Having explained the rather self-evident fact that death or migration always wipes out the surplus which cannot be fed, it is now in order to take up the other end of the problem - the fluctuations of the birth rate from the natural tendency to keep the land full to overflowing. That is, the birth rate is a delicately regulated governor instantly responding to the need for overpopulation. It is very large where losses are tremendous or where the offspring can find room - it is low in the opposite conditions of civilization, and moreover it fluctuates to suit circumstances. It must keep every country overcrowded.
That the birth rate is intensely sensitive to changes in national prosperity has been proved by G. Udny Yule in a paper read before the Royal Statistical Society in London, in 1905. He studied the marriage and birth rates of the previous half century in England and Wales, and found that after periods of trade depression fewer babies are born, but as soon as the tide turned and prices improved, babies began to appear to share the surplus. There was no doubt that in bad times people could not afford babies. There was a remarkable drop after the financial troubles of 1873.
Hunter, in his work on Poverty, states that in every country of Europe it has been observed that emigration has never reduced the population, but on the other hand has increased the birth rate. Every man who leaves for America makes room for another baby, and there were 20,000,000 born who would not have existed if the 20,000,000 emigrants had not made room.* The life-saving advocated by Hunter's book is, of course, necessary, but it will only cause a lessened birth rate, and will not lessen overpopulation any more than emigration from Europe has lessened their poverty problem. He shows that sanitation would save 25,000 lives yearly in New York City, but then there would be 25,000 fewer babies born, if we can judge from European experience. Our immigration has, so far, prevented the birth of 20,000,000 in America! In comparison to the 40,000,000 killed in the wars of the last two or three centuries, it is quite evident that emigration is merely an attempt of a few to survive and in the end does very little good to the home country beyond eliminating the least efficient. When, in 1835, plague destroyed the natives of Cairo and Alexandria at the rate of 2,000 a day, it merely made a small gap which was almost instantly filled. When Frederick the Great was taxed with the loss of life in his wars, he merely replied that one night in Berlin would restore the balance, for it is a well-known phenomenon that wars are followed by a greater birth rate. Even though they are due to overcrowding, the condition is instantly restored.