Archeeopteryx macrura. 44.7 
two ways, entirely opposite. In the one case they are 
shortened, so as to become useless or serviceable only as 
organs of prehension ; in the other they are lengthened, so 
as to become organs of flight. 
In Vertebrates we see the tendency to an upright position 
developed in Dinosaurs and Birds among the Sauropsids, and 
among Mammals in Kangaroos, Jerboas, and the Anthropo- 
morphs, including Man. It is to be remarked that this ten- 
dency is, without doubt, very ancient: Dinosaurs appeared 
so early as the Triassic beds, and Kangaroos, since they are 
Marsupials, may make us believe that some of the Jurassic 
Marsupials—resembling the Kangaroo-rats (Hypsiprymnus) in 
their dentition—may have already shown the peculiar struc- 
ture of animals that leap. Anyhow, the tendency is shown by 
the greater fixity and solidity of the pelvis, which is made up 
of a greater number of sacral vertebre, by the increased 
length and thickness of the leg-bones, and lastly, except in 
the Anthropomorphs, by the progressive diminution in the 
number of the digits, which, on the other hand, become 
thicker and longer. The Anthropomorphs alone form an 
exception from this point of view ; and the diminution in the 
number of digits being a general law for derivative types, we 
may say that they are, as regards these members, conservative 
beyond all others. As for the rest, this diminution, which 
carries with it that of the tarsal and metatarsal bones, is con- 
stantly shown. 
The adaptation to flight 1s wholly independent of an up- 
right position. The Pterosaurs and Bats, beyond question 
excellent fliers, prove this proposition in a decisive way. 
Both have the hind-feet very weak, short, furnished with 
slender digits, which are quite separate and armed with claws. 
We have only to observe the difficulty with which a Bat 
walks to convince ourselves directly that it could never hold 
itself upright on its hind-limbs ; and comparing the skeleton 
of a Pterodactyl or of a Rhamphorhynchus with that of a Bat, 
we shall be directly persuaded that the Pterosaurs were well 
able to cling to any thing by their hind feet, but never to hold 
themselves upright. The structure of the hind-lmbs, as seen 
