“Had all these forms continued unchanged, the world would now be just as then. But a change came, due, we must suppose (from analogy), not to a change in the radical nature of these organisms, but to a change in the external conditions affecting some of them. The food environment, or the temperature environment, or the electrical environment surrounding some of these organisms, or some other unknown but always external influence, wrought a change in some of these lowly forms of life. The mother of Form—Differentiation—was the result. “The organisms affected by Differentiation had to reproduce themselves by producing other organisms in a slightly different form, either lower than themselves, or on the same plane as themselves, or higher than themselves. “Had they taken the first course, Differentiation would have meant destruction and death to all the organisms it touched. The second course was absolutely impossible. A simple organism cannot alter itself without ascending or descending; if it becomes the least degree more complex, it ascends; if it becomes the least degree less complex, it descends. It cannot alter its nature or its form in a horizontal direction. It is absolutely condemned to the vertical, and must go up or down. “These basic simple organisms, then, that formed the foundation for all life, must have responded to their change in environment by ascending, that is, by becoming more complex. They must have done this, or else have descended to death. They were making for the great goal, Sex. “How they reached that goal may be a story yet to be read by Science, but reach it they did on the day that two of these simple-minded organisms reproduced themselves, not by individual fission, but by mutual union. “It was not a radical change in the life of the organisms; it was only a radical change in the method by which that life was reproduced. “It was a change in business methods. It was co-operation, pure and simple, between two organisms in the production of other organisms. Before that day, the whole business had to be done by one individual; after that day it was done by partners, one called Male, the other Female. [Sidenote: Sex a Partnership] “Now, what is the essence of partnership? Mutual assistance. In a labour partnership where the business is in the least complex, two men would be of very little assistance to each other who insisted always on doing the same job, or the same part of a job. There must always be a top sawyer and a bottom sawyer, a man who does what the other cannot do, or gives what the other has not got. “It is exactly the same in the business of life-production, and the instant that Form could demonstrate them, the two partners appeared, and the instant that the new business of Life originated by this partnership became acute and competitive, the partners found themselves leagued together not only for the production of life, but for the defence of that life.