: if one of them, in the process of perfecting itself, should attain the degree of complexity possessed beforehand by the species one step higher, this does not mean that the latter has thereby been overtaken, because, carried onward by the same momentum, it cannot avoid perfecting itself to an equivalent degree: There will be a continual and more or less slow progress of all the species towards a superior perfection, with the result that all the degrees of the scale will be continually variable within a determined and constant relation. .