Moreover, if spontaneous generation occurs as a resultant of any forces, like forces would produce it again. We have never known it to occur. Should it occur the organisms thus produced would have no bonds of blood-relationship with those already in existence. With these they should show no homology, as they could have no inheritance in common. But all known organisms have common homologies. The factors of organic evolution are essentially the same for all. The unity of life amid all its diversity seems to point to origin from a common stock. If not from one stock, the lines of division between one and another are hidden from us. The study of embryology breaks down the time-honored branch lines of vertebrates, articulates, mollusks, and radiates. The groups of animals are more numerous, more complex, and more intertangled than Cuvier and Agassiz thought. The number of primary branches of animals or plants is uncertain, their boundaries undefined. If spontaneous generation exists, it is a factor in evolution. If it is a factor, our explanation of the meaning and nature of homology must be fundamentally changed. But it may be that it should be changed. We cannot show that spontaneous generation does not exist. All we know is that we have no means of recognizing it. If there is now spontaneous generation of protoplasm, it cannot take the form of any creature we know. An organism fresh from the mint of creation would be too small for us to see with any microscope. It would be too simple for us to trace by any instrumentality now in our possession. It could contain but a few molecules, and a molecule in a drop of water is as small as an orange beside the sun. Our race of creatures, spontaneously generated, without concessions to environment, would grow hoary with the centuries before it came to our notice. Its descendants would have belonged for ages to the unnumbered hosts of microbes before we should be aware of its creation. Evolution is not a creed or a body of doctrine to be believed on authority. There is no saving grace in being an evolutionist. There are many who take this name and have no interest in finding out what it means or in making any application of its principles to the affairs of life. For one who cares not to master its ideas, there is no power in the word. Evolution is not a panacea or a medicine to be applied to social or personal ills. It is simply an expression of the teaching of enlightened common sense as to the order of changes in life. If its principles are mastered a knowledge of evolution is an aid in the conduct of life, as knowledge of gravitation is essential in the building of machinery. There is nothing "occult" in the science of evolution. It is not the product of philosophic meditation or of speculative philosophy. It is based on hard facts, and with hard facts it must deal.