INTRODUCTION Tol 
1). Hitherto 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 
for 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-Ptenormithes—the first flying Birds (Urflugvégel), of 
which, in all likelihood, there were many varied forms, though Arche- 
opteryx (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 (héhere Fluqvégel). 
This type was already established in the Cretaceous Ichthyornis (page 652), 
and includes the vast majority of existing Birds commonly grouped as 
Carinatx ; 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 vertebrz ; 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 Deuteropten- 
ormithes were differentiated, 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 afforded for the dwind- 
ling away of the volant power, and hence arose the different types which 
are commonly grouped as Ratite, and may be called Deuter-Aptenornithes, or 
secondary Flightless Birds (secwndér fluglos Végel). Again, says the author, 
if the retrogression extended only to a limited degree, as in recent cases like 
the Impennes, Alca tmpennis, certain Rallidx, the Dididx, Stringops and 
others, in whose structure this or that Carinate character is very apparent, 
these form the Trit-Aptenornithes or Flightless Carinates (fluglose Carinaten). 
But in Nature no sharp boundary exists between the Deuter- and Trit- 
Aptenornithes ; Cnemiornis and still more likely Gastornis and Aptornis 
