Genus and species possess in common, (as we have said,) the being predicated of many things, but species must be taken as species only, and not as genus, if the same thing be both species and genus. Moreover, it is common to them both to be prior to what they are predicated of, and to be each a certain whole; but they differ, because genus indeed comprehends species, but species are comprehended by, and do not comprehend genera, for genus is predicated of more than species. Besides, it is necessary that genera should be presupposed, and when formed by specific differences, that they should consummate species, whence also genera are by nature prior. They also co-subvert, but are not co-subverted, for species existing, genus also entirely exists, but genus existing there is not altogether species; genera too, are indeed univocally predicated of speciesunder them, but not species of genera. Moreover, genera exceed, from comprehending the species which are under them, but species exceed genera by their proper differences; besides, neither can species become most generic, nor genus most specific.