There is, moreover, no such profound difference between the egg-cell and other cells as Weismann postulates. Not only the egg-cell but every cell of the body may, and in many cases certainly does, contain germ-plasm sufficient to reproduce the individual under proper conditions. Regeneration and vegetative reproduction is thus explained in a simple and natural way. When the whole course of development is conceived of as a series of reactions to external stimuli and when we consider what an astonishing variety of such stimuli act upon every cell, the reason becomes plain why variation is the invariable law of heredity and development. When we consider how slight a cause may alter development, the introduction of a minute quantity of substance from an insect's sting causing a plant to produce a gall whose structure is totally different from anything prescribed by the hereditary history of the plant, we may well wonder that species and races are as constant as they are. Only the general uniformity of conditions can account for it. Of the two factors of heredity, inherited tendencies and environment, the Preformationist lays weight on the former, the Epigenesist on the latter. The one emphasizes internal, the other external forces. Evidently there is truth in both sides. The epigenetic theory, however, appeals to the investigator as a more direct and natural explanation of the facts, affording a sound basis for experiment, elastic enough to cover all cases, and in touch everywhere with observed fact. Moreover, the progress of investigation points with growing emphasis to the environment as a determining factor in development. It is true that the manner in which the egg reacts depends on its inherited structure. It is not necessary, however, to assume with Weismann that this structure depends on a fixed and wonderfully complicated arrangement of granules. Such a conception is irreconcilable with the facts of metabolism. It is far more probable that its chemical composition is the significant thing. Indeed the whole question is rapidly approaching a physico-chemical basis. It is clearly seen that the study of heredity will in future be an investigation of the physical and chemical reactions of the cell with its environment and that here the whole explanation lies hidden. The great problems of heredity are, therefore, cellproblems and these in turn are problems of physics and chemistry. For their solution the physicist and chemist must prepare the way: the biologist may then hope to approach by experimental methods a step nearer to the ultimate causes of inheritance and development. In view of the wide-spread popular interest in the subject and its fundamental importance in many fields a guide to the most important literature is greatly needed. The following list was prepared for the Botanical Seminary which devoted the past year to the study of heredity and will continue the work during the present one. This list includes only a few of the most important accessible publications bearing on the topics mentioned. A card catalogue of the most important literature of heredity has been prepared for the use of the Seminary and is at the service of all who may wish to consult it. GENERAL: Wilson, E. B.: The Cell, 296-330. Haacke, W.: Grundriss der Entwickelungsmechanik, 172-185. Thompson, J. A.: The History and Theory of Heredity. Proceedings of the Royal Society of Edinburgh, 1889. Brooks, W. K.: Heredity, 16-80. Hertwig, O.: Aeltere und neuere Entwickelungs-theorien. Berlin, 1892. ARE ACQUIRED CHARACTERS INHERITED? Romanes, G. J.: Darwin and after Darwin, II. 39-158. Weismann, A.: The Germ-plasm, 352-391. Delage, Y.: L' Hérédité, 186-216, 235-241. Darwin, Ch.: Animals and Plants under Domestication, II. 9-41, 81-108, and 302-428. Osborn, H. F.: American Naturalist, 1892, 537-567. NON-RESEMBLANCE OF PARENT AND OFFSPRING. Delage, Y.: L'Hérédité, 222-233, 242-259. Darwin, Ch.: Animals and Plants under Domestication, II. Chap. XIII-XIX. Darwin, Ch.: Origin of Species, Chap. IX. Bailey, L. H.: Plant Breeding. Bateson, W. Materials for the Study of Variation. Nägeli, C.: Die Bastardbildung im Pflanzenreich. Sitzungsberichte der königlichen bayerischen Akademie der WissenDecember 15th, 1865. Also Botan schaften in München. ische Mittheilungen, II. 187. THEORIES OF DARWIN AND HIS PREDECESSORS. Darwin's Predecessors. Delage, Y.: L'Hérédité, 354-358. Whitman, C. O.: Wood's Holl Biological Lectures. 1894. Brooks, W. K.: Heredity, 20-27. Osborn, H. F.: From the Greeks to Darwin. Wolff, Caspar Friedrich: Theoria generationis; Uebersetzt und herausgegeben von Dr. Paul Samassa. Leipzig, 1896. Darwin's Theory. Darwin, Ch.: Animals and Plants under Domestication, II. 428-481. Brooks, W. K.: Heredity, especially Chap. II and III. Delage, Y.: L'Hérédité, 534-550, et seq. THE THEORY OF W. HIS. His, W.: Unsere Körperform und das physiologische Problem ihrer Entstehung. Leipzig, 1874. Delage, Y.: L'Hérédité, 468. Wilson, E. B.: The Cell, 297. THE THEORY OF C. NAEGELI. Nägeli, C.: Mechanisch-physiologische Theorie der Abstammungs- Nägeli's Theory of Organic Evolution: Summary. Open Court Delage, Y.: L'Hérédité, 592-643. Wilson, E. B.: The Cell, 300. Wiesner, J.: Die Elementarstructur und das Wachsthum der lebenden Substanz. THE THEORY OF H. DE VRIES. de Vries, H.: Intracellulare Pangenesis. Wilson, E. B.: The Cell, 303. Haacke, W.: Gestaltung und Vererbung. THE THEORY OF A. WEISMANN. Weismann, A.: The Germ-plasm. Hertwig, O.: The Biological Problem of To-day. Delage, Y.: L'Hérédité, 512, 667, 724. Romanes, G. J.: An Examination of Weismannism. Roux, W.: Gesammelte Abhandlungen, Vol. I. 135-422; Vol. THE THEORY OF W. HAACKE. Haacke, W.: Gestaltung und Vererbung. Grundriss der Entwicklungsmechanik. THE THEORY OF O. HERTWIG. Hertwig, O.: The Biological Problem of To-day. Wilson, E. B.: The Cell, 312-317. UNITY IN VARIABILITY. By HUGO DE VRIES.* Alle Gestalten sind ähnlich Doch keine gleichet der andren Und so deutet das Chor Auf ein geheimes Gesetz. The idea which underlies these words of Goethe had been expressed long before by Cats in these lines: Als van twee gepaarde Schelpen G'eene breekt of wel verliest, Hoe ge zoekt of hoe ge kiest; † and afterwards by Darwin in his well known assertion that no two individuals of the same kind are alike. We may therefore consider it as a generally acknowledged and fundamental principle of heredity, that individual differences are everywhere present, affecting every character of the organism. But the general acknowledgment of this fact does not disclose to us the law by which it is governed. The "geheimes Gesetz," the existence of which was already * An inaugural address delivered by Hugo de Vries, Professor of Botany at the University of Amsterdam, in his capacity of Rector Magnificus on the last dies natalis of that University, January 8, 1898. It appeared originally in the "Album der Natuur," No. 3, 1898, publisher H. V. Tjeenk Willink, Haarlem, Holland. Translated from the Dutch by H. T. A. Hus, and revised by the author. This translation was made for the use of the Botanical Seminary, and was read September 28, 1898. † If of a pair of shells, one is lost or broken, it will be impossible to find one which will exactly replace it. |