LIFE IN THE PAST 165 
can use this oxygen in the burning of its own sub- 
stance to produce its own activities, can act in re- 
sponse to sensation gained from outside, can throw 
off its waste matter produced by its own activities, and 
can grow. When the proper time comes its nucleus 
can split in two, the cell itself enclosing the nucleus 
can separate into two cells, each of which can grow 
to the size of the parent cell and repeat its life. This 
is as simple an animal as we have yet discovered. 
Every kitchen drain swarms with such creatures. On 
a summer day the stagnant pools are full of them. 
The simplest microscope will show them clearly. This 
is life in its lowest terms with which we are ac- 
quainted. With such life, it seems to us, the animal 
and plant world must have started their existence, 
when first the earth began to teem with living matter. 
If, then, we may form any judgment concerning the 
first living things upon the globe by considering the 
simplest creatures that live here to-day, certain facts 
seem clear. In the first place, life began in the water, 
and for a long time was only to be found in the water. 
Single cells are so small and dry out so easily that 
it is necessary to their existence that they should be 
kept entirely moist by the presence of water all about 
them. It is true many of them will stand drying, but 
while they are thus dried they can scarcely be said to 
be much more than just alive. They are utterly in- 
