the increase of surface and mass, and it cannot be denied that these
relations do largely determine the size of the body. A beetle could
never reach the size of an elephant, because, constituted as it is, it
would be incapable of existence if it attained such dimensions. But
nevertheless the relations between surface and mass do not form the
only reason why any given individual does not exceed the average size
of its species. Each individual does not strive to grow to the largest
possible size, until the absorption from its digestive area becomes
insufficient for its mass; but it ceases to grow because its cells
cannot be sufficiently nourished in consequence of its increased size.
The giants which occasionally appear in the human species prove that
the plan upon which man is constructed can also be carried out on a
scale which is far larger than the normal one. If the size of the body
chiefly depends upon amount of nutriment, it would be possible to make
giants and dwarfs at will. But we know, on the contrary, that the size
of the body is hereditary in families to a very marked extent; in fact
so much so that the size of an individual depends chiefly upon
heredity, and not upon amount of food.
These observations point to the conclusion that the size of the
individual is in reality pre-determined, and that it is potentially
contained in the egg from which the individual developes.
We know further that the growth of the individual depends chiefly upon
the multiplication of cells and only to a slight extent upon the growth
of single cells. It is therefore clear that a limit of growth is
imposed by a limitation in the processes by which cells are increased,
both as regards the number of cells produced and the rate at which they
are formed. How could we otherwise explain the fact that an animal
ceases to grow long before it has reached the physiologically
attainable maximum of its species, without at the same time suffering
any loss of vital energy?
In many cases at least, the most important duty of an organism, viz.
reproduction, follows upon the attainment of full size—a fact which
induced Johannes Müller to reject the prevailing hypothesis which
explained the death of animals as due to ‘the influences of the
inorganic environment, which gradually wear away the life of the
individual.’ He argued that, if this were the case, ‘the organic energy
of an individual would steadily decrease from the beginning,’ while the
facts indicate that this is not so[5].
If it is further asked why the egg should give rise to a fixed number
of cell-generations, although perhaps a number which varies widely
within certain limits, we may now refer to the operation of natural
selection upon the relation of surface to mass, and upon other
physiological necessities which are peculiar to the species. Because a
certain size is the most favourable for a certain plan of organization,
the process of natural selection determined that such a size should be
within certain variable limits, characteristic of each species. This
size is then transmitted from generation to generation, for when once
established as normal for the species, the most favourable size is
potentially present in the reproductive cell from which each individual
is developed.
If this conclusion holds, and I believe that no essential objection can
be raised against it, then we have in the limitation in space a process
which is exactly analogous to the limitation in time, which we have
already considered. The latter limitation—the duration of life—also
depends upon the multiplication of cells, the rapid increase of which
first gave rise to the characteristic form of the mature body, and then
continued at a slower rate. In the mature animal, cell-reproduction
still goes on, but it no longer exceeds the waste; for some time it
just compensates for loss, and then begins to decline. The waste is not
compensated for, the tissues perform their functions incompletely, and
thus the way for death is prepared, until its final appearance by one
of the three great _Atria mortis_.
I admit that facts are still wanting upon which to base this
hypothesis. It is a pure supposition that senile changes are due to a
deficient reproduction of cells: at the same time this supposition
gains in probability when we are enabled to reduce the limitations of
the organism in both time and space to one and the same principle. It
cannot however be asserted under any circumstances that it is a pure
supposition that the ovum possesses a capacity for cell-multiplication
which is limited both as to numbers produced and rate of production.
The fact that each species maintains an average size is a sufficient
proof of the truth of this conclusion.
Hitherto I have only spoken of animals and have hardly mentioned
plants. I should not have been able to consider them at all, had it not
happened that a work of Hildebrand’s [See Note 12] has recently
appeared, which has, for the first time, provided us with exact
observations on the duration of plant-life.
The chief results obtained by this author agree very well with the view
which I have brought before you to-day. Hildebrand shows that the
duration of life in plants also is by no means completely fixed, and
that it may be very considerably altered through the agency of the
external conditions of life. He shows that, in course of time, and
under changed conditions of life, an annual plant may become perennial,
or _vice versa_. The external factors which influence the duration of
life are here however essentially different, as indeed we expect them
to be, when we remember the very different conditions under which the
animal and vegetable kingdoms exist. During the life of animals the
destruction of mature individuals plays a most important part, but the
existence of the mature plant is fairly well secured; their chief
period of destruction is during youth, and this fact has a direct
influence upon the degree of fertility, but not upon the duration of
life. Climatic considerations, especially the periodical changes of
summer and winter, or wet and dry seasons, are here of greater
importance.
It must then be admitted that the dependence of the duration of life
upon the external conditions of existence is alike common to plants and
animals. In both kingdoms the high multicellular forms with
well-differentiated organs contain the germs of death, while the low
unicellular organisms are potentially immortal. Furthermore, an undying
succession of reproductive cells is possessed by all the higher forms,
although this may be but poor consolation to the conscious individual
which perishes. Johannes Müller is therefore right, when in the
sentence quoted at the beginning of my lecture, he speaks of an
‘appearance of immortality’ which passes from each individual into that
which succeeds it. That which remains over, that which persists, is not
the individual itself,—not the complex aggregate of cells which is
conscious of itself,—but an individuality which is outside its
consciousness, and of a low order,—an individuality which is made up of
a single cell, which arises from the conscious individual. I might here
conclude, but I wish first, in a few words, to protect myself against a
possible misunderstanding.
I have repeatedly spoken of immortality, first of the unicellular
organism, and secondly of the reproductive cell. By this word I have
merely intended to imply a duration of time which appears to be endless
to our human faculties. I have no wish to enter into the question of
the cosmic or telluric origin of life on the earth. An answer to this
question will at once decide whether the power of reproduction
possessed by these cells is in reality eternal or only immensely
prolonged, for that which is without beginning is, and must be, without
end.
The supposition of a cosmic origin of life can only assist us if by its
means we can altogether dispense with any theory of spontaneous
generation. The mere shifting of the origin of life to some other
far-off world cannot in any way help us. A truly cosmic origin in its
widest significance will rigidly limit us to the statement—_omne vivum
e vivo_—to the idea that life can only arise from life, and has always
so arisen,—to the conclusion that organic beings are eternal like
matter itself.
Experience cannot help us to decide this question; we do not know
whether spontaneous generation was the commencement of life on the
earth, nor have we any direct evidence for the idea that the process of
development of the living world carries the end within itself, or for
the converse idea that the end can only be brought about by means of
some external force.
I admit that spontaneous generation, in spite of all vain efforts to
demonstrate it, remains for me a logical necessity. We cannot regard
organic and inorganic matter as independent of each other and both
eternal, for organic matter is continually passing, without residuum,
into the inorganic. If the eternal and indestructible are alone without
beginning, then the non-eternal and destructible must have had a
beginning. But the organic world is certainly not eternal and
indestructible in that absolute sense in which we apply these terms to
matter itself. We can, indeed, kill all organic beings and thus render
them inorganic at will. But these changes are not the same as those
which we induce in a piece of chalk by pouring sulphuric acid upon it;
in this ease we only change the form, and the inorganic matter remains.
But when we pour sulphuric acid upon a worm, or when we burn an oak
tree, these organisms are not changed into some other animal and tree,
but they disappear entirely as organized beings and are resolved into
inorganic elements. But that which can be completely resolved into
inorganic matter must have also arisen from it, and must owe its
ultimate foundation to it. The organic might be considered eternal if
we could only destroy its form, but not its nature.
It therefore follows that the organic world must once have arisen, and
further that it will at some time come to an end. Hence we must speak
of the eternal duration of unicellular organisms and of reproductive
cells in the Metazoa and Metaphyta in that particular sense which
signifies, when measured by our standards, an immensely long time.
Yet who can maintain that he has discovered the right answer to this
important question? And even though the discovery were made, can any
one believe that by its means the problem of life would be solved? If
it were established that spontaneous generation did actually occur, a
new question at once arises as to the conditions under which the
occurrence became possible. How can we conceive that dead inorganic
matter could have come together in such a manner as to form living
protoplasm, that wonderful and complex substance which absorbs foreign
material and changes it into its own substance, in other words grows
and multiplies?
And so, in discussing this question of life and death, we come at
last—as in all provinces of human research—upon problems which appear
to us to be, at least for the present, insoluble. In fact it is the
quest after perfected truth, not its possession, that falls to our lot,
that gladdens us, fills up the measure of our life, nay! hallows it.
APPENDIX.
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 facts which I have been able to collect, as
a slight contribution towards such a compilation.
* * * * *
Small singing birds live from eight to eighteen years: the nightingale,
in captivity, eight years, but longer according to some writers: the
blackbird, in captivity, twelve years, but both these birds live longer
in the natural state. A ‘half-bred nightingale built its nest for nine
consecutive years in the same garden’ (Naumann, ‘Vögel Deutschlands,’
p. 76).
Canary birds in captivity attain an age of twelve to fifteen years (l.
c., p. 76).
Ravens have lived for almost a hundred years in captivity (l. c., Bd.
I. p. 125).
Magpies in captivity live twenty years, and, ‘without doubt,’ much
longer in the natural state (l. c., p. 346).
Parrots ‘in captivity have reached upwards of a hundred years’ (l. c.,
p. 125).
A single instance of the cuckoo (alluded to in the text) is mentioned
by Naumann as reaching the age of thirty-two years (l. c., p. 76).
Fowls live ten to twenty years, the golden pheasant fifteen years, the
turkey sixteen years, and the pigeon ten years (Oken, ‘Naturgeschichte,
Vögel,’ p. 387).
A golden eagle which ‘died at Vienna in the year 1719, had been
captured 104 years previously’ (Brehm, ‘Leben der Vögel,’ p. 72).
A falcon (species not mentioned) is said to have attained an age of 162
years (Knauer, ‘Der Naturhistoriker,’ Vienna, 1880).
A white-headed vulture which was taken in 1706 died in the Zoological
Gardens at Vienna (Schönbrunn) in 1824, thus living 118 years in
captivity (l. c.).
The example of the bearded vulture, mentioned in the text, is quoted
from Schinz’s ‘Vögel der Schweiz,’ p. 196.
The wild goose must live for upwards of 100 years, according to Naumann
(l. c., p. 127). The proof of this is not, however, forthcoming. A wild
goose which had been wounded reached its eighteenth year in captivity.
Swans are said to have lived 300 years(?), (Naumann, l. c., p. 127).
It is evident that observations upon the duration of life in wild birds
can only rarely be made, and that they are usually the result of chance
and cannot be verified. It is on this account all the more to be
desired that every ascertained fact should be collected.
If the long life of birds has been correctly interpreted as
compensation for their feeble fertility and for the great mortality of
their young, it will be possible to estimate the length of life in a
species, without direct observation, if we only know its fertility and
the percentage of individuals destroyed. This percentage can, however,
at best, be known only as an average. If we consider, for example, the
enormous number of sea birds which breed in summer on the rocks and
cliffs of the northern seas, and if we remember that the majority of
these birds lay but one, or at most two eggs yearly, and that their
young are exposed to very many destructive agencies, we are forced to
the conclusion that they must possess a very long life, so that the
breeding period may be many times repeated. Their number does not
diminish. Year after year countless numbers of these birds cover the
rocks, from summit to sea line; millions of them rest there, and rise
in the air like a thick cloud whenever they are disturbed. Even in
those localities which are every year visited by man in order to effect
their capture, the number does not appear to decrease, unless the birds
are disturbed and are therefore prompted to seek other breeding-places.
From the small island of St. Kilda, off Scotland, 20,000 young gannets
(_Sula_) and an immense number of eggs are annually collected; and
although this bird only lays a single egg yearly and takes four years
to attain maturity, the numbers do not diminish[6]. 30,000 sea-gulls’
eggs and 20,000 terns’ eggs are yearly exported from the
breeding-places on the island of Sylt, but in this case it appears that
a systematic disturbance of the birds is avoided by the collectors, and
no decrease in their numbers has yet taken place[7]. The destruction of
northern birds is not only caused by man, but also by various
predaceous mammals and birds. Indeed the dense mass of birds which
throng the cliffs is a cause of destruction to many of the young and to
the eggs, which are pushed over the edge of the rocks. According to
Brehm the foot of these cliffs is ‘always covered with blood and the
dead bodies of fledglings.’
Such birds must attain a great age or they would have been exterminated
long ago: the minimum duration of life necessary for the maintenance of
the species must in their case be a very high one.
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.
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 entirely suspended for a time by freezing (_Anabiosis_:
Preyer[8]), or it is at any rate never more than a _vita minima_, with
a reduction of assimilation to its lowest point.
The following account does not make any claim to contain all or even
most of the facts scattered through the enormous mass of entomological
literature, and much less all that is privately known by individual
entomologists. It must therefore be looked upon as merely a first
attempt, a nucleus, around which the principal facts can be gradually
collected. It is unnecessary to give any special information as to the
duration of larval life, for numerous and exact observations upon this
part of the subject are contained in all entomological works.
I. Orthoptera.
Gryllotalpa. The eggs are laid in June or July, and the young are
hatched in from two to three weeks; they live through the winter, and
become sexually mature in the following May or June. ‘When the female
has deposited her eggs, her body collapses, and afterwards she does not
survive much longer than a month.’ ‘According as the females are
younger or older, they live a longer or shorter life, and hence some
females are even found in the autumn’ (Rösel, ‘Insektenbelustigungen,’
Bd. II. p. 92). Rösel believes that the female watches the eggs until
they are hatched, and this explains the fact that she outlives the
process of oviposition by about a month. It is not stated whether the
males die at an earlier period.
_Gryllus campestris_ becomes sexually mature in May, and sings from
June till October, ‘when they all die’ (Oken, ‘Naturgeschichte,’ Bd.
II. Abth. iii. p. 1527). It is hardly probable that any single
individual lives for the whole summer; probably, as in the case of
_Gryllotalpa_, the end of the life of those individuals which first
become mature, overlaps the beginning of the life of others which reach
maturity at a later date.
_Locusta viridissima_ and _L. verrucivora_ are mature at the end of
August; they lay their eggs in the earth during the first half of
September and then die. It is probable that the females do not live for
more than four weeks in the mature state. It is not known whether the
males of this or other species of locusts live for a shorter period.
I have found _Locusta cantans_ in plenty, from the beginning of
September to the end of the month. In captivity they die after
depositing their eggs: the males are probably more short-lived, for
towards the middle and end of September they are much less plentiful
than the females.
_Acridium migratorium_ ‘dies after the eggs are laid’ (Oken,
‘Naturgeschichte’).
The male _Termes_ probably live for a short time only, although exact
observations upon the point are wanting. The females ‘seem sometimes to
live four or five years,’ as I gather from a letter from Dr. Hagen, of
Cambridge, Mass., U.S.A.
_Ephemeridae._ Rösel, speaking of _Ephemera vulgata_
(‘Insektenbelustigungen,’ Bd. II. der Wasserinsekten, 2^{te} Klasse, p.
60 et seq.), says:—‘Their flight commences at sunset, and comes to an
end before midnight, when the dew begins to fall.’ ‘The pairing
generally takes place at night and lasts but a short time. As soon as
the insects have shed their last skin, in the afternoon or evening,
they fly about in thousands, and pair almost immediately; but by the
next day they are all dead. They continue to emerge for many days, so
that when yesterday’s swarm is dead, to-day a new swarm is seen
emerging from the water towards the evening.’ ‘They not only drop their
eggs in the water, but wherever they may happen to be,—on trees,
bushes, or the earth. Birds, trout and other fish lie in wait for them.’
Dr. Hagen writes to me—‘It is only in certain species that life is so
short. The female _Palingenia_ does not live long enough to complete
the last moult of the sub-imago. I believe that a female imago has
never been seen. The male imago, often half in its sub-imago skin,
fertilizes the female sub-imago and immediately the contents of both
ovaries are extruded, and the insect dies. It is quite possible that
the eggs pass out by rupturing the abdominal segments.’
_Libellula._ All dragon-flies live in the imago condition for some
weeks; at first they are not capable of reproduction, but after a few
days they pair.
_Lepisma saccharina._ An individual lived for two years in a pill-box,
without any food except perhaps a little _Lycopodium_ dust[9].
II. Neuroptera.
_Phryganids_ ‘live in the imago stage for at least a week and probably
longer, apparently without taking food’ (letter from Dr. Hagen).
According to the latest researches _Phrygane grandis_[10] never
contains food in its alimentary canal, but only air, although it
contains the latter in such quantities that the anterior end of the
chylific ventricle is dilated by it.
III. Strepsiptera.
The larva requires for its development a rather shorter time than that
which is necessary for the grub of the bee into the body of which it
has bored. The pupa stage lasts eight to ten days. The male, which
flies about in a most impetuous manner, lives only two to three hours,
while the female lives for some days. Possibly the pairing does not
take place until the female is two to three days old. The viviparous
female seems to produce young only once in a lifetime, and then dies:
it is at present uncertain whether she also produces young
parthenogenetically (cf. Siebold, ‘Ueber Paedogenesis der
Strepsipteren,’ Zeitschr. f. Wissensch. Zool., Band. XX, 1870).
IV. Hemiptera.
_Aphis._ Bonnet (‘Observations sur les Pucerons,’ Paris, 1745) had a
parthenogenetic female of _Aphis euonymi_ in his possession for
thirty-one days, from its birth, during which time it brought forth
ninety-five larvae. Gleichen kept a parthenogenetic female of _Aphis
mali_ fifteen to twenty-three days.
_Aphis foliorum ulmi._ The mother of a colony which leaves the egg in
May is 2‴ long at the end of July: it therefore lives for at least two
and a half months (De Geer, ‘Abhandlungen zur Geschichte der Insekten,’
1783, III. p. 53).
_Phylloxera vastatrix._ The males are merely ephemeral sexual
organisms, they have no proboscis and no alimentary canal, and die
immediately after fertilizing the female.
_Pemphigus terebinthi._ The male as well as the female sexual
individuals are wingless and without a proboscis; they cannot take food
and consequently live but a short time,—far shorter than the
parthenogenetic females of the same species (Derbès, ‘Note sur les
aphides du pistachier térébinthe,’ Ann. des sci. nat., Tom. XVII, 1872).
_Cicada._ In spite of the numerous and laborious descriptions of the
Cicadas which have appeared during the last two centuries, I can only
find precise statements as to the duration of life in the mature insect
in a single species. P. Kalm, writing upon the North American _Cicada
septemdecim_, which sometimes appears in countless numbers, states that
‘six weeks after (such a swarm had been first seen) they had all
disappeared.’ Hildreth puts the life of the female at from twenty to
twenty-five days. This agrees with the fact that the Cicada lays many
hundred eggs (Hildreth states a thousand); sixteen to twenty at a time
being inserted into a hole which is bored in wood, so that the female
takes some time to lay her eggs (Oken, ‘Naturgeschichte,’ 2^{ter} Bd.
3^{te} Abth. p. 1588 et seq.).