FOUND LINKS. 119 



merits of a good hypothesis these requirements being 

 that it explains all the facts and is contrary to none. 

 This end the theory of evolution attains in explaining 

 both the likenesses and the dissimilarities of living 

 nature. 



Returning, after this needful digression, to the case 

 of birds and reptiles, let us firstly note the structural 

 points in which these classes agree. To begin with, 

 the skull of both is joined to the spine by one bony 

 process or condyle. There are two of these processes 

 in frogs and their neighbours, and a similar number 

 in quadrupeds, including man. Then, secondly, the 

 lower jaw of a reptile agrees with that of a bird in its 

 compound nature. The jaw, instead of being simple 

 and composed of two simple halves (as in quadrupeds), 

 consists in birds and reptiles of from eight to twelve 

 distinct pieces, which are amalgamated to form one 

 bone. Furthermore, whilst the quadruped's lower 

 jaw is joined directly, and of itself, to the skull, that 

 of the bird and reptile is attached to the skull through 

 the medium of a distinct bone, which is named the 

 quadrate bone. Curiously enough, this bone in the 

 quadruped is pushed upwards into the middle of the 

 skull in the course of development, and becomes one 

 of the three small bones (malleus) of the internal ear. 

 Again, reptiles and birds agree in possessing lungs 

 alone as their breathing organs. No gills are de- 

 veloped (as in frogs and fishes) at any period of reptile 

 or bird-life, although both, like quadrupeds, possess 

 gill-clefts in the neck in early life. These "gill- 



