This section is from the "Nature Cure: Philosophy and Practice Based on the Unity of Disease and Cure" book, by Henry Lindlahr.
We shall now consider the three primary causes of disease one by one.
There is a well-defined limit to the running of a watch. When the wound spring has spent its force, the mechanism stops.
So also the living forms of vegetable, animal and human life seem to be wound by Nature to run a certain length of time, in accordance with the laws governing their growth and development. Even the healthiest of animals living in the most congenial surroundings in the freedom of Nature do not much exceed their allotted span of life, nor do they fall much below it. As a rule, the longer the period between birth and maturity, the longer the life of the animal.
All the different families of mammalia, when living in freedom, live closely up to the life period allotted to them by Nature. Man is the only exception. It is claimed that according to the laws of longevity his average length of life should be considerably over one hundred years, while according to life insurance statistics, the average is at present  thirty-seven years.This shows an immense discrepancy between the possible and the actual longevity of man.
Even this brief span of life means little else than weakness, physical and mental suffering and degeneracy for the majority of mankind. Visiting physicians of the public schools in our large cities report that seventy-five percent of all school children show defective health in some way. Diagnosis from the Eye proves that the remaining twenty-five percent are also more or less affected by hereditary and acquired disease conditions. Christian Science says, "There is no disease." Nature's records in the iris of the eye say there is no perfect health.
These established facts of greatly impaired longevity and universal abnormality of the human race would of themselves indicate that there is something radically wrong somewhere in the life habits of man, and that there is ample reason for the great health-reform movement which was started about the middle of the last century by the pioneers of Nature Cure in Germany, and which has since swept, under many different forms and guises, all portions of the civilized world.
When people in general grow better acquainted with the laws underlying prenatal and postnatal child culture, natural living and the natural treatment of diseases, human beings will approach much more closely the normal in health, strength, beauty and longevity. Then will arise a true aristocracy, not of morbid, venous blue blood, but pulsating with the rich red blood of health.
However, to reach this ideal of perfect physical, mental and moral health, succeeding generations will have to adhere to the natural ways of living and of treating their ailments. It cannot be attained by the present generation. The enthusiasts who claim that they can, by their particular methods, achieve perfect health and live the full term of human life, are destined to disappointment. We are so handicapped by the mistakes of the past that the best which most of us adults can do is to patch up, to attain a reasonable measure of health and to approach somewhat nearer Nature's full allotment of life.
Wild animals living in freedom retain their full vigor unimpaired almost to the end of life. Hunters report that among the great herds of buffalo, elk and deer, the oldest bucks are the rulers and maintain their sovereignty over the younger males of the herd solely by reason of their superior strength and prowess. Premature old age, among human beings, as indicated by the early decay of physical and mental powers, is brought on solely by their violation of Nature's Laws in almost all the ordinary habits of life.