Archeeopteryx macrura. 451 
or folding the flying-membrane are necessary. We thus see 
on the one hand a corresponding modification of the shoulder- 
girdle, which is the supporting point around which the muscles 
are disposed, and on the other hand a modification of the digits. 
In the Pterosaurs, which were, no doubt, weak fliers, there 
was only a single digit employed. In the Bats four digits 
are made use of, and only one, the thumb, keeps its original 
nature. 
The series of halting-places in the progressive adaptation 
for flight by means of feathers is less complete. 
Everybody knows the bony framework of the Bird’s wing 
(fig. 4). The sternum is very large, and commonly furnished 
with a keel; the coracoid is very strong; the clavicles are 
united together to form the furcula; the humerus 1s often, 
and the fore-arm always, elongated ; the carpus is formed by 
two insignificant bones only ; the only two metacarpals are 
united together; and only one long stiff median digit is 
developed, while the two other digits are rudimentary or 
may even be wholly wanting. The elongation, with the view 
of gaining space for the insertion of feathers, the reduced 
number of digits, the greater fixity of the wing-joint, and 
the establishment of considerable surfaces for the insertion of 
muscles characterize this modification, which reaches its 
utmost in the Birds which are good fliers. 
The halting-place of Archeopteryx may be in some measure 
compared with that of Galeopithecus in the former series. 
It is, however, a step further forward in the march of adap- 
tation. The number of digits and the single carpal depart 
from the normal structure of Reptiles. The digits, without 
doubt, are altogether of the most decidedly Reptilian confor- 
mation ; but they are reduced to the normal number possessed 
by Birds, and the middle digit is the longest of the three. 
Modification thus begins to be perceptible in the skeleton, 
and is not confined to the skin only, as in the Flying-squirrels ; 
but this beginning is so weak and so insignificant that one 
might have doubts about it had the feathers not been pre- 
served. Archeopteryx, without doubt, enjoyed the faculty 
of active flight; but, judging from its weak shoulder-girdle, 
2H2 
