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Show 388 PROVISIONAL HYPOTHESIS CHAP. XXVII. As, during all the stages of development, the tissues of plants consist of cells, and as new cells are not known to be formed between, or independently of, pre-existing ce~ls, we must conclude that the gemmules derived from the formgn pollen do not become developed merely in contact with pre-existing cells, but actually penetrate the nascent cells of the mother-plant. This process may be compared with the ordinary act of fertilisation, during which the contents of the pollen-tubes penetrate the closed embryonic sack within the ovule, and determine the development of the embryo. According to this view, the cells of the mother-plant may almost literally be said to be fertilised by the gemmules derived from the foreign pollen. With all organisms, as we sball presently see, the cells or organic units of the embryo during the successive stages of development may in like manner be said to be fertilised by the gemmules of the cells, whic.h come next in the order of formation. Animals, when capable of sexual reproduction, are fully developed, and it is scarcely possible that the male element should affect the tissues of the mother in the same direct manner as with plants; nevertheless it is certain that her ova.ria are sometimes affected by a previous impregnation, so that the ovules subsequently fertilised by a distinct male are plainly influenced in character ; and this, as in the case of foreign pollen, is intelligible through the diffusion, retention, and action of the gemmules included within the spermatozoa of the pre-vious male. Each organism reaches maturity through a longer or shorter course of development. The changes may be small and insensibly slow, as when a child grows into a man, or many, abrupt, and slight, as in the metamorphoses of certain ephemerous insects, or again few and strongly marked, as with most other insects. Each part may be moulded within a previously existing and corresponding part, and in this case it will appear, falsely as I believe, to be formed from the old part ; or it may be developed within a wholly distinct part of the body, as in the extreme cases of metagenesis. An eye, for instance, may be developed at a spot where no eye previously existed. We have also seen that allied organic beings in the course of their metamorphoses sometimes attain nearly the same structure after passing CliAP. XXVII. OF P Al"'fGENESIS. 389 through widely different forms; or converse~y, after pa~sing through nearly the same early f~r~s, arrr:e at a WI~ely different termination. In these cases It IS very difficult to bebeve that the early cells or units possess the inherent power, independently of any external agent, of prod~cing new structures wholly different in form, position~ and functwn .. Burt these. cas~s become plain on the hypotheSIS of pangeneSIS. rhe orgamc units, during each stage of development, throw off gemmules, which, multiplying, are transmitted to the offspring. In the offspring, as soon as any particular cell or unit ~n th~ pro~er order of development becomes partially developed, It unites with (or to speak metaphorically is fertilised by) the g~mmule of the next succeeding cell, and so onwards. Now, supposmg that at any stage of development, certain cells or aggregates of cells had been slightly modified by the action of some disturbing cause, the castoff gemmules or atoms of the cell-contents could hardly fail to be similarly affected, and consequently would reproduce the same modification. This process might be repeated until the structure of the part at this particular stage of development became greatly changed, but this would not necessarily affect other parts whether previously or subsequently developed. In this manner we can understand the remarkable independence of structure in the successive metamorphoses, and especially in the successive metageneses of many animals. rrhe term growth ought strictly to be connned to mere increase of size, and development. to change of structure.43 Now, a child is said to grow into a man, and a foal into a horse, but, as in these cases there is much change of structure, the process properly belongs to the order of development. We have indirect evidence of this in many variations and diseases supervening during so-called growth at a particular period, and being inherited at a corresponding period. In the case, however, of diseases which supervene during old age, subsequently to the ordinary period of procreation, and which nevertheless are sometimes inherited, as occurs with brain and heart complaints, we 43 Various physiologists have insisted on this distinction between growth and development. Prof. Marshall ('Phil. Transact.,' 1864, p. 544) gives a good instance in microcephalous idiots, in which the brain continues to grow after having been arrested in its development. |