Now, there is not the least doubt that the same is true of the order and manner of the _first appearance_ of the natural forces in the phylogenic series. In the history of the evolution of the cosmos, the forces of Nature have appeared successively and suddenly when conditions became favorable. There was a time in the history of the earth when only physical forces existed, chemical affinity being held in abeyance by the intensity of the heat.[41] By gradual cooling, chemical affinity at a certain stage came into being--was born, a new form of force, with new and peculiar phenomena, though doubtless derived from the preceding. Ages upon ages passed away until the time was ripe and conditions were favorable, and life appeared--a new and higher form of force, producing a still more peculiar group of phenomena, but still, as I believe, derived from the preceding. Ages upon ages again passed away, during which this life-force took on higher and higher forms--in the highest foreshadowing and simulating reason itself--until finally, when the time was fully ripe and conditions were exceptionally favorable, spirit, self-conscious, self-determining, rational, and moral, appeared--a new and still higher form of force, but still, as I am persuaded, derived from the preceding. Now, that these forces are really of derivative origin is proved by the fact that we see every step of this process taking place daily under our very eyes. I pass over the conversion of physical into chemical force because this is admitted on all hands. I begin, therefore, with vital force. Sunlight falling on green leaves disappears as light and reappears as life--is consumed in doing the work of decomposing CO2, H2O, and NH3, and the C, H, O, and N thus set free from previous combination unite to form living protoplasm.[42] Again, in the embryonic history of every animal we see the next change take place--i. e., the emergence of the psychic out of the vital. In the germ-cell, in the egg, and even in the early stages of the embryo, there is no distinctive animal life--i. e., no consciousness, nor volition, nor response of any kind to stimulus. At a certain stage distinctive animal or psychic life appears. We call it quickening. Materials for psychology are now present for the first time. In man alone, and that only some time after physical birth, we see the last change. The new-born child has animal life only. The emergence of self-consciousness--a change so wonderful that it may well be called the birth of spirit--takes place only at the age of two to three years. Now for the first time we have phenomena distinctive of humanity.