The process of fission in the _Diatomaceae_ seems to me to be theoretically important, because here, as in the previously-mentioned _Monothalamia_ (_Euglypha_, etc.), the new silicious skeleton is built up within the primary organism, but not, as in _Euglypha_, for the new individual only, but for both parent and daughter-cell alike[31]. If we compare the diatom shell to a box, then the two halves of the old shell would form two lids, one for each of the products of fission, while a new box is built up afresh for each of them. In this case there is an absolute equality between the products of fission, so far as the shell is concerned. Note 11. Regeneration. A number of experiments have been recently undertaken, in connection with a prize thesis at Würzburg, in order to test the powers of regeneration possessed by various animals. In all essential respects the results confirm the statements of the older observers, such as Spallanzani. Carrière has also proved that snails can regenerate not only their horns and eyes, but also part of the head when it has been cut off, although he has shown that Spallanzani's old statement that they can regenerate the whole head, including the nervous system, is erroneous[32]. Note 12. The Duration of Life in Plants. The title of the work on this subject mentioned in the Text is ‘Die Lebensdauer und Vegetationsweise der Pflanzen, ihre Ursache und ihre Entwicklung,’ F. Hildebrand, Engler’s botanische Jahrbücher, Bd. II. 1. und 2. Heft, Leipzig, 1881. Note 13. [Many interesting facts and conclusions upon the subject of this essay will be found in a volume by Professor E. Ray Lankester, ‘On comparative Longevity in Man and the lower Animals,’ Macmillan and Co., 1870.—E. B. P.] ------------------------------------------------------------------------ Footnotes for the Appendix to Essay I. Footnote 1: Humboldt’s ‘Ausichten der Natur.’ Footnote 2: This estimate is derived from observation of the time during which these insects are to be seen upon the wing. Direct observations upon the duration of life in this species are unknown to me. Footnote 3: [Sir John Lubbock has now kept a queen ant alive for nearly 15 years. See note 2 {note 18 below} on p. 51.—E. B. P.] Footnote 4: [After reading these proofs Dr. A. R. Wallace kindly sent me an unpublished note upon the production of death by means of natural selection, written by him some time between 1865 and 1870. The note contains some ideas on the subject, which were jotted down for further elaboration, and were then forgotten until recalled by the argument of this Essay. The note is of great interest in relation to Dr. Weismann’s suggestions, and with Dr. Wallace’s permission I print it in full below. ‘The Action of Natural Selection in Producing Old Age, Decay, and Death.