Later on, beginning with Cuvier, the identity of species was to be determined in the same way by a set of differences, but the differences were in this case to emerge from the background of the great organic unities possessing their own internal systems of dependencies (skeleton, respiration, circulation); the invertebrates were to be defined, not only by their lack of vertebrae, but also by a certain mode of respiration, by the existence of a type of circulation, and by a whole organic cohesiveness outlining a positive unity.