348 THE BIOLOGY OF BIRDS 



posterior part of the skull, and a delicate rod of bone (the 

 columella) passing from the drum or tympanum to the inner 

 ear. All of these are characters of some importance, indica- 

 tive of genuine affinity. And the point becomes clearer 

 when it is noted that not one of these features, common to 

 birds and reptiles, is to be seen in' mammals. For in 

 mammals there are two occipital condyles, the mandible is 

 one bone on each side, it articulates with the squamosal, the 

 quadrate has probably become one of the three ear-ossicles 

 (the incus), and the place of the columella is probably taken 

 by another ear-ossicle — the stapes. 



In some other regions of the body, both in hard and soft 

 parts, affinities with reptiles can be detected, but it may be 

 enough to give two other instances. When a reptile moves 

 its ankle-joint, it is working the upper or proximal row of 

 tarsal bones against the lower or distal row. This is called 

 an inter-tarsal type of ankle-joint, in contrast to what is 

 found in mammals {cniro-tarsal), where the lower end of the 

 tibia works against the astragalus, and the lower end of the 

 fibula, it may be, against the os calcis. (The astragalus and 

 OS calcis form the two tarsal or ankle bones of the upper or 

 proximal row.) Now in birds there are no ankle-bones left 

 in the fully-formed skeleton, and yet the ankle-joint is ijiter- 

 tarsal as in reptiles. For the proximal tarsals fuse to the 

 lower end of the tibia and the distal tarsals fuse to the fused 

 metatarsals, the plane of ankle-movement remaining between 

 the two rows of tarsals, though the bones as such have dis- 

 appeared. This is the kind of exception that proves the 

 rule. 



As regards soft parts, it should be recalled that the 

 crocodilians have a four-chambered heart as birds have, 

 though they do not make so much of it ; that the blood is 

 much the same in bird and reptile ; that air-sacs, so character- 

 istic of birds, are seen in the chamaeleon among lizards ; and 

 that the urogenital system of birds is plainly derivable from 

 the reptilian type. 



