CHAPTER XXVI.

LAWS OF VARIATION, continued -SUMMARY.

THE FUSION OF HOMOLOGOUS PARTS-THE VARIABILITY OF MULTIPLE AND HOMOLOGOUS PARTS-COMPENSATION OF GROWTH-MECHANICAL PRESSURE -RELATIVE POSITION OF FLOWERS WITH RESPECT TO THE AXIS, AND OF SEEDS IN THE OVARY, AS INDUCING VARIATION-ANALOGOUS OR PARALLEL VARIETIES-SUMMARY OF THE THREE LAST CHAPTERS.

The Fusion of Homologous Parts.-Geoffroy Saint-Hilaire formerly propounded what he called la loi de l'affinité de soi pour soi, which has been discussed and illustrated by his son, Isidore, with respect to monsters in the animal kingdom,1 and by Moquin-Tandon, with respect to monstrous plants. This law seems to imply that homologous parts actually attract one another and then unite. No doubt there are many wonderful cases, in which such parts become intimately fused together. This is perhaps best seen in monsters with two heads, which are united, summit to summit, or face to face, or Janus-like, back to back, or obliquely side to side. In one instance of two heads united almost face to face, but a little obliquely, four ears were developed, and on one side a perfect face, which was manifestly formed by the fusion of two half-faces. Whenever two bodies or two heads are united, each bone, muscle, vessel, and nerve on the line of junction appears as if it had sought out its fellow, and had become completely fused with it. Lereboullet, who carefully studied the development of double monsters in fishes, observed in fifteen instances the steps by which two heads gradually became united into one. In all such cases it is now thought by the greater number of capable judges that the homologous parts do not attract each other, but that in the words of Mr. Lowne: 3 As union takes place before the differentia

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1 Hist. des Anomalies,' 1832, tom. i. pp. 22, 537-556; tom. iii. p. 462. 2 Comptes Rendus,' 1855, pp. 855,

3 'Catalogue of the Teratological Series in the Museum of the R. Coll. of Surgeons,' 1872, p. xvi.

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"tion of distinct organs occurs, these are formed in continuity "with each other." He adds that organs already differentiated probably in no case become united to homologous ones. M. Dareste does not speak quite decisively against the law of soi pour soi, but concludes by saying, "On se rend parfaitement compte de la formation des monstres, si l'on admet que les embryons qui se soudent appartiennent à un même auf; "qu'ils s'unissent en même temps qu'ils se forment, et que la soudure ne se produit que pendant la première période de la "vie embryonnaire, celle ou les organes ne sont encore constitués que par des blastèmes homogènes."

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By whatever means the abnormal fusion of homologous parts is effected, such cases throw light on the frequent presence of organs which are double during an embryonic period (and throughout life in other and lower members of the same class) but which afterwards unite by a normal process into a single medial organ. In the vegetable kingdom MoquinTandon gives a long list of cases, showing how frequently homologous parts, such as leaves, petals, stamens, and pistils, flowers, and aggregates of homologous parts, such as buds, as well as fruit, become blended, both normally and abnormally, with perfect symmetry into one another.

The Variability of Multiple and Homologous Parts.-Isidore Geoffroy insists that, when any part or organ is repeated many times in the same animal, it is particularly liable to vary both in number and structure. With respect to number, the proposition may, I think, be considered as fully established; but the evidence is chiefly derived from organic beings living under their natural conditions, with which we are not here concerned. Whenever such parts as the vertebræ or teeth, the rays in the fins of fishes, or the feathers in the tails of birds, or petals, stamens, pistils, or seeds, are very numerous, the number is generally variable. With respect to the structure of multiple parts, the evidence of variability is not so decisive; but the fact, as far as it may be trusted,

4 Archives de Zoolog. Expér.,' Jan., 1874, p. 78.

Teratologie Veg.,' 1841, livre iii.

6 Hist. des Anomalies,' tom. iii, PP. 4, 5, 6.

probably depends on multiple parts being of less physiological importance than single parts; consequently their structure has been less rigorously guarded by natural selection.

Compensation of Growth, or Balancement.-This law, as applied to natural species, was propounded by Goethe and Geoffroy Saint-Hilaire at nearly the same time. It implies that, when much organised matter is used in building up some one part, other parts are starved and become reduced. Several authors, especially botanists, believe in this law; others reject it. As far as I can judge, it occasionally holds good; but its importance has probably been exaggerated. It is scarcely possible to distinguish between the supposed effects of such compensation, and the effects of long-continued selection which may lead to the augmentation of one part, and simultaneously to the diminution of another. Anyhow, there can be no doubt that an organ may be greatly increased without any corresponding diminution of an adjoining part. To recur to our former illustration of the Irish elk, it may be asked what part has suffered in consequence of the immense development of the horns?

It has already been observed that the struggle for existence does not bear hard on our domesticated productions, and consequently the principle of economy of growth will seldom come into play, so that we ought not to expect to find with them frequent evidence of compensation. We have, however, some such cases. Moquin-Tandon describes a monstrous bean,7 in which the stipules were enormously developed, and the leaflets apparently in consequence completely aborted; this case is interesting, as it represents the natural condition of Lathyrus aphaca, with its stipules of great size, and its leaves reduced to mere threads, which act as tendrils. De Candolle has remarked that the varieties of Raphanus sativus which have small roots yield numerous seed containing much oil, whilst those with large roots are not productive in oil; and so it is with Brassica asperifolia. The varieties of

7 Tératologie Vég., p. 156. See also my book on The Movements and Habits of Climbing Plants,' 2nd edit,

1875, p. 202.

8 Mémoires du Muséum,' &c.. tom. viii. p. 178.

Cucurbita pepo which bear large fruit yield a small crop, according to Naudin; whilst those producing small fruit yield a vast number. Lastly, I have endeavoured to show in the eighteenth chapter that with many cultivated plants unnatural treatment checks the full and proper action of the reproductive organs, and they are thus rendered more or less. sterile; consequently, in the way of compensation, the fruit becomes greatly enlarged, and, in double flowers, the petals are greatly increased in number.

With animals, it has been found difficult to produce cows which yield much milk, and are afterwards capable of fattening well. With fowls which have large top-knots and beards the comb and wattles are generally much reduced in size; though there are exceptions to this rule. Perhaps the entire absence of the oil-gland in fantail pigeons may be connected with the great development of their tails.

Mechanical Pressure as a Cause of Modifications.-In some few cases there is reason to believe that mere mechanical pressure has affected certain structures. Vrolik and Weber 9 maintain that the shape of the human head is influenced by the shape of the mother's pelvis. The kidneys in different birds differ much in form, and St. Ange 10 believes that this is determined by the form of the pelvis, which again, no doubt, stands in close relation with their power of locomotion. In snakes, the viscera are curiously displaced, in comparison with their position in other vertebrates; and this has been attributed by some authors to the elongation of their bodies; but here, as in so many previous cases, it is impossible to disentangle a direct result of this kind from that consequent on natural selection. Godron has argued that the abortion of the spur on the inner side of the flowers in Corydalis, is caused by the buds at a very early period of growth whilst underground being closely pressed against one another and against the stem. Some botanists believe that the singular difference in the shape both of the seed and corolla, in the

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series, tom. xix. p. 327.

11 Comptes Rendus,' Dec. 1864, p. 1039.

interior and exterior florets in certain Compositous and Umbelliferous plants, is due to the pressure to which the inner florets are subjected; but this conclusion is doubtful.

The facts just given do not relate to domesticated productions, and therefore do not strictly concern us. But here is a more appropriate case: H. Müller 12 has shown that in shortfaced races of the dog some of the molar teeth are placed in a slightly different position to that which they occupy in other dogs, especially in those having elongated muzzles; and as he remarks, any inherited change in the arrangement of the teeth deserves notice, considering their classificatory importance. This difference in position is due to the shortening of certain facial bones and the consequent want of space; and the shortening results from a peculiar and abnormal state of the embryonal cartilages of the bones.

Relative Position of Flowers with respect to the Axis, and of Seeds in the Ovary, as inducing Variation.

In the thirteenth chapter various peloric flowers were described, and their production was shown to be due either to arrested development, or to reversion to a primordial condition. MoquinTandon has remarked that the flowers which stand on the summit of the main stem or of a lateral branch are more liable to become peloric than those on the sides; 13 and he adduces, amongst other instances, that of Teucrium campanulatum. In another Labiate plant grown by me, viz. the Galeobdolon luteum, the peloric flowers were always produced on the summit of the stem, where flowers are not usually borne. In Pelargonium, a single flower in the truss is frequently peloric, and when this occurs I have during several years invariably observed it to be the central flower. This is of such frequent occurrence that one observer" gives the names of ten varieties flowering at the same time, in every one of which the central flower was peloric. Occasionally more than one flower in the truss is peloric, and then of course the additional ones must be lateral. These flowers are interesting as showing how the whole structure is correlated. In the common Pelargonium the upper sepal is produced into a nectary which coheres with the flowerpeduncle; the two upper petals differ a little in shape from the three lower ones, and are marked with dark shades of colour; the stamens are graduated in length and upturned. In the peloric

12Ueber fötale Rachites," Würzburger Medicin. Zeitschrift,' 1860, B. i. s. 265.

13 Teratologie Vég.,' p. 192.

14 Journal of Horticulture,' July 2nd, 1861, p. 253.