In this article we propose a revision of the concepts of developmental constraints and variation , in consideration of recent empirical and theoretical work, particularly on Hox genes, phylotypic stage, and morphogenesis. We argue that the notion of isotropic and unlimited variation associated to Darwinian and Neo Darwinian theories must be rethought in the light of recent contributions that come from developmental biology. In this view, variation would be constrained and biased. This conceptual reformulation of variation goes in hand with a reformulation of the concept of developmental constraints : they should be understood as positive causal factors in evolution, in contrast with the way they are usually understood in biology.