Some Microscopic Builders. 



from its own substance, and thus constructs a flask- 

 shaped ' test ' having a short neck and a single large 

 orifice. Another picks up the finer grains and puts 

 them together with the same cement into perfectly 

 spherical * tests ' of the most extraordinary finish, 

 perforated with numerous small pores disposed at 

 pretty regular intervals. Another selects the minutest 

 sand grains and the terminal portions of sponge spicules, 

 and works these up together apparently with no cement 

 at all, but by the mere * laying ' of the spicules into 

 perfect white spheres, like homoeopathic globules, each 

 having a single fissured orifice. And another, which 

 makes a straight, many-chambered * test/ the conical 

 mouth of each chamber projecting into the cavity of 

 the next, while forming the walls of its chambers of 

 ordinary sand grains rather loosely held together, shapes 

 the conical mouths of the successive chambers by firmly 

 cementing to each other the quartz grains which border 

 it. To give these actions the vague designation * in- 

 stinctive ' does not in the least help us to account for 

 them, since what we want to discover is the mechanism 

 by which they are worked out ; and it is most difficult 

 to conceive how so artificial a selection can be made 

 by creatures so simple." 



Of these Foraminifera whose shells are perforated by 

 multitudes of minute tubules, the Globigerina, whose 

 microscopic shell consists of an assemblage of nearly 

 spherical chambers with coarsely perforated walls, is of 

 particular interest, for its ancestors were abundant in 

 the seas of a past geological epoch. To-day the Globi- 

 gerina occurs in extraordinary abundance at great depths 

 from 1,260 to 3,000 fathoms over wide areas of the 



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