How can we understand the construction of an organism?

Since a dozen years, biology is in a state of permanent technical and conceptual excitement. The pendulum is swinging back from the selectionist populationist biology of Darwin--the organism is a black box: viewed from the outside--to the mechanistic embryology of Aristotle--viewed from the inside of the organism--leading to a new interpretation of old concepts. This short text tries to get to the meaning of these events by putting them in historical and epistemological perspectives, through such concepts as teleology and differentiation, on the way describing several paradoxes: experimental results yield a detailed description of purposeless mechanical devices, explaining Nature, which, to us, appears purposeful.