36 THE MEANING OF EVOLUTION 
arithmetic ratio. That is, while the food might in- 
crease like 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 question 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- 
