X INTRODUCTION. 



girder principle ; to discover in the skull, sometimes most 

 beautifully cleaned up by small crustaceans, an index — 

 perhaps the most trustworthy of many — to the bird's 

 blood-relationships. 



In a recently published anatomical investigation it is 

 shown that the heart of the Ptarmigan, which is a bird 

 of the mountains, is in certain respects distinctly stronger 

 than the heart of its first cousin, the Willow-Grouse, which 

 is a bird of the lowlands ; and in a working out of this 

 interesting adaptation to the strain of living at high 

 altitudes, we have an instance of the thousand and one 

 contributions which precise anatomy has made to a deeper 

 understanding of the bird and its life. The case we have 

 just mentioned illustrates what is certainly part of the 

 pleasure of 'structural analysis,' whether it be in taking 

 a beautifully made mechanism — a watch or a motor- 

 engine — to bits, or in dissecting out the structure of a 

 bird. The pleasure is partly that of discovering how the 

 structure is suited to the activity, and this is true in 

 regard to both mechanism and organism. But to this 

 has to be added, in the case of the organism only, that 

 the study of structure throws light on blood-relationships. 

 To take a simple case : we clean up the leg of a Red- 

 throated Diver which has been drowned in a fishing-net 

 and throAvn up on the shore. Just above the knee we 

 find a remarkable spur-like projection from the top of 

 the main bone of the lower leg ; to this (cnemial crest 

 of the tibia) strong muscles are fastened, and a simple 

 experiment in the process of cleaning-up shows that the 

 structural peculiarity gi\es the leg an unusual leverage in 

 striking the water. Now in Grebes, which are related to 

 Divers, we find the same kind of lever-arrangement — the 

 same and yet different, for the knee-pan or patella, which 

 is very small in Divers, is large in Grebes. And the 



