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Show 380 PROVISIONAL HYPOTHESIS CHAP. XXVII. each reproduce::; the form of the part whence derived. But this non-diffusion of the gemmules from bud to bucl may 1e only apparent, depending, as we shall hereafter f:lee, on the nature of the first-formed cells in the buds. The assumed elective affinity of each gemmule for that particular cell which precedes it in the order of development is supported by many analogies. In all ordinary cases of sexual reproduction the male and female elements have a mutual affinity for each other: thus, it is believed that about ten thousand species of Compositre exist, and there can be no doubt that if the pollen of all these species could be, simultaneously or successively, placed on the stigma of any one species, this one would elect with unerring certainty its own pollen. This elective capacity is all the more wonderful, as it must have been acquired since the many species of this great group of plants branched off from a common progenitor. On any view of the nature of sexual reproduction, the protoplasm contained within the ovules and within the sperm-cells (or the "spermatic force" of the latter, if so vagne a. term be preferred) must act on each other by some law of special affinity, either during or subsequently to impregnation, so that corresponding parts alone affect each other; thus, a calf produced from a short-horned cow by a long-horned bull has its horns and not its horny hoofs affected by the union of-the two forms, and the offspring from two birds with differently coloured tails have their tails and not their whole plumage affected. . The. various t~ss~es of the body plainly show, as many physiOlogists have rns1sted,36 an affinity for special organic sub~ tances, whether natural or foreign to the body. We see this rn the cells of the kidneys attracting urea from the blood; in the worrara poison affecting the nerves; upas and digitalis the muscles; the Lytta vesicatoria the kidneys; and in the poisonous matter of many diseases, as small-pox, scarlet-fever, hoopingcoug~, glanders, cancer, and hydrophobia, affecting certain defimte parts of the body or certain tissues or glands. The affinity of various parts of the body for each other during ~6 Paget, 'Lectures on Pathology,' p. 27 ; Virchow, 'Cellular Patholocry' translat. by Dr. Chance, pp. 123, i26, 294 ; Claude Bernard, 'Des 'fissus Vivants,' liP· 177, 210, 337; Miillcr·s ' Physiology,' Eng. trnnslat., p. 290. CHAP. XXVII. OF PANGENESIS. 381 their early development was shown m . the last chapter, when discussing the tendency to fusion in homologous parts. This affinity displays itself in the normal fusion of organs which are separate at an early embryonic age, and still more plainly in those marvellous cases of double monsters in which each bone, muscle, vessel, and nerve in the one embryo, blends with the corresponding part in the other. The affinity between homologous organs may come into action with single parts, or with the entire individual, as in the case of flowers or fruits which are symmetrically blended together with all their parts doubled, but without any other trace of fusion. It has also been assumed that the development of each gemmule depends on its union with another cell or unit which has just commenced its development, and which, from preceding it in order of growth, is of a somewhat different nature. Nor is it a very improbable assumption that the development of a gemmule is determined by its union with a cell slightly different in nature, for abundant evidence was given in the seyenteenth chapter, showing that a slight degree of differentiation in the male and female sexual elements favours in a marked manner their union and subsequent development. But what determines the development of the gemmules of the first-formed or primordial cell in the unimpregnated ovule, is beyond conjecture. It must also be admitted that analogy fails to guide us towards any determination on several other points : for instance, whether cells, derived from the same parent-cell, may, in the regular course of growth, become developed into different structures, from absorbing peculiar kinds of nutriment, independently of their union with distinct gemmules. We shall appreciate this difficulty if we call to mind, what complex yet symmetrical growths the cel1s of plants yield when they are inoculated by the poison of a gall-insect. \Vith animals various polypoid excrescences and tumours are now generally admitted 37 to be the direct product, through proliferation, of normal cells which have become abnormal. In t.he regular growth and repair of bones, the tissues undergo, as Virchow remarks,38 a whole series of permutations anll substitutions. "The cartilage-cells may be 37 Virchow,' Cellulftr Pathology,' trans. by Dr. Chance, 1860, pp. 60, 162, 245, 441, 454. 38 Iuem, pp. 412-·126. |