In other words, we are faced here with the same pattern of reculer pour mieux sauter, 'step back to leap', which we have encountered at the critical turning points in the evolution of science and art. It is as if the stream of life had momentarily reversed its course, flowing uphill for a while towards its original source then opened up a new stream-bed - leaving the koala bear stranded on his tree like a discarded hypothesis. It involves a retreat from specialized adult forms to earlier, less committed and more plastic stages in the development of organisms - followed by a sudden advance in a new direction. Paedomorphosis - or juvenilization - thus appears to play an important part in the grand strategy of evolution. Sir Gavin de Beer compared the process to the re-winding of a biological clock when evolution is in danger of running down and coming to a standstill: 'A race may become rejuvenated by pushing the adult stage of its individuals off from the end of their ontogenies, and such a race may then radiate out in all directions.' It has two aspects: the animal starts to breed while still in a larval or juvenile stage and it never reaches the fully adult stage, which is dropped off - eliminated from its life cycle ('terminal abbreviation'). Now this lowering of the age of sexual maturity is a well-known evolutionary phenomenon called neoteny. The crucial event in this process is the appearance at the foetal, larval or juvenile stage of some useful evolutionary novelty which is carried over into the adult stage of the organism's progeny.
It indicates that at certain critical stages evolution can retrace its steps, as it were, along the path which led to the dead end and make a fresh start in a new, more promising direction. It was described by Garstang in the 1920s, and taken up by several biologists but although the existence of the phenomenon is generally accepted, it made little impact on the orthodox theory and is rarely mentioned in the textbooks. One line of escape from the maze of blind alleys is of particular relevance to our theme: a phenomenon which goes under the name of 'paedomorphosis'.
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