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 seventeenth 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 cells of plants yield when they are inoculated by the poison of a gall-insect. With animals various polypoid excrescences and tumours are now generally admitted[[910]] to be the direct product, through proliferation, of normal cells which have become abnormal. In the regular growth and repair of bones, the tissues undergo, as Virchow remarks,[[911]] a whole series of permutations and substitutions. "The cartilage-cells may be
converted by a direct transformation into marrow-cells, and continue as such; or they may first be converted into osseous and then into medullary tissue; or lastly, they may first be converted into marrow and then into bone. So variable are the permutations of these tissues, in themselves so nearly allied, and yet in their external appearance so completely distinct." But as these tissues thus change their nature at any age, without any obvious change in their nutrition, we must suppose in accordance with our hypothesis that gemmules derived from one kind of tissue combine with the cells of another kind, and cause the successive modifications.
It is useless to speculate at what period of development each organic unit casts off its gemmules; for the whole subject of the development of the various elemental tissues is as yet involved in much doubt. Some physiologists, for instance, maintain that muscle or nerve-fibres are developed from cells, which are afterwards nourished by their own proper powers of absorption; whilst other physiologists deny their cellular origin; and Beale maintains that such fibres are renovated exclusively by the conversion of fresh germinal matter (that is the so-called nuclei) into "formed material." However this may be, it appears probable that all external agencies, such as changed nutrition, increased use or disuse, &c., which induced any permanent modification in a structure, would at the same time or previously act on the cells, nuclei, germinal or formative matter, from which the structures in question were developed, and consequently would act on the gemmules or cast-off atoms.
There is another point on which it is useless to speculate, namely, whether all gemmules are free and separate, or whether some are from the first united into small aggregates. A feather, for instance, is a complex structure, and, as each separate part is liable to inherited variations, I conclude that each feather certainly generates a large number of gemmules; but it is possible that these may be aggregated into a compound gemmule. The same remark applies to the petals of a flower, which in some cases are highly complex, with each ridge and hollow contrived for special purposes, so that each part must have been separately modified, and the modifications transmitted; consequently, separate gemmules, according to our hypothesis,
must have been thrown off from each cell or part. But, as we sometimes see half an anther or a small portion of a filament becoming petaliform, or parts or mere stripes of the calyx assuming the colour and texture of the corolla, it is probable that with petals the gemmules of each cell are not aggregated together into a compound gemmule, but are freely and separately diffused.
Having now endeavoured to show that the several foregoing assumptions are to a certain extent supported by analogous facts, and having discussed some of the most doubtful points, we will consider how far the hypothesis brings under a single point of view the various cases enumerated in the First Part. All the forms of reproduction graduate into each other and agree in their product; for it is impossible to distinguish between organisms produced from buds, from self-division, or from fertilised germs; such organisms are liable to variations of the same nature and to reversion of character; and as we now see that all the forms of reproduction depend on the aggregation of gemmules derived from the whole body, we can understand this general agreement. It is satisfactory to find that sexual and asexual generation, by both of which widely different processes the same living creature is habitually produced, are fundamentally the same. Parthenogenesis is no longer wonderful; in fact, the wonder is that it should not oftener occur. We see that the reproductive organs do not actually create the sexual elements; they merely determine or permit the aggregation of the gemmules in a special manner. These organs, together with their accessory parts, have, however, high functions to perform; they give to both elements a special affinity for each other, independently of the contents of the male and female cells, as is shown in the case of plants by the mutual reaction of the stigma and pollen-grains; they adapt one or both elements for independent temporary existence, and for mutual union. The contrivances for these purposes are sometimes wonderfully complex, as with the spermatophores of the Cephalopoda. The male element sometimes possesses attributes which, if observed in an independent animal, would be put down to instinct guided by sense-organs, as when the
spermatozoon of an insect finds its way into the minute micropyle of the egg, or as when the antherozoids of certain algæ swim by the aid of their ciliæ to the female plant, and force themselves into a minute orifice. In these latter cases, however, we must believe that the male element has acquired its powers, on the same principle with the larvæ of animals, namely by successive modifications developed at corresponding periods of life: we can hardly avoid in these cases looking at the male element as a sort of premature larva, which unites, or, like one of the lower algæ, conjugates, with the female element. What determines the aggregation of the gemmules within the sexual organs we do not in the least know; nor do we know why buds are formed in certain definite places, leading to the symmetrical growth of trees and corals, nor why adventitious buds may be formed almost anywhere, even on a petal, and frequently upon healed wounds.[[912]] As soon as the gemmules have aggregated themselves, development apparently commences, but in the case of buds is often afterwards suspended, and in the case of the sexual elements soon ceases, unless the elements of the opposite sexes combine; even after this has occurred, the fertilised germ, as with seeds buried in the ground, may remain during a lengthened period in a dormant state.
The antagonism which has long been observed,[[913]] though exceptions occur,[[914]] between active growth and the power of sexual reproduction—between the repair of injuries and gemmation—and with plants, between rapid increase by buds, rhizomes, &c., and the production of seed, is partly explained by the gemmules not existing in sufficient numbers for both processes.