In the eighteenth century naturalists had already begun to realize that the diversity of living forms was so great that they could not be arranged into a single linear hierarchy. The relationships were best represented as a two-dimensional map, or ─ when the time dimension was added ─ as a branching tree. Darwin used the 'tree of life' but because his theory of natural selection explained evolution in terms of adaptation to local environments his tree had no central trunk leading to humanity. Evolution was a utilitarian process in which success depended on gaining an advantage in dealing with the environment. Progress towards higher levels of organization occurred in many different ways, although only a few adaptations resulted in an overall increase in complexity. In the early twentieth century palaeontologists studying the history of life showed that progress was episodic, occurring only when a major biological innovation led to the emergence of a new class of organisms. These events were unpredictable on the basis of previous trends and could be compared to the inventions that have transformed human societies. Darwinism was a form of 'creative evolution' in which innovations were multifarious and unpredictable.