Memoirs, Horticultural Society of New York (1902)
Hybridism vs. Selection
F. W. Burbidge
Curator, Trinity College Botanical Gardens, Dublin, Ireland
Hybrids are of two descriptions, those produced naturally or spontaneousy in the wilds, and those raised artificially in the garden but there is no real distinction between them. We are told that a hybrid is the offspring of two species, one or both of which at times may be either pollen or seed parent. But the day of a rigid belief in "pure species" of plants is past. and to say any plant is a species, simply means the expression of one's own. or somebody else's, judgment or opinion. A species is merely some botanists decision, and not nature's decision, for how can nature be decided. seeing that evolution is continually going on? A species often includes an enormous number of individual plants varying more or less among themselves. and which come more or less true from seed. A species is, in fact, often a very variable quantity, and its capacity for variation is absolutely unknown, except as it is experimented upon in the garden or elsewhere. Our ignorance of the natural history of plants is profound. Two so-called species grow in the same soil and situation, and belong to the same natural group or order, and yet while one is a useful food plant. the other is a virulent poison to men or other animals.
Again, two plants called species, may grow on the Andes or Himalayas side by side, and yet, when brought to American or European gardens, the one may he quite hardy, while the other dies unless sheltered in an artificial temperature. Why plants thus vary in their secretions or products, and in hardihood, no one knows; that they do so is a fact patent to the most ordinary observer, and these problems await solution from the biologists of the future.
Again, two related species will, when hybridized together, sometimes produce fertile offspring, and in other cases barren ones. Sometimes Species A will fertilize species B, but species B will not fertilize species A, but why, no one knows. In other cases, two or more species will be reciprocally fertile, but why this is so, neither physicist nor physiologist can say, any more than they can tell us why one plant secretes or makes sugar and another starch, and others wine and oil—nutritious food—healing medicines or deadly poisons. These so far are the secrets of nature's great laboratory.
But let us come to the hybrid. The whole history of hybrids is obscure, and in many cases the so-called records are most unreliable. In the case of so-called spontaneous or wild hybrids what has happened is this: The arm-chair botanist, knowing nothing of the circumstances of their origin or native environment, has simply named and described them as pure species! Now and then, as in the case of some orchids, a guess as to their parentage has been made by collectors abroad and even rarely by botanists at borne, and we have instances where orchids and other wild hybrids have been made over again by fertilizing the parent species in our hothouses here at home. Nevertheless there are thousands of wild hybrids lying obscured under Latin specific names in our books and herbaria throughout Europe and America to-day. As to garden hybrids, in the beginning of the past century it was thought impious to molest nature, and the early botanists and students plant hybridism were pretty much in the position of the surgical vivisectionists who, rightly or wrongly, experiment on living animals to-day. In a word, they worked in secret and scarcely wished or dared to tell the truth! It is curious to observe that while physical unions such as marching or inoculation, budding and grafting, were looked upon as quite respectable and clever, the physiological unions by cross pollination were universally tabooed, and in Northern Europe more especially. I say in Northern Europe, because in South Europe, North Africa and the East, the necessity for fertilizing the fig tree and the dioecious date palm artificially had been carried out from very early times.
|*See Gardeners' Chronicle, 1890, July 26, page 103.|
In English gardens hybrids have been reared designedly for a period of well night two hundred years. The first of garden hybrids recorded, in England was Fairchild's Mule Pink, said to have been raised at Hoxton near London, before 1719, between Dianthus caryophyllus, and D. barbatus; that is to say, between the Carnation and the Sweet William. This and many other early hybrids were called "mules" from an erroneous belief that like the horse and ass hybrid so called, all vegetable hybrids were likewise sterile. The early history of garden hybrids has been obscured by the secretive character of the early experimenters and the jealousy they felt of each other.* Also by a more or less superstitious fear of revealing or recording what at the time was regarded as an irreligious or sacrilegious interference with nature. There were also later on trade jealousies, and hybrids were either said to have come from abroad, whence their parents had come before them, or their origin was disguised and concealed under specific Latin names. One remarkable instance of these latter tactics being adopted on a large scale occurred when Messrs. Rollison, of Tooting, and other growers of Cape heaths, at a time when they were nearly as popular as orchids and begonias are to-day, reared numerous hybrids and seedlings all of which were credited to the Cape of Good Hope and duly christened with Latin names.
Another potent source of error as to garden hybrids is due to the fact that, fertilization having been effected by wind or insects, the seedlings that varied were assumed to have been hybrids. In a word, the seeming intermediates were assigned the most probable or obvious parentage without any real proof.
This brings me to the point of this paper, viz.: That the parentage of an enormous quantity of hybrids depends on mere "guesses at truth" and not on any accurate records whatever. It is difficult to estimate the dire results of this practice as a source of error, because intermediates are often produced in gardens by ordinary seminal variation, and without any hybridizing operation whatever.
We are apt to attribute too much to hybridism as a motive power in producing variations, and even in the blending of characters among cultivated plants .
Before we can be sure of what hybridism effects, we must know exactly how far the parent species themselves can vary as self-pollinated. It is self-evident that some species which so far as we know have never been hybridized can and do vary infinitely as cultivated. In a word, cultivation and the inter-crossing of varieties yield results at times almost, even if not quite, as great as does hybridism. The Chinese Primula, Cyclamen latifolium, the Gloxinia or Sinningia, many root crops and cultivated vegetables, which so far as we know have never been hybridized, yet vary as much as those plants which have been so originated. We have only to look at the immense variations in apples, pears and other domestic fruits in order to recognize the great central fact that cultivation—the crossing of seminal varieties-and human selection are quite as potent as, or even more so than, hybridism alone. Of course, hybridism as confined to so-called species and the cross breeding of varieties differ only in degree, both being sexual and physiological processes. Many of our type species even as wild plants are extremely variable from seed, just as many seminal garden varieties come practically true from seed.
As a matter of fact the distinction between species and varieties is an arbitrary one, but it is for the present a convenience to keep up the nominal distinction. Some day it will he recognized universally that garden species artificially reared are quite as distinct botanically and often a great deal more useful than the native or wild ones. Hybridism often, it is true, gives us a splendid starting place—a spring board, or a new eid of variation as it were, but that field must be further improved by cross breeding and selection or the highest and best of practical results are lost, or in any case not actually realized.
We must clearly grasp the fact that the three great factors in the making of plant products more useful or suitable to our daily wants are cultivation, the cross breeding of varieties, and a careful selection of the most suitable or desirable seedling kinds. Even cultivation and selection alone from wild plants, as in the carrot and parsnip of Vilmorin's and Buckland's experiments, will work wonderful transformations in only a few generations.
The fourth factor, viz., hybridism, is potent in the origination of new races, as illustrated in the Tuberous rooted Begonias, the large flowered Cannas, the Gladiolus, Marliac's colored Water Lilies, and many other things; but the initial gain still depends on the other three factors for its full development
I doubt very much whether the newly discovered 'Mendel's law" will be of much practical service to the ordinary hybridist, or whether it will lead to a more precise and exact system of working among hybridists or breeders in the future. In conclusion, I may put forth the following suggestions to those who live in this country, fertile as it is in experimental stations and gardens of all kinds. As a rule, I know the best practical results in hybridizing and cross breeding have been obtained by going direct to the point, but the losses have also been very great under this plan. The scientific way is to do one thing at a time, and work from the simple to the complex. In this way I would suggest that ten or a dozen suitable species should be selected for experimentation.
One plot of individuals should be well cultivated and self-fertilized, their seeds being again sown so as to get at the simple results of good cultivation and selection alone. The plants in plot 2, under the same conditions, should be carefully hybridized, reciprocally if possible, and the seeds of these should be again sown and well grown. Selection might be made in both cases, the object in view being to decide whether the simple selection of self-fertilized seedlings does not play a larger and hybridization alone a smaller part in the evolution of garden plants than is at present believed to be the case. The experiments could then be continued with the same material, so as to determine the importance of the part played by the cross breeding of the selected varieties in both cases. As it is, we are in "going direct" working with unknown factors. We must first of all find out how our parent species behave under, 1, culture; 2, selection; 3, cross breeding, and 4, hybridism, instead of hybridizing first and trusting to chance for our results. When we see the wonderful results attained among live stock, cattle and poultry, as well as among fruits, vegetables and flowers, by cross fertilization and selection, we may realize that after all hybridism is not everything in the evolution of the most useful animals and plants of both the farm and the garden.