INTRODUCTORY 3 



the riddles of the rocks. Let us take the anatomist's evidence first. 

 According to him, the peculiarities which distinguish the bird have 

 been derived from the reptiles, and this is nowhere more evident 

 than in the skull. As in the reptile, it joins the neck by a single, 

 rounded boss of bone ; while in the mammals (the great class to which 

 we ourselves belong, the class distinguished by the body-covering of 

 hair, and the fact that the young are suckled by milk) the skull joins 

 the neck by two such bosses. In the form of the backbone and of 

 the hip-girdle, and in the structure of the legs, birds also agree with 

 the reptiles. To state in full, the evidence on which these conclusions 

 are founded would be wearisome to those who are not particularly 



— Tibia 



1st row of ankle'boaes 



Joint 



2nd row of ankle- boaes 



Bones of the foot 



Fig. I.— Bones of the-Foot and Ankle of a Young Fowl, showing the Separate 



Elements thereof. 



interested in dry bones, but we may indicate the nature of the argument 

 by a comparison of the hind limb of the bird and reptile. This limb 

 then, in both, is peculiar in that the ankle-joint is formed in such a 

 way that when the foot is bent the joint turns on a hinge formed 

 between two rows of ankle-bones, while in the mammals the joint 

 is formed by the hinging of the shank of the leg upon the uppermost 

 row of ankle-bones. But the bird's leg, we may remark, is peculiar 

 in that these two rows of ankle-bones have undergone great modi- 

 fications, and can be seen in their originally separate condition only 

 by examining the chick some time before hatching, though traces 

 yet remain in, say, a young fowl of three months old (see fig. i). 



