Clearly, then, that man is misinformed who defends a wrong by referring to its age and reasons that, if certain things were harmful, they would not have survived. To the young thinker the existence of harmful ideas and habits among mankind may at first seem inconsistent with the principles of the survival of the fittest, but this difficulty will disappear upon further investigation. [Sidenote: Factors that determine the survival of races] Since the beginning of recorded history many factors have helped to determine what kind of individuals and races should survive. War, economic wealth and poverty, intellectual beliefs, religions, and social institutions have all been potent factors in determining who should survive. With wealth and conquest came the opportunity to gratify tastes and passions of which the poor individuals of weaker races could not avail themselves. [Sidenote: Many habits and customs detrimental to life and health] Many of the habits and customs which man has developed are not necessary to life, and may be positively detrimental to health and longevity. They have been handed down from generation to generation, not because of their benefit to man, but in spite of their detriment. Such condition of affairs would not be possible if man were not the dominant animal. Man's intellectual supremacy has given him power over the rest of nature, which has resulted in making his struggle for existence much less severe. His use of weapons and of artificial protection from natural destructive forces, as severe heat or cold, has made it possible for him to live and to produce offspring in spite of wrong habits and wrong methods of living, and the natural resistance of life. [Sidenote: Man's organs have a limited power of adaptation] A prevalent error that is due to an incomplete knowledge of the facts of evolution is the belief that organs readily change or adapt themselves to the habits or environment of the individual. This is not true to the extent that it is ordinarily believed. Each individual has a certain limited power of adaptation. He may develop his lungs to a greater breathing capacity, or train his hand for certain skilled work, but these particular acquired habits of the individual are not inherited. Evolution of the race proceeds by the law of natural selection. Thus, if those who are born with great vigor and strong lungs are enabled to live where their weak-lunged neighbors will die, the result will be that their offspring, having greater lung capacity, will form a race with increased lung capacity. But the individual training of the lungs, or of the hand, or of any other organ of the body, will not of itself change the inherited tendency, or, to use a common term of the scientist, the germ-plasm of the race.