The law of reason which requires us to seek for systematic unity is a necessary law, since without it we should have no reason at all, and without reason no coherent employment of the understanding, and in the absence of this, no sufficient criterion of empirical truth. In other words we must assume our rules conform to nature. If the appearances which present themselves were of so great a variety that understanding could never by comparison detect the slightest similarity, we would not have any universal concept and the understanding itself would not exist. The logical principle of genera is balanced by the Principle of Species which prescribes to the understanding that it attend to diversity no less than to identity. Aims at the systematic completeness of knowledge.
Reason demands that no species be the lowest.
This law cannot be derived from experience. It is also only on the assumption of differences in nature that we can have any faculty of understanding. In order to complete the systematic unity, reason has another law, the Principle of Continuity which prescribes that we proceed from each species to every other by a gradual increase in the diversity. Arises from the union of the other two by prescribing that even amidst the utmost manifoldness we observe homogeneity in the gradual transition from one species to another and thus recognize a relationship of the different branches all springing from the same stem. No difference is the smallest.