Darwin s Artificial and Natural Selection in 



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- 



