OCR Text |
Show 268 CAUSES OF VARIABILITY. CHAP. XXII. variation the action cannot have been through the reproductive system. With respect to the part which the reproductive system takes in cau~ing variability, we have seen in the eighteenth chapter that. even ~hght changes in the conditions of life have a remarkable pow.er m causmg a greater or less degree of sterility. Hence it seems not IIDprobable that beings generated through a system so easil! affe?t~d should themselves be affected, or should fail to inherit, or mhent m excess, c~ara~ters proper to their parents. We know that cer~ain groups ~f orgamc bemgs, but with exceptions in each group, have then· reproductive systems m~ch more easily affected by changed conditions than. other groups; for mstance, carnivorous birds more readily than carmvorous ~amma~s, and parrots more readily than pigeons; and t!J!s fac~ harmo_n1zes With the apparently capricious manner and degree m which varwus groups of animals and plants vary under domestication. Ki:ilreuter 47 was struck with the parallelism between the excessive variability of hybrids when crossed and recrossed in various ways,these hybrids having their reproductive powers more or les~ affectcd,and the variability of anciently cultivated plants. Max WIChura 4 ~ has gone one step farther, and shows that with many of our highly cultivated plants, such as the hyacinth, tulip, auricula, snapdragon, potato, cabbage, &c., which there is no reason to believe have been hybridized, the anthers contain many irregular pollen-grains, in the same state as in hybrids. He finds also in certain wild forms, the same coincidence between tho state of the pollen and a high degree of variability, as in many species of Rubus; but in R. cresius and idreus, which are not highly variable species, the pollen is sound. It is also notorious that many cultivated plants, such as the banana, pine-apple, breadfruit, and others previously mentioned, have their reproductive organs so seriously affected as to be generally quite sterile; and when they do yield seed, the seedlings, judging from the large number of cultivated races which exist, must be variable in an extreme degree. These facts indicate that there is some relation between the state of the reproductive organs and a tendency to variability; but we must not conclude that the relation is strict. Although many of our highly cultivated plants may have their pollen in a deteriorated condition, yet, as we have previously seen, they yield more seed, and our anciently domesticated animals are more prolific, than the corresponding species in a state of nature. The peacock is almost the only bird which is believed to be less fertile under domestication than in its native state, and it has varied in a remarkably small degree. From these considerations it would seem that changes in the conditions of life lead either to sterility or to variability, or to both; and not that sterility induces variability. On the whole it is probable that any cause affecting the organs of reproduction would likewise affect their product,-that is, the offspring thus generated. ~7 'Dritte Fortsetzung,' &c., 1766,s.85. 48 ' Die Bnstardbefruchtung,' &c., 1865, s. 92: see also the Rev. M. J. Berkeley on the same subject, in 'Jomnal of Royal Hort. Soc.,' 1866, p. 80. CHAP, XXII. CAUSES OF VARIABILITY. 269 The period of life at which the causes that induce variability act, is another obscure subject, which has been discussed by various authorS.49 In some of the cases, to be given in the following chapter, of modifications from the direct action of changeJ conditions, which are inherited, there can be no doubt that the causes have acted on the mature or nearly mature animal. On the other hand, monstrosities, which cannot be distinctly separated from lesser variations, are often caused by the embryo being injured whilst in the mother's womb or in the egg. Thus I. Geoffroy St. Hilaire 50 asserts that poor women who work hard during their pregnancy, and the mothers of illegitimate children troubled in their minds and forced to conceal their state, are far more liable to give birth to monsters than women in easy circumstances. The eggs of the fowl when placed upright or otherwise treated unnaturally frequently produce monstrous chickens. It would, however, appear that complex monstrosities are induced more frequently during a rather late than during a very early period of embryonic life; but this may partly result from some one part, which has been injured during an early period, affecting by its abnormal growth other parts subsequently developed; and this would be less likely to occur with parts injured at a later period.51 When any part or organ becomes monstrous through abortion, a rudiment is generally left, and this likewise indicates that its development had already commenced. Insects sometimes have their antennal or legs in a monstrous condition, and yet the larval from which they are metamorphosed do not possess either antennal or legs; and in these cases, as Quatrefages 52 believes, we are enabled to see the precise period at which the normal progress of development has been troubled. But the nature of the food given to a caterpillar sometimes affects the colours of the moth, without the caterpillar itself being affected; therefore it seems possible that other chr.racters in the mature insect might be indirectly modified through the larval. There is no reason to suppose that organs which have been rendered monstrous have always been acted on during their development; the cause may have acted on the organisation at a much earlier stage. It is even probable that either the male or female sexual elements, or both, before their union, may be affected in such a manner as to lead to modifications in organs developed at a late period of life; in nea1·ly the 8ame manner as a child may inherit from his father a disease which does not appear until old age. In accordance with the facts above given, which prove that in many cases a close relation exists between variability and the sterility following from changed conditions, we may conclude that the exciting cause often acts at the earliest possible period, namely, on the sexual elements, before impregnation has taken place. That an affection of the female sexual element may induce variability we may likewise infer as probable from the occurrence of bud-variations; for a bud seems to be tho analogue of an ovule. But the male element is apparently much oftener affected by changed con- ~9 Dr. P. Lucas has givcu a history of opinion on this subjeet : 'Hered. Nat.,' 1847, tom. i. p. 175. 50 'Hi st. des Anomalies,' tom. iii. p. 499. 51 Idem, tom. iii. pp. 392, 502. 52 See his iuteresting work, 'Metamorphoses de l' Homme,' &c., 1862, p. 129. |