286 LIFE AND DEATH. 



the anatomical form characterizes the animal and the 

 plant. In both cases, form — regarded as a method 

 of distribution of the parts — indicates the individual 

 and allows us to diagnose it with more or less facility. 



Parentage of Living Beings and Mineral Parent- 

 age. — Still another analogy has been noted. In 

 animals and plants similarity in forrn indicates 

 similarity in descent, community of origin, and 

 proximity in any scheme of classification. In the 

 same way identity of crystalline form indicates 

 mineral relationship. Substances chemically analo- 

 gous show identical, geometrically superposable 

 forms, and are thus arranged in family or generic 

 groups recognizable at a glance. 



Isomorphism and the Faculty of Cross-breeding. — 

 And further, the possibility in the case of isomorphous 

 bodies, of their replacing each other in the same crystal 

 during the process of formation and of thus mingling, 

 so to speak, their congenital elements, may be 

 compared with the possibility of inter-breeding with 

 living beings of the same species. Isomorphism is 

 thus a kind of faculty of crossing. And as the 

 impossibility of crossing is the touchstone of taxo- 

 nomic relationship, testing it, and separating stocks 

 that ought to be separated, so the operation of 

 crystallization is also a means of separating from an 

 accidental mixture of mineral species the pure forms 

 which are blended therein. Crystallization is the 

 touchstone of the specific purity of minerals ; it is the 

 great process in chemical purification. 



Other Analogies. — The analogies between crystal- 

 line and living forms have been pushed still further 

 even to the verge of exaggeration. 



The internal and external symmetry of animals 



