32 THE SIMPLEST FORMS OF LIFE 



often have noticed a thin green or blueish-green deposit on 

 damp rocks. This consists of millions of minute living 

 things, which may run down to the five-thousandth of an 

 inch in diameter. Others float in our ponds, others in the 

 sea. There are hundreds of species of these tiny beings, 

 and millions of them in a small visible clump of floating 

 matter. They are the lowest forms of life that we know 

 to-day; though, of course, improved microscopes may reveal 

 still simpler forms in time. Little more than half a century 

 ago they themselves were unknown. We may take one of 

 these tiny creatures, magnify it a thousand times, and 

 observe the movements in which its life consists. 



This is, surely, the correct way to attack our problem. 

 If the earliest living things were at least not more complex 

 than they, we must certainly choose the lowest of them for 

 study. Sir Oliver Lodge does not follow that method. The 

 lowest form of life he ever mentions is the amoeba or an 

 amoeboid cell. The lowest organisms are much simpler 

 than this. The amoeba has a very characteristic system of 

 locomotion and a fair degree of sensitiveness. It thrusts 

 out parts of its jelly-like substance, and uses them as arms 

 or legs in case of need. It wraps these temporary projec- 

 tions round particles of food, and it pulls itself along by 

 means of them. Further, the amoeba seems to perceive 

 that the particles of food are near it when it thrusts out 

 these arms to enclose them ; and if a drop of irritant acid is 

 put in its water-bath, the amoeba curls up, or literally rolls 

 up, into a tiny ball. It is not conscious, of course, but it is 

 sensitive. Moreover, in its very structure the amoeba 



