manifestly affected, they produce seedlings which vary greatly, departing from the parental type in many characters.[63] Metzger, as stated in the ninth chapter, found that certain kinds of wheat brought from Spain and cultivated in Germany, failed during many years to reproduce themselves truly; but at last, when accustomed to their new conditions, they ceased to be variable,—that is, they became amenable to the power of inheritance. Nearly all the plants which cannot be propagated with any approach to certainty by seed, are kinds which have been long propagated by buds, cuttings, offsets, tubers, etc., and have in consequence been frequently exposed during what may be called their individual lives to widely diversified conditions of life. Plants thus propagated become so variable, that they are subject, as we have seen in the last chapter, even to bud-variation. Our domesticated animals, on the other hand, are not commonly exposed during the life of the individual to such extremely diversified conditions, and are not liable to such extreme variability; therefore they do not lose the power of transmitting most of their characteristic features. In the foregoing remarks on non-inheritance, crossed breeds are of course excluded, as their diversity mainly depends on the unequal development of character derived from either parent or their ancestors.
_Conclusion._ It has been shown in the early part of this chapter how commonly new characters of the most diversified nature, whether normal or abnormal, injurious or beneficial, whether affecting organs of the highest or most trifling importance, are inherited. It is often sufficient for the inheritance of some peculiar character, that one parent alone should possess it, as in most cases in which the rarer anomalies have been transmitted. But the power of transmission is extremely variable. In a number of individuals descended from the same parents, and treated in the same manner, some display this power in a perfect manner, and in some it is quite deficient; and for this difference no reason can be assigned. The effects of injuries or mutilations are occasionally inherited; and we shall see in a future chapter that the long-continued use and disuse of parts produces an inherited effect. Even those characters which are considered the most fluctuating, such as colour, are with rare exceptions transmitted much more forcibly than is generally supposed. The wonder, indeed, in all cases is not that any character should be transmitted, but that the power of inheritance should ever fail. The checks to inheritance, as far as we know them, are, firstly, circumstances hostile to the particular character in question; secondly, conditions of life incessantly inducing fresh variability; and lastly, the crossing of distinct varieties during some previous generation, together with reversion or atavism-that is, the tendency in the child to resemble its grand-parents or more remote ancestors instead of its immediate parents. This latter subject will be discussed in the following chapter.
[2] Mr. Buckle, in his ‘History of 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 ‘Domestic Animals of the British Mr. Tegetmeier, 1866, p. 123. For pigs, _ see_ Mr. Sidney’s edition of 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.
respect to France, Report by Pariset in ‘Comptes Rendus,’ 1847, p.
592.
[10] Girou de Buzareingues, ‘De la Génération,’ p. 282. I have given an analogous case in my book on ‘The Expression of the Emotions.’
[11] The works which I have read and found most useful are Dr. Prosper 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,’ 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. 8th, 1853, gives a curious case of hereditary bleeding in four generations.
[12] Marshall, quoted by Youatt in his work on Cattle, p. 284.
[13] Almost any other organ might have been selected. For instance Mr.
many instances with teeth, and others have been communicated to me.
[16] This affection, as I hear from Mr. Bowman, has been ably described and spoken of as hereditary by Dr. Donders of Utrecht, whose work was published in English by the Sydenham Society in 1864.
[17] M. Giraud-Teulon has recently collected abundant statistical showing that short sight is due to the habit of viewing objects from a short distance, _c’est le travail assidu, de près._ [18] Quoted by Mr. Herbert Spencer, ‘Principles of Biology,’ vol. i.
p. 244.
[20] Dr. Osborne, Pres. of Royal College of Phys. in Ireland, published this case in the ‘Dublin Medical Journal,’ for 1835.
[21] These various statements are taken from the following works and 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 Buzareingues, Veterinary;’ Roberts in vol. ii. p. 144; M. Marrimpoey vol. ii. p.
387; Mr. Karkeek, vol. iv. p. 5; Youatt on Goitre in Dogs, vol. v. p.
483: Youatt in vol. vi. pp. 66, 348, 412; M. Bernard, vol. xi. p. 539; Dr. Samesreuther, on Cattle, in vol. xii. p. 181; Percivall, in vol.
xiii. p. 47. With respect to blindness in horses _see also_ a whole row of authorities in Dr. P. Lucas’s great work, tom. i. p. 399. Mr.
Baker in ‘The Veterinary,’ vol. xiii. p. 721, gives a strong case of hereditary imperfect vision and of jibbing.
[22] Knight on ‘The Culture of the Apple and Pear,’ p. 34. Lindley’s [23] These statements are taken from the following works in vol. viii. p. 50. With respect to Robson, _see_ ‘The Veterinary,’ vol.
by Cecil, 1851; Baron Cameronn, quoted in ‘The Veterinary,’ vol. x. p.
500.
[27] Vrolik has discussed this point at full length in a work published in Dutch, from which Sir J. Paget has kindly translated for me passages. _See, also,_ Isidore Geoffroy St. Hilaire’s ‘Hist. des [28] ‘Massachusetts Medical Society,’ vol. ii. No. 3; and ‘Proc.
Boston Soc. of Nat. Hist.,’ vol. xiv. 1871, p. 154.
[29] Dr. J. W. Ogle gives a case of the inheritance of deficient phalanges during four generations. He adds references to various recent papers on inheritance, ‘Brit. and For. Med.-Chirurg. Review,’ April 1872.
[30] For these several statements, _see_ Dr. Struthers ‘Edinburgh New Phil. Journal,’ July, 1863, especially on intermissions in the line of descent. Prof. Huxley, ‘Lectures on our Knowledge of Organic Nature,’ 1863, p. 97. With respect to inheritance, _see_ Dr. Prosper Lucas, ‘Intermarriage,’ 1838, p. 140, gives a case of five generations; as does Mr. Sedgwick in ‘Brit. and Foreign Medico-Chirurg. Review,’ April, 1863, p. 462. On the inheritance of other anomalies in the extremities _see_ Dr. H. Dobell, in vol. xlvi. of ‘Medico-Chirurg.
Transactions,’ 1863; also Mr. Sedgwick in op. cit., April, 1863, p.
460. With respect to additional digits in the negro _see_ Prichard, ‘Physical History of Mankind.’ Dr. Dieffenbach (‘Jour. Royal Geograph.
Soc.,’ 1841, p. 208) says this anomaly is not uncommon with the Polynesians of the Chatham Islands; and I have heard of several cases with Hindus and Arabs.
[31] Meckel and Isid G. St. Hilaire insist on this fact. _See also_ M.
A. Roujou, ‘Sur quelques Analogies du Type Humain,’ p. 61; published, I believe, in the ‘Journal of the Anthropolog. Soc. of Paris,’ Jan.
1872.
[33] The statements in this paragraph are taken from Isidore Geoffroy St. Hilaire, ‘Hist. des Anomalies,’ tom. i. pp. 688-693. Mr. Goodman gives, ‘Phil. Soc. of Cambridge,’ Nov. 25th, 1872, the case of a cow with three well developed toes on each hind limb, besides the ordinary rudiments; and her calf by an ordinary bull had extra digits. This calf also bore two calves having extra digits.
183, 189.
similar statement in ‘Proc. Nat. of Philadelphia,’ 1872, p. 235.
prostrate trees produced from these seeds.
[42] Verlot, op. cit., p. 93.
[44] These statements are taken from Alph. De Candolle, ‘Bot.
[45] Verlot, op. cit., p. 38.
[49] Darwin in ‘Journal of Proc. Linn. Soc. Bot.,’ 1862, p. 94.
a great breeder of canaries, informs me that he believes that these statements are correct.
200-204. Mr. Sedgwick has given such full details on this subject, with ample references, that I need refer to no other authorities.
[54] Mr. Sproule, in ‘British Medical Journal,’ April 18th, 1863.
[56] Nevertheless Mr. Wetherell states, ‘Nature,’ Dec. 1870, p. 168, that when he visited fifteen years ago the Sioux Indians, he was informed “by a physician, who has passed much of his time with these tribes, that sometimes a child was born with these marks. This was confirmed by the U.S. Government Indian Agent.”
this last paper. It appears that Obersteiner, ‘Stricker’s Med.
Jahrbücher,’ 1875, No. 2, has confirmed Brown-Séquard’s observations.
[59] This last case is quoted by Mr. Sedgwick in ‘British and Foreign tom. ii. p. 492. Also, ‘Transact. Linn. Soc.,’ vol. ix. p. 323. Some curious cases are given by Mr. Baker in the ‘Veterinary,’ vol. xiii.
p. 723. Another curious case is given in the ‘Annales des Scienc.
[61] The Mot-mot habitually bites the barbs off the middle part of the two central tail-feathers, and as the barbs are congenitally somewhat reduced on the same part of these feathers, it seems extremely probable, as Mr. Salvin remarks (‘Proc. Zoolog. Soc.’ 1873, p. 429), that this is due to the inherited effects of long-continued mutilation.
43, 72.
CHAPTER XIII. INHERITANCE _continued_—REVERSION OF ATAVISM.
DIFFERENT FORMS OF REVERSION—IN PURE OR UNCROSSED BREEDS, AS IN PIGEONS, FOWLS, HORNLESS CATTLE AND SHEEP, IN CULTIVATED PLANTS—REVERSION IN FERAL ANIMALS AND PLANTS—REVERSION IN CROSSED VARIETIES AND SPECIES—REVERSION THROUGH BUD-PROPAGATION, AND BY SEGMENTS IN THE SAME FLOWER OR FRUIT—IN DIFFERENT PARTS OF THE BODY IN THE SAME ANIMAL—THE ACT OF CROSSING A DIRECT CAUSE OF REVERSION, VARIOUS CASES OF, WITH INSTINCTS—OTHER PROXIMATE CAUSES OF DEVELOPMENT OF THE TWO SIDES OF THE BODY—APPEARANCE WITH ADVANCING AGE OF CHARACTERS DERIVED FROM A CROSS—THE GERM, WITH ALL ITS LATENT CHARACTERS, A WONDERFUL OBJECT—MONSTROSITIES—PELORIC FLOWERS DUE IN SOME CASES TO REVERSION.
The great principle of inheritance to be discussed in this chapter has been recognised by agriculturists and authors of various nations, as shown by the scientific term _ Atavism,_ derived from atavus, an ancestor; by the English terms of _Reversion,_ or _Throwing-back_; by the French _ Pas-en-Arrière_; and by the German _Rückschlag,_ or _Rückschritt._ When the child resembles either grandparent more closely than its immediate parents, our attention is not much arrested, though in truth the fact is highly remarkable; but when the child resembles some remote ancestor or some distant member in a collateral line,—and in the last case we must attribute this to the descent of all the members from a common progenitor,—we feel a just degree of astonishment. When one parent alone displays some newly-acquired and generally inheritable character, and the offspring do not inherit it, the cause may lie in the other parent having the power of prepotent transmission. But when both parents are similarly characterised, and the child does not, whatever the cause may be, inherit the character in question, but resembles its grandparents, we have one of the simplest cases of reversion. We continually see another and even more simple case of atavism, though not generally included under this head, namely, when the son more closely resembles his maternal than his paternal grand-sire in some male attribute, as in any peculiarity in the beard of man, the horns of the bull, the hackles or comb of the cock, or, as in certain diseases necessarily confined to the male sex; for as the mother cannot possess or exhibit such male attributes, the child must inherit them, through her blood, from his maternal grandsire.
The cases of reversion may be divided into two main classes which, however, in some instances, blend into one another; namely, first, those occurring in a variety or race which has not been crossed, but has lost by variation some character that it formerly possessed, and which afterwards reappears. The second class includes all cases in which an individual with some distinguishable character, a race, or species, has at some former period been crossed, and a character derived from this cross, after having disappeared during one or several generations, suddenly reappears. A third class, differing only in the manner of reproduction, might be formed to include all cases of reversion effected by means of buds, and therefore independent of true or seminal generation. Perhaps even a fourth class might be instituted, to include reversions by segments in the same individual flower or fruit, and in different parts of the body in the same individual animal as it grows old. But the two first main classes will be sufficient for our purpose.
_Reversion to lost Characters by pure or uncrossed forms._—Striking instances of this first class of cases were given in the sixth chapter, namely, of the occasional reappearance, in variously-coloured breeds of the pigeon, of blue birds with all the marks characteristic of the wild _Columba livia._ Similar cases were given in the case of the fowl. With the common ass, as the legs of the wild progenitor are almost always striped, we may feel assured that the occasional appearance of such stripes in the domestic animal is a case of simple reversion. But I shall be compelled to refer again to these cases, and therefore here pass them over.
The aboriginal species from which our domesticated cattle and sheep are descended, no doubt possessed horns; but several hornless breeds are now well established. Yet in these—for instance, in Southdown sheep—“it is not unusual to find among the male lambs some with small horns.” The horns, which thus occasionally reappear in other polled breeds, either “grow to the full size,” or are curiously attached to the skin alone and hang “loosely down, or drop off.”[1] The Galloways and Suffolk cattle have been hornless for the last 100 or 150 years, but a horned calf, with the horn often loosely attached, is occasionally produced.[2] There is reason to believe that sheep in their early domesticated condition were “brown or dingy black;” but even in the time of David certain flocks were spoken of as white as snow. During the classical period the sheep of Spain are described by several ancient authors as being black, red, or tawny.[3] At the present day, notwithstanding the great care which is taken to prevent it, particoloured lambs and some entirely black are occasionally, or even frequently, dropped by our most highly improved and valued breeds, such as the Southdowns. Since the time of the famous Bakewell, during the last century, the Leicester sheep have been bred with the most scrupulous care; yet occasionally grey-faced, or black-spotted, or wholly black lambs appear.[4] This occurs still more frequently with the less improved breeds, such as the Norfolks.[5] As bearing on this tendency in sheep to revert to dark colours, I may state (though in doing so I trench on the reversion of crossed breeds, and likewise on the subject of prepotency) that the Rev. W. D. Fox was informed that seven white Southdown ewes were put to a so-called Spanish ram, which had two small black spots on his sides, and they produced thirteen lambs, all perfectly black. Mr. Fox believes that this ram belonged to a breed which he has himself kept, and which is always spotted with black and white; and he finds that Leicester sheep crossed by rams of this breed always produce black lambs: he has gone on recrossing these crossed sheep with pure white Leicesters during three successive generations, but always with the same result.
Mr. Fox was also told by the friend from whom the spotted breed was procured, that he likewise had gone on for six or seven generations crossing with white sheep, but still black lambs were invariably produced.
Similar facts could be given with respect to tailless breeds of various animals. For instance, Mr. Hewitt[6] states that chickens bred from some rumpless fowls, which were reckoned so good that they won a prize at an exhibition, “in a considerable number of instances were furnished with fully developed tail-feathers.” On inquiry, the original breeder of these fowls stated that, from the time when he had first kept them, they had often produced fowls furnished with tails; but that these latter would again reproduce rumpless chickens.
Analogous cases of reversion occur in the vegetable kingdom; thus “from seeds gathered from the finest cultivated varieties of Heartsease (_Viola tricolor_), plants perfectly wild both in their foliage and their flowers are frequently produced;”[7] but the reversion in this instance is not to a very ancient period, for the best existing varieties of the heartsease are of comparatively modern origin. With most of our cultivated vegetables there is some tendency to reversion to what is known to be, or may be presumed to be, their aboriginal state; and this would be more evident if gardeners did not generally look over their beds of seedlings, and pull up the false plants or “rogues” as they are called. It has already been remarked, that some few seedling apples and pears generally resemble, but apparently are not identical with, the wild trees from which they are descended. In our turnip[8] and carrot-beds a few plants often “break ”—that is, flower too soon; and their roots are generally hard and stringy, as in the parent-species. By the aid of a little selection, carried on during a few generations, most of our cultivated plants could probably be brought back, without any great change in their conditions of life, to a wild or nearly wild condition: Mr. Buckman has effected this with the parsnip;[9] and Mr. Hewett C. Watson, as he informs me, selected, during three generations, “the most diverging plants of Scotch kail, perhaps one of the least modified varieties of the cabbage; and in the third generation some of the plants came very close to the forms now established in England about old castle-walls, and called indigenous.”
_Reversion in Animals and Plants which have run wild._—In the cases hitherto considered, the reverting animals and plants have not been exposed to any great or abrupt change in their conditions of life which could have induced this tendency; but it is very different with animals and plants which have become feral or run wild. It has been repeatedly asserted in the most positive manner by various authors, that feral animals and plants invariably return to their primitive specific type.
It is curious on what little evidence this belief rests. Many of our domesticated animals could not subsist in a wild state; thus, the more highly improved breeds of the pigeon will not “field” or search for their own food. Sheep have never become feral, and would be destroyed by almost every beast of prey.[10] In several cases we do not know the aboriginal parent-species, and cannot possibly tell whether or not there has been any close degree of reversion. It is not known in any instance what variety was first turned out; several varieties have probably in some cases run wild, and their crossing alone would tend to obliterate their proper character. Our domesticated animals and plants, when they run wild, must always be exposed to new conditions of life, for, as Mr. Wallace[11] has well remarked, they have to obtain their own food, and are exposed to competition with the native productions.
Under these circumstances, if our domesticated animals did not undergo change of some kind, the result would be quite opposed to the conclusions arrived at in this work. Nevertheless, I do not doubt that the simple fact of animals and plants becoming feral, does cause some tendency to reversion to the primitive state; though this tendency has been much exaggerated by some authors.
I will briefly run through the recorded cases. With neither horses nor cattle is the primitive stock known; and it has been shown in former chapters that they have assumed different colours in different countries. Thus the horses which have run wild in South America are generally brownish-bay, and in the East dun-coloured; their heads have become larger and coarser, and this may be due to reversion. No careful description has been given of the feral goat. Dogs which have run wild in various countries have hardly anywhere assumed a uniform character; but they are probably descended from several domestic races, and aboriginally from several distinct species. Feral cats, both in Europe and La Plata, are regularly striped; in some cases they have grown to an unusually large size, but do not differ from the domestic animal in any other character. When variously-coloured tame rabbits are turned out in Europe, they generally reacquire the colouring of the wild animal; there can be no doubt that this does really occur, but we should remember that oddly-coloured and conspicuous animals would suffer much from beasts of prey and from being easily shot; this at least was the opinion of a gentleman who tried to stock his woods with a nearly white variety; if thus destroyed, they would be supplanted by, instead of being transformed into, the common rabbit. We have seen that the feral rabbits of Jamaica, and especially of Porto Santo, have assumed new colours and other new characters. The best known case of reversion, and that on which the widely spread belief in its universality apparently rests, is that of pigs. These animals have run wild in the West Indies, South America, and the Falkland Islands, and have everywhere acquired the dark colour, the thick bristles, and great tusks of the wild boar; and the young have reacquired longitudinal stripes. But even in the case of the pig, Roulin describes the half-wild animals in different parts of South America as differing in several respects. In Louisiana the pig[12] has run wild, and is said to differ a little in form, and much in colour, from the domestic animal, yet does not closely resemble the wild boar of Europe. With pigeons and fowls,[13] it is not known what variety was first turned out, nor what character the feral birds have assumed. The guinea-fowl in the West Indies, when feral, seems to vary more than in the domesticated state.
With respect to plants run wild, Dr. Hooker[14] has strongly insisted on what slight evidence the common belief in their reversion to a primitive state rests. Godron[15] describes wild turnips, carrots, and celery; but these plants in their cultivated state hardly differ from their wild prototypes, except in the succulency and enlargement of certain parts,— characters which would certainly be lost by plants growing in poor soil and struggling with other plants. No cultivated plant has run wild on so enormous a scale as the cardoon (_Cynara cardunculus_) in La Plata. Every botanist who has seen it growing there, in vast beds, as high as a horse’s back, has been struck with its peculiar appearance; but whether it differs in any important point from the cultivated Spanish form, which is said not to be prickly like its American descendant, or whether it differs from the wild Mediterranean species, which is said not to be social (though this may be due merely to the nature of the conditions), I do not know.
_Reversion to Characters derived from a Cross, in the case of Sub-varieties, Races, and Species._—When an individual having some recognisable peculiarity unites with another of the same sub-variety, not having the peculiarity in question, it often reappears in the descendants after an interval of several generations. Every one must have noticed, or heard from old people of children closely resembling in appearance or mental disposition, or in so small and complex a character as expression, one of their grandparents, or some more distant collateral relation. Very many anomalies of structure and diseases[16] of which instances have been given in the last chapter, have come into a family from one parent, and have reappeared in the progeny after passing over two or three generations. The following case has been communicated to me on good authority, and may, I believe, be fully trusted: a pointer-bitch produced seven puppies; four were marked with blue and white, which is so unusual a colour with pointers that she was thought to have played false with one of the greyhounds, and the whole litter was condemned; but the gamekeeper was permitted to save one as a curiosity. Two years afterwards a friend of the owner saw the young dog, and declared that he was the image of his old pointer-bitch Sappho, the only blue and white pointer of pure descent which he had ever seen. This led to close inquiry, and it was proved that he was the great-great-grandson of Sappho; so that, according to the common expression, he had only 1/16th of her blood in his veins. I may give one other instance, on the authority of Mr. R. Walker, a large cattle-breeder in Kincardineshire. He bought a black bull, the son of a black cow with white legs, white belly and part of the tail white; and in 1870 a calf the gr.-gr.-gr.-gr.-grandchild of this cow was born coloured in the same very peculiar manner; all the intermediate offspring having been black. In these cases there can hardly be a doubt that a character derived from a cross with an individual of the same variety reappeared after passing over three generations in the one case, and five in the other.
When two distinct races are crossed, it is notorious that the tendency in the offspring to revert to one or both parent-forms is strong, and endures for many generations. I have myself seen the clearest evidence of this in crossed pigeons and with various plants. Mr. Sidney[17] states that, in a litter of Essex pigs, two young ones appeared which were the image of the Berkshire boar that had been used twenty-eight years before in giving size and constitution to the breed. I observed in the farmyard at Betley Hall some fowls showing a strong likeness to the Malay breed, and was told by Mr. Tollet that he had forty years before crossed his birds with Malays; and that, though he had at first attempted to get rid of this strain, he had subsequently given up the attempt in despair, as the Malay character would reappear.
This strong tendency in crossed breeds to revert has given rise to endless discussions in how many generations after a single cross, either with a distinct breed or merely with an inferior animal, the breed may be considered as pure, and free from all danger of reversion.
No one supposes that less than three generations suffices, and most breeders think that six, seven, or eight are necessary, and some go to still greater lengths.[18] But neither in the case of a breed which has been contaminated by a single cross, nor when, in the attempt to form an intermediate breed, half-bred animals have been matched together during many generations, can any rule be laid down how soon the tendency to reversion will be obliterated. It depends on the difference in the strength or prepotency of transmission in the two parent-forms, on their actual amount of difference, and on the nature of the conditions of life to which the crossed offspring are exposed. But we must be careful not to confound these cases of reversion to characters which were gained by a cross, with those under the first class, in which characters originally common to BOTH parents, but lost at some former period, reappear; for such characters may recur after an almost indefinite number of generations.
The law of reversion is as powerful with hybrids, when they are sufficiently fertile to breed together, or when they are repeatedly crossed with either pure parent-form, as in the case of mongrels. It is not necessary to give instances. With plants almost every one who has worked on this subject, from the time of Kölreuter to the present day, has insisted on this tendency. Gärtner has recorded some good instances; but no one has given more striking ones than Naudin.[19] The tendency differs in degree or strength in different groups, and partly depends, as we shall presently see, on whether the parent-plants have been long cultivated. Although the tendency to reversion is extremely general with nearly all mongrels and hybrids, it cannot be considered as invariably characteristic of them; it may also be mastered by long-continued selection; but these subjects will more properly be discussed in a future chapter on Crossing. From what we see of the power and scope of reversion, both in pure races, and when varieties or species are crossed, we may infer that characters of almost every kind are capable of reappearing after having been lost for a great length of time. But it does not follow from this that in each particular case certain characters will reappear; for instance, this will not occur when a race is crossed with another endowed with prepotency of transmission. Sometimes the power of reversion wholly fails, without our being able to assign any cause for the failure: thus it has been stated that in a French family in which 85 out of above 600 members, during six generations, had been subject to night-blindness, “there has not been a single example of this affection in the children of parents who were themselves free from it.”[20] _Reversion through Bud-propagation—Partial Reversion, by segments in the same flower or fruit, or in different parts of the body in the same individual animal._—In the eleventh chapter many cases of reversion by buds, independently of seminal generation, were given—as when a leaf-bud on a variegated, a curled, or laciniated variety suddenly reassumes its proper character; or as when a Provence-rose appears on a moss-rose, or a peach on a nectarine-tree. In some of these cases only half the flower or fruit, or a smaller segment, or mere stripes, reassume their former character; and here we have reversion by segments. Vilmorin[21] has also recorded several cases with plants derived from seed, of flowers reverting by stripes or blotches to their primitive colours: he states that in all such cases a white or pale-coloured variety must first be formed, and, when this is propagated for a length of time by seed, striped seedlings occasionally make their appearance; and these can afterwards by care be multiplied by seed.
The stripes and segments just referred to are not due, as far as is known, to reversion to characters derived from a cross, but to characters lost by variation. These cases, however, as Naudin[22] insists in his discussion on disjunction of character, are closely