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 ovaria 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 previous 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
through widely different forms; or conversely, after passing through nearly the same early forms, arrive at a widely different termination. In these cases it is very difficult to believe that the early cells or units possess the inherent power, independently of any external agent, of producing new structures wholly different in form, position, and function. But these cases become plain on the hypothesis of pangenesis. The organic 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 in the proper order of development becomes partially developed, it unites with (or to speak metaphorically is fertilised by) the gemmule of the next succeeding cell, and so onwards. Now, supposing that at any stage of development, certain cells or aggregates of cells had been slightly modified by the action of some disturbing cause, the cast-off 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.
The term growth ought strictly to be confined to mere increase of size, and development to change of structure.[[916]] 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
must suppose that the organs were in fact affected at an earlier age and threw off at this period affected gemmules; but that the affection became visible or injurious only after the prolonged growth of the part in the strict sense of the word. In all the changes of structure which regularly supervene during old age, we see the effects of deteriorated growth, and not of true development.
In the so-called process of alternate generation many individuals are generated asexually during very early or later stages of development. These individuals may closely resemble the preceding larval form, but generally are wonderfully dissimilar. To understand this process we must suppose that at a certain stage of development the gemmules are multiplied at an unusual rate, and become aggregated by mutual affinity at many centres of attraction, or buds. These buds, it may be remarked, must include gemmules not only of all the succeeding but likewise of all the preceding stages of development; for when mature they have the power of transmitting by sexual generation gemmules of all the stages, however numerous these may be. It was shown in the First Part, at least in regard to animals, that the new beings which are thus at any period asexually generated do not retrograde in development—that is, they do not pass through those earlier stages, through which the fertilised germ of the same animal has to pass; and an explanation of this fact was attempted as far as the final or teleological cause is concerned. We can likewise understand the proximate cause, if we assume, and the assumption is far from improbable, that buds, like chopped-up fragments of a hydra, are formed of tissue which has already passed through several of the earlier stages of development; for in this case their component cells or units would not unite with the gemmules derived from the earlier-formed cells, but only with those which came next in the order of development. On the other hand, we must believe that, in the sexual elements, or probably in the female alone, gemmules of certain primordial cells are present; and these, as soon as their development commences, unite in due succession with the gemmules of every part of the body, from the first to the last period of life.
The principle of the independent formation of each part, in
so far as its development depends on the union of the proper gemmules with certain nascent cells, together with the superabundance of the gemmules derived from both parents and self-multiplied, throws light on a widely different group of facts, which on any ordinary view of development appears very strange. I allude to organs which are abnormally multiplied or transposed. Thus gold-fish often have supernumerary fins placed on various parts of their bodies. We have seen that, when the tail of a lizard is broken off, a double tail is sometimes reproduced, and when the foot of the salamander is divided longitudinally, additional digits are occasionally formed. When frogs, toads, &c., are born with their limbs doubled, as sometimes occurs, the doubling, as Gervais remarks,[[917]] cannot be due to the complete fusion of two embryos, with the exception of the limbs, for the larvæ are limbless. The same argument is applicable[[918]] to certain insects produced with multiple legs or antennæ, for these are metamorphosed from apodal or antennæless larvæ. Alphonse Milne-Edwards[[919]] has described the curious case of a crustacean in which one eye-peduncle supported, instead of a complete eye, only an imperfect cornea, out of the centre of which a portion of an antenna was developed. A case has been recorded[[920]] of a man who had during both dentitions a double tooth in place of the left second incisor, and he inherited this peculiarity from his paternal grandfather. Several cases are known[[921]] of additional teeth having been developed in the palate, more especially with horses, and in the orbit of the eye. Certain breeds of sheep bear a whole crowd of horns on their foreheads. Hairs occasionally appear in strange situations, as within the ears of the Siamese hairy family; and hairs "quite natural in structure" have been observed "within the substance of the brain."[[922]] As many as five spurs have been seen on both legs in certain Game-fowls. In the Polish fowl the male is ornamented with a topknot of hackles
like those on his neck, whilst the female has one of common feathers. In feather-footed pigeons and fowls, feathers like those on the wing arise from the outer side of the legs and toes. Even the elemental parts of the same feather may be transposed; for in the Sebastopol goose, barbules are developed on the divided filaments of the shaft.
Analogous cases are of such frequent occurrence with plants that they do not strike us with sufficient surprise. Supernumerary petals, stamens, and pistils, are often produced. I have seen a leaflet low down in the compound leaf of Vicia sativa converted into a tendril, and a tendril possesses many peculiar properties, such as spontaneous movement and irritability. The calyx sometimes assumes, either wholly or by stripes, the colour and texture of the corolla. Stamens are so frequently converted, more or less completely, into petals, that such cases are passed over as not deserving notice; but as petals have special functions to perform, namely, to protect the included organs, to attract insects, and in not a few cases to guide their entrance by well-adapted contrivances, we can hardly account for the conversion of stamens into petals merely by unnatural or superfluous nourishment. Again, the edge of a petal may occasionally be found including one of the highest products of the plant, namely the pollen; for instance, I have seen in an Ophrys a pollen-mass with its curious structure of little packets, united together and to the caudicle by elastic threads, formed between the edges of an upper petal. The segments of the calyx of the common pea have been observed partially converted into carpels, including ovules, and with their tips converted into stigmas. Numerous analogous facts could be given.[[923]]