336 SPAKKS FROM A GEOLOGIST'S HAMMER. 



culatory structures exist. If we extend the comparison 

 to the fish, we have only to drop, in addition, the structure 

 and function of aerial respiration, and all the rest presents 

 a complete correspondence between the fish, alligator and 

 ox. Thus, in short, the entire world of backboned animals 

 is shown to be united by profound structural and func- 

 tional relations. 



If we stray from our starting point so far as to bring 

 an insect or a worm into the comparison, we find still 

 an immense preponderance of resemblances. All the in- 

 vertebrates, like the vertebrates, possess the faculty of 

 voluntary motion; they hunger and feed; they perceive; 

 they have relations to each other and to the inanimate 

 world; they breathe; they digest; they reproduce; they 

 provide for, protect and defend their young; their whole 

 system of physiological activity and coordinated structure 

 is the same; they have mouth, oesophagus, stomach, intes- 

 tine, liver; they digest by secretion of gastric juice, and 

 imbibe the nutritive products of digestion; they appropri- 

 ate the oxygen of the air, and aerate a fluid answering 

 to blood; they have nerves which ramify to the various 

 organs; they move by means of contractile muscular fibers; 

 they rest from their labors at certain intervals, and sleep. 

 Certainly, if a genetic relationship unites the different 

 classes of vertebrates, it must also embrace the other ani- 

 mals which possess such a preponderance of resemblances 

 with the vertebrates. 



All classification is based on these resemblances. Classi- 

 fication, if true and correct, is therefore nothing more 

 than the building of a genealogical tree. 



There are numerous independent features of the mor- 

 phological evidence. The heart of the warm-blooded verte- 



