846 Dr Barry on the Unity of Structure in 



ner of the change, is probably the same throughout the animal kingdom, how- 

 ever much, 3dly, The direction (or type) and degree of development may differ, 

 and thus produce variety in structure ; which, however, there is good reason 

 to believe, is, Itftly, In essential character, fundamentally the same. Yet, 5thly, 

 That no two individuals can have precisely the same innate susceptibilities of 

 structure, or plastic properties ; and therefore, Gthly, That though all the 

 individuals of a species, may take, in their development, the same general di- 

 rection, there is a particular direction in development, and, therefore, a 

 particular structure, proper to each individual. Tthly, That structures com- 

 mon to a whole class must, in a modified form, re-appear in individual develop- 

 ment ; and, lastly, That they can re-appear in a certain order only ; viz. in the 

 order of their generality in the animal kingdom. 



These conclusions, especially the two last, with the reasoning 

 from which they are derived, sufficiently explain why, in the 

 embryonal life of the more elaborate animals, there occur tem- 

 porary resemblances in certain parts of structure, to the perma- 

 nent states of corresponding parts, in animals less wrought out. 



A Diagram will serve to illustrate some of these conclusions. 



The Tree of Animal Development ; 



Shewing fundamental Unity in Structure, and the causes of variety ; the 

 latter consisting in Direction and Degree of development. 

 Fig. 12. 



