There is still another graphic way of seeing natural selection at work, by an examination of the infant mortality alone. Imagine a thousand babies coming into the world on a given day. It is known that under average American conditions more than one-tenth of them will die during the first year of life. Now if those who die at this time are the inherently weaker, then the death-rate among survivors ought to be correspondingly less during succeeding years, for many will have been cut down at once, who might otherwise have lingered for several years, although doomed to die before maturity. On the other hand, if only a few die during the first year, one might expect a proportionately greater number to die in succeeding years. If it is actually found that a high death-rate in the first year of life is associated with a low death-rate in succeeding years, then there will be grounds for believing that natural selection is really cutting off the weaker and allowing the stronger to survive. E. C. Snow[56] analyzed the infant mortality registration of parts of England and Prussia to determine whether any such conclusion was justified. His investigation met with many difficulties, and his results are not as clear-cut as could be desired, but he felt justified in concluding from them that "the general result can not be questioned. Natural selection, in the form of a selective death-rate, is strongly operative in man in the early years of life. We assert with great confidence that a high mortality in infancy (the first two years of life) is followed by a correspondingly low mortality in childhood, and vice-versa.... Our work has led us to the conclusion that infant mortality _does_ effect a 'weeding out' of the unfit." "Unfitness" in this connection must not be interpreted too narrowly. A child may be "unfit" to survive in its environment, merely because its parents are ignorant and careless. Such unfitness makes more probable an inheritance of low intelligence. Evidence of natural selection was gathered by Karl Pearson from another source and published in 1912. He dealt with material analogous to that of Dr. Snow and showed "that when allowance was made for change of environment in the course of 50 years, a very high association existed between the deaths in the first year of life and the deaths in childhood (1 to 5 years). This association was such that if the infantile death-rate _increased_ by 10% the child death rate _decreased_ by 5.3% in males, while in females the _fall_ in the child death-rate was almost 1% for every 1% _rise_ in the infantile death-rate." To put the matter in the form of a truism, part of the children born in any district in a given year are doomed by heredity to a premature death; and if they die in one year they will not be alive to die in some succeeding year.