Darwin's Artificial and Natural Selection 111 
when about thirty years old and goes on until ninety years, 
bringing forth six young in the interval, after 750 years 
there will be nearly nineteen million elephants alive which 
have descended from the first pair. 
Obviously, then, if all the descendants of all the individuals 
of a species were to remain alive, the world would be over- 
crowded in a very short time, and the want of room would in 
itself lead to the destruction of countless individuals, if for 
no other reason than lack of food. We can easily carry out 
on a small scale an experiment that shows how the overstock- 
ing, resulting from favorable conditions, comes about, and how 
it checks itself. If we make a meat broth suitable for the 
life of a particular bacterium, and sow in the broth a very 
few individuals, we find in the course of several days the fluid 
swarming with the descendants of the original individuals. 
Thus it has been shown that, if we start with a few hundred 
bacteria, there will be five thousand after twenty-four hours, 
and twenty thousand, forty-eight hours later; and after four 
days they are beyond calculation. 
Cohn found that a single bacterium produces two individ- 
uals in one hour, and four in two hours, and if they continue 
to multiply at this rate there will be produced at the end of 
three days 4,772 billions of descendants. If these are reduced 
to weight, they would weigh seventy-five hundred tons. Thus 
when the conditions are favorable, bacteria are able to in- 
crease at such an enormous rate that they could cover the 
surface of the earth in a very few days. The reason that 
they do not go on increasing at this rate is that they soon 
exhaust the food supply, and the rate of increase slows 
down, and will finally cease altogether. If the bacteria 
were dependent on a continuous supply of food, they would 
perish after the supply had been exhausted, so that the 
rapid rate of multiplication would serve only to bring 
the career of the organism to an untimely end. If the 
weaker individuals were to die first, the products of their dis- 
