It is probable, from recent experiments by De Vries, Correns, and others, that a remarkable regularity, expressed by Mendel in the form of a law, obtains in the variations which result from hybridising.
In considering these illustrative cases, it is necessary to thoroughly apprehend that two procedures are involved. In the first place we have the cross-pollination leading to the formation of the hybrid plant by cross-fertilisation. But experience shows that this would lead to very uncertain results if the plant-breeder did not supplement them by the second and extremely important process of rigid selection - i.e. by choosing the best of the progeny and breeding from them apart from the parent-forms, and gradually intensifying, as it were, the variations in certain directions which have been started by the crossing.
It is by selection, careful culture, and repeated selection that so much has been done in obtaining the innumerable new varieties of roses, sweet-peas, orchids, orchard fruits, cereals, grapes, strawberries, melons, tomatoes, early potatoes, etc., brought forward by numerous breeders of plants in all countries, as will readily be understood if reference be made to the work of Hays and Webber in America; Saunders in Canada; Garton, Sutton, Veitch, Bateson, and others in this country.
Nor is it necessary that the new materials for selection to work upon should be started by hybridisation. Grafting, change of conditions, and even variations so vaguely understood that we term them "spontaneous," may supply the starting-points for changes in the characters of plants, so remarkable after intensification by breeding that people find it difficult to believe they can have come from one stock.
Here, however, I must conclude, merely remarking that the above sketch is a mere outline of the subjects modern agriculture and horticulture concern themselves with. There are hundreds of problems connected with the germination of seeds, on which valuable recent work has been done by Klebs, Green, Horace Brown, and others; with the resistance of seeds and seedlings to high and low temperatures, a subject opened out by Sachs, Kny, De Vries, Krasan, Just, Hohnel, Dewar, Dyer, and others; with the conditions of vegetation which affect the various functions of growth, respiration, assimilation, transpiration, and so forth, on which I cannot even touch in these pages.
Meanwhile I hope I have succeeded in impressing upon you the grand fact that the plant is a living and very complex engine, driven by the radiant energy of the sun, and capable of doing work thereby, and this just as truly as any heat-engine is driven by chemical energy gained by means of the sun's rays, or as a water-mill is driven by power which must be referred to the energy of potential in the head of water placed in position by the sun's work in evaporation. Fundamentally the whole of life and work on our planet is to be referred to the one great source of energy which renders possible the establishment of differences of potential.
This machine, then, doing work in various ways, adapts itself - or goes to the wall - to the conditions of its work among competing organisms or opposing circumstances. Curiously enough, while in some cases it suffers from the competition, in others it is benefited by its life-actions fitting in between those of other organisms, which in their turn supplement it. In other words new types of this engine, capable of doing the work in various ways, are obtainable; some are good types for the conditions afforded, others are bad ones.
Examples of both will occur in the further exposition of the subject.
Man's position in regard to the struggle is that of an intelligent being who steps in at certain stages and protects, fosters, and in every way favours the agricultural plant - the living machine -and sees that every opportunity is given it to do its best work in the best way - from his points of view!
The foundation of any course of reading on hybridisation and selection should be Darwin's Effects of Cross and Self-Fertilisation in the Vegetable Kingdom, which, with his books On the Origin of Species by means of Natural Selection and The Variation of Animals and Plants under Domestication, will prepare the student for the long course of reading necessary for a full appreciation of what has been done in this department of science.
From the numerous works which followed these I should select Bailey's Survival of the Unlike, London, 1896, and Evolution of our Native Fruits, New York, 1898, as especially useful for the reader of this book, to which may also be added Plant Breeding, New York, 1896, by the same author, as giving numerous facts and practical directions of value. Further, the "Hybrid Conference Report," Journ. Roy. Hort. Soc., 1900, abounds in facts and information. Rimpau, Landw. fahrbvol. xx., 1891, p. 239. The student who wishes to get towards the root of the matter will hardly be able to dispense with Strasburger's Neue Untersuchungen uber die Befruchtungsvorgang bei den Phanerogamen, Jena, 1884. An interesting summary of recent work on Xenia and "double fertilisation" will be found in Bull. No. 22, U.S. Dept. of Agric., 1900. See also Nature, Mar. 15, 1900, p. 470.
If he wishes to explore the vast region of controversial literature that opens up from these points, and which is far beyond the purpose of this book, he may consult the literature collected in Kassowitz' Allge7neine Biologie, Wien, 1899, B. II., and the references in the works quoted; also, Strasburger, "The Periodic Reduction of Chromosomes in Living Organisms," Ann. Botviii., 1894, p. 281. For "Mendel's Law," see Correns in Ber. d. deutsch. bot. Gesellsch., vol. xviii., 1900, p. 158.