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What could be more striking than the parallel between the rise of the mammals to dominance over the reptiles, and the rise of the motor vehicle to dominance over that drawn by horses?

In both cases, a comparatively long period in which the new type is in a precarious and experimental stage, only just managing to exist, of small size and rare occurrence, and in no real sense a serious rival to its old-established competitors. Then, suddenly, a change. It reaches a level at which it can effectively compete with them. What happens? In the case both of man-made machine and evolving vertebrate group, there is first a sudden increase in numbers of the new, a corresponding decrease in numbers of the old type. The upper level of size of the new type also begins to increase, and it begins to split up into a great number of differentiated sub-types. Some of these sub-types become extinct, others, on the other hand, are gradually improved, while still others undergo such rapid change as to merit the style of new sub-types. The upper level of size, complexity, and efficiency increase, both in animal and machine.