Part 16

A shortening of the normal duration of life is also possible; this is shown in every case of sudden death, after the deposition of the whole of the eggs at a single time. This occurs among certain insects, while nearly allied forms of which the oviposition lasts over many days therefore possess a correspondingly long imago-life. The _Ephemeridae_ and Lepidoptera afford many examples of this, and in an earlier work I have collected some of them[92]. The humming-bird hawk-moth flies about for weeks laying an egg here and there, and, like the allied poplar hawk-moth and lime hawk-moth, probably dies when it has deposited all the eggs which can be matured with the amount of nutriment at its disposal. Many other Lepidoptera, such as the majority of butterflies, fly about for weeks depositing their eggs, but others, such as the emperor-moths and lappet-moths, lay their eggs one after another and then die. The eggs of the parthenogenetic _Psychidae_ are laid directly after the imago has left the cocoon, and death ensues immediately, so that the whole life of the imago only lasts for a few hours. No one could look upon this brief life as a primitive arrangement among Lepidoptera, any more than we do upon the absence of wings in the female _Psychidae_; shortening of life here is therefore clearly explicable.

In such cases have we any right to speak of the fatal effect of reproduction? We may certainly say that these insects die of exhaustion; their vital strength is used up in the last effort of laying eggs, and in the case of the males, in the act of copulation. Reproduction is here certainly the most apparent cause of death, but a more remote and deeper cause is to be found in the limitation of vital strength to the length and the necessary duties of the reproductive period. The fact that there are female Lepidoptera which, like the emperor-moths, do not feed in the imago-state, proves the truth of this statement. They still possess a mouth and a complete alimentary canal, but they have no spiral ‘tongue,’ and do not take food of any kind, not even a drop of water. They live in a torpid condition for days or weeks until fertilization is accomplished, and then they lay their eggs and die. The habit of extracting honey from flowers—common to most hawk-moths and butterflies—would not have thus fallen into disuse, if the store of nutriment, accumulated in the form of the fat-bodies, during the life of the caterpillar, had not been exactly sufficient to maintain life until the completion of oviposition. The fact that the habit of taking food has been thus abandoned is a proof that the duration of life beyond the reproductive period would not be to the advantage of the species.

The protraction of existence into old age among the higher Metazoa proves that death is not a necessary consequence of reproduction. It seems to me that Götte’s statement ‘that the appearances of senility must not be regarded as the general cause of death’ is not in opposition to my opinions but rather to those which receive general acceptance. I have myself pointed out that ‘death is not always preceded by senility or a period of old age[93].’

The materials are wanting for a comprehensive investigation of the causes which first introduced this period among the higher Metazoa; in fact the most fundamental data are absent, for we do not even know the part of the animal kingdom in which it first appeared: we cannot even state the amount by which the duration of life exceeds that of the period of reproduction, or what is the value to the species of this last stage in the life of the individual.

It is in these general directions that we must seek for the significance of old age. It is obviously of use to man, for it enables the old to care for their children, and is also advantageous in enabling the older individuals to participate in human affairs and to exercise an influence upon the advancement of intellectual powers, and thus to influence indirectly the maintenance of the race. But as soon as we descend a step lower, if only as far as the apes, accurate facts are wanting, for we are, and shall probably long be, ignorant of the total duration of their life, and the point at which the period of reproduction ceases.

* * * * *

I must here break off in the midst of these considerations, rather than conclude them, for much still remains to be said. I hope, nevertheless, that I have thrown new light upon some important points, and I now propose to conclude with the following short abstract of the results of my enquiry.

I. Natural death occurs only among multicellular beings; it is not found among unicellular organisms. The process of encystment in the latter is in no way comparable with death.

II. Natural death first appears among the lowest Heteroplastid Metazoa, in the limitation of all the cells collectively to one generation, and of the somatic or body-cells proper to a restricted period: the somatic cells afterwards in the higher Metazoa came to last several and even many generations, and life was lengthened to a corresponding degree.

III. This limitation went hand in hand with a differentiation of the cells of the organism into reproductive and somatic cells, in accordance with the principle of division of labour. This differentiation took place by the operation of natural selection.

IV. The fundamental biogenetic law applies only to multicellular beings; it does not apply to unicellular forms of life. This depends on the one hand upon the mode of reproduction by fission which obtains among the Monoplastides (unicellular organisms), and on the other upon the necessity, induced by sexual reproduction, for the maintenance of a unicellular stage in the development of the Polyplastides (multicellular organisms).

V. Death itself, and the longer or shorter duration of life, both depend entirely on adaptation. Death is not an essential attribute of living matter; it is neither necessarily associated with reproduction, nor a necessary consequence of it.

* * * * *

In conclusion, I should wish to call attention to an idea which is rather implied than expressed in this essay:—it is, that reproduction did not first make its appearance coincidently with death. Reproduction is in truth an essential attribute of living matter, just as is the growth which gives rise to it. It is as impossible to imagine life enduring without reproduction as it would be to conceive life lasting without the capacity for absorption of food and without the power of metabolism. Life is continuous and not periodically interrupted: ever since its first appearance upon the earth, in the lowest organisms, it has continued without break; the forms in which it is manifested have alone undergone change. Every individual alive to-day—even the very highest—is to be derived in an unbroken line from the first and lowest forms.

Footnotes for Chapter III.

Footnote 59:

‘Ueber den Ursprung des Todes,’ Hamburg and Leipzig, 1883.

Footnote 60:

As in the case of the bodies of monks on the Great St. Bernard, or the dried-up bodies in the well-known Capuchine Monastery at Palermo.

Footnote 61:

Professor Gruber informs me that among the Infusoria of the harbour of Genoa, he has observed a species which encysts upon one of the free-swimming Copepoda. He has often found as many as ten cysts upon one of these Copepods, and has observed the escape of their contents whenever the water under the cover-glass began to putrefy. Here advantage is probably gained in the rapid transport of the cyst by the Crustacean.

Footnote 62:

The views of most biologists who have worked at this subject agree in all essentials with that expressed above. Bütschli says (Bronn’s ‘Klassen und Ordnungen des Thierreichs,’ Protozoa, p. 148): ‘The process of encystment does not appear to have originally borne any direct relation to reproduction: it appears on the contrary to have taken place originally,—as it frequently does at the present day,—either for the protection of the organism against injurious external influences, such as desiccation or the fatal effects of impure water, etc.; and also to enable the organism, after taking up an unusually abundant supply of food, to assimilate it in safety.’ Balbiani (‘Journ. de Micrographie,’ Tom. V. 1881, p. 293) says in reference to the Infusoria, ‘Un petit nombre d’espèces, au lieu de se multiplier à l’état de vie active, se reproduisent dans une sorte d’état de repos, dit état d’enkystement. Ces sortes de kystes peuvent être désignés sous le nom de kystes de reproduction, par opposition avec d’autres kystes, dans lesquels les Infusoires se renferment pour se soustraire à des conditions devenues défavorables du milieu qu’ils habitent, le manque d’air, le dessèchement, etc.—ceux-ci sont des kystes de conservation....’

Footnote 63:

This is of importance in so far as single individuals might be thus compelled to encyst even when the existing external conditions of life do not require it. The substance which _Actinosphaerium_, for example, employs in the secretion of its thick siliceous cyst must have been gradually accumulated by means of a process peculiar to the species. We can scarcely be in error if we assume that the silica accumulated in the organism cannot increase to an unlimited extent without injury to the other vital processes and that the secretion of the cyst must take place as soon as the accumulation has exceeded a certain limit. Thus we can understand that encystment may occur without any external necessity. Similarly, certain Entomostraca (_e. g._ _Moina_) produce winter-eggs in a particular generation, and these are formed even when the animals are kept in a room protected from cold and desiccation.

Footnote 64:

Upon this point Professor Gruber intends to publish an elaborate memoir.

Footnote 65:

This view has not even been proved for _Actinosphaerium_, upon which Götte chiefly relies. The observations which we now possess merely indicate that the animal contracts to the smallest volume possible. Compare F. E. Schulze, ‘Rhizopodenstudien,’ I, Arch. f. mikr. Anat. Bd. 10, p. 328; and Karl Brandt, ‘Ueber Actinosphaerium Eichhornii,’ Inaug. Diss.; Halle, 1877.

Footnote 66:

The conception of Protozoa and Metazoa does not correspond exactly with that of unicellular and multicellular beings, for which Götte has proposed the names Mono- and Polyplastides.

Footnote 67:

Among the Rhizopoda encystment is only known in fresh-water forms, and not in a single one of the far more numerous marine forms which possess shells (see Bütschli, ‘Protozoa,’ p. 148); the marine Rhizopoda are not exposed to the effects of desiccation or frost, and thus the strongest motives for the process of encystment do not exist, at least among forms possessing a shell.

Footnote 68:

I trust that it will not be objected that the germ-cells cannot be immortal, because they frequently perish in large numbers, as a result of the natural death of the individual. There are certain definite conditions under which alone a germ-cell can render its potential immortality actual, and these conditions are for the most part fulfilled with difficulty (fertilization, etc.). It follows from this fact that the germ-cells must always be produced in numbers which reach some very high multiple of the necessary number of offspring, if these latter are to be ensured for the species. If in the natural death of the individual the germ-cells must also die, the _natural_ death of the _soma_ becomes a cause of _accidental_ death to the germ-cells.

Footnote 69:

l. c., p. 78.

Footnote 70:

l. c., p. 47.

Footnote 71:

‘Entwicklungsgeschichte der Unke,’ Leipzig, 1875, p. 65.

Footnote 72:

Id., p. 842.

Footnote 73:

‘Ursprung des Todes,’ p. 79.

Footnote 74:

l. c., p. 42.

Footnote 75:

‘Contributions à l’histoire des Mesozoaires. Recherches sur l’organisation et le développement embryonnaire des Orthonectides,’ Arch. de Biologie, vol. iii. 1882.

Footnote 76:

l. c., p. 37.

Footnote 77:

Julin does not enter into further details on this point, and it is not quite clear at what precise time the cells of the ectoderm atrophy; but this is irrelevant to the origin of death, since the granular mass surrounding the egg-cells at any rate belongs to the _soma_ of the mother.

Footnote 78:

Leuckart finds such a great resemblance between the newly born young of _Distoma_ and the Orthonectides, that he is inclined to believe that the latter are Trematodes, ‘which in spite of sexual maturity have not developed further than the embryonic condition of the _Distoma_’ (‘Zur Entwicklungsgeschichte des Leberegels,’ Zool. Anzeiger, 1881, No. 99). In reference to the Dicyemidae, which resemble the Orthonectides in their manner of living and in their structure, Gegenbaur has stated his opinion that they belong to a ‘stage in the development of Platyhelminthes’ (Grundriss d. vergleich. Anatomie). Giard includes both in the ‘phylum Vermes,’ and regards them as much degenerated by parasitism; and Whitman—the latest investigator of the Dicyemids—speaks of them in a similar manner in his excellent work ‘Contributions to the Life-history and Classification of Dicyemids’ (Leipzig, 1882).

Footnote 79:

‘Dauer des Lebens;’ translated as the first essay in this volume.

Footnote 80:

See the first essay upon ‘The Duration of Life,’ p. 22 et seq.

Footnote 81:

‘Ursprung des Todes,’ p. 29.

Footnote 82:

l. c., p. 5.

Footnote 83:

See the preceding essay ‘On Heredity.’

Footnote 84:

The problem is very easily solved if we seek assistance from the principle of panmixia developed in the second essay ‘On Heredity.’ As soon as natural selection ceases to operate upon any character, structural or functional, it begins to disappear. As soon, therefore, as the immortality of somatic cells became useless they would begin to lose this attribute. The process would take place more quickly, as the histological differentiation of the somatic cells became more useful and complete, and thus became less compatible with their everlasting duration.—A. W. 1888.

Footnote 85:

See the preceding essay ‘On Heredity.’

Footnote 86:

See the first essay on ‘The Duration of Life.’

Footnote 87:

See the first essay on ‘The Duration of Life.’

Footnote 88:

These assumptions can be authenticated among the Infusoria. The encysted _Colpoda cucullus_, Ehrbg. divides into two, four, eight, or sixteen parts; _Otostoma Carteri_, into two, four, or eight; _Tillina magna_, Gruber, into four or five; _Lagynus_ sp. Gruber, into two; _Amphileptus meleagris_, Ehrbg. into two or four. The last two species and many others frequently do not divide at all during the encysted condition. But while any further increase in the number of divisions within the cyst does not occur in free-swimming Infusoria, the interesting case of _Ichthyophthirius multifiliis_, Fouquet, shows that parasitic habits call forth a remarkable increase in the number of divisions. This animal divides into at least a thousand daughter individuals.

Footnote 89:

True development also takes place in the above-mentioned _Ichthyophthirius_. While in other Infusoria the products of fission exactly resemble the parent, in _Ichthyophthirius_ they have a different form; the sucking mouth is wanting while provisional clasping cilia are at first present. In this case therefore the word germ may be rightly applied, and _Ichthyophthirius_ affords an interesting example of the phyletic origin of germs among the lower Flagellata and Gregarines. Cf. Fouquet, ‘Arch. Zool. Expérimentale,’ Tom. V. p. 159. 1876.

Footnote 90:

Bütschli, long ago, doubted the application of the fundamental law of biogenesis to the Protozoa (cf. ‘Ueber die Entstehung der Schwärmsprösslings der Podophrya quadripartita,’ Jen. Zeit. f. Med. u. Naturw. Bd. X. p. 19, Note). Gruber has more recently expressed similar views, and in fact denies the presence of development in the Protozoa, and only recognizes growth (‘Dimorpha mutans, Z. f. W. Z.’ Bd. XXXVII. p. 445). This proposition must however be restricted, inasmuch as a development certainly occurs, although one which is coenogenetic and not palingenetic.

Footnote 91:

See the first essay on ‘The Duration of Life,’ p. 23 _et seq._

Footnote 92:

See Appendix to the first essay on ‘The Duration of Life,’ pp. 43-46.

Footnote 93:

See the first essay on ‘The Duration of Life,’ p. 21.

IV.

THE CONTINUITY OF THE GERM-PLASM AS THE FOUNDATION OF A THEORY OF HEREDITY.

1885.

CONTINUITY OF THE GERM-PLASM, &c.

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), until after I had finished writing my essay, and I could not therefore make as much use of it as I should otherwise have done. Furthermore, the paper by Kölliker on ‘The Significance of the Nucleus in the Phenomena of Heredity’ (Die Bedeutung der Zellkerne für die Vorgänge der Vererbung), did not appear until after the completion of my manuscript. The essential treatment of the subject would not, however, have been altered if I had received the papers at an earlier date, for as far as the most important point—the significance of the nucleus—is concerned, my views are in accordance with those of both the above-named investigators; while the points upon which our views do not coincide had already received attention in the manuscript.

A. W.

Freiburg I. Breisgau,

June 16, 1885.

CONTINUITY OF THE GERM-PLASM, &c.

CONTENTS.

Introduction 165

I. The Germ-Plasm 174

1. Historical development of the theory as to the 174 localization of the germ-plasm in the nucleus

2. Nägeli’s ‘idioplasm’ is not identical with Weismann’s 180 ‘germ-plasm’

3. A retransformation of somatic idioplasm into germ- 183 idioplasm does not take place

4. Confirmation of the theory as to the significance of the 185 nuclear substance afforded by Nussbaum’s and Gruber’s experiments on regeneration in Infusoria

5. The nucleoplasm changes during ontogeny according to a 186 certain law

6. The identity of the daughter-nuclei produced by indirect 187 nuclear division, as assumed by Strasburger, is not necessary for my theory

7. The gradual decrease in complexity of the structure of 190 the nucleus during ontogeny

8. Nägeli’s view on the germs (‘Anlagen’) in the idioplasm 192

9. The manner in which germ-cells arise from somatic cells 194

10. ’Embryonic’ cells in the mature organism 196

11. The rule of probability is against a retransformation of 198 somatic idioplasm into germ-plasm

12. The regular cyclical development of the idioplasm 199 founded upon phylogeny by Nägeli

13. It follows from phyletic considerations that the germ- 201 cells have not arisen at the end of ontogeny

14. They originally arose at the beginning of ontogeny, but 202 at a later period the time of their origin was displaced

15. A continuity of the germ-cells does not now exist in 205 most cases

16. But there is a continuity of the germ-plasm 205

17. Strasburger’s objection to my supposition that the germ- 209 plasm passes along distinct routes

18. The cell-body may remain unchanged when the nucleus is 210 changed

19. It is conceivable that all somatic nuclei may contain 211 some germ-plasm

II. The Significance of the Polar Bodies 212

1. The egg-cell contains two kinds of idioplasm; germ-plasm 213 and histogenetic nucleoplasm

2. The expulsion of the polar bodies signifies the removal 214 of the histogenetic nucleoplasm

3. Other theories as to the significance of the polar 214 bodies

4. The modes of occurrence of polar bodies 217

5. Their possible occurrence in male germ-cells 219

6. There are two kinds of nucleoplasm in the male germ- 219 cells

7. Polar bodies in plants 222

8. Morphological origin of polar bodies 223

III. On the Nature of Parthenogenesis 225

1. The phenomena exhibited in the maturation of the egg are 225 identical in parthenogenetic and sexual development

2. The difference between parthenogenetic and sexual cells 226 must be of a quantitative nature

3. The quantity of the germ-plasm determines development 227

4. The expulsion of polar bodies depends upon the 230 antagonism between germ-plasm and ovogenetic nucleoplasm

5. Fertilization does not act dynamically 231

6. An insufficient quantity of germ-plasm arrests 232 development

7. Relation of the nucleus to the cell 234

8. The case of the bee does not constitute any objection to 234 my theory

9. Strasburger’s views upon parthenogenesis 237

10. Parthenogenesis does not depend upon abundant nutrition 239

11. The indirect causes of sexual and parthenogenetic 241 reproduction

12. The direct causes 242

13. Explanation of the formation of nutritive cells 243

14. Identity of the germ-plasm in male and female germ-cells 246

Note 249

IV.

THE CONTINUITY OF THE GERM-PLASM AS THE FOUNDATION OF A THEORY OF HEREDITY.

Introduction.