166 THE STORY OF LIFE'S MECHANISM. 



justified in this conclusion when we find that 

 the relationships which we draw between the 

 types of life now in existence run parallel to 

 the history of these types as revealed to us by 

 fossils, and at the same time disclosed by the 

 study of embryology. 



This subject of comparative anatomy includes 

 a consideration of what is called homology, and 

 perhaps a concrete example may be instructive 

 both in illustration and as suggesting the course 

 which nature adopts in constructing her machines. 

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

 as homologous, although they are wonderfully 

 different in appearance and adapted to different 

 duties. They are called homologous because they 

 have similar parts in similar relations. This can 

 be seen in Figs. 47 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 others lost. Their similarity 

 points to a relationship, but their dissimilarity 

 tells us that the relationship is a distant 

 one, and that their common point of origin 

 must have been quite far back in history. 

 Now if we follow back the history of these 

 two kinds of appendages, as shown to us by 

 fossils, we find them approaching a common 

 point. The arm can readily be traced to a 

 walking appendage, while the bird's wing, by 

 means of some interesting connecting links, can 

 in a similar way be traced to an appendage with 

 its five fingers all free and used for walking. 

 Fig. 49 shows one of these connecting links re- 

 presenting the earliest type of bird, where the 



