  Instructive influence of environment on heredity has been a debated topic for
centuries. Darwin's identification of natural selection coupled to chance
variation as the driving force for evolution, against a formal interpretation
proposed by Lamarck, convinced most scientists that environment does not
specifically instruct evolution in an oriented direction. This is true for
multicellular organisms. In contrast, bacteria were long thought of as prone to
receive oriented influences from their environment, although much was in favour
of the Darwinian route (1). In this context Cairns et al. raised a passionate
debate by suggesting that bacteria generate mutations oriented by the
environmental conditions (2). Several independent pieces of work subsequently
demonstrated that mutations overcoming specific defects arised as a consequence
of cultivation on specific media (3-7). Two diametrically opposed
interpretations were proposed to explain these observations : either induction
of mutations instructed by the environment (e.g. by a process involving a
putative reverse transcription) or selection of variants among a large set of
mutant bacteria generated when stress conditions are present. The experiments
presented below indicate that the Darwinian paradigm is the most plausible.
