In fact, as far as the tree of life is concerned, the distance between two species belonging to the same subclass (branch) must always be smaller than the distance between one of these species and any species that does not belong to the subclass. In order to ensure that this property is always true, we propose to attribute a variable length for edges: an important length is given to edges close to the tree root, and the length is successively reduced when the edge gets away from the root. A geometric progression can be implemented here. The length of edges at the tree root is 1. The length of any other edge equals half the length of the parent edge (Fig. 3 , right insert (B)).