Essays Upon Heredity and Kindred Biological Problems

Essays Upon Heredity and Kindred Biological Problems
August Weismann

August Weismann

Essays Upon Heredity and Kindred Biological Problems / Authorised Translation

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 upon the causes which regulate the duration of life in various forms, I was drawn on to the consideration of fresh questions which demanded further research. These considerations and the results of such research form the subject-matter of all the subsequent essays.

I am here making use of the word ‘research’ in a sense somewhat different from that in which it is generally employed in natural science; for it is commonly supposed to imply the making of new observations. Some of these essays, especially Nos. IV, V, and VI, essentially depend upon new discoveries. But in most of the remaining essays the researches are of a more abstract nature, and consist in bringing forward new points of view, founded upon a variety of well-known facts. I believe, however, that the history of science proves that advance is not only due to the discovery of new facts, but also to their correct interpretation: a true conception of natural processes can only be arrived at in this way. It is chiefly in this sense that the contents of these essays are to be looked upon as research.

The fact that they contain the record of research made it impossible to introduce any essential alterations in the translation, even in those points about which my opinion has since changed to some extent. I should to-day express some of the points in Essays I, IV, and V, somewhat differently; but had I made such alterations, the relation between the essays as a whole would have been rendered less clear, for each of the earlier ones formed the foundation of that which succeeded it. Even certain errors of interpretation are on this account left uncorrected. Thus, for instance, in Essay IV it is assumed that the two polar bodies expelled by sexual eggs are identical; for at that time there was no reason for doubting that they were physiologically equivalent. The discovery of the numerical law of the polar bodies described in Essay VI, led to what I believe to be a truer knowledge of them. In this way the causes of parthenogenesis, as developed in Essay V, received an important addition in the fact published in Essay VI, that only one polar body is expelled by parthenogenetic eggs. This fact alone explains why sexual eggs cannot as a rule develope without fertilization.

Hence the reader must not take the individual essays as the full and complete expression of my present opinion; but they must rather be looked upon as stages in research, as steps towards a more perfect knowledge.

I must therefore express the hope that the essays may be read in the same order as that in which they appeared, and in which they are arranged in the present volume. The reader will then follow the same road which I traversed in the development of the views here set forth; and even though he may be now and then led away from the direct route, perhaps such deviations may not be without interest.

I should wish to express my warm thanks to Mr. Poulton for the great trouble he has taken in editing the translation, which in many places presented exceptional difficulties. The greater part of the text I have looked through in proof, and I believe that it well expresses the sense of the original; although naturally I cannot presume to judge concerning the niceties of the English language. I am especially grateful to the three gentlemen who have brought these essays before an English public, because I believe that many English naturalists, even when thoroughly conversant with the German tongue, might possibly misinterpret many points in the original; for the difficulty of the questions treated of greatly increases the difficulty of the language.

If the readers of this book only feel half as much pleasure in its perusal as I experienced in writing it, I shall be more than satisfied.

    AUGUST WEISMANN.

Freiburg I. Breisgau,

January, 1889.

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 by Professor Lankester at the meeting of the British Association at Manchester in 1887,—a discussion in which Professor Weismann himself took part. The deep interest which has everywhere been expressed in a subject which concerns the very foundations of evolution, has encouraged the Editors to hope that a volume containing a collection of all Professor Weismann’s essays upon heredity and kindred problems would supply a real want. At the present time, when scientific periodicals contain frequent references to these essays, and when the various issues which have been raised by them are discussed on every occasion at which biologists come together, it is above all things necessary to know exactly what the author himself has said. And there are many signs that discussion has already suffered for want of this knowledge.

A translation of Essays I and II was commenced by Mr. A. E. Shipley during his residence at Freiburg in the winter of 1884. His work was greatly aided by the kind assistance of Dr. van Rees of Amsterdam, to whom we desire to express our most sincere thanks. The translation was laid aside until the summer of 1888, when Mr. Shipley was invited to co-operate with the other editors in the preparation of the present volume; the Clarendon Press having consented to publish the complete series of essays as one of their Foreign Biological Memoirs.

We think it probable that this work may interest many who are not trained biologists, but who approach the subject from its philosophical or social aspects. Such readers would do well to first study Essays I, II, VII, and VIII, inasmuch as some preparation for the more technical treatment pursued in the other essays will thus be gained.

The notes signed A. W. and dated, were added by the author during the progress of the translation. The notes included in square brackets were added by the Editors; the authorship being indicated by initials in all cases.

In conclusion, it is our pleasant duty to thank those who have kindly helped us by reading the proof-sheets and making valuable suggestions. Our warmest thanks are due to Mrs. Arthur Lyttelton, Mr. W. Hatchett Jackson, Deputy Linacre Professor in the University of Oxford, Mr. J. S. Haldane, and Professor R. Meldola. Important suggestions were also made by Professor E. Ray Lankester, Mr. Francis Galton, and Dr. A. R. Wallace. Professor W. N. Parker also greatly helped us by looking over the proof-sheets with Professor Weismann.

E. B. P.

S. S.

A. E. S.

Oxford, February, 1889.

I.

THE DURATION OF LIFE.

1881

THE DURATION OF LIFE.

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 without an essential change of form, and therefore I have not put into the text a note which ought otherwise to have been there, and which is now to be found in the Appendix, as Note 8. It fills up a gap which was left in the text, for the above-mentioned reason, by attempting to give an explanation of the normal death of cells of tissues—an explanation which is required if we are to maintain that unicellular organisms are so constituted as to be potentially immortal.

The other parts of the Appendix contain, partly further expansions, partly proofs of the views brought forward in the text, and above all a compilation of all the observations which are known to me upon the duration of life in several groups of animals. I am indebted to several eminent specialists for the communication of many data, which are among the most exact that I have been able to obtain. Thus Dr. Hagen of Cambridge (U.S.A.) was kind enough to send me an account of his observations upon insects of different orders: Mr. W. H. Edwards of West Virginia, and Dr. Speyer of Rhoden—their experience with butterflies. Dr. Adler of Schleswig sent me data upon the duration of life in Cynipidae, which have a special value, as they are accompanied by very exact observations upon the conditions of life in these animals; hence in this case we can directly examine the factors upon which, as I believe, the duration of life is chiefly based. Sir John Lubbock in England, and Dr. August Forel of Zürich, have had the kindness to send me an account of their observations upon ants, and S. Clessin of Ochsenfurth his researches upon our native land and fresh-water Mollusca.

In publishing these valuable communications, together with all facts which I have been able to collect from literature upon the subject of the duration of life, and the little which I have myself observed upon this subject, I hope to provide a stimulus for further observation in this field, which has been hitherto much neglected. The views which I have brought forward in this paper are based on a comparatively small number of facts, at least as far as the duration of life in various species is concerned. The larger the number of accurate data which are supplied, and the more exactly the duration of life and its conditions are ascertained, the more securely will it be possible to establish our views upon the causes which determine the duration of life.

    A. W.

Naples, Dec. 6, 1881.

I.

THE DURATION OF LIFE

With your permission, I will bring before you to-day some thoughts upon the subject of the duration of life. I can scarcely do better than begin with the simple but significant words of Johannes Müller: ‘Organic bodies are perishable; while life maintains the appearance of immortality in the constant succession of similar individuals, the individuals themselves pass away.’

Omitting, for the time being, any discussion as to the precise accuracy of this statement, it is at any rate obvious that the life of an individual has its natural limit, at least among those animals and plants which are met with in every-day life. But it is equally obvious that the limits are very differently placed in the various species of animals and plants. These differences are so manifest that they have given rise to popular sayings. Thus Jacob Grimm mentions an old German saying, ‘A wren lives three years, a dog three times as long as a wren, a horse three times as long as a dog, and a man three times as long as a horse, that is eighty-one years. A donkey attains three times the age of a man, a wild goose three times that of a donkey, a crow three times that of a wild goose, a deer three times that of a crow, and an oak three times the age of a deer.’

If this be true a deer would live 6000 years, and an oak nearly 20,000 years. The saying is certainly not founded upon exact observation, but it becomes true if looked upon as a general statement that the duration of life is very different in different organisms.

The question now arises as to the causes of these great differences. How is it that individuals are endowed with the power of living long in such very various degrees?

One is at first tempted to seek the answer by an appeal to the differences in morphological and chemical structure which separate species from one another. In fact all attempts to throw light upon the subject which have been made up to the present time lie in this direction.

All these explanations are nevertheless insufficient. In a certain sense it is true that the causes of the duration of life must be contained in the organism itself, and cannot be found in any of its external conditions or circumstances. But structure and chemical composition—in short the physiological constitution of the body in the ordinary sense of the words—are not the only factors which determine duration of life. This conclusion forces itself upon our attention as soon as the attempt is made to explain existing facts by these factors alone: there must be some other additional cause contained in the organism as an unknown and invisible part of its constitution, a cause which determines the duration of life.

The size of the organism must in the first place be taken into consideration. Of all organisms in the world, large trees have the longest lives. The Adansonias of the Cape Verd Islands are said to live for 6000 years. The largest animals also attain the greatest age. Thus there is no doubt that whales live for some hundreds of years. Elephants live 200 years, and it would not be difficult to construct a descending series of animals in which the duration of life diminishes in almost exact proportion to the decrease in the size of the body. Thus a horse lives forty years, a blackbird eighteen, a mouse six, and many insects only a few days or weeks.

If however the facts are examined a little more closely it will be observed that the great age (200 years) reached by an elephant is also attained by many smaller animals, such as the pike and carp. The horse lives forty years, but so does a cat or a toad; and a sea anemone has been known to live for over fifty years. The duration of life in a pig (about twenty years) is the same as that in a crayfish, although the latter does not nearly attain the hundredth part of the weight of a pig.

It is therefore evident that length of life cannot be determined by the size of the body alone. There is, however, some relation between these two attributes. A large animal lives longer than a small one because it is larger; it would not be able to become even comparatively large unless endowed with a comparatively long duration of life.

Apart from all other reasons, no one could imagine that the gigantic body of an elephant could be built up like that of a mouse in three weeks, or in a single day like that of the larva of certain flies. The gestation of an elephant lasts for nearly two years, and maturity is only reached after a lapse of about twenty-four years.

Furthermore, to ensure the preservation of the species, a longer time is required by a large animal than by a small one, when both have reached maturity. Thus Leuckart and later Herbert Spencer have pointed out that the absorbing surface of an animal only increases as the square of its length, while its size increases as the cube; and it therefore follows that the larger an animal becomes, the greater will be the difficulty experienced in assimilating any nourishment over and above that which it requires for its own needs, and therefore the more slowly will it reproduce itself.

But although it may be stated generally that the duration of the period of growth and length of life are longest in the largest animals, it is nevertheless impossible to maintain that there is any fixed relation between the two; and Flourens was mistaken when he considered that the length of life was always equivalent to five times the duration of the period of growth. Such a conclusion might be accepted in the case of man if we set his period of growth at twenty years and his length of life at a hundred; but it cannot be accepted for the majority of other Mammalia. Thus the horse lives from forty to fifty years, and the latter age is at least as frequently reached among horses as a hundred years among men; but the horse becomes mature in four years, and the length of its life is thus ten or twelve times as long as its period of growth.

The second factor which influences the duration of life is purely physiological: it is the rate at which the animal lives, the rapidity with which assimilation and the other vital processes take place. Upon this point Lotze remarks in his Microcosmus—‘Active and restless mobility destroys the organized body: the swift-footed animals hunted by man, as also dogs, and even apes, are inferior in length of life to man and the larger beasts of prey, which satisfy their needs by a few vigorous efforts.’ ‘The inertness of the Amphibia is, on the other hand, accompanied by relatively great length of life.’