Essays Upon Heredity and Kindred Biological Problems. Weismann August
by the greater transparency of its younger shell, it is nevertheless impossible to admit that the characteristics of the two animals are in any way different, for just before the separation of the two individuals a circulation of the protoplasm through both shells takes place after the manner described in the text, and there is therefore a complete intermingling of the substance of the two bodies.
The difference between the products is even greater after transverse fission of the Infusoria, for a new anus must be formed at the anterior part and a new mouth posteriorly. It is not known whether any circulation of the protoplasm takes place, as in Euglypha. But even if this does not occur, there is no reason for believing that the two products of division possess a different duration of life.
The process of fission in the Diatomaceae seems to me to be theoretically important, because here, as in the previously-mentioned Monothalamia (Euglypha, etc.), the new silicious skeleton is built up within the primary organism, but not, as in Euglypha, for the new individual only, but for both parent and daughter-cell alike31. If we compare the diatom shell to a box, then the two halves of the old shell would form two lids, one for each of the products of fission, while a new box is built up afresh for each of them. In this case there is an absolute equality between the products of fission, so far as the shell is concerned.
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 whole head, including the nervous system, is erroneous32.
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.
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.]
II.
ON HEREDITY.
1883
ON HEREDITY.
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 propose to treat of the whole problem of heredity, but only of a certain aspect of it—the transmission of acquired characters which has been hitherto assumed to occur. In taking this course I may say that it was impossible to avoid going back to the foundation of all the phenomena of heredity, and to determine the substance with which they must be connected. In my opinion this can only be the substance of the germ-cells; and this substance transfers its hereditary tendencies from generation to generation, at first unchanged, and always uninfluenced in any corresponding manner, by that which happens during the life of the individual which bears it. If these views, which are indicated rather than elaborated in this paper, be correct, all our ideas upon the transformation of species require thorough modification, for the whole principle of evolution by means of exercise (use and disuse), as proposed by Lamarck, and accepted in some cases by Darwin, entirely collapses.
The nature of the present paper—which is a lecture and not an elaborate treatise—necessitates that only suggestions and not an exhaustive treatment of the subject could be given. I have also abstained from giving further details in the form of an appendix, chiefly because I could hardly have attempted to complete a treatment of the whole range of the subject, and I hope to refer again to these questions in the future, when new experiments and observations have been made.
I am very glad to see that such an important authority as Pflüger33 has in the meantime come to the same opinion, from an entirely different direction—an opinion which forms the foundation of the views here brought forward, namely, that heredity depends upon the continuity of the molecular substance of the germ from generation to generation.
II.
ON HEREDITY
With your permission I wish to bring before you to-day my views on a problem of general biological interest—the problem of heredity.
Heredity is the process which renders possible that persistence of organic beings throughout successive generations, which is generally thought to be so well understood and to need no special explanation. Nevertheless our minds cannot fail to be much perplexed by the multiplicity of its manifestations, and to be greatly puzzled as to its real nature. A celebrated German physiologist says34, ‘Although many hands have at all times endeavoured to break the seal which hides the theory of heredity from our view, the results achieved have been but small; and we are in a certain degree justified in looking with little hope upon new efforts undertaken in this direction. We must nevertheless endeavour from time to time to ascertain how far we have advanced towards a complete explanation.’
Such a course is in every way advisable, for we are not dealing with phenomena which from their very nature are incomprehensible by man. The great complexity of the subject has alone rendered it hitherto insuperable, but in the province of heredity we certainly have not reached the limits of attainable knowledge.
From this point of view heredity bears some resemblance to certain anatomical and physiological problems, e. g. the structure and function of the human brain. Its structure—with so many millions of nerve-fibres and nerve-cells—is of such extraordinary complexity that we might well despair of ever completely understanding it. Each fibre is nevertheless distinct in itself, while its connection with the nearest nerve-cell can be frequently traced, and the function of many groups of cell elements is already known. But it would seem to be impossible to unravel the excessively complex network into which the cells and fibres are knit together; and hence to arrive at the function of each single element appears to be also beyond our reach. We have not however commenced to untie this Gordian knot without some hope of success, for who can say how far human perseverance may be able to penetrate into the mechanism of the brain, and to reveal a connected structure and a common principle in its countless elements? But surely this work will be most materially assisted by the simultaneous investigation of the structure and function of the nervous system in the lower forms of life—in the polypes and jelly-fish, worms and Crustacea. In the same way we should not abandon the hope of arriving at a satisfactory knowledge of the processes of heredity, if we consider the simplest processes of the lower animals as well as the more complex processes met with in the higher forms.
The word heredity in its common acceptation, means that property of an organism by which its peculiar nature is transmitted to its descendants. From an eagle’s egg an eagle of the same species developes; and not only are the characteristics of the species transmitted to the following generation, but even the individual peculiarities. The offspring resemble their parents among animals as well as among men.
On what does this common property of all organisms depend?
Häckel was probably the first to describe reproduction as ‘an overgrowth of the individual,’ and he attempted to explain heredity as a simple continuity of growth. This definition might be considered as a play upon words, but it is more
Cf. Victor Hensen, ‘Physiologie d. Zeugung,’ p. 152.
Cf. J. Carrière, ‘Ueber Regeneration bei Landpulmonaten,’ Tagebl. der 52. Versammlg. deutsch. Naturf. pp. 225-226.
Pflüger, ‘Ueber den Einfluss der Schwerkraft auf die Theilung der Zellen und auf die Entwicklung des Embryo,’ Arch. f. Physiol. Bd. XXXII. p. 68, 1883.
Victor Hensen in his ‘Physiologie der Zeugung,’ Leipzig, 1881, p. 216.