44 



CHAPTERS ON EVOLUTION. 



for every known fish, just as the diagram of the jointed animal, 

 constructed from the details of the lobster, served to represent the 

 essential anatomy of every known crustacean. 



If, now, we examine the structure of a frog or other amphibian, 

 or that of any reptile, we shall find that, like the fish, these animals 

 have a nervous system lying along the back, and enclosed within a 

 bony tube. Again, their hearts are lowest, their digestive system 



a 



B 



FIG. 7. DIAGRAM OF ECHINODERM, CCELENTERATE, AND PROTOZOON. 



occupies the median position, and their limbs are never more than 

 four, and are invariably developed in pairs. Hence the diagram of 

 the fish represents the essential anatomy of other two distinct classes, 

 namely, frogs and reptiles. But it is easy to show that the fish type 

 is also represented in animals still more widely removed from all 



apparent relationship with the finny tribes. Between a bird and a 

 fish there seems at first to be no relationship, absolute or comparative ; 

 yet the diagram of the fish will serve to express all the structural 

 features of the bird. The latter exhibits, in short, the same arrange- 

 ment of its nervous, digestive, and blood systems as does the fish ; 

 its nervous system is similarly protected by a bony axis ; and its limbs 

 are likewise in pairs. And ascending, last of all, to the highest 



