36 THE MEANING OF EVOLUTION 



arithmetic ratio. That is, while the food might in- 

 crease hke the series 2-4-6-8-10, the population would 

 increase like the series 2-4-8-16-32. On this basis it 

 is only a (juestion of time when the earth will be too 

 full of people for it to be possible for the food to 

 sustain them. Malthus added many observations and 

 suggestions, but this is as much of the book as inter- 

 ests us in this connection. Here was the idea that 

 suggested to Darwin his agency for producing the 

 change of the animals of the past into those of the 

 present. 



The number of animals of any particular species re- 

 mains practically the same. There may be a few 

 more one year, and a few less another, but on the 

 average, year by year, the number of toads, the num- 

 ber of blacksnakes, the number of fieldmice, remains 

 sensibly the same. Sometimes the rise of man brings 

 an end to the wild population, and so in the past ani- 

 mals have dropped out of the race. Yet in the long 

 run and for a considerable time the number of any 

 species is constant. But each animal produces off- 

 spring in quantities sufficient to far more than replace 

 himself as he dies out. In other words, animals in- 

 crease not by addition but by multiplication. Too 

 many are born for all of them to live. What becomes 

 of the great mass of them? The answer is they die; 

 most of them die young. Only a few fortunate in- 



