INTRODUCTION loi 



1). Hitlaerto all these ancient animals, whether having four feet or two, 

 moved on the ground or, at most, and this especially in the case of the 

 smaller forms, climbed trees. Among those that possessed this habit, the 

 befeathering (which as yet had, like the hair of Mammals, served only 

 foi warmth) presumably entered upon a higher step, the feathers becom- 

 ing larger on certain parts of the body, particularly on the fore limbs 

 and tail, so as to begin to act as a parachute, and allow of a safe gliding 

 descent from a height. By successive increase in stiffness and size of the 

 feathers, and corresponding modification and strengthening of the skeleton 

 and muscles, the possibility of incipient but real flight was afforded to 

 these Birds, the Proto-Ptenornithes — the first flying Birds {Urflugvogel), of 

 which, in all likelihood, there were many varied forms, though Archee- 

 oiAeryz (page 278) is the single type known to us. The faculty of flight, 

 thus acquired, went on improving. The remiges grew stronger and 

 stronger, and, in correlation therewith, the distal wing-bones (the meta- 

 carpals coalescing) gained greater rigidity, and the muscles connected 

 with them, as well as the processes giving origin and insertion thereto, 

 increased in size. In proportion as the fore limbs specialized into highly- 

 developed wings, and the pectoral arch approached the Carinate type, the 

 original faculty of the former as grasping organs was lost. Simultaneously 

 as the remiges acquired strength, the tail shortened and was consolidated, 

 the posterior vertebrae becoming united as a pygostyle (page 753). Thus 

 originated those forms which may be denominated Deutero-Ptenornithes or 

 Euptenornithes — the higher or better Birds of Flight (hohere Flugvogel). 

 This type was already established in the Cretaceous Ichthyornis (page 652), 

 and includes the vast majority of existing Birds commonly grouped as 

 Garinatae, ; but these only in later times developed their various higher modi- 

 fications, which were rendered possible by the saving of material and weight, 

 —more elaborate vertebrae ; the loss of teeth ; the gain in pneumacity 

 of the body — especially in larger forms ; the suitable configuration of 

 parts of the skeleton, and the greater importance of smooth muscle com- 

 pensating for the diminished performance of striped muscle (page 602). 



During the period in which the Protoptenornithes and Deuterojoten- 

 ornithes were difterentiated, there came about, as almost everywhere in 

 Nature^ retrograde movement. All Birds did not reach the highest degree 

 of faculty of flight. Many stopped, as it were, half way, when a retro- 

 gression of the power already attained took place ; or, if the power were 

 reached, it could not be maintained — an easy life and absence of rivalry 

 inducing an increased bulk of the body, until the utmost exertion of 

 muscular strength could no longer sustain it in the air. Thus when 

 this retrograde development began, occasion was afl'orded for the dwind- 

 ling away of the volant power, and hence arose the different types which 

 are commonly grouped as Ratitse, and may be called Deuter-Aptenornithes, or 

 secondary Flightless Birds {secunddr Jluglos Vogel). Again, says the author, 

 if the retrogression extended only to a limited degree, as in recent cases like 

 the Impennes, Alca impennis, certain Eallidse, the Dididee, Stringops and 

 others, in whose structure this or that Carinate character is very apparent, 

 these form the Trit-Aptenornithes or Flightless Carinates (Jluglose Carinaten). 

 But in Nature no sharp boundary exists between the Deuter- and Trit- 

 Aptenornithes ; Cnemiornis and still more likely Gastornis and Aptornis 



