THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 105 



of certain doctrines that have long been dominant, and have 

 still a wide currency. 



Among the Vertebrata, as among the Mollusca, homogenesis 

 is universal. The two sub-kingdoms are like one another 

 and unlike the remaining sub-kingdoms in this, that in 

 all the types they severally include, a single fertilized ovum 

 produces only a single individual. It is true that as the 

 eggs of certain Gasteropods occasionally exhibit spontaneous 

 fission of the vitelline mass, which may or may riot result in 

 the formation of two individuals ; so among vertebrate ani- 

 mals we now and then meet with double monsters, which 

 appear to imply such a spontaneous fission imperfectly car- 

 ried out. But these anomalies serve to render conspicu- 

 ous the fact, that in both these sub-kingdoms the normal 

 process is the integration of the whole germ-mass into a 

 single organism, which at no phase of its development dis- 

 plays any tendency to separate into two or more parts. 



Equally as throughout the Mollmca there holds throughout 

 the Vertebrata, the correlative fact, that not even in its lowest 

 any more than in its highest types, is the body divisible into 

 homologous segments. The vertebrate animal, under its 

 simplest as under its most complex form, is like the mollusc- 

 ous animal in this, that you cannot cut it into transverse 

 slices, each of which contains a digestive organ, a respiratory 

 organ, a reproductive organ, &c. The organs of the least- 

 developed fish as well as those of the most-developed 

 mammal, form but a single physiological whole ; and they 

 show not the remotest trace of having ever been divisible 

 into two or more physiological wholes. That segmentation 

 which the vertebrate animal usually exhibits throughout 

 part of its organization, is the same in origin and meaning 

 as the segmentation of a Chiton's shell ; and no more implies 

 in the vertebrate animal a composite structure, than do the 

 successive pairs of branchiae of the Doto or the transverse rows 

 of branchias in the Eolis, imply composite structure in the 

 molluscous animal. To some this will seem a very question- 



