. Elements of Comparative Anatomy. 416 COMPAEATIVE AN-ATOjMY. division undergoes differentiation; a number, generally not more than five, terminal joints can be distinguisbed as fingers and toes. As a part of tbe body wbicb generally projects towards the exterior is more under modifying influences than any other part, we find a large number of adaptations in them; few parts of the body present so many metamorphoses as these terminal parts of the appendages— the hand and foot. The primitive union of the fingers or of the toes into a swim- ming-plate, represented by the hand and foot, is retaine
. Elements of Comparative Anatomy. 416 COMPAEATIVE AN-ATOjMY. division undergoes differentiation; a number, generally not more than five, terminal joints can be distinguisbed as fingers and toes. As a part of tbe body wbicb generally projects towards the exterior is more under modifying influences than any other part, we find a large number of adaptations in them; few parts of the body present so many metamorphoses as these terminal parts of the appendages— the hand and foot. The primitive union of the fingers or of the toes into a swim- ming-plate, represented by the hand and foot, is retained in the natatory membrane of many Reptiles, in the hind-limbs of many Birds, and also in a number of Mammals, where it is always con- nected wath the adaptation of these appendages to the function of a swimming organ. The angulation of the limb at- tainedto,in connection with terres- A ^ i^x: B ^. Fig. 215. Diagram to show the differen- tiation and alteration in the direction of the axes of the limbs in the Vertebrata. .4 Fish. B Amphibian (the side-view, which one has been obliged to give so as to compare it with the rest, gives to this and to the next figure the appearance of tlse body being raised up). C Reptile. D Mammal, a Shoulder girdle. ^Pelvic girdle. trial locomotion, and which is also advantageous in aquatic locomo- tion, becomes gradually different in the case of the two extremities; the difference corresponds to the functions performed by the an- terior and posterior extremities when moving about on land. These relations are distinctly seen even in the Amphibia (B); but the difference in position between the upper and forearm, and thigh and leg, is not well marked. The upper arm and thigh are turned outwards to almost the same extent. There is a greater difference between them in the Reptilia ((7), and this is still more marked in the Mam- malia, where the planes in which the angles of the limbs of either side are set are parallel to the vertical median plane of
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