198 OYSTERS, AND ALL ABOUT THEM. 



character. It is probable that naturalists would have 

 avoided much confusion of ideas if they had more fre- 

 quently borne the necessary limitations of our knowledge 

 in mind. But while it may safely be admitted that we are 

 acquainted with only the morphological characters of the 

 vast majority of species the functional, or physiological, 

 peculiarities of a few have been carefully investigated, and. 

 the result of that study forms a large and most interesting 

 portion of the physiology of reproduction. (/") 



Now it is an interesting fact, that with the substance 

 of our globe shells have much to do. " Of all classes," 

 says Lyell, "the testacea are the most generally diffused 

 in a fossil state, and may be called the medals principally 

 employed by nature in recording the chronology of past 

 events." Lying upon and filling up depressions or basins 

 of the chalk, is our great tertiary formation, including many 

 tertiary shells, all apparently extinct. The most common 

 shell is a massive gigantic oyster, sometimes over a foot in 

 diameter. The beds comprising this formation are covered 

 by others of a peculiar soft, white .stone, including much 

 gypsum, and resembling chalk, but really of the nature of 

 pumice stone. This substance is very remarkable, from its 

 being formed, to at least one-tenth of its bulk, of animal- 

 cules ; and Professor Ehrenberg has recognised in it no 

 fewer than thirty marine creatures. 



Darwin describes the geology of Patagonia as worthy 

 of attention. It differs from Europe, where the tertiary 

 formation appears to have accumulated in the bays ; for 

 here, along hundreds of miles of coast, there is one great 

 deposit, including many tertiary shells, all apparently 



(f) "The Origin of Species," "Lay Sermons,'' c., by Professor 



T. H. Huxley, pp. 258-60. 



