120 NATCEE STUDIES. 



clefts," seen in the early life of man himself, are to be 

 viewed as feeble survivals of the aquatic ancestry 

 from which, according to evolution, all Vertebrate 

 animals have sprung. Furthermore, instead of the 

 ankle-joint (as in man and quadrupeds) being situated 

 between the end of the leg, so to speak, and the 

 beginning of the ankle-bones, this joint in reptiles 

 and birds exists in the middle of the ankle-bones 

 themselves. This curious feature will be farther 

 alluded to later on. 



The technical naturalist would enumerate other 

 points of agreement between birds and reptiles, but 

 sufficient has been said to show the close affinities 

 which lie just beneath the surface of their organisa- 

 tion. Their differences, however, are also of pro- 

 nounced type. The causes to which in the far-back 

 past the evolutionist conceives the likeness between 

 these animals to be due, have operated, through 

 variation, at a less remote period, to produce the 

 divergent lines of development. Thus we discover 

 that birds are warm-blooded, whilst reptiles possess 

 cold blood; the bird's feathers are unknown in the 

 reptile-world; and the perfect heart and circulation 

 of the bird similar to that of man are also unrepre- 

 sented in reptiles. Crocodiles, which possess a 

 four-chambered heart, like birds and quadrupeds, 

 nevertheless exhibit the same imperfect and " mixed'* 

 circulation seen equally in frogs and reptiles. Tho 

 lungs of birds are of " open " structure, and part of 

 the air inspired passes through the lungs to fill certain 



