I do not know how physiologists look at such facts as the foregoing. According to the doctrine of pangenesis, the free and superabundant gemmules of the transposed organs are developed in the wrong place, from uniting with wrong cells or aggregates of cells during their nascent state; and this would follow from a slight modification in the elective affinity of such cells, or possibly of certain gemmules. Nor ought we to feel much surprise at the affinities of cells and gemmules varying
under domestication, when we remember the many curious cases given, in the seventeenth chapter, of cultivated plants which absolutely refuse to be fertilised by their own pollen or by that of the same species, but are abundantly fertile with pollen of a distinct species; for this implies that their sexual elective affinities—and this is the term used by Gärtner—have been modified. As the cells of adjoining or homologous parts will have nearly the same nature, they will be liable to acquire by variation each other's elective affinities; and we can thus to a certain extent understand such cases as a crowd of horns on the heads in certain sheep, of several spurs on the leg, and of hackles on the head of the fowl, and with the pigeon the occurrence of wing-feathers on their legs and of membrane between their toes; for the leg is the homologue of the wing. As all the organs of plants are homologous and spring from a common axis, it is natural that they should be eminently liable to transposition. It ought to be observed that when any compound part, such as an additional limb or an antenna, springs from a false position, it is only necessary that the few first gemmules should be wrongly attached; for these whilst developing would attract others in due succession, as in the regrowth of an amputated limb. When parts which are homologous and similar in structure, as the vertebræ in snakes or the stamens in polyandrous flowers, &c., are repeated many times in the same organism, closely allied gemmules must be extremely numerous, as well as the points to which they ought to become united; and, in accordance with the foregoing views, we can to a certain extent understand Isid. Geoffroy St. Hilaire's law, namely, that parts, which are already multiple, are extremely liable to vary in number.
The same general principles apply to the fusion of homologous parts; and with respect to mere cohesion there is probably always some degree of fusion, at least near the surface. When two embryos during their early development come into close contact, as both include corresponding gemmules, which must be in all respects almost identical in nature, it is not surprising that some derived from one embryo and some from the other should unite at the point of contact with a single nascent cell or aggregate of cells, and thus give rise to a single part or organ. For instance, two embryos might thus come to have on their
adjoining sides a single symmetrical arm, which in one sense will have been formed by the fusion of the bones, muscles, &c., belonging to the arms of both embryos. In the case of the fish described by Lereboullet, in which a double head was seen gradually to fuse into a single one, the same process must have taken place, together with the absorption of all the parts which had been already formed. These cases are exactly the reverse of those in which a part is doubled either spontaneously or after an injury; for in the case of doubling, the superabundant gemmules of the same part are separately developed in union with adjoining points; whilst in the case of fusion the gemmules derived from two homologous parts become mingled and form a single part; or it may be that the gemmules from one of two adjoining embryos alone become developed.
Variability often depends, as I have attempted to show, on the reproductive organs being injuriously affected by changed conditions; and in this case the gemmules derived from the various parts of the body are probably aggregated in an irregular manner, some superfluous and others deficient. Whether a superabundance of gemmules, together with fusion during development, would lead to the increased size of any part cannot be told; but we can see that their partial deficiency, without necessarily leading to the entire abortion of the part, might cause considerable modifications; for in the same manner as a plant, if its own pollen be excluded, is easily hybridised, so, in the case of a cell, if the properly succeeding gemmules were absent, it would probably combine easily with other and allied gemmules. We see this in the case of imperfect nails growing on the stumps of amputated fingers,[[924]] for the gemmules of the nails have manifestly been developed at the nearest point.
In variations caused by the direct action of changed conditions, whether of a definite or indefinite nature, as with the fleeces of sheep in hot countries, with maize grown in cold countries, with inherited gout, &c., the tissues of the body, according to the doctrine of pangenesis, are directly affected by the new conditions, and consequently throw off modified gemmules, which are transmitted with their newly acquired peculiarities to the offspring. On any ordinary view it is unintelligible how changed
conditions, whether acting on the embryo, the young or adult animal, can cause inherited modifications. It is equally or even more unintelligible on any ordinary view, how the effects of the long-continued use or disuse of any part, or of changed habits of body or mind, can be inherited. A more perplexing problem can hardly be proposed; but on our view we have only to suppose that certain cells become at last not only functionally but structurally modified; and that these throw off similarly modified gemmules. This may occur at any period of development, and the modification will be inherited at a corresponding period; for the modified gemmules will unite in all ordinary cases with the proper preceding cells, and they will consequently be developed at the same period at which the modification first arose. With respect to mental habits or instincts, we are so profoundly ignorant on the relation between the brain and the power of thought that we do not know whether an inveterate habit or trick induces any change in the nervous system; but when any habit or other mental attribute, or insanity, is inherited, we must believe that some actual modification is transmitted;[[925]] and this implies, according to our hypothesis, that gemmules derived from modified nerve-cells are transmitted to the offspring.
It is generally, perhaps always, necessary that an organism should be exposed during several generations to changed conditions or habits, in order that any modification in the structure of the offspring should ensue. This may be partly due to the changes not being at first marked enough to catch the attention, but this explanation is insufficient; and I can account for the fact, only by the assumption, which we shall see under the head of reversion is strongly supported, that gemmules derived from each cell before it had undergone the least modification are transmitted in large numbers to successive generations, but that the gemmules derived from the same cells after modification, naturally go on increasing under the same favouring conditions, until at last they become sufficiently numerous to overpower and supplant the old gemmules.
Another difficulty may be here noticed; we have seen that