On the Structure of Diatoms. 25 



mentation would exclude the idea of a blind process of chemical 

 crystallization. Analogy should teach that they are secreted for a 

 protective covering for the tender animal-like plant, as among 

 higher forms. If this is true, the surface markings ought to be so 

 distributed as to give additional strength to the shell without 

 greatly adding to its weight. It would also be expected that some 

 of the larger shells would be perforated with holes. This idea, 

 of course, would have to admit into the discussion considerations of 

 habits of growth, and environments. Those contained in gelatinous 

 envelopes should be less developed in strength of shell and bracing. 

 Those growing on algae, and in exposed localities, should be strong 

 to resist fracture. On those moving free the bracing would be 

 in proportion to the weakness of the shell ; larger shells being 

 relatively more liable to be broken. Here as elsewhere nature, 

 without waste of material, combines utility with beauty. — The 

 American Naturalist, May. 



