On the whole we may conclude that whatever part or character is most valued-- whether the leaves, stems, tubers, bulbs, flowers, fruit, or seed of plants, or the size, strength, fleetness, hairy covering, or intellect of animals-- that character will almost invariably be found to present the greatest amount of difference both in kind and degree. And this result may be safely attributed to man having preserved during a long course of generations the variations which were useful to him, and neglected the others.
I will conclude this chapter by some remarks on an important subject. With animals such as the giraffe, of which the whole structure is admirably co- ordinated for certain purposes, it has been supposed that all the parts must have been simultaneously modified; and it has been argued that, on the principle of natural selection, this is scarcely possible. But in thus arguing, it has been tacitly a.s.sumed that the variations must have been abrupt and great. No doubt, if the neck of a ruminant were suddenly to become greatly elongated, the fore limbs and back would have to be simultaneously strengthened and modified; but it cannot be denied that an animal might have its neck, or head, or tongue, or fore-limbs elongated a very little without any corresponding modification in other parts of the body; and animals thus slightly modified would, during a dearth, have a slight advantage, and be enabled to browse on higher twigs, and thus survive. A few mouthfuls more or less every day would make all the difference between life and death. By the repet.i.tion of the same process, and by the occasional intercrossing of the survivors, there would be some progress, slow and fluctuating though it would be, towards the admirably coordinated structure of the giraffe. If the short- faced tumbler-pigeon, with its small conical beak, globular head, rounded body, short wings, and small feet--characters which appear all in harmony--had been a natural species, its whole structure would have been viewed as well fitted for its life; but in this case we know that inexperienced breeders are urged to attend to point after point, and not to attempt improving the whole structure at the same time. Look at the greyhound, that perfect image of grace, symmetry, and vigour; no natural species can boast of a more admirably co-ordinated structure, with its tapering head, slim body, deep chest, tucked- up abdomen, rat-like tail, and long muscular limbs, all adapted for extreme fleetness, and for running down weak prey. Now, from what we see of the variability of animals, and from what we know of the method which different men follow in improving their stock--some chiefly attending to one point, others to another point, others again correcting defects by crosses, and so forth--we may feel a.s.sured that if we could see the long line of ancestors of a first-rate greyhound up to its wild wolf-like progenitor, we should behold an infinite number of the finest gradations, sometimes in one character and sometimes in another, but all leading towards our present perfect type. By small and doubtful steps such as these, nature, as we may confidently believe, has progressed, on her grand march of improvement and development.
A similar line of reasoning is as applicable to separate organs as to the whole organisation. A writer (20/91. Mr. J.J. Murphy in his opening address to the Belfast Nat. Hist. Soc. as given in the "Belfast Northern Whig" November 19, 1866. Mr. Murphy here follows the line of argument against my views previously and more cautiously given by the Rev. C. Pritchard, Pres. Royal Astronomical Soc., in his sermon Appendix page 33 preached before the British a.s.sociation at Nottingham 1866.) has recently maintained that "it is probably no exaggeration to suppose that in order to improve such an organ as the eye at all, it must be improved in ten different ways at once. And the improbability of any complex organ being produced and brought to perfection in any such way is an improbability of the same kind and degree as that of producing a poem or a mathematical demonstration by throwing letters at random on a table." If the eye were abruptly and greatly modified, no doubt many parts would have to be simultaneously altered, in order that the organ should remain serviceable.
But is this the case with smaller changes? There are persons who can see distinctly only in a dull light, and this condition depends, I believe, on the abnormal sensitiveness of the retina, and is known to be inherited. Now if a bird, for instance, receive some great advantage from seeing well in the twilight, all the individuals with the most sensitive retina would succeed best and be the most likely to survive; and why should not all those which happened to have the eye itself a little larger, or the pupil capable of greater dilatation, be likewise preserved, whether or not these modifications were strictly simultaneous? These individuals would subsequently intercross and blend their respective advantages. By such slight successive changes, the eye of a diurnal bird would be brought into the condition of that of an owl, which has often been advanced as an excellent instance of adaptation. Short- sight, which is often inherited, permits a person to see distinctly a minute object at so near a distance that it would be indistinct to ordinary eyes; and here we have a capacity which might be serviceable under certain conditions, abruptly gained. The Fuegians on board the Beagle could certainly see distant objects more distinctly than our sailors with all their long practice; I do not know whether this depends upon sensitiveness or on the power of adjustment in the focus; but this capacity for distant vision might, it is probable, be slightly augmented by successive modifications of either kind. Amphibious animals which are enabled to see both in the water and in the air, require and possess, as M. Plateau has shown (20/92. On the Vision of Fishes and Amphibia, translated in "Annals and Mag. of Nat. Hist." volume 18 1866 page 469.), eyes constructed on the following plan: "the cornea is always flat, or at least much flattened in the front of the crystalline and over a s.p.a.ce equal to the diameter of that lens, whilst the lateral portions may be much curved." The crystalline is very nearly a sphere, and the humours have nearly the same density as water. Now as a terrestrial animal became more and more aquatic in its habits, very slight changes, first in the curvature of the cornea or crystalline, and then in the density of the humours, or conversely, might successively occur, and would be advantageous to the animal whilst under water, without serious detriment to its power of vision in the air. It is of course impossible to conjecture by what steps the fundamental structure of the eye in the Vertebrata was originally acquired, for we know nothing about this organ in the first progenitors of the cla.s.s. With respect to the lowest animals in the scale, the transitional states through which the eye at first probably pa.s.sed, can by the aid of a.n.a.logy be indicated, as I have attempted to show in my "Origin of Species." (20/93. Sixth edition 1872 page 144.)
CHAPTER 2.XXI.
SELECTION, continued.
NATURAL SELECTION AS AFFECTING DOMESTIC PRODUCTIONS.
CHARACTERS WHICH APPEAR OF TRIFLING VALUE OFTEN OF REAL IMPORTANCE.
CIRc.u.mSTANCES FAVOURABLE TO SELECTION BY MAN.
FACILITY IN PREVENTING CROSSES, AND THE NATURE OF THE CONDITIONS.
CLOSE ATTENTION AND PERSEVERANCE INDISPENSABLE.
THE PRODUCTION OF A LARGE NUMBER OF INDIVIDUALS ESPECIALLY FAVOURABLE.
WHEN NO SELECTION IS APPLIED, DISTINCT RACES ARE NOT FORMED.
HIGHLY-BRED ANIMALS LIABLE TO DEGENERATION.
TENDENCY IN MAN TO CARRY THE SELECTION OF EACH CHARACTER TO AN EXTREME POINT, LEADING TO DIVERGENCE OF CHARACTER, RARELY TO CONVERGENCE.
CHARACTERS CONTINUING TO VARY IN THE SAME DIRECTION IN WHICH THEY HAVE ALREADY VARIED.
DIVERGENCE OF CHARACTER, WITH THE EXTINCTION OF INTERMEDIATE VARIETIES, LEADS TO DISTINCTNESS IN OUR DOMESTIC RACES.
LIMIT TO THE POWER OF SELECTION.
LAPSE OF TIME IMPORTANT.
MANNER IN WHICH DOMESTIC RACES HAVE ORIGINATED.
SUMMARY.
NATURAL SELECTION, OR THE SURVIVAL OF THE FITTEST, AS AFFECTING DOMESTIC PRODUCTIONS.
We know little on this head. But as animals kept by savages have to provide throughout the year their own food either entirely or to a large extent, it can hardly be doubted that in different countries, varieties differing in const.i.tution and in various characters would succeed best, and so be naturally selected. Hence perhaps it is that the few domesticated animals kept by savages partake, as has been remarked by more than one writer, of the wild appearance of their masters, and likewise resemble natural species. Even in long-civilised countries, at least in the wilder parts, natural selection must act on our domestic races. It is obvious that varieties having very different habits, const.i.tution, and structure, would succeed best on mountains and on rich lowland pastures. For example, the improved Leicester sheep were formerly taken to the Lammermuir Hills; but an intelligent sheep-master reported that "our coa.r.s.e lean pastures were unequal to the task of supporting such heavy- bodied sheep; and they gradually dwindled away into less and less bulk: each generation was inferior to the preceding one; and when the spring was severe, seldom more than two-thirds of the lambs survived the ravages of the storms."
(21/1. Quoted by Youatt on "Sheep" page 325. See also Youatt on "Cattle" pages 62, 69.) So with the mountain cattle of North Wales and the Hebrides, it has been found that they could not withstand being crossed with the larger and more delicate lowland breeds. Two French naturalists, in describing the horses of Circa.s.sia, remark that, subjected as they are to extreme vicissitudes of climate, having to search for scanty pasture, and exposed to constant danger from wolves, the strongest and most vigorous alone survive. (21/2. MM.
Lherbette and De Quatref.a.ges in "Bull. Soc. d"Acclimat." tome 8 1861 page 311.)
Every one must have been struck with the surpa.s.sing grace, strength, and vigour of the Game-c.o.c.k, with its bold and confident air, its long, yet firm neck, compact body, powerful and closely pressed wings, muscular thighs, strong beak ma.s.sive at the base, dense and sharp spurs set low on the legs for delivering the fatal blow, and its compact, glossy, and mail-like plumage serving as a defence. Now the English game-c.o.c.k has not only been improved during many years by man"s careful selection, but in addition, as Mr.
Tegetmeier has remarked (21/3. "The Poultry Book" 1866 page 123. Mr.
Tegetmeier, "The Homing or Carrier Pigeon" 1871 pages 45-58.), by a kind of natural selection, for the strongest, most active and courageous birds have stricken down their antagonists in the c.o.c.kpit, generation after generation, and have subsequently served as the progenitors of their race. The same kind of double selection has come into play with the carrier pigeon, for during their training the inferior birds fail to return home and are lost, so that even without selection by man only the superior birds propagate their race.
In Great Britain, in former times, almost every district had its own breed of cattle and sheep; "they were indigenous to the soil, climate, and pasturage of the locality on which they grazed: they seemed to have been formed for it and by it." (21/4. "Youatt on Sheep" page 312.) But in this case we are quite unable to disentangle the effects of the direct action of the conditions of life,--of use or habit--of natural selection--and of that kind of selection which we have seen is occasionally and unconsciously followed by man even during the rudest periods of history.
Let us now look to the action of natural selection on special characters.
Although nature is difficult to resist, yet man often strives against her power, and sometimes with success. From the facts to be given, it will also be seen that natural selection would powerfully affect many of our domestic productions if left unprotected. This is a point of much interest, for we thus learn that differences apparently of very slight importance would certainly determine the survival of a form when forced to struggle for its own existence. It may have occurred to some naturalists, as it formerly did to me, that, though selection acting under natural conditions would determine the structure of all important organs, yet that it could not affect characters which are esteemed by us of little importance; but this is an error to which we are eminently liable, from our ignorance of what characters are of real value to each living creature.
When man attempts to make a breed with some serious defect in structure, or in the mutual relation of the several parts, he will partly or completely fail, or encounter much difficulty; he is in fact resisted by a form of natural selection. We have seen that an attempt was once made in Yorkshire to breed cattle with enormous b.u.t.tocks, but the cows perished so often in bringing forth their calves, that the attempt had to be given up. In rearing short- faced tumblers, Mr. Eaton says (21/5. "Treatise on the Almond Tumbler" 1851 page 33.), "I am convinced that better head and beak birds have perished in the sh.e.l.l than ever were hatched; the reason being that the amazingly short- faced bird cannot reach and break the sh.e.l.l with its beak, and so perishes."
Here is a more curious case, in which natural selection comes into play only at long intervals of time: during ordinary seasons the Niata cattle can graze as well as others, but occasionally, as from 1827 to 1830 the plains of La Plata suffer from long-continued droughts and the pasture is burnt up; at such times common cattle and horses perish by the thousand, but many survive by browsing on twigs, reeds, etc.; this the Niata cattle cannot so well effect from their upturned jaws and the shape of their lips; consequently, if not attended to, they perish before the other cattle. In Columbia, according to Roulin, there is a breed of nearly hairless cattle, called Pelones; these succeed in their native hot district, but are found too tender for the Cordillera; in this case, however, natural selection determines only the range of the variety. It is obvious that a host of artificial races could never survive in a state of nature;--such as Italian greyhounds,--hairless and almost toothless Turkish dogs,--fantail pigeons, which cannot fly well against a strong wind,--barbs and Polish fowls, with their vision impeded by their eye wattles and great topknots,--hornless bulls and rams, which consequently cannot cope with other males, and thus have a poor chance of leaving offspring,--seedless plants, and many other such cases.
Colour is generally esteemed by the systematic naturalist as unimportant: let us, therefore, see how far it indirectly affects our domestic productions, and how far it would affect them if they were left exposed to the full force of natural selection. In a future chapter I shall have to show that const.i.tutional peculiarities of the strangest kind, entailing liability to the action of certain poisons, are correlated with the colour of the skin. I will here give a single case, on the high authority of Professor Wyman; he informs me that, being surprised at all the pigs in a part of Virginia being black, he made inquiries, and ascertained that these animals feed on the roots of the Lachnanthes tinctoria, which colours their bones pink, and, excepting in the case of the black varieties, causes the hoofs to drop off. Hence, as one of the squatters remarked, "we select the black members of the litter for raising, as they alone have a good chance of living." So that here we have artificial and natural selection working hand in hand. I may add that in the Tarentino the inhabitants keep black sheep alone, because the Hyperic.u.m crispum abounds there; and this plant does not injure black sheep, but kills the white ones in about a fortnight"s time. (21/6. Dr. Heusinger "Wochenschrift fur die Hei1kunde" Berlin 1846 s. 279.)
Complexion, and liability to certain diseases, are believed to run together in man and the lower animals. Thus white terriers suffer more than those of any other colour from the fatal distemper. (21/7. Youatt on the "Dog" page 232.) In North America plum-trees are liable to a disease which Downing (21/8. "The Fruit-trees of America" 1845 page 270: for peaches page 466.) believes is not caused by insects; the kinds bearing purple fruit are most affected, "and we have never known the green or yellow fruited varieties infected until the other sorts had first become filled with the knots." On the other hand, peaches in North America suffer much from a disease called the "yellows,"
which seems to be peculiar to that continent, and more than nine-tenths of the victims, "when the disease first appeared, were the yellow-fleshed peaches.
The white-fleshed kinds are much more rarely attacked; in some parts of the country never." In Mauritius, the white sugar-canes have of late years been so severely attacked by a disease, that many planters have been compelled to give up growing this variety (although fresh plants were imported from China for trial), and cultivate only red canes. (21/9. "Proc. Royal Soc. of Arts and Sciences of Mauritius" 1852 page 135.) Now, if these plants had been forced to struggle with other competing plants and enemies, there cannot be a doubt that the colour of the flesh or skin of the fruit, unimportant as these characters are considered, would have rigorously determined their existence.
Liability to the attacks of parasites is also connected with colour. White chickens are certainly more subject than dark-coloured chickens to the "gapes," which is caused by a parasitic worm in the trachea. (21/10.
"Gardener"s Chronicle" 1856 page 379.) On the other hand, experience has shown that in France the caterpillars which produce white coc.o.o.ns resist the deadly fungus better than those producing yellow coc.o.o.ns. (21/11. Quatref.a.ges "Maladies Actuelles du Ver a Soie" 1859 pages 12, 214.) a.n.a.logous facts have been observed with plants: a new and beautiful white onion, imported from France, though planted close to other kinds, was alone attacked by a parasitic fungus. (21/12. "Gardener"s Chronicle" 1851 page 595.) White verbenas are especially liable to mildew. (21/13. "Journal of Horticulture" 1862 page 476.) Near Malaga, during an early period of the vine-disease, the green sorts suffered most; "and red and black grapes, even when interwoven with the sick plants, suffered not at all." In France whole groups of varieties were comparatively free, and others, such as the Cha.s.selas, did not afford a single fortunate exception; but I do not know whether any correlation between colour and liability to disease was here observed. (21/14. "Gardener"s Chronicle"
1852 pages 435, 691.) In a former chapter it was shown how curiously liable one variety of the strawberry is to mildew.
It is certain that insects regulate in many cases the range and even the existence of the higher animals, whilst living under their natural conditions.
Under domestication light-coloured animals suffer most: in Thuringia (21/15.
Bechstein "Naturgesch. Deutschlands" 1801 b. 1 s. 310.) the inhabitants do not like grey, white, or pale cattle, because they are much more troubled by various kinds of flies than the brown, red, or black cattle. An Albino negro, it has been remarked (21/16. Prichard "Phys. Hist. of Mankind" 1851 volume 1 page 224.), was peculiarly sensitive to the bites of insects. In the West Indies (21/17. G. Lewis "Journal of Residence in West Indies" "Home and Col.
Library" page 100.) it is said that "the only horned cattle fit for work are those which have a good deal of black in them. The white are terribly tormented by the insects; and they are weak and sluggish in proportion to the white."
In Devonshire there is a prejudice against white pigs, because it is believed that the sun blisters them when turned out (21/18. Sidney"s edition of Youatt on the "Pig" page 24. I have given a.n.a.logous facts in the case of mankind in my "Descent of Man" 2nd edition page 195.); and I knew a man who would not keep white pigs in Kent, for the same reason. The scorching of flowers by the sun seems likewise to depend much on colour; thus, dark pelargoniums suffer most; and from various accounts it is clear that the cloth-of-gold variety will not withstand a degree of exposure to sunshine which other varieties enjoy. Another amateur a.s.serts that not only all dark-coloured verbenas, but likewise scarlets, suffer from the sun: "the paler kinds stand better, and pale blue is perhaps the best of all." So again with the heartsease (Viola tricolor); hot weather suits the blotched sorts, whilst it destroys the beautiful markings of some other kinds. (21/19. "Journal of Horticulture" 1862 pages 476, 498; 1865 page 460. With respect to the heartsease "Gardener"s Chronicle" 1863 page 628.) During one extremely cold season in Holland all red-flowered hyacinths were observed to be very inferior in quality. It is believed by many agriculturists that red wheat is hardier in northern climates than white wheat. (21/20. "Des Jacinthes, de leur Culture" 1768 page 53: on wheat "Gardener"s Chronicle" 1846 page 653.)
With animals, white varieties from being conspicuous are the most liable to be attacked by beasts and birds of prey. In parts of France and Germany where hawks abound, persons are advised not to keep white pigeons; for, as Parmentier says, "it is certain that in a flock the white always first fall victims to the kite." In Belgium, where so many societies have been established for the flight of carrier-pigeons, white is the one colour which for the same reason is disliked. (20/21. W.B. Tegetmeier "The Field" February 25, 1865. With respect to black fowls see a quotation in Thompson "Nat. Hist.
of Ireland" 1849 volume 1 page 22.) Prof. G. Jaeger (21/22. "In Sachen Darwin"s contra Wigand" 1874 page 70.) whilst fishing found four pigeons which had been killed by hawks, and all were white; on another occasion he examined the eyrie of a hawk, and the feathers of the pigeons which had been caught were all of a white or yellow colour. On the other hand, it is said that the sea-eagle (Falco ossifragus, Linn.) on the west coast of Ireland picks out the black fowls, so that "the villagers avoid as much as possible rearing birds of that colour." M. Daudin (20/23. "Bull. de la Soc. d"Acclimat." tome 7 1860 page 359.), speaking of white rabbits kept in warrens in Russia, remarks that their colour is a great disadvantage, as they are thus more exposed to attack, and can be seen during bright nights from a distance. A gentleman in Kent, who failed to stock his woods with a nearly white and hardy kind of rabbit, accounted in the same manner for their early disappearance. Any one who will watch a white cat prowling after her prey will soon perceive under what a disadvantage she lies.
The white Tartarian cherry, "owing either to its colour being so much like that of the leaves, or to the fruit always appearing from a distance unripe,"
is not so readily attacked by birds as other sorts. The yellow-fruited raspberry, which generally comes nearly true by seed, "is very little molested by birds, who evidently are not fond of it; so that nets may be dispensed with in places where nothing else will protect the red fruit." (21/24. "Transact.
Hort. Soc." volume 1 2nd series 1835 page 275. For raspberries see "Gardener"s Chronicle" 1855 page 154 and 1863 page 245.) This immunity, though a benefit to the gardener, would be a disadvantage in a state of nature both to the cherry and raspberry, as dissemination depends on birds. I noticed during several winters that some trees of the yellow-berried holly, which were raised from seed from a tree found wild by my father remained covered with fruit, whilst not a scarlet berry could be seen on the adjoining trees of the common kind. A friend informs me that a mountain-ash (Pyrus aucuparia) growing in his garden bears berries which, though not differently coloured, are always devoured by birds before those on the other trees. This variety of the mountain-ash would thus be more freely disseminated, and the yellow-berried variety of the holly less freely, than the common varieties of these two trees.
Independently of colour, trifling differences are sometimes found to be of importance to plants under cultivation, and would be of paramount importance if they had to fight their own battle and to struggle with many compet.i.tors.
The thin-sh.e.l.led peas, called pois sans parchemin, are attacked by birds (21/25. "Gardener"s Chronicle" 1843 page 806.) much more commonly than ordinary peas. On the other hand, the purple-podded pea, which has a hard sh.e.l.l, escaped the attacks of tomt.i.ts (Parus major) in my garden far better than any other kind. The thin-sh.e.l.led walnut likewise suffers greatly from the tomt.i.t. (21/26. Ibid 1850 page 732.) These same birds have been observed to pa.s.s over and thus favour the filbert, destroying only the other kinds of nuts which grew in the same orchard. (21/27. Ibid 1860 page 956.)
Certain varieties of the pear have soft bark, and these suffer severely from wood-boring beetles; whilst other varieties are known to resist their attacks much better. (21/28. J. De Jonghe in "Gardener"s Chronicle" 1860 page 120.) In North America the smoothness, or absence of down on the fruit, makes a great difference in the attacks of the weevil, "which is the uncompromising foe of all smooth stone-fruits;" and the cultivator "has the frequent mortification of seeing nearly all, or indeed often the whole crop, fall from the trees when half or two-thirds grown." Hence the nectarine suffers more than the peach. A particular variety of the Morello cherry, raised in North America, is, without any a.s.signable cause, more liable to be injured by this same insect than other cherry-trees. (21/29. Downing "Fruit-trees of North America" pages 266, 501: in regard to the cherry page 198.) From some unknown cause, certain varieties of the apple enjoy, as we have seen, the great advantage in various parts of the world of not being infested by the coccus. On the other hand, a particular case has been recorded in which aphides confined themselves to the Winter Nelis pear and touched no other kind in an extensive orchard. (21/30.
"Gardener"s Chronicle" 1849 page 755.) The existence of minute glands on the leaves of peaches, nectarines, and apricots, would not be esteemed by botanists as a character of the least importance for they are present or absent in closely-related sub-varieties, descended from the same parent-tree; yet there is good evidence (21/31. "Journal of Horticulture" September 26, 1865 page 254; see other references given in chapter 10.) that the absence of glands leads to mildew, which is highly injurious to these trees.
A difference either in flavour or in the amount of nutriment in certain varieties causes them to be more eagerly attacked by various enemies than other varieties of the same species. Bullfinches (Pyrrhula vulgaris) injure our fruit-trees by devouring the flower-buds, and a pair of these birds have been seen "to denude a large plum-tree in a couple of days of almost every bud;" but certain varieties (21/32. Mr. Selby in "Mag. of Zoology and Botany"
Edinburgh volume 2 1838 page 393.) of the apple and thorn (Crataegus oxyacantha) are more especially liable to be attacked. A striking instance of this was observed in Mr. Rivers"s garden, in which two rows of a particular variety of plum (21/33. The Reine Claude de Bavay "Journal of Horticulture"
December 27, 1864 page 511.) had to be carefully protected, as they were usually stripped of all their buds during the winter, whilst other sorts growing near them escaped. The root (or enlarged stem) of Laing"s Swedish turnip is preferred by hares, and therefore suffers more than other varieties.
Hares and rabbits eat down common rye before St. John"s-day-rye, when both grow together. (21/34. Mr. Pusey in "Journal of R. Agricult. Soc." volume 6 page 179. For Swedish turnips see "Gardener"s Chronicle" 1847 page 91.) In the south of France, when an orchard of almond-trees is formed, the nuts of the bitter variety are sown, "in order that they may not be devoured by field- mice" (21/35. G.o.dron "De l"Espece" tome 2 page 98.); so we see the use of the bitter principle in almonds.
Other slight differences, which would be thought quite unimportant, are no doubt sometimes of great service both to plants and animals. The Whitesmith"s gooseberry, as formerly stated, produces its leaves later than other varieties, and, as the flowers are thus left unprotected, the fruit often fails. In one variety of the cherry, according to Mr. Rivers (21/36.
"Gardener"s Chronicle" 1866 page 732.), the petals are much curled backwards, and in consequence of this the stigmas were observed to be killed by a severe frost; whilst at the same time, in another variety with incurved petals, the stigmas were not in the least injured. The straw of the Fenton wheat is remarkably unequal in height; and a competent observer believes that this variety is highly productive, partly because the ears from being distributed at various heights above the ground are less crowded together. The same observer maintains that in the upright varieties the divergent awns are serviceable by breaking the shocks when the ears are dashed together by the wind. (21/37. "Gardener"s Chronicle" 1862 pages 820, 821.) If several varieties of a plant are grown together, and the seed is indiscriminately harvested, it is clear that the hardier and more productive kinds will, by a sort of natural selection, gradually prevail over the others; this takes place, as Colonel Le Couteur believes (21/38. "On the Varieties of Wheat" page 59.), in our wheat-fields, for, as formerly shown, no variety is quite uniform in character. The same thing, as I am a.s.sured by nurserymen, would take place in our flower-gardens, if the seed of the different varieties were not separately saved. When the eggs of the wild and tame duck are hatched together, the young wild ducks almost invariably perish, from being of smaller size and not getting their fair share of food. (21/39. Mr. Hewitt and others, in "Journal of Hort." 1862 page 773.)
Facts in sufficient number have now been given showing that natural selection often checks, but occasionally favours, man"s power of selection. These facts teach us, in addition, a valuable lesson, namely, that we ought to be extremely cautious in judging what characters are of importance in a state of nature to animals and plants, which have to struggle for existence from the hour of their birth to that of their death,--their existence depending on conditions, about which we are profoundly ignorant.
CIRc.u.mSTANCES FAVOURABLE TO SELECTION BY MAN.
The possibility of selection rests on variability, and this, as we shall see in the following chapters, mainly depends on changed conditions of life, but is governed by infinitely complex and unknown laws. Domestication, even when long continued, occasionally causes but a small amount of variability, as in the case of the goose and turkey. The slight differences, however, which characterise each individual animal and plant would in most, probably in all, cases suffice for the production of distinct races through careful and prolonged selection. We see what selection, though acting on mere individual differences, can effect when families of cattle, sheep, pigeons, etc., of the same race, have been separately bred during a number of years by different men without any wish on their part to modify the breed. We see the same fact in the difference between hounds bred for hunting in different districts (21/40.
"Encyclop. of Rural Sports" page 405.), and in many other such cases.
In order that selection should produce any result, it is manifest that the crossing of distinct races must be prevented; hence facility in pairing, as with the pigeon, is highly favourable for the work; and difficulty in pairing, as with cats, prevents the formation of distinct breeds. On nearly the same principle the cattle of the small island of Jersey have been improved in their milking qualities "with a rapidity that could not have been obtained in a widely extended country like France." (21/41. Col. Le Couteur "Journal Roy.
Agricult. Soc." volume 4 page 43.) Although free crossing is a danger on the one side which every one can see, too close interbreeding is a hidden danger on the other side. Unfavourable conditions of life overrule the power of selection. Our improved heavy breeds of cattle and sheep could not have been formed on mountainous pastures; nor could dray-horses have been raised on a barren and inhospitable land, such as the Falkland Islands, where even the light horses of La Plata rapidly decrease in size. It seems impossible to preserve several English breeds of sheep in France; for as soon as the lambs are weaned their vigour decays as the heat of the summer increases (21/42.
Malingie-Nouel "Journal R. Agricult. Soc." volume 14 1853 pages 215, 217.): it would be impossible to give great length of wool to sheep within the tropics; yet selection has kept the Merino breed nearly true under diversified and unfavourable conditions. The power of selection is so great, that breeds of the dog, sheep, and poultry, of the largest and smallest size, long and short beaked pigeons, and other breeds with opposite characters, have had their characteristic qualities augmented, though treated in every way alike, being exposed to the same climate and fed on the same food. Selection, however, is either checked or favoured by the effects of use or habit. Our wonderfully- improved pigs could never have been formed if they had been forced to search for their own food; the English racehorse and greyhound could not have been improved up to their present high standard of excellence without constant training.
As conspicuous deviations of structure occur rarely, the improvement of each breed is generally the result of the selection of slight individual differences. Hence the closest attention, the sharpest powers of observation, and indomitable perseverance, are indispensable. It is, also, highly important that many individuals of the breed which is to be improved should be raised; for thus there will be a better chance of the appearance of variations in the right direction, and individuals varying in an unfavourable manner may be freely rejected or destroyed. But that a large number of individuals should be raised, it is necessary that the conditions of life should favour the propagation of the species. Had the peac.o.c.k been reared as easily as the fowl, we should probably ere this have had many distinct races. We see the importance of a large number of plants, from the fact of nursery gardeners almost always beating amateurs in the exhibition of new varieties. In 1845 it was estimated (21/43. Gardener"s Chronicle" 1845 page 273.) that between 4000 and 5000 pelargoniums were annually raised from seed in England, yet a decidedly improved variety is rarely obtained. At Messrs. Carter"s grounds, in Ess.e.x, where such flowers as the Lobelia, Nemophila, Mignonette, etc., are grown by the acre for seed, "scarcely a season pa.s.ses without some new kinds being raised, or some improvement effected on old kinds." (21/44. "Journal of Horticulture" 1862 page 157.) At Kew, as Mr. Beaton remarks, where many seedlings of common plants are raised, "you see new forms of Laburnums, Spiraeas, and other shrubs." (21/45. "Cottage Gardener" 1860 page 368.) So with animals: Marshall (21/46. "A Review of Reports" 1808 page 406.), in speaking of the sheep in one part of Yorkshire, remarks, "as they belong to poor people, and are mostly in small lots, they never can be improved." Lord Rivers, when asked how he succeeded in always having first-rate greyhounds, answered, "I breed many, and hang many." This, as another man remarks, "was the secret of his success; and the same will be found in exhibiting fowls,-- successful compet.i.tors breed largely, and keep the best." (21/47. "Gardener"s Chronicle" 1853 page 45.)
It follows from this that the capacity of breeding at an early age and at short intervals, as with pigeons, rabbits, etc., facilitates selection; for the result is thus soon made visible, and perseverance in the work encouraged.
It can hardly be an accident that the great majority of the culinary and agricultural plants which have yielded numerous races are annuals or biennials, which therefore are capable of rapid propagation, and thus of improvement. Sea-kale, asparagus, common and Jerusalem artichokes, potatoes, and onions, must be excepted, as they are perennials: but onions are propagated like annuals, and of the other plants just specified, none, with the exception of the potato, have yielded in this country more than one or two varieties. In the Mediterranean region, where artichokes are often raised from seed, there are several kinds, as I hear from Mr. Bentham. No doubt fruit- trees, which cannot be propagated quickly by seed, have yielded a host of varieties, though not permanent races; but these, judging from prehistoric remains, have been produced at a comparatively late period.