A simple illustration will serve to justify the procedure we are about to adopt. Suppose that the whole of our literary and pictorial references to earlier stages in the development of the bicycle, the locomotive, or the loom, were destroyed. We should still be able to retrace the phases of their evolution, because we should discover specimens belonging to those early phases lingering in our museums, in backward regions, and elsewhere. They might yet be useful in certain environments into which the higher machines have not penetrated. In the same way, if all the remains of prehistoric man and early civilisation were lost, we could still fairly retrace the steps of the human race, by gathering the lower tribes and races, and arranging them in the order of their advancement. They are so many surviving illustrations of the stages through which mankind as a whole has passed. Just in the same way we may marshal the countless species of animals and plants to-day in such order that they will, in a general way, exhibit to us the age-long procession of life. From the very start of living evolution certain forms dropped out of the onward march, and have remained, to our great instruction, what their ancestors were millions of years ago. People create a difficulty for themselves by imagining that, if evolution is true, all animals must evolve. A glance at our own fellows will show the error of this. Of one family of human beings, as a French writer has said, one only becomes a Napoleon; the others remain Lucien, Jerome, or Joseph. Of one family of animals or trees, some advance in one or other direction; some remain at the original level. There is no "law of progress." The accidents of the world and hereditary endowment impel some onward, and do not impel others. Hence at nearly every great stage in the upward procession through the ages some regiment of plants or animals has dropped out, and it represents to-day the stage of life at which it ceased to progress. In other words, when we survey the line of the hundreds of thousands of species which we find in nature to-day, we can trace, amid their countless variations and branches, the line of organic evolution in the past; just as we could, from actual instances, study the evolution of a British house, from the prehistoric remains in Devonshire to a mansion in Park Lane or a provincial castle.