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AUTHOR’S PREFACE.
AUTHOR’S PREFACE.
The essays which now appear for the first time in the form of a single volume were not written upon any prearranged plan, but have been published separately at various intervals during the course of the last seven years. Although when writing the earlier essays I was not aware that the others would follow, the whole series is, nevertheless, closely connected together. The questions which each essay seeks to explain have all arisen gradually out of the subjects treated in the first. Reflecting up
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EDITORS’ PREFACE.
EDITORS’ PREFACE.
The attention of English biologists and men of science was first called to Professor Weismann’s essays by an article entitled ‘Death’ in ‘The Nineteenth Century’ for May, 1885, by Mr. A. E. Shipley. Since then the interest in the author’s arguments and conclusions has become very general; having been especially increased by Professor Moseley’s two articles in ‘Nature’ (Vol. XXXIII, p. 154, and Vol. XXXIV, p. 629), and by the discussion upon ‘The Transmission of Acquired Characters,’ introduced b
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PREFACE.
PREFACE.
The following paper was read at the meeting of the Association of German Naturalists at Salzburg, on September 21st, 1881; and it is here printed in essentially the same form. A somewhat longer discussion of a few points has been now intercalated; these were necessarily omitted from the lecture itself for the sake of brevity, and are, therefore, not contained in the account printed in the Proceedings of the fifty-fourth meeting of the Association. Further additions would not have been admissible
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Note 1. The Duration of Life among Birds.
Note 1. The Duration of Life among Birds.
There is less exact knowledge upon this subject than we might expect, considering the existing number of ornithologists and ornithological societies with their numerous publications. It has neither been possible nor necessary for my purpose to look up all the widely-scattered references which are to be found upon the subject. Many of these are doubtless unknown to me; for we are still in want of a compilation of accurately determined observations in this department of zoology. I print the few fa
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Note 2. The Duration of Life among Mammals.
Note 2. The Duration of Life among Mammals.
The statements upon this subject in the text are taken from many sources; from Giebel’s ‘Säugethiere,’ from Oken’s ‘Naturgeschichte,’ from Brehm’s ‘Illustrirtem Thierleben,’ and from an essay of Knauer in the ‘Naturhistoriker,’ Vienna, 1880....
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Note 3. The Duration of Life among Mature Insects.
Note 3. The Duration of Life among Mature Insects.
A short statement of the best established facts which I have been able to find is given below. I have omitted the lengthening of imaginal life which is due to hybernation in certain species. In almost all orders of insects there are certain species which emerge from the pupa in the autumn, but which first reproduce in the following spring. The time spent in the torpid condition during winter cannot of course be reckoned with the active life of the species, for its vital activity is either entire
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Note 4. The Duration of Life of the Lower Marine Animals.
Note 4. The Duration of Life of the Lower Marine Animals.
I have only met with one definite statement in the literature of this part of the subject. It concerns a sea anemone,—which is a solitary and not a colonial form. The English zoologist Dalyell, in August, 1828, removed an Actinia mesembryanthemum from the sea and placed it in an aquarium [21] . It was a very fine individual, although it had not quite attained the largest size; and it must have been at least seven years old, as proved by comparison with other individuals reared from the egg. In t
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Note 5. The Duration of Life inIndigenous Terrestrial and Fresh-water Mollusca.
Note 5. The Duration of Life inIndigenous Terrestrial and Fresh-water Mollusca.
I am indebted to Herr Clessin—the celebrated student of our mollusca—for some valuable notes upon our indigenous snails and bivalves ( Lamellibranchiata ). I could not incorporate them in the text, for a number of necessary details as to the conditions of life are at present entirely unknown, or are at least only known in a very fragmentary manner. No statistics as to the amount of destruction suffered by the young are available, and even the number of eggs produced annually is only known for a
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Note 6. Unequal Length of Life in the two Sexes.
Note 6. Unequal Length of Life in the two Sexes.
This inequality is frequently found among insects. The males of the remarkable little parasites infesting bees, the Strepsiptera , only live for two to three hours in the mature condition, while the wingless, maggot-like, female lives eight days: in this case, therefore, the female lives sixty-four times as long as the male. The explanation of these relations is obvious; a long life for the male would be useless to the species, while the relatively long life of the female is a necessity for the
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Note 7. Bees.
Note 7. Bees.
It has not been experimentally determined whether the workers, which are usually killed after some months, would live as long as the queen, if they were artificially protected from danger in the hive; but I think that this is probable, because it is the case among ants, and because the peculiarity of longevity must be latent in the egg. As is well known, the egg which gives rise to the queen is identical with that which produces a worker, and differences in the nutrition alone decide whether a q
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Note 8. Death of the Cells in higher Organisms.
Note 8. Death of the Cells in higher Organisms.
The opinion has been often expressed that the inevitable appearance of normal ‘death’ is dependent on the wearing out of the tissues in consequence of their functional activity. Bertin says, referring to animal life [25] :—‘L’observation des faits y attache l’idée d’une terminaison fatale, bien que la raison ne découvre nullement les motifs de cette nécessité. Chez les êtres qui font partie du règne animal l’exercise même de la rénovation moléculaire finit par user le principe qui l’entretient s
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Note 9. Death by Sudden Shock.
Note 9. Death by Sudden Shock.
The most remarkable example of this kind of death known to me, is that of the male bees. It has been long known that the drone perishes while pairing, and it was usually believed that the queen bites it to death. Later observations have however shown that this is not the case, but that the male suddenly dies during copulation, and that the queen afterwards bites through the male intromittent organ, in order to free herself from the dead body. In this case death is obviously due to sudden excitem
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Note 10. Intermingling during the Fission of Unicellular Organisms[30].
Note 10. Intermingling during the Fission of Unicellular Organisms[30].
Fission is quite symmetrical in Amoebae , so that it is impossible to recognise mother and daughter in the two resulting organisms. But in Euglypha and allied forms the existence of a shell introduces a distinguishing mark by which it is possible to discriminate between the products of fission; so that the offspring can be differentiated from the parent. The parent organism, before division, builds the parts of the shell for the daughter form. These parts are arranged on the surface of that part
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Note 11. Regeneration.
Note 11. Regeneration.
A number of experiments have been recently undertaken, in connection with a prize thesis at Würzburg, in order to test the powers of regeneration possessed by various animals. In all essential respects the results confirm the statements of the older observers, such as Spallanzani. Carrière has also proved that snails can regenerate not only their horns and eyes, but also part of the head when it has been cut off, although he has shown that Spallanzani's old statement that they can regenerate the
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Note 12. The Duration of Life in Plants.
Note 12. The Duration of Life in Plants.
The title of the work on this subject mentioned in the Text is ‘Die Lebensdauer und Vegetationsweise der Pflanzen, ihre Ursache und ihre Entwicklung,’ F. Hildebrand, Engler’s botanische Jahrbücher, Bd. II. 1. und 2. Heft, Leipzig, 1881....
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Note 13.
Note 13.
[Many interesting facts and conclusions upon the subject of this essay will be found in a volume by Professor E. Ray Lankester, ‘On comparative Longevity in Man and the lower Animals,’ Macmillan and Co., 1870.—E. B. P.]...
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PREFACE.
PREFACE.
The following essay was my inaugural lecture as Pro-Rector of the University of Freiburg, and was delivered publicly in the hall of the University, on June 21, 1883; it first appeared in print in the following August. Only a few copies of the first edition were available for the public, and it is therefore now reprinted as a second edition, which only differs from the first in a few not unimportant improvements and additions. The title which I have chosen requires some explanation. I do not prop
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PREFACE.
PREFACE.
The following paper was first printed as an academic lecture in the summer of the present year (1883), with the title ‘Upon the Eternal Duration of Life’ (‘Über die Ewigkeit des Lebens’). In now bringing it before a larger public in an expanded and improved form, I have chosen a title which seemed to me to correspond better with the present contents of the paper. The stimulus which led to this biological investigation was given in a memoir by Götte, in which this author opposes views which I had
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PREFACE.
PREFACE.
The ideas developed in this essay were first expressed during the past winter in a lecture delivered to the students of this University (Freiburg), and they were shortly afterwards—in February and the beginning of March—written in their present form. I mention this, because I might otherwise be reproached for a somewhat partial use of the most recent publications on related subjects. Thus I did not receive Oscar Hertwig’s paper—‘Contributions to the Theory of Heredity’ (Zur Theorie der Vererbung
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Introduction.
Introduction.
When we see that, in the higher organisms, the smallest structural details, and the most minute peculiarities of bodily and mental disposition, are transmitted from one generation to another; when we find in all species of plants and animals a thousand characteristic peculiarities of structure continued unchanged through long series of generations; when we even see them in many cases unchanged throughout whole geological periods; we very naturally ask for the causes of such a striking phenomenon
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I. The Germ-plasm.
I. The Germ-plasm.
I must first define precisely the exact meaning of the term germ-plasm. In my previous writings in which the subject has been alluded to, I have simply spoken of germ-plasm without indicating more precisely the part of the cell in which we may expect to find this substance—the bearer of the characteristic nature of the species and of the individual. In the first place such a course was sufficient for my immediate purpose, and in the second place the number of ascertained facts appeared to be ins
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II. The Significance of the Polar Bodies.
II. The Significance of the Polar Bodies.
We have already seen that the specific nature of a cell depends upon the molecular structure of its nucleus; and it follows from this conclusion that my theory is further, and as I believe strongly, supported, by the phenomenon of the expulsion of polar bodies, which has remained inexplicable for so long a time. For if the specific molecular structure of a cell-body is caused and determined by the structure of the nucleoplasm, every kind of cell which is histologically differentiated must have a
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III. On the Nature of Parthenogenesis.
III. On the Nature of Parthenogenesis.
It is well known that the formation of polar bodies has been repeatedly connected with the sexuality of germ-cells, and that it has been employed to explain the phenomena of parthenogenesis. I may now, perhaps, be allowed to develope the views as to the nature of parthenogenesis at which I have arrived under the influence of my explanation of polar bodies. The theory of parthenogenesis adopted by Minot and Balfour is distinguished by its simplicity and clearness, among all other interpretations
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NOTE.
NOTE.
It is of considerable importance for the proper appreciation of the views advanced in the present essay, to ascertain whether a polar body is or is not expelled from eggs which develope parthenogenetically. I wish therefore to briefly state that I have recently succeeded in proving the formation of a polar body of distinctly cellular structure in the summer-eggs of Daphnidae . I propose to publish a more detailed account in a future paper. June 22, 1885....
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PREFACE.
PREFACE.
The greater part of the present essay was delivered at the first general meeting of the Association of German Naturalists, at Strassburg, on September 18th, 1885, and is printed in the Proceedings of the fifty-eighth meeting of that Society. The form of a lecture has been retained in the present publication, but its contents have been extended in many ways. Besides many small and a few large additions to the text, I have added six appendices in order to treat of certain subjects more fully than
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APPENDICES.
APPENDICES.
Although the mere fact that parthenogenesis occurs at all is, in my opinion, sufficient to disprove the theory of rejuvenescence, it is well to remember that parthenogenesis is now the only method of reproduction in many species (although we do not know the period of time over which these conditions have extended), and is nevertheless unattended by any perceptible decrease in fertility. From all these considerations we may draw the conclusion that the process of rejuvenescence, as described abov
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APPENDICES.
APPENDICES.
When I describe Nägeli’s theory of transformation as due to active causes lying within the organism, as a phyletic force of transformation, I do not mean to imply that it is one of those mysterious principles which, according to some writers, constitute the unconscious cause which directs the transformation of species. Nägeli’s idioplasm, which changes from within itself, is conceived as a thoroughly scientific, mechanically operating principle. This cause is undoubtedly capable of theoretical c
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PREFACE.
PREFACE.
The following paper stands in close relation to a series of short essays which I have published from time to time since the year 1881. The first of these treated of ‘The Duration of Life,’ and the last of ‘The Significance of Sexual Reproduction.’ The present essay is most intimately connected with that upon ‘The Continuity of the Germ-plasm,’ and has, in fact, grown out of the explanation of the meaning of polar bodies in the animal egg, brought forward in that essay. The explanation rested upo
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I. Parthenogenetic and Sexual Egg.
I. Parthenogenetic and Sexual Egg.
Hitherto no value has been attached to the question whether an animal egg produces one or two polar bodies. Several observers have found two such bodies in many different groups of animals, both high and low in the scale of organization. In certain species only one has been observed, in others again three, four, or five (e. g. Bischoff, in the rabbit). Many observers did not even record the number of polar bodies found by them, and simply spoke of ‘polar bodies.’ As long as their formation was l
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II. The Significance of the Second Polar Body.
II. The Significance of the Second Polar Body.
I have already discussed the physiological importance of the first polar body, or rather of the first division undergone by the nucleus of the egg, and I have explained it as the removal of ovogenetic nuclear substance which has become superfluous and indeed injurious after the maturation of the egg. I do not indeed know of any other meaning which can be ascribed to this process, now that we know of the occurrence of a first division of the nucleus in parthenogenetic as well as in sexual eggs. A
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III. The Foregoing Considerations Applied To the Male Germ-Cells.
III. The Foregoing Considerations Applied To the Male Germ-Cells.
If the result of the previous considerations be correct, and if the number of ancestral germ-plasms contained in the nucleus of the egg-cell destined for fertilization must be reduced by one half, there can be no doubt that a similar reduction must also take place, at some time and by some means, in the germ-plasms of the male germ-cells. This must be so if we are correct in maintaining that the young germ-cells of a new individual contain the same nuclear substance, the same germ-plasm, which w
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IV. The Foregoing Considerations applied To Plants.
IV. The Foregoing Considerations applied To Plants.
It remains to briefly consider the case of plants. Obviously, the ‘reducing division’ of the germ-nuclei, if it takes place at all, cannot be restricted to the germ-cells of animals. There must be a corresponding process in plants, for sexual reproduction is essentially the same in both kingdoms; and if fertilization must be preceded by the expulsion of half the number of ancestral germ-plasms from the eggs of animals, the same necessity must hold in the case of plants. But whether the process a
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V. Conclusions with regard to Heredity.
V. Conclusions with regard to Heredity.
The ideas developed in the preceding paragraphs lead to remarkable conclusions with regard to the theory of heredity,—conclusions which do not harmonize with the ideas on this subject which have been hitherto received. For if every egg expels half the number of its ancestral germ-plasms during maturation, the germ-cells of the same mother cannot contain the same hereditary tendencies, unless of course we make the supposition that corresponding ancestral germ-plasms are retained by all eggs—a sup
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VI. Recapitulation.
VI. Recapitulation.
To bring together shortly the results of this essay:—the fundamental fact upon which everything else is founded is the fact that two polar bodies are expelled, as a preparation for embryonic development, from all animal eggs which require fertilization, while only one such body is expelled from all parthenogenetic eggs. This fact in the first place refutes every purely morphological explanation of the process. If it were physiologically valueless, such a phyletic reminiscence of the two successi
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