134 



Dr Barry on the Unity of Structure 



internal or vegetative, and an external or animal portion of the 

 body. 



The following diagram is intended to illustrate fundamental 

 unity, and the causes of subsequent variety in structure ; the 

 latter being acquired in development, and development being 

 represented by curves. 



Let the point A, re- Fig. 11. 



present the supposed 

 coincidence in funda- 

 mental form, of four 

 germs of the Class 

 Vertebrata. 



The curves drawn 

 from the point A, to 

 the points B, C, D, E, 

 represent, respectively, 



the development of / / '>/ 



Fishes, Reptiles, 



Birds, and Mam- C / 



mals. 



The identity of the curves, in curvature, corresponds to iden- 

 tity in the manner of development (i. e. in the manner of his- 

 tological and morphological separation). 



The lengths of the curves, together with the degrees of un- 

 dulation, measure the degree of aggregate elaboration attained 

 by each of the above, in the course of its development.* 



The dotted continuations of the curves, measure the diffe- 

 rences in the degree of elaboration. 



The different directions of the curves, both general and par- 

 ticular, represent the different directions taken in development ; 

 i. e. modifications of the type of the Vertebrata ; and serve to 

 shew, that while there exist resemblances, identities are impos- 

 sible. 



The amount of divergence, measures the difference in struc- 

 ture, arising from the different directions taken in development, 

 (difference in kind). 



This illustration is, however, a very coarse one. We cannot, for in- 

 stance, represent the fact, that certain parts of structure in birds, are more 

 wrought out, than corresponding parts in mammals. The term aggregate ela- 

 boration, has therefore been employed. 



