MARÍA EUGENIA ZAVALA DE COSÍO
The foundations of the theory of demographic transition have recently been discussed in the light of changing fertility in the developing countries, and the universal applicability of this theory has been called into question, particularly in the case of Africa ( Blake 1985; Tabutin 1985; Locoh 1986). A study by Jean-Claude Chesnais, based on a masterly reconstruction of world-wide demographic and economic time series, attempts to reaffirm the validity of the demographic transition theory for the countries of the developing world ( Chesnais 1986b). Chesnais emphasizes propositions derived from the original formulation of the theory ( Landry 1934; Notestein 1945 and 1953): 'Hence, apart from differences in context or rhythm, the demographic transition in the poor countries in reality follows the same fundamental mechanisms as in Europe' ( Chesnais 1986b). None the less, Chesnais admits certain deficiencies in the theory, such as the underestimation of the roles of mortality and international migration, its excessive focus on natality, and the absence of an explanatory framework for demographic change: 'In spite of its robustness, the original transition theory only provides a relatively imprecise and rather inexplicit framework for the overall functional mechanism and for the structural causes of the observed demographic mutations' ( Chesnais 1986b).
In point of fact, thinking on this issue has happened at two different levels: the one analyses the fundamental mechanisms of demographic change during the transition process; the other identifies the socio-economic and cultural variables that explain the process of change. Unfortunately, these two aspects have not been very clearly separated, either in defence or criticism of the theory.
The definition of population dynamics or regimes is the first important contribution of the theory of demographic transition. The empirical scheme is the shift from a traditional regime of high mortality and fertility to a modern one of reduced mortality and fertility. The richness of the theory lies in its considerable broadening of the concept of demographic regulation, in which the dynamics of a population do not depend only on mortality and fertility, but also incorporate other parameters. The different variables interact among themselves, leading to