III.] INDEPENDENT SIMILARITIES OF STRUCTURE. 109 



a law of descent ; that is, by considering such similar forms 

 as the descendants of atoms which inhabited one special 

 part of the primitive nebular cosmos, each considerable 

 space of which may be supposed to have been under the 

 influence of somewhat different conditions. 



Surely, however, there can be no real parity between 

 the relationship of existing minerals to nebular atoms, and 

 the relationship of existing animals and plants to the ear- 

 liest organisms. In the first place, the latter have pro- 

 duced others by generative multiplication, which mineral 

 atoms never did. In the second, existing animals and 

 plants spring from the living tissues of preceding animals 

 and plants, while existing minerals spring from the chemi- 

 cal affinity of separate elements. Carbonate of soda is not 

 formed, by a process of reproduction, from other carbonate 

 of soda, but directly by the suitable juxtaposition of car- 

 bon, oxygen, and sodium. 



Instead of approximating animals and minerals in the 

 mode suggested, it may be that they are to be approx- 

 imated in quite a contrary fashion ; namely, by attributing 

 to mineral species an internal innate power. For, as we 

 must attribute to each elementary atom an innate power 

 and tendency to form (under the requisite external con- 

 ditions) certain unions with other atoms, so we may at- 

 tribute to certain mineral species as crystals an innate 

 power and tendency to exhibit (the proper conditions being 

 supplied) a definite and symmetrical external form. The 

 distinction between animals and vegetables on the one 

 hand, and minerals on the other, is that, while in the or- 

 ganic world close similarity is the result sometimes of in- 

 heritance, sometimes of direct production independently 

 of parental action, in the inorganic world the latter is the 

 constant and only mode in which such similarity is pro- 

 duced. 



When we come to consider the relations of species to 



