ON THE NATURE OF THE CHANGES IN THE CONDITIONS OF LIFE WHICH INDUCE VARIABILITY.
From a remote period to the present day, under climates and circumstances as different as it is possible to conceive, organic beings of all kinds, when domesticated or cultivated, have varied. We see this with the many domestic races of quadrupeds and birds belonging to different orders, with goldfish and silkworms, with plants of many kinds, raised in various quarters of the world. In the deserts of northern Africa the date-palm has yielded thirty-eight varieties; in the fertile plains of India it is notorious how many varieties of rice and of a host of other plants exist; in a single Polynesian island, twenty-four varieties of the bread-fruit, the same number of the banana, and twenty-two varieties of the arum, are cultivated by the natives; the mulberry- tree in India and Europe has yielded many varieties serving as food for the silkworm; and in China sixty-three varieties of the bamboo are used for various domestic purposes. (22/5. On the date-palm see Vogel 'Annals and Mag. of Nat. Hist.' 1854 page 460. On Indian varieties Dr. F. Hamilton 'Transact. Linn. Soc.' volume 14 page 296. On the varieties cultivated in Tahiti see Dr. Bennett in Loudon's 'Mag. of N. Hist.' volume 5 1832 page 484. Also Ellis 'Polynesian Researches' volume 1 pages 370, 375. On twenty varieties of the Pandanus and other trees in the Marianne Island see 'Hooker's Miscellany' volume 1 page 308. On the bamboo in China see Huc 'Chinese Empire' volume 2 page 307.) These facts, and innumerable others which could be added, indicate that a change of almost any kind in the conditions of life suffices to cause variability—different changes acting on different organisms.
Andrew Knight (22/6. 'Treatise on the Culture of the Apple' etc. page 3.) attributed the variation of both animals and plants to a more abundant supply of nourishment, or to a more favourable climate, than that natural to the species. A more genial climate, however, is far from necessary; the kidney- bean, which is often injured by our spring frosts, and peaches, which require the protection of a wall, have varied much in England, as has the orange-tree in northern Italy, where it is barely able to exist. (22/7. Gallesio 'Teoria della Riproduzione Veg.' page 125.) Nor can we overlook the fact, though not immediately connected with our present subject, that the plants and shells of the Arctic regions are eminently variable. (22/8. See Dr. Hooker's Memoir on Arctic Plants in 'Linn. Transact.' volume 23 part 2. Mr. Woodward, and a higher authority cannot be quoted, speaks of the Arctic mollusca in his 'Rudimentary Treatise' 1856 page 355 as remarkably subject to variation.) Moreover, it does not appear that a change of climate, whether more or less genial, is one of the most potent causes of variability; for in regard to plants Alph. De Candolle, in his 'Geographie Botanique' repeatedly shows that the native country of a plant, where in most cases it has been longest cultivated, is that where it has yielded the greatest number of varieties.
It is doubtful whether a change in the nature of the food is a potent cause of variability. Scarcely any domesticated animal has varied more than the pigeon or the fowl, but their food, especially that of highly-bred pigeons, is generally the same. Nor can our cattle and sheep have been subjected to any great change in this respect. But in all these cases the food probably is much less varied in kind than that which was consumed by the species in its natural state. (22/9. Bechstein in his 'Naturgeschichte der Stubenvogel' 1840 s. 238, has some good remarks on this subject. He states that his canary-birds varied in colour, though kept on uniform food.)
Of all the causes which induce variability, excess of food, whether or not changed in nature, is probably the most powerful. This view was held with regard to plants by Andrew Knight, and is now held by Schleiden, more especially in reference to the inorganic elements of the food. (22/10. 'The Plant' by Schleiden translated by Henfrey 1848 page 169. See also Alex. Braun in 'Bot. Memoirs' Ray Soc. 1853 page 313.) In order to give a plant more food it suffices in most cases to grow it separately, and thus prevent other plants robbing its roots. It is surprising, as I have often seen, how vigorously our common wild species flourish when planted by themselves, though not in highly manured land; separate growth is, in fact, the first step in cultivation. We see the converse of the belief that excess of food induces variability in the following statement by a great raiser of seeds of all kinds (22/11. Messrs. Hardy and Son of Maldon in 'Gardener's Chronicle' 1856 page 458. Carriere 'Production et Fixation des Varietes' 1865 page 31.): "It is a rule invariably with us, when we desire to keep a true stock of any one kind of seed, to grow it on poor land without dung; but when we grow for quantity, we act contrary, and sometimes have dearly to repent of it." According also to Carriere, who has had great experience with flower-garden seeds, "On remarque en general les plantes de vigeur moyenne sont celles qui conservent le mieux leurs caracteres."
In the case of animals the want of a proper amount of exercise, as Bechstein remarked, has perhaps played, independently of the direct effects of the disuse of any particular organ, an important part in causing variability. We can see in a vague manner that, when the organised and nutrient fluids of the body are not used during growth, or by the wear and tear of the tissues, they will be in excess; and as growth, nutrition, and reproduction are intimately allied processes, this superfluity might disturb the due and proper action of the reproductive organs, and consequently affect the character of the future offspring. But it may be argued that neither an excess of food nor a superfluity in the organised fluids of the body necessarily induces variability. The goose and the turkey have been well fed for many generations, yet have varied very little. Our fruit-trees and culinary plants, which are so variable, have been cultivated from an ancient period, and, though they probably still receive more nutriment than in their natural state, yet they must have received during many generations nearly the same amount; and it might be thought that they would have become habituated to the excess. Nevertheless, on the whole, Knight's view, that excess of food is one of the most potent causes of variability, appears, as far as I can judge, probable.
Whether or not our various cultivated plants have received nutriment in excess, all have been exposed to changes of various kinds. Fruit-trees are grafted on different stocks, and grown in various soils. The seeds of culinary and agricultural plants are carried from place to place; and during the last century the rotation of our crops and the manures used have been greatly changed.
Slight changes of treatment often suffice to induce variability. The simple fact of almost all our cultivated plants and domesticated animals having varied in all places and at all times, leads to this conclusion. Seeds taken from common English forest-trees, grown under their native climate, not highly manured or otherwise artificially treated, yield seedlings which vary much, as may be seen in every extensive seed-bed. I have shown in a former chapter what a number of well-marked and singular varieties the thorn (Crataegus oxycantha) has produced: yet this tree has been subjected to hardly any cultivation. In Staffordshire I carefully examined a large number of two British plants, namely Geranium phaeum and pyrenaicum, which have never been highly cultivated. These plants had spread spontaneously by seed from a common garden into an open plantation; and the seedlings varied in almost every single character, both in their flower and foliage, to a degree which I have never seen exceeded; yet they could not have been exposed to any great change in their conditions.
With respect to animals, Azara has remarked with much surprise (22/12. 'Quadrupedes du Paraguay' 1801 tome 2 page 319.) that, whilst the feral horses on the Pampas are always of one of three colours, and the cattle always of a uniform colour, yet these animals, when bred on the unenclosed estancias, though kept in a state which can hardly be called domesticated, and apparently exposed to almost identically the same conditions as when they are feral, nevertheless display a great diversity of colour. So again in India several species of fresh-water fish are only so far treated artificially, that they are reared in great tanks; but this small change is sufficient to induce much variability. (22/13. M'Clelland on Indian Cyprinidae 'Asiatic Researches' volume 19 part 2 1839 pages 266, 268, 313.)
Some facts on the effects of grafting, in regard to the variability of trees, deserve attention. Cabanis asserts that when certain pears are grafted on the quince, their seeds yield a greater number of varieties than do the seeds of the same variety of pear when grafted on the wild pear. (22/14. Quoted by Sageret 'Pom. Phys.' 1830 page 43. This statement, however, is not believed by Decaisne.) But as the pear and quince are distinct species, though so closely related that the one can be readily grafted and succeeds admirably on the other, the fact of variability being thus caused is not surprising; as we are here enabled to see the cause, namely, the very different nature of the stock and graft. Several North American varieties of the plum and peach are well known to reproduce themselves truly by seed; but Downing asserts (22/15. 'The Fruits of America' 1845 page 5.), "that when a graft is taken from one of these trees and placed upon another stock, this grafted tree is found to lose its singular property of producing the same variety by seed, and becomes like all other worked trees;"—that is, its seedlings become highly variable. Another case is worth giving: the Lalande variety of the walnut-tree leafs between April 20th and May 15th, and its seedlings invariably inherit the same habit; whilst several other varieties of the walnut leaf in June. Now, if seedlings are raised from the May-leafing Lalande variety, grafted on another May-leafing variety, though both stock and graft have the same early habit of leafing, yet the seedlings leaf at various times, even as late as the 5th of June. (22/16. M. Cardan in 'Comptes Rendus' December 1848 quoted in 'Gardener's Chronicle' 1849 page 101.) Such facts as these are well fitted to show on what obscure and slight causes variability depends.