THE TRIGGER-FISH 



and structure-building peculiar to itself. The pro- 

 duction of a skeleton is evidence of this process. The 

 apparatus, since it has the function of supporting, 

 must have a compactness, a power of resistance, which 

 can be dispensed with by other organs. The ordinary 

 tissues seldom have sufficient firmness, and therefore 

 a process of mineralisation is applied to its formation, 

 which consists of adding to itself mineral substances 

 which will ultimately render it more solid. These it 

 draws from its environment, taking them with the 

 materials of its general nutriment. It keeps them 

 within itself instead of rejecting them, and adds them 

 as a reinforcement to itself. 



To its separate parts, thus constituted, the skeleton 

 gives a form in keeping with the symmetry of the 

 organism. Although the substances at its disposal 

 for this purpose are not many (they are confined to 

 silica and certain salts found normally in solution in 

 the waters of the globe), the creature utilizes them and 

 attains the necessary end. Usually the amount of 

 this natural solution is very small, a litre of sea- 

 water containing only one and a half centigrams of 

 dissolved silica, and one and a half grams of lime 

 salts, while most fresh water has even less. But still 

 the process of vital assimilation collects these mineral 

 products as they are taken in with food, keeps them, 

 distributes them to the required point, and finally 

 establishes their ultimate condition. 



Even many of the unicellular beings, in spite of 

 their usually microscopical size and the simplicity 

 of their structure, have a skeleton. In it we find a 

 kind of opposition, even of exclusion, between silica 

 and lime. The mineralizing substance is either one 

 or the other, it is never both at once. It looks as 

 though, in creatures quite closely related, assimilation 

 took two different, divergent directions, having as 

 their only point of resemblance the main purpose 

 behind them; one concerning itself with the fixation 

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