Popular Science Monthly/Volume 66/January 1905/Some Experiments of Luther Burbank
|SOME EXPERIMENTS OF LUTHER BURBANK.|
By President DAVID STARR JORDAN,
LELAND STANFORD JUNIOR UNIVERSITY.
MR. LUTHER BURBANK, of Santa Rosa, California, is doubtless the most skilful experimenter in the field of the formation of new forms of plant life by the process of crossing and selection. He is the creator of many of our most useful plant forms: roots, nuts, fruits, grains and grasses, as well as of many of our most beautiful flowers. His methods are the practical application of the theories of Darwin and his followers, and to a degree wholly exceptional among plant breeders, Mr. Burbank has kept in touch with most modern work in the field of bionomics, and very much of his time and energy is devoted to experiments of scientific interest not likely of themselves to yield immediate practical results. In the nature of things, the demands of his work, and the necessity for the sale of new forms produced by him, have prevented the keeping of detailed records of his work, although steps have been taken toward the provision of explicit records in the future. For the rest, Mr. Burbank's success in practical achievement gives weight to his views on theoretical questions.The present writer has recently at different times spent three days in Mr. Burbank's company at his gardens at Santa Rosa and Sebastopol, keeping note of things seen and of Mr. Burbank's views concerning them. In this paper, selections are made from these notes, the forms of plants referred to having been examined by the writer and the report of explanations having been verified by Mr. Burbank. All quotation marks refer to conversations with Mr. Burbank, the statement of which Mr. Burbank has verified or corrected. The accompanying illustrations are all from photographs by Mr. Burbank.
The process of formation of new types may be grouped under four heads: selection, crossing, hybridization and mutation (or saltation). The process of artificial selection is used in all cases, those varying strains likely to prove useful being preserved, the others destroyed. The word 'crossing' may be advantageously used for the mingling of strains within a species, and 'hybridization' for the breeding together of members of different species. The name 'mutation' (or preferably 'saltation') is applied to sudden changes of characters for which no immediate cause is apparent.
Not many of Mr. Burbank's results are due to unassisted selection, as the processes of crossing and hybridization save time by the increase of the rate or degree of variation. There is, however, no evident limit to the results to be obtained by simple selection. New and permanent species of wheat have, without a shadow of doubt, been produced by selection alone.
In the California poppy (Eschscholtzia californica), stripes of crimson are never seen on the inside. Mr. Burbank once found a seedling in which the outside crimson had struck through like a crimson thread which had been misplaced. In other generations, by selection, this red was more and more increased, until finally out of it is developed a crimson poppy, of which Mr. Burbank has now many specimens, seeding more or less true to the type. The 'Shirley' poppy (Papaver rhæas) is well on the way to blue by selection.
It is questioned whether competition in minor details, or 'intraspecific selection' can form species permanent as wild species are. As to this, Mr. Burbank notes that the cultivated species produced after the fashion of his crimson eschscholtzia 'have a very brief history compared with the wild species, and, moreover, they are constantly being placed in a new environment by man, being influenced by new soils, new climates, new fertilizers and the like. "Breeding to a fixed line will bring fixed results. Man's desultory breeding is brief, the struggle for existence is mostly absent, and new ideals and new uses are required instead of ability to endure under natural conditions. Man's efforts at selective breeding are fluctuating, with frequent saltations."
Dr. De Vries notes that in the common sugar beet, which is a biennial species, there are from one to ten per cent, of plants which bear seed the first year. None of these is ever chosen for seed, and yet the long-continued process of natural selection has never succeeded in rooting them out. As to this Mr. Burbank observes: "This long-fixed tendency to insure continued existence in the past is not yet bred out. Analogous to this is the tendency in flocks to produce black sheep, and the appearance of zebra stripes on horses—ancestral traits not yet bred out."
From the pale yellow Iceland poppy (Papaver nudicaule) are developed white, yellow and orange forms, and some with striped petals and a strong tendency to become double. Selecting the Iceland poppy for size alone, flowers three and one-half inches across have been developed. A large scarlet poppy, Papaver glaucum, closes its two inner petals when a bee or two have entered, shutting in the bees, which buzz angrily and cover themselves with pollen until they are set free. If not visited by bees, the flowers do not close.
A wild form of one of the Liliacæ, Brodiæa terrestris, was made white by selection of the palest among the pale wild ones. Brodiæa lactea taken from the high Sierras where it is a dwarf, becomes, after two years of cultivation, more than twice as high as the original stock, but not nearly as high as the same species grown in the valley.
"Crossing is done to secure a wealth of variation. By this means we get the species into a state of perturbation or 'wabble,' and take advantage of the 'wabbling' to guide the life forces into the desired habits or channels. The first crossing is generally a step in the direction in which we are going, but repeated crossing is often necessary and judicious selection always necessary to secure valuable practical results. Crossing may give the best or the worst qualities of the parent, or any other qualities; and previous crossings often affect the results."
"Hybridization differs from ordinary crossing only in degree. A species is only a race which has assumed greater fixity. The purposes and results of crossing within the species and of hybridization of different species are essentially alike. The formation of the new individual by the sexual relation of two parents is in itself a species of crossing, giving each new individual in its degree new traits or new combinations."
"Bees and other insects, as well as the wind, cross plants, but they do not work intelligently, therefore rarely to any advantage economically to man. No mechanic could invent such devices as those which tend to prevent self-crossing in plants. All evolution and improvement are dependent on crossing, therefore nature has produced more wonderful devices for this purpose than for any other."
"Mutations, or saltations, are often found; that is, fixed forms springing up, generally from unknown causes, forms which are not hybrids, and which remain constant; as, for instance, colored flowers which yield white forms, these yielding white constantly in their progeny. These mutations can be produced at will by any of the various means which disturb the habits of the plant. It comes out when the conditions are ripe. New conditions bring out latent traits. I should expect mutations to arise in the American primrose and most other plants under wholly new conditions. Extra food or growth force as well as crossing favors variation, as does abrupt change of conditions of any kind. Five or six generations will usually fix a mutation. Sometimes it is fixed at once."
"On the average, perhaps about six generations fairly fix a variation, but this varies greatly, depending upon previously acquired hereditary tendencies. Bringing a species into a new environment disturbs
its fixity. Rich soil especially gives rise to variations in growth which seem to be new, and by repetition become inherently fixed. Sometimes ancestral states are brought about by good soil; sometimes (perhaps oftener), also, by starvation; new variations oftenest by rich soil and general prosperity. There is no evidence of any limit in the production of variation through artificial selection, especially if preceded by crossing. Mutations are probably due to the sudden appearance of latent tendencies in new combinations, producing novel effects analogous to new chemical combinations."
"Mutation is not a period, but a state induced by various hereditary and external conditions. It is not by any means certain that there is any period in the life-history of the species when it is more subject to mutation than at other times, other conditions being similar. By crossing different species we can form more variations and mutations in half-a-dozen generations than will be developed by ordinary variation in a hundred or even a thousand generations."
"The La France and some other roses, as well as dahlias, callas and many other plants, every once in a while throw out, on some particular year, a number of unusual sports in various localities. This is probably a matter of season, the forces outside bringing about parallel mutations. The evolution of species is largely dependent on crossing the variations contained within it. Forms too closely bred soon run out, because generally only by crossing does variation appear. It is of great advantage to have the parents a certain distance apart in their hereditary tendencies. If too close together there is not range enough of variety. If too far apart, the developed forms are unfitted for existence because too unstable. Correlated changes work together to produce the effect of mutations. Environment effects a permanent change in species by selection of those which fit it or by producing changes in individuals which are better equipped to survive. Heredity is the sum of all past environment, conditions both latent and apparent. Latent traits often arise when circumstances make them possible. Environment of a lifetime does not necessarily or usually appear in another lifetime, but continues in the same direction and will strike into the nature of the plant in time. We may refer to Emerson's remark on the 'baking into the picture of the pigment laid down by environment.' Selection is 'cumulative environment.' Fortuitous variations occur everywhere. They come up all the time, from past environments, past heredity and present opportunity. No two individuals are alike. Where there is a marked tendency in one direction, we have the case of a persistent effect of environment. Monstrosities are engorgements of force. They are generally a thousand times more likely to. develop another sort of monstrosity than normal individuals are. You are likely to get from sports and monstrosities either extreme of variance. They do not, however, maintain themselves, because heredity pulls back their descendants. A wide variance is more easily pulled back than a slight variance. There are cases where the monstrosity might pull back its species. This is more likely to happen if the forces of natural or artificial selection were in its favor. There are many cases where the variant in minor points is prepotent and outweighs the original stock. Monstrosities produced by crossing often perpetuate themselves as well as the species does."
"One difficulty with the mutation theory of Dr. De Vries, in my opinion, is lack of sufficiently wide experimentation. Fuller investigations will certainly show that the 'sports' or 'chance' variations come under the same law as that of 'fluctuating' variations, mutations being only fluctuating variations carried beyond the critical point where past fluctuating variations can not withstand the accumulated forces without disintegration, thus bending them in a new direction."
"Professor Hubrecht is certainly in error in stating that the mean fluctuations can not be carried into the extreme or 'sport' variations by selection. Professor Hubrecht speaks of two divergent processes, 'fluctuating variations' and 'mutations' which he says: 'Darwin has not sufficiently kept separate.' They are not separate; one is only a tendency toward the other, and which continued, though latent, may, or will, at last become dominant, so as to swing the fluctuating variations fully out of the old orbit into the 'mutation' or 'sport' condition. Radical changes of environment for a series of generations will produce a tendency to sport, but hybridization will bring it about far more abruptly, and for practical plant or animal breeding or for scientific study of all these variations, far more satisfactorily."
" The misunderstanding evidently comes from not having a clear knowledge of latent and dominant hereditary forces. A knowledge of these explains the whole matter and makes harmony between Darwin and Wallace, leaving Professor De Vries's careful experiments good, but coming to different conclusions on the results."
"Professor Hubrecht also states that 'now for the first time—forty years after the appearance of the 'Origin of Species'—the actual birth of a species has been observed by him.' As I have produced several good species by hybridization, as good as nature herself has produced, and as others have done the same by selection alone, the above sentence is hardly true. But as before stated, hybridization followed by selection is the shortest plan by which valid new species can be produced. In other words, the 'period of mutation' can he produced at will!"
"The mutation theory of the origin of species seems like a step backward towards the special creation theory, and without any facts as yet adequate to support it as a universal theory, however valuable and suggestive the experiments of Dr. De Vries may be."
"There is a remarkably close analogy between hybridization and grafting. Bringing. over from France a prunus (P. mirobolana var. pissadi), of which there was no other specimen in America, it was grafted on to the Kelsey plum, a variety of Prunus triflorus. The graft itself did not bloom, but the presence of the graft brought about in the tree a cross of the two species. This is the only case known to me in which the graft affected the reproductive system of the plant, forming a cross between forms which had never crossed. Many hundred descendants of this cross are now living. Darwin accepted with reservations the account of the graft hybrids in potatoes, and there still remains some doubt of reliable testimony of the supposed fact. He also speaks of a now well-known graft hybrid of a yellow and purple cytisus, which is perhaps the most remarkable fact in this line on record."
Diagram showing the Zone of Life and Parallelism of Results in Crossing and Grafting. (L. B.)
Utter refusal to unite under any circumstances, either by crossing or grafting. (Outside of zone of possible union.)
|Pollen acts as a poison.||Grafts blight and die as if poisoned.|
|Union partial, mosaic or temporary; seed rarely produced; seedlings generally inherit tendencies and qualities of one parent only; second or later generations revert fully.||Grafts often form a temporary union but are not in a normal condition. Avoided by nurserymen and planters with great care, as results are often disastrous to the grower.|
|Union free; seedlings show unbalanced condition, varying widely; often unusually vigorous; best condition for scientific or natural selection. Good qualities can be made permanent to the race.||Mendelian
|Grafts unite readily but separate under unusual stress—drought, overbearing, lack of nourishment, etc. Avoided by nurserymen and planters.|
|Unite freely; seed of superior germinating quality produced abundantly. Seedlings normal with ordinary amount of variability.||Unusual
|Grafts unite readily, thriving well; sometimes better than when grafted on their own stock.|
|Ordinary plant life as oftenest met with.||Normal.||Grafts unite and thrive as we oftenest see them.|
|Self-fertile; seed produced, but as there are very limited opportunities for profitable variations, this state ultimately ends in||Grafts grow on their own roots.|
All these varying states shade off from one to the other, with few hard and fast lines of separation.
"In some directions the strains of heredity are much more unbalanced than in others. An impulse from outside forces may bring about new combinations. This is illustrated by De Yries by a ball with many facets, which, if lightly touched, will return to its original position, if vigorously touched will turn over. Burbank once crossed a pole bean (Phaseolus vulgaris) with a lima bean (Phaseolus lunatus var. macro-carpus). There was no visible effect in the appearance of the pod or the bean, but, when planted, each bean developed a cotyledon, part of one species and part of the other. The lima bean represented the end of the cotyledon, and was united to the lower part by serrated edges; below was the smaller and striped cotyledon of the pole bean. The cotyledons finally parted at the joints between the two, the upper portion falling off, as is often the case with grafts which are uncongenial. The forms were tremendously vigorous, but all came back to the common pole or horticultural bean after the second generation, as though it were an uncongenial graft hybrid, the alien portion being finally entirely rejected. It often happens in grafting, that the branch will be united thoroughly at the point of grafting, but in great stress,. as the overbearing of fruit, the grafted portion will separate and entirely fall off."
"In one sense, hybridization is only a mode of grafting, both being a more or less permanent combination. The different results from hybridization are shown in the diagram below."
"Where the plants are very different, having a different line of descent, and consequently different structure, there will be no hybridization at all. From this we have every gradation to the point where the individuals are very closely alike, and here we have scarcely any variation at all in the progeny, a condition which favors extinction. Again, in grafting, we have every intergradation between total inability to unite and absolutely perfect blend."
"Sometimes a graft strengthens a plant by increasing the body of foliage and thus strengthening the roots. Grafting a Japanese pear on the Bartlett pear will give the latter new life through the increase in the foliage, which gives material for root action and further extension."
As illustrations of the results of crossing and hybridization, the following notes were taken on plants in Mr. Burbank's gardens:
In the beginning of his work Mr. Burbank crossed all sorts of beans and had a half acre of them. Some climbed to the height of twenty or thirty feet, producing all sorts of pods—some with pods long and slender and stems so short that the pods doubled up on the ground. These forms could have been fixed in time, though the variations were unusually persistent and very amazing in their variety and abundance.
Crossing the red and white pole bean, two or three of the beans grew large and bore striped pods, the beans themselves being jet black. From this cross many varieties were developed having all the colors known in beans.
The results of selection are often so simple as to form a mathematical rule, as in the case of Mendel's peas, which holds good with the tribe of peas (Pisum), but not generally with others so far experimented on. At other times they are so complicated that to follow them requires the highest skill, or may be utterly impossible.
A rubus (R. cratægifolius) from Siberia has fruit the size of a large half pea, brownish, seedy and tasteless. Hybridizing with the California blackberry (R. vitifolius), some of the hybrids have the best qualities of both berries combined, and a perfect balance of characters. Out of over five thousand second generation hybrid seedlings, every one is true to the seed. This refers to the Primus blackberry, which is now fully as true a species as any classified species of Rubus.
The raspberry has been hybridized with a strawberry: the results were thornless plants with trifoliate leaves looking like a strawberry plant and sending out underground stolons like the strawberry. At last, however, the plants send up canes three to five feet high bearing panicles of flowers more profuse in number than those on either parent. After flowering the plant never produces a berry, the fruit forming a small knob, with no effort at maturity.
In the hybrid of the strawberry and raspberry, the resultant plants bore three or four times as many flowers as the raspberry, seven or eight times as many as the strawberry.
Tendencies strong in the parent, even though for a time latent,
usually come out strong in the descendants. Ordinary hybrids of forms closely related generally form a perfect blend from both parents. When the parents are far apart all sorts of variations occur, the so called Mendelian condition being one phase of the results.
Hybridizing the iceberg white blackberry with the Cuthbert raspberry develops a plant with foliage and growth midway. About half the plants bear fruit which is red like the raspberry, about half bear fruit which is white like the iceberg blackberry; the quality is midway between the blackberry and the raspberry. In the crossed fruit (first generation) the flavor is not superior, but it is quite intermediate between blackberry and raspberry. The form of the receptacle is intermediate. Some of the fruitlets separate at the base, but not above. In crossing it makes no difference which sex is taken as the male parent; it all depends upon the hereditary tendencies of the sex.
Crosses of wild species yield results similar to those from cultivated species, hut the latter are more available. The white blackberry is a wild variation crossed with the Lawton for size and vigor; the result is a
much clearer white than the wild one, larger, and very much more productive, in these respects fully equal to its staminate parent, the Lawton.
Apples brought up from the south temperate zone are entirely confused here, yielding leaves, buds, flowers and small apples at various seasons. One of these apples in time, however, became adapted to the conditions and developed into one of the best apples in Mendocino County.
"Animals or plants changed by transference from one country to another never quite go back to the old conditions, even if placed in them again, as hereditary tendencies acquired under the new environments, even though latent for many generations may be called forth again under favoring conditions. Exceptions seem to be as important as the rules in this work. Nature leaves so many loopholes that there is almost no rule without exceptions. She does not tie herself up to any unvarying conditions. Adaptability is more important than perseverance."
A blackberry plant with an immense mass of fruit developed from a seedling from the Himalayas. One plant covers 150 square feet, is 8 feet high, and has a bushel or more of fruit. This is only a young, small plant; when full grown this variety is many times larger.
A purple larkspur reared by Mr. Burbank is produced by crossing a native blue with a native scarlet, the color being entirely a blend. The blackberry was crossed with apples and with all the various rosaceous plants. Over five thousand plants were produced. The apple-blackberry cross came out essentially apples in foliage and growth, though raised from blackberry seeds. Only two of them ever bloomed, all were thornless, one of them bearing rose-colored flowers. From the mountain ash and blackberry a salmon-colored fruit with no thorns and no albumen in the seed was developed. A hybrid between the English and the black walnut grows fully four times as fast as the English walnut; it bears little fruit. The seedlings from the fruit produce some English, some black, and some hybrid walnuts, and not rarely entirely new forms. Crossing often brings about great vegetative life at the expense of reproductive life, or the reverse. The young (second generation) hybrids of the black walnut and the English walnut show very great variation in their leaves, resembling neither parent. The hybrids of the English and California black walnuts are most rapidly growing trees and unusually productive. The first hybrid, of the English with the Japanese walnut, Juglans sieboldi, is largely like the Japanese in the nuts, but rather more like the English in foliage, the second generation being very variable as usual.
By crossing types already crossed, we may often bring out the original stock which had been lost in cultivation. The English walnut has usually five leaflets, the black walnut fifteen to nineteen. The first
generation hybrid has eleven, with a fragrance to the leaves that no original walnut has. This tendency or trait is just as real as any other. The American walnut (Juglans nigra) and the California black walnut (J. californica) are closely related species and when hybridized yield fruit of very large size and in enormous quantities.
Descendants of hybrids usually revert to either one or the other parent or break up in all directions. A cross of the eastern black walnut (Juglans nigra) with the California nut (J. californica) yields a hybrid which is a very great grower. From the seed of this tree a surprising variety of mutations are developed, not only resembling
Some time since, a hybrid mesembryanthemum was developed, and lasted for four years, forming an attractive plant with a profusion of white flowers. Then all individuals, whereever located, died at once, doubtless because conditions were adverse: but there was no visible cause of soil, of insect pest, of fungus or of climate. These plants all died from the root up. A hybrid of petunia and nicotiana has abundance of flowers and a large vigorous leaves, but the roots are inadequate. A hybrid red poppy is formed by uniting the opium poppy with the oriental poppy. These hybrids bloom every day of the year, while the blooming season of either parent is only a few weeks, but they yield no seed. The seed capsules are developed in great variety, some of them four to six times as large as the capsules of either parent. Others are scarcely thicker than the stem which hears them, while others are absolutely and completely absent. This hybrid poppy is tall and generally branches like the opium poppy. It is perennial, although its pistillate or seed ancestor is a short-lived annual. This red poppy can even be divided at the root and multiplied like the perennial oriental poppy. These hybrids have generally a dark mark at the base of the scarlet petals as in the oriental poppy; in some the leaves are smoothish and glaucous, as in the opium poppy; in most, deep green and hairy, more as in the other. Many flowers have their stems coalescent with that of the neighboring flower.
"These second generation hybrid poppy plants unexpectedly all proved to be perennials, and are now making a tremendous growth; the clusters of foliage of some of them are fourteen to eighteen inches across already. Among this second generation hybrid lot of poppies each single plant seems to be different from every other plant in the lot and strange to say the leaves now resemble not only poppy leaves, but celandine, various thistles, primroses, turnips, mustards and numerous other plants are very closely imitated, showing most astounding variations."
The striped amaryllis, vittata, hybridized with a Mexican species, formosissima, has narrow twisted petals of a very deep scarlet and nearly plain. The leaves are much narrower than in the vittata, the stalks more slender, and the plants more profuse bloomers.Hybridizing crinum with amaryllis develops a plant with a fine
Leaves from Hybrid Poppies, showing unusual variations in foliage even for second generation hybrids. Blossoms vary about as much as the leaves—the habits of the plants also. These are an average random selection from about two thousand second generation seedlings.
Capsules of Second Generation Hybrid Poppies, showing series of variations from complete absence of capsules to capsules of unusual size and to double capsules of unusual size. These selected at random from about 2,000 plants. The individual plants which produce these types generally follow them in all the capsules.
flower but no seeds. Crossing the small hardy white calla with a yellow one which is not hardy, develops, with selection, a hardy yellow calla.
A crinum from Florida is hardy but not handsome. Crossing this with a handsome crinum from Mexico, the plants were selected for those which should be both hardy and handsome. The desired qualities of the two species have been combined and other valuable new qualities incidentally developed as regeneration and selection proceeded.
In hybridizing callas, the yellow ones with the white, to form a hardy yellow race, some of the resultant plants have pale flowers, some light yellow, and those chosen are made deep yellow by selection from second and later generations. Both parent plants in this case have leaves blotched with white, and this is found in all the descendants.
Hybridizing the wild flower, Erysimum arkansanum, which is yellow, with a native wild white species, resulted in the first generation a perfect blend of yellow and white; with a second generation the species separate completely, about five per cent, of those examined being yellow, the other ninety-five per cent, white; white dominant. With a hybrid Thalictrum, seed pods are developed more abundantly than with either parent, but the seeds are not viable.
We may expect variations in form, size, color, quality, fragrance, vigor or any other characteristic. To get variation in any one direction is to open the door to anything else. Hybridizing the Japanese quince with the common quince, we have large-leaved seedlings which look quite different from the parent (common quince). The final result is a seedling looking like the Japanese quince, without the power of continued growth (too wide a cross to blend permanently or profitably).
Some of the black raspberries when hybridized with some of the blackberries usually die when the time comes to bear fruit. Many hybrids perish under the stress of reproduction. The Amaryllis vittata is now eight to eleven inches across, being nearly four times as broad as before the work of selection for size was begun, and with vigor and freedom of growth and bloom amazingly increased. On a strip of poor land it grows very small, with narrow leaves and slender flowers, but on the same poor land some of the hybrid variants grow very large and pay no attention to the soil. A variant of Ampelopsis quinquefolia has very large leaves, highly colored in the fall, but no fruit. Mimulus tigrinus of Europe has very many variations. Its flowers are yellow, with patches of orange and other colors. When crossed with some oij our native species, the seedlings are greatly improved in all respects, even in blooming, yet rarely produce seeds.It is generally much easier to develop variations in seedlings from variegated flowers than from those of solid color (the variegation shows a lack of complete amalgamation). A double mimulus is formed of the hose-en-hose sort. One hybrid poppy produces an abortive flower
inside the capsule. All seedlings always vary more or less. With the same parent, one fruit may be two and one half or more times the diameter of the other, of a different color, flavor or differing in almost all respects. "There is no prepotency of male or female as such. Prepotency depends wholly on heredity. We can not rely on the stoneless types being prepotent, but a certain number of trees producing stoneless fruit usually come from crossing them with those having stones. The prepotency to produce a stone, or a half stone, having been more thoroughly fixed by ages of stone-producing trees, will perhaps be about ninety-nine times out of one hundred. But other things being equal, there is absolutely no balance in favor of either sex. This may be set down as fixed."
With plum-almond crosses there is every kind of variation in the flowers. Some have all stamens, some have many petals or none, some never open, and some have pistils only.
The Climax plum is a cross of the bitter, flat, tomato-shaped Chinese plum, Prunus simoni, and the Japanese plum, Prunus triflora. The Chinese plum produces almost no pollen; hardly a grain of it is known, not more than one could put in his eye without feeling it; but the whole fruit shipping industry of the world has been changed by this hybrid plum (Climax) produced by it. With many crosses of many things it is certain that forms of great importance will come out every year, though never in profusion.
In developing a spineless cactus for stock-feeding, selections were made from the three hardy northern species, Opuntia rafinesquii, O, mesacantha and O. vulgaris; these were crossed with O. tuna, O. ficusindica and with a Small opuntia from Central America, almost thornless.
The cactus has smooth cotyledons, but the first bud is covered with thorns. These thorns have also been eliminated by selecting the smoothest individual seedlings without crossing. Crossing in this case generally interrupts the process, as it brings out well-fixed ancestral traits, but later, to combine the best qualities of several species, crossing and selection must be resorted to. Examples seen were shoots of the original stock, prickly; the second generation, slightly prickly; the third, without thorns; and later the spicules even within the substance of the cactus have been removed so as to make the cactus very excellent food for cattle. This will have very great value in the arid regions. Some cacti lose the thorns on the plant but retain them on the fruit; others vice versa. By crossing and extensive and intensive selection a cactus may be improved in various ways besides being deprived of thorns and of the internal spicules in six or less generations; these, by means of cuttings, may be multiplied rapidly to any extent, but the process, to be complete, generally takes longer. This thornless cactus should prove of very great value in the development of desert regions as Arizona or Sonora, as the quantity of food produced per acre is enormous.
The Bartlett plum has the flavor of a Bartlett pear, but even more strongly developed. The 'rice seed' plum has extremely small seeds. The stoneless plum is a cross of the French prune with a wild plum having the stone almost eliminated by a fortuitous variation. The result thus far is a great number of stoneless plums of good size, but in flavor inferior to the best cultivated ones. These are being crossed again to improve the flavor, and new selections made.Crossing the Japan and the New England chestnut (Castanea
japonica and C. americana), the trees, leaves, growth and nuts are midway; second generation and later generations as usual show more varied combinations and variations. To breed the burrs off from chestnuts is dangerous, because it allows the birds to get in at the nuts. The
burr is originally intended to keep off the birds. In developing a superior variety of the Persian (often called English) walnut (Juglans regia), the shell was made too thin, so that the birds could break in. It was necessary to make new selections and crossings to thicken the shell and still retain its other superior qualities.
The Pierce grape was a bud sport from the Isabella, producing much larger fruit. This bud sport remains constant. All the seedlings even from it are similar to the Pierce grape, following the bud sport (Pierce) and not reverting to the real parent form of the Isabella. Some ripen early, some late; some are pale, and some are black; but all resemble the Pierce more than the Isabella. Cultivating a choke cherry, the seeds all from one parent tree, many variations are found, although the soil in which they are placed is uniform. Among them was found one variant less bitter than usual; others earlier or later ripening and with larger or smaller fruit or leaves, and an almost bewildering number and variety of other variations. A peach-almond cross often develops a tree as large as ten peach trees or almond trees of the same age. Sometimes a similar cross with different individuals of the same species will produce opposite or totally different results, owing to past heredity, either recent or far back. Crosses are sometimes more vigorous than either parent and more than any descendant, but other cases are just the reverse. The more variant crosses are often less vigorous, and sometimes yield seedlings that can not exist. Sometimes all die in the fruiting season. A peach named 'Quality' is one of the best peaches extant—a cross of the Muir and the Crawford. A cross of the nectrine and peach also produces variant types of value. In some hybrids of petunia and tobacco, the roots fail while the tops may be of unusual vigor. These individuals can only be kept alive for any length of time by grafting, another instance—if other were needed—of the parallelism of crossing and grafting.
"A character may be latent through many generations or centuries, appearing when the right cross brings it out; or it may appear under specially favorable or peculiar conditions of growth."
According to Burbank, "the facts of plant life demand a kinetic theory of evolution, a slight change from Huxley's statement that 'matter is a magazine of force,' to that of matter being force alone. The time will come when the theory of ions will be thrown aside and no line left between force and matter. We can not get the right perspective in science unless we go beyond our senses. A dead material universe moved by outside forces is in itself highly improbable, but a universe of force alone is probable, but requires great effort to make it conceivable, because we must conceive it in the terms of our sense experience."