1 8 ELEMENTS OF MAMMALIAN ANATOMY. 



operations of the nervous system, constitutes the science 

 of Psychology. 



The same species of animals are not found in all parts 

 of the world. The lion and tiger are found wild only in 

 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 

 location 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 ani- 

 mals and the dogs are less marked, while the differences 

 are more striking. 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 differences furnishes a basis of classifi- 

 cation, 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 features 

 marking them as a connecting-link between the inver- 

 tebrates and vertebrates. Tunicates or sea squirts, 

 Amphioxus. 



