In the case of the rabbit, which has become feral on the island of Porto Santo, we are at first strongly tempted to attribute the whole change--the greatly reduced size, the altered tints of the fur, and the loss of certain characteristic marks--to the definite action of the new conditions to which it has been exposed. But in all such cases we have to consider in addition the tendency to reversion to progenitors more or less remote, and the natural selection of the finest shades of difference.
The nature of the food sometimes either definitely induces certain peculiarities, or stands in some close relation with them. Pallas long ago a.s.serted that the fat-tailed sheep of Siberia degenerate and lose their enormous tails when removed from certain saline pastures; and recently Erman (23/39. "Travels in Siberia" English translation volume 1 page 228.) states that this occurs with the Kirgisian sheep when brought to Orenburgh.
It is well known that hemp-seed causes bullfinches and certain other birds to become black. Mr. Wallace has communicated to me some much more remarkable facts of the same nature. The natives of the Amazonian region feed the common green parrot (Chrysotis festiva, Linn.) with the fat of large Siluroid fishes, and the birds thus treated become beautifully variegated with red and yellow feathers. In the Malayan archipelago, the natives of Gilolo alter in an a.n.a.logous manner the colours of another parrot, namely, the Lorius garrulus, Linn., and thus produce the Lori rajah or King-Lory. These parrots in the Malay Islands and South America, when fed by the natives on natural vegetable food, such as rice and plaintains, retain their proper colours. Mr. Wallace has, also, recorded (23/40. A.R. Wallace "Travels on the Amazon and Rio Negro"
page 294.) a still more singular fact. "The Indians (of S. America) have a curious art by which they change the colours of the feathers of many birds.
They pluck out those from the part they wish to paint, and inoculate the fresh wound with the milky secretion from the skin of a small toad. The feathers grow of a brilliant yellow colour, and on being plucked out, it is said, grow again of the same colour without any fresh operation."
Bechstein (23/41. "Naturgeschichte der Stubenvogel" 1840 s. 262, 308.) does not entertain any doubt that seclusion from light affects, at least temporarily, the colours of cage-birds.
It is well known that the sh.e.l.ls of land-mollusca are affected by the abundance of lime in different districts. Isidore Geoffroy Saint-Hilaire (23/42. "Hist. Nat Gen." tome 3 page 402.) gives the case of Helix lactea, which has recently been carried from Spain to the South of France and to the Rio Plata, and in both countries now presents a distinct appearance, but whether this has resulted from food or climate is not known. With respect to the common oyster, Mr. F. Buckland informs me that he can generally distinguish the sh.e.l.ls from different districts; young oysters brought from Wales and laid down in beds where "natives" are indigenous, in the short s.p.a.ce of two months begin to a.s.sume the "native" character. M. Costa (23/43. "Bull.
de La Soc. Imp. d"Acclimat." tome 8 page 351.) has recorded a much more remarkable case of the same nature, namely, that young sh.e.l.ls taken from the sh.o.r.es of England and placed in the Mediterranean, at once altered their manner of growth and formed prominent diverging rays, like those on the sh.e.l.ls of the proper Mediterranean oyster. The same individual sh.e.l.l, showing both forms of growth, was exhibited before a society in Paris. Lastly, it is well known that caterpillars fed on different food sometimes either themselves acquire a different colour or produce moths differing in colour. (23/44. See an account of Mr. Gregson"s experiments on the Abraxus grossulariata "Proc.
Entomolog. Soc." January 6, 1862: these experiments have been confirmed by Mr.
Greening in "Proc. of the Northern Entomolog. Soc." July 28, 1862. For the effects of food on caterpillars see a curious account by M. Michely in "Bull.
De La Soc. Imp. d"Acclimat." tome 8 page 563. For a.n.a.logous facts from Dahlbom on Hymenoptera see Westwood "Modern Cla.s.s. of Insects" volume 2 page 98. See also Dr. L. Moller "Die Abhangigkeit der Insecten" 1867 s. 70.)
It would be travelling beyond my proper limits here to discuss how far organic beings in a state of nature are definitely modified by changed conditions. In my "Origin of Species" I have given a brief abstract of the facts bearing on this point, and have shown the influence of light on the colours of birds, and of residence near the sea on the lurid tints of insects, and on the succulency of plants. Mr. Herbert Spencer (23/45. "The Principles of Biology" volume 2 1866. The present chapters were written before I had read Mr. Herbert Spencer"s work, so that I have not been able to make so much use of it as I should otherwise probably have done.) has recently discussed with much ability this whole subject on general grounds. He argues, for instance, that with all animals the external and internal tissues are differently acted on by the surrounding conditions, and they invariably differ in intimate structure. So again the upper and lower surfaces of true leaves, as well as of stems and petioles, when these a.s.sume the function and occupy the position of leaves, are differently circ.u.mstanced with respect to light, etc., and apparently in consequence differ in structure. But, as Mr. Herbert Spencer admits, it is most difficult in all such cases to distinguish between the effects of the definite action of physical conditions and the acc.u.mulation through natural selection of inherited variations which are serviceable to the organism, and which have arisen independently of the definite action of these conditions.]
Although we are not here concerned with the definite action of the conditions of life on organisms in a state of nature, I may state that much evidence has been gained during the last few years on this subject. In the United States, for instance, it has been clearly proved, more especially by Mr. J.A. Allen, that, with birds, many species differ in tint, size of body and of beak, and in length of tail, in proceeding from the North to the South; and it appears that these differences must be attributed to the direct action of temperature.
(23/46. Professor Weismann comes to the same conclusion with respect to certain European b.u.t.terflies in his valuable essay "Ueber den Saison- Dimorphismus" 1875. I might also refer to the recent works of several other authors on the present subject; for instance to Kerner"s "Gute und schlechte Arten" 1866.) With respect to plants I will give a somewhat a.n.a.logous case: Mr. Meehan (23/47. "Proc. Acad. Nat. Soc. of Philadelphia" January 28, 1862.), has compared twenty-nine kinds of American trees with their nearest European allies, all grown in close proximity and under as nearly as possible the same conditions. In the American species he finds, with the rarest exceptions, that the leaves fall earlier in the season, and a.s.sume before their fall a brighter tint; that they are less deeply toothed or serrated; that the buds are smaller; that the trees are more diffuse in growth and have fewer branchlets; and, lastly, that the seeds are smaller--all in comparison with the corresponding European species. Now considering that these corresponding trees belong to several distinct orders, and that they are adapted to widely different stations, it can hardly be supposed that their differences are of any special service to them in the New and Old worlds; and if so such differences cannot have been gained through natural selection, and must be attributed to the long continued action of a different climate.
GALLS.
Another cla.s.s of facts, not relating to cultivated plants, deserves attention.
I allude to the production of galls. Every one knows the curious, bright-red, hairy productions on the wild rose-tree, and the various different galls produced by the oak. Some of the latter resemble fruit, with one face as rosy as the rosiest apple. These bright colours can be of no service either to the gall-forming insect or to the tree, and probably are the direct result of the action of the light, in the same manner as the apples of Nova Scotia or Canada are brighter coloured than English apples. According to Osten Sacken"s latest revision, no less than fifty-eight kinds of galls are produced on the several species of oak, by Cynips with its sub-genera; and Mr. B.D. Walsh (23/48. See Mr. B.D. Walsh"s excellent papers in "Proc. Entomolog. Soc. Philadelphia"
December 1866 page 284. With respect to the willow see ibid 1864 page 546.) states that he can add many others to the list. One American species of willow, the Salix humilis, bears ten distinct kinds of galls. The leaves which spring from the galls of various English willows differ completely in shape from the natural leaves. The young shoots of junipers and firs, when punctured by certain insects, yield monstrous growths resembling flowers and fir-cones; and the flowers of some plants become from the same cause wholly changed in appearance. Galls are produced in every quarter of the world; of several sent to me by Mr. Thwaites from Ceylon, some were as symmetrical as a composite flower when in bud, others smooth and spherical like a berry; some protected by long spines, others clothed with yellow wool formed of long cellular hairs, others with regularly tufted hairs. In some galls the internal structure is simple, but in others it is highly complex; thus M. Lacaze-Duthiers (23/49.
See his admirable "Histoire des Galles" in "Annal. des Sc. Nat. Bot." 3rd series tome 19 1853 page 273.) has figured in the common ink-gall no less than seven concentric layers, composed of distinct tissue, namely, the epidermic, sub-epidermic, spongy, intermediate, and the hard protective layer formed of curiously thickened woody cells, and, lastly, the central ma.s.s, abounding with starch-granules on which the larvae feed.
Galls are produced by insects of various orders, but the greater number by species of Cynips. It is impossible to read M. Lacaze-Duthiers" discussion and doubt that the poisonous secretion of the insect causes the growth of the gall; and every one knows how virulent is the poison secreted by wasps and bees, which belong to the same group with Cynips. Galls grow with extraordinary rapidity, and it is said that they attain their full size in a few days (23/50. Kirby and Spence "Entomology" 1818 volume 1 page 450; Lacaze- Duthiers ibid page 284.); it is certain that they are almost completely developed before the larvae are hatched. Considering that many gall-insects are extremely small, the drop of secreted poison must be excessively minute; it probably acts on one or two cells alone, which, being abnormally stimulated, rapidly increase by a process of self-division. Galls, as Mr.
Walsh (23/51. "Proc. Entomolog. Soc. Philadelphia" 1864 page 558.) remarks, afford good, constant, and definite characters, each kind keeping as true to form as does any independent organic being. This fact becomes still more remarkable when we hear that, for instance, seven out of the ten different kinds of galls produced on Salix humilis are formed by gall-gnats (Cecidomyidae) which "though essentially distinct species, yet resemble one another so closely that in almost all cases it is difficult, and in most cases impossible, to distinguish the full-grown insects one from the other." (23/52.
Mr. B.D. Walsh ibid page 633 and December 1866 page 275.) For in accordance with a wide-spread a.n.a.logy we may safely infer that the poison secreted by insects so closely allied would not differ much in nature; yet this slight difference is sufficient to induce widely different results. In some few cases the same species of gall-gnat produces on distinct species of willows galls which cannot be distinguished; the Cynips fecundatrix, also, has been known to produce on the Turkish oak, to which it is not properly attached, exactly the same kind of gall as on the European oak. (23/53. Mr. B.D. Walsh ibid 1864 pages 545, 411, 495; and December 1866 page 278. See also Lacaze-Duthiers.) These latter facts apparently prove that the nature of the poison is a more powerful agent in determining the form of the gall than the specific character of the tree which is acted on.
As the poisonous secretion of insects belonging to various orders has the special power of affecting the growth of various plants; as a slight difference in the nature of the poison suffices to produce widely different results; and lastly, as we know that the chemical compounds secreted by plants are eminently liable to be modified by changed conditions of life, we may believe it possible that various parts of a plant might be modified through the agency of its own altered secretions. Compare, for instance, the mossy and viscid calyx of a moss-rose, which suddenly appears through bud-variation on a Provence-rose, with the gall of red moss growing from the inoculated leaf of a wild rose, with each filament symmetrically branched like a microscopical spruce-fir, bearing a glandular tip and secreting odoriferous gummy matter.
(23/54. Lacaze-Duthiers ibid pages 325, 328.) Or compare, on the one hand, the fruit of the peach, with its hairy skin, fleshy covering, hard sh.e.l.l and kernel, and on the other hand one of the more complex galls with its epidermic, spongy, and woody layers, surrounding tissue loaded with starch granules. These normal and abnormal structures manifestly present a certain degree of resemblance. Or, again, reflect on the cases above given of parrots which have had their plumage brightly decorated through some change in their blood, caused by having been fed on certain fishes, or locally inoculated with the poison of a toad. I am far from wishing to maintain that the moss-rose or the hard sh.e.l.l of the peach-stone or the bright colours of birds are actually due to any chemical change in the sap or blood; but these cases of galls and of parrots are excellently adapted to show us how powerfully and singularly external agencies may affect structure. With such facts before us, we need feel no surprise at the appearance of any modification in any organic being.
[I may, also, here allude to the remarkable effects which parasitic fungi sometimes produce on plants. Reissek (23/55. "Linnaea" volume 17 1843; quoted by Dr. M.T. Masters, Royal Inst.i.tution, March 16, 1860.) has described a Thesium, affected by an Oecidium, which was greatly modified, and a.s.sumed some of the characteristic features of certain allied species, or even genera.
Suppose, says Reissek, "the condition originally caused by the fungus to become constant in the course of time, the plant would, if found growing wild, be considered as a distinct species or even as belonging to a new genus." I quote this remark to show how profoundly, yet in how natural a manner, this plant must have been modified by the parasitic fungus. Mr. Meehan (23/56.
"Proc. Acad. Nat. Sc., Philadelphia" June 16, 1874 and July 23, 1875.) also states that three species of Euphorbia and Portulaca olereacea, which naturally grow prostrate, become erect when they are attacked by the Oecidium.
Euphorbia maculata in this case also becomes nodose, with the branchlets comparatively smooth and the leaves modified in shape, approaching in these respects to a distinct species, namely, the E. hypericifolia.]
FACTS AND CONSIDERATIONS OPPOSED TO THE BELIEF THAT THE CONDITIONS OF LIFE ACT IN A POTENT MANNER IN CAUSING DEFINITE MODIFICATIONS OF STRUCTURE.
I have alluded to the slight differences in species naturally living in distinct countries under different conditions; and such differences we feel at first inclined to attribute, probably often with justice, to the definite action of the surrounding conditions. But it must be borne in mind that there exist many animals and plants which range widely and have been exposed to great diversities of climate, yet remain uniform in character. Some authors, as previously remarked, account for the varieties of our culinary and agricultural plants by the definite action of the conditions to which they have been exposed in the different parts of Great Britain; but there are about 200 plants (23/57. Hewett C. Watson "Cybele Britannica" volume 1 1847 page 11.) which are found in every single English county; and these plants must have been exposed for an immense period to considerable differences of climate and soil, yet do not differ. So, again,, some animals and plants range over a large portion of the world, yet retain the same character.
[Notwithstanding the facts previously given on the occurrence of highly peculiar local diseases and on the strange modifications of structure in plants caused by the inoculated poison of insects, and other a.n.a.logous cases; still there are a mult.i.tude of variations--such as the modified skull of the niata ox and bulldog, the long horns of Caffre cattle, the conjoined toes of the solid-hoofed swine, the immense crest and protuberant skull of Polish fowls, the crop of the pouter-pigeon, and a host of other such cases--which we can hardly attribute to the definite action, in the sense before specified, of the external conditions of life. No doubt in every case there must have been some exciting cause; but as we see innumerable individuals exposed to nearly the same conditions, and one alone is affected, we may conclude that the const.i.tution of the individual is of far higher importance than the conditions to which it has been exposed. It seems, indeed, to be a general rule that conspicuous variations occur rarely, and in one individual alone out of millions, though all may have been exposed, as far as we can judge, to nearly the same conditions. As the most strongly marked variations graduate insensibly into the most trifling, we are led by the same train of thought to attribute each slight variation much more to innate differences of const.i.tution, however caused, than to the definite action of the surrounding conditions.
We are led to the same conclusion by considering the cases, formerly alluded to, of fowls and pigeons, which have varied and will no doubt go on varying in directly opposite ways, though kept during many generations under nearly the same conditions. Some, for instance, are born with their beaks, wings, tails, legs, etc., a little longer, and others with these same parts a little shorter. By the long-continued selection of such slight individual differences which occur in birds kept in the same aviary, widely different races could certainly be formed; and long-continued selection, important as is the result, does nothing but preserve the variations which arise, as it appears to us, spontaneously.
In these cases we see that domesticated animals vary in an indefinite number of particulars, though treated as uniformly as is possible. On the other hand, there are instances of animals and plants, which, though they have been exposed to very different conditions, both under nature and domestication, have varied in nearly the same manner. Mr. Layard informs me that he has observed amongst the Caffres of South Africa a dog singularly like an arctic Esquimaux dog. Pigeons in India present nearly the same wide diversities of colour as in Europe; and I have seen chequered and simply barred pigeons, and pigeons with blue and white loins, from Sierra Leone, Madeira, England, and India. New varieties of flowers are continually raised in different parts of Great Britain, but many of these are found by the judges at our exhibitions to be almost identical with old varieties. A vast number of new fruit-trees and culinary vegetables have been produced in North America: these differ from European varieties in the same general manner as the several varieties raised in Europe differ from one another; and no one has ever pretended that the climate of America has given to the many American varieties any general character by which they can be recognised. Nevertheless, from the facts previously advanced on the authority of Mr. Meehan with respect to American and European forest-trees it would be rash to affirm that varieties raised in the two countries would not in the course of ages a.s.sume a distinctive character. Dr. M. Masters has recorded a striking fact (23/58. "Gardener"s Chronicle" 1857 page 629.) bearing on this subject: he raised numerous plants of Hybiscus syriacus from seed collected in South Carolina and the Holy Land, where the parent-plants must have been exposed to considerably different conditions; yet the seedlings from both localities broke into two similar strains, one with obtuse leaves and purple or crimson flowers, and the other with elongated leaves and more or less pink flowers.
We may, also, infer the prepotent influence of the const.i.tution of the organism over the definite action of the conditions of life, from the several cases given in the earlier chapters of parallel series of varieties,--an important subject, hereafter to be more fully discussed. Sub-varieties of the several kinds of wheat, gourds, peaches, and other plants, and to a limited extent sub-varieties of the fowl, pigeon, and dog, have been shown either to resemble or to differ from one another in a closely corresponding or parallel manner. In other cases, a variety of one species resembles a distinct species; or the varieties of two distinct species resemble one another. Although these parallel resemblances no doubt often result from reversion to the former characters of a common progenitor; yet in other cases, when new characters first appear, the resemblance must be attributed to the inheritance of a similar const.i.tution, and consequently to a tendency to vary in the same manner. We see something of a similar kind in the same monstrosity appearing and reappearing many times in the same species of animal, and, as Dr. Maxwell Masters has remarked to me, in the same species of plant.]
We may at least conclude, that the amount of modification which animals and plants have undergone under domestication does not correspond with the degree to which they have been subjected to changed circ.u.mstances. As we know the parentage of domesticated birds far better than of most quadrupeds, we will glance through the list. The pigeon has varied in Europe more than almost any other bird; yet it is a native species, and has not been exposed to any extraordinary change of conditions. The fowl has varied equally, or almost equally, with the pigeon, and is a native of the hot jungles of India. Neither the peac.o.c.k, a native of the same country, nor the guinea-fowl, an inhabitant of the dry deserts of Africa, has varied at all, or only in colour. The turkey, from Mexico, has varied but little. The duck, on the other hand, a native of Europe, has yielded some well-marked races; and as this is an aquatic bird, it must have been subjected to a far more serious change in its habits than the pigeon or even the fowl, which nevertheless have varied in a much higher degree. The goose, a native of Europe and aquatic like the duck, has varied less than any other domesticated bird, except the peac.o.c.k.
Bud-variation is, also, important under our present point of view, in some few cases, as when all the eyes on the same tuber of the potato, or all the fruit on the same plum-tree, or all the flowers on the same plant, have suddenly varied in the same manner, it might be argued that the variation had been definitely caused by some change in the conditions to which the plants had been exposed; yet, in other cases, such an admission is extremely difficult.
As new characters sometimes appear by bud-variation, which do not occur in the parent-species or in any allied species, we may reject, at least in these cases, the idea that they are due to reversion. Now it is well worth while to reflect maturely on some striking case of bud-variation, for instance that of the peach. This tree has been cultivated by the million in various parts of the world, has been treated differently, grown on its own roots and grafted on various stocks, planted as a standard, trained against a wall, or under gla.s.s; yet each bud of each sub-variety keeps true to its kind. But occasionally, at long intervals of time, a tree in England, or under the widely different climate of Virginia, produces a single bud, and this yields a branch which ever afterwards bears nectarines. Nectarines differ, as every one knows, from peaches in their smoothness, size, and flavour; and the difference is so great that some botanists have maintained that they are specifically distinct. So permanent are the characters thus suddenly acquired, that a nectarine produced by bud-variation has propagated itself by seed. To guard against the supposition that there is some fundamental distinction between bud and seminal variation, it is well to bear in mind that nectarines have likewise been produced from the stone of the peach; and, reversely, peaches from the stone of the nectarine. Now is it possible to conceive external conditions more closely alike than those to which the buds on the same tree are exposed? Yet one bud alone, out of the many thousands borne by the same tree, has suddenly, without any apparent cause, produced a nectarine. But the case is even stronger than this, for the same flower-bud has yielded a fruit, one-half or one-quarter a nectarine, and the other half or three-quarters a peach. Again, seven or eight varieties of the peach have yielded by bud-variation nectarines: the nectarines thus produced, no doubt, differ a little from one another; but still they are nectarines. Of course there must be some cause, internal or external, to excite the peach-bud to change its nature; but I cannot imagine a cla.s.s of facts better adapted to force on our minds the conviction that what we call the external conditions of life are in many cases quite insignificant in relation to any particular variation, in comparison with the organisation or const.i.tution of the being which varies.
It is known from the labours of Geoffroy Saint-Hilaire, and recently from those of Dareste and others, that eggs of the fowl, if shaken, placed upright, perforated, covered in part with varnish, etc., produce monstrous chickens.
Now these monstrosities may be said to be directly caused by such unnatural conditions, but the modifications thus induced are not of a definite nature.
An excellent observer, M. Camille Dareste (23/59. "Memoire sur la Production Artificielle des Monstruosites" 1862 pages 8-12; "Recherches sur les Conditions, etc., chez les Monstres" 1863 page 6. An abstract is given of Geoffroy"s Experiments by his son, in his "Vie, Travaux" etc. 1847 page 290.), remarks "that the various species of monstrosities are not determined by specific causes; the external agencies which modify the development of the embryo act solely in causing a perturbation--a perversion in the normal course of development." He compares the result to what we see in illness: a sudden chill, for instance, affects one individual alone out of many, causing either a cold, or sore-throat, rheumatism, or inflammation of the lungs or pleura.
Contagious matter acts in an a.n.a.logous manner. (23/60. Paget "Lectures on Surgical Pathology" 1853 volume 1 page 483.) We may take a still more specific instance: seven pigeons were struck by rattle-snakes (23/61. "Researches upon the Venom of the Rattle-snake" January 1861 by Dr. Mitch.e.l.l page 67.): some suffered from convulsions; some had their blood coagulated, in others it was perfectly fluid; some showed ecchymosed spots on the heart, others on the intestines, etc.; others again showed no visible lesion in any organ. It is well known that excess in drinking causes different diseases in different men; but in the tropics the effects of intemperance differ from those caused in a cold climate (23/62. Mr. Sedgwick "British and Foreign Medico-Chirurg. Review"
July 1863 page 175.); and in this case we see the definite influence of opposite conditions. The foregoing facts apparently give us as good an idea as we are likely for a long time to obtain, how in many cases external conditions act directly, though not definitely, in causing modifications of structure.
SUMMARY.
There can be no doubt, from the facts given in this chapter, that extremely slight changes in the conditions of life sometimes, probably often, act in a definite manner on our domesticated productions; and, as the action of changed conditions in causing indefinite variability is acc.u.mulative, so it may be with their definite action. Hence considerable and definite modifications of structure probably follow from altered conditions acting during a long series of generations. In some few instances a marked effect has been produced quickly on all, or nearly all, the individuals which have been exposed to a marked change of climate, food, or other circ.u.mstance. This has occurred with European men in the United States, with European dogs in India, with horses in the Falkland Islands, apparently with various animals at Angora, with foreign oysters in the Mediterranean, and with maize transported from one climate to another. We have seen that the chemical compounds of some plants and the state of their tissues are readily affected by changed conditions. A relation apparently exists between certain characters and certain conditions, so that if the latter be changed the character is lost--as with the colours of flowers, the state of some culinary plants, the fruit of the melon, the tail of fat-tailed sheep, and the peculiar fleeces of other sheep.
The production of galls, and the change of plumage in parrots when fed on peculiar food or when inoculated by the poison of a toad, prove to us what great and mysterious changes in structure and colour, may be the definite result of chemical changes in the nutrient fluids or tissues.
We now almost certainly know that organic beings in a state of nature may be modified in various definite ways by the conditions to which they have been long exposed, as in the case of the birds and other animals in the northern and southern United States, and of American trees in comparison with their representatives in Europe. But in many cases it is most difficult to distinguish between the definite result of changed conditions, and the acc.u.mulation through natural selection of indefinite variations which have proved serviceable. If it profited a plant to inhabit a humid instead of an arid station, a fitting change in its const.i.tution might possibly result from the direct action of the environment, though we have no grounds for believing that variations of the right kind would occur more frequently with plants inhabiting a station a little more humid than usual, than with other plants.
Whether the station was unusually dry or humid, variations adapting the plant in a slight degree for directly opposite habits of life would occasionally arise, as we have good reason to believe from what we actually see in other cases.
The organisation or const.i.tution of the being which is acted on, is generally a much more important element than the nature of the changed conditions, in determining the nature of the variation. We have evidence of this in the appearance of nearly similar modifications under different conditions, and of different modifications under apparently nearly the same conditions. We have still better evidence of this in closely parallel varieties being frequently produced from distinct races, or even distinct species; and in the frequent recurrence of the same monstrosity in the same species. We have also seen that the degree to which domesticated birds have varied, does not stand in any close relation with the amount of change to which they have been subjected.
To recur once again to bud-variations. When we reflect on the millions of buds which many trees have produced, before some one bud has varied, we are lost in wonder as to what the precise cause of each variation can be. Let us recall the case given by Andrew Knight of the forty-year-old tree of the yellow magnum bonum plum, an old variety which has been propagated by grafts on various stocks for a very long period throughout Europe and North America, and on which a single bud suddenly produced the red magnum bonum. We should also bear in mind that distinct varieties, and even distinct species,--as in the case of peaches, nectarines, and apricots,--of certain roses and camellias,-- although separated by a vast number of generations from any progenitor in common, and although cultivated under diversified conditions, have yielded by bud-variation closely a.n.a.logous varieties. When we reflect on these facts we become deeply impressed with the conviction that in such cases the nature of the variation depends but little on the conditions to which the plant has been exposed, and not in any especial manner on its individual character, but much more on the inherited nature or const.i.tution of the whole group of allied beings to which the plant in question belongs. We are thus driven to conclude that in most cases the conditions of life play a subordinate part in causing any particular modification; like that which a spark plays, when a ma.s.s of combustibles bursts into flame--the nature of the flame depending on the combustible matter, and not on the spark. (23/63. Professor Weismann argues strongly in favour of this view in his "Saison-Dimorphismus der Schmetterlinge" 1875 pages 40-43.)
No doubt each slight variation must have its efficient cause; but it is as hopeless an attempt to discover the cause of each, as to say why a chill or a poison affects one man differently from another. Even with modifications resulting from the definite action of the conditions of life, when all or nearly all the individuals, which have been similarly exposed, are similarly affected, we can rarely see the precise relation between cause and effect. In the next chapter it will be shown that the increased use or disuse of various organs produces an inherited effect. It will further be seen that certain variations are bound together by correlation as well as by other laws. Beyond this we cannot at present explain either the causes or nature of the variability of organic beings.
CHAPTER 2.XXIV.
LAWS OF VARIATION--USE AND DISUSE, ETC.
NISUS FORMATIVUS, OR THE CO-ORDINATING POWER OF THE ORGANISATION.
ON THE EFFECTS OF THE INCREASED USE AND DISUSE OF ORGANS.
CHANGED HABITS OF LIFE.
ACCLIMATISATION WITH ANIMALS AND PLANTS.
VARIOUS METHODS BY WHICH THIS CAN BE EFFECTED.
ARRESTS OF DEVELOPMENT.
RUDIMENTARY ORGANS.
In this and the two following chapters I shall discuss, as well as the difficulty of the subject permits, the several laws which govern Variability.
These may be grouped under the effects of use and disuse, including changed habits and acclimatisation--arrest of development--correlated variation--the cohesion of h.o.m.ologous parts-the variability of multiple parts--compensation of growth--the position of buds with respect to the axis of the plant--and lastly, a.n.a.logous variation. These several subjects so graduate into one another that their distinction is often arbitrary.
It may be convenient first briefly to discuss that coordinating and reparative power which is common, in a higher or lower degree, to all organic beings, and which was formerly designated by physiologists as nisus formativus.
[Blumenbach and others (24/1. "An Essay on Generation" English translation page 18; Paget "Lectures on Surgical Pathology" 1853 volume 1 page 209.) have insisted that the principle which permits a Hydra, when cut into fragments, to develop itself into two or more perfect animals, is the same with that which causes a wound in the higher animals to heal by a cicatrice. Such cases as that of the Hydra are evidently a.n.a.logous to the spontaneous division or fissiparous generation of the lowest animals, and likewise to the budding of plants. Between these extreme cases and that of a mere cicatrice we have every gradation. Spallanzani (24/2. "An Essay on Animal Reproduction" English translation 1769 page 79.) by cutting off the legs and tail of a Salamander, got in the course of three months six crops of these members; so that 687 perfect bones were reproduced by one animal during one season. At whatever point the limb was cut off, the deficient part, and no more, was exactly reproduced. When a diseased bone has been removed, a new one sometimes "gradually a.s.sumes the regular form, and all the attachments of muscles, ligaments, etc., become as complete as before." (24/3. Carpenter "Principles of Comp. Physiology" 1854 page 479.)
This power of regrowth does not, however, always act perfectly; the reproduced tail of a lizard differs in the form of the scales from the normal tail: with certain Orthopterous insects the large hind legs are reproduced of smaller size (24/4. Charlesworth "Mag. of Nat. Hist." volume 1 1837 page 145.): the white cicatrice which in the higher animals unites the edges of a deep wound is not formed of perfect skin, for elastic tissue is not produced till long afterwards. (24/5. Paget "Lectures on Surgical Pathology" volume 1 page 239.) "The activity of the nisus formativus," says Blumenbach, "is in an inverse ratio to the age of the organised body." Its power is also greater with animals, the lower they stand in the scale of organisation; and animals low in the scale correspond with the embryos of higher animals belonging to the same cla.s.s. Newport"s observations (24/6. Quoted by Carpenter "Comp. Phys." page 479.) afford a good ill.u.s.tration of this fact, for he found that "myriapods, whose highest development scarcely carries them beyond the larva of perfect insects, can regenerate limbs and antennae up to the time of their last moult;" and so can the larvae of true insects, but, except in one order, not in the mature insect. Salamanders correspond in development with the tadpoles or larvae of the tailless Batrachians, and both possess to a large extent the power of regrowth; but not so the mature tailless Batrachians.