In consequence of the nature of organised beings which has been set forth, teleology, as the assumption of the adaptation of every part to its end, is a perfectly safe guide in considering the whole of organised nature; on the other hand, in a metaphysical regard, for the explanation of nature beyond the possibility of experience, it must only be regarded as valid in a secondary and subsidiary manner for the confirmation of principles of explanation which are otherwise established: for here it belongs to the problems which have to be given account of. Accordingly, if in some animal a part is found of which we do not see any use, we must never venture the conjecture that nature has produced it aimlessly, perhaps trifling, or out of mere caprice. Certainly it is possible to conceive something of this kind under the Anaxagorean assumption that the disposition of nature has been brought about by means of an ordering understanding, which, as such, obeys a foreign will; but not under the assumption that the true inner being (_i.e._, outside of our idea) of every organism is simply and solely _its own will_; for then the existence of every part is conditioned by the circumstance that in some way it serves the will which here lies at its foundation, expresses and realises some tendency of it, and consequently in some way contributes to the maintenance of this organism. For apart from _the will which manifests itself in it_, and the conditions of the external world under which this has voluntarily undertaken to live, for the conflict with which its whole form and disposition is already adapted, nothing can have influenced it and determined its form and parts, thus no arbitrary power, no caprice. On this account everything in it must be designed; and therefore final causes (_causæ finales_) are the clue to the understanding of organised nature, as efficient causes (_causæ efficientes_) are the clue to the understanding of unorganised nature. It depends upon this, that if in anatomy or zoology, we cannot find the end or aim of an existing part, our understanding receives a shock similar to that which it receives in physics from an effect whose cause remains concealed; and as we assume the latter as necessary, so also we assume the former, and therefore go on searching for it, however long we may already have done so in vain. This is, for example, the case with the spleen, as to the use of which men never cease inventing hypotheses, till some day one shall have proved itself correct. So is it also with the large spiral‐ formed teeth of the babyroussa, the horn‐shaped excrescences of certain caterpillars, and more of the like. Negative cases are also judged by us according to the same rule; for example, that in a class which, as a whole, is so uniform as that of lizards, so important a part as the bladder is present in many species, while it is wanting in others; similarly that dolphins and certain cetacea related to them are entirely