2 ELEMEiNTS OF MAMMALIAN ANATOMY 



representatives. A consideration of this geographical and stra- 

 tigraphical location of organisms forms the science of Distribu- 

 tion. The science of Phylogeny seeks to discover the geological 

 ancestral history of an organism. 



A casual glance shows at once striking similarities and differ- 

 ences 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 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 resem- 

 blances and differences furnishes a basis of classification, the 

 treatment of which forms the science of Taxonomy. 



THE PHYLOGENY AND CLASSIFICATION OF 



VERTEBRATES 



Vertebrates have existed upon the earth for a very long time, 

 so long, indeed, that the time and manner of their origin is very 

 uncertain From what great group of invertebrates they were 

 derived can only be surmised from embryological and morpho- 

 logical data. The evidence from paleontology is very incom- 

 plete and unsatisfactory. Within the class of vertebrates, 

 however, the geological history is more certain. As a matter 

 of fact, the detail in which the phylogeny of such groups as 

 the horse j the camel> and the elephant can be reconstructed 

 from their fossil remains is indeed marvelous. 



Geologic time has been conveniently divided into eras, pe- 

 riods, epochs, ages, etc. The following table has been inserted 

 for reference in connection with the geological history of the 

 vertebrate types. Certain dates have been assigned to the 

 several eras more as a reminder that the mind must think in 

 terms of millions of years than that they are even approxi- 

 mated correct. 



