Biological laws are much more subtle and difficult to trace and formulate than chemical and mechanical laws. Hence the student of organic evolution can rarely arrive at the demonstrable certainties in this field that he can in the sphere of chemistry and mechanics. It is very doubtful if life can ever be explained in terms of these things. Life works through chemical combinations and affinities, and yet is it not more than chemistry? It works with and through mechanical principles and forces, and yet it is evidently more than mechanics. It is manifested through matter, and yet no analysis of matter can reveal its secret. It comes and goes while matter stays; we destroy life, but cannot destroy matter. It is as fugitive as the wind which fills all sails one minute and is gone the next. It avails itself of fluids and gases and the laws which govern them, but fluids and gases do not explain it. It waits upon the rains and the dews, but it is more than they are; it follows in the footsteps of the decay and disintegration of the inorganic, and yet it is not the gift of these things; it transforms the face of the earth, and yet the earth has been and will be when it was not and when it will not be. Through his knowledge and his science man performs wonders every day; he can reduce mountains to powder and seas to dry land, but he cannot create or start de novo the least throb of life. At least, he has not yet done so. With all his vast resources of mechanics and chemistry, and his insight into the mechanism of the universe, he has not yet made the least particle of inorganic matter thrill with the mysterious something we call life. There must have been a time when life was not upon the earth and there must again come a time when it will not be. It has probably vanished from the moon and all inferior planets, and it has not yet come to the superior planets, except maybe to Mars. It must be and must have always been potential in matter, but this fact leaves the mystery as profound as ever.