As regards their anatomy, these birds are very much alike ; so much so that it is a matter of extreme difficulty to arrive at a satisfactory scheme of classification for them. This is partly because the birds are a comparatively recent group, and because they have been so successful in the walk of life to which they have become adapted that little extinction has taken place. Most of them are strictly aerial animals. Others such as the common fowl and the extinct Dodo of Mauritius have secondarily become terrestrial as a result of reduction of the powers of flight. Many have become adapted to swimming on water, and have developed webs of skin between the toes of the feet, which are then described as webbed feet. This has taken place independently in a great many groups, such as the Petrels, Penguins, Divers, Cormorants, Flamingos, Ducks, and Gulls.

In the Penguins, in addition, the wings have been modified into paddles or flippers and the birds can no longer fly. It is worth noticing that these aquatic birds, with the exception of the penguins, have their powers of flight in no way impaired by the adaptation of webbed feet ; this is a consequence of the avine structure of the wing which leaves the hind limbs unencumbered. A peculiar modification which may occur is the absence of the 5th flight-feather carried on the ulna (5th secondary remex). This condition is known as aquintocubitalism or diastataxy, and it occurs sporadically in some groups and not in others, or even in some members and not in others of the same group. The significance of this modification which is so peculiar is a mystery.

The Palaeognathae include the Struthiones (Ratites) or " flightless birds " and the Tinamus. In the former the flight- feathers have lost the hamuli on the barbules, so that the vanes are no longer resistant to the air. This character is associated with the loss of the power of flight on the part of these birds, and accounts for the well-known structure of the ostrich plume. Also connected with their flightless condition, the Palaeognathae (again with the exception of the Tinamus) have lost the keel on the sternum. These characters are specialisations and losses from a more primitive type of flying bird. At the same time the palate of the Palaeognathae is more primitive than that of Neognathae, and some preserve primitive characters such as the claws at the ends of the fingers in the wing in the ostrich. In addition to the ostrich (Struthio, South African and Arabian), the Struthiones include the Rhea (South American), the Emu and Cassowary (Australasian), and the Kiwi (Apteryx, New Zealand). The extinct Moa of New Zealand had reduced its wings altogether.

It is interesting to note that some Neognathae also lost the power of flight, such as the extinct Dodo of Mauritius, and the Solitaire of Rodriguez. Flightless birds, whether Palaeognathae or Neognathae, are more or less restricted to the southern hemisphere or to islands, where there is little competition to fear from other animals. Their degeneracy was therefore, so to speak, allowed because of the greater leniency of natural selection in these regions.

Heilmann, G. The Origin of Birds. Witherby, London, 1926. Pycraft, W. P. A History of Birds. Methuen, London, 1910.

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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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