144 THE STORY OF THE LIVING MACHINE. 



We speak of a monkey's arm and a bird's wing as 

 homologous, although they are wonderfully dif- 

 ferent in appearance and adapted to different 

 duties. They are called homologous because they 

 have similar parts in similar rela- 

 tions. This can be seen in Figs. 47 



FIG. 47. FIG. 48. FIG. 49. 



FIG. 47. The arm of a monkey, a prehensile appendage. 

 FIG. 48. The arm of a bird, a flying appendage. In life covered 



with feathers. 

 FIG. 49. The arm of an ancient half-bird half -reptile animal. In 



life covered with feathers and serving as a wing. 



and 48, where it will be seen that each has the 

 same bones, although in the bird's wing some 

 of the bones have been fused together and oth- 

 ers lost. Their similarity points to a relation- 

 ship, but their dissimilarity tells us that the re- 

 lationship is a distant one, and that their com- 

 mon point of origin must have been quite far 

 back in history. Now if we follow back the his- 

 tory of these two kinds of appendages, as shown 

 to us by fossils, we find them approaching a com- 

 mon point. The arm can readily be traced to a 



