The Variation of Animals and Plants Under Domestication, Volume II (of 2). Darwin Charles
family, various defects in either the right or left eye; and Mr. White Cooper has often seen peculiarities of vision confined to one eye reappearing in the same eye in the offspring.17
The following cases are taken from an able paper by Mr. W. Sedgwick, and from Dr. Prosper Lucas.18 Amaurosis, either congenital or coming on late in life, and causing total blindness, is often inherited; it has been observed in three successive generations. Congenital absence of the iris has likewise been transmitted for three generations, a cleft-iris for four generations, being limited in this latter case to the males of the family. Opacity of the cornea and congenital smallness of the eyes have been inherited. Portal records a curious case, in which a father and two sons were rendered blind, whenever the head was bent downwards, apparently owing to the crystalline lens, with its capsule, slipping through an unusually large pupil into the anterior chamber of the eye. Day-blindness, or imperfect vision under a bright light, is inherited, as is night-blindness, or an incapacity to see except under a strong light: a case has been recorded, by M. Cunier, of this latter defect having affected eighty-five members of the same family during six generations. The singular incapacity of distinguishing colours, which has been called Daltonism, is notoriously hereditary, and has been traced through five generations, in which it was confined to the female sex.
With respect to the colour of the iris: deficiency of colouring matter is well known to be hereditary in albinoes. The iris of one eye being of a different colour from that of the other, and the iris being spotted, are cases which have been inherited. Mr. Sedgwick gives, in addition, on the authority of Dr. Osborne,19 the following curious instance of strong inheritance: a family of sixteen sons and five daughters all had eyes "resembling in miniature the markings on the back of a tortoiseshell cat." The mother of this large family had three sisters and a brother all similarly marked, and they derived this peculiarity from their mother, who belonged to a family notorious for transmitting it to their posterity.
Finally, Dr. Lucas emphatically remarks that there is not one single faculty of the eye which is not subject to anomalies; and not one which is not subjected to the principle of inheritance. Mr. Bowman agrees with the general truth of this proposition; which of course does not imply that all malformations are necessarily inherited; this would not even follow if both parents were affected by an anomaly which in most cases was transmissible.
Even if no single fact had been known with respect to the inheritance of disease and malformations by man, the evidence would have been ample in the case of the horse. And this might have been expected, as horses breed much quicker than man, are matched with care, and are highly valued. I have consulted many works, and the unanimity of the belief by veterinaries of all nations in the transmission of various morbid tendencies is surprising. Authors, who have had wide experience, give in detail many singular cases, and assert that contracted feet, with the numerous contingent evils, of ring-bones, curbs, splints, spavin, founder and weakness of the front legs, roaring or broken and thick wind, melanosis, specific ophthalmia, and blindness (the great French veterinary Hazard going so far as to say that a blind race could soon be formed), crib-biting, jibbing, and ill-temper, are all plainly hereditary. Youatt sums up by saying "there is scarcely a malady to which the horse is subject which is not hereditary;" and M. Bernard adds that the doctrine "that there is scarcely a disease which does not run in the stock, is gaining new advocates every day."20 So it is in regard to cattle, with consumption, good and bad teeth, fine skin, &c. &c. But enough, and more than enough, has been said on disease. Andrew Knight, from his own experience, asserts that disease is hereditary with plants; and this assertion is endorsed by Lindley.21
Seeing how hereditary evil qualities are, it is fortunate that good health, vigour, and longevity are equally inherited. It was formerly a well-known practice, when annuities were purchased to be received during the lifetime of a nominee, to search out a person belonging to a family of which many members had lived to extreme old age. As to the inheritance of vigour and endurance, the English race-horse offers an excellent instance. Eclipse begot 334, and King Herod 497 winners. A "cock-tail" is a horse not purely bred, but with only one-eighth or one-sixteenth impure blood in his veins, yet very few instances have ever occurred of such horses having won a great race. They are sometimes as fleet for short distances as thoroughbreds, but as Mr. Robson, the great trainer, asserts, they are deficient in wind, and cannot keep up the pace. Mr. Lawrence also remarks, "perhaps no instance has ever occurred of a three-part-bred horse saving his 'distance' in running two miles with thoroughbred racers." It has been stated by Cecil, that when unknown horses, whose parents were not celebrated, have unexpectedly won great races, as in the case of Priam, they can always be proved to be descended on both sides, through many generations, from first-rate ancestors. On the Continent, Baron Cameronn challenges, in a German veterinary periodical, the opponents of the English race-horse, to name one good horse on the Continent which has not some English race-blood in his veins.22
With respect to the transmission of the many slight, but infinitely diversified characters, by which the domestic races of animals and plants are distinguished, nothing need be said; for the very existence of persistent races proclaims the power of inheritance.
A few special cases, however, deserve some consideration. It might have been anticipated, that deviations from the law of symmetry would not have been inherited. But Anderson23 states that a rabbit produced in a litter a young animal having only one ear; and from this animal a breed was formed which steadily produced one-eared rabbits. He also mentions a bitch, with a single leg deficient, and she produced several puppies with the same deficiency. From Hofacker's account24 it appears that a one-horned stag was seen in 1781 in a forest in Germany, in 1788 two, and afterwards, from year to year, many were observed with only one horn on the right side of the head. A cow lost a horn by suppuration,25 and she produced three calves which had on the same side of the head, instead of a horn, a small bony lump attached merely to the skin; but we here approach the doubtful subject of inherited mutilations. A man who is left-handed, and a shell in which the spire turns in the wrong direction, are departures from the normal though a symmetrical condition, and they are well known to be inherited.
Polydactylism.– Supernumerary fingers and toes are eminently liable, as various authors have insisted, to transmission, but they are noticed here chiefly on account of their occasional regrowth after amputation. Polydactylism graduates26 by multifarious steps from a mere cutaneous appendage, not including any bone, to a double hand. But an additional digit, supported on a metacarpal bone, and furnished with all the proper muscles, nerves, and vessels, is sometimes so perfect, that it escapes detection, unless the fingers are actually counted. Occasionally there are several supernumerary digits; but usually only one, making the total number six. This one may represent either a thumb or finger, being attached to the inner or outer margin of the hand. Generally, through the law of correlation, both hands and feet are similarly affected. I have tabulated the cases recorded in various works or privately communicated to me, of forty-six persons with extra digits on one or both hands and feet; if in each case all four extremities had been similarly affected, the table would have shown a total of ninety-two hands and ninety-two feet each with six digits. As it is, seventy-three hands and seventy-five feet were thus affected. This proves, in contradiction to the result arrived at by Dr. Struthers,27 that the hands are not more frequently affected than the feet.
The presence of more than five digits is a great anomaly, for this number is not normally exceeded by any mammal, bird, or existing reptile.28 Nevertheless, supernumerary digits are strongly inherited; they have been transmitted through five generations; and in some cases, after disappearing for one, two, or even three generations, have reappeared through reversion. These facts are rendered, as Professor Huxley has observed, more remarkable from its being known in most cases that the affected person had not married one similarly
Quoted by Mr. Herbert Spencer, 'Principles of Biology,' vol. i. p. 244.
'British and Foreign Medico-Chirurg. Review, 'April, 1861, p. 482-6; 'l'Héréd. Nat.,' tom. i. pp. 391-408.
Dr. Osborne, Pres. of Royal College of Phys. in Ireland, published this case in the 'Dublin Medical Journal' for 1835.
These various statements are taken from the following works and papers: – Youatt on 'The Horse,' pp. 35, 220. Lawrence, 'The Horse,' p. 30. Karkeek, in an excellent paper in 'Gard. Chronicle,' 1853, p. 92. Mr. Burke, in 'Journal of R. Agricul. Soc. of England,' vol. v. p. 511. 'Encyclop. of Rural Sports,' p. 279. Girou de Buzareignues, 'Philosoph. Phys.,' p. 215.
Knight on 'The Culture of the Apple and Pear,' p. 31. Lindley's 'Horticulture,' p. 180.
These statements are taken from the following works in order: – Youatt on 'The Horse,' p. 48; Mr. Darvill, in 'The Veterinary,' vol. viii. p. 50. With respect to Robson,
'Recreations in Agriculture and Nat. Hist.,' vol. i. p. 68.
'Ueber die Eigenschaften,' &c., 1828, s. 107.
Bronn's 'Geschichte der Natur,' band ii. s. 132.
Vrolik has discussed this point at full length in a work published in Dutch, from which Mr. Paget has kindly translated for me passages.
'Edinburgh New Phil. Journal,' July, 1863.
Some great anatomists, as Cuvier and Meckel, believe that the tubercle one side of the hinder foot of the tailless Batrachians represents a sixth digit. Certainly, when the hinder foot of a toad, as soon as it first sprouts from the tadpole, is dissected, the partially ossified cartilage of this tubercle resembles under the microscope, in a remarkable manner, a digit. But the highest authority on such subjects, Gegenbaur (Untersuchung. zur vergleich. anat. der Wirbelthiere: Carpus et Tarsus, 1864, s. 63), concludes that this resemblance is not real, only superficial.