The Variation of Animals and Plants Under Domestication, Volume II (of 2). Darwin Charles

follows from any degree of close interbreeding has been denied by many persons; but rarely by any practical breeder; and never, as far as I know, by one who has largely bred animals which propagate their kind quickly. Many physiologists attribute the evil exclusively to the combination and consequent increase of morbid tendencies common to both parents: that this is an active source of mischief there can be no doubt. It is unfortunately too notorious that men and various domestic animals endowed with a wretched constitution, and with a strong hereditary disposition to disease, if not actually ill, are fully capable of procreating their kind. Close interbreeding, on the other hand, induces sterility; and this indicates something quite distinct from the augmentation of morbid tendencies common to both parents. The evidence immediately to be given convinces me that it is a great law of nature, that all organic beings profit from an occasional cross with individuals not closely related to them in blood; and that, on the other hand, long-continued close interbreeding is injurious.

      Various general considerations have had much influence in leading me to this conclusion; but the reader will probably rely more on special facts and opinions. The authority of experienced observers, even when they do not advance the grounds of their belief, is of some little value. Now almost all men who have bred many kinds of animals and have written on the subject, such as Sir J. Sebright, Andrew Knight, &c.,246 have expressed the strongest conviction on the impossibility of long-continued close interbreeding. Those who have compiled works on agriculture, and have associated much with breeders, such as the sagacious Youatt, Low, &c., have strongly declared their opinion to the same effect. Prosper Lucas, trusting largely to French authorities, has come to a similar conclusion. The distinguished German agriculturist Hermann von Nathusius, who has written the most able treatise on this subject which I have met with, concurs; and as I shall have to quote from this treatise, I may state that Nathusius is not only intimately acquainted with works on agriculture in all languages, and knows the pedigrees of our British breeds better than most Englishmen, but has imported many of our improved animals, and is himself an experienced breeder.

      Evidence of the evil effects of close interbreeding can most readily be acquired in the case of animals, such as fowls, pigeons, &c., which propagate quickly, and, from being kept in the same place, are exposed to the same conditions. Now I have inquired of very many breeders of these birds, and I have hitherto not met with a single man who was not thoroughly convinced that an occasional cross with another strain of the same sub-variety was absolutely necessary. Most breeders of highly-improved or fancy birds value their own strain, and are most unwilling, at the risk, in their opinion, of deterioration, to make a cross. The purchase of a first-rate bird of another strain is expensive, and exchanges are troublesome; yet all breeders, as far as I can hear, excepting those who keep large stocks at different places for the sake of crossing, are driven after a time to take this step.

      1

      'Medical Notes and Reflections,' 3rd edit., 1855, p. 267.

      2

      Mr. Buckle, in his grand work on 'Civilisation,' expresses doubts on the subject owing to the want of statistics. See also Mr. Bowen, Professor of Moral Philosophy, in 'Proc. American Acad. of Sciences,' vol. v. p. 102

      3

      For greyhounds, see Low's 'Domest. Animals of the British Islands,' 1845, p. 721. For game-fowls, see 'The Poultry Book,' by Mr. Tegetmeier, 1866, p. 123. For pigs, see Mr. Sidney's

1

'Medical Notes and Reflections,' 3rd edit., 1855, p. 267.

2

Mr. Buckle, in his grand work on 'Civilisation,' expresses doubts on the subject owing to the want of statistics. See also Mr. Bowen, Professor of Moral Philosophy, in 'Proc. American Acad. of Sciences,' vol. v. p. 102

3

For greyhounds, see Low's 'Domest. Animals of the British Islands,' 1845, p. 721. For game-fowls, see 'The Poultry Book,' by Mr. Tegetmeier, 1866, p. 123. For pigs, see Mr. Sidney's edit. of 'Youatt on the Pig,' 1860, pp. 11, 22.

4

'The Stud Farm,' by Cecil, p. 39.

5

'Philosophical Transactions,' 1755, p. 23. I have seen only second-hand accounts of the two grandsons. Mr. Sedgwick, in a paper to which I shall hereafter often refer, states that four generations were affected, and in each the males alone.

6

Barbara Van Beck, figured, as I am informed by the Rev. W. D. Fox, in Woodburn's 'Gallery of Rare Portraits,' 1816, vol. ii.

7

'Proc. Zoolog. Soc.,' 1833, p. 16

8

Hofacker, 'Ueber die Eigenschaften,' &c., 1828, s. 34. Report by Pariset in 'Comptes Rendus,' 1847, p. 592.

9

Hunter, as quoted in Harlan's 'Med. Researches,' p. 530. Sir A. Carlisle, 'Phil. Transact.,' 1814, p. 94.

10

Girou de Buzareignues, 'De la Génération,' p. 282.

11

'Macmillan's Magazine,' July and August, 1865.

12

The works which I have read and found most useful are Dr. Prosper Lucas's great work, 'Traité de l'Hérédité Naturelle,' 1847. Mr. W. Sedgwick, in 'British and Foreign Medico-Chirurg. Review,' April and July, 1861, and April and July, 1863: Dr. Garrod on Gout is quoted in these articles. Sir Henry Holland, 'Medical Notes and Reflections,' 3rd edit., 1855. Piorry, 'De l'Hérédité dans les Maladies,' 1840. Adams, 'A Philosophical Treatise on Hereditary Peculiarities,' 2nd edit., 1815. Essay on 'Hereditary Diseases,' by Dr. J. Steinan, 1843. See Paget, in 'Medical Times,' 1857, p. 192, on the Inheritance of Cancer; Dr. Gould, in 'Proc. of American Acad. of Sciences,' Nov. 8, 1853, gives a curious case of hereditary bleeding in four generations. Harlan, 'Medical Researches,' p. 593.

13

Marshall, quoted by Youatt in his work on Cattle, p. 284.

14

'Philosoph. Transact.,' 1814, p. 94.

15

'Medical Notes and Reflections,' 3rd edit., p. 33.

16

This affection, as I hear from Mr. Bowman, has been ably described and spoken of as hereditary by Dr. Dondera, of Utrecht, whose work was published in English by the Sydenham Society in 1864.

17

Quoted by Mr. Herbert Spencer, 'Principles of Biology,' vol. i. p. 244.

18

'British and Foreign Medico-Chirurg. Review, 'April, 1861, p. 482-6; 'l'Héréd. Nat.,' tom. i. pp. 391-408.

19

Dr. Osborne, Pres. of Royal College of Phys. in Ireland, published this case in the 'Dublin Medical Journal' for 1835.

20

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,'

246

For Andrew Knight, see A. Walker, on 'Intermarriage,' 1838, p. 227. Sir J. Sebright's Treatise has just been quoted.