74 THE SCIENCE OF LIFE. 



forms, founded on plans of structure or structural ideas. 

 Classes comprise all forms which agree simply in a special 

 modification of the type to which they belong. The 

 type represents the plan, but there may be several ways 

 of executing the plan, and these ways illustrate the 

 classes. In human works of art " there are certain ar- 

 chitectonic types, including edifices of different materials, 

 with an infinite variety of architectural details and ex- 

 ternal ornaments ; but the flat roof and the colonnade 

 are typical of all Grecian temples, whether built of mar- 

 ble or granite or wood, whether Doric or Ionic or Corin- 

 thian, whether simple and massive or light and orna- 

 mental ; and, in like manner, the steep roof and pointed 

 arch are the typical characters of all Gothic cathedrals, 

 whatever be the material or the details. The architect- 

 ural conception remains the same in all its essential ele- 

 ments, however the more superficial features vary. Such 

 relations as these edifices bear to the architectural idea 

 that includes them all, do classes bear to the primary di- 

 visions," or types.* Thus Fishes, Amphibians, Reptiles, 

 Birds, and Mammals are classes under the Vertebrate 

 type of animal life. 



An Order is a group of families, or genera, related 

 to one another by a common structure. Thus Cats, 

 Dogs, Hyenas, and Bears are linked together, since their 

 teeth, stomachs, and claws show the carnivorous habits 

 of the order Carnivora. 



A Family, or Tribe, does not allude to the progeny of 

 a known stock, but refers to a group of genera having 

 similarity of form. The term was first introduced into 



* Agassiz, " Methods of Study." 



