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Show 37G PlWYISIOKAL llYl'OTHESIS C HA P. XX\'11. Before proceeding to show, firt;tly, how far these a~sumptions are in themselves probal)le, and seeoll(lly, how far they connect and explain the ntrious groups of facts with which we are concerned, it may be useful to give an illustration of the hypothesis. If one of the simplest Protozoa be formed, as appears under the microscope, of a small mnss of homogeneous gelatinous matter, a minute atom thrown off from any part and nourished under favourable circumstances \vould naturally reproduce the whole; but if the upper and lower surfaces were to differ in texture from the central I_Jortion, then all three parts would have to throw off atoms or gemmules, which when nggregated by mutual aGJnity would form either buds or the sexuctl elements. Precisely tho same view may be extended to one of the higher animals; although in this case many thous<And gemmules must l>e thrown off from the various parts of the body. Now, when the log, for instance, of a salamander is cut off, a slight crust forms over the wound, and beneath this crust the uninjured cells or units of bone, muscle. nerves, &c., are supposed to unite with the diffused gemmules of those cells which in ~he perfect leg come next in order; and these as they l>ecome shghtly developed unite \Yith others, and so on until a papilla of soft cellular tissue, the " budding leg," is formeo, and 111 time a perfect leg.30 'fhus, that portion of the leg which had " we ha\'e two groups of physiological "units, ~li ghtly differcut in .their ;;true" tures." ... " It is not obvious tllat " change in the form of the part, caused '' bychangedaction,involvessuchchange " in the physiological units throughout "the organism, tktt these, when groups " of them are ihrown off in the shape of "reproductive cent res, will unfold into "organisms tha,t have this part similarly · " changed in form. Indeed, when treat" ing of Adaptation, we saw that an "orgu,u modified l>y iucreasc or decrease "of fun ction can but slowly so react on "the system at large as to bring about " those correlative changes required to "produce a new equilibrium; and yet "only when such new equilibrium has "been established, can we expect it to be '· ful~y expressed in the modified phy:;io'' loglcal units of which the orgauism is "built-only then can we count on a ''complete transfer of the modification "to descendants." .. . . . " That the "change in the offspring must, other · " things equal, be in the same direction "as the change in the parent, we may :: dimly see is implied by the fact, that ,. the cl 1ange propagated throughout the ·parental system is a change towards " a new sta te of equilibrium- a change " tending to bring the actions of all " organs, reproductive included, into " harmony with these new actions." 30 lVI. Philipeaux (' Comptes Rend us,' Oct. 1, 1866, p. 576, and June, 1867) has lately shown that when the entire fore-limb, including the scapula, is ex·· tirpated, the power of regrowth is lost. From this he concludes that it is necessary for regrowth that a small portion of the limb should be ldt. But a~ in CHAP. XXVII. OF PANGENESIS. 377 been cut off, neither more nor less, would be reproduced. If the tail or leg of a young animal had been cut off, a young tail or leg would have been reproduced, as :-~ctually occurs with the amputated tail of the t adpole; for gemmules of all the units whj ch compose the tail are diffused throughout the body at all ages. But during the adult state the gemmules of the larval tail would remain dormant, for they woulrl not meet with preexisting cells in a proper state of development with which to unite. If from changed conditions or any other cause any part of the body should become permanently mocJified. the gemmuletl, whieh are merely minute portions of the contents of the cells forming the part, would naturally reproduce the same modification. But gemmules previously derived from the same part before it had undergone any change, would still be diffused throughout the organisation, and would be transmitted from generation to generation, so that under favourable circumstances they might be redeveloped, and then the new modification would be for a time or for ever lost. The aggregation of gemmules derived from every part of the body, through their mutual affinity, would form buds, and their aggregation in some special manner, apparently in small quantity, together probably with the presence of gemmules of certain primordial cells, would constitute the sexual elements. By means of these illustrations the hypothesis of pangenesis has, I hope, been rendered intelligible. Physiologists maintain, as we have seen, that each cell, though to a large extent dependent on others, is likewise, to a certain extent, independent or autonomous. I go one small step further, and assume that each eell casts off a free gemmule, which is capab1e of reproducing a similar cell. There is some analogy between this view and what we see in compound animals and in the flower-buds on the same tree ; for these are distinct individuals capable of true or seminal reproduction, yet' have parts in common and are dependent on each other; thus the lower animals the whole body may be bisected and both halves be reproduced, this belief does not seem probable. lVIay not the early closing of a deep wound, as in the case of the extirpation of the scapula, prevent the formation or protrusion of the naseent limb ? |