colony at the beginning of May: the larvæ, which hatch from the first
eggs, which are about fifteen in number, become pupæ at the beginning
of June, and the imagos appear towards the end of the same month. These
are all small ‘workers,’ and they perform such good service in tending
the second brood, that the latter attain the size of the female which
founded the colony; only differing from her in the perfect condition of
their wings, for by this time her wings are greatly worn away.
The males appear at the beginning of July; their spermatozoa are mature
in August, and pairing then takes place with certain ‘special females
which require fertilization’ which have in the meantime emerged from
their cocoons. These are the females which live through the winter and
found new colonies in the following spring. The old females of the
previous winter die, and do not live beyond the summer at the beginning
of which they founded colonies. At the first appearance of frost, the
young fertilized females seek out winter quarters; the males which
never survive the winter, do not take this course, but perish in
October. The parthenogenetic females, which remain in the nest during
the nuptial flight, also perish.
The males of _Polistes gallica_ do not live longer than three
months—from July to the beginning of October; the parthenogenetic
females live a fortnight longer at the outside—from the middle of June
to October, but the later generations have a shorter life. The sexual
females alone live for about a year, including the winter sleep.
A similar course of events takes place in the genus _Vespa_. In both
these genera the possibility of reproduction is not restricted to a
single female in the nest, but is shared by a number of females. In the
genus _Apis_ alone is the division of labour complete, so that only a
single female (the queen) is at any one time capable of reproduction, a
power which differentiates it from the sterile workers.
Note 4. The Duration of Life of the Lower Marine Animals.
I have only met with one definite statement in the literature of this
part of the subject. It concerns a sea anemone,—which is a solitary and
not a colonial form. The English zoologist Dalyell, in August, 1828,
removed an _Actinia mesembryanthemum_ from the sea and placed it in an
aquarium[21]. It was a very fine individual, although it had not quite
attained the largest size; and it must have been at least seven years
old, as proved by comparison with other individuals reared from the
egg. In the year 1848, it was about thirty years old, and in the twenty
years during which it had been in captivity it had produced 334 young
Actiniae. Prof. Dohrn, of Naples, tells me that this Actinia is still
living to-day, and is shown as a curiosity to those who visit the
Botanical Gardens in Edinburgh. It is now (1882) at least sixty-one
years old[22].
Note 5. The Duration of Life in
Indigenous Terrestrial and Fresh-water Mollusca.
I am indebted to Herr Clessin—the celebrated student of our
mollusca—for some valuable notes upon our indigenous snails and
bivalves (_Lamellibranchiata_). I could not incorporate them in the
text, for a number of necessary details as to the conditions of life
are at present entirely unknown, or are at least only known in a very
fragmentary manner. No statistics as to the amount of destruction
suffered by the young are available, and even the number of eggs
produced annually is only known for a few species. I nevertheless
include Herr Clessin’s very interesting communications, as a
commencement to the life statistics of the Mollusca.
(1) ‘_Vitrinae_ are annual; the old animals die in the spring, after
having produced the spawn from which the young develope. These continue
to grow until the following spring.’
(2) ‘The _Succineae_ are mostly biennial; _Succinea putris_ probably
triennial. Fertilization takes place from June till the beginning of
August, and the young develope until the autumn. _Succinea Pfeifferi_
and _S. elegans_ live through the winter, and the fact is proved by
very distinct annual markings. Reproduction takes place in July and
August of the following year, and they die in the autumn. They continue
to grow until their death.’
(3) ‘The shells of our native species of _Pupa_, _Clausilia_, and
_Bulimus_ (with the exception of _Bulimus detritus_) show but faint
annual markings. They can hardly require more than two years for their
complete development. The great number of living individuals with
full-sized shells belonging to these genera, as compared with the
number which possess smaller shells, makes it probable that these
animals live in the mature condition longer than our other _Helicidae_.
I have always found full-sized shells present in at least two-thirds of
the individuals of these genera characterized by much-coiled shells—a
proportion which I have never seen among our larger _Helicidae_.
Nevertheless direct observations as to the length of life in the mature
condition are still wanting.’
(4) ‘The _Helicidae_ live from two to four years; _Helix sericea_, _H.
hispida_, two to three years; _H. hortensis_, _H. nemoralis_, _H.
arbustorum_, as a rule three years; _H. pomatia_ four years.
Fertilization is not in these species strictly confined to any one time
of year, but in the case of old animals takes place in the spring, as
soon as the winter sleep is over; while in the two-year-old animals it
also happens later in the summer.’
(5) ‘The _Hyalineae_ are mostly biennial: they seldom live three years,
and even in the largest species such an age is probably exceptional.
The smallest _Hyalineae_ and _Helicidae_ live at most two years. The
length of life is dependent upon the time at which the parents are
fertilized, for this decides whether the young begin to shift for
themselves early in the summer or later in the autumn, and so whether
the first year’s growth is large or small.’
(6) ‘The species of _Limnaeus_, _Planorbis_, and _Ancylus_ live two to
three years, that is they take two to three years to attain the full
size. _L. auricularis_ is mostly biennial, _L. palustris_ and _L.
pereger_ two to three years: I have found that the latter, in the
mountains at Oberstorf in the Bavarian Alps, may exceptionally attain
the age of four years, that is, it may possess three clearly defined
annual markings, whilst the specimens from the plain never showed more
than two.’
(7) ‘The _Paludinidae_ attain an age of three or four years.’
(8) ‘The smaller bivalves, _Pisidium_ and _Cyclas_, do not often live
for more than two years: the larger _Najadae_, on the other hand, often
live for more than ten years, and indeed they are not full grown until
they possess ten to fourteen annual markings. It is possible that
habitat may have great influence upon the length of life in this order.’
‘_Unio_ and _Anodonta_ become sexually mature in the third to the fifth
year.’
As far as I am aware but few statements exist upon the length of life
in marine mollusca, and these are for the most part very inexact. The
giant bivalve _Tridacna gigas_ must attain an age of 60 to 100
years[23]. All _Cephalopods_ live for at least over a year, and most of
them well over ten years; and the giant forms, sometimes mistaken for
‘sea-serpents,’ must require many decades in which to attain such a
remarkable size. L. Agassiz has determined the length of life in a
large sea snail, _Natica heros_, by sorting a great number of
individuals according to their sizes: he places it at 30 years[24].
I am glad to be able to communicate an observation made at the
Zoological Station at Naples upon the length of life in _Ascidians_.
The beautiful white _Cionea intestinalis_ has settled in great numbers
in an aquarium at the Station, and Professor Dohrn tells me that it
produces three generations annually, and that each individual lives for
about five months, and then reproduces itself and dies. External
conditions accounting for this early death have not been discovered.
It is known that the freshwater _Polyzoa_ are annual, but it is not
known whether the first individuals produced from a colony in the
spring, live for the whole summer. The length of life is also unknown
in single individuals of any marine Polyzoon.
Clessin’s accurate statements upon the freshwater Mollusca, previously
quoted, show that a surprisingly short length of life is the general
rule. Only those forms of which the large size requires that many years
shall elapse before the attainment of sexual maturity, live ten years
or over (_Unio_, _Anodonta_); indeed, our largest native snail (_Helix
pomatia_) only lives for four years, and many small species only one
year, or two years if the former time is insufficient to render them
sexually mature. These facts seem to indicate, as I think, that these
molluscs are exposed to great destruction in the adult state, indeed to
a greater extent than when they are young, or, at any rate, to an equal
extent. The facts appear to be the reverse of those found among birds.
The fertility is enormous; a single mussel contains several hundred
thousand eggs; the destruction of young as compared with the number of
eggs produced is distinctly smaller than in birds, therefore a much
shorter duration of the life of each mature individual is rendered
possible, and further becomes advantageous because the mature
individuals are exposed to severe destruction.
However it can only be vaguely suggested that this is the case, for
positive proofs are entirely absent. Perhaps the destruction of single
mature individuals does not play so important a part as the destruction
of their generative organs. The ravages of parasitic animals
(_Trematodes_) in the internal organs of snails and bivalves are well
known to zoologists. The ovaries of the latter are often entirely
filled with parasites, and such animals are then incapable of
reproduction.
Besides, molluscs have many enemies, which destroy them both on land
and in water. In the water,—fish, frogs, newts, ducks and other
water-fowl, and on land many birds, the hedgehog, toads, etc., largely
depend upon them for food.
If the principles developed in this essay apply to the freshwater
Mollusca, we must then infer that snails which maintain the mature
condition—the capability of reproduction—for one year, are in this
state more exposed to destruction from the attacks of enemies than
those species which remain sexually mature for two or three years, or
that the latter suffer from a greater proportional loss of eggs and
young.
Note 6. Unequal Length of Life in the two Sexes.
This inequality is frequently found among insects. The males of the
remarkable little parasites infesting bees, the _Strepsiptera_, only
live for two to three hours in the mature condition, while the
wingless, maggot-like, female lives eight days: in this case,
therefore, the female lives sixty-four times as long as the male. The
explanation of these relations is obvious; a long life for the male
would be useless to the species, while the relatively long life of the
female is a necessity for the species, inasmuch as she is viviparous,
and must nourish her young until their birth.
Again, the male of _Phylloxera vastatrix_ lives for a much shorter
period than the female, and is devoid of proboscis and stomach, and
takes no food: it fertilizes the female as soon as the last skin has
been shed and then dies.
Insects are not the only animals among which we find inequality in the
length of life of the two sexes. Very little attention has been
hitherto directed to this matter, and we therefore possess little or no
accurate information as to the duration of life in the sexes, but in
some cases we can draw inferences either from anatomical structure or
from the mode of development. Thus, male _Rotifers_ never possess
mouth, stomach, or intestine, they cannot take food, and without doubt
live much shorter lives than the females, which are provided with a
complete alimentary canal. Again, the dwarf males of many parasitic
_Copepods_—low Crustacea—and the ‘complementary males’ of _Cirrhipedes_
(or barnacles) are devoid of stomach, and must live for a much shorter
time than the females; and the male _Entoniscidae_ (a family of which
the species are endo-parasitic in the larger Crustacea), although they
can feed, die after fertilizing the females; while the latter then take
to a parasitic life, produce eggs, and continue to live for some time.
It is supposed that the dwarf male of _Bonellia viridis_ does not live
so long by several years as the hundred times larger female, and it too
has no mouth to its alimentary canal. These examples might be further
increased by reference to zoological literature.
In most cases the female lives longer than the male, and this needs no
special explanation; but the converse relation is conceivable, when,
for instance, the females are much rarer than the males, and the latter
lose much time in seeking them. The above-mentioned case of _Aglia tau_
probably belongs to this category.
We cannot always decide conclusively whether the life of one sex has
been lengthened or that of the other shortened; both these changes must
have taken place in different cases. There is no doubt that a
lengthening of life in the female has arisen in the bees and ants, for
both sexes of the saw-flies, which are believed to be the ancestors of
bees, only live for a few weeks. But among the _Strepsiptera_ the
shorter life of the male must have been secondarily acquired, since we
only rarely meet with such an extreme case in insects.
Note 7. Bees.
It has not been experimentally determined whether the workers, which
are usually killed after some months, would live as long as the queen,
if they were artificially protected from danger in the hive; but I
think that this is probable, because it is the case among ants, and
because the peculiarity of longevity must be latent in the egg. As is
well known, the egg which gives rise to the queen is identical with
that which produces a worker, and differences in the nutrition alone
decide whether a queen or a worker shall be formed. It is therefore
probable that the duration of life in queen and worker is potentially
the same.
Note 8. Death of the Cells in higher Organisms.
The opinion has been often expressed that the inevitable appearance of
normal ‘death’ is dependent on the wearing out of the tissues in
consequence of their functional activity. Bertin says, referring to
animal life[25]:—‘L’observation des faits y attache l’idée d’une
terminaison fatale, bien que la raison ne découvre nullement les motifs
de cette nécessité. Chez les êtres qui font partie du règne animal
l’exercise même de la rénovation moléculaire finit par user le principe
qui l’entretient sans doute parceque le travail d’échange ne
s’accomplissant pas avec une perfection mathématique, il s’établit dans
la figure, comme dans la substance de l’être vivant, une déviation
insensible, et que l’accumulation des écarts finit par amener un type
chimique ou morphologique incompatible avec la persistance de ce
travail.’
Here the replacement of the used-up elements of tissue by new ones is
not taken into account, but an attempt is made to show that the
functions of the whole organism necessarily cause it to waste away. But
the question at once arises, whether such a result does not depend upon
the fact that the single histological elements,—the cells,—are worn out
by the exercise of function. Bertin admits this to be the case, and
this idea of the importance of changes in the cells themselves is
everywhere gaining ground. But although we must admit that the
histological elements do, as a matter of fact, wear out, in
multicellular animals, this would not prove that, nor explain why, such
changes must follow from the nature of the cell and the vital processes
which take place within it. Such an admission would merely suggest the
question:—how is it that the cells in the tissues of higher animals are
worn out by their function, while cells which exist in the form of free
and independent organisms possess the power of living for ever? Why
should not the cells of any tissue, of which the equilibrium is
momentarily disturbed by metabolism, be again restored, so that the
same cells continue to perform their functions for ever:—why cannot
they live without their properties suffering alteration? I have not
sufficiently touched upon this point in the text, and as it is
obviously important it demands further consideration.
In the first place, I think we may conclude with certainty from the
unending duration of unicellular organisms, that such wearing out of
tissue cells is a secondary adaptation, that the death of the cell,
like general death, has arisen with the complex, higher organisms.
Waste does not depend upon the intrinsic nature of the cells, as the
primitive organisms prove to us, but it has appeared as an adaptation
of the cells to the new conditions by which they are surrounded when
they come into combination, and thus form the cell-republic of the
metazoan body. The replacement of cells in the tissues must be more
advantageous for the functions of the whole organism than the unlimited
activity of the same cells, inasmuch as the power of single cells would
be much increased by this means. In certain cases, these advantages are
obvious, as for example in many glands of which the secretions are made
up of cast-off cells. Such cells must die and be separated from the
organism, or the secretion would come to an end. In many cases,
however, the facts are obscure, and await physiological investigation.
But in the meantime we may draw some conclusions from the effects of
growth, which are necessarily bound up with a certain rate of
production of new cells. In the process of growth a certain degree of
choice between the old cells which have performed their functions up to
any particular time, and the new ones which have appeared between them,
is as it were left to the organism.
The organism may thus, figuratively speaking, venture to demand from
the various specific cells of tissues a greater amount of work than
they are able to bear, during the normal length of their life, and with
the normal amount of their strength. The advantages gained by the whole
organism might more than compensate for the disadvantages which follow
from the disappearance of single cells. The glandular secretions which
are composed of cell-detritus, prove that the cells of a complex
organism may acquire functions which result in the loosening of their
connexion with the living cell-community of the body, and their final
separation from it. And the same facts hold with the blood corpuscles,
for the exercise of their function results in ultimate dissolution.
Hence it is not only conceivable, but in every way probable, that many
other functions in the higher organisms involve the death of the cells
which perform them, not because the living cell is necessarily worn out
and finally killed by the exercise of any ordinary vital process, but
because the specific functions in the economy of the cell community
which such cells undertake to perform, involve the death of the cells
themselves. But the fact that such functions have appeared,—involving
as they do the sacrifice of a great number of cells,—entirely depends
upon the replacement of the old by newly formed cells, that is by the
process of reproduction in cells[26].
We cannot _a priori_ dispute the possibility of the existence of
tissues in which the cells are not worn out by the performance of
function, but such an occurrence appears to be improbable when we
recollect that the cells of all tissues owe their constitution to a
very far-reaching process of division of labour, which leaves them
comparatively one-sided, and involves the loss of many properties of
the unicellular, self-sufficient organism. At any rate we only know of
potential immortality in the cells which constitute independent
unicellular organisms, and the nature of these is such that they are
continually undergoing a complete process of reformation.
If we did not find any replacement of cells in the higher organism, we
should be induced to look upon death itself as the direct result of the
division of labour among the cells, and to conclude that the specific
cells of tissues have lost, as a consequence of the one-sided
development of their activities, the power of unending life, which
belongs to all independent primitive cells. We should argue that they
could only perform their functions for a certain time, and would then
die, and with them the organism whose life is dependent upon their
activity. The longer they are occupied with the performance of special
functions, the less completely do they carry out the phenomena of life,
and hence they lead to the appearance of retrogressive changes. But the
replacement of cells is certain in many tissues (in glands, blood,
etc.), so that we can never seek a satisfactory explanation in the
train of reasoning indicated above, but we must assume the existence of
limits to the replacement of cells. In my opinion, we can find an
explanation of this in the general relations of the single individual
to its species, and to the whole of the external conditions of life;
and this is the explanation which I have suggested and have attempted
to work out in the text.
Note 9. Death by Sudden Shock.
The most remarkable example of this kind of death known to me, is that
of the male bees. It has been long known that the drone perishes while
pairing, and it was usually believed that the queen bites it to death.
Later observations have however shown that this is not the case, but
that the male suddenly dies during copulation, and that the queen
afterwards bites through the male intromittent organ, in order to free
herself from the dead body. In this case death is obviously due to
sudden excitement, for when the latter is artificially induced, death
immediately follows. Von Berlepsch made some very interesting
observations on this point, ‘If one catches a drone by the wings,
during the nuptial flight, and holds it free in the air without
touching any other part, the penis is protruded and the animal
instantly dies, becoming motionless as though killed by a shock. The
same thing happens if one gently stimulates the dorsal surface of the
drone on a similar occasion. The male is in such an excited and
irritable condition that the slightest muscular movement or disturbance
causes the penis to be protruded[27].’ In this case death is caused by
the so-called nervous shock. The humble-bees are not similarly
constituted, for the male does not die after fertilizing the female,
‘but withdraws its penis and flies away.’ But the death of male bees,
during pairing, must not be regarded as normal death. Experiment has
shown that these insects can live for more than four months[28]. They
do not, as a matter of fact, generally live so long; for—although the
workers do not, as was formerly believed, kill them after the
fertilization of the queen, by direct means—they prevent them from
eating the honey and drive them from the hive, so that they die of
hunger[29].
We must also look upon death which immediately, or very quickly,
follows upon the deposition of eggs as death by sudden shock. The
females of certain species of _Psychidae_, when they reproduce
sexually, may remain alive for more than a week waiting for a male:
after fertilization, however, they lay their eggs and die, while the
parthenogenetic females of the same species lay their eggs and die
immediately after leaving the cocoon; so that while the former live for
many days, the latter do not last for more than twenty-four hours. ‘The
parthenogenetic form of _Solenobia triquetrella_, soon after emergence,
lays all her eggs together in the empty case, becomes much shrunken,
and dies in a few hours.’ (Letter from Dr. Speyer, Rhoden.)
Note 10. Intermingling during the Fission of Unicellular Organisms[30].