446 EVOLUTIONARY MORPHOLOGY 



vertebrae, and a lower jaw perforated by a foramen as in croco- 

 diles. It is possible that Archaeopteryx had cartilaginous 

 uncinate processes on its ribs. 



Nothing can be regarded as more certain than that the birds 

 were evolved from reptiles of the Sauropsidan branch, and the 

 only point left to consider in this connexion, is which. 

 Birds share with : — 



Crocodiles : the structure of the heart and arteries, the 

 arrangement of the ccelom, the large corpus striatum, 

 the foramen in the mandible (of Archaeopteryx) ; and 

 according to the precipitation blood-tests a high degree 

 of blood-relationship ; 

 Dinosaurs : the prelachrymal fossa, the mesotarsal 

 articulation, the hollow bones, and the bipedal mode of 

 progression ; 

 Pterosaurs : the structure of the cerebellum ; 

 Rhynchocephalia : the uncinate processes of the ribs ; 

 with all the above-mentioned reptiles, the two temporal 

 vacuities. 

 Now, the ancestors of all these reptiles were probably 

 closely allied to the Pseudosuchia, of which Euparkeria is an 

 example from the Triassic ; and it is very probable that these 

 forms were the ancestors of the birds also. 



The characteristic of birds is their peculiar method of 

 flight, and most if not all of their modifications are adaptations 

 to life in the air, foremost among which are the feathers. 



Feathers are one of the most marvellous cases of adaptation 

 known in the animal kingdom. Firmly anchored by the rachis 

 or quill to the skeleton of the forelimb, the vane of the feather 

 gives perfect air-resistance at the downstroke coupled with 

 lack of resistance in the upstroke. The feather is proverbially 

 light, and the structure of the hooks or hamuli and barbules 

 enables the feather to be repaired if damaged, by the bird itself, 

 by simply drawing the vane through the beak. The result of 

 this process is to rehook the hamuli on to the barbules should 

 the barbs have become torn apart. 



Other feathers are smaller and serve not for flight but as 

 a non-conducting layer for heat, and enable the bird to maintain 



