Dr Alison on the Principle of Vital Affinity, 275 



deposited in them from the same nourishing fluid ; and I ap- 

 prehend, that, on this point, we must come to the same con- 

 clusion which Cuvier drew from examining, throughout the 

 animal kingdom, the structure of the different glands, the 

 vessels entering them, and the ducts passing out of them, 

 viz., that there is no difference of structure or of composition, 

 corresponding, in the slighest degree, to the great difference 

 of the products which appear. All cells in the vegetable 

 kingdom appear to consist of the same matter, cellulose, and 

 in the animal kingdom of the same matter, protein ; and in 

 the first instance they are quite similar to one another. 

 When we attend to the early stages of the existence of a liv- 

 ing body, when the difference of textures is only beginning 

 to appear, we find only that a fluid passing through similar 

 capillary vessels, and effused into similar cells, in different 

 parts of the structure, acquires different properties. And 

 when we carry our inquiries farther back, and observe the 

 first development of cells themselves out of the granular mat- 

 ter inclosed within the sac of the yolk, it appears obvious that 

 the particles of this matter are attracted, not into cells al- 

 ready existing, but to points where cells are about to be formed. 

 The facts known as to the evolution of the chick in ovo from 

 the matter that lies in contact with the germinal membrane, 

 sufliciently indicate that the powers which effect the separa- 

 tion of the different component parts of that matter, so as to 

 form the beginning of the different textures and organs, re- 

 side, not in pre-existing cells of different composition or struc- 

 ture, but simply in different points of a pre-existing mem- 

 brane, which, in the first instance, is homogeneous. The ex- 

 pression of Liebig, that " the chemical forces in living bodies 

 are subject to the invisible cause by which the forms of or- 

 gans are produced," when the action of that cause is duly 

 considered, implies, that they are subject to a cause which 

 undoubtedly acts differently at different points of the same 

 matter; but the difference of the action of which, at these 

 points, is determined by no other condition, that we can see, 

 than their position. 



This mode oQimitation of the vital affinities, by which the 

 selection and appropriation of living matter is effected, is only 



