In explaining the theory of Evolution, science says that there are two principal factors in the process of evolution; the first is the tendency to vary, which exists in all living forms whether vegetable or animal; the second is the tendency of environment to influence that variation, either favorably or unfavorably. Without the first, evolution of any kind would be absolutely impossible. But the cause of that innate tendency to vary is still unknown to science. Upon the second depends the law of natural selection. The variation must be adapted to favorable conditions of life; consequently either the germ of life will select suitable environments or vary itself in order to suit the surrounding conditions, if they are unfavorable. But the agent of this selective process is the struggle for existence, which is a no less important factor. Thus Evolution depends on these three laws: Tendency to vary, or variation, natural selection, and struggle for existence. Science tries to explain through these three laws the physical, mental, intellectual, moral and spiritual evolution of mankind. But the theory of Evolution will remain unintelligible until science can trace the cause of that innate "tendency to vary" which exists in every stage of all living forms. If we study closely we find that man's "self" consists of two natures, one animal and the other moral or spiritual. Animal nature includes all the animal propensities, desire for sense enjoyments, love of self, fear of death and struggle for existence. Each of these is to be found in lower animals as well as in human beings, the difference being only in degree and not in kind. In a savage tribe the expression of this animal nature is simple and natural, while in a highly civilized nation it is expressed not in a simple and straightforward manner, but in an artful and refined way. In a civilized community the same nature working through varied device, policy and plan brings the same results in a more polished form. In the struggle for existence amongst lower animals and savage tribes, those who are physically strong survive and gain advantage over those who are physically weak; while in the civilized world the same result is obtained, not by displaying physical force, but by art, diplomacy, policy, strategy and skill. Various kinds of defensive and offensive weapons have been invented to conquer those who are less skillful in using them, although they may be physically stronger. The simple expression of animal nature which we notice in savages and lower animals, by the natural process of evolution has gradually become more and more complex, as we find in the civilized nations of the world. The energy of the lower human nature is spent chiefly in the struggle for material existence.