_efficient_ cause always ultimately rests upon something that cannot be fathomed, a force of nature, _i.e._, a _qualitas occulta_, and, therefore, it can only give a _relative_ explanation; while the final cause within its sphere affords a sufficient and perfect explanation. It is true we are only perfectly content when we know both the efficient cause, also called by Aristotle ἡ αιτια εξ αναγκης, and the final cause, ἡ χαριν του βελτιονος, at once and yet separately, as their concurrence, their wonderful working together, then surprises us, and on account of it the best appears as the absolutely necessary, and the necessary again as if it were merely the best and not necessary; for then arises in us the dim perception that both causes, however different may be their origin, are yet connected in the root, in the nature of the thing in itself. But such a twofold knowledge is seldom attainable; in _organised_ nature, because the efficient cause is seldom known to us; in _unorganised_ nature, because the final cause remains problematical. However, I will illustrate this by a couple of examples as good as I find within the range of my physiological knowledge, for which physiologists may be able to substitute clearer and more striking ones. The louse of the negro is black. Final cause: its own safety. Efficient cause: because its nourishment is the black _rete Malpighi_ of the negro. The multifarious, brilliant, and gay colouring of the plumage of tropical birds is explained, although only very generally, from the strong effect of the light in the tropics, as its efficient cause. As the final cause I would assign that those brilliant feathers are the gorgeous uniform in which the individuals of the innumerable species there, often belonging to the same genus, may recognise each other; so that each male may find his female. The same holds good of butterflies of different zones and latitudes. It has been observed that consumptive women, in the last stage of their illness, readily become pregnant, that the disease stops during pregnancy, but after delivery appears again worse than before, and now generally results in death: similarly that consumptive men generally beget another child in the last days of their life. The _final cause_ here is that nature, always so anxiously concerned for the maintenance of the species, seeks to replace by a new individual the approaching loss of one in the prime of life; the _efficient cause_, on the other hand, is the unusually excited state of the nervous system which occurs in the last period of consumption. From the same final cause is to be explained the analogous phenomenon that (according to Oken, _Die Zeugung_, p. 65) flies poisoned with arsenic still couple, and die in the act of copulation. The final cause of the pubes in both sexes, and of the Mons Veneris in the female, is that even in the case of very thin subjects the Ossa pubis shall not be