72 DIATOMS. 



avoiding contact with foreign substances. When we 

 have the two j^henomena of motion and sensitiveness 

 combined in any organism, we obtain results closely 

 allied to the manifestations of intelligence. The vital 

 processes of these organisms are exceedingly active. 

 Professor Bischoff 'has estimated that in order to form 

 their valves they must take into their Interior the same 

 quantity of water in proportion to their mass as would be 

 swallowed by a man drinking a cubic foot of water per 

 second. (A cubic foot of water I believe contains about 

 seven gallons). You are doubtless aware that most wa- 

 ters, fresh and salt, contain only a trace of silica and yet 

 from this minute trace the shells must be bailt. So 

 rapidly do they grow that you may frequently find 

 myriads of them where not one could be found the day 

 before, and it is never safe to leave a lot of them expect- 

 ing to collect them the next day. You may find no 

 trace of them when you visit the spot then. 



They propagate in two ways, both very simple, by 

 fission or splitting apart and by conjugation and forma- 

 tion of spores. 



When we get down toward the beginnings of life we 

 find all the functions reduced to their simplest forms. 

 For example, the simplest known animal is the amoeba. 

 This little creature is a minute mass of jelly or proto- 

 plasm and nothing else. It moves without any organs of 

 locomotion, eats without a mouth and digests without a 

 stomach. Whenever the amoeba runs across a suitable 

 article of food — let us suppose a diatom — it proceeds to 

 wrap itself around it, shutting off its supply of water, 

 and the diatom soon dies and falls apart. The amoeba 

 absorbs the contents of the shells and having no further 

 use for the bands and valves it unwraps itself and looks 

 for another meal. 



When the creature after a succession of such repasts 

 has grown large and unwieldy it simply splits in two and 



lO 



