PART SECOND. 

 THE FIVE GREAT ANIMAL GROUPS. 



CHAPTER VI. 



THE IDEAL TYPES. ALL ANIMALS BILATEKAL. 



WHAT I have said thus far did not especially involve the 

 discussion of the relative rank or grade of animals as a whole ; 

 the point to determine was whether animals ever develop as I 

 have stated. We come now to the discussion of the question as 

 to whether the higher animals have any relation to the lower 

 ones, and what those relations are. Are they relations of mere 

 similarity, simply because they all exhibit life ? Are they such 

 relations as would indicate that the higher have developed from 

 the lower, or vice versa ? Or are they such relations as would 

 seem to show that they have affinities with each other by groups ? 

 All naturalists admit that they are allied to each other by groups, 

 in some form or other. 



Lamarck arranged animals in two principal groups, namely : 

 the Vertebrata, or such as have a backbone or vertebral column ; 

 and the Invertebrata, or such as have no backbone, but have, as 

 he says, a skeleton on the outside. Cuvier considered the 

 Animal Kingdom to be divided into four groups. Vertebrata, 

 Mollusca, Articulata, Zoophyta. The last three correspond to 

 Lamarck's Invertebrata. 



Lamarck believed the classes of his groups to be genetically 

 related to each other; and he constructed a tabulation of the 

 various groups, which should exhibit in what way, or through 

 what lines or passages, these minor groups most probably have 



