There are few persons in America to-day who reach the age of forty sound and normal in every part of the body, especially if we include among abnormalities the minor ailments. The extent to which minor ills are prevalent among those who pass for "well" people is not generally appreciated. Once we penetrate beneath conventional acquaintance we almost invariably learn of some functional trouble, such as impairment of heart, circulation, liver, kidneys, stomach; or gallstones, constipation, diarrhea; or insomnia, neurasthenia, neuritis, neuralgia, sick-headache; or tonsillitis, bronchitis, hay fever, catarrh, grippe, colds, sore throat; or rupture, enlarged glands, skin eruptions; or rheumatism, lumbago, gout, obesity; or decayed teeth, baldness, deafness, eye ailments, spinal curvature, flat foot, lameness; or sundry other "troubles." These ailments, though regarded as "minor," should be recognized promptly and accepted as the signal that the person is moving in the wrong direction. There is no need for alarm provided this warning is heeded. Otherwise disaster is almost certain sooner or later to follow. The laws of physiology are just as inexorable as the laws of physics. There is no compromising with Nature. No man can disobey the laws of health to which he has been bred by Nature without paying for it--any more than a man can sign a check against his bank account without reducing the amount. He may not be immediately bankrupt, and until he exhausts his account he may not experience any inconvenience from his great extravagance, but Nature keeps her balances very accurately, and in the end all claims must be paid. [Sidenote: The Personal Equation] It is true, of course, that some persons have greater resistance than others. If we had a convenient barometer by which to measure daily the state of our vitality, we might register the effect of every unhygienic act. But it is so seldom that endurance is accurately measured that few people appreciate the enormous differences in people and the variations of the same person at different times. These differences and variations have a range of many hundred per cent. Some people can not walk upstairs or run across the street without being out of breath, while others will climb the Matterhorn without overstrain. The fact that certain people have lived to the century-mark in spite of unhygienic living is sometimes cited to prove that hygiene is ineffective. One might as well cite the fact that certain trees are not blown down in a gale or are not quickly destroyed by insect-pests to prove that gales have no tendency to blow down or insects to destroy trees. [Sidenote: Over-confidence]