Very little is known about the use children make of temporal concepts in their explanations of reality. The experiment presented here concerns the development of diachronic thinking in children, i.e. their ability to situate an object of knowledge along a temporal dimension and to conceive changes of this object over time. The experimental situation concerns forest disease and explores the child's ability to understand this phenomenon and to reconstruct the stages of its evolution. A total of 52 children aged from 8 to 11 years were asked questions about the past and future states of a diseased tree. The results show that important changes take place between 8 and 11 years in the conception of the evolution involved and reveal the components of a diachronic perspective. This perspective does not only consist of imagining and ordering steps in an evolutive process. It must also include the ability to establish a link between the steps by conceiving internal transformations that produce external changes. The changes constitute a continuous and gradual sequence, going from past to future. This diachronic perspective is not completely mastered before the age of 10-11 years. Before this, children have a punctual representation, imagine a small number of steps, a short time span and do not establish links between past and future.The genetic method in psychology, first adopted by Baldwin (1906), and later by Piaget (1924Piaget ( ,1970, is at the origin of developmental psychology today. This method has contributed a great deal to the study of cognition. It involves the application to the field of psychology of a perspective which is found in other sciences (e.g. physical chemistry, Prigogine & Stengers, 1981) and which is sometimes called evolutionist (Morss, 1990;Spencer, 1855) in reference to the model constituted by the theory of biological evolution. We prefer to use the more general term of diachronic perspective, which we define as the ability to situate an event on a temporal axis and to consider it as a stage in an evolutive process.The question we wish to address here concerns the foundations of this diachronic perspective and its contributions to the development of children's thinking. Such a question, in the field of developmental psychology has, of course epistemological implications, particularly as regards the problem of the contribution of the diachronic perspective to the knowledge of reality. These implications justify our interest in * Requests for reprints.

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