TRANSLATIONS
OF
FOREIGN BIOLOGICAL MEMOIRS
IV.
------------------------------------------------------------------------
London
_HENRY FROWDE_
OXFORD UNIVERSITY PRESS WAREHOUSE
AMEN CORNER, E.C.
------------------------------------------------------------------------
ESSAYS UPON HEREDITY
AND KINDRED
BIOLOGICAL PROBLEMS
BY
Dr. AUGUST WEISMANN
PROFESSOR IN THE UNIVERSITY OF FREIBURG IN BREISGAU
_AUTHORISED TRANSLATION_
EDITED BY
EDWARD B. POULTON, M.A., F.L.S., F.G.S.
TUTOR OF KEBLE COLLEGE, OXFORD
LECTURER IN NATURAL SCIENCE, JESUS COLLEGE, OXFORD
SELMAR SCHÖNLAND, Ph.D.
SUB-CURATOR OF THE FIELDING HERBARIUM IN THE UNIVERSITY OF OXFORD
AND
ARTHUR E. SHIPLEY, M.A., F.L.S.
FELLOW AND LECTURER OF CHRIST'S COLLEGE, CAMBRIDGE
DEMONSTRATOR OF COMPARATIVE ANATOMY IN THE UNIVERSITY OF CAMBRIDGE
Oxford
AT THE CLARENDON PRESS
1889
------------------------------------------------------------------------
AUTHOR’S PREFACE.
The essays which now appear for the first time in the form of a single
volume were not written upon any prearranged plan, but have been
published separately at various intervals during the course of the last
seven years. Although when writing the earlier essays I was not aware
that the others would follow, the whole series is, nevertheless,
closely connected together. The questions which each essay seeks to
explain have all arisen gradually out of the subjects treated in the
first. Reflecting upon the causes which regulate the duration of life
in various forms, I was drawn on to the consideration of fresh
questions which demanded further research. These considerations and the
results of such research form the subject-matter of all the subsequent
essays.
I am here making use of the word ‘research’ in a sense somewhat
different from that in which it is generally employed in natural
science; for it is commonly supposed to imply the making of new
observations. Some of these essays, especially Nos. IV, V, and VI,
essentially depend upon new discoveries. But in most of the remaining
essays the researches are of a more abstract nature, and consist in
bringing forward new points of view, founded upon a variety of
well-known facts. I believe, however, that the history of science
proves that advance is not only due to the discovery of new facts, but
also to their correct interpretation: a true conception of natural
processes can only be arrived at in this way. It is chiefly in this
sense that the contents of these essays are to be looked upon as
research.
The fact that they contain the record of research made it impossible to
introduce any essential alterations in the translation, even in those
points about which my opinion has since changed to some extent. I
should to-day express some of the points in Essays I, IV, and V,
somewhat differently; but had I made such alterations, the relation
between the essays as a whole would have been rendered less clear, for
each of the earlier ones formed the foundation of that which succeeded
it. Even certain errors of interpretation are on this account left
uncorrected. Thus, for instance, in Essay IV it is assumed that the two
polar bodies expelled by sexual eggs are identical; for at that time
there was no reason for doubting that they were physiologically
equivalent. The discovery of the numerical law of the polar bodies
described in Essay VI, led to what I believe to be a truer knowledge of
them. In this way the causes of parthenogenesis, as developed in Essay
V, received an important addition in the fact published in Essay VI,
that only one polar body is expelled by parthenogenetic eggs. This fact
alone explains why sexual eggs cannot as a rule develope without
fertilization.
Hence the reader must not take the individual essays as the full and
complete expression of my present opinion; but they must rather be
looked upon as stages in research, as steps towards a more perfect
knowledge.
I must therefore express the hope that the essays may be read in the
same order as that in which they appeared, and in which they are
arranged in the present volume. The reader will then follow the same
road which I traversed in the development of the views here set forth;
and even though he may be now and then led away from the direct route,
perhaps such deviations may not be without interest.
I should wish to express my warm thanks to Mr. Poulton for the great
trouble he has taken in editing the translation, which in many places
presented exceptional difficulties. The greater part of the text I have
looked through in proof, and I believe that it well expresses the sense
of the original; although naturally I cannot presume to judge
concerning the niceties of the English language. I am especially
grateful to the three gentlemen who have brought these essays before an
English public, because I believe that many English naturalists, even
when thoroughly conversant with the German tongue, might possibly
misinterpret many points in the original; for the difficulty of the
questions treated of greatly increases the difficulty of the language.
If the readers of this book only feel half as much pleasure in its
perusal as I experienced in writing it, I shall be more than satisfied.
AUGUST WEISMANN.
Freiburg I. Breisgau,
_January, 1889_.
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EDITORS’ PREFACE.
The attention of English biologists and men of science was first called
to Professor Weismann’s essays by an article entitled ‘Death’ in ‘The
Nineteenth Century’ for May, 1885, by Mr. A. E. Shipley. Since then the
interest in the author’s arguments and conclusions has become very
general; having been especially increased by Professor Moseley’s two
articles in ‘Nature’ (Vol. XXXIII, p. 154, and Vol. XXXIV, p. 629), and
by the discussion upon ‘The Transmission of Acquired Characters,’
introduced by Professor Lankester at the meeting of the British
Association at Manchester in 1887,—a discussion in which Professor
Weismann himself took part. The deep interest which has everywhere been
expressed in a subject which concerns the very foundations of
evolution, has encouraged the Editors to hope that a volume containing
a collection of all Professor Weismann’s essays upon heredity and
kindred problems would supply a real want. At the present time, when
scientific periodicals contain frequent references to these essays, and
when the various issues which have been raised by them are discussed on
every occasion at which biologists come together, it is above all
things necessary to know exactly what the author himself has said. And
there are many signs that discussion has already suffered for want of
this knowledge.
A translation of Essays I and II was commenced by Mr. A. E. Shipley
during his residence at Freiburg in the winter of 1884. His work was
greatly aided by the kind assistance of Dr. van Rees of Amsterdam, to
whom we desire to express our most sincere thanks. The translation was
laid aside until the summer of 1888, when Mr. Shipley was invited to
co-operate with the other editors in the preparation of the present
volume; the Clarendon Press having consented to publish the complete
series of essays as one of their Foreign Biological Memoirs.
We think it probable that this work may interest many who are not
trained biologists, but who approach the subject from its philosophical
or social aspects. Such readers would do well to first study Essays I,
II, VII, and VIII, inasmuch as some preparation for the more technical
treatment pursued in the other essays will thus be gained.
The notes signed A. W. and dated, were added by the author during the
progress of the translation. The notes included in square brackets were
added by the Editors; the authorship being indicated by initials in all
cases.
In conclusion, it is our pleasant duty to thank those who have kindly
helped us by reading the proof-sheets and making valuable suggestions.
Our warmest thanks are due to Mrs. Arthur Lyttelton, Mr. W. Hatchett
Jackson, Deputy Linacre Professor in the University of Oxford, Mr. J.
S. Haldane, and Professor R. Meldola. Important suggestions were also
made by Professor E. Ray Lankester, Mr. Francis Galton, and Dr. A. R.
Wallace. Professor W. N. Parker also greatly helped us by looking over
the proof-sheets with Professor Weismann.
E. B. P.
S. S.
A. E. S.
Oxford, _February, 1889_.
------------------------------------------------------------------------
CONTENTS.
_Translator._ A. E. Shipley
I. The Duration of Life, 1881 1
II. On Heredity, 1883 67
III. Life and Death, 1883 107
_Translator._ Selmar Schönland
IV. The Continuity of the Germ-plasm as the 161
Foundation of a Theory of Heredity, 1885
V. The Significance of Sexual Reproduction 251
in the Theory of Natural Selection, 1886
VI. On the Number of Polar Bodies and their 333
Significance in Heredity, 1887
VII. On the Supposed Botanical Proofs of the 385
Transmission of Acquired Characters,
1888.
VIII. The Supposed Transmission of 419
Mutilations, 1888
------------------------------------------------------------------------
_Abstracts of Professor Weismann’s Essays on Heredity and Kindred
Problems, already Published in this Country._
I. A short abstract in ‘Nature,’ Vol. XXXVII, pp.
541-542, by P. C. Mitchell.
II. A short abstract in ‘Nature,’ Vol. XXXVIII, pp.
156-157, by P. C. Mitchell.
III. A short article on the subject of this Essay in
‘The Nineteenth Century’ for May, 1885, by A. E.
Shipley.
IV. Abstract in ‘Nature,’ Vol. XXXIII, pp. 154-157, by
Professor Moseley.
V. Abstract in ‘Nature,’ Vol. XXXIV, pp. 629-632, by
Professor Moseley.
VI. Abstract in ‘Nature,’ Vol. XXXVI, pp. 607-609, by
Professor Weismann.
VII, VIII. The Essays being of so recent a date no
abstract has yet appeared in this country.
A criticism of Professor Weismann’s theories will be found in ‘The
Physiology of Plants,’ by Professor Vines, Lecture XXIII, pp. 660 et
seqq.
------------------------------------------------------------------------
I.
THE DURATION OF LIFE.
1881.
------------------------------------------------------------------------
THE DURATION OF LIFE.
PREFACE.
The following paper was read at the meeting of the Association of
German Naturalists at Salzburg, on September 21st, 1881; and it is here
printed in essentially the same form. A somewhat longer discussion of a
few points has been now intercalated; these were necessarily omitted
from the lecture itself for the sake of brevity, and are, therefore,
not contained in the account printed in the Proceedings of the
fifty-fourth meeting of the Association.
Further additions would not have been admissible without an essential
change of form, and therefore I have not put into the text a note which
ought otherwise to have been there, and which is now to be found in the
Appendix, as Note 8. It fills up a gap which was left in the text, for
the above-mentioned reason, by attempting to give an explanation of the
normal death of cells of tissues—an explanation which is required if we
are to maintain that unicellular organisms are so constituted as to be
potentially immortal.
The other parts of the Appendix contain, partly further expansions,
partly proofs of the views brought forward in the text, and above all a
compilation of all the observations which are known to me upon the
duration of life in several groups of animals. I am indebted to several
eminent specialists for the communication of many data, which are among
the most exact that I have been able to obtain. Thus Dr. Hagen of
Cambridge (U.S.A.) was kind enough to send me an account of his
observations upon insects of different orders: Mr. W. H. Edwards of
West Virginia, and Dr. Speyer of Rhoden—their experience with
butterflies. Dr. Adler of Schleswig sent me data upon the duration of
life in _Cynipidae_, which have a special value, as they are
accompanied by very exact observations upon the conditions of life in
these animals; hence in this case we can directly examine the factors
upon which, as I believe, the duration of life is chiefly based. Sir
John Lubbock in England, and Dr. August Forel of Zürich, have had the
kindness to send me an account of their observations upon ants, and S.
Clessin of Ochsenfurth his researches upon our native land and
fresh-water Mollusca.
In publishing these valuable communications, together with all facts
which I have been able to collect from literature upon the subject of
the duration of life, and the little which I have myself observed upon
this subject, I hope to provide a stimulus for further observation in
this field, which has been hitherto much neglected. The views which I
have brought forward in this paper are based on a comparatively small
number of facts, at least as far as the duration of life in various
species is concerned. The larger the number of accurate data which are
supplied, and the more exactly the duration of life and its conditions
are ascertained, the more securely will it be possible to establish our
views upon the causes which determine the duration of life.
A. W.
Naples, _Dec. 6, 1881_.
I.
THE DURATION OF LIFE.
With your permission, I will bring before you to-day some thoughts upon
the subject of the duration of life. I can scarcely do better than
begin with the simple but significant words of Johannes Müller:
‘Organic bodies are perishable; while life maintains the appearance of
immortality in the constant succession of similar individuals, the
individuals themselves pass away.’
Omitting, for the time being, any discussion as to the precise accuracy
of this statement, it is at any rate obvious that the life of an
individual has its natural limit, at least among those animals and
plants which are met with in every-day life. But it is equally obvious
that the limits are very differently placed in the various species of
animals and plants. These differences are so manifest that they have
given rise to popular sayings. Thus Jacob Grimm mentions an old German
saying, ‘A wren lives three years, a dog three times as long as a wren,
a horse three times as long as a dog, and a man three times as long as
a horse, that is eighty-one years. A donkey attains three times the age
of a man, a wild goose three times that of a donkey, a crow three times
that of a wild goose, a deer three times that of a crow, and an oak
three times the age of a deer.’
If this be true a deer would live 6000 years, and an oak nearly 20,000
years. The saying is certainly not founded upon exact observation, but
it becomes true if looked upon as a general statement that the duration
of life is very different in different organisms.
The question now arises as to the causes of these great differences.
How is it that individuals are endowed with the power of living long in
such very various degrees?
One is at first tempted to seek the answer by an appeal to the
differences in morphological and chemical structure which separate
species from one another. In fact all attempts to throw light upon the
subject which have been made up to the present time lie in this
direction.
All these explanations are nevertheless insufficient. In a certain
sense it is true that the causes of the duration of life must be
contained in the organism itself, and cannot be found in any of its
external conditions or circumstances. But structure and chemical
composition—in short the physiological constitution of the body in the
ordinary sense of the words—are not the only factors which determine
duration of life. This conclusion forces itself upon our attention as
soon as the attempt is made to explain existing facts by these factors
alone: there must be some other additional cause contained in the
organism as an unknown and invisible part of its constitution, a cause
which determines the duration of life.
The size of the organism must in the first place be taken into
consideration. Of all organisms in the world, large trees have the
longest lives. The Adansonias of the Cape Verd Islands are said to live
for 6000 years. The largest animals also attain the greatest age. Thus
there is no doubt that whales live for some hundreds of years.
Elephants live 200 years, and it would not be difficult to construct a
descending series of animals in which the duration of life diminishes
in almost exact proportion to the decrease in the size of the body.
Thus a horse lives forty years, a blackbird eighteen, a mouse six, and
many insects only a few days or weeks.
If however the facts are examined a little more closely it will be
observed that the great age (200 years) reached by an elephant is also
attained by many smaller animals, such as the pike and carp. The horse
lives forty years, but so does a cat or a toad; and a sea anemone has
been known to live for over fifty years. The duration of life in a pig
(about twenty years) is the same as that in a crayfish, although the
latter does not nearly attain the hundredth part of the weight of a pig.
It is therefore evident that length of life cannot be determined by the
size of the body alone. There is, however, some relation between these
two attributes. A large animal lives longer than a small one because it
is larger; it would not be able to become even comparatively large
unless endowed with a comparatively long duration of life.
Apart from all other reasons, no one could imagine that the gigantic
body of an elephant could be built up like that of a mouse in three
weeks, or in a single day like that of the larva of certain flies. The
gestation of an elephant lasts for nearly two years, and maturity is
only reached after a lapse of about twenty-four years.
Furthermore, to ensure the preservation of the species, a longer time
is required by a large animal than by a small one, when both have
reached maturity. Thus Leuckart and later Herbert Spencer have pointed
out that the absorbing surface of an animal only increases as the
square of its length, while its size increases as the cube; and it
therefore follows that the larger an animal becomes, the greater will
be the difficulty experienced in assimilating any nourishment over and
above that which it requires for its own needs, and therefore the more
slowly will it reproduce itself.
But although it may be stated generally that the duration of the period
of growth and length of life are longest in the largest animals, it is
nevertheless impossible to maintain that there is any fixed relation
between the two; and Flourens was mistaken when he considered that the
length of life was always equivalent to five times the duration of the
period of growth. Such a conclusion might be accepted in the case of
man if we set his period of growth at twenty years and his length of
life at a hundred; but it cannot be accepted for the majority of other
Mammalia. Thus the horse lives from forty to fifty years, and the
latter age is at least as frequently reached among horses as a hundred
years among men; but the horse becomes mature in four years, and the
length of its life is thus ten or twelve times as long as its period of
growth.
The second factor which influences the duration of life is purely
physiological: it is the rate at which the animal lives, the rapidity
with which assimilation and the other vital processes take place. Upon
this point Lotze remarks in his Microcosmus—‘Active and restless
mobility destroys the organized body: the swift-footed animals hunted
by man, as also dogs, and even apes, are inferior in length of life to
man and the larger beasts of prey, which satisfy their needs by a few
vigorous efforts.’ ‘The inertness of the Amphibia is, on the other
hand, accompanied by relatively great length of life.’
There is certainly some truth in these observations, and yet it would
be a great mistake to assume that activity necessarily implies a short
life. The most active birds have very long lives, as will be shown
later on: they live as long as and sometimes longer than the majority
of Amphibia which reach the same size. The organism must not be looked
upon as a heap of combustible material, which is completely reduced to
ashes in a certain time the length of which is determined by size, and
by the rate at which it burns; but it should be rather compared to a
fire, to which fresh fuel can be continually added, and which, whether
it burns quickly or slowly, can be kept burning as long as necessity
demands.
The connection between activity and shortness of life cannot be
explained by supposing that a more rapid consumption of the body
occurs, but it is explicable because the increased rate at which the
vital processes take place permit the more rapid achievement of the aim
and purpose of life, viz. the attainment of maturity and the
reproduction of the species.
When I speak of the aim and purpose of life, I am only using figures of
speech, and I do not mean to imply that nature is in any way working
consciously.
When I was speaking of the relation between duration of life and the
size of the body, I might have added another factor which also exerts
some influence, viz. the complexity of the structure. Two organisms of
the same size, but belonging to different grades of organization, will
require different periods of time for their development. Certain
animals of a very lowly organization, such as the Rhizopoda, may attain
a diameter of ·5 mm. and may thus become larger than many insects’
eggs. Yet under favourable circumstances an Amoeba can divide into two
animals in ten minutes, while no insect’s egg can develope into the
young animal in a less period than twenty-four hours. Time is required
for the development of the immense number of cells which must in the
latter case arise from the single egg-cell.
Hence we may say that the peculiar constitution of an animal does in
part determine the length of time which must elapse before reproduction
begins. The period before reproduction is however only part of the
whole life of an animal, which of course extends over the total period
during which the animal exists.
Hitherto it has always been assumed that the duration of this total
period is solely determined by the constitution of the animal’s body.
But the assumption is erroneous. The strength of the spring which
drives the wheel of life does not solely depend upon the size of the
wheel itself or upon the material of which it is made; and, leaving the
metaphor, duration of life is not exclusively determined by the size of
the animal, the complexity of its structure, and the rate of its
metabolism. The facts are plainly and clearly opposed to such a
supposition.
How, for instance, can we explain from this point of view the fact that
the queen-ant and the workers live for many years, while the males live
for a few weeks at most? The sexes are not distinguished by any great
difference in size or complexity of body, or in the rate of metabolism.
In all these three particulars they must be looked upon as precisely
the same, and yet there is this immense difference between the lengths
of their lives.
I shall return later on to this and other similar cases, and for the
present I assume it to be proved that physiological considerations
alone cannot determine the duration of life. It is not these which
alone determine the strength of the spring which moves the machinery of
life; we know that springs of different strengths may be fixed in
machines of the same kind and quality. This metaphor is however
imperfect, because we cannot imagine the existence of any special force
in an organism which determines the duration of its life; but it is
nevertheless useful because it emphasises the fact that the duration of
life is forced upon the organism by causes outside itself, just as the
spring is fixed in its place by forces outside the machine, and not
only fixed in its place, but chosen of a certain strength so that it
will run down after a certain time.
To put it briefly, I consider that duration of life is really dependent
upon adaptation to external conditions, that its length, whether longer
or shorter, is governed by the needs of the species, and that it is
determined by precisely the same mechanical process of regulation as
that by which the structure and functions of an organism are adapted to
its environment.
Assuming for the moment that these conclusions are valid, let us ask
how the duration of life of any given species can have been determined
by their means. In the first place, in regulating duration of life, the
advantage to the species, and not to the individual, is alone of any
importance. This must be obvious to any one who has once thoroughly
thought out the process of natural selection. It is of no importance to
the species whether the individual lives longer or shorter, but it is
of importance that the individual should be enabled to do its work
towards the maintenance of the species. This work is reproduction, or
the formation of a sufficient number of new individuals to compensate
the species for those which die. As soon as the individual has
performed its share in this work of compensation, it ceases to be of
any value to the species, it has fulfilled its duty and may die. But
the individual may be of advantage to the species for a longer period
if it not only produces offspring, but tends them for a longer or
shorter time, either by protecting, feeding, or instructing them. This
last duty is not only undertaken by man, but also by animals, although
to a smaller extent; for instance, birds teach their young to fly, and
so on.
We should therefore expect to find that, as a rule, life does not
greatly outlast the period of reproduction except in those species
which tend their young; and as a matter of fact we find that this is
the case.
All mammals and birds outlive the period of reproduction, but this
never occurs among insects except in those species which tend their
young. Furthermore, the life of all the lower animals ceases also with
the end of the reproductive period, as far as we can judge.
Duration of life is not however determined in this way, but only the
point at which its termination occurs relatively to the cessation of
reproduction. The duration itself depends first upon the length of time
which is required for the animal to reach maturity—that is, the
duration of its youth, and, secondly, upon the length of the period of
fertility—that is the time which is necessary for the individual to
produce a sufficient number of descendants to ensure the perpetuation
of the species. It is precisely this latter point which is determined
by external conditions.
There is no species of animal which is not exposed to destruction
through various accidental agencies—by hunger or cold, by drought or
flood, by epidemics, or by enemies, whether beasts of prey or
parasites. We also know that these causes of death are only apparently
accidental, or at least that they can only be called accidental as far
as a single individual is concerned. As a matter of fact a far greater
number of individuals perish through the operation of these agencies
than by natural death. There are thousands of species of which the
existence depends upon the destruction of other species; as, for
example, the various kinds of fish which feed on the countless minute
Crustacea inhabiting our lakes.
It is easy to see that an individual is, _ceteris paribus_, more
exposed to accidental death when the natural term of its life becomes
longer; and therefore the longer the time required by an individual for
the production of a sufficient number of descendants to ensure the
existence of the species, the greater will be the number of individuals
which perish accidentally before they have fulfilled this important
duty. Hence it follows, first, that the number of descendants produced
by any individual must be greater as the duration of its reproductive
period becomes longer; and, secondly, the surprising result that nature
does not tend to secure the longest possible life to the adult
individual, but, on the contrary, tends to shorten the period of
reproductive activity as far as possible, and with this the duration of
life; but these conclusions only refer to the animal and not to the
vegetable world.
All this sounds very paradoxical, but the facts show that it is true.
At first sight numerous instances of remarkably long life seem to
refute the argument, but the contradictions are only apparent and
disappear on closer investigation.
Birds as a rule live to a surprisingly great age. Even the smallest of
our native singing birds lives for ten years, while the nightingale and
blackbird live from twelve to eighteen years. A pair of eider ducks
were observed to make their nest in the same place for twenty years,
and it is believed that these birds sometimes reach the age of nearly
one hundred years. A cuckoo, which was recognised by a peculiar note in
its call, was heard in the same forest for thirty-two consecutive
years. Birds of prey, and birds which live in marshy districts, become
much older, for they outlive more than one generation of men.
Schinz mentions a bearded vulture which was seen sitting on a rock upon
a glacier near Grindelwald, and the oldest men in Grindelwald had, when
boys, seen the same bird sitting on the same rock. A white-headed
vulture in the Schönbrunn Zoological Gardens had been in captivity for
118 years, and many examples are known of eagles and falcons reaching
an age of over 100 years. Finally, we must not forget Humboldt’s[1]
Atur parrot from the Orinoco, concerning which the Indians said that it
could not be understood because it spoke the language of an extinct
tribe.
It is therefore necessary to ask how far we can show that such long
lives are really the shortest which are possible under the
circumstances.
Two factors must here be taken into consideration; first, that the
young of birds are greatly exposed to destructive agencies; and,
secondly, that the structure of a bird is adapted for flight and
therefore excludes the possibility of any great degree of fertility.
Many birds, like the stormy petrel, the diver, guillemot, and other
sea-birds, lay only a single egg, and breed (as is usually the case
with birds) only once a year. Others, such as birds of prey, pigeons,
and humming-birds, lay two eggs, and it is only those which fly badly,
such as jungle fowls and pheasants, which produce a number of eggs
(about twenty), and the young of these very species are especially
exposed to those dangers which more or less affect the offspring of all
birds. Even the eggs of our most powerful native bird of prey, the
golden eagle, which all animals fear, and of which the eyrie, perched
on a rocky height, is beyond the reach of any enemies, are very
frequently destroyed by late frosts or snow in spring, and, at the end
of the year in winter, the young birds encounter the fiercest of foes,
viz. hunger. In the majority of birds, the egg, as soon as it is laid,
becomes exposed to the attacks of enemies; martens and weasels, cats
and owls, buzzards and crows are all on the look out for it. At a later
period the same enemies destroy numbers of the helpless young, and in
winter many succumb in the struggle against cold and hunger, or to the
numerous dangers which attend migration over land and sea, dangers
which decimate the young birds.
It is impossible directly to ascertain the exact number which are thus
destroyed; but we can arrive at an estimate by an indirect method. If
we agree with Darwin and Wallace in believing that in most species a
certain degree of constancy is maintained in the number of individuals
of successive generations, and that therefore the number of individuals
within the same area remains tolerably uniform for a certain period of
time; it follows that, if we know the fertility and the average
duration of life of a species, we can calculate the number of those
which perish before reaching maturity. Unfortunately the average length
of life is hardly known with certainty in the case of any species of
bird. Let us however assume, for the sake of argument, that the
individuals of a certain species live for ten years, and that they lay
twenty eggs in each year; then of the 200 eggs which are laid during
the ten years, which constitute the lifetime of an individual, 198 must
be destroyed, and only two will reach maturity, if the number of
individuals in the species is to remain constant. Or to take a concrete
example; let us fix the duration of life in the golden eagle at 60
years, and its period of immaturity (of which the length is not exactly
known) at ten years, and let us assume that it lays two eggs a
year;—then a pair will produce 100 eggs in 50 years, and of these only
two will develope into adult birds; and thus on an average a pair of
eagles will only succeed in bringing a pair of young to maturity once
in fifty years. And so far from being an exaggeration, this calculation
rather under-estimates the proportion of mortality among the young; it
is sufficient however to enforce the fact that the number of young
destroyed must reach in birds a very high figure as compared with the
number of those which survive [See Note 1].
If this argument holds, and at the same time the fertility from
physical and other grounds cannot be increased, it follows that a
relatively long life is the only means by which the maintenance of the
species of birds can be secured. Hence a great length of life is proved
to be an absolute necessity for birds.
I have already mentioned that these animals demonstrate most clearly
that physiological considerations do not by any means suffice to
explain the duration of life. Although all vital processes take place
with greater rapidity and the temperature of the blood is higher in
birds than in mammals, yet the former greatly surpass the latter in
length of life. Only in the largest Mammalia,—the whales and the
elephants—is the duration of life equal to or perhaps greater than that
of the longest lived birds. If we compare the relative weights of these
animals, the Mammalia are everywhere at a disadvantage. Even such large
animals as the horse and bear only attain an age of fifty years at the
outside; the lion lives about thirty-five years, the wild boar
twenty-five, the sheep fifteen, the fox fourteen, the hare ten, the
squirrel and the mouse six years [See Note 2]; but the golden eagle,
though it does not weigh more than from 9-12 pounds, and is thus
intermediate as regards weight between the hare and the fox, attains
nevertheless an age which is ten times as long. The explanation of this
difference is to be found first in the much greater fertility of the
smaller Mammalia, such as the rabbit or mouse, and secondly in the much
lower mortality among the young of the larger Mammalia. The minimum
duration of life necessary for the maintenance of the species is
therefore much lower than it is among birds. Even here, however, we are
not yet in possession of exact statistics indicating the number of
young destroyed; but it is obvious that Mammalia possess over birds a
great advantage in their intra-uterine development. In Mammalia the
destruction of young only begins after birth, while in birds it begins
during the development of the embryo. This distinction is in fact
carried even further, for many mammals protect their young against
enemies for a long time after birth.
It is unnecessary to go further into the details of these cases, or to
consider whether and to what extent every class of the animal kingdom
conforms to these principles. Thus to consider all or even most of the
classes of the animal kingdom would be quite impossible at the present
time, because our knowledge of the duration of life among animals is
very incomplete. Biological problems have for a long time excited less
interest than morphological ones. There is nothing or almost nothing to
be found in existing zoological text books upon the duration of life in
animals; and even monographs upon single classes, such as the Amphibia,
reptiles, or even birds, contain very little on this subject. When we
come to the lower animals, knowledge on this point is almost entirely
wanting. I have not been able to find a single reference to the age in
Echinodermata, and very little about that of worms, Crustacea, and
Coelenterata [See Note 4]. The length of life in many molluscan species
is very well known, because the age can be determined by markings on
the shell [See Note 5]. But even in this group, any exact knowledge,
such as would be available for our purpose, is still wanting concerning
such necessary points as the degree of fertility, the relation to other
animals, and many other factors.
Data the most exact in all respects are found among the insects [See
Note 3], and to this class I will for a short time direct your special
attention. We will first consider the duration of larval life. This
varies very greatly, and chiefly depends upon the nature of the food,
and the ease or difficulty with which it can be procured. The larvae of
bees reach the pupal stage in five to six days; but it is well known
that they are fed with substances of high nutritive value (honey and
pollen), and that they require no great effort to obtain the food,
which lies heaped up around them. The larval life in many
_Ichneumonidae_ is but little longer, being passed in a parasitic
condition within other insects; abundance of accessible food is thus
supplied by the tissues and juices of the host. Again, the larvae of
the blow-fly become pupae in eight to ten days, although they move
actively in boring their way under the skin and into the tissues of the
dead animals upon which they live. The life of the leaf-eating
caterpillars of butterflies and moths lasts for six weeks or longer,
corresponding to the lower nutritive value of their food and the
greater expenditure of muscular energy in obtaining it. Those
caterpillars which live upon wood, such as _Cossus ligniperda_, have a
larval life of two to three years, and the same is true of
hymenopterous insects with similar habits, such as _Sirex_.
Furthermore, predaceous larvae require a long period for attaining
their full size, for they can only obtain their prey at rare intervals
and by the expenditure of considerable energy. Thus among the
dragon-flies larval life lasts for a year, and among many may-flies
even two or three years.
All these results can be easily understood from well-known
physiological principles, and they indicate that the length of larval
life is very elastic, and can be extended as circumstances demand; for
otherwise carnivorous and wood-eating larvae could not have survived in
the phyletic development of insects. Now it would be a great mistake to
suppose that there is any reciprocal relation between duration of life
in the larva and in the mature insect, or imago; or, to put it
differently, to suppose that the total duration of life is the same in
insects of the same size and activity, so that the time which is spent
in the larval state is, as it were, deducted from the life of the
imago, and _vice versa_. That this cannot be the case is shown by the
fact already alluded to, that among bees and ants larval life is of the
same length in males and females, while there is a difference of some
years between the lengths of their lives as imagos.
The life of the imago is generally very short, and not only ends with
the close of the period of reproduction, as was mentioned above, but
this latter period is also itself extremely short [See Note 3].
The larva of the cockchafer devours the roots of plants for a period of
four years, but the mature insect with its more complex structure
endures for a comparatively short time; for the beetle itself dies in
about a month after completing its metamorphosis. And this is by no
means an extreme case. Most butterflies have an even shorter life, and
among the moths there are many species (as in the _Psychidae_) which
only live for a few days, while others again, which reproduce by the
parthenogenetic method, only live for twenty-four hours. The shortest
life is found in the imagos of certain may-flies, which only live four
to five hours. They emerge from the pupa-case towards the evening, and
as soon as their wings have hardened, they begin to fly, and pair with
one another. Then they hover over the water; their eggs are extruded
all at once, and death follows almost immediately.
The short life of the imago in insects is easily explained by the
principles set forth above. Insects belong to the number of those
animals which, even in their mature state, are very liable to be
destroyed by others which are dependent upon them for food; but they
are at the same time among the most fertile of animals, and often
produce an astonishing number of eggs in a very short time. And no
better arrangement for the maintenance of the species under such
circumstances can be imagined than that supplied by diminishing the
duration of life, and simultaneously increasing the rapidity of
reproduction.
This general tendency is developed to very different degrees according
to conditions peculiar to each species. The shortening of the period of
reproduction, and the duration of life to the greatest extent which is
possible, depends upon a number of co-operating circumstances, which it
is impossible to enumerate completely. Even the manner in which the
eggs are laid may have an important effect. If the larva of the may-fly
lived upon some rare and widely distributed food-plant instead of at
the bottom of streams, the imagos would be compelled to live longer,
for they would be obliged—like many moths and butterflies—to lay their
eggs singly or in small clusters, over a large area. This would require
both time and strength, and they could not retain the rudimentary mouth
which they now possess, for they would have to feed in order to acquire
sufficient strength for long flights; and—whether they were carnivorous
like dragon-flies, or honey-eating like butterflies—their feeding would
itself cause a further expenditure of both time and strength, which
would necessitate a still further increase in the duration of life. And
as a matter of fact we find that dragon-flies and swift-flying
hawk-moths often live for six or eight weeks and sometimes longer.
We must also remember that in many species the eggs are not mature
immediately after the close of the pupal stage, but that they only
gradually ripen during the life of the imago, and frequently, as in
many beetles and butterflies, do not ripen simultaneously, but only a
certain number at a time. This depends, first, upon the amount of
reserve nutriment accumulated in the body of the insect during larval
life; secondly, upon various but entirely different circumstances, such
as the power of flight. Insects which fly swiftly and are continually
on the wing, like hawk-moths and dragon-flies, cannot be burdened with
a very large number of ripe eggs. In these cases the gradual ripening
of the eggs becomes necessary, and involves an increase in the duration
of life. In Lepidoptera, we see how the power of flight diminishes step
by step as soon as other circumstances permit, and simultaneously how
the eggs ripen more and more rapidly, while the length of life becomes
shorter, until a minimum is reached. Only two stages in the process of
transformation can be mentioned here.
The strongest flyers—the hawk-moths and butterflies—must be looked upon
as the most specialised and highest types among the Lepidoptera. Not
only do they possess organs for flight in their most perfect form, but
also organs for feeding—the characteristic spiral proboscis or ‘tongue.’
There are certain moths (among the Bombyces) of which the males fly as
well as the hawk-moths, while the females are unable to use their large
wings for flight, because the body is too heavily weighted by a mass of
eggs, all of which reach maturity at the same time. Such species, as
for instance _Aglia tau_, are unable to distribute their eggs over a
wide area, but are obliged to lay them all in a single spot. They can
however do this without harm to the species, because their caterpillars
live upon forest trees, which provide abundant food for a larger number
of larvae than can be produced by the eggs of a single female. The eggs
of _Aglia tau_ are deposited directly after pairing, and shortly
afterwards the insect dies at the foot of the tree among the
moss-covered roots of which it has passed the winter in the pupal
state. The female moth seldom lives for more than three or four days;
but the males which fly swiftly in the forests, seeking for the less
abundant females, live for a much longer period, certainly from eight
to fourteen days[2].
The females of the _Psychidae_ also deposit all their eggs in one
place. The grasses and lichens upon which their caterpillars live grow
close at hand upon the surface of the earth and stones, and hence the
female moth does not leave the ground, and generally does not even quit
the pupa-case, within which it lays its eggs; as soon as this duty is
finished, it dies. In relation to these habits the wings and mouth of
the female are rudimentary, while the male possesses perfectly
developed wings.
The causes which have regulated the length of life in these cases are
obvious enough, yet still more striking illustrations are to be found
among insects which live in colonies.
The duration of life varies with the sex in bees, wasps, ants, and
termites: the females have a long life, the males a short one; and
there can be no doubt that the explanation of this fact is to be found
in adaptation to external conditions of life.
The queen-bee—the only perfect female in the hive—lives two to three
years, and often as long as five years, while the male bees or drones
only live four to five months. Sir John Lubbock has succeeded in
keeping female and working ants alive for seven years—a great age for
insects[3],—while the males only lived a few weeks.
These last examples become readily intelligible when we remember that
the males neither collect food nor help in building the hive. Their
value to the colony ceases with the nuptial flight, and from the point
of view of utility it is easy to understand why their lives should be
so short [See Note 7 and Note 9]. But the case is very different with
the female. The longest period of reproduction possible, when
accompanied by very great fertility, is, as a rule, advantageous for
the maintenance of the species. It cannot however be attained in most
insects, for the capability of living long would be injurious if all
individuals fell a prey to their enemies before they had completed the
full period of life. Here it is otherwise: when the queen-bee returns
from her nuptial flight, she remains within the hive until her death,
and never leaves it. There she is almost completely secure from enemies
and from dangers of all kinds; thousands of workers armed with stings
protect, feed, and warm her; and in short there is every chance of her
living through the full period of a life of normal length. And the case
is entirely similar with the female ant. In neither of these insects is
there any reason why the advantages which follow from a lengthened
period of reproductive activity should be abandoned [See Note 6].
That an increase in the length of life has actually taken place in such
cases seems to be indicated by the fact that both sexes of the
saw-flies—the probable ancestors of bees and ants—have but a short
life. On the other hand, the may-flies afford an undoubted instance of
the shortening of life. Only in certain species is life as short as I
have indicated above; in the majority it lasts for one or more days.
The extreme cases, with a life of only a few hours, form the end of a
line of development tending in the direction of a shortened life. This
is made clear by the fact that one of these may-flies (_Palingenia_)
does not even leave its pupa-skin, but reproduces in the so-called
sub-imago stage.
It is therefore obvious that the duration of life is extremely
variable, and not only depends upon physiological considerations, but
also upon the external conditions of life. With every change in the
structure of a species, and with the acquisition of new habits, the
length of its life may, and in most cases must, be altered.