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Show 358 PROVISIONAL HYPOTHESIS CHAP, XXVII. plies that the whole organisation, in the sense of every separate atom or unit, reproduces itself. Hence ovules and pollongrains,- the fertilised seed or egg, as well as buds,-include and consist of a multitude of germs thrown off from each separate atom of the organism. In the First Part I will enumerate as briefly as I can the groups of facts which seem to demand connection; but certain subjects, not hitherto discussed, must be treated at disproportionate length. In the Second Part tho hypothesis will be given ; and we shall see, after considering how far the necessary assumptions are in themselves improbable, whether it serves to bring under a single point of view the various facts. PART I. Reproduction may be divided into two main classes, namely, sex.ual and asexual. The latter is effected in many ways-by gemmation, that is by the formation of buds of various kinds, and by fissiparous generation, that is by spontaneous or artificial division. It is notorious that some of the lower animals, when cut into many pieces, reproduce so many perfect individuals : Lyonnet cut a N ais or freshwater worm into nearly forty pieces, and these all reproduced perfect animals.1 It is probable that segmentation could be carried much further in some of the protozoa, and with some of the lowest plants each cell will reproduce the parent-form. Johannes 1\iiiller thought that there was an important distinction between gemmation ancl fission; for in the latter case the divided portion, however small, is more perfectly organised; but most physiologists are now convinced that the tvvo processes are essentially alike.~ Prof. Huxley 'remarks, " fission is little more than a peculiar 1 Quoted by Paget, 'Lectures on Pathology,' 1853, p. 159. 2 Dr. Lachmann, also, observes ('Annals and Mag. of Nat. History,' 2nd series, vol. xix., 1857, p. 231) with respect to infusoria, that "fissa,, tion and gemmation pass into each " other almost imperceptibly." Again, M:r. W. C. M:inor (' Annals and 1\'Iag. of Nat. }list.,' 3rd series, vol. xi. p. 328) shows that with Annelids the distinction that has been made between fission and budding is not a fundamental one. See Bonnet,' CEuvres d'Hist. Nat.,' tom. v., 1781, p. 339, for remarks on the budding-out of the amputated limbs of Salamanders. See, also, Professor Clark's work '1\'Iind in Nature,' New York, 1865, pp. 62, 84. Cl!AP. XXVIL. OF PAl~GENESIS. 359 "mode of budding," and Prof. H. J. Clark, who has especially attended to this su~ject, shows in detail that there is sometimes "a compromise between self-division and budding." When a limb is amputated, or when the whole body is bisected, the cut extremities are said to bud forth ; and as the papilla, which is first formed, consists of undeveloped cellular tissue like that forming an ordinary bud, the expression is apparently correct. We see the connection of the two processes in another way ; for Trembley observed that with the hydra the reproduction of the head after amputation was checked as soon a.s the animal began to bud.3 Between the production, by fissiparous generation, of two or more complete individuals, and the repair of even a very slight injury, we have, as remarked in a former chapter, so perfect aud insensible a gradation, that it is impossible to doubt that they are connected processes. Between the power which repairs a trifling injury in any part, and the power which; previously "was occupied in its maintenance by the contmued " mutation of its particles," there cannot be any great difference ; and we may follow Mr. Paget in believing them to be the selfsame power. As at each stage of growth an amputated part is replaced by one in the same state of development, we must likewise follow Mr. Paget in admitting " that the powers " of development from the embryo are identical with those " exercised for the restoration from injuries : in other words, "that the powers are the same by which perfection is first " achieved, and by which, when lost, it is recoverecl." 4 Finally, we may conclude that the several forms of gemmation, and of fissiparous generation, the repair of injuries, the maintenance of each part in its proper state, and. the growth or progressive development of the whole structure of the embryo, are all essentially the results of one and the same great power. Sexual Generation.-The union of the two sexual elements seems to make a broad distinction between sexual ancl asexual reproduction. But the well-ascertained cases of Parthenogenesis prove that the distinction is not really so great as it at first appears ; for oyules occasionally, and even in some cases fre-a Paget, 'Lectures on Patlwlogy,' 1853, p. 158. " Idem, pp. 152, 164. |