Chapter X: Preface (3)

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.

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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.

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IV.

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

1885.

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CONTINUITY OF THE GERM-PLASM, &c.