PROGRESS OF ANIMAL MORPHOLOGY. 495 



and since that time the division has been commonly 

 accepted among naturalists (x). 



Sect. 3. Attempts to establish the Identity of the 

 Types of Animal Forms. 



SUPPOSING this great step in Zoology, of which we 

 have given an account, the reduction of all ani- 

 mals to four types or plans, to be quite secure, we 

 are then led to ask whether any further advance is 

 possible ; whether several of these types can be 

 referred to one common form by any wider effort of 

 generalization. On this question there has been a 

 considerable difference of opinion. Geoffrey Saint- 

 Hilaire", who had previously endeavoured to show 

 that all vertebrate animals were constructed so 

 exactly upon the same plan as to preserve the 

 strictest analogy of parts in respect to their osteo- 

 logy, thought to extend this unity of plan by demon- 

 strating, that the hard parts of crustaceans and 

 insects are still only modifications of the skeleton 

 of higher animals, and that therefore the type of 

 vertebrata must be made to include them also : 

 the segments of the articulata are held to be strictly 

 analogous to the vertebrae of the higher animals, and 

 thus the former live within their vertebral column 

 in the same manner as the latter live without it. 

 Attempts have even been made to reduce mollus- 

 cous and vertebrate animals to a community of type 

 as we shall see shortly. 



11 Mr. Jenyns, Brit. Assoc. Rep. iv. 150. 



