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 felt, which might excite antipathy; the efficient cause, on the other hand, is to be sought in the fact that wherever the mucous membrane passes over to the outer skin, hair grows in the vicinity; and, secondly, also that the head and the genitals are to a certain extent opposite poles of each other, and therefore have various relations and
