2 ELEMENTS OF MAMMALIAN ANATOMY. 



the old world, while the opossum is confined to the new 

 world. Again, many species of animals whose fossil re- 

 mains indicate their existence on earth several millions of 

 years ago, have at present no living representatives. A 

 consideration of this geographical and stratigraphical loca- 

 tion of organisms forms the science of Distribution. The 

 science of Phytogeny seeks to discover the geological 

 ancestral history of an organism. 



A casual glance shows at once striking similarities and 

 differences between the common cat, the lion, and the tiger. 

 All have retractile claws, the same number and kind of 

 teeth, and the same number of toes. On the other hand, 

 the resemblances between these cat-like animals and the 

 dogs are less marked, while the differences are more strik- 

 ing. The cats and dogs resemble each other more closely 

 than either does a horse. Horses, dogs, and cats have 

 numerous characters in common which are not present in 

 birds. The recognition of such resemblances and differ- 

 ences furnishes a basis of classification, the treatment of 

 which forms the science of Taxonomy. 



The following is a brief classification of the animal 

 kingdom : 



Invertebrata : Animals with no skeletal axis and without a central 

 nervous system entirely dorsal of the alimentary canal. 

 Protochordata: Small marine forms having, during part of their life 

 at least, a rudimentary skeletal axis and other fea- 

 tures marking them as a connecting-link between the 

 invertebrates and vertebrates. Tunicates or sea 

 squirts, Amphioxus. 

 Vertebrata: Forms with a skeletal axis and dorsal nerve-cord 



whose anterior end is dilated into a brain. 

 PISCES (fish). 



AMPHIBIA (frogs, toads, and salamanders). 

 REPTILIA (snakes and lizards). 

 AVES (birds). 



