Yet who can maintain that he has discovered the right answer to this important question? And even though the discovery were made, can any one believe that by its means the problem of life would be solved? If it were established that spontaneous generation did actually occur, a new question at once arises as to the conditions under which the occurrence became possible. How can we conceive that dead inorganic matter could have come together in such a manner as to form living protoplasm, that wonderful and complex substance which absorbs foreign material and changes it into its own substance, in other words grows and multiplies? And so, in discussing this question of life and death, we come at last—as in all provinces of human research—upon problems which appear to us to be, at least for the present, insoluble. In fact it is the quest after perfected truth, not its possession, that falls to our lot, that gladdens us, fills up the measure of our life, nay! hallows it. APPENDIX. Note 1. The Duration of Life among Birds. There is less exact knowledge upon this subject than we might expect, considering the existing number of ornithologists and ornithological societies with their numerous publications. It has neither been possible nor necessary for my purpose to look up all the widely-scattered references which are to be found upon the subject. Many of these are doubtless unknown to me; for we are still in want of a compilation of accurately determined observations in this department of zoology. I print the few facts which I have been able to collect, as a slight contribution towards such a compilation. * * * * * Small singing birds live from eight to eighteen years: the nightingale, in captivity, eight years, but longer according to some writers: the blackbird, in captivity, twelve years, but both these birds live longer in the natural state. A ‘half-bred nightingale built its nest for nine consecutive years in the same garden’ (Naumann, ‘Vögel Deutschlands,’ p. 76). Canary birds in captivity attain an age of twelve to fifteen years (l. c., p. 76). Ravens have lived for almost a hundred years in captivity (l. c., Bd. I. p. 125). Magpies in captivity live twenty years, and, ‘without doubt,’ much longer in the natural state (l. c., p. 346). Parrots ‘in captivity have reached upwards of a hundred years’ (l. c., p. 125). A single instance of the cuckoo (alluded to in the text) is mentioned by Naumann as reaching the age of thirty-two years (l. c., p. 76). Fowls live ten to twenty years, the golden pheasant fifteen years, the turkey sixteen years, and the pigeon ten years (Oken, ‘Naturgeschichte, Vögel,’ p. 387). A golden eagle which ‘died at Vienna in the year 1719, had been captured 104 years previously’ (Brehm, ‘Leben der Vögel,’ p. 72).