66 THE DURATION OF LIFE. 



formed at the anterior part and a new mouth posteriorly. It 

 is not known whether any circulation of the protoplasm takes 

 place, as in Euglypha. But even if this does not occur, there is 

 no reason for believing that the two products of division possess 

 a different duration of life. 



The process of fission in the Diatoniaceae 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 \ 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 ii. Regeneration. 

 A number of experiments have been recently undertaken, in 

 connection with a prize thesis at Wiirzburg, 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. Carriere 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'^. 



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 bota- 

 nische Jahrbucher, Bd. II. i. 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 & Co., 1870.— E. B. P.] 



^ Cf. Victor Hensen, ' Phj^siologie d. Zeugung,' p. 152. 

 '^ Cf. J. Carriere, ' Ueber Regeneration bei Landpulmonaten,' Tagebl. 

 der 52. Versammlg. deutscher Naturf. pp. 225-226. 



