THE EVOLUTION OF THE REPTILES 443 



On the other hand, the other reptiles such as the Chelonia, 

 together with the Squamata, and all the Diapsida have a hook- 

 shaped 5th metatarsal. The birds were derived from a Diapsid 

 stock, and so it may be said, therefore, that the line of avine 

 descent is characterised by the possession of a hook-shaped 

 5th metatarsal. Actually what this modification means or 

 what function it serves is unknown, but it is to be noticed that 

 among the animals possessing it are forms which live on land, 

 in the water, and in the air, so that it would seem not to have 

 an adaptive significance, nor to be capable of modification by 

 different modes of life. It looks, therefore, as if it could be 

 used as a diagnostic feature inherited from a common ancestor 

 by all the forms possessing it. This common ancestor was 

 probably a late Anapsidan, and the importance of this matter 

 is that from this point onwards the reptiles were divided into 

 two main and divergent branches. One branch which may be 

 called the Sauropsidan includes the Chelonia, the Parapsida, 

 the Diapsida, and the birds. The other or Theropsidan branch 

 includes the Synapsida perhaps (the Synaptosauria), and the 

 mammals. As a result of these considerations, it appears that 

 the term " Reptilia " is applied not so much to a unified group 

 of related animals as to two divergent stocks. It therefore 

 refers to a grade of structure and degree of evolution ; and when 

 the knowledge of fossil forms is more complete, it will be 

 possible to abolish the class " Reptilia," or to restrict it to the 

 primitive Anapsida, and to substitute the classes " Sauropsida " 

 and " Theropsida," containing the birds and mammals 

 respectively. 



That these conclusions are sound is shown by a considera- 

 tion of the aortic arches. It is to be noticed that all the living 

 reptiles belong to the Sauropsidan branch, and in all of them 

 the systemic aorta is split into two right down to the ventricle 

 of the heart. The result is that there are right and left systemic 

 arches springing respectively from the left and right sides of 

 the ventricle. The condition of the bird fits into this scheme, 

 for it differs from the arrangement in the crocodile only by the 

 loss of the left arch. Now, in the mammal, the systemic aorta 

 is single and undivided. The point is that it is impossible to 



