72 THE OYSTER. 



form which unites in itself the characteristics of several 

 widely separated descendants, and is thus enabled to 

 test and to verify the conclusions which he has reached 

 by comparative study. 



In this way, through the study of details too numer- 

 ous and minute to be described here, it can be shown 

 that the oyster is descended from a mollusc which 

 was furnished with locomotor organs and sense organs, 

 and which wandered about in search of food, and had 

 altogether a much wider and more varied life than that 

 of the oyster. Its gills were very simple and were 

 nothing but breathing organs, and the many uses 

 which they serve were provided for by distinct organs. 

 Very long ago, as we measure time, but quite late 

 in the history of the mollusca, as the continental areas 

 were elevated and became covered with terrestrial vege- 

 tation, and fringed by bays and sounds of brackish 

 water, it gradually became modified in such a way as 

 to fit it for life in these estuaries. Its locomotor organs 

 and its organs for discovering and capturing food 

 were gradually lost, as it learned to feed upon the 

 microscopic life of the mud-flats. The gills then 

 gradually became modified and fitted for maintaining 

 the circulation of water, and for filtering out the minute 

 food particles it contains. 



Food is most abundant on the muddy bottom, but 

 in estuaries this is so deep and soft that a locomotor 

 animal would sink and smother in it, so the oyster 

 has gradually become converted into a fixture, and 

 has learned to fasten itself when young to something 



