the Animal Kingdom. 351 



viating fulfilment of the same laws, though but a rudimental 

 structure be produced. 



The absence of the sternum and the costal cartilages in most 

 Fishes and the " lower" Reptiles,* proves that these parts are 

 not necessary to the general type of the Vertebrata; further 

 evidence of which, is given in the late acquirement of them by 

 those animals in which they are present. 



In Man the encephalon is small in comparison with the spinal 

 cord and the rest of the nervous system ; the cerebellum is small 

 compared with the medulla oblongata ; and the corpora quadri- 

 gemina are enormous in proportion to all the other parts of the 

 encephalon. f Corresponding states are found through life in 

 many animals. Now, this is just what we should expect. For 

 certain parts of the cerebro- spinal axis, such as the spinal cord, 

 the medulla oblongata, and corpora quadrigemina, must, in ful- 

 filment of the law determining the order in which structures 

 common, essentially, to a whole class, reappear modified in the 

 development of an individual organism, precede, in their appear- 

 ance, those that are of a more specific character, such as the 

 large volume of the hemispheres in Man. 



Having seen why, in corresponding stages of development, 

 parts of the human organism should resemble, in their structure, 

 corresponding parts in many perfect but less elaborate animals ; 

 it is obvious, that if human development be arrested in any of 

 those stages, the resemblances become permanent. Hence mal- 

 formations of defect. But sometimes the development of certain 

 parts proceeds beyond the normal limits ; and hence malforma- 

 tions of excess.^ 



Examples occur especially in the vascular system. Thus, the human heart 

 with a single cavity, is somewhat analogous, in its simplicity, to the heart of the 

 Insecta, Crustacea, and Brachiopoda. A single auricle and a single ventricle, 

 afford some resemblance to what we find the normal state in most molluscous 

 animals and Fishes. The Batrachians have two auricles and a ventricle ; the 

 incomplete state of the intervenlricular septum, being analogous to what is 

 regular in the more elaborate Reptiles. Anomalies in the pulmonary artery 

 and the aorta, afford examples of the same kind of resemblance ; as do also 



" Meckel, 1. c. p. 400. t Ibid. p. 401. 



t J. F. Meckel was the first who satisfactorily explained congenital anomalies. 

 Handbuch der Pathologischen Anatomic. Leipzig, 1612. 



