BIOLOGY

LIBRARY

Lectures on Biology

DE. CUET THESING

TKANSLATED FROM THE SECOND EDITION BY

W. E. BOELTEE

WITH THE ORIGINAL COLOURED AND OTHER ILLUSTRATIONS

Xonfcon
JOHN BALE, SONS & DANIELSSON, LTD.

OXFORD HOUSE

83-91, GKEAT TITCHFIELD STKEET, OXFORD STREET, W.

1911

BIOLOGY
R
G

PREFACE.

THIS book consists of Lectures which were given by me
during the winter terms of 1905 to 1907 at the Humboldt
Academy and the Urania in Berlin.

My object in delivering them was to show that it is no
longer possible, having regard to the advances of modern research,
to find complete satisfaction in being an out-and-out believer
in the Darwinian or Lamarckian or any other theory.

Each of these doctrines has rendered important service in
promoting our knowledge, but the marks of their limitations have
of recent years become increasingly distinct.

The factors which the different doctrines assume to be at
work in the genesis and evolution of the organic world are,
however, not exclusive, for the Theory of Selection, the Doctrine
of Adaptation and of Use and Non-use, and finally de Vries'
Theory of Mutations, offer each for a certain section of organic
evolution a sufficient and satisfactory explanation.

That, on the other hand, a vast field of vital processes remains
still hidden and unexplained only dogmatic prejudice will venture
to deny.

If I have succeeded in conveying to a wider circle this
conception of conflicting theories, and if I have further succeeded
in showing that "incomprehensible" does not mean " impossible
to comprehend," my object in publishing these Lectures in
book form has been attained.

CURT THESING.

466582

ERRATA.
Page 6, line 21 from top, for " animals " read " mammals."

9 ,, 20 ,, ,, ,, "but rather" read "as."

„ 58 „ 9 ,, „ „ " (Triebewegung) " read "(Triebbewegung)."
,, 136, lines 7 and 10 for " ues centrale " read "os centrale"
,, 138, line 14 from top, for " princates " read "primates."
,, 169 „ 9 „ „ „ "trees "read "tree," and line 20 for "if "read
,, 178 ,, 1, read "use and natural selection."
,, 181 ,, 5 from top, insert after " Aid " " — a factor/'
,, 293, lines 14 and 15 from bottom are transposed.

CONTENTS.

CHAPTER I.

PAGE

FROM THALES TO LAMARCK 1

CHAPTER II.
PHENOMENA AND CONDITIONS OF LIFE .".. ... 27

CHAPTER III.
THE FORCES IN THE ORGANISM 50

CHAPTER IV.
THE BUILDING-STONES OF THE ORGANIC WORLD 60

CHAPTER V.
THE ORIGIN OF LIFE 85

CHAPTER VI.
THE EVOLUTION THEORY .. 103

CHAPTER VII.
THE FACTORS OF EVOLUTION 140

CHAPTER VIII.
THE CONSERVATION OF LIFE ... 211

CHAPTER IX.
HP-PRODUCTION AND HEREDITY . ... „. ,.. 25 J

LIST OF ILLUSTKATIONS.

FIG. 1. — The Lancelot (Branchiostoma lanceolatum) ... ... ... 13

FIG. 2. — Haddock ( Gadus cBglefinus] ; and Tub-fishes (Trigla hirundo)... 15

FIG. 3.— The African Mud-jumper (Periophthalmus hoelreuteri) 18

FIG. 4.— Evolution of the Extremities face 20

FIG. 5. — Skeletons of the Anterior Extremities of various Vertebrates ... 21

FIG. 6. — Flying-foxes (Pteropus edulis) 25

FIG. 7. — Skeleton of the Anterior Extremities of the Bat and the Flying

Saurian Pterodactylus ... , ... ... ... ... ... 26

FIG. 8. — Amoeba Umax at different Temperatures 32

FIG. 9. — Neoscopelus macro lepidotus... ... ... ... ... ... 35

FIG. 10. — Bear-animalcules (Macrobiotus Imfelandi) ... ... ... 38

FIG. 11. — The Mud-fish (Protopterus annectens) 39

FIG. 12.— Fowl in a State of Cataplexy 44

FIG. 13.— Crayfish in a State of Cataplexy 45

FIG. 14. — Longitudinal Section of a Thighbone ... ... ... ... 52

FIG. 15.— Struggle of the Germ-cells in the Male Glands of a Cuttle-fish

(Rossia macrosoma) ... ... ... ... ... ... 55

FIG. 16. — Anthozoa ... ... ... face 61

FIG. 17. — Fresh-water Polyp (H ydra viridis) 63

FIG. 18. — Symbiosis of Sea-anemone and Unicellular Algae ... ... 64

FIG. 19. — Venus's Fly-trap (Dionceamuscipula] ... .;.,'... ... 66

FIG. 20.— Structure of a Cell 71

FIG. 21. — Leucocytes of the Frog devouring a Bacillus 73

FIG. 22.— Cells-forms 78

FIG. 23.— Mitosis face 80

FIG. 24. — Bathybius HaecUi .... 98

FIG. 25.— Trilobites 105

FIG. 26.— Shells of Extinct Cephalopoda 107

FIG. 27. — Archaeopteryx face 110

FIG. 28. — Peripatus capensis Ill

FIG. 29. —Development of the Foot of the Horse 113

FIG. 30.— Tooth-development of the Horse 114

FIG. 31.— Ova of Different Animals 116

FIG. 32.— Embryos of Vertebrates 118

FIG. 33. — Amblystoma tigrinum ... ... ... ... ... ... 121

FIG. 34. — Shifting of the Eyes during the Development of a Young

Flat-fish 123

FIG. 35. — The Common Eorqual (Balcenoptera musculus) 124

FIG. 36. — Rudimentary Organs face 126

FIG. 37. — Echinodermata ,, 128

FIG. 38.— Water- vascular System of a Starfish ... 129

FIG. 39. — Larv* of Echinodermata 130

Vlll. LIST OF ILLUSTKATIONS

PAGK

FIG. 40. — Fornaria-larva of a Marine- worm (Balanof/lossus) 131

FIG. 41. — Larval Form a Crinoid 131

FIG. 42. — Trochophora Larva of Polygordius, a Marine Annelid ... ... 132

FIG. 43. — Nauplius Stage of Penceus potimirum 133

FIG. 44, —Zosea Stage of Crab ... ' 135

FIG. 45. — Life-history of the Liver-fluke (Fasciola [Distomum} liepatica) 154

FIG. 46. — Marine Jelly-fish face 162

FIG. 47. — Colour-adaptation and Mimicry „ 168

FIG. 48.— The Taguan (Pteromys petaurista) 178

FIG. 49. — Sexual Dimorphism face 178

FIG. 50. — Influence of Temperature during the Pupa Stage upon Colour

and Design of Butterfly 187

FIG. 51. — Amoeba proteus in various Stages of Motion ... ... ... 213

FIG. 52. — Nummulites ... 219

FIG. 53. — A Micro-aquarium 220

FIG. 54. — Trumpet-animalcule (Stentor coeruleus) ... ... ... ... 223

FIG. 55. — Stentor cceruleus 224

FIG. 56. — Conjugation of Par amsecium in Six Stages 227

FIG. 57. — Life- cycle of the Malaria Parasite of Man ... ... ... 235

FIG. 58. — The Malaria Mosquito (Anopheles claviger) 239

FIG. 59. — Volvox globator 246

FIG. 60. — Green Fresh-water Polyp (Hydra viridis) 256

FIG. 61. — Salpa democratica ... ... ... ... ... ... face 259

FIG. 62. — Development of Aurelia aurita 260

FIG. 63. — Caterpillar covered with Cocoons of Microgaster glomeratus... 272

FIG. 64. — Ammophila liirsuta paralysing a Caterpillar ... ... ... 272

FIG. 65.— (1) Bhyncliites populi, with Leaf-roll. (2) Leaf-rolls of

Khynchites betulte... ... ... ... ... ... ... 273

FIG. 66.— Mud-fly (Eristalis tenax] 275

FIG. 67. — G-eophagiis brachyurus 277

FIG. 68. — Hippocampus antiquorum ... ... ... ... ... ... 278

FIG. 69. — Brazilian Frog (Pipa americana), with young 280

FIG. 70. — Spermatozoa of Different Animals face 283

FIG. 71. — Change of a Spermatid in a Scuttle-fish (Octopus Defilippi]

into a Spermatozoon 292

'FiG. 72. — Diagram of a Maturation Division 294

FIG. 73.— Fertilization face 296

FIG. 74.— Mendel's Law 318

COLOUEED PLATES.

PLATE I. — Animal Life in the Antarctic ... ... ... to face page 32

PLATE II. — Deep-sea Life „ „ 42

PLATE III. — Reptiles of the Triassic and Jurassic Periods ,, ,, 108

PLATE IV.— Flat-fish „ „ 122

PLATE V. — Domesticated Pigeons „ „ 142

PLATE VI.— Symbiosis „ „ 182

LECTURES ON BIOLOGY.

CHAPTER I.
FKOM THALES TO LAMAECK.

Two characteristics are ineradicably fixed in the human
mind : the desire for action, and the thirst for knowledge. We
see their origin in the infant when in the first awakening of
consciousness it longingly puts forth its tiny hands. The play
of the child is but the consciousness of its desire to be busy.
Play to the child means living. In play it forgets itself and
its bodily wants : a day of enforced idleness will harm it more
than a day without food.

When, later, the growing child breaks up its playthings, we
observe in that action the awakening of the desire for know-
ledge. It is no longer content with the bare fact that its doll
squeaks when it is squeezed ; it wants to know how the doll
looks inside, why it squeaks. Destructiveness and inquisitive-
ness are to the child as yet identical. In the act of destruction
reveals itself the desire to analyse, to dissect a phenomenon
into its elements, to fathom the cause of things.

As in the early life of the individual, so we perceive in the
history of the nations and of mankind the need to trace each
phenomenon to a cause — in other words, we observe the con-
scious pursuit of knowledge. It is true that, as with the
child, so with primitive man, the impulse is as yet not clearly
perceived ; but it exists, nevertheless, no matter how far we
may go back in the history of primitive man. Like the child
1

2 LECTURES ON BIOLOGY

questioning persistently because it wants to know, so savage
man asks after the Why of all things. Why does the sun rise
morning after morning ? Why does the sun set in the evening
behind the mountains in the distant horizon ? Why is there
sickness, why death ? Why is one year healthy and fruitful,
why does the next bring famine and epidemics ?

But the greatest question of all that have agitated the mind
of man is that of his own place in Nature, of his origin, and
of the future of his soul after death. Unable to find a satisfying
answer to his yearning, he created gods in his own image,
regarding them as the cause of all that is, to whom the world
and he owed their existence, and into whose presence his soul
could fly when death had claimed him.

As man is, so is his god ; the degree of man's knowledge
determines the perfection of his deity. The gods of the savage
and barbarous tribes, even those of the Greeks and Romans, are
endowed with every human passion and weakness, and it is not
until the Jewish-Christian period that we find a god who rises to
the purity of a perfect being. All that man has perceived to be
high and noble : mercifulness, love, righteousness, wisdom, truth,
and goodness, he attributes to his deity. ' First, man uncon-
sciously creates god in his image/ says Feuerbach in a brilliant
paraphrase, 'then this god consciously creates man in his image.'

In this dark yearning after knowledge and truth, in this
eternal desire of the human mind to find an answer to the
innumerable questions with which he is confronted on all sides,
and a solution of the great riddle of his existence and life, we
find the origin of all religion. But there we find also the
beginning of science, for science, too, searches after a cause for
each phenomenon, and after the final cause of all phenomena.
Thus religion and science have the same aim though they
approach it by different roads : to find the cause of all causes.

Why is it, we may ask, that in spite of like objects and
like aims there exists between them a growing abyss ? Why
do science and religion oppose each other in bitter strife ? As
both arise from the same desire, why do they not help each
other? Why did the Church in the dark Middle Ages put to
the stake or sword men who had found and proclaimed the
truth? To-day, when manners are milder, the stake and sword

FKOM THALES TO LAMARCK 3

are replaced by harsh and bitter words which the protagonists
in both camps hurl at each other unsparingly.

The cause of this deadly enmity lies deep down in the nature
of both schools of thought. For whilst religion — -that is, religious
dogma — represents the ' origin of all that is ' as something
absolute, unalterable, and visits with severe punishment _any
enquiry after that which lies beyond, science is ever conscious,
or should be, that each hypothesis promulgated, each law formu-
lated, are but steps in the pursuit of knowledge, the aim of
which lies, an eternal task, in the infinite. ' There is no finality
to our pursuit of the knowledge of life, and though we may now
and then attempt to cast a balance, we know that even the best
which we are able to give can but be a step to the better.'
These words, which form the introduction to the Lectures on
the Evolution Theory by the great zoologist of Freiburg, August
Weismann, fittingly describe not only the value of biological
research, but also of all science.

It is not our object to deal with the various creation-theories
contained in the mythology of the Indians, Assyrians, and other
nations, nor need we refer to the biblical story of the creation
of the world and man, for on the one hand it is familiar to all,
and on the other of no value for the advancement of scientific
knowledge. We may therefore at once direct our attention to
the classic period of the Ionic Natural Philosophers.

At the head of the Greek philosophy, and, indeed, as the
first Greek philosopher, is usually mentioned Thales of Miletus
who was counted one of the Seven Wise Men. In his teachings
we find, for the first time, an attempt to establish a uniform
principle of all phenomena, a universal theory of reality. Thales
declares that water is the original principle from which all pro-
ceeds ; his successors gradually establish the Infinite, the Fire,
the Air, and the Atoms as fundamental principles. These are
according to modern conceptions crude and primitive theories,
but they are nevertheless the first attempts to comprehend the
reality scientifically.

There will be no time in this lecture to deal with every
phase of this period of more than 2,000 years, nor shall we be
able to deal, except in the briefest manner, with the most
important epochs and the men to whom biology owes its

4 LECTUKES ON BIOLOGY

greatest advancement. The object of this lecture is not to give
a complete record but rather to present briefly the gradual rise
of the natural sciences, the first dawn of the evolution theory ;
afterwards, the rapid decay of science, and finally, its brilliant
development during the eighteenth century.

If one looks back upon the history of natural sciences in the
Middle Ages and modern times, their rapid rise, in particular
that of the descriptive branches, above all of zoology and botany,
seems almost inconceivable. On one side we see the grossest
superstition, or, at the best, defective and inaccurate knowledge ;
on the other, an almost inexhaustible wealth of facts and accepted
principles. One may indeed regard with pride the results of
the untiring labour of the two preceding centuries.

According to Ben Akiba there is nothing new under the sun.
Looking backwards from the ignorance and suspicion of the
Middle Ages, the loftiness of Greek civilization, evidenced by
the high state of their knowledge of nature, seems equally incom-
prehensible ; and it is still more difficult to understand that such
great wealth of knowledge as was stored in the mind of an
Aristotle should pass from the memory of man, almost without
leaving a trace behind.

Anaximander, a contemporary, disciple, and countryman of
Thales, already taught the gradual natural evolution and change
of the Universe and the organisms. He perceives the original
principle in the 'Apeiron,' the Infinite. It has no origin and
no end, changing only its forms. He conceived that from this
unlimited original matter there proceeded by spontaneous division
the warm and the cold. From these was formed the moist, and
out of it, through drying, the earth, the air, and an all-surrounding
fireball which finally burst and gave rise to the sun and the
other heavenly bodies. The influence of the sunrays developed
out of the primordial mud the first degree of organic life, certain
vesicular formations which gradually developed into fish-like
animals. In proportion as the drying process went on, some
of the fishes left the moist element and under the influence of
changed external conditions and a new mode of life changed into
the various species of land animals. Finally the species man
naturally evolved from animal, ancestors. ' But whence all
beings have sprung thence they must return, as a penance for

FROM THALES TO LAMARCK 5

the injustice of their existence, in the order of time.' Thus
worlds follow worlds in a never-ending cycle.

It is remarkable to encounter thus early a theory of the
mechanical creation of the organic world, and a hypothesis of
a natural evolution. A philosopher of the fifth century B.C.,
Empedocles of Acragas, equally famous by his teachings as by
his death which he sought and found in the crater of j3£tna, does
not only take his stand on the theory of descent but sounds also
the first distinct note of the Darwinian Theory of the Survival of
the Fittest. These doctrines are, indeed, as yet intermixed with
many crude and phantastic conceptions, but their appearance is
all the more surprising when one considers the very small store
of information in natural history which was then available.

According to Empedocles the origin of all things lies in the
four elements of the ancients : water, fire, air and earth. In the
beginning of time all four elements rested with one another,
unseparated and unmixed, completely held together in the form
of a self-contained sphere by the union of love, or as we should
say to-day, by the force of attraction. But hatred found an
entrance, and with it came separation which led to the founda-
tion of the world and all organisms. Through this eternal strife
between love and hatred, and the consequent perpetual mingling
and separating of the four elements, there gradually arose on the
earth organic life. First the plants sprouted from the lap of the
maternal earth, then came the animals. But according to
Empedocles only single organs developed in the beginning, legs
without bodies, noses and eyes without faces, heads without
trunks, and so forth. At first different organs became united by
chance, thus originating chiefly awful monstrosities unfit to live
and doomed to death as soon as they had begun to exist. But
as like attracts like and repels unlike, here and there certain
creatures originated whose organs fitted and complemented each
other. Thus organisms were formed which were fit to live, and
if the fitness had been complete capable of reproducing their
kind and transmitting their own qualities to their descendants.

However extravagant and wild these ideas may at the first
glance appear, they contain a profound thought : the mechanical
origin of the fittest in nature and the survival of the fittest
forms. It is the same thought which 2,000 years after arose to

LECTURES ON BIOLOGY

new life with Darwin and, cleansed of all dross of mysticism,
opened to research undreamt-of paths.

The zenith of natural philosophy in classic time is reached
with the age of Aristotle. Posterity has justly bestowed upon
this great man the name of Father of Natural Sciences. In him
we do not only find embodied the sum-total of the knowledge
of his time, but there are also numerous branches of knowledge
which he vastly enriched. It was he who first attempted a
classification of the animal kingdom according to the degree of
relationship. The boldness of this undertaking will be better
understood when we call to mind the fact that Aristotle knew
only some 500 different species, whilst we know to-day between
300,000 to 400,000. Yet so clearly saw this master-mind in spite
of this difficulty that the Aristotelian division has in its main
points been retained up to the present day.

Aristotle was aided by a surprising store of detailed informa-
tion, much of which was re-discovered during the last century
by the great naturalist of Berlin, Johannes Miiller. Thus
Aristotle knew, to mention only two instances, that many sharks
are viviparous, and that whales and dolphins are not fishes but
animals. That such a mind should ponder on the origin of
the organic world and endeavour to find a natural explanation
cannot therefore excite surprise.

With the death of the great Stagirite natural sciences rapidly
decay ; instead of scientific research we find the grossest super-
stition. Even Pliny the Elder who perished during an erup-
tion of Vesuvius A.D. 79 whilst commanding the Roman fleet at
Misenum is at the best only a very untrustworthy compiler.
If of anyone, it may be said of him that

Viele Dinge wusste er freilich,
Doch alle wusste er schlecht.

Pliny piled up whatever he found, without troubling himself
about the scientific value of his ' finds.' There is nothing of the
Aristotelian mind in him, and his division of the natural world
into ' land, water and air animals ' is no more scientific than
if he had divided it, as Weismann tersely remarks, according
to the alphabet.

The decay of sciences proceeded apace, first under the Roman

FROM THALES TO LAMARCK 7

emperors, still more during the Middle Ages. Even Galen, the
founder of scientific physiology, who lived in the third century,
was unable to arrest it. Nothing will better indicate the
low ebb at which science had arrived than a few quotations
from the so-called 'Physiologus,' which was during the Middle
Ages the most celebrated and best known work on natural
history. It was written about the third century and has been
translated into almost every civilized language. It draws its
facts of natural history mainly from the Bible and various
Roman and Greek authors. Altogether forty-one animals are
mentioned in this book, and of each we hear remarkable news.
Thus it is said of the panther : ' He is multi-coloured, after a
meal he sleeps three days, wakes up roaring and emits such
pleasant odours that all animals go to him/ Of the lion we are
told that ' after his birth he is dead three days, but on the third
day his father comes, blows into his face and thus ^Wakens him
into life.' Similar stories are told of the hyena, the pelican, the
phoenix, ttie unicorn, the siren and other * species of the animal
kingdom.' '^

Blind belief in dogma and all theories, encouraged by a
fanatic priesthood, is the sign of that period. Only with the
founder of astronomy, Nicolas Copernicus, with Galilei and his
undying * E pur si muove ' flung from his dungeon out into
the world, with Kepler's discovery of the course of the planets,
and finally with Newton's demonstration of the Law of Gravita-
tion, the blind belief in authorities breaks down, the tyranny of
the Church collapses and we see the dawn of the new age of
scientific research. , . •

Prominent among the leaders in this new era is Cuvier,
famous alike as the founder of comparative anatomy and as the
bitterest and most powerful opponent of the Doctrine of Descent.

His chief merit is that he, for the first time, examined the
different species from the standpoint of comparative anatomy
and thus built up a system which has remained the basis of
modern classification. He formed the following four groups : —

(1) Vertebrata.

(2) Articulata.

(3) Mollusca.

(4) Radiata.

8 LECTUEES ON BIOLOGY

According to Cuvier, each of these groups is entirely
independent of all others, forms a distinct type, and is in no wise
correlated with another type. Moreover, not only are these main
groups entirely distinct, but each individual species again repre-
sents a definite invariable unit. This constancy and immu-
tability of the species is the corner-stone of Cuvier's theory.
To this he clung with incredible tenacity up to the hour of his
death, defending it in the face of all facts and with the most
patent sophisms. When, for instance, palaeontology had proved
that in past periods animal forms had existed widely differing
from the present species, Cuvier established his Doctrine of
Catastrophism, in order to preserve his dogma of the immuta-
bility of the species. According to this theory each period of
the earth's history had been distinguished by a fauna and flora
peculiar to itself. Enormous revolutions of the surface of the
earth, floods, earthquakes, volcanic eruptions, and glacial floods
had then at one blow wiped out the entire plant and animal
world, and thus terminated one period. On the new virgin soil
a new fauna and flora had been created, by a divine act, thus
possessing no connection either with the organisms in the pre-
ceding or in the subsequent earth periods.

During Cuvier's time the great naturalist, Jean Baptiste
Lamarck, and with him, Geoffrey St.-Hilaire, strongly opposed
this dogma of the fixity of the species, but could not avail
against the almost unlimited authority of Cuvier who had then
been raised to the peerage of France. On the contrary, the
theory of descent suffered complete defeat in the debate between
Cuvier and St.-Hilaire before the French Academy in 1830, and
consequently disappeared for years from the scientific agenda.
How deeply this controversy affected the leading spirits of those
restless times is shown by a dialogue between Goethe, then a
very old man, and his friend Soret. It was on August 2, 1830 ;
the news of the expulsion of Charles X. from the throne of
France had just reached Germany and all the world was talking
about it. On the morning of this day Soret had called on
Goethe who greeted him with these words: 'Ah! what do you
think of this great event ? The volcano is in full eruption,
everything is aflame, and it is no longer possible to deal with the
matter behind locked doors.'

FROM THALES TO LAMARCK 9

'It is indeed a terrible thing,' replied Soret ; 'but what
else could you expect, with such conditions and such a Cabinet,
than the expulsion of the Koyal Family ? '

' You seem to misunderstand me,' said Goethe, ' I am
not thinking of those people ; I am speaking of the rupture
which has taken place before the French Academy between
Cuvier and St.-Hilaire ! This matter is of the utmost importance
to science and you can have no idea of my feelings at these
news. We have now in St.-Hilaire a powerful ally.'

It was only some time after Cuvier's death, when K. E. A.
Hoff and the great English geologist Charles Lyell had proved
irrefutably that the evolution of the earth had taken place in
obedience to the same laws that still govern it and that the
various periods of the earth's history were connected by gradual
stages, that the theory of catastrophism fell, and with it the
hypothesis of the invariability of the species.

That even such a clear thinker as Cuvier came to assume the
coming, at certain periods, of enormous catastrophes which
annihilated all life was probably not so much due to the in-
fluence of traditions — Indian, Arabic, Biblical — but rather to the
fact that at that time it was general greatly to underestimate
the periods which had elapsed since the earth had become a unit
in the universe. To compress within a few thousand years the
evolution of what Nature had used millions of years to achieve
meant of necessity the assumption of all-destroying catastrophes.
Indeed, the history of the world would be nothing but a blood-
stained record of murder and destruction if we were to assume
that it had covered but a few decades.

The first man who clearly enunciated the idea of evolution
is the poet and naturalist Dr. Erasmus Darwin, the grand-
father of Charles Darwin. The works of this man, in par-
ticular his 'Zoonornia, or The Laws of Organic Life' (1794),
contain by the side of numerous profound observations, often
almost of a prophetic character, many childlike conceptions even
of the simplest processes of life. On one side we find the free
outlook of the far-seeing naturalist ; on the other, the short-
sightedness of teleological dogma. But that fact cannot lessen
his merits, for Erasmus Darwin was the first who taught the
variability of the species and the adaptation of an organ to its

10 LECTUKES ON BIOLOGY

use, though what the forces were that affected variation and
adaptation was a question to which he was unable to give a
satisfactory answer.

In Germany it was above all Goethe who advocated the
theory of descent in numerous important works, but the most
prominent predecessor of the great Darwin is doubtless Jean
Baptiste Lamarck. In vain Lamarck waited during a long
laborious life, the last seventeen years of which were spent in
total blindness, for the recognition of his contemporaries. His
numerous writings remained either unknown or were misunder-
stood. Only now, when almost a century has passed since his
death, it seems as if his work shall at last receive its due share
of praise, for during recent years a strong tendency has shown
itself not only among botanists but also zoologists to turn back
to the teachings of this great, unhappy man.

According to Lamarck, the simplest organisms originated by
spontaneous generation (Urzeugung) out of inorganic matter
when the earth had sufficiently cooled to permit of the existence
of life. In the course of vast periods the living species of
plants and animals developed out of these lowest forms by
slow and gradual changes. The last and highest development
was man. In contrast to modern biogenetic conceptions which
represent the evolution of the organic world in the shape of ;i
tree of many branches, Lamarck conceived the animal world as a
uniform series of stages ascending from the unicellular protozoon
to man.

Lamarck perceived the cause of the changes in the organic
world and the origin of new species to be that with the pro-
gressing cooling of the earth's crust the conditions of life for both
plants and animals underwent a continuous if gradual change.
If the organisms had not possessed the faculty of adapting them-
selves to the new conditions it would have been inevitable that
after the lapse of thousands or millions of years forms that once
were well adapted would cease to be so and perish. But we can
clearly see that the animals of the present day are as well adapted
to present conditions as were the animals of earlier periods to the
conditions then existing. It is, therefore, indubitable that hand
in hand with the changes of the earth there must have gone on
a continuous change in the organisms. Hence the question arises,

FROM THALES TO LAMARCK 11

Which forces have been responsible for this development ? To
this Lamarck replies, that ' it is the change of function of the
individual organs which brings about this seeming miracle.'

We may at any time observe in our own body the effects of
the use or disuse of an organ. Systematic exercise produces
better and stronger muscles ; the fibres not only increase in
volume, but are also able to act and contract more quickly. Con-
versely, want of action produces slackness and degeneration of
the fibres. A person whom illness or accident keeps to the bed
and from all action knows that after a few weeks the legs will
be so weakened as scarcely to be able to support the weight of
the body. Like a child, the patient must perforce once more
strengthen the muscles by gradual exercises, that is, he must
again learn to walk. Supposing that it were possible to transmit
to the next generation the primary constituents (Anlagen) of such
characters newly acquired by exercise — (in this case the invigora-
tion or degeneration of the muscle fibres) — and that this and
following generations would act similarly, organisms might
eventually be found which would be widely differentiated from
their original parents ; for it is obvious that a radical modifica-
tion of the muscles would in one way or another influence also
the development of the other parts of the body.

Who does not know Lamarck's instance of the giraffe? In
earlier times giraffes were probably, like other ruminants, of normal
build, nor differing greatly in the neck from their congeners.
The severe competition arising on the ordinary grazing grounds
from the presence of other herbivorous animals forced the
giraffes to profit by their comparative height and seek nourish-
ment on the branches of trees. The continual stretching and
exercising of the neck caused a richer circulation, a better
feeding of the muscles and other tissues, and a slight addition
to its length which was transmitted to the offspring. Continued
in subsequent generations, this process gradually produced in the
course of many thousands of years an abnormally long neck such
as we see to-day. — The web between the toes of the waterfowl
is said to have been caused through the habit of these birds of
seeking their prey on the water and of spreading out their toes
as widely as possible in order to swim better. The continued
stimulus acting upon the skin between the toes gradually effected

12 LECTURES ON BIOLOGY

a stronger development of these parts. Similarly one might
explain with more or less probability of truth the origin of most
organs.

On the other hand, as we have already seen, disuse leads to
reversion and degeneration of the affected parts. If an organ
was deprived of work owing to altered conditions of life, gradual
degeneration followed of necessity. Thanks to the conservatism
of heredity, the organs may for some time be retained in the
successors, but finally the last .vestige is lost. The numerous
rudimentary organs form ample illustrations of this phenomenon.
We need only mention, among many other similar instances,
the loss of eyes in many cave-animals, the degenerate eyes of
the mole, the breast glands of male animals, and the muscle of
the human ear, whereby it can be pulled upwards or twitched
forward or backward.

In all nature there is hardly another case in which one may
so clearly see the adaptation of an organ to its functions and
the improving or degenerating influence of use and disuse as
in the development of the arms and legs of the vertebrates.
However different the extremities of vertebrates may externally
appear, it is possible to demonstrate, if not in the adult, at
any rate in the embryonic stage, that on the whole they possess
similar bones and are thus proved to be of common origin.

Fishes represent the lowest stage of the vertebrates. In
these denizens of the water the extremities have developed into
instruments of rowing, or fins. These are either paired or
unpaired. We know that the unpaired extremities are derived
from a continuous dorsal fringe which commences immediately
behind the head and continues to the anus. In earliest youth
this uniform fringe divides itself in most fishes into dorsal and
anal fins, and the great locomotor fin of the tail. In a similar
manner the paired fins originate from two at first continuous
lateral folds which divide later into the two anterior pectoral and
ventral fins. Soon afterwards solid skeleton parts appear in the
skin folds, giving to the whole the necessary stability. Of these
two kinds of extremities, the unpaired are phylogeneticaily the
earlier, for we find them already in the lower types of fishes, the
primitive fish Branchiostoma (Amphioxus), and in the Lampreys
(Cyclostoma), which are as yet without paired fins. Being

FROM THALES TO LAMARCK 13

specially adapted to a life in the water, the unpaired fins dis-
appear with the departure from the moist element. In the
amphibians we find in
the larval stage a com-
plete rudimentary
fringe, though without
skeleton parts ; but it
becomes, as a rule, de-
generate in the adult

stage. The paired FIG. I.— THE LANCELET (Brancliiostvma

extremities, however, lanceoiatum).

reach in the higher

vertebrates a more and more perfect development, for they help

the animals to add to the conquest of the water that of the land

and the air. They are therefore of special interest from the

standpoint of comparative anatomy.

All extremities of the higher vertebrates, however widely
they may differ in construction, may be traced back biogenetic-
ally to the so-called Ichthyopterygium, as we see it in the fins
of the lower shark-like fishes. Unequal growth of the single
skeleton parts and a considerable reduction in their numbers
transformed the Ichthyopterygium into the five-fingered extremity
characteristic of all vertebrates from the amphibians upwards.
But yet another change is demanded by the changed conditions
of life. As long as the extremities served as oars or rudders it was
an advantage that they developed into broad ' blades,' acting with
their entire surface. But on land the arms and legs are to act
as levers in carrying and moving the body. To do this, the
extremities must divide into small parts united together and
movable one upon the other ; in other words, they must form
joints.

Everyone who has seen the skeleton of a bird will no
doubt object that in the denizen of the air nothing is to be seen
of a five-fingered extremity, and in most cases indeed nothing
but one finger. This objection is true, but it is obviously the
different function which causes the deviation from the rule and
produces frequently in the skeletons of adult animals considerable
reversion or a combination of the single skeleton parts, thus
making the affected parts better adapted to their special work.

14 LECTUEES ON BIOLOGY

But it can be proved biogenetically and phylogenetically that most
of these animals have descended from five-fingered ancestors, or
at any rate from such as had more than one finger. At an
early stage of incubation of the embryo of the penguin we can
readily distinguish three well-developed fingers and the rudiments
(Anlagen) of a fourth. The Archseopteryx of the Jurassic period
when fully developed had three movable fingers. Let us
endeavour to make clear the radical influence of changed condi-
tions of life by the consideration of a few prominent instances.

If we observe a fish in its natural element and notice
how perfectly adapted are its fins to life and movement in the
water we would hardly think it possible that these specially
developed organs could be used for other than swimming actions.
Yet we know fishes who, with the aid of their fins, are not only
able to run, climb, and jump, but even to traverse not inconsider-
able distances flying. It was, of course, necessary for the fins
of these privileged animals to undergo extensive changes to
qualify them for their different functions.

In fishmongers' shops one may now and then see a very
quaint and awkward-looking fish, the Sapphirine Gurnard or
Tub-fish (Trigla hirundo). It is from 40 to 60 cm. long and a
well-known inhabitant of the North Sea. It has a rounded body
and a thick, square head, protected by an armour of rough skin.
When touched it makes a peculiar noise which reminds one
of the creaking of a badly-oiled door, by rubbing together its
hard gill-covers. We know that the fins of fishes are supported
by cartilaginous rays which are generally firmly joined to the fin.
But in the gurnard the strongly developed pectoral fins have in
front three free, fin-like rays which seem fixed to the body after
the manner of joints. In swimming, these free fin-rays are
folded backwards against the body, but on the sea-floor the fish
uses them skilfully as legs and is thus able to walk. It is a
curious sight to see it deliberately putting one ray before the
other and drawing its plump body slowly along the sea-floor when
in search of its prey.

A near relative of the gurnard is the Flying Gurnard (Dactyl-
opterus volitans), abundant in the Mediterranean, and a repre-
sentative of the flying-fishes. The structure of this fish reminds
one strongly of his cousin of the North, but is much smaller, at

FKOM THALES TO LAMARCK

15

the most 40 cm. It is distinguished from its relative by the
extraordinary development of its pectoral fins, the posterior
section of which broadens into a fan and is supported by cartila-
ginous fin-rays which are nearly 30 cm. long. Every passenger
through the Mediterranean may in clear weather sometimes see
great shoals of these fishes suddenly emerge from the water,
rise to a height of about 18 ft. and traverse fairly quickly a

FIG. 2. — ABOVE, HADDOCK (Gadus <zglefinus)\ BELOW, TUB-FISHES (Trigld hirundo).

distance of more than 300 ft. They then descend and disappear
under the water to rise again in another place. Sometimes
these fishes miss their direction and fall on the deck of the
ship which happens to cross their course.

This curious mode of locomotion cannot be properly described
as flying, for it is rather a rising and gliding like that of a paper
kite. By means of a vigorous jump these fishes propel themselves

16 LECTURES ON BIOLOGY

with widely extended breast-fins from the water and are car-
ried in a direction opposite to that of the wind and kept
suspended for some distance by the current which is caught
under the wing-like fins. It is also probable that they assist
their flight by an up-and-down movement of the breast-fins. It
seems that the flying-fishes use their ability chiefly for escaping
from their numerous enemies among the fishes of prey, often
only to fare worse. For no sooner has a swarm of gurnards
risen than it is attacked by the ever-ravenous sea-gulls. Some-
times they rise in the air in play, like our trout, which on fine
summer evenings leap from the water with audacious jumps.
: In the seas of the Tropics the flying-fishes are still more
common. As soon as a ship has passed the Equator it is during
fine weather accompanied by shoals of these curious creatures.
There are in all more than fifty species of flying-fishes, of
which the Swallow-fish (Exoccztus volitans), an inhabitant of
the Mediterranean, is most widely known next to the Flying
Gurnard.

Whilst the extremities, and m particular the paired pectoral
fins, of the flying-fishes have developed to large dimensions, we
know on the other hand numerous species in which the fins
have degenerated into small insignificant forms which are of little
use in locomotion. All such fishes— I will only mention eels,
murcence, ribbon-fishes and needle -fishes — are distinguished by a
striking elongation of the body-axis. This phenomenon may be
easily explained, with Lamarck, as a result of use and disuse.
When body and tail are growing longer they commence to
participate in the locomotion by snake-like twists and turns,
thus tending more and more to relieve the extremities ; when the
elongation of the axis becomes extreme, the extremities cease
to be active and consequently become degenerate.

All that is said here of the fishes is equally applicable to the
higher classes of the vertebrates. The short, tailless frogs have
very strong legs which are equally well adapted to swimming,
jumping, and climbing. In the long-tailed salamanders and newts
the extremities are less strongly developed ; in the Amphiuma
(A. means), and still more so in the snake-like Csecilians
(Gymnophiona) , they become rudimentary or disappear altogether-
The same observation applies to the reptiles. Tortoises, lizards,

FEOM THALES TO LAMARCK 17

and crocodiles possess very strongly developed front and hind
legs. In the long thin Skinks (Chalcides) only four feet-stumps
are developed which have no functions. Reversion has gone
still further in the blind-worms, which are even without external
vestiges of extremities, whilst finally in the snakes not only the
extremities themselves, but also their connections with the ver-
tebral column, shoulder, and pelvic girdle have been lost. Only
the pythons and boas still possess rudiments (Arilagen) of a
pelvis and the hind extremities ; these are, however, no longer
preserved as means of locomotion, but for purposes of copula-
tion. It is interesting to note that in the snakes the want of
extremities is not only balanced by the enormous elongation of
the body, but also of the ribs, which, having become free and
movable, are made to assist in locomotion. This law does not
even fail in the highest vertebrates, the mammals, for it is well
known that, for instance, monkeys with very long tails possess
proportionally weaker arms and legs than the short-tailed or
tailless baboons and anthropoid apes.

But let us return to the fishes. One of the quaintest among
them, a veritable clown among fishes, is the African Mud-
jumper (Periophthalmus koelreuteri) (fig. 3). It is found all
along the West Coast of Africa, but varies according to its habitat
in colour and marking. It is usually greyish-green or brown?
with silver spots and stripes. The fins are blue, the very
prominent eyes red. Its length does not exceed 15 cm. Its
life-habits are more those of a newt or frog than of a fish,
for it feels both at home in the water and out of it. At ebb-
tide one may see them in tribes of dozens, lying on the wet sand
in numerous parts of the West African coast, or moving along by
means of curious short jumps. Because of their temporary
sojourn on the land the pectoral fins of the Mud-jumper have
assumed a leg-like appearance and are used even for climbing,
for they may frequently be seen ascending the air-roots of man-
grove trees on which they hold their siesta. Still more wonderful
than the change of their fins into instruments of walking and
climbing is the adaptation of their respiratory organs to a life in
the air. We know how sensitive most fishes are to want of
water and how soon they perish when removed from their proper
element. But the Mud-jumper is able to loaf about on the land

2

18

LECTURES ON BIOLOGY

for whole half-days, hunting its prey and moving as freely as in
the water. In order to be able to do so it carries under the
tightly closed gill-covers a good supply of water so that even on
dry land the gills are kept continually moist.

This adaptation to a terrestrial life has made still further
progress in the Lung-fishes (Dipnoi), and particularly in an
Australian species, the Barramunda (Neoceratodus forsteri).

FIG. 3. — THE AFRICAN MUD-JUMPER (Periophthalmus koelreuteri).

These fishes do not only possess the normal respiratory apparatus,
the gills, but their swim-bladders, too, have been transformed
into lungs, so that they are able to breathe both in the water and
on land. The two pairs of extremities of Neoceratodus are
uniformly developed, one might almost say, like legs, but they
are probably too weak to support the big body on land, for the
Barramunda reaches a length of 6 ft. It is therefore question-

FROM THALES TO LAMARCK 19

able whether this fish leaves the water voluntarily for any length
of time.

Evolution having reached this point, the transition to a ' life
ashore,' to amphibious animals, was not difficult. But with
a sojourn on dry land the extremities are confronted with the
solution of fresh and more difficult tasks, and thus we see in
the higher vertebrates, especially in the typical terrestrial
animals, the Keptiles, Birds, and Mammals, that these organs
assume a marvellous wealth of forms and functions.

It will only be possible for us to consider one order, the lizards.
The typical lizards are running animals, possessing five-fingered
extremities, but exhibiting great differences according to their
different modes of life. The South American Teju (Tupi-
nambis teguixin), a large scaly lizard, about 3 ft. long, has
assumed the habit of digging, under the roots of trees, deep caves,
into which it may retire in a moment of danger. Consequently
its front legs are strongly developed and furnished with sharp
claws, thus forming powerful digging-tools. In species that live
in deserts we frequently find that the toes have been broadened
into a shovel-like shape and furnished with lateral fringes so
that the animals are* able to run even over the finest sand without
danger of sinking into it. Numerous species of Geckos, among
them the common Wall Gecko (Tarentola mauritanica), an
inhabitant of the Mediterranean countries, are furnished with
adhesive digits which enable them to run about on steep, smooth
walls, and even on the ceiling. In many districts in Africa
Geckos are kept as domestic animals, for as soon as night has
come they emerge from their hiding-places to prey on spiders,
flies, mosquitoes, and other pests of man. But in spite of their
usefulness they are persecuted by man in the most unreasonable
manner.

Typical inhabitants of trees are the Chameleon tidae, famous
for their striking colour-changes and fantastic shapes. Their
extremities, too, have become transformed corresponding to their
mode of life, for in them the toes have grown together in twos
and threes into uniform plates (laminae) which stand opposite
to each other, as the parts of pincers. They possess, therefore,
an ideal gripping and climbing organ by means of which they
are able to walk safely even on thin branches.

20 LECTURES ON BIOLOGY

Let us now turn to the birds and observe the flight of the
sea-gull. It skims over the surface of the water, now rises into
the air. circles round and round, or hangs in mid-air, motionless.
Suddenly it darts down, quick as lightning, to capture its prey
with unerring aim. All is lightness, elegance, power. Let us
see how the development of its extremities will demonstrate the
fitness of its organization and the transforming influence of
function.

The first feature noticed in a comparison of the wing of
a bird with the forefoot, for instance, of a salamander or lizard
is the great elongation of the various skeleton-parts. Everything
seems to have been arranged with a view to obtaining a long
and strong lever to which to join the feathers. The more skilful
the flier, the more pronounced the lengthening of the bones of
the arm and hand. We also find that not only the carpal and
metacarpal bones but also those of the fingers have decreased
in number and that some have become fused together into
a uniform bone. This fusion caused a considerable lengthening
of the skeleton of the hand without affecting its firmness. The
skeleton of the Archaeopteryx shows that this bird was a very
awkward flyer, making only a faint attempt to conquer the air ;
it also proves its descent from lizard-like ancestors, for its hand
had three well developed free-moving fingers armed with sharp
claws which probably enabled it to climb rocks and trees.

In spite of the wonderful adaptation of the wing of birds to
flying, numerous birds have felt themselves induced to return
to terra firma and to depend more upon their legs. The cause of
this action is obscure. But we find that a more frequent use
and consequently stronger development of the hind-organs entail
a degeneration of the wings and a corresponding decrease in the
power to fly. There is a complete chain, perfect in every link,
which leads from the Gallinaceous Birds to the Ostriches and
from them to the Kiwis. Whilst the Pteroclidae (Sand-Grouse)
are still able to fly well and move chiefly by means of their
wings, the Phasianidae (game-birds, fowls, &c.) occupy a half-way
position, for they use fore and hind extremities in about equal
proportions. The Tinamous (Tinamus), finally, greatly dislike
flying, and in case of danger prefer to trust to their legs. These
different degrees in the use of wings are indicated externally
by their decreasing size.

FIG. 4. — THE DEVELOPMENT OP THE EXTREMITIES. CHAMELEON

vulgaris) • WALL-GECKO (Tarentola mauritanica) ; EYED LIZARD (Lacertaocellata),
AND Zamenis gemonensis viridiflavus.

FIG. 5. — SKELETONS OF THE ANTEEIOR EXTREMITIES OF VARIOUS VERTEBRATES.

(1) Diagram of a typical five-fingered extremity ; (2) extinct bird Archceopteryx ;
(3) eagle ; (4) penguin ; (5) greeriland whale ; (6) man.

22 LECTURES ON BIOLOGY

A further stage of wing-degeneration is reached in the
Ostriches (Struthionidae). The skeleton-parts of the arm are
still existent but far too weak to exercise their functions and
raise the heavy body aloft. Ostriches have entirely lost the power
of flight and have become typical runners. But the effect of
this change in the mode of life has not been confined to the
extremities, for the bones have lost their pneumatic structure
and become massive. The breastbone of the normal bird carries
a strong, median ridge, or keel, the so-called ' crista,' to which
are joined the powerful muscles of flying. But as the ostrich
has no longer any use for the crista it has entirely disappeared.

But of all birds the Kiwis (Apterygidse) of New Zealand supply
the most remarkable instance of transformations brought about
by the relinquishing of the power of flight. In these birds the
feathers have come to resemble hair and the wings become
mere stumps which fulfil no functions and are barely noticeable
externally.

Still more remarkable than the degeneration of wings due to
disuse is their tranformation into fins, a phenomenon which
may be observed in the Penguins (see coloured plate). A more
radical change in a bird can hardly be conceived. If it were
not for the plumage it would be difficult to say whether these
creatures really are birds. The legs have been shifted far back,
thus giving the penguins on land an erect appearance ; in the
water they are extended backwards and employed as a rudder.
The tail is furnished with short, stiff feathers which have to
support the body in the sitting attitude. In the place of the
large jointed wings we find in the penguins broad uniform
' rowing-plates ' or paddles covered with scale-like feathers.

Those who see a penguin for the first time in its watery element
swimming and diving like a fish and keeping for minutes under
water would feel convinced that these birds had in the begin-
ning been specially endowed for a ' life on the ocean wave.'
But anatomy proves the exact contrary, for it shows that they
have gradually developed from typical flying birds : the fins of the
penguin still contain the same skeleton-parts as the well-known
fliers (one upper-arm, two fore-arm, and two carpal bones, and
three hand and two finger bones which have become fused
together). But while in these the tendency is all towards the

FROM THALES TO LAMARCK 23

lengthening of the wing, in the penguin the single bone-parts
have become shorter and stronger, and flattened, in order to
provide a broad 'rowing-surface.' (See fig. 5, 3 and 4.)

There are other birds which provide, as it were, the transition
from the aerial to the aquatic life. The wings of the Common
Guillemot (Uria troile), a bird of the Northern Seas, often found
in Heligoland hatching its eggs, show a distinct shortening, the
bones a broadening. But these birds still possess well-developed
wings and use them for short-distance flights. They derive,
however, greater advantage from their wings on their extensive
swimming tours on and under the water by using them as oars.

Another step further has been taken by the Great Auk (Alca
impennis), once very common in the Northern Sea, but now
almost extinct, owing to the senseless persecution of this bird
by sailors. Its fish-like wings still bear feathers, but they are
much too short for flight and serve now only for locomotion in
the water.

The best proof for the statement that it is the function wrhich
determines the form of all organs is supplied by animals of
different classes but with the same life-habits. Whales and
dolphins are even to-day regarded by many as fishes, so com-
pletely has their shape been altered by a continual sojourn in the
water. Their spindle-like body, the absence of hair, the strong
tail terminating in a double fin, the formation of an unpaired
dorsal and paired pectorals, and finally the absence of a neck
make this mistake pardonable. But the zoologist knows that
whales and dolphins are mammals. They are viviparous and
suckle their young. They have warm blood and breathe air
through lungs like other mammals. There are, however, sharp
distinctions. All other mammals have two pairs of extremities,
but dolphins and whales have only two pectoral fins, which may
perhaps be compared to forelegs ; of a second pair, however,
there is no external trace. But as whales have been gradually
transformed from mammals into denizens of the deep we find on
examination that the breast fins, while externally resembling the
unjointed fins of fishes, are internally supported by the same
skeleton-part as are, for instance, the arms of man or the fore-
leg of a lion. (Compare fig. 5.) But in order to obtain a strong
' rowing-surface ' the bones are much shortened and flattened,

24 LECTUEES ON BIOLOGY

and instead of being jointed are held rigidly together by strong
bands. The hind extremities, however, being relieved of their
functions through the immense lengthening of the body and the
formation of a fleshy tail-fin, have in most species degenerated
without leaving a trace behind. Only in the Greenland Whale
(Balcena mysticetus) and a few others do we find hidden away
in the flesh and unconnected with the vertebral column the
insignificant remains of a short thigh and leg.

As Uria troile forms the transition from the air to the
water, so are the seals (Pinnipedia) the transition form between
land and sea. We still find in them four . strongly developed
extremities which have been transformed into fins. The
pectoral fins not only contain internally the complete parts
of the mammalian arm, but have also five fingers which are
armed with claws and firmly jointed together by exceedingly
strong webs. The posterior fins, which also clearly betray their
origin, are turned backwards and fulfil in swimming the function
of a caudal fin: they serve as a rudder. Locomotion on land
is effected chiefly by an alternate lowering and raising of the fore
and hind part of the body and an eel-like bending and stretching
of the whole body. The legs are only used for support in
climbing inclines.

Among the many other known instances it will only be
possible to mention the Mole (Talpa europoea), whose short and
strong arms, combined with a hand armed with sharp shovel-
claws, seem admirably adapted for digging his long subterranean
runs. It is curious to observe that similar habits of life have
brought about a similar development in the forelegs of an insect,
the Mole-cricket (Gryllotalpa vulgaris).

The conquest by mammals of the elements became complete
with their conquest of the air. The hair of mammals, as com-
pared with feathers, does not seem adapted to flight, but Nature,
never at a loss, knows how to overcome this difficulty, and thus
we see bats and flying-foxes share with birds the ' liberty of the
air.' In the absence of feathers the skin is used for the forma-
tion of a flying-apparatus. This necessitates a radical transfor-
mation of the arm-skeleton ; it is not enough that the single
bones of the anterior extremities are enormously elongated, but it
is also necessary to provide a strong frame on which the delicate
flying-membrane may be expanded. This task is solved by a

FROM THALES TO LAMARCK

25

most remarkable elongation of the finger-joint, which is so con-
siderable that three of the inner fingers reach a length equal
to the joint length of the upper and fore arm. As the thumb has
no function to fulfil in flying it remains short but develops a
strong claw, by means of which the animals can climb or suspend
themselves. Between the fingers of this giant hand on one side

FIG. 6. — FLYING-FOXES (PteTOpUS

and the hind extremities and the body on the other the thin skin
is expanded like a parachute.

Some years ago geologists found in the upper strata of the
so-called lithographic slate of Bavaria (belonging to the Jurassic
Period) the fossil remains of a strange animal which was called
a Pterodactylus. A layman would at the first glance have
unhesitatingly declared it to be a bat, because of the enormous

26

LECTUEES ON BIOLOGY

elongation of one finger, and because the wing-membrane
extended between the hand and the body, exactly as in bats.
But as a matter of fact these fossils are the remains of a
long-extinct genus of reptiles, the Flying Saurians. Here, again,
like functions have produced likeness of organs in widely differing
classes of animals.

Having reviewed
these facts, we can now
understand why Lam-
arck came to proclaim
his theory of evolution.
The most important
question which we shall
have to answer before
we can adopt his views
is, whether a trans-
mission of functional
increase or modification

FIG. 7. — SKELETON OF THE ANTEEIOE EX-
TREMITIES OF THE BAT AND THE FLYING SAURIAN
PTERODACTYLUS.

of an organ to subse-

quent generations is

conceivable, or, in other words, whether we have a right to
assume the transmission by heredity of acquired characters.
With an answer to this question Lamarck's theory stands or
falls. We shall attempt to decide the important question in
the seventh lecture, where we shall also see how Darwin
explained the fitness of the organic world and the adaptation of
animals and plants to the ruling conditions of life.

27

CHAPTER II.
PHENOMENA AND CONDITIONS OF LIFE.

THE Ancients regarded motion as the most important
criterion of life. ' All that moves lives.' The murmuring well,
the burning fire, the rapid lightning were to them living beings.
The same conception governs the untaught even to-day. We
read in ' Eobinson Crusoe ' that Friday, the child of Nature,
who has never come in contact with civilization and does not
know the use of fire, explains to himself the bubbling of the
boiling water by believing that it is stirred up by an animal
which is hiding in the kettle.

Motion is doubtless the most prominent characteristic of
living substance, and we may define life as a series of extremely
complex and varied motions. But for all that the conception of
motion is not identical with the conception of life nor are we
able to say of any form of motion that it is specific to life,
for as there are inorganic bodies the parts of which are in active
motion, so there are living organisms which exhibit no trace of
any perceptible movement. It often needs prolonged investiga-
tion to decide whether a certain object is dead or alive ; super-
ficial inspection frequently leads to wrong conclusions.

If we observe under the microscope a drop of human blood
we perceive at first in the fluid only the red and white corpuscles.
But soon numerous minute, round, pear- or star-shaped par-
ticles become visible, which dart about with a constant vibrat-
ing motion. No one would doubt that these minute motile
bodies are alive, for as no external cause of their movements is
perceptible we seem justified in assuming that they are actively
motile. But this assumption would be wrong, for these minute
bodies are nothing but the products of decomposition of the red
blood corpuscles, and their motion is due to the molecular

28 LECTURES ON BIOLOGY

movement present in all fluids. We may obtain a similar effect
by adding very fine particles of soot to a drop of water, when
these tiny grains of soot will be observed to dance and vibrate
within the field of vision as if they were alive.

That, on the other hand, rest, or the absence of perceptible
motion, may make it very difficult to distinguish between life and
death, has been demonstrated by the following experiment made
by Verworn. If we examine under the microscope a drop of
the sediment in a bottle of white Berlin beer we shall see in the
fluid numerous minute globules of a pale colour, being suspended
singly or in rows of twos and threes. However long we may
continue our examination, these globules always remain in perfect
rest, without exhibiting the least trace of movement. Similar
formations, exactly alike in appearance, may be observed in milk.
But whilst the latter are nothing but inanimate particles of fat,
the globules in the beer are organized unicellular vegetable
growths, the familiar yeast-cells which cause fermentation of the
beer. Many other analogous .cases might be mentioned. Even
the trained microscopist frequently finds it difficult to decide
whether certain minute bacilli, spirilla, or cocci occurring in
preparations are bacteria or decomposition-products of the sur-
rounding tissues. Mere appearance is not sufficient for deciding
whether an organism is an animal or a plant, or for determining
whether an object is animate or inanimate.

Physiology has therefore always striven to determine the
difference between dead matter and an organism. But the results
at which the different investigators arrived were widely diver-
gent, according to the view-point of the investigator. While
one side denies that there is anything in common between the
two and will at the most admit that organisms are built up
with the same chemical elements that are found in inorganic
nature, the other side recognizes none of these distinctions as
valid. The extreme standpoint among modern physiologists is
taken by Verworn, who even refuses to accept metabolism as an
exclusive characteristic of organic substance.

As long as each organism lives it performs work, and work
requires force. As a steam engine can only continue to work
as long as it is fed with fuel, so each organism must consume
food, from the combustion of which its energy is obtained,

PHENOMENA AND CONDITIONS OF LIFE 29

unless its life-functions are to cease temporarily or permanently.
Whilst the substance of the body is renewed from the food, the
matter that has become decomposed and useless is voided. This
continued breaking-down and building-up of organic substance
is called metabolism ; it is on this process that all the life
phenomena of animals and plants rest, and it is therefore
generally regarded as a specific function of life.

According to Verworn, however, metabolism is only a charac-
teristic which distinguishes the living organism from the dead,
but not from the inorganic substances, for it is possible to
observe the same process in inorganic bodies. As an example
this scientist chooses the behaviour of nitric acid in the manu-
facture of so-called English sulphuric acid. If we bring together
nitric acid with the anhydride of sulphurous acid, the sulphurous
acid takes away oxygen from the nitric acid, thus becoming sul-
phuric acid, while the nitric acid becomes hyponitric acid. If
we now regularly admit fresh air and water, nitric acid continues
to develop from the hyponitric acid, and gives off part of its
oxygen to new volumes of sulphurous acid, so that the mole-
cule of the nitric acid is continually being disintegrated by
giving off oxygen, and restored by absorbing oxygen. In this
manner we are able, with a definitive quantity of nitric acid, to
change unlimited quantities of sulphurous acid into sulphuric
acid. Verworn thinks, therefore, that the process observed here
in the simplest form, that is, in a simple chemical compound,
the succession of decomposition and reconstruction of the sub-
stance with taking-in and giving-off matter, corresponds in every
detail to the metabolism of organism.

Bat it seems to me that in spite of the apparent similarity
there is a fundamental difference in these two processes, for
whilst in an organism metabolism proceeds automatically, it
proceeds here on the supposition that the constant access of
air and water is being duly regulated ; in other words, it proceeds
with the assistance of a rational being. As soon as this assis-
tance fails, as soon as the supply of air and water is not so
regulated as to bring the hyponitric acid in a certain manner
and in certain proportions successively into contact with air,
sulphurous acid and water, metabolism ceases. But it is in
the spontaneity that the essence of the life-processes rests, and

30 LECTURES ON BIOLOGY

the definition of metabolism holds, therefore, still good as a
function specific only to living substance.

Nevertheless, this criterion does not suffice for the strict
definition of the animate and inanimate, for though real
metabolism is not to be found in dead nature, we shall see
later that there are nevertheless organisms in which this
function is, for shorter or longer periods, absent.

We have already heard that the normal course of life
depends upon the presence of certain conditions, the absence of
which leads inevitably to death, or at least to the temporary
discontinuance of the life-functions. One of these conditions is
nutrition. Without nutrition there is no life. The substances
which furnish the different organisms with the required force and
heat and renew the substances which have been used up, are of
a very varied nature ; further, the substances required for the
nutrition of animals are different from those required by plants.
Whilst the former are only able to use complex organic com-
pounds for building up living albumen, the food-sources of plants
lie in the surrounding air, water, and the salts of the soil. But
however great the wealth of such food-sources may be, the plants
are unable to use them except under the influence of the sun-
light. It is only with the help of the sunlight that the chloro-
phyll is able to decompose the carbon dioxide of the air into
carbon and oxygen — the first step that must be taken in the
building up of the organic compounds. And as all nutrition of
animals depends finally upon this activity of the plant cell, we
must see in the sun one of the fundamental conditions of life.

Next to nutrition stands respiration as a function specific to
all organisms. All that lives breathes. In a room wholly
devoid of air, in water wholly devoid of gas, life is impossible.
But just as no particular article of nutrition can be mentioned as
being of absolute necessity to all organisms, so there is no gas
which is absolutely necessary to the life of all organisms. Often
that which is necessary to the life of one brings death to another.
Formerly it was thus believed that oxygen is needed for the
growth of all organisms. This is still true of all higher plants
and animals, for though plants give off oxygen during the day
they absorb it again, like animals, during darkness. We know,
however, thanks to Pasteur, that among the lowest known

PHENOMENA AND CONDITIONS OF LIFE 31

organisms, the bacteria, there are many for which life is only
possible if free oxygen is completely excluded — indeed, they
die when brought into contact with it. They are known as
anaerobic bacteria. It is, however, not improbable that even
anaerobic microbes use oxygen which they obtain from the
various stable compounds of their nutrient media, and that only
a liberal supply of free oxygen is fatal to their existence.

Another of the fundamental conditions of life is water.
Whatever animal or plant we may examine, we shall always find
that their cell-protoplasm contains water. Indeed, water is the
chief component of all organisms. The human body contains
about sixty parts of it, the tissues of the muscles from seventy-five
to eighty. That many plants and parts of plants, in particular
fruits, consist largely of water is well known. Many denizens of
the sea, especially the jellyfishes and ctenophores, contain two
parts of solid matter and ninety- eight of water. It is remarkable
how under such conditions a structure can be maintained. One
may frequently see on the sea-shore a large brilliantly coloured
medusa, stranded high and dry, in a short time disappear before
one's very eyes, leaving nothing behind but a little heap of
crumpled membrane.

Warmth is another conditio sine qua non. The limits of
temperature between which active life is possible are very narrow,
for great heat and great cold are equally hostile to it. Though
many organisms may, owing to the warmth of their bodies, exist
for considerable periods in a temperature far below zero, all
active life is doomed to cease as soon as the interior of the body
has cooled down to the freezing-point of water, for only when
water can retain its liquid form nutrition and respiration are
possible. In the higher organisms life ceases in temperatures
which coagulate albumen. Between these two extremes the life-
functions of all organisms operate, though the optimum of well-
being is, in the various species of plant or animal, subject to
considerable variations of temperature.

If we observe a protozoon, the minute Amoeba Umax, in a
temperature of + 0° C. we shall see that it rests suspended in its
drop of water, contracted into a globule, devoid of any action.
It is said to be in a state of * chill-coma.' But if we now
increase the temperature we observe that at 2° to 5° C. a slight

32

LECTURES ON BIOLOGY

protoplasmic motion takes place in the interior of the cell :
slowly and lazily a short pseudopodium emerges here and there,
though the movements are as yet so slow that they are capable
of demonstration only by persistent observation. But the higher
the temperature rises the more active become the functions until
the maximum is reached at 26° C. If we now continue to
increase the temperature beyond that point we observe that a
retrogressive process is taking place : the protoplasmic motion
slows down, the pseudo-feet are retracted, the amoeba gradually
assumes its globular form, and when an upper limit of 35° C.
has been reached, become once more motionless ; it now lies in
a state of ' heat-coma.'

6

8

FIG. 8. — Amoeba Umax AT DIFFERENT TEMPERATURES.

(1) In a state of chill-coma at + 0° C. ; (2-8) the temperature is being gradually
increased ; (4) + 26° C., maximum of vital activity ; (8) in a state of heat-coma at
+ 35°C.

Like warmth, the conditions of air and water pressure have
a far-reaching influence upon the normal course of life. Here,
too, there are upper and lower limits which may not be trans-
gressed except at peril to life. Unfortunately our knowledge
of the effects of increased or decreased air-pressure is as yet very
primitive, but the experience gained in various balloon ascents
is nevertheless of interest. It has been shown that it is impos-
sible, at any rate for a human-being, to ascend beyond a certain

Plate 1

Yellow-billed

Animal Life in the Antarctic.

d Albatros (Diomedea chlororhynchus), Rock-hopper (Eudyptes chrysocome)
and Elephant Seals (Macrorhimis leonmus).

PHENOMENA AND CONDITIONS OF LIFE 33

height without seriously endangering his life by the resultant
decrease in the air-pressure. We must, however, bear in mind
that the evidence on this point is far from conclusive, as in
addition to the diminished air-pressure there are other injurious
factors to be taken into account, as, for instance, the want of
oxygen and the lowered temperature.

One of the most instructive ascents was the unfortunate
journey of the French aeronauts, Sivel, Croce-Spinelli and
Tissandier. They ascended from Paris on April 15, 1875, and
soon reached a height of 7,000 metres. On ascending higher,
according to Tissandier, the only survivor, they began to feel a
faintness which increased from second to second. Soon they were
unable to make the least movement, even speaking having become
impossible. The senses, however, remained acute and the mind
clear. Having passed a height of 8,000 metres, they lost conscious-
ness. When they had regained it they found that they were
drifting at an altitude of 7,000 metres. Again they threw out
ballast, again the balloon rose, and again these aeronauts lost con-
sciousness. When at last Tissandier had once more recovered
consciousness the balloon was floating at a height of 6,000 metres.
His two comrades were dead.

Sixteen years later Berson and Siiring reached the greatest
height ever gained by man, 11,000 metres, the barometer showing
193 mm. This ascent confirmed the experience that if a certain
height is exceeded consciousness is lost and other functions are
seriously disturbed.

Bert has shown that the maximum and minimum of pres-
sure, under which animals may safely live, is in a certain degree
dependent upon the volume of oxygen in the air. We know that
the atmosphere of the earth consists (apart from other com-
ponents) of 21 parts of oxygen and 78.06 of nitrogen. The
pressure exercised by this amount of air upon the earth surface
is called an atmosphere and has been calculated as being 1.033
kilogrammes to the square centimetre. It is generally constant for
all terrestrial animals and plants. But the researches of Bert and
others have shown that it is possible, even in the case of the
higher animals, to alter these conditions without disturbing the
vital activity. For instance, mammals, including man, can per-
manently exist in an atmosphere which contains only fourteen
3

34 LECTUEES ON BIOLOGY

parts of oxygen. If, however, this percentage is lowered to ten,
we observe in man and the higher mammals serious difficulties
of breathing. A further decrease down to seven per cent,
inevitably produces death by suffocation.

There is a wide divergence in the behaviour under equal con-
ditions of warm-blooded and cold-blooded animals. If we place
a rabbit in a room containing an insufficient percentage of oxygen
it will after a few minutes have convulsions and die ; frogs, how-
ever, will remain alive for many hours, even in an atmosphere
from which all oxygen has been removed. These facts are
obviously connected with the varying consumption of oxygen
by the various species. While, for instance, the small songsters
consume 12 grammes of oxygen per kilogramme of body-weight
per hour, the rabbit consumes 1 gramme, man about 0*5, and the
frog only 0'07.

Just as we are able to decrease the supply of oxygen without
a resultant disturbance of the functions of life, so we are similarly
able to increase it. Thus mammals are able under ordinary
pressure to live in pure oxygen. If, however, the pressure is
increased to three atmospheres, death results, curiously enough,
from suffocation. In ordinary air, however, animals can support
a pressure up to fifteen atmospheres. The researches by Bert
show, further, that a low percentage of oxygen may be balanced
by an increase in air-pressure, whilst, conversely, the deleterious
effects of high pressure may be balanced by a decrease in the
supply of oxygen. This operates, however, only within certain
very narrow limits.

The power of resistance of aquatic animals to water-pressure
is remarkable. The results of deep-sea expeditions undertaken
by various Governments have clearly shown that the greatest
depths of ocean, abysses of 6,000 to 8,000 metres, are inhabited
by organisms able to withstand a pressure of several hundred
atmospheres. And not only do we find at these depths primitive
forms, but also representatives of the higher species, cuttle-
fishes, crustaceans, and even fishes. (See coloured plate, No. 2.)
Plants are found at a depth of about 4,000 metres, but beyond
that depth existence becomes to them impossible, owing to the
total absence of light.

If fishes from such depths are rapidly brought to the surface

PHENOMENA AND CONDITIONS OF LIFE 35

in nets, it frequently happens that owing to the tremendous
decrease in the external pressure the air in the swim-bladder
expands so suddenly and enormously that the whole fish explodes,
or that at any rate its abdomen and pharynx are forced forward and
outward, into the shape of a drum (fig. 9). This phenomenon may
often be observed in the numerous fishes of the deep Alpine
lakes, in particular in Coregonus hiemalis. According to von
Siebold these fishes live at a depth of 80 to 90 metres. Con-
sequently its air-filled swim-
bladder has to sustain a
constant pressure of eight to
nine atmospheres, that is,
a weight of 8 kilogrammes
to the square centimetre.
If now this fish is suddenly
brought to the surface the
air in the swim - bladder
suffers a decrease in pressure

of seven to eiffht atmo- FIG' & — Neoscopelus Macrolepidotus

BROUGHT TO THE SURFACE FROM A DEPTH

spheres and must expand in OF 1,500 METRES. (AFTER KELLER.)
proportion as the pressure

diminishes. But as the walls of the swim-bladder and the
abdomen are unable to withstand such pressure the body swells
so enormously that death soon follows.

The researches made by Regnard into the effects upon
different organisms of extreme pressure shows that under a
pressure of 700 atmospheres the activity of putrefactive organisms
completely ceases. Meat and blood which had been kept
under such pressure exhibited no signs of putrefaction even after
the lapse of several weeks, whilst similar quantities, kept for
purposes of comparison under ordinary conditions, had long
since become decomposed. Various infusorians, vermes, crusta-
ceans and molluscs, exposed to similar enormous pressure, ex-
hibited after a few minutes a retarding and finally a cessation
of all vital functions. If after a short time they were brought
back to normal conditions, life sometimes returned, but a pro-
longed stay under extreme pressure always produced death.

The question why the inhabitants of the deep sea can safely
sustain such abnormal conditions is answered by the considera-

36 LECTURES ON BIOLOGY

tion that there a gradual adaptation has taken place, whilst in
the experiments mentioned the organisms were exposed suddenly
to extreme pressure. Nevertheless, we are justified in assuming
that there exists an upper limit of pressure beyond which life
becomes impossible.

Besides these external conditions of life there are internal
conditions of structure and chemical composition of the organic
substance. As far as we 'know, all life that exists to-day on this
earth is bound to the form of the cell, and to the reciprocal action
of protoplasm and nucleus. It may be that in earlier earth-
periods there existed organisms which had not yet reached the
organization of the cell but were a shapeless mass of plasm,
though possessing the function of metabolism and reproduction.
This theory has indeed been retained by Haeckel until the
present day. Accurate investigations, however, have always
demonstrated that these so-called monera already exhibited a
differentiation into protoplasm and cell-nucleus. Nucleus and
protoplasm must, therefore, according to the present state of
knowledge, be regarded as essential to the running of the engine
of life. Only where all these external and internal conditions are
present life becomes possible.

As varied as is the structure of organisms, as numerous are
the disturbances which are caused by a change of these con-
ditions. We have already heard that protoplasm is rich in
water. The withdrawal of water renders it hard and tough,
and its vital activities become suspended, but are not necessarily
destroyed ; for numerous organisms may be gradually called
back to life from complete 'dry- coma' by the supply of water.
Many cells are able to retain life for many years in such a state
of rest. The spores of many species of bacteria, for instance,
remain alive up to ten years, and dry-preserved grains of corn
germinated even after twenty years. But this rest is not un-
limited, as many seem inclined to think. The stories of the
germination of wheat taken from Egyptian pyramids where they
were said to have rested for more than 5,000 years have been
exposed as gross deception. When the native guides saw that
the foreign tourists were willing customers for this ' wheat from
the Pyramids,' they put this knowledge to a highly remunera-
tive use and smuggled into the venerable monuments the ordinary

PHENOMENA AND CONDITIONS OF LIFE 37

commodity. No wonder that this wheat grew directly after it
had been sown. The real wheat from the Pyramids has a dark,
almost black, colour and when placed into water dissolves intc
a powder.

It has long been a much debated question whether the vital
activity in the resting seed has ceased absolutely or has only
been lowered to that point where our senses are not able to
perceive it. If life is still existent, then respiration and
metabolism must of necessity go on. The following experi-
ment by W. Koch seems, however, to prove that this is not the
case.

He placed a large quantity of plant seed into a glass tube,
obtained as perfect a vacuum as possible, and then fused both ends
of the tube. If respiration had taken place even to the most
minute degree, traces of carbondioxide would have been observ-
able, but when after several months the tube was opened it was
found impossible, even with the most exact methods, to demon-
strate the presence of a vestige of carbondioxide. Though life,
therefore, had been completely at a standstill, the seeds were
nevertheless found to have retained their power of germination.

It seems to me that no fundamental objection can be raised
against this assumption. Just as much as we are not prepared
to deny that a falling stone, brought to a halt by an intervening
obstacle, will continue to fall as soon as such obstacle has been
removed, and is therefore at that moment not at perfect rest
but in a state of motion however infinitesimal, just as little
cause is there for denying that the functions of life may some-
times come to a temporarily absolute standstill. But as we may
not call such organisms alive, because life implies the presence
of life-functions, nor dead, because they contain within them
potential life, we must introduce a new term and call them
lifeless or apparently dead (schemtot).

Preyer vividly illustrates this difference between dead and
lifeless by the action of a clock. Whilst the lifeless organism
is comparable to a clock, with spring wound and pendulum
arrested, but needing only the right impetus to set it in motion,
so is the dead animal or plant comparable to a clock, the works
of which have been irreparably destroyed.

The phenomenon of ' dry-coma ' is most remarkable in the

38

LECTURES ON BIOLOGY

various animalcules which owing to their frequent occurrence
in the lichen on roofs or in the dirt of gutters are comprised
in the name of ' gutter fauna.' As early as the end of the
seventeenth century the famous Dutch zoologist Leeuwenhoek
discovered by means of a home-made microscope that in the dirt
of gutters or lichen-covered rocks and trees there lived minute
organisms which were able to dry up into insignificant particles
of dust and come to life again when moistened by the rain.
Recent investigations have removed all doubts that were long
cast upon this discovery, and also demonstrated the existence
of other similar organisms.

If, for instance,
we place the dust-
crust of dried moss
on a piece of glass
and examine the
whole under the mi-
croscope we shall see
after a short time
that some of the
seeming grains of
sand begin to swell,
extend, and show
distinct signs of in-
dependent motion
(fig. 10). Soon the
stillness of death be-

FIG. 10.— BEAR-ANIMALCULES (MdCroUotUS hufeldndl), COm6S ^placed by
ALIVE, AND IN A STATE OF DRY-COMA. actlVC life, and nU~

merous awkward

bear-animalcules, dainty rotatoria and minute amoebae crawl and
swim about in search of food. But no sooner do we let the water
evaporate than the movements of these strange organisms
become slower and slower, their forms shrivel up, and we observe
once more nothing but particles of dust and sand.

The power of resistance of these minute organisms is extra-
ordinary. Particles of dust wThich had been kept dry several years
developed into Wheel and Bear-animalcules and even minute
crustaceans. They seem equally insensible to heat and cold and

PHENOMENA AND CONDITIONS OF LIFE

39

to dryness. All these phenomena we have to regard as perfect
adaptations to the conditions of life, for as these animalcules
have to sustain in their normal habitation, the roof-gutter, not
only frequent changes of moisture and dryness, but also,
unprotected, the heat of the sun and the cold of winter, life
without such high power of resistance would to them become
impossible.

PIG. 11. — THE MUD-FISH (Protopterus Annectens). ON THE LEFT, A FISH BESTING

IN ITS MUD-SHELL.

Cases of ' dry-coma ' are known even among the higher
animals. The best instance is that of the mud-fishes or Dipnoi,
a small order of fishes that do not only breathe through gills
but also through lungs, the air-bladder having become trans-
formed into a single or double lung. When the dry weather
comes these curious animals build out of mud and slime a solid
shell in which they roll themselves up and pass the dry period,
apparently quite dead. As this state lasts for several months it
has been found possible to bring them, shell and all, to Europe.

40 LECTURES ON BIOLOGY

As soon as the rainy period commences they awaken from their
torpor, the mud-shells dissolve in the water, and though at first
the fishes appear sleepy and awkward, in a few hours they have
regained their natural vivacity. (Fig. 11.)

The frequently described reviving of the ' Rose of Jericho '
does not fall into this category. There we have to do with a
completely dead organism, and the unfolding of the dry branches,
the so-called blossoming, is a purely mechanical process due
to the swelling of the leaf-stalks. But the Californian Miracle-
flower (Selaginella rediviva), which is now frequently seen in
Europe, is a ' resurrection plant ' in the truest sense of the
word. For months, even years, this pretty plant, shrivelled up
into an insignificant greyish ball, may lie about at the bottom
of a drawer or on the top of a shelf, but no sooner is it taken
out and moistened with water or planted in a flower-pot and well
watered, than it unfolds its branches after a few hours, becomes
green, and begins to sprout. The instantaneous rise of a vege-
tation on the bare, dark rocks of the Cordilleras after a short
rainfall, mentioned by many travellers, is due to the sudden re-
awakening of Selaginella.

Some organisms show also an astonishing power of resistance
to high and low temperatures. Whilst the protoplasm of the
higher animals and plants is highly sensitive to the effects of
heat and cold, coagulating at + 50° C. and thereby losing its
ability to live, certain of the lower plants, many algae and bacteria,
can, without injury, permanently sustain a temperature of more
than + 70C C. Indeed, some of them find there the most
favourable conditions of their life. In the water of the Karlsbad
Sprudel one may observe numerous filamentous algse (Oscillaria),
and in the hot springs of Albano and in the Solfatarra near
Naples I observed with astonishment quite a flora of simple
plants, and, at a temperature of + 60° C., even minute crustaceans
and larvae of insects, all in a flourishing condition. But most
astonishing of all is that in the hot springs of Yellowstone Park
various species of algae and bacteria thrive at a temperature
of + 75° C.

A still more remarkable power of resistance is observed in
bacteria. The spores of many species will sustain without injury
being exposed for hours to a heat of 1QO° C. Dry spores of

PHENOMENA AND CONDITIONS OF LIFE 41

anthrax are killed only after having been exposed for three hours
to a heat of 140° C. Even seeds of grass and corn, which had
previously been desiccated, sustained for hours a dry heat of
100° to 110° C. without losing their power of germination.

A similar tenacity is displayed by many of the lower organisms
in the presence of cold. According to MacFadyen and others,
certain species of bacteria survived being exposed for seven days
to a temperature of- 190° to -225° C.

That many multicellular organisms are almost insensitive to
rapid changes of temperature is shown by the behaviour of the
' gutter-fauna,' in particular, the Wheel and Bear-animalcules.
We are not only able to expose these delicate organisms to severe
frost, but also to a heat of over 100° C., and yet revive them by
restoring the normal conditions of their life.

One might think that such extraordinary tenacity is specific
only to the simplest and lowest organisms. In a certain sense
that is true, for sensitiveness to external influences increases
with increasing differentiation, a fact that may be observed
in everyday life. The coarse strength of the robust country-
man will easily endure hardships under which the more finely
organized and therefore more delicate townsman collapses. But
the countryman is denied many pleasures which to the more
highly differentiated townsman are his ' real life.' Thus everything
in the organic world is interdependent, and each ' higher ' in one
regard is balanced by one * lower ' in another. In natural science
we should not, therefore, speak of higher and lower organisms,
but of simple and complex. Indeed, if we took the degree of
success in adaptation as our standard we should have to regard
protozoa and bacteria as the highest and most perfect organisms.

Turning for a brief moment to the vertebrates, we find among
them similar cases of great resistance to unusual external con-
ditions. Like a romance sounds the history of a newt, as related
by Erber. Destined to serve as food for a snake, it had suc-
ceeded in escaping its intended fate, and was found many weeks
after in a corner of the room, completely shrivelled up. Placed
in its natural element it revived rapidly, ate well, and soon
regained its original appearance. But there was worse in store
for the newt, for an unexpected night-frost came, froze the water
in the aquarium, and the next morning found the newt frozen

42 LECTURES ON BIOLOGY

inside a solid block of ice. It was dead at last, but since it
seemed befitting to preserve its body in spirit, the piece of ice
was placed in a pot on an oven to thaw and — forgotten. When
at last some one went to take the newt for its final funeral it
was trying its hardest to escape from what seemed very much
like a Turkish bath. None the worse for being nearly frozen
and nearly boiled to death, the little animal lived in the best
of health until it died of natural causes.

I have personally repeatedly observed that fishes, in par-
ticular the mud-fishes (Cobitis fossilis) and frogs, may be several
times successively frozen and thawed without suffering any fatal
injuries, provided that the thawing is done very slowly and
carefully.

It was formerly believed that as soon as the interior of the
body of animals had cooled down to zero death was inevitable,
but numerous experiments made with various animals by Power,
Preyer, Pictet, myself and many .others contradict this belief.
Preyer, for instance, slowly froze two frogs, gradually lowering
the temperature to — 2J° C. One frog was then opened, and it
was shown that all organs, even heart and blood, were frozen
to ice. The frog was then carefully taken into a warm room,
when the heart began to beat and drive the blood which had
again become liquid through the artieries. (On account of its
extraordinary resistance the frog is a favoured object for experi-
ments of this kind, for its heart will continue to beat long
after the animal is dead, even when it has been removed from
the body.) When the second frog, which had been exposed to the
same conditions, had been thawed, it revived and did not appear to
have in any way suffered. Pictet's recent experiments prove
that frogs and snakes are not adversely affected by a tempera-
ture of —28° C., and that snails can successfully withstand a
temperature of — 120° C., even when exposed to it for several
days. As at such low temperatures all chemical processes cease,
we have here another proof that life has temporarily come
to an absolute standstill.

What we attained by artificial means Nature performs
regularly year after year in all countries with a cold climate,
for a large number of mammals, such as the bat, hedgehog,
badger, hamster, and brown bear, become torpid as soon as the

Plate 2

Deep-sea Life.

Deep-sea jelly-fish, Atolla; Melanocetus, a fish belonging to the family of the Lophiidae;
Deep-sea 'crustacean, Nematocarcinus. On the sea-floor, on the left; a glass-sponge,
Hexactinellida, in the centre, a starfish, Brisinga elegans, and a crustacean, Geryon.
On the right, skeleton-part of a glass-sponge, Aphrocallistes ; an Alcyonaria, and a

Crinoid, Melacrinus.

PHENOMENA AND 'CONOTTIONS' Otf 'Lltffi ' 43

cold weather sets in. These animals hibernate during a period,
the extent of which varies with the different species, but there is
always noticeable a great decrease in metabolism. Eespiration
ceases, the actions of the heart are hardly perceptible, and
the body temperature of the sleepers, which otherwise about
equals that of man, almost falls .to freezing point. When the
mild weather returns vital activity gradually recommences, and
emaciated and exhausted the animals come out of their winter-
quarters.

For many animals hibernation is a direct necessity. As their
food consists of insects or plants, and as they cannot, like some
birds, undertake long journeys to some more hospitable country,
they would die from hunger if they could not sleep the time
away. But as in hibernating animals metabolism is not entirely
suspended but only reduced, a certain quantity of substance is
naturally used up even during sleep. The animals live during
this time on the fat which they have accumulated during the
good season. If we examine a bat which we find hibernating in
a cellar or chimney, we notice that all organs, muscles, intes-
tines, &c., are surrounded with thick layers of fat. But in
spring, on re-awakening, the bat is thin and emaciated, having
lost about a quarter of its body-weight.

Some of the hibernating animals make the period of hunger
easier to sustain by carrying together, in the autumn, stores of
food which they consume whenever a sunny winter day awakens
them. While most animals, when hibernating, are insensible
to almost every external stimulus, and most difficult to rouse,
others — as, for instance, the bear — awaken at the least suspicious
noise.

To the decrease of vital activity corresponds a considerable
increase in insensibility to injuries. Wounds, to which these
animals would in the waking state succumb almost instan-
taneously, are frequently sustained for days. The heart of a
hibernating hedgehog beats for hours after the spinal cord has
been severed.

We have so far seen that the withdrawal of water, variation of
air-pressure, and extreme cold and heat, will cause in animals tem-
porary torpor. But the same effect can also be produced by purely

44

s" : v ;_: *

'* OV. BIOLOGY

psychic influences. Already in olden times it was said that many
persons possessed the faculty voluntarily to reduce their vital
activity short of total cessation, to lapse into a state of torpor from
which they would re-awaken after a longer or shorter period.
The best known illustration is offered by the Indian fakirs and
Egyptian dervishes who, it is said, even cause themselves to be
buried in the earth, and rise from their graves after having been
buried for weeks and even months.

However, as often as such ' miracles ' have been thoroughly
investigated they have usually been found to be clever deceptions.
Nevertheless, it would be unwise to dispute the possibility of
such phenomena. We know that as a result of illness persons

not unfrequently fall
into a state of such
complete debility that
a superficial exami-
nation is unable to
find even a ves-
tige of vital activity.
Cases of apparent
death or suspended
animation are known
which lasted for
several days. The
only miraculous part

in the action of fakirs and dervishes is, therefore, that they
are seemingly able to place themselves into a state of hidden life
voluntarily, and for considerable periods. But though we cannot
explain these things they may be true, nevertheless. Even to-day
" there are more things in heaven and earth than are dreamt
of in our philosophy." Let us only remember the strange
phenomena of hypnosis which are still awaiting a satisfactory
explanation ; here, too, vital functions may be so reduced as to
lead to a state of complete catalepsy.

In the animal kingdom we observe similar phenomena due
to hypnotic influences. Many will here recall the celebrated
11 experimentum mirabile de imaginatione gallince," described in
1646 by Athanasius Kircher in his " Ars magna lucis et umbra"
If we firmly grasp a fowl and carefully put it on its back it

PIG. 12. — FOWL IN A STATE OF CATAPLEXY.

PHENOMENA AND CONDITIONS OF LIFE 45

will at first make feeble defensive movements, but then lie still
as if paralyzed. Kircher prescribed that this experiment should
be conducted in the following manner : " Tie together the legs of
a fowl and place it on the floor, and it will at first endeavour to
free itself from its fetters by beating its wings and moving its
body. But after a few futile attempts to escape it will remain
quiet. Then draw on the floor a straight chalk line, com-
mencing from the eye of the fowl, and remove its fetters,
and though the fowl is no longer bound it will not run away
even if it be encouraged to do so." The phenomenon is caused
by the effects of an intense feeling of fright — the fowl is in a
state of ' cataplexy,' as Preyer calls it.

It has been known for
thousands of years that
the dreaded cobra, whose
bite invariably causes
death, may be paralyzed
by a slight pressure
exerted in the region of
the neck, and that its
body may then be placed

into any desired shape. / FiG. 13.— CRAYFISH IN A STATE

The ancient miracle, per- \ OF CATAPLEXY.

formed by Moses by
changing the serpent into a staff, thus probably finds a natural
explanation.

Every one knows that small animals at the sight of a serpent
become so frightened as to remain quite motionless, thus falling
an easy prey. Moreover, a great fright has a paralyzing effect
upon a large number of animals. If one suddenly seizes a cray-
fish and presses it on the floor in the manner shown in the
illustration (fig. 13) it will remain in this unnatural position for
a considerable time without moving a limb. Many similar
cases could be quoted.

If in the instances mentioned the animals derived no advan-
tage from this state of cataplexy, other animals owe to it often
escape from dire peril, and we are therefore justified in regarding
cataplexy as a form of adaptation to the conditions of life. It is
very common among insects and spiders, and its advantages are

46 LECTURES ON BIOLOGY

obvious. Many beetles and spiders collapse on the floor as soon
as they are touched, draw in the limbs close to the body and
4 pretend to be dead.' In this state they frequently remain
a considerable time. As a resting object is notoriously more
difficult to discern than a moving one, especially if its colour
does not greatly differ from that of its surroundings, they
frequently escape by this artifice from the persecutions of their
enemies. As, further, many animals seize only living and moving
prey, but pass by motionless objects, this ' pretence to be dead '
is of considerable advantage to those animals which practise it.

An interesting observation is reported by Belt. Several
locusts had met a detachment of the murderous Driver-ants.
Some of the locusts forthwith became paralyzed with fright, and
being rigid and motionless, escaped death ; others sought salva-
tion in flight, but were quickly caught, overpowered and devoured.

It is necessary to say a word about the attitude of many
organisms to chemical influences. Here we see that conditions
which are fatal to one organism have often not the least dele-
terious effect upon others. In zoological practice pure alcohol
and osmic acid are two of the strongest and most frequently
used means of preserving. With the majority of the smaller
animals an immersion lasting only a fewT seconds, or, at the
most, minutes, is sufficient to kill them and harden the tissues.
But if we place the aquatic larva of Corethra plumicornis, a
species of gnat, into either of these liquids it swims placidly
about, just as if it were in its proper element. After a con-
siderable time it begins to show signs of unrest, but frequently it
is only after several days that this delicate little animal succumbs
to the effects of the poison.

Still more wonderful appears the life of a little worm
(Anguillula aceti) which lives in vinegar. Where no other life
can exist this tiny worm, which does not exceed 2 mm. in length,
grows and multiplies, feeling as comfortable as a fish in water.
If one holds up a bottle of vinegar to the light one may frequently
see many of these ' thin threads ' swimming gaily about.

Nature places before us riddle after riddle. Inexhaustible are
her ways and means of providing for her creatures suitable con-
ditions of existence, and even to unlock for them such regions
as according to our idea of living organisms would seem to

PHENOMENA AND CONDITIONS OF LIFE 47

preclude every possibility of living. For do not numerous para-
sitic bacteria, protozoa and vermes live in the digestive juices of
the intestinal canal of the higher animals, which are fatal to all
other organic life? The question has often been asked, How
is it possible that the intestinal parasites are not digested in the
same manner as all other organic substances introduced into the
intestines? But this fact is no more remarkable than the fact
that the intestinal cells are not themselves decomposed by the
juices produced by them. This does actually occur in cases of intes-
tinal affections, for it is frequently observed, especially during
the summer season, in a human corpse examined soon after death,
that certain larger or smaller portions of the walls of the stomach
and intestines have been digested.

Formerly it was believed, on the ground of these observations,
that each living organism possessed a protective agent in a special
vital force (vis vitalis), and that digestion of the stomach-wall
set in after the death of this protective force. As for a long time
no other reasonable explanation was forthcoming, this belief was
held even in expert circles. It is a common weakness of the
human mind to substitute for the mysterious an equally mys-
terious phrase, pretentious of being an explanation, and thus
to deceive ourselves into believing that we know, rather than
frankly admit our ignorance of that subject. When it was
proved by various experiments — first by Pavy, who introduced
the ear of a living rabbit into the gastric fistula of a dog — that
parts of an animal still alive are digested by the juices of the
stomach, the convenient assumption of the effects of a mys-
terious vital force had to be abandoned. To-day it is generally
believed that the impregnability of the healthy intestinal mucous
membrane, as well as the power of resistance of intestinal para-
sites, rests in their ability to secrete a substance which acts
as an antidote to the gastric juices and renders them harmless.

This assumption is in perfect accord with our knowledge of
the effect of poisons. It is well known that every animal and
human body is able to become accustomed to poisons — as for
instance nicotine, alcohol, morphia, opium, arsenic — by gradually
increasing the dose to such a degree that a quantity of poison,
fatal under natural conditions, will cause very little or no harm.
It is said of Mithridates, the great ruler of Pontus, that having

48 LECTUEES ON BIOLOGY

hardened himself in this manner against poison, as a protective
measure against the poisoner, no poison was found strong enough
to kill him when on the collapse of his sanguinary career he
wanted to make an end of his miserable life.

The power of the living organism to become accustomed to
poisons rests on the faculty peculiar to it of creating for each
toxin an antitoxin which will neutralize the ill-effects of the
toxin, unless the quantity introduced has been too large. But
prudent Nature always forms more antitoxins than are required
for the relevant purposes of neutralization. This surplus is
stored up in the tissues of the body where it lies ready for
any subsequent attack. Unfortunately, the curative effect of
these antitoxins is strictly specific, so that, for instance, an
acquired habit of taking arsenic is a protection only against
arsenical poisoning, but not against the ill-effects of morphia
or opium.

We know to-day that the devastations of many species of
bacteria — the causes of most of the infectious diseases — are
due to the effects of certain specific toxins which, themselves
the results of the mode of life of these minute organisms, far
exceed in destructive power all other animal, plant, and mineral
poisons. Whilst 120 to 130 milligrammes of strychnine consti-
tute a fatal dose, only 0'23 milligramme, i.e., about the five-
hundredth part, of Bacillus tetani, the cause of lockjaw, is
sufficient to kill a man of about 70 kilogrammes body- weight.
But in spite of this truly awful power the animal body is capable,
to a certain extent, of becoming accustomed to bacterial poisons.
An instance of this experience is protective inoculation, as it has
been practised in Germany and in other countries against small-
pox and other infectious diseases for many years. Vaccination
does nothing else but cause in the body of the vaccinated
individual the production of a small-pox antidote which retains
for several years the power to annihilate, in the event of an
epidemic, automatically any small-pox disease germs that may
invade the organism. We cause, in fact, at first artificially
such slight cases of small-pox as the organism is able to with-
stand, in order to enable it in the future to escape severer
attacks. The great reduction in the number of cases of small-
pox in Germany since the passing of the Compulsory Vaccination

PHENOMENA AND CONDITIONS OF LIFE 49

Law is the best proof that this method is right. Everyone
knows that persons who have had the measles, scarlet-fever,
whooping-cough, &c., are almost perfectly immune against
renewed infection, a phenomenon which can only be explained
by the formation in the body of a protective substance which
remains effective for some time.

This immunity which, as we have seen, may be acquired
artificially by the living organism against various toxins is a
natural property of many animals. We find that many infec-
tious diseases fatal to man are not transmissible to animals, and
vice versa. Further, while among closely related animals one
species is highly susceptible to a certain disease, another species
is absolutely immune. It is still more remarkable that the
same phenomenon is observed in individuals of the same species.
Why that should be so remains for the present obscure.

Many serpent-hunters, among them the useful hedgehog, are
almost immune against the effects of snake poisons. One may
often see the hedgehog devour, with the greatest relish and
without the least discomfort to himself, a viper, poison-glands
and all. That the bite of the viper does not injure the hedgehog
I have been able to ascertain on various occasions.

A still more remarkable instance of immunity is presented by
a tiny beetle. The fresh juicy leaves of the deadly nightshade,
Belladonna atropa, almost seem to provoke the appetite of all
plant-eaters. Yet, no matter how scarce the food supply may be,
the nightshade is carefully avoided by all the large grazing
animals, for its leaf and juice contain a deadly poison. Without
this terrible weapon of defence this plant would long ago have
been defeated in the struggle for existence. But even the most
powerful protection can only grant a qualified security, for the
poison which frightens away the largest animals has no terrors
for the little Haltica atropce, for the leaves and juices of the
deadly nightshade constitute its sole food.

50

CHAPTER III.
THE FORCES IN THE ORGANISM.

No living naturalist is able to give an accurate definition of
an organism or life itself. However great the progress made in
biology during the last century, however great the store of obser-
vations made and facts discovered, we are as far as ever from
the desired goal of a definition of life and from a real knowledge
of the conditions of life, the most important object of our
researches. Though girders, stones, and mortar are indispens-
able to the builder of a house, the main thing is the plan of
the architect who unites the scattered parts in one harmonic
whole. While former ages and naturalists were content to
describe the structure and shape of animals and plants, to give
them names and classify them according to the degree of simi-
larity, we demand to-day to know not only how an organism is
constructed, but also why it is so constructed, how it originated,
how its various organs were evolved, what their functions are,
and by what laws the different vital functions are regulated.

Not so long ago there existed between the inanimate, in-
organic Nature and the world of animals and plants an abyss
over which no bridge led, nor, indeed, could lead, according to
the then almost general belief. Whilst in inorganic Nature
all changes took place according to the unalterable laws of
mechanics, physics and chemistry, the phenomena of life remained
a profound mystery. But as science was unwilling to renounce
even an attempt to explain the seemingly inexplicable, there
arose the convenient assumption of the presence in each
organism of a specific, mysterious, unknowable source of force,
which under the sonorous name of vital force — vis vitalis — long
haunted the realms of science. Though by this assumption
knowledge had not been advanced a single step, and though

THE FORCES IN THE ORGANISM 51

this vis vitalis expressed no rational thought, there existed
now at any rate a sounding phrase which covered a mass of
ignorance : —

" Denn eben wo Begriffe fehlen,
Da stellt ein Wort zur rechten Zeit sich ein."

Only slowly and gradually did the conviction gain ground that
there is no foundation for such a radical difference as that
which superficial observation had created between the organic
and inorganic world.

The school of Vitalism suffered a severe blow when chemistry,
daily extending the boundaries of its knowledge, proved that the
bodies of all living beings are built up with the same element
as dead matter, and that the difference between organic and
inorganic compounds is only the difference in the combination of
the individual elements. And when in 1828 the famous chemist
Wohler succeeded in producing artificially in the laboratory
an organic compound, urine, by synthesis from inorganic bodies,
the doctrine of vitalism lost one supporter after another ; further
discoveries completed its decay. Other animal and vegetable
compounds — spirits of wine, vinegar, sugar, starch, &c. — were
produced out of their elements by chemical synthesis, and
we are now within measurable distance of the day when
chemists will succeed in making artificially the foundation of
all phenomena of life, the albumen. Quite recently an important
advance in this direction has been made by the brilliant work
of Emil Fischer.

But not only do similar chemical elements build up alike
animate and inanimate matter, both are also subject to the
same physico-mechanical laws. With each advance of science
this fact becomes clearer and more convincing. A few instances
will suffice.

On looking at a longitudinal section of a human femur we
observe that the fibre-ducts and bone-corpuscles are arranged
on strict architectonic principles. The femur is a tube with
strong walls. In the so-called neck and head of this bone — the
parts which carry the greatest load — the spongy cancellous tissue
is arranged from wall to wall in slender bars or lamellae, which
unite together to form an open lattice-work, like that of an iron

LECTUEES ON BIOLOGY

bridge, running exactly in the direction of the greatest pressure.
"Nature," aptly says J. Wolff, " has built the bones as the
engineer builds a bridge, obtaining a maximum of strength and
suitability of form with a mimimum of material."

But it may be objected
that this instance refutes
our arguments and proves
what we are refuting.
Granted that the strength
of the bone rests on simple
mechanical laws ; but to
what cause is its perfect
construction due ? Does
not the very perfection
testify to the influence of
some mysterious source of
force ? Is it conceivable
that blind mechanical
forces can create perfec-
tion?— The credit of hav-
ing shown the way out of
this difficulty to a natural
explanation of the appar-
ently inexplicable belongs
to Eoux and his Theory of
the Struggle of the Parts
in the organism.
We have already seen that constant use strengthens our
organs, whilst non-use leads to degeneration and atrophy. This
rule does not only apply to the muscles and other organs of the
body, but also, as everyone may ascertain in himself, to our
mental faculties, in particular the memory.

Eoux stated that this strengthening is produced by a
specific function of the organ itself — in other words, that it is
the functional stimulus which strengthens the organs. For
though the increased use and more intense activity cause an
increased metabolism, this is not only balanced, but there is even
a large credit balance, owing to increased power of assimilation.
It may therefore be said that the stimulus has a direct nutritive

FIG. 14. — LONGITUDINAL SECTION OF A
THIGHBONE.

a, Marrow-shaft; b, spongy tissue. (After
v. Hanstein, " Naturgeschichte des Tierreichs.'')

THE FOECES IN THE ORGANISM 53

effect. If, on the other hand, the normal stimulus is absent
there follows of necessity a weakening and decrease. The
much-exercised muscle grows in volume and correspondingly
in working power, the unused muscle becomes powerless and
atrophied.

Most clearly is the degeneration of muscular tissue observed
in pathological changes, as, for instance, in a badly healed joint.
Here the blood supply is exactly the same as in the healthy
organs, but the functional stimulus is absent, and thus degenera-
tion sets in. A similar result is observed when we sever the
nerve conveying the stimulus : decrease in volume and symptoms
of degeneration become quickly noticeable in the muscle, and
complete destruction of the structure is not infrequently the
final result. Or, to give another instance, when in a human being
one kidney degenerates and loses its functions, or is removed by
an operation, the other kidney, owing to the greater demands
made upon it, increases enormously in volume, often to more
than double its usual size.

But as the organism has at its disposal, at any rate
within certain limits, a circumscribed amount of building
material in which all its various parts share, it follows that
the excess of food used by a certain organ or tissue, owing to
increased activity, must be taken away from another less used
organ. In proportion, therefore, to their importance or " out-
put of work " the various parts of the animal body maintain
an unstable equilibrium, and, on the whole, prevent that one part
exceeds its normal ''allowance" at the expense of the rest, at
any rate as long as the welfare of the whole body does not
justify the excess. There is going on, metaphorically speaking,
a restless competition — a struggle of the parts — for the available
sources of food in the living body. Though perhaps this point
would be more appropriately mentioned in another place, it
seems nevertheless advisable, on account of its great importance,
to anticipate some of the points and deal with them here.

An organ in which this struggle of the parts may be most
clearly observed — for it is here the entire cells which enter into
direct competition — is the male and female generative gland of
many animals. A little cuttle-fish from the Gulf of Naples may
serve as an illustration. If we observe thin transverse sections

54 LECTURES ON BIOLOGY

of the testicles of this animal, under the microscope, magnified
about 2,000 times, we can distinguish numerous heterogeneous
cell-elements, which represent different stages of development
from the so-called spermatoblast to the fully developed sper-
matozoon which is furnished with a thin caudal filament.
The spermatoblasts develop by direct metamorphosis from the
so-called protoblastic cells, i.e., the cells which evolve from the
epithelial covering of the vesicles. , Each of these spermato-
blasts splits up into two cells of equal value, called sperma-
tocytes of the first degree, which in their turn split up each
into two cells, called spermatocytes of the second degree,
or spermatides. These, again, change by a highly complex
metamorphosis into the complete spermatozoa. From each
spermatoblast originate, therefore, normally four complete sper-
matozoa— supposing that the cells are favoured by luck, for many
cells perish at an early stage. How are we to explain this
remarkable phenomenon ? When I observed this development
for the first time in Rossia macrosoma I felt greatly puzzled,
but accurate comparisons with the conditions obtaining in other
species of cuttle-fishes and many tedious researches solved the
mystery.

It is known that in the generative glands of most animals,
— in a most highly developed form in insects, crustaceans, &c. —
there is found, in addition to the sex-cells proper, a large
number of accessory cells whose task it is to supply the growing
spermatozoa and ova abundantly with food. Of the numerous
methods by which these cells fulfil their task I will mention only
the two most important. They either take nutriment from the
surrounding body-cells of the organism and transmit it to the
sex-cells, thus playing the part of a middleman, or they them-
selves serve as food and are gradually devoured by the sperma-
tozoa and ova.

No such special feeding apparatus has been evolved in the
cuttle-fish, but Nature knows how to help herself without it.
At certain times there are formed in the seminal tubules of the
testicles of Eossia enormous numbers of germ-cells. As these
cells are themselves only sparsely furnished with food, the store
being insufficient for the growth and metamorphosis .of the com-
plete spermatozoa, there takes place in consequence among this

THE FORCES IN THE ORGANISM

55

vast number of germ-cells a fierce competition for the food con-
tained in the vesicles. But as there are among the germ-cells,
as among animals, strong and weak individuals it happens that
the stronger cells seize great quantities of food at the expense
of the weaker cells. The result of this struggle is that a large
number of the germ-cells, owing to insufficient nutrition, atrophy

FIG. 15. — THE STRUGGLE OF THE GERM-CELLS IN THE MALE GLANDS OF A

CUTTLE-FISH (Rossia macrosonia) .

(1) Spermatoblasts in process of disintegration : (a) normal; (b) disintegrating;
(c) two young spermatozoa (spermatides) which have entered into the disintegrating
mass. (2) The completely disintegrated cells, forming an amorphous lump of food,
at the expense of which the surviving spermatozoa develop. (3) and (4) The "food-
mass " is being gradually devoured. (After Thesing, "Notes on the Spermato-
genesis of the Cephalopods.")

and finally disintegrate. One may sometimes observe entire
colonies of many hundreds of cells in this state of incipient disso-
lution. First the margins become indistinct, then the nucleus
disintegrates and its component parts mix with the rest of the
cell-fluid. Finally, confluence takes place of the adjoining cells,
thus forming a uniform substance (see fig. 15).

56 LECTURES ON BIOLOGY

Forthwith the more favoured germ-cells take advantage of the
unfortunate position of their brothers ; numerous normal and
healthy spermatozoa penetrate with their heads into the disin-
tegrated cells and commence to feed vigorously and to grow. At
no stage of their development are the germ- cells quite secure
against destruction, for it may not infrequently be observed that
almost fully developed spermatozoa disintegrate and are assimilated
by the survivors. It is a veritable struggle for existence in which
apparently the fittest gain the victory here as elsewhere in Nature.

This secret fight to the death is of double importance for the
species. In the first place, it is only the healthy, vigorous sper-
matozoa which will reach the final stage of development, and
these, again, are during the entire period of growth abundantly
supplied with food. But vigorous and healthy germ-cells are
the surest guarantee of a vigorous generation, and a rational
result is thus obtained by a crude, mechanical selection of the
strongest cells.

But let us return to the starting point of this examination,
the conditions observed in the femur. Here, too, we see that
the purposeful arrangement of the individual pillars and arches
proceeds from a struggle between the individual bone-forming
cells. We have already seen that an increased stimulus conse-
quent upon an increased normal function of an organ or part of
an organ leads to an increase in assimilation and a strengthen-
ing and better formation of the affected tissues. The bone,
however, does not consist of a homogeneous substance but of
heterogeneous elements ; in this case, therefore, the burden
placed upon the femur is not equally distributed but stimulates
different parts to a different degree. It is therefore quite natural
i,hat the largest amount of bone-tissue is formed and deposited
in the direction of the strongest pressure, for there the bone-
forming cells are most frequently and intensely stimulated into
activity and growth. The interspersed parts, however, which
participate in the work but slightly or not at all, suffer a corre-
sponding want of nutrition and lag behind in their development.
In this way we obtain a typical pillar-and-arch structure, as
shown in the picture, reminding us of the rational plan of the
expert engineer.

In one respect, however, the natural structure far sur-

THE FORCES IN THE ORGANISM 57

passes all human work — in being able to adapt itself suitably
to altered conditions of pressure. The necessity for this arises
very frequently in those cases of fracture in which the two ends
have healed together obliquely. The course of the pillars and
arches is now no longer in the direction of the strongest pressure
and pull. The parts which previously did most of the work,
qualified for it by their stronger formation, have now become
inactive and useless. But this state does not last long, for soon
the course of the lamellae once more proceeds in what is now
the direction of the strongest pressure. But everything has
happened quite naturally, and there is no need to assume the
existence of a mysterious vital force in order to explain the
purposeful construction of the bones or any other organs.

A different question is : What enables an organic substance
to react suitably upon a functional stimulus? How does it
happen that the awakened need carries with it its own satis-
faction? It seems as if we are here confronted with a fund.i-
mental difference between the living, and the dead and inorganic
matter. But as we observe hourly that living matter changes
into inorganic substance, and as we are forced to assume that in
distant earth-periods living matter originated from inorganic,
there is no necessity for believing this faculty of the living body to
be a new force created mysteriously out of nothing, but we must
rather assume that it is an essential quality of matter which is
non- apparent in the inorganic. Just as electric energy, slumber
ing potentially in bodies, only reveals its presence and effects
when a certain combination of matter has taken place, so it is
with this assumed force of living matter.

We cannot escape the conclusion that the inorganic carries
within itself the conditions of its organization, nor can we deny
the justice of the claims advanced by those inclined to postu-
late a specific psychic form of energy besides the chemical,
mechanical, electric energy.

It is true that modern biologists confess to an open distrust
of everything psychological ; they regard it as mystic, and
many would prefer to exclude it entirely from the field of
natural science. But that would at the same time exclude an
understanding of most of the physical processes and pheno-
mena. "Because," says Wundt, "a superficial observation of the

58 LECTURES ON BIOLOGY

phenomena of evolution easily leads to the conclusion that with
the perfection of the physical organization an increase takes place
in the psychic activities, we meet with the still current belief
that the former is the cause of the latter. But a deeper insight
into the evolution of psychical phenomena leads to the opposite
belief; through the motion which it causes the stimulus reacts
upon the physical organization, and leaves behind in it those
permanent traces which facilitate at first the renewal of the
stimulus-motion (Triebewegung], and then, when the reactions
of other stimulus-actions are added, permits the origination of
more complex stimulus-expressions." We are even " impelled to
believe that the physical development is not the cause but the
effect of the psychic development." In fact, without this
assumption an understanding of the purposefulness of the life-
phenomena is impossible. It will be clear that this statement
is only apparently anti-mechanical, for the postulated psychic
energy does not represent something imperceptible like the vital
force, but is like every other form of energy definable quan-
titatively and thus perceivable scientifically.

If we now attempt to define the aim and object of modern
biology we shall say that they consist in reducing all functional
processes comprised in the term "life" to chemico-physical
causes. But only when we have succeeded in reducing an
organism to simple mathematical formulae, to predetermine its
life-phenomena, changes, &c., just as the astronomer calculates
the course of the stars and predicts eclipses of the sun and
moon accurately to the minute, only then can we claim to have
satisfied this demand. That this goal is as yet far distant, that
neither modern chemistry nor physics is able to perform this
task, and that in the end we shall only be able to approach our
aim but never reach it, need not be specially mentioned. The
phenomena of life are so many, the structure and conditions of
the living substance so exceedingly complex, that in the majority
of cases we shall have to be content with a mere description,
because no explanation is available.

But however far we may be privileged to penetrate into
the mysteries, one will always remain hidden — consciousness.
We may possess a perfect knowledge of the structure of the

THE FORCES IN THE ORGANISM 59

human eye, understand every detail of the laws of optics, know
how the rays which focus an image on the retina are converged,
refracted and reflected ; but how it comes about that we have
a sensation of the image, how we become conscious of it still
remains an impenetrable mystery. We may know the delicate
structure of the brain, follow each change, every chemical pro-
cess and motion of the atoms during the process of thinking ;
but how a thought arises, how we become conscious of an
object, remains a question to which there is no answer, nor
ever will be. As Du Bois Keymond said : " It will for ever
remain inexplicable why a certain number of atoms of carbon,
hydrogen, nitrogen, and oxygen should not be utterly indifferent
as to how they lie and move, how they did lie and move, and
how they will lie and move. It is impossible to see how their
correlated action produces consciousness. The mental processes
which are going on in the brain side by side with the material
processes offer no explanation acceptable to our reason. They
stand outside the law of causality, and are for that reason alone
as incomprehensible as a perpetuum mobile. As the world and
all individual thought originate in consciousness, so conscious-
ness is the final law to which we can reduce a phenomenon."
But though this greatest riddle may remain unsolved, the need,
deeply ingrained in our mind, of conceiving all processes and
changes as effects, and of searching for the causes of these
effects, finds, nevertheless, a high degree of satisfaction in the
endeavour to reduce each phenomenon to its cause. Even the
conviction that we shall never be able to fathom the truth will
not prevent us from striving after the truth for ever.

60

CHAPTER IY.
THE BUILDING-STONES OF THE ORGANIC WORLD.

IT often happens that long before patient research has
reached, at a snail's pace, a new goal of knowledge, genius
has attained it by the mere force of thought, by speculative
thinking. Goethe taught that " nothing that lives is an indi-
vidual but a plurality ; even in so far as it appears to us as
individual it remains an aggregation of living independent beings
which are alike in conception and disposition (Anlage), but may
in appearance become like or similar, unlike or dissimilar. These
beings are either originally correlated or they meet and become
united. They part and once more seek one another, and thus
effect an infinite production of every kind and on all sides. The
more imperfect a being is, the more alike or similar are these
parts to each other and the more alike are they to the whole.
The more perfect the being becomes, the more dissimilar become
the parts. In that case the whole is more or less like the parts,
in this the whole is dissimilar to the parts. The more similar
the parts are to each other, the less they are subordinated to
one another. The subordination of the parts indicates the more
perfect being." These words embody the views of the modern
naturalist, for they contain already the essential elements of the
cell-theory.

What is life ? Every time that a great discovery has been
made in natural science there is new hope that a nearer approach
has been gained to this question which is all-important to the
mind of man. Never seemed the hope of solving the riddle of
life nearer its realization than when the invention of the micro-
scope opened new paths to scientific research. So overwhelmed
were the first investigators by all the wonders of this new world
of minutest organisms revealed by this remarkable instrument

FIG. 16. — ANTHOZOA.

On the left, below, fixed on the rock, Noble or Red Coral (Corallium nobile s.
rubrum) ; above, two Tunicates (Cynthia papillosa) ; in the corner, a Sea-anemone
(Anemonia sulcata) ; on the right, above, a colony of Barnacles (Lepas anatifera) ;
below, in the back ground, Leather-coral (Alcyonium palmatum) ; in front of it,
Sea-pen (Pennatula phosphorea) ; in the centre, Sea-anemone (Cerianthus mem-
branaceus) ; in the foreground, on the right, three Sea-anemones (Adamsia ronde-
letii).

THE BUILDING-STONES OF THE ORGANIC WORLD 61

that they regarded it as sacrilege to peer deeper into the ' secrets
of Nature which, according to Divine will, were to be for ever
hidden from human eyes.' It was only very gradually that
objective investigation availed itself in full of the new achieve-
ments of technical skill, and to-day we owe almost all the vast
progress of modern science to the insight into the interior of
organic bodies granted by the microscope.

When we observe the world of animal and plant organisms
we find in the structure of their bodies a fundamental and
complete likeness : they are composed of an immense number
of minute homogeneous building-stones which we are accus-
tomed to call cells. Whichever organ we may examine with
the microscope, we shall always find that it is composed of
single cells ; and when we descend into the world of invisible
organisms, known to most men not even by name, and observe
the simplest and lowest animals and plants, we shall find that
they are nothing else but free-living single cells.

But the most important discovery was that even the
highest and most complex organism, composed of many million
cells, whether man, animal, or plant, consists at the starting-
point of its individual existence of but a single cell — the ovum,
or, more accurately, the impregnated cell, for, as a rule, it is
necessary that two cells, the female ovum and the male sperma-
tozoon, become united in order to render the production of a
new organism possible. Through many divisions of the egg-
cell, the differentiation of the individual parts, according to the
duties which they will later be called upon to perform, and the
fusion of homogeneous cells, tissues are formed ; these unite
to form the various organs which constitute the fully developed
organism, whether animal or plant.

We may conveniently point here to an error committed by
many persons. If they are asked to state the difference between
an animal and a plant they will not hesitate to mention a
large number of seeming distinctions. They do so because most
persons think in such a question only of the types of both natural
kingdoms known to them, that is, the most highly organized and
most complex. The distinctive features between a pear-tree or
a poplar, a fern or a blade of grass on one side, and a lion, horse,
worm, butterfly or crab on the other are, of course, so obvious and

62 LECTURES ON BIOLOGY

so radical that the existence of any relationship seems excluded.
But the boundary becomes less distinct when we come to simpler
forms. Let us, for instance, examine the coral-stock with its
tree-like branches, its many hundreds of individual inhabitants
and, simultaneously, builders — the polyps — which peep like tiny
blossoms from the branches of this tree ; or the closely related
sea-anemone, large numbers of which clothe the rocks at the
bottom of the Southern seas as with a brilliant flower-carpet.
Here even a trained observer might often be doubting whether
he is looking at an animal or a plant. As recently as the end
of the seventeenth century even naturalists counted both corals
and sea-anemones among plants. When the French physician
and zoologist, Peysonnel, endeavoured to prove the animal
nature of these organisms, and submitted his discovery to the
Academy of Sciences in Paris, the President of the Academy, the
famous physicist Reaumur felt it his duty to suppress the
name of the author of such an improbable assertion. And yet
the organization of the polyps shows that they are undoubtedly
animals.

Everyone knows the skeleton of the bath-sponge. Who
would think that sponges are animals ? Firmly fixed to the
bottom of the sea, the sponge is betraying with not a single
motion that it is a living creature. Only the most accurate
microscopic examination and observation of its development
enable us to demonstrate its animal nature. The young sponges
leave the body of their mother as minute ciliated larvae and swim
gaily about. This free life lasts several days, and then the little
animals attach themselves to stones or similar objects and become
motionless sponges.

These few instances, and particularly a glance at our illus-
tration, will show that external appearance alone is not sufficient
to enable us to distinguish between animal and plant. Let us
now briefly consider the various distinctive features upon which
science has based the division of the organic world into two large
kingdoms.

The greatest distinction is represented by the difference in
the process of metabolism. Whilst the animals are for their
nutrition restricted to organic matter, that is> other living things,
we have already seen that plants possess the faculty of extracting

THE BUILDING-STONES OF THE ORGANIC WORLD

63

their food directly from the surrounding atmospheric air. Thanks
to the presence of a certain substance contained in the plant-
cells, the chlorophyll or plant-green, plants are enabled to absorb
the carbon dioxide of the air and, under the chemical influence of
the sunlight, split it up into carbon and oxygen. The oxygen
is then largely exhaled whilst the carbon
enters with the water introduced into the plant
from the earth through the roots into the first
visible organic compound, starch. Starch is
the first and the only product of assimilation
from which all other organic compounds of
the plant are formed by means of chemical
metamorphosis. From starch we obtain all
other carbohydrates, fats, and, finally, the
highly complex albumen. Animals, on the
other side, inhale oxygen and exhale carbonic
acid, so that between the two kingdoms a
never-ending exchange takes place. It seems,
therefore, as if the presence or absence of the
chlorophyll, and the kind of alimentation con-
ditioned by it, is a fundamental factor in
differentiating between plant and animal.

But there is found in every ditch and pool
a minute relative of the magnificent sea-
anemone, the little fresh-water polyp Hydra
viridis. This delicate little organism is distinguished by its
beautiful green colour, which on examination proved to be due to
chlorophyll. When this discovery was first made it seemed to
destroy the most important factor in distinguishing between
animal and plant, until it was further discovered that these
chlorophyll-corpuscles are not produced by the Hydra and do
not belong to its body, but are independent unicellular plants
(Algae), which penetrate from the outside into the body of the
Hydra and grow and multiply.

Whenever we observe in Nature such intimate living-together
of two different organisms our first thought is that we have
before us a case of parasitism in which one part lives at the
expense of the other. But in this case we are observing an
intimate alliance, a so-called symbiosis, into which Hydra and

FIG. 17.

FRESH -WATER POLYP

(Hydra viridis) ON

THE ROOTS OF DUCK-
WEED (Lemna).

(From v. Hanstein,
" Naturgeschichte
des Tierreichs. ")

64

LECTUEES ON BIOLOGY

Algae have entered to their mutual advantage. Confined within
the cells of the polyp the Algae do not only find shelter for an
undisturbed growth, but share at the same time in the carbon
dioxide which the polyp exhales. In return the Algae supply the
polyp with the necessary oxygen, and also with a part of the
starch produced by them.

In some of the marine polyps which, like the Hydra, give
shelter to Algae, mutual adaptation has made such progress that,
for instance, the sea-anemones have lost the power of taking their

-~ec

c--

FIG. 18. — SYMBIOSIS OF SEA-ANEMONE AND UNICELLULAR ALGJE.

A, Section of Sea-anemone : o, Mouth aperture ; t, tentacles ; g, stomach
cavity ; ec, ectoderm ; en, endoderm ; g, supporting lamella. B, Transverse
section of the body-wall (magnified) : c, Unicellular algse within the endoderm.
C, Unicellular algse (greatly magnified). (After O. Hertwig.)

food independently, but are fed entirely by their ' lodgers/
Experiments have shown that without Algae they are invariably
doomed to die from starvation. We may even regard Algae and
the polyp as one individual, for these minute plants even invade
the growing ovum of the polyp and are thus ' inherited ' by
the descendants from the mother.

There is, however, a large class of plants the species of which
possess no chlorophyll at all. These are the fungi. As a result
of this defect fungi have lost the power of extracting their food
from the inorganic world and are, like animals, restricted to

THE BUILDING-STONES OF THE ORGANIC WORLD 65

organic food. Numerous other parasitic growths have no
chlorophyll, but are nevertheless true plants.

Since the time of Linnaeus the faculties of nutrition and
reproduction have been generally ascribed to plants, whilst
animals were said to possess in addition the faculties of sensation
and motion. These distinctions, however, can no longer be
maintained. , -,

We have already seen that numerous lower animals — sponges,
corals, and sea-anemones — possess no power of locomotion.
Sponges do not even exhibit any externally noticeable signs of
motion, and are all but insensible to every kind of stimulus.
Even among the higher animals many have given up their free
life ; various species of crabs, for instance, Acorn-shells and Bar-
nacles, as well as numerous tunicates which for various reasons
are regarded as the precursors of the vertebrates, are fixed to the
ground. On the other hand, there are plants which in response
to certain stimuli are able to execute distinct vivid movements.

In hot-houses we often meet with the famous sensitive plant
(Mimosa pudica), known in tropical countries as a most objec-
tionable weed. The leaves of this plant are 'beautifully divided,
again and again pinnate, with a great number of small leaflets
of which the pairs close upwards when touched. If we shake
the plant, the leaflets close together, the pinnae sink down, then
the leaf-stalks sink down, and the whole leaf hangs as if
withered.' For this reason popular fancy has called the mimosa
the ' chaste flower/

Striking and apparently voluntary movements are executed
by various ' carnivorous ' plants of which we know at present
about five hundred species. The ability to react to a stimulus
has in some of these plants reached a very high degree. Darwin's
investigation showed that in the well-known Sundew (Drosera
rotimdifolia) a curving of the glandular ' hairs ' took place in
response to a stimulus exerted by a particle of hair ^ mm.
in length, or of 30,o00 nig. of phosphate of ammonia. It is a
remarkable fact that it is impossible to deceive these ' tentacles '
and entice them to useless exertions, for the movements are
only executed if the leaves are touched with a nitrogenous body,
that is, one that may be used for food ; when touched with non-
nitrogenous minerals no reaction takes place.

5

66

LECTUEES ON BIOLOGY

More striking still are the movements of the Venus's Fly-
trap (Dioncea muscipula), a native of North America, where it
occurs on moorlands. ' It has a circle of more or less prostrate
leaves round the base of the stalk which rises 4 to 6 in. from
the ground. The leaves, about 4 in. in length, consist of

FIG. 19. — VENUS'S FLY-TRAP (Dioncea muscipula}.

Above, two leaves of Dion sea, one prepared for a capture, the other with a
captured insect.

a spatulate stalk which is constricted to the mid-rib at its
junction with the broad blade. The halves of the blade are
movable on one another along the mid-rib, and close together,
as this book would do if fitted with an automatic closing spring.
Bound each margin are twelve to twenty long teeth which inter-

THE BUILDING-STONES OF THE ORGANIC WORLD 67

lock in rat-trap fashion with those of the opposite side. The
centre of the leaf bears numerous digestive glands, and there are
on each half of the blade three sensitive hairs which rise obliquely,
but bend flat on a basal joint when the leaf closes. The blade
shuts up in eight to ten seconds when one of the sensitive hairs
is stimulated, and if an insect is' caught in the fly-trap a profuse
secretion is exuded from the glands ; after a week or a fortnight
the insect is digested and the leaf then reopens.' — (Thomson.)

It has long been known that the leaves can carry out move-
ments calculated to place themselves in the most favourable
position to the light. Haberlandt's investigations showed that
these movements are made by organs situated in the upper
epidermis, which not only perceive the light-stimulus but also the
direction of the light. In the simplest forms the entire upper
epidermis of the leaf has become a light-sensitive epithelium.
In other plants, for instance Fittonia verschaffelti, a few speci-
fically formed cells have assumed the role of primitive eyes,
called ocelli, like the analogous organs in the lower animals.
In Fittonia these ocelli consist of two cells, one above the other.
Protruding from the other homogeneous epidermis-cells are in
different places large cells with strongly vaulted outer walls, each
of which carries on its head a small biconvex lens-cell. The
interior of this lens-cell is filled with a clear, highly refractive
substance. Probably these ocelli were evolved from cilia. But
not only light-sensitive organs but also specific forms for the
reception of mechanical stimuli and stimuli of gravitation have
been made known as the result of laborious research, and thus
what once appeared to be a fundamental distinction between
plant and animal has become a new connecting link.

Many experiments have further demonstrated the inaccuracy
of the assertion that only plants are able to form cellulose, for
this substance is not only found in the protective covering of
many of the lower animals but forms also the chief constituent
of the mantle of the tunicates.

More vague still does the line of demarcation become when
we come to unicellular organisms ; for here in the majority of
cases it becomes really a matter of choice whether we count
certain organisms among animals or plants. With each advance
made by science it becomes clearer that a natural division of

68 LECTUEES ON BIOLOGY

the organic world into two separate kingdoms does not exist.
However much each highly organized animal may differ from
the higher plant, the history of their development proclaims
their common origin. Animals and plants are but two branches
of one tree of life. If, nevertheless, a line of demarcation is
still drawn, such a proceeding is justified only by considerations
of utility.

The discovery and establishment of the modern cell-theory
is of comparatively recent date, for it is only since the begin-
ning of the past century that the work of Mathias Schleiden,
Schwann, v. Mohl, Max Schultze and others brought about a
complete revolution in the methods of scientific research and
made possible an undreamt-of advance in the whole of our
knowledge of Nature.

As early as 1667 an English scientist, Robert Hooke,
succeeded in demonstrating by means of 'a microscope con-
structed by himself that a particle of cork is built up of an
immense number of single homogeneous parts. He it was, too,
who first used the name of cell in this connection, because of
the similarity of these forms with the cells of bees and wasps.
Not many years afterwards (1672) the English botanist Grew
published a comprehensive work (" Anatomy of Plants "), with
numerous microscopic pictures of sections of various parts
of plants, in which the cell-like structure may be distinctly
observed. Even then it was already perceived that the indi-
vidual ' bee-cells ' differed in structure and appearance, and
that homogeneous cells joined together and formed larger struc-
tures ; nor did these investigators fail to observe that in growing
parts the cells grew in size and multiplied in number. But
the true cause of all these phenomena, the seat of life in the
plant organisms, remained nevertheless hidden in darkness, for
what the early investigators saw and described was but a dead,
unimportant component, a secretion of the cell, its membrane.
Only with the discovery by v. Mohl and Schleiden of the living
contents, the cell-body, with the perception of the uniform
construction of the whole plant-world out of homogeneous ele-
mentary organs, and finally with Schwann's demonstration of
analogous conditions in animals, the cell-theory took the field
and gradually gained its present importance.

THE BUILDING-STONES OF THE ORGANIC WORLD 69

Each cell we must conceive to be a minute chemical laboratory
within the narrow space of which there goes on the sum total of
all the processes, changes, and movements which we are wont to
designate as life. Whenever and wherever on earth we may find
life, we shall always find it bound to a cell, that is, to an ele-
mentary organism, the body of which consists of two different
substances or parts, the protoplasm and the nucleus.

Until recently numerous investigators maintained the exist-
ence of more primitive organisms, so-called Monera or Cytodes,
which had not yet attained the organization of the cell. But
since Haeckel established his class of the Monera, the number of
those elementary organisms which were said to be without a
nucleus has decreased year by year in proportion as our optical
instruments and staining methods have become more perfect. It
is not difficult with modern instruments to demonstrate a typical
nucleus-cell in many cells which were formally believed to be
without a nucleus, for instance, various species of amoebae, myxo-
mycetes, &c. It is not necessary for the nucleus to be formed
homogeneously; in many cases, for instance in the protozoon
Pelomyxa pallida, the nuclear substance is distributed in
numberless minute granules throughout the entire protoplasm.
Of other cells which at the mature stage are without a nucleus
— in particular, the red blood-corpuscles of mammals — we know
that they have originated from nucleated cells.

The only remaining non-nucleated cells, therefore, are the
bacteria and the yeasts, but evidence is accumulating tending to
show that even these are typical nucleated cells. Quite recently
Wagner succeeded in demonstrating not only that yeast-cells
have a nucleus, but also that they are cells with a highly
organized nuclear apparatus. According to the investigations
made by Biitschli it seems further probable that the bacteria,
too, are differentiated into plasm and nucleus. This excellent
investigator found that in some of the larger bacteria — spirilla,
spirochaetse, &c. — it is possible with a high-power microscope
and the aid of special nuclear dyes to demonstrate the presence
in the body of the bacterium of two different substances, one of
which represents the protoplasm, the other probably a primi-
tive nucleus. All but those that are bound by prejudice must
therefore admit that science knows not a single non-nucleated

70 LECTUEES ON BIOLOGY

organism, and that there is therefore no justification for retain-
ing the belief in the existence of such ' simplest organism.'
When and whether Monera lived in times past can only be
a matter of speculation.

The size of the different species of cells varies between fairly
wide limits. By far the larger majority is minute, standing close
to the limits of visibility. Often it is just barely possible to
observe them with the naked eye or a magnifying glass as minute
points. But there are cells so incomprehensively minute that
they can be observed only with the most powerful instruments,
magnifying 2,000 to 3,000 times. To these infinitely minute
bodies belong numerous bacteria, several protozoa, and the male
germ-cells of many animals. Some of these protozoa and bacteria
attain only a length of ^^ mm. The cocci of pus attain a
volume of only 1 700>0100 000 c.mm. In one drop of water there is
ample room for thousands of millions of these bacteria, and a
glass of water would to them be more immense than the earth
is to the human race. In spite of this minuteness, attempts
have been made to calculate the weight of one of these cocci ;
the result given is e^b~6To"oo^ob m£- ^n order, therefore, to obtain
1 gramme of them it would be necessary to heap up six billions
of these minute organisms. But even with these minute
measures the lowest limit of life has apparently not yet been
reached, for the germs of pleuro-pneumonia, for instance, are
so minute that they pass even through the densest porcelain
filter; indeed, there seems little hope of our ever perceiving them
with modern microscopes ; we know the effects of these dan-
gerous organisms, but the organisms themselves remain invisible.

Many cells, on the other hand, reach considerable dimensions,
and may be easily perceived with the naked eye. Veritable
giants among them are the eggs of different animals, in particular
of reptiles and birds, for they are nothing but single enormous
cells. It must, however, be taken into consideration that by
far the greater part of these eggs consist of the store of food
which is to serve later for the support of the growing embryo.

In plants, too, we know cells of considerable magnitude, as,
for instance, the bast-cells, which reach a length of 10 to 20 cm.
Caulerpa, one of the marine Algae, though it often reaches a
length of 1 metre and has a body with stem and leaves, is
nevertheless but a single multi-nucleated cell.

THE BUILDING-STONES OF THE ORGANIC WORLD

71

In the living state the cell represents a minute drop of light-
coloured, almost opaque liquid. Like every other drop of liquid,
this cell has in the free state the desire to contract itself into the
smallest possible volume, that is, to assume globular form. The
sphere is theoretically the ideal form of the cell, but we find it
only rarely realized in Nature. Even the simplest protozoa
assume the form of a sphere only temporarily when about to
rest. In the body of the higher
organisms there is hardly a shape
which the cell is not able to
assume. Here we find cubic and
flat, cylindrical and oval, spindle-
shaped and branched cells, and
even in the unicellular organisms
the variety of forms is astonishing,
But however different the various
species may appear, however
radical the difference may be in
the form of the protozoon and
the unicellular Alga or the cell of
the metazoon, there is one point
in which their structure always
agrees : their composition of protoplasm and nucleus.

The most important and largest part of the cell, the substance
with which all functions of life appear to be connected, is the
protoplasm. But when we ask what this protoplasma really is
we are regretfully compelled to admit that at present we know
very little of its nature.

Chemically, protoplasm is no homogeneous body, but a highly
complex compound in a continual state of change, consisting
mainly of albumen, which is formed chiefly of the five elements
hydrogen, carbon, oxygen, nitrogen, and sulphur. It is always
rich in water. When we withdraw the water, protoplasm
becomes hard and tough and the functions of life cease, without,
however, necessarily becoming extinct. We have already seen
that many cells are able in such a state of rest to remain alive
for a great number of years and, on a renewed supply of water,
to awaken to renewed activity.

In slightly magnified protoplasm we can easily distinguish

FIG. 20. — STRUCTURE OF A CELL.

72 LECTUEES ON BIOLOGY

a homogeneous opaque main substance, and embedded in it
numerous strongly refractive granules ; higher magnification
shows that the refractive granules lie on the fine meshes of a
delicate network embedded in the opaque main substance.
According to modern opinion we must regard these meshes as
the optical transverse section of a cell-system, in other words,
we must assume that the cell is built after the manner of a
beehive. • The walls of the single cells are formed of a more
solid substance, whilst their interior is filled with a clear liquid,
the cell-juice. But not all cells have such complex organization.
In many cases even the most powerful microscope reveals only
the hyaline main substance with numberless granules distributed
throughout the cell.

In order to gain an insight into the life-functions of these
minute elementary organisms let us examine a simple protozoon,
an Amoeba, which we may find at any time in a ditch. At first,
frightened by the dazzling light in the microscope, it contracts
itself into a sphere, and only the trained eye is able to decide
whether the motionless little lump is a living organism or a
particle of dirt. But soon the Amoeba adapts itself to the altered
circumstances ; it becomes hungry, and with the hunger, active.
Slowly, almost cautiously, the protoplasm begins to bulge in one
spot and forms a minute pseudopodium. Kemaining undisturbed,
the Amoeba gains courage and stretches its little foot further
forward; soon other feet follow. At last the little animal is in
full activity and moves along with flowing, crawling, lazy
movements.

But in addition to these external amoeboid movements there
is the restless motion of the protoplasm itself : the granules move
backwards and forwards, here and there, as ants in an ant-heap.
This flowing of the granules is characteristic of the protoplasmic
movements. It can also be demonstrated in the cells of the
higher animals and plants, whose form usually becomes rigid
in consequence of the secretion of a firm cell- wall, which prevents
the amoeboid motion. It is unnecessary to mention that these
movements of the granules are not automatic, but are effected by
the flow of the cell-juice.

This process may be very distinctly observed in protozoa with
long pseudopodia, as, for instance, the sun-animalcules (Heliozoa).

THE BUILDING-STONES OF THE ORGANIC WORLD

73

Like persons promenading, these granules slowly wander away
from the body on one side of the little foot, and then turn round
and come back on the other side. But as all persons do not walk
at the same pace, so of these granules some travel quickly, some
slowly ; sometimes one overtakes the others, then apparently rests
itself and yields the lead to others. We are able to influence the
rapidity of motion, increase it or arrest it, through stimuli of
different kinds, chemical influences, or the effects of warmth
or cold. This is the best proof that protoplasm is irritable,
that it possesses sensation. How, indeed, could life exist without
the possibility of purposeful reaction, and how, again, could this
be possible without sensation. If the Amoeba felt no sensation of
hunger, what could suggest to her the "thought " of eating ?

These animalcules have no oral aper-
ture, but they are able to absorb nourish-
ment in an exceedingly simple manner.
When on its chase the Amoeba encoun-
ters an organism suitable for food, it seizes
it with its pseudopodia and wraps itself
around it. Then in the interior of its
protoplasm all the nutritive parts of the
prey are extracted and the indigestible re-
mainder ejected. Certain cells of the
body of higher animals — the white blood-
corpuscles or leucocytes — feed in this
primitive manner, to the great advantage
of the organism, for the leucocytes repre-
sent, as it were, the natural bodyguard
whose duty is to take care that no dan-
gerous germs settle down in the body. Differing but slightly
in outward appearance from the Amoebae, and being, like them,
motile, the leucocytes leave the blood-channel and traverse all
the tissues of the body. When on these scouting expeditions
they encounter any bacteria or any other minute parasites they
dispose of them by the simple process of eating them up. Many
illnesses are probably warded off by the watchfulness of these
phagocytes (see fig. 21). The leucocytes are also believed to
fulfil a useful part in the nutrition of the body-cells by charging
themselves with liquid food from the intestinal glands and

FIG. 21. — LEUCOCYTES OF
THE FROG DEVOURING A

(After E. Metchnikoff.)

74 LECTURES ON BIOLOGY

distributing it throughout the body. Sometimes, however, when
morbid changes of the organism facilitate an excessive increase
in the number of leucocytes they may become a serious danger.
They are then no longer content to feed on bacteria and senile
body-cells which are no longer able to do useful work, but make
raids upon the healthy tissue. In contrast to the phagocytes,
most of the other body-cells are unable to absorb solid par-
ticles but are restricted to fluid food.

If we observe an Amoeba under the microscope we might
easily be led to conclude from the fact that the changes take place
only in the protoplasm that only this substance is essential
to the course of the functions of life. This would, however, be
wrong, for though invisible it is the cell-nucleus which regulates
the various processes. Only when nucleus and protoplasm co-
operate normal life becomes possible.

I shall here only be able to give a very general description
of the nucleus. Like the cell itself, it is greatly variable in size.
As a rule, its size is proportioned to the amount of surrounding
protoplasm. Thus we find in the large ganglion cells, and in
particular in immature ova, nuclei of considerable dimensions,
which we can observe with the naked eye and take out of the
cell-body with a pin. But this rule is not without exception, for
in ripe and fertilized ova the nucleus becomes sometimes so
minute that it is difficult to demonstrate its presence. The
normal position of the nucleus is in the centre of the cell, but
sometimes it leaves its place and wanders to other positions.
But wherever it may be, it remains always surrounded by proto-
plasm. According to their behaviour towards certain stains we
distinguish in the nucleus two main substances, of which one,
chromatin, exhibits a strong inclination towards dyes such as
haematoxylin, carmine and others, and stains intensively, whilst
the other, achromatin or limn, does not as a rule stain.

In its morphology the nucleus repeats on a minute scale the
structure of the whole cell (see fig. 20). Here, too, the main
substance is a light-coloured, filamentous fluid which is known as
the nuclear juice. Its volume mainly decides the size of the
nucleus. Embedded in the nuclear fluid is a very fine mesh-
work formed of linin-threads which according to many investi-
gators must be regarded as the optical expression of the alveolar

THE BUILDING-STONES OF THE ORGANIC WORLD 75

or " cell " structure. In the resting nucleus the chromatin is
usually lying on the linin-threads in the form of granules. This
is particularly noticeable at the corners of the meshes.

Of all the parts of the nucleus the chromatin requires the
most thorough consideration, for it is said to be the agent which
transmits the qualities of the parents through the germ-cells
to the children. We shall in another place consider at greater-
length the reasons which compel us to this assumption. That
the chromatin is a most important substance is proved by the
striking part played by it in the reproduction of the cell.

Whether the nucleus is divided from the protoplasm by a
special membrane is a question which is at present undecided ;
probably different nuclei act in a different manner.

It remains to mention a minute globule of a deep black
colour frequently found in the nucleus, the 'nucleus-corpuscle.'
What influence it has upon the life of the cell is at present
unknown, but as it is absent from many cells it is doubtful
whether any great importance is to be attached to it.

Each cell contains usually only one nucleus, but we know
many vegetable and animal cells which contain two, three, and
even a hundred nuclei.

Next to the nucleus we observe in most animal cells a little
strongly refractory granule surrounded by a halo. This is the
so-called centrosome which, according to many investigators,
represents the organ of locomotion. We shall deal with it in
detail when we come to consider the division of the cell.

The organization of plant-cells differs from that of animal-
cells in that in the former we have to deal regularly with a
cell-wall or membrane secreted by the protoplasm. In addi-
tion the plant-cell contains the chlorophyll, which we have
already mentioned. It is these minute green corpuscles which
render it possible for all animal life on earth to exist, for only
they possess the faculty of changing inorganic matter into
organic compounds.

Let us now endeavour to gain an insight into the function of
the single cell-parts. Though the layman will no doubt wonder
how it is possible to obtain accurate information from examining
such minute forms, he may rest assured that the task looks more

76 S LECTUEES ON BIOLOGY

difficult than it really is. There are two methods by which we
can gain this knowledge — direct observation, and experiment.

If, for instance, we observe under the microscope a plant-cell
about to strengthen its membrane in any one point by fresh
lamellae of cellulose it will be noticed that the nucleus leaves its
normal position in the centre of the cell and wanders to the point
under discussion. Here it remains until the work is completed,
when it returns to its original place. Does not this observation
suggest that the nucleus takes an active part in the construction
of the cell-wall ? Korschelt showed, further, that in the developing
ova of many animals the nucleus sends forth during the prepara-
tion of the food-yolk long pseudopodia-like shoots, and that the
production of yolk is most intensive in the direction of these
pseudopodia. It is therefore very probable that the nucleus
exercises also a certain influence upon the formation of yolk.

Still more convincing are the replies which we obtain to
our questions from experiments ; minute though the protozoa are,
they are not so small that we cannot perform certain operations
with them. If we dissect the body of an Amoeba into a nucleated
and non-nucleated half the fate of the parts is very different.
At first the two animalcules thus created seem to have suffered
in nowise from this operation ; they round themselves off, pro-
trude their pseudopodia, crawl about, and may even be observed
to absorb food like healthy Amoebae. But whilst the nucleated
part digests and assimilates the nutriment the non-nucleated
part is unable to derive any advantage from its food which
remains undigested in the protoplasm. Soon we observe in the
non-nucleated part distinct signs of decay, and finally it disinte-
grates into a minute heap of granules. In the meantime the
other Amoeba has repaired the damage and grown once more to
normal size. Hence it is impossible to doubt that the protoplasm
without a nucleus cannot live, and that the nucleus plays the
leading part in metabolism, the nutrition of the cell. We
must, however, be careful to guard against an exaggeration of
the significance of the nucleus. As the protoplasm needs the
nucleus, so the nucleus needs the protoplasm. Verworn suc-
ceeded, by a clever operation, in taking the nucleus from a marine
protozoon, Thalassicola pelagica, a little ball of nearly 0'5 cm. in
diameter. This experiment proved that the nucleus invariably

THE BUILDING-STONES OF THE ORGANIC WORLD 77

dies after having been entirely separated from protoplasm, whilst
it was able to reconstruct another complete animal as long as
only a trace of protoplasm had been left adhering to it. We are
therefore able to say, by way of summing up, that the functions
of the cell-body are chiefly locomotion and the absorption of food,
whilst the nucleus controls the metabolism and regulates most
of the vital functions of the cell. Only as long as both parts col-
laborate will the life-machine be able to do its work.

However simple the structure of the cell may appear from
the foregoing remarks, it can nevertheless assume the most mani-
fold forms. Just as the body of a higher organism is divided into
a great number of single organs and tissues, and as each of these
parts performs a definite vital task, so we observe in the single
cell a far-reaching division of labour. This is most striking in
the unicellular protozoa in which the one cell, depending upon
itself alone, must minister to all the wants of life. We find,
therefore, that in them the protoplasm frequently proceeds to
the formation of certain minute organs of motion, organella,
as we will call the different instruments of the single cell, as
distinguished from the multicellular ' organs ' of the metazoans
and metaphytes. We are able principally to distinguish two
types : thin delicate hairs or cilia, which are mostly distributed
in immense numbers along the surface of the body, or a few
long, whip-like forms, or flagella. By means of whipping
movements these cilia or flagella enable the cell to glide easily
through the water. In numerous protozoa we find, further,
that the cell-body is divided into several sharply defined layers,
each of which performs a separate task. The external plasm
is capable of becoming rigid, thus forming a protecting cover ;
another part forms special weapons of defence, and only the
inner protoplasm containing the nucleus continues to perform
its original task of assimilation. We even know numerous
unicellular protozoa which have proceeded to the formation of
a cell-mouth and cell-anus, possess a complex system of excretion,
and even simple organella of sight and touch. Like the protozoa,
the body-cells of the higher animals may be differently con-
structed, corresponding to their special functions. An ovum
differs in appearance from a spermatozoon, a nerve-cell from an
epithelial cell. A glance at the illustration (p. 78) will show
these radical differences.

78

LECTURES ON BIOLOGY

When we describe the cells as the building-stones of the
organic world we are doing justice only to one side of their
nature, for they are not only building-stones, but at the same

FIG. 22. — CELL-FORMS.

(1) Flagellate cell of a sponge; (2) ganglion-cell from the spinal cord of man;

(3) epithelium of a flagellate cell from the stomach cavity of a sea-anemone;

(4) ciliated cell of a mussel; (5) cell from the intestinal epithelium of man;
(6) striated muscle-cells from cardiac wall of frog.

time builders. The growth of each higher multicellular organism
depends upon the power of each individual cell to enlarge and
multiply, to produce out of itself new building material. It

THE BUILDING-STONES OF THE ORGANIC WORLD 79

needed the laborious research of many decades before science
gained an accurate conception of the multiplication of the cell,
and even to-day we are still far from an accurate understanding
of all the details of this process which is of the utmost
importance for the preservation of life.

Formerly it was usual to compare cells with crystals. It is
probably due to this wrong conception that Schleiden and
Schwann, the founders of the cell-theory, formed such faulty
ideas regarding the genesis of new cells : they thought that cells
crystallized, as it were, out of a mother-liquor, the germinal
matter. By his observations Schleiden had been driven to the
conclusion that the light-coloured fluid which fills the adult
plant-cell forms this germinal matter ; according to him this
liquid first forms by condensation a minute granule — the
nucleolus, around which afterwards the nuclear substance aggre-
gates. The nucleus then becomes a formative centre for the
protoplasm. Schwann went yet a step further in his error and
taught that free cell-formation also permanently takes place
outside the cells, in the so-called intercellular substance of the
animal body. The first scientist who opposed this doctrine of
the spontaneous generation (Urzeugung) of the cell was the botanist
v. Mohl. He demonstrated that at least in the plant kingdom
the new formation of cells can only take place in correlation
with, and by the division of, already existing cells. It took,
however, many years before this erroneous conception was
finally disposed of by Kemack, Virchow, and others who
demonstrated that the law enunciated by Mohl applies to the
whole organic world, and that therefore no free cell-formation
can take place in the body of animals. Virchow coined the
famous sentence, " Omnis cellula e cellula, " a pronouncement
which has been brilliantly justified by all subsequent researches.
But not only do the cells not form themselves anew, their most
important organ, the nucleus, too, owes its origin to a pre-existing
nucleus, and we may therefore further say " Omnis nucleus e nucleo."

We have already had an opportunity of dealing with the white
blood-corpuscles, the leucocytes. In addition to this ' sanitary
police ' which represents only a very small part of the blood we
find in human blood enormous numbers of small round discs, the
red blood-corpuscles. Though they are non-nucleated in the

80 LECTUKES ON BIOLOGY

adult stage, they have nevertheless descended from typical
nucleated cells, having lost the nucleus as they grew up : as
their task has been fulfilled they are no longer in need of it.
After having circulated for a short time through the arteries and
supplied the tissues of the body with oxygen from the lungs these
red blood-corpuscles decay and are disintegrated in the liver,
other new corpuscles taking their places. When they are young
we are able to observe quite distinctly the process of reproduction.
First the body of the cell extends longitudinally and the nucleus
assumes the form of a dumb-bell. The groove in the centre
grows deeper and deeper, and soon we observe in the proto-
plasm, in the place of one large nucleus, two smaller nuclei.
Now the two nuclei commence to move, each to an opposite
pole. Here they remain, for their part in the process of
division is over. While these alterations went on in the nucleus
the protoplasm did not remain idle. We saw that the cell-
body commenced to extend longitudinally. Soon it begins to
constrict in the centre of its axis, and the originally round
blood-corpuscle gradually assumes dumb-bell form. We have now
before us two small nucleated cells held together by a thin cord of
protoplasm. Finally, this last connecting link is severed and the
act of reproduction is over : one blood-corpuscle has become two,
which soon replace by growth the loss of substance and then
proceed in their turn to division, or develop into red blood-
corpuscles.

But division proceeds only in rare cases in this simple manner,
for only cells whose descendants will shortly cease to live are able
to transmit to them parts of the nuclear substance in this rough-
and-ready manner. That with this primitive method of multipli-
cation the distribution of the nuclear substance is very unequal
must be clear to everyone. We see, however, that as a general
rule the process of division is much more difficult. In place of
the crude constriction there is a mechanism which carries out the
division with wonderful accuracy, effecting an absolutely just
and equal distribution of the chromatin. This process is called
indirect nuclear division, or mitosis. Generally mitosis takes
place in four different phases which have been described as
prophasis, metaphasis, anaphasis, and telephasis.

In a resting cell (compare fig. 20) all the chromatin is

FIG. 23. — INDIRECT CELL AND NUCLEUS DIVISION.

(For explanation of the figures see text.)

THE BUILDING-STONES OF THE ORGANIC WORLD 81

distributed in the nucleus along the nuclear framework. The
first step to the indirect nucleus-division is that the fine chromatin
granules arrange themselves into one long much-intertwined coil
of thread. Simultaneously characteristic changes appear in the
centrosome. It divides in two minute grains which become
the centre of a system of most delicate fibres ranged like rays ;
these two central bodies lie in the protoplasm like two little suns
(fig. 23,0.

Gradually the distance between them increases, and slowly they
wander to opposite poles of the nucleus. During this process
the ray-system becomes more and more distinct. The origin of
these rays or, as they are usually called, 'spindle-figures' is a
much-disputed subject. While some say that they come from
the protoplasm, others believe that they arise from the ' linin-
frame ' of the nucleus. Probably their origin is different in
different cell-species ; they may be formed either by the protoplasm
or the nuclear substance, or it may be that both substances
participate in their production.

The next important change affects the nucleus. The long
uniform chromatin-thread suddenly breaks up into a definite
number of separate sections, the nuclear-loops, or chromosomes.
The number of the chromosomes is always the same in all body-
cells of the same animal or plant species, and all organisms with
bisexual reproduction possess always an even number of
chromosomes. I would mention in anticipation that this
phenomenon finds its explanation in the fact that one half of the
chromosomes is of paternal, the other of maternal origin. For
instance, anematode common in the horse, Ascaris megalocephala
univalens, possesses in all body-cells only two chromosomes,
whilst another of the nematodes, Acaris megalocephala bivalens,
has four. The human body-cells have sixteen, Helix pomatiat
some Amphibians, the mouse, the lily, and numerous other
organisms have twenty-four. In Artemia, a small crustacean, the
number of chromosomes reaches one hundred and sixty-eight.
The statement that the number of chromosomes is in all body-
cells of the same animal species always the same expresses a
rule to which one species of cells, the mature male and female
sex-cells, forms a remarkable exception. In these the number of

82 LECTUEES ON BIOLOGY

chromosomes is reduced by one-half. We shall deal with this
fact later on.

If we observe highly magnified chromosomes we may some-
times see a fine crevice extending over their entire length,
but in most cases they appear as yet uniform. Gradually
the nuclear membranes become indistinct, and finally disappear
altogether. It now seems as if the chromosomes lay perfectly
free in the cell-protoplasm. Further observation will, however,
show that to each chromosome there have become attached
some of the ' spindle-filaments ' radiating from the central
bodies. Along these rays the chromosomes wander towards
the centre, and finally range themselves in a plane perpen-
dicular to the longitudinal axis of the spindle, the so-called ' equa-
torial plane. ' This process finishes the first stage of mitosis, the
prophase, and now commences the metaphase, the climax of
indirect nuclear division (fig. 23, 2 and 3).

It is a strange and surprising spectacle which unrolls itself
before our eyes. Suddenly, and with absolute — one might almost
say mathematical exactitude — all the chromosomes split along
their length, like logs of wood. This division proceeds with
such precision that it seems unquestionably a process of vital
importance. The result of it is that the chromatin of the mother-
nucleus is distributed among the two newly formed daughter-
nuclei on the principles of strictest justice. There is no other
substance in the whole cell the distribution of which is watched
over with such care, and it is therefore not surprising that a
special function is ascribed to it. Quite early it was dis-
covered that chromatin was the ' bearer of the hereditary
qualities,' an hypothesis which, as we shall see later on, was
afterwards confirmed by many investigators.

Events now rapidly tend to the end. Just as in a dolls'
theatre the operator regulates at will the movements of his
marionettes by means of numerous strings held in his hands, so the
central bodies appear here to control each step of the chromo-
somes. One gains the impression that the spindle-fibres adher-
ing to the chromosomes suddenly shrink, as if impelled by an
invisible force, thus forcing the longitudinal sections of the
chromosomes nearer to the corresponding central bodies. In this
way we obtain the stage of the two daughter-plates (fig. 23, 4).

THE BUILDING-STONES OF THE ORGANIC WORLD 83

At first the two chromosome-halves are still connected with
each other by fine colourless filaments or fibres, but soon a
dividing wall arises from the margin of the cell towards the
centre, and with it the last connection between them disappears
(fig. 23,5). The division is now essentially complete; it only
remains for the chromosomes in the young daughter-cells to
change into new resting nuclei. This is done in the last act of
indirect nucleus-division, the telephase. As soon as the chromo-
somes have arrived in the neighbourhood of the central bodies
they all range themselves in close formation. They form pro-
jections and in the interior we observe the appearance of vesicles
which become greatly enlarged by the absorption of fluid from
the cell and drive apart the chromatin particles. Thus the
original chromatin frame is once more restored. Finally a
new nucleus membrane is formed, and with that act the nucleus
reaches once more a stage of rest (fig. 23, e) .

There is now no trace of individual chromosomes to be
seen : they have been completely dissolved into their chromatic
constituents. But this is only apparently so, for as in each new
division the chromosomes appear once more in the same form
and number, we may assume that, though invisible, they retain
their individuality in the nucleus at rest. Several facts confirm
this hypothesis. If it accidentally happens during a division
that a supernumerary chromosome enters iuto one nuclear half,
it will reappear afterwards in all descendants of this particular
nucleus.

On reviewing this wonderful process of mitosis one fact seems
to be incontrovertible, namely, that all these complex processes
have for their final object the most accurate distribution of the
chromatin, the hereditary substance, among the nuclei of the
daughter-cells. But the nature of the forces here at work,
tending with such wonderful precision towards one definite aim, is
a deep mystery. It is true that we are able to observe accurately
with a microscope every morphological change that takes place
during the nucleus-division ; we gain the immediate impression
that the central bodies exercise a controlling influence upon all
processes and are the real ' kinetic centre ' from which the
various motions result, but from a real understanding of this
strange play of the forces we are as yet far distant. We are
here confronted with one of the deepest riddles of the Universe,

84 LECTURES ON BIOLOGY

The attempt has often been made to explain the nucleus-
division mechanically. Comparisons have been instituted be-
tween the nuclear ' spindle-figures ' and magnetic force-lines ; it
has been possible to produce similar figures artificially in gela-
tine, &c., but no success has ever been recorded in passing in
such attempts beyond superficial analogies. We must agree with
Yves Delage who says that " it would be as futile to regard the
Lion, Balance and Fishes in the zodiac as a real lion, a real balance
and real fishes, as to regard mechanically formed cell-structures
and nucleus-division forms as real cell-structures and real nuclei-
division forms." We must, therefore, for the present be content
with a simple description of the processes, in the hope that
the restless research of later times may lead us to a deeper
understanding.

85

I

CHAPTER Y.
THE ORIGIN OF LIFE.

ALL our speculations about the first appearance of life on
our planet must be based upon the assumption that in times
immemorial the earth revolved in the universe as an incandes-
cent sphere like those nebulous spots which we still see on the
firmament. The process of cooling and condensation brought
about the present form of sun, earth, and the other stars. The
Law of Evolution, to which every living thing is subject, still
rules the world-systems, and if there is one statement in science
which we may make with every appearance of truth it is that
our earth, which is to the simple mind the prototype of the
permanent, was not created spontaneously but has evolved in
accordance with this law.

It is true that experience deserts us here ; it tells us nothing,
or only very little, about the history of the world-bodies, but we
may, nevertheless, regard the Nebular Theory of Kant-Laplace, at
least in its fundamental principles, as one of the surest founda-
tions of our conception of Nature.

Science can never dispense with hypothesis. The most
important foundation of our natural philosophy, the Law of
Causality, which is so intimately connected with our conscious-
ness that without it we are unable to conceive any change, is
but a hypothesis which we formulate and force upon Nature so
that we may bring order into the chaos of phenomena. To direct
observation the causal nexus between two changes is as deeply
hidden as, for instance, the origin of the solar system ; what we
see and observe is only a variety, a multitude of concurrent or
successive phenomena. When we observe that a certain change
is regularly followed by another certain change we formulate the
hypothesis that the second change is a necessary consequence

86 LECTUEES ON BIOLOGY

of the first, and we now describe the first change as cause and
the second as effect. By conceiving the Law of Causality to
be necessary and universal we admit that it does not proceed
from experience, for experience can only give us probability and
comparative universality, but never necessity.

Let us consider the atomistic theory. An atom of the above
has never been observed by any human eye, yet on the hypothesis
of the atom has been reared the proud edifice of modern chemistry
which owes to the atomistic theory its greatest successes. But
what precisely are atoms ? They are the ultimate particles, divis-
ible neither chemically nor physically, from which all elements are-
said to to be constructed. But indivisibility is inconceivable.
However minute we may conceive atoms to be, they always remain
bodies possessing all the properties of bodies — volume, form,
weight, solidity, &c. But a body possessing volume and being
indivisible is in itself a contradiction. For if we persist in dividing
a body into more and more minute particles we shall finally arrive
at a point where mechanical divisibility seems no longer possible,
though the mind does not recognize a limit to the idea of divisi-
bility. We see, therefore, that even the doctrine of the atom is a
hypothesis which to a certain extent is in conflict with our con-
ceptions, yet it has achieved enormous advances in our knowledge.
But just as once the Ptolemaic system, though proceeding
from false premises, provided a satisfactory explanation for most
astral phenomena, and ruled science for thousands of years until
it was replaced by the more comprehensive system of Copernicus,
so it is conceivable that in future times scientific progress will
remove the bounds of the atomistic theory and substitute for it
a more comprehensive law.

Without hypothesis there is no science, nor is there any justi-
fication for fighting shy of hypotheses. One might even be
tempted to say to the supporters of pure empirical research, who
seek in experience alone the ' last word of wisdom,' that it is
theory, not experience, which grants absolute certainty. How
often are we not deceived by our senses ? How often does not
experience desert us ? And even if according to Kant all our
perception begins with experience, it is not by any means all
knowledge that springs from experience. Hypothesis, however,
must not be carried to the limitless, but at every stage prove
its value by the results of experience and observation.

THE ORIGIN OF LIFE 87

So far as direct observation goes, it teaches us that every
living organism which originates to-day presupposes another
homogeneous organism from which it originates. The old doc-
trine of the great English physiologist Harvey, Omne vivum
ex ovo, or more correctly, Omne vivum e vivo, is true to-day if
we are content to consult our own experience gained from obser-
vation. It would, of course, be most convenient to assume that
life on earth has existed in its present variety since eternity —
in other words, that this wealth of animal and plant life was
called forth by a spontaneous divine act of creation, and had main-
tained itself by reproduction. Indeed, as long as man clung to
the traditions of the Church this theory remained victorious.
Even to-day we find in certain orthodox circles the attempt to
defend the biblical creation myths as scientific facts. Science,
however, has long since passed over these endeavours ' and pro-
ceeded to the first item of the business of the day.'

To-day we can admit no justification for tendencies of this
kind without doing violence to our reason. The results of
researches made in geology, palaeontology, comparative anatomy
and biology impel us in an equally convincing manner to assume
for the world of organisms a gradual law-governed development
from the simplest forms to more and more complex and higher
organisms.

The number of investigators who have tried the edge of their
speculative powers since ancient times on this most difficult and
most interesting problem of all is legion. We have already
seen that Anaximander and Empedocles attempted to give a
natural explanation of the origin of organisms, but the state
of contemporary knowledge being what it was, these theories
did not exceed the value of an ingenious fancy sketch. We are
even inclined to regard with derision the conception of Aristotle,
according to which even the higher organized animals, such as
eels and frogs, were generated spontaneously out of mud, or
insects out of putrifying wood. But we have very little cause
for feeling superior to him, for it is not so long since similar
views were discussed seriously even in scientific circles. Every-
one knows that in mediaeval times, when alchemy was flourish-
ing, serious men were endeavouring to generate in the retort of
the chemical laboratory a little mannikin, the Homunculus.

88 LECTURES ON BIOLOGY

' ' A man is being made ! ' . . .

' Heaven forbid ! ' As nonsense we declare
The ancient precreative mode ;
The tender point, life's spring, the gentle strength
That took and gave, that from within has pressed
" And seized, intent itself to manifest.

The nearest first, the more remote at length, —

This from its dignity is not dethroned !

The brute indeed may take delight therein,

But man, by whom such mighty gifts are owned

Must have a purer, higher origin.

It flashes, see 1 — Now may we trustful hold

That if of substances a hundredfold

Through mixture — for on mixture it depends- -

The human substance duly we compose

And then in a retort enclose

And cohobate, in still repose,

The work is perfected, our labour ends.

What Nature's mystery we once did style

That now to test our reason tries ;

And what she organized erstwhile,

We now are fain to crystallize."

— GOETHE, Faust II., Act ii.

He who had been created in this miraculous manner was to
repay his human creator for this labour of love by rendering
unnecessary all further speculating and giving him an answer
to every question. The days of the Homunculus are gone,
but even to-day the firm belief remains, especially among the
agricultural population, that fleas and other vermin are generated
by pouring urine on sawdust.

But let us relinquish these ' grey theories ' and turn to the
question : How must science conceive the first appearance on
earth of organisms ? That there was a time once when no life
existed on earth can no longer remain doubtful, for we are forced
on all sides to the conclusion that the earth itself has passed
through radical changes during a development which is still
going on. If we make these considerations our starting-point,
how are we to conceive the origin of life ? When we have
definitely declined a divine creative act there are obviously two
possibilities : either life has been transmitted to our planet from
outside, from another cosmic body, and has gradually developed

THE ORIGIN OF LIFE 89

to its present variety, or it originated on the earth itself from
inorganic matter by spontaneous generation (Urzengung).

But even here we proceed from the assumption that life
originated. Is there no other possibility? Even if we assume
that the earth was once a mass of molten matter, are we in that
case compelled to assume an origin of organisms ? Is it not
possible that even in the red heat of the revolving nebular masses
there was already life, and that therefore our questions are not
justified ?

However improbable this theory may sound, it has neverthe-
less found powerful advocates. No less a man than the eminent
physiologist Preyer, an original and acute thinker, strongly
advocated this idea in his book, " Die Hypothesen iiber den
Ursprung des Lebens," published in 1880, nor did he hesitate
to accept the last consequences of his theory. Basing his argu-
ments on Harvey's law that every living organism descends
from another living organism, Preyer thinks it arbitrary and
inconsistent to assume that this wTas ever otherwise. The human
eye has never seen an organism which was not descended from
another organism, but observation and experiment show how
dead inorganic matter is daily and hourly formed of living matter.
Why should we not assume that it was always so ? Why
should we not be more justified in concluding that living sub-
stance was first, and that inorganic elements were later excreted
from it ? " It is only because of our narrow conception of life,"
says Preyer, " that this idea appears to us strange and fantastic.
But if we free ourselves from the entirely arbitrary thought,
rendered probable by not a single fact, that protoplasm always
possessed the nature which it does now, and give up the ancient
prejudice, due only to laziness in thinking, that in the beginning
there was only the inorganic, we shall not hesitate to take
another step forward, drop the theory of abiogenesis, and acknow-
ledge that there is no beginning of life. Omne vivum e vivo."

To Preyer's imagination the entire incandescent mass of the
solar system appears as one gigantic organism. Its life is the
motion of its matter. Out of it came our earth. This, too,
consisted at first only of animated matter, and only later, as the
cooling process proceeds, the substances, which can no longer
remain at the molten stage, are deposited as a solid nucleus.

90 LEOTUEES ON BIOLOGY

They do no longer participate in the life-motion of the whole,
but have become the first inorganic matter.

" Only when these combinations in the course of time became
rigid on the earth-surface — i.e., died and became extinct — com-
pounds were formed of the elements that so far had remained
gaseous and liquid, which become more and more similar to the
protoplasm which is to-day the basis of everything that lives.
With the fall in the temperature and the diminution of dissocia-
tions it was necessary that there were formed still more complex
compounds, chemical substitutions, still more compact bodies,
still more complicated interacting motions of parts becoming
more intimately correlated, and only these primary forms of
the plant and animal kingdom, made possible by progressing
differentiation, were able to persist. We do not say (continues
Preyer) that protoplasm existed as such since the beginning
of the formation of the earth, nor that without a beginning it
immigrated as such from a point outside in the cosmic space
to the cooled earth, much less that it originated from inorganic
matter on the planet, as the theory of spontaneous generation
would have us believe ; but we contend that the beginningless
motion in the universe is life, that protoplasm necessarily
remained after the bodies which we now call inorganic had been
excreted on its cooling surface by the more intensive life-activity
of the red-hot planet. The heavy metals, once organic elements,
were unable to melt again or to re-enter into the circulation
from which they had been excreted. They are the symptoms
of rigor mortis of prehistoric, gigantic, red-hot organisms whose
breath was glowing iron-vapour, whose blood molten metal, and
whose food probably meteorites."

Do we not seem to see the over-heated dreams of an exorcist
rather than the calm arguments of a serious scientific mind?
The conception of the ancients that life proceeded from the fire
is here once more revived. All that moves, lives. Life is said
to be without beginning, but finds its end in death. For that the
state of life in which our earth, the sun, and the planets are at
the present time is only temporary, and that the bodies of the
universe hasten with inexorable necessity towards a state of
complete torpidity such as the moon has shown for thousands of
years, even Preyer could not doubt. Life is eternal, it is true,

THE OKIGIN OF LIFE 91

but it is a limited eternity, an eternity which finds its end in
time. To what ends will fancy and speculation lead an otherwise
acute mind when it leaves the safe grounds of reality !

Apart from these considerations, natural science has very few
objections to Preyer's theory. He has a perfect right to define
life as he does, and no one can gainsay him if he regards molten
metal as animate ; though in that case he understands by life
something very different from what all other men mean by it.
But when all has been said, the whole fight is one about words.
In any case, Preyer's theory rests on the accurate perception
that, as Fechner had already demonstrated, there is no hard-and-
fast dividing line between organic and inorganic nature, but that
both are different phenomena of the same law.

According to modern knowledge the conception of life is
identical with that of metabolism, and so far as our experience
goes this is always bound up with certain albuminoids. In that
case, however, neither sun nor any other astral body can be
called a living organism. Matter is eternal and indestructible.
But as it is certain that the earth will some day sink into the
icy night of death, so there was certainly once a time when no
life existed on earth nor could exist. The soil must first be
prepared before the seed can germinate.

We are, therefore, again confronted with the alternative that
either life on earth is the result of spontaneous generation
(Urzengung], or that it originated on other bodies in the universe
from which it reached the earth. If this latter assumption, the
so-called cosmozoic theory, has few prominent supporters to-day,
we must nevertheless mention it, if only for the sake of the
famous men who have broken a lance on its behalf — Sir William
Thomson (now Lord Kelvin), Liebig, and Helmholtz. According
to this hypothesis the first germs of life reached the earth hidden
in the crevices of meteorites, and then developed on earth.

It was in the year 1871, at the Congress of the British Asso-
ciation, that Thomson, in his presidential address, endeavoured
to prove the scientific admissibility of his hypothesis. " When
we retrace," he said, " the physical history of the earth according
to strict dynamic laws we arrive finally at a red-hot molten
sphere upon which there was as yet no possibility for life to
exist. When afterwards conditions on earth became suitable

92 LECTUKES ON BIOLOGY

for the existence of life no living beings existed. There were
solid and disintegrated rocks, water, and air all around, a warm-
ing and illuminating sun, and everything ready to become a
garden. Did grass, trees, and flowers in all their wealth of
variety and beauty spring into existence by the fiat of a creative
power, or did vegetation originate from seeds which had been
sown upon the earth ?

" If a volcanic island emerges from the sea and after a few
years is clothed with vegetation we do not hesitate to believe
that the seed was carried to it through the air or by the sea. Is
it not possible, and, if possible, is it not probable, that the begin-
ning of plant-life on earth can be explained in a similar manner ?
Each year there fall upon the earth thousands, probably millions,
of fragments of solid matter. Where do they come from ?

" If two large bodies collide in space it is certain that a large
part thereof is fused, but it appears equally certain that in many
cases a large mass of debris must be hurled in all directions ; and
that among them many suffered no greater injury than parts of
a rock in a landslip, or during blasting operations. If the
moment when our earth collides with an equally large body
should occur whilst it is still clothed with vegetation, many large
and small fragments bearing seed, living plants and animals
would doubtless be distributed throughout space. Therefore, and
because we all firmly believe that there exist at present, besides
our own, many other worlds with life, and have existed since
time immemorial, we must regard it as highly probable that
innumerable seed-bearing meteorites are moving through space.
If, therefore, no life existed upon earth at this moment a
meteorite falling upon this planet would lead to the production
of vegetation by what we call a natural cause.

" The hypothesis that life originated on our earth through
seed-bearing fragments from the ruins of another world may
appear strange and fanciful, but I contend that it is not
unscientific."

The theory of a cosmic origin of life on earth arose probably
from the fact that it is sometimes possible to demonstrate in
meteorites traces of carbon compounds equal to those which
come from organisms. Even humus-like matter has been found
preserved in the interior of meteorites. If these formations were

THE OKIGIN OF LIFE 93

able to survive the red heat to which meteorites are exposed
when entering the atmosphere of the earth there is no funda-
mental objection to the assumption that primitive germs of life
may reach us in this manner uninjured by the heat. Some
investigators, endowed with a particularly rich imagination,
were even able to recognize in thin slabs of meteorites the
most diverse organisms, such as corals, star-fishes, &c., which
looked as much like their terrestrial cousins as one egg looks
like another, except that their dimensions were considerably
smaller; but this pretty support soon crumbled away when
more accurate investigation showed that these alleged animals
and plants were nothing more nor less than crystalline forma-
tions which anyone may at any time produce by artificial means.
Other objections of a fundamental nature were soon raised
against the cosmozoic theory from the most different directions.
Thus it was said that it provided no solution at all of the problem
but merely removed the question of the origin of life from our
earth to some other star, thus leaving the riddle itself unaffected.
This objection, however, cannot be regarded as sound, for there
is no cogent reason, assuming the possibility of transmission of
living germs from one star to another, why life should not be
eternal, like inorganic matter. But as space is conceived as
infinite and as the number of astral bodies is infinite, the assump-
tion that life has come to this earth from another star which in its
turn received it from yet another star, and so on, does not remove
the question to another fixed period of time, but to eternity. And
with that the question of the origin of life becomes a question of
transcendental philosophy ; it is now as incomprehensible as the
origin of the world or the conception of matter, for it is placed
beyond the boundaries of the possibilities of our perception.

There is another reason which compels us to decline this
hypothesis. Though it might be possible, according to Helm-
holtz, for the germs hidden in the interior of the meteorites to
pass through the atmosphere of the earth without losing their
vitality or, if they adhered to the surface of the meteorites, to be
wafted down by the air when the fragments entered the earth-
atmosphere and thus descend slowly and safely to the earth-
surface, it is yet almost inconceivable that any organism should
be able to endure the intense cold and absolute desiccation to

94 LECTURES ON BIOLOGY

which it would be exposed during a journey lasting untold years
through the icy space entirely devoid of water. It is true that
in a former chapter we have become acquainted with living
organisms which exhibit an astonishing power of resistance to
all injurious influences of the external world, heat as well as
cold, and preserve latent life for a considerable time, but in all
these cases the span of life is not without limit, and even in the
most favourable cases restricted to a few years.

But even though we cannot refute the cosmozoic hypothesis
on strictly scientific grounds — just as science does not enable us
to prove that a divine creative act is an impossibility — it appears
nevertheless highly improbable. To fall back upon it would
mean doing violence to reason, because we possess in the doctrine
of spontaneous generation (Urzengung} of the organisms a theory
which stands in better accord with the facts derived from
observation.

We have thus come back once more to earth itself as the
place of the origin of life. As we have already seen, the
investigators of the earliest times had no difficulty in explain-
ing the spontaneous generation of even highly organized animals
— insects, fishes, frogs and snakes — and even in mediaeval times
a line of demarcation between inorganic matter and animated
nature was unknown. But the more science investigated the
nature of the living organisms, the clearer became the view
that, if any, only the most primitive unicellular organisms could
be considered from the point of view of abiogenesis.

It is but a few decades ago that it was a universally accepted
dogma that infusoria were generated by infusions of water upon
hay, and that bacteria originated in putrifying flesh or other
fermenting matter by spontaneous generation. Even at the
end of the nineteenth century a French investigator, Pouchet,
endeavoured to produce such organisms artificially in the retort.
According to him it was only necessary to mix together the
various parts which composed the body of a living being—' For
on the mixture it depends ' — and to procure the most favourable
external conditions, in order to obtain unicellular organisms.
Truly an attempt worthy to be compared with those of the
alchemists. No one has so far succeeded in producing by syn-

THE ORIGIN OF LIFE 95

thesis the simplest albuminoid ; even the chemical structure of
the organic substance still remains an impenetrable mystery, and
yet some one will attempt to make a living plant or animal.

These experiments by Pouchet, which sometimes really
appeared to be crowned with success, were soon proved to have
been abortive. Subsequent experiments showed beyond doubt
that the fungi, bacteria, &c., which sometimes did appear in the
cultures had developed from germs which had entered the test
tubes from without. As long ago as 1838 Ehrenberg demon-
strated that the entire atmosphere is filled with desiccated animal
and plant germs, that these are carried by wind and weather over
vast distances until by chance they fall upon favourable soil and
there re-awaken to new life. Moreover, Ehrenberg had rendered
it highly probable that in all those cases in which infusoria
or fungi are observed it is not a generation out of nothing or
out of inorganic matter which we are observing, but merely a
development out of already existing germs.

But an opinion once formed is not easily eradicated. As
Schopenhauer aptly observed, " A hypothesis leads in the mind
in which it has settled, or was perhaps created, a life which
resembles that of an organism, in so far as it absorbs from the
outer world only that which is homogeneous and digestible and
rejects the heterogeneous and injurious, or excretes it in toto if
it was unavoidably introduced." In the same manner the idea
of generation without ancestors, and even of an artificial genera-
tion of unicellular organisms, reappears from time to time in a
few fantastic minds.

Later investigators — at first Theodor Schwann, afterwards
Ferdinand Schultze, Pasteur and others — proved definitely that
neither bacteria, nor fungi, nor any protozoa could be generated
in vessels with infusions of meat or hay in which at first all
germs of life had been carefully killed by exposing them for
some time to a boiling temperature and then enclosing them in
air-tight tubes. (This is the well-known process adopted in the
preservation of various foodstuffs which retain their wholesome-
ness for a very considerable time if sterilization has been
sufficiently thorough.) But if afterwards the tubes which had
thus been rendered free from germs were opened and thus
enabled freely to admit ordinary air it did not, as a rule, take

96 LECTUEES ON BIOLOGY

long before a rich variety of life began to spring up in the
tubes in spite of the preceding boiling. When, however, air
was admitted which had previously been passed through a red-
hot tube or sulphuric acid or caustic potash and thus rendered
germ-free, no organism was ever observed to originate.

These experiments were almost universally regarded in
scientific circles as an absolute proof against the spontaneous
generation of these organisms. But this is in my opinion a
hasty conclusion, for it is not only possible but even certain
that the air undergoes in its composition, as a result of the
various processes, more or less radical changes. The carbon
dioxide of the air is, for instance, more or less decomposed by
caustic potash, and the ammonium by the sulphuric acid, and
there is, therefore, at least the possibility that these changes,
no matter how slight, are the cause of rendering an abiogenesis
of organisms impossible. At any rate, the possibility is not
excluded, even if the probability has become extremely slight.

However, as shown later by Schroder and by Dusch, it is
not even necessary to adopt such circumstantial precautionary
measures in order to prevent an appearance of living organisms
in sterilized test-tubes, for it is sufficient merely to close the
tubes with pads of cotton-wool. Like extremely fine filters these
plugs of wadding keep back all germs and permit only the
purified air to enter. Further, according to Pasteur's exhaustive
experiments, it is only necessary to conduct the air to the germ-
free nutrient medium through long sinuous tubes in order to
prevent fermentation and putrefaction. The air itself passes
freely through the windings of the tubes, but all particles of
dust or germs of organisms are deposited at the lowest points of
the bends.

Confronted with these results, even a sceptic must now
admit that abiogenesis of bacteria and protozoa does apparently
not take place. Indeed, if at the time when these investiga-
tions excited scientific circles scientists had possessed that
knowledge of the structure of the protozoa and of the still
more delicate structure of bacteria which we possess to-day
no one would probably ever have conceived the idea that these
differentiated forms could owe their existence to a transforma-
tion from inorganic matter. We shall have to deal with these

THE ORIGIN OF LIFE 97

lower organisms later on in a more detailed manner. We shall
then see that unicellular animals and plants are not by any
means primitive but highly complex organisms, and that even
the lowest bacteria and protozoa whose structure the micro-
scope has revealed to us bear such distinct vestiges of a long
historical development that it is impossible to regard them as
owing their existence to spontaneous generation.

When we ask ourselves how those organisms would be con-
stituted that might have been ' crystallized ' from inorganic
matter, the only answer which sound reason can find is that they
must stand on the lowest conceivable step of organic life. We
shall agree with Ernest Haeckel, the great but dogmatic pro-
tagonist of the theory of descent, " that the organisms that came
into existence by spontaneous generation were not cells at all,
but perfectly homogeneous, structureless, amorphous lumps of
albumen." But all organisms, animals as well as plants, w^hich
are known to-day show structures, and differentiations in their
structures, and have, therefore, apparently reached the stage of
cells. On the ground of Pasteur's experiments and these con-
siderations we seem, therefore, justified to infer that for all these
organisms the old saying applies : Omni vivum e vivo.

But are we now entitled to deny the possibility of a genera-
tion without ancestors? I am inclined to answer, No. It is
true that the perfection of optical instruments has unlocked
a whole world of life of which former generations had no con-
ception. Our microscopes, with a power of magnifying 3,000
times, enable us to perceive organisms so minute that they far
transcend our power of imagination, and yet we know that the
last boundary of the infinitely small is as yet far off, and that
there exists life which lies beyond the present limit of our
senses. Indeed, in order to prove the existence of many
organisms which singly are too minute to be made visible we
are compelled to offer them in suitable artificial nutrient media
an opportunity for enormous multiplication, so that they may
betray their presence by the clouding of the medium, or by
other signs. Of their structures, however, whether they are
cells or structureless lumps of albumen, we know nothing.
Whether such organisms originate to-day is a question which lies
where experience ceases and faith begins. A number of years
7

98 LECTUEES ON BIOLOGY

ago it was thought that an undoubted case of spontaneous
generation had been found in a remarkable structure described
by the English zoologist Huxley, the celebrated Bathybius
haeckeli. During a survey of the ocean-bed sailors found in
the greatest depths of the sea, in abysses between 4,000 and
8,000 metres, that the whole bottom of the sea was covered for
long distances by a formless mass of non-differentiated,
apparently organic slime which executed slow, crawling move-
ments. According to Haeckel and other investigators this
Bathybius represented a primitive organism,
originating permanently at the bottom of the
sea by abiogensis.

Unfortunately this apparently conclusive
proof rested on an error. For the celebrated
Bathybius is nothing but gelatinous gyp-
FIG. 24.— Bathybius sum, a sediment of the sea-water, which
may be artificially produced by mixing sea-
water with alcohol. With what joy this
discovery was hailed in certain circles, and how capital was made
out of it against the Theory of Descent is only too well known.
Where facts are absent, every little error made by an opponent
is used by some people to serve as a weapon with which to
beat him.

It is more for the sake of completeness than because of any
particular importance which this observation possesses that
I mention the latest attempt to produce life artificially.
Eadium, which since its discovery has astonished us with so
many remarkable phenomena, is this time the magic wand
which is to call forth life. Burke, of the Cavendish Laboratory
in Cambridge, who made this discovery, describes it thus : A
test-tube with the culture-medium, a nutrient gelatine such as
is used for the culture of bacteria, was sterilized in the most
careful manner possible. A small quantity of radium salt was
then added. After a lapse of twenty-four hours there appeared
on the surface a peculiar culture-like structure which during the
next fourteen days continued to grow downwards. A casual
observer would have taken it for a bacterial growth, but (according
to Burke) this assumption was wrong, because the mysterious
growth was soluble in water.

THE ORIGIN OF LIFE 99

What could it mean ? The fact that there was an unin-
terrupted growth and continuous development by division proved
that the structures were not crystals, as had been suggested by
various observers. They were apparently more than simple
aggregates because they appeared capable not only of growth
but also of subdivision, and certainly of disintegration. " But
if these products are not crystals," said Burke, " we are not
able to refuse the inference that we have before us living,
highly organized bodies which owe their existence to the
independent, self-acting influence of the radium salt upon the
culture-medium." In order to indicate the history of the
genesis of these structures and their similarity to microbes, the
discoverer bestowed upon them the name of 'radiobes.'

But however well sounding this name may be, however
acceptable this ' fact ' communicated by Burke might have
been as a support of the theory had it been proved to be true,
it cannot stand before criticism. No less a man than the famous
physicist Ramsay was chosen to commit the ' radiobes ' to merited
oblivion. According to his experiments the radium salt added
by Burke to his nutrient gelatine did in fact' cause peculiar
alterations, but these had nothing whatever to do with life and
growth. Through the emanations of radium the water of the
culture-fluid is decomposed into oxygen and hydrogen, causing
at the same time a coagulation of the albumen. Thus there
are produced in the nutrient gelatine small air-bubbles sur-
rounded by a film of coagulated albumen which naturally
increase hand in hand with the increase in the development of
gas, thus deceiving the uncritical eye — into the belief that it is
observing the growth of organisms. When will the fantastic
hope be abandoned that we shall be able to tear away the veil
from this greatest problem of the origin of life by such coarse
methods ?

If we must now admit that up to the present hour spon-
taneous generation has never been directly observed and that
it is very questionable whether spontaneous generation under
modern conditions of life is at all possible, we must, neverthe-
less, accept abiogenesis as a just postulate of reason unless we
are prepared on this sole point to put a miracle in the place of

100 LECTURES ON BIOLOGY

a natural, law-governed development. At what period in the
earth's history life appeared for the first time and what were
the external conditions will always remain a matter of specu-
lation. Nevertheless, the attempt to form an idea of the con-
ditions which then existed might not prove entirely futile. Let
us, therefore, return in imagination to the original state of our
earth, to the time when it was a sphere of incandescent gases.

It is known that very high temperatures suspend all chemical
compounds and decompose them into their single elements. We
must, therefore, assume that this glowing nebular mass contained
within itself all the numerous constituents disintegrated into the
last elements, which build up the earth and all objects at present
upon it. It is even conceivable that, seeing the enormous tem-
peratures which then existed, the substanceswhich we regard at
the present state of our knowledge as being incapable of further
division — i.e., carbon hydrogen, oxygen, nitrogen and sulphur
or, iron, lead, gold and silver — were decomposed into still more
elementary substances. This fiery ball, continually emitting its
heat into the icy space, continually lost warmth ; thus slowly
but steadily the cooling process went on. Then came the time
when the first chemical compounds came into existence, followed
by the period of the formation of a strong earth-crust. Never-
theless, we must assume that the temperature of the earth and
the gases surrounding it — the atmosphere — was at that time yet
so high that water could only be present in the atmosphere in a
gaseous form.

The period of the formation of the earth-crust was probably
followed by a very long waterless period. When afterwards,
corresponding to the continued loss of heat, water became
deposited on the solid earth-crust and when this primeval
ocean had further cooled there were then at last present the
conditions of the possibility of existence of organisms like those
that exist to-day. But who would venture to say what enormous
periods of time had to elapse even then before the first life
appeared on the earth? It is possible to defend with good reason
the assumption, though it may appear at the first glance as a
contradictio in adjecto, that there first followed a time in which
organic compounds were formed before plants and animals came
into existence.

THE OEIGIN OF 'LIFE 101

The problem of the origin of life has probably been most
thoroughly considered by Pfluger, to whom we are indebted for
a luminous theory dealing most comprehensively with the
chemical details of this subject. But as his arguments pre-
suppose a comprehensive knowledge of chemistry we must
unfortunately omit it here.

Since chemistry has proved that the body of an organism is
constructed of the same chemical elements of which inorganic
bodies are composed, and since Wohler succeeded, first of all, in
producing artificially an organic compound, urine, out of inorganic
constituents, the hypothesis of spontaneous generation does no
longer constitute a leap into the dark, but follows as a logical
conclusion. For, we may ask, is it a lesser miracle that a tree
absorbs the carbon dioxide of the air, water and different salts
from the soil and uses them all for the formation of its organs,
stem, leaves, flowers and fruits? Life, highly complicated and
differentiated, is here being created from inorganic matter. The
tiny millet seed which we confide in spring to the soil becomes
in a few weeks a plant of considerable size with many leaves,
representing an enormous quantity of organic substance which
has in a short time been formed in this minute natural labora-
tory. It is true that the change in these cases proceeds in, and
by the agency of, an already existing organism. But though it is
not possible to trace accurately each single process of change
and repeat it experimentally we know, nevertheless, that there
is no mystic something, no unknowable vitalistic force at play,
but that it is strict chemical processes which produce the effects
observed. Is it, therefore, so very extraordinary to assume that
in earlier earth-periods, when the temperature, the composition
of the atmosphere, and the conditions of pressure were utterly
different from those existing to-day, they formed the conditions
requisite for the free generation of simple organized living matter ?

It must be pointed out once more that of the manner
in which the formation of these original ancestors of modern
animals and plants took place we do not and cannot know
anything. Experience deserts us here. It is a doctrine of faith
that a Creator outside the cosmos, an omnipotent God, made by
his fiat the first germs of life arise. But it is also a doctrine
of naturalists when we assume that the lowest organisms have

102 LECTUEES ON BIOLOGY

come into existence out of dead, inorganic substance by spon-
taneous generation. Here, as there, we move on the unstable
ground of hypothesis, but with this exception that the creation-
myth denotes to our minds an inexplicable miracle, a breaking
through the laws of Nature, whilst the doctrine of a generation
without ancestors, even if we cannot prove it, may, nevertheless,
be brought into accord with the laws of Nature, and — this is the
least that may be said in its favour — does not come into conflict
with logic. The hypothesis of spontaneous generation is after
all the best theory which has been promulgated for the solution
of this difficult question. Until science shows a better way we
must assume the doctrine of spontaneous generation to be true.
But though we cannot say with certainty whence life came we
know at any rate how life has been preserved after it had been
generated. With that subject I shall deal in another lecture.

103

CHAPTER VI.
THE EVOLUTION THEORY.

WHEN we review the history of research in the last two
centuries we see that there are two opposite views : on one side
that of Cuvier and his school who try to prove the immutability
of the species; on the other, Lamarck, Geoffrey St. Hilaire,
Charles Darwin and their supporters who declare a species to
be an artificial arrangement carried into nature by man, and
proclaim the descent of all living beings, man included, from
the simplest organisms by gradual changes.

Before we examine the different theories which undertake to
explain the causes of this assumed evolution of the organisms,
we may suitably enquire whether such change of species and
progressive development has actually taken place ; in other
words, whether the Evolution Theory can be proved.

Let us go back to those far-distant earth-periods from which
have come to us the first vestiges of organic life, but let us also
moderate our expectations, for during the many cataclysms to
which the earth's crust has been exposed most germs of life
have been destroyed without leaving a trace behind. We can
see in our cemeteries how rapidly decay takes place ; frequently
after a few decades a few bones are the only signs left to show
that once a human corpse was here committed to its final rest ;
very soon these too will crumble into dust.

Thus it seems almost miraculous that so many prehistoric
organisms have in the form of fossils been preserved for our
days. It is obvious that the older the strata are and the more
remote their date in the history of the earth, the rarer become
the fossils until at last they cease altogether. It is further clear
that almost without exception only those animals were preserved
that had a firm endo-skeleton, like the vertebrates ; strong armour

104 LECTUEES ON BIOLOGY

of chifcin or lime, like the arthropods or echinoderms ; or shells
and houses of lime, silicic acid, &c., like snails, mussels, and
many species of unicellular protozoans. It is very rare to find
animals with soft bodies preserved, or at any rate their forms
impressed on stone, as for instance in the case of the famous find
of Medusa in the lithographic slates of the Upper Jura, near
Solenhofen and Eichstatt. The impressions left by these delicate
animals on the hard stone are sometimes so perfect as to make
an exact systematic determination possible. But unfortunately
such treasure-troves are rare exceptions for which the inaccuracy
of the fossil-documents found elsewhere cannot compensate us.
Above all, the most primitive forms of life which had probably
not yet reached the organization of the cell and would therefore
be of particular interest to us have completely and for all times
been expunged from the geological record : a fundamental reason
why we shall never be able to substitute exact knowledge for
the present vague guesses concerning the first appearance of
organisms upon our earth.

Another proof of the inadequacy of the palseontological evidence
is that generally only the remains of aquatic animals were
preserved in large numbers, but terrestrial animals only excep-
tionally in favourable places, that is, where there was a possibility
that the animal corpses would be rapidly covered by a crust of
mud or earth, and thus saved from destruction.

Geologically the oldest stratum in which we meet with
fossilized life is the Cambrian stage. We see here chiefly
quaintly constructed crustaceans, called Trilobites, on account of
the tri-partition of their bodies. These ancient animals were
thorough cosmopolitans, for they, or related species, have been
found in the Cambrian strata of North America, England, and
Bohemia. As they cannot be credited with having been able to
undertake long journeys we must assume, for the purpose of
explaining their world-wide distribution, that they were probably
carried by sea and wind in the larval state over enormous spaces.

One of the most important types of the Cambrian Trilobite
fauna is a specimen of the genus Paradoxides (fig. 25, 1). The
many distinctive features are the divisions of the body into three
parts, an impaired median axis, the so-called spindle, and the paired
lateral parts or pleurae. The anterior end is formed by a cephalic

THE EVOLUTION THEORY

105

shield which in Paradoxides is ornamented with long horns. The
thorax consists of sixteen to twenty segments. At the posterior
end is the tail or pygidium, which in this type is small, but in
other Trilobites sometimes of considerable dimensions. As with
their modern edible cousins, the crabs and lobsters, the dorsal
surface of the body is protected by a strong calcareous crust,
whilst the ventral surface, with the legs and gills, is relatively
soft.

It is remarkable that ap- 1 2

parently all Cambrian Trilo-
bites were blind, whereas in
the next formation,, the Silu-
rian, species are found that
have highly developed facetted
eyes. This phenomenon may
be explained in two ways.
The Cambrian Trilobites were
either denizens of the deep
sea, living in depths which no
ray of light can reach, or, what
is still more probable, the at-
mosphere in those early days
of the earth-history contained
as yet so much vapour that
there was a permanent twi-
light which the sunrays could
not penetrate.

Of other animals found in
the Cambrian system we must

notice the Brachiopods, several species of mussels and snails, and
finally some low forms of Echinoderms, the Cystoidea. Higher
animals are completely absent. The Brachiopoda are only
represented by some of the lower forms. They nourish in the
subsequent systems — there are many species in the Devonian,
Carboniferous, and Permian systems — and then become gradually
extinct. To-day they form an unimportant class with but a
few species which live at enormous depths of the sea and are
apparently becoming extinct. The Brachiopoda are distinguished
from the mussels by having dorsal and ventral, instead of right

FIG. 25. — TRILOBITES.

(I) Paradoxides bohemicus (Cambrian) ;

(2) Phacops schlotheimi (Devonian) ;

(3) The same, coiled up.

106 LECTUEES ON BIOLOGY

and left shell-valves. Further, most species have in the interior
of the dorsal valve a limy ribbon of very varied shape, which
forms the supporting organ of the fleshy mouth-arms.

A similar poverty and primitive character is exhibited by the
Cambrian flora, which so far as we know consists exclusively of
simple marine Algae.

In the second stratum, the Silurian system, there is a far
greater variety and a higher development of organisms. But
here again Trilobites are the principal species, forming, when
contrasted with their Cambrian progenitors, a veritable classic
example of progressing development and adaptation to changed
conditions of life. We saw already that Trilobites of the Silurian
Period had well-developed eyes, whilst the Cambrian species were
blind. But it is more surprising that they have now acquired
the faculty of curling up, precisely as is done by one of the little
terrestrial crustaceans of the family of wood-lice (Armadillidium
vulgare) (fig. 25, 2 and 3).

This acquisition is capable of a simple explanation. As Trilo-
bites were in the Cambrian system the only large animals there
was no need to fear hostile attacks, and consequently no need
of special protective measures. But it is different in the Silurian
and Devonian strata. Here we find other large animals, mighty
Cephalopoda, near relations of the modern Nautilus and the
cuttle-fishes, and like them dangerous and voracious robbers.
There is no longer peace and safety. An active defence is
impossible against these animals which are often more than
two metres long, for the soft-skinned ventral surface of the crabs
•offers a favourable and dangerous point of attack. The faculty
of rolling themselves up now becomes an effective protection.
When a rapacious enemy came near in search of a toothsome
meal the Trilobites simply folded together the cephalic shield and
pygidium (fig. 26), and were thus enclosed in a solid armour which
offered effective resistance to their enemy's teeth.

Besides the Trilobites we find here higher crustaceans,
enormous Eurypterides, often more than one metre long. But
most important of all, we find here and in subsequent systems
the Cephalopoda which show in the structure of their shells a
gradually increasing complexity.

Of special interest are the first representatives of the verte-

THE EVOLUTION THEORY

107

brates met with in the Silurian Period, a low species of fishes
related to the sharks. This class assumes greater importance in
the Devonian Period. They are mostly fantastic forms, widely
differing from present species, called Placodermata on account
of the powerful osseous armour which covers the anterior
half of their body. In the Devonian formation are also found
transition-forms from the fishes to the amphibians, the so-called
lung-fishes or Dipnoi. Species of this remarkable Order still
exist, dispersed over the whole world. Thus we find on the

FIG. 26.— SHELLS OF EXTINCT CEPHALAPODA.

(1) Orthoceros timidum (Upper Silurian) ; (2 and 3) Ceratites
nodosus (Upper Muschelkalk, Triassic), front and side view.

river courses of the Australian continent the well-known Ceratodus
forsteri, in Africa the Protopterus annectens, and in South America
a third species, Lepidosiren paradoxa.

The most important distinctive feature of the Dipnoi is
that, unlike fishes, they are not restricted to breathing through
gills. For if the waters which they inhabit should dry up the
gills cease to be active and the swim-bladder becomes a respira-
tory organ, a lung. As we must probably trace the amphibians
phylogenetically from similar forms, it is important to find
that they are preceded by them in the geological formations.
Whether the fossil-ancestors of the Dipnoi already possessed
lung-respiration must be regarded as doubtful. Their distribu-

108 LECTUEES ON BIOLOGY

tion contradicts this assumption, for they are largely inhabitants
of extensive basins for which the danger of a sudden drying-up
did not exist. Lung-respiration would therefore have been
useless to them.

The Carboniferous system is characterized by a much higher
degree of organization. There are land-plants, ferns, lycopodia
that grow to the size of big trees, giant ' horse-tails ' and palms ;
in the forests, spiders, scorpions, myriapoda and numerous
species of insects which appear here for the first time in con-
siderable numbers. In the swamps we see the first amphibians,
the forerunners of the Stegocephala, an Order which develops and
flourishes in the Permian, and becomes extinct in the Triassic
formation. To them belong the most powerful amphibians
that ever existed, as for instance the ungainly Anthracosaurus,
an animal of a large salamander-like structure, or the still
more enormous Mastodon, whose skull alone exceeded one metre
in length. Wonderfully preserved skeleton-parts and skulls of
this prehistoric monster have been found in the alum slates of
Oedendorf, in Wiirtemberg.

The Carboniferous is followed by the Permian system.
Here the amphibians flourish and forerunners of the reptiles
appear, to reach afterwards in the Triassic and Jurassic forma-
tions their world-wide distribution and highest development. Of
all fossils the reptiles attract the particular interest of laymen
and scientists alike on account of their weird forms and enor-
mous dimensions. Among them we find the true giants of
prehistoric times, who conquered land, air, and water. The
reptiles of the Permian system are still unimportant forms,
belonging to the two Orders of Ehynchocephalians and Thero-
morphs. Of the former only one genus has survived, the famous
Tuatara of New Zealand (Hatteria punctata), but owing to the
persecution by Maoris and the hog it is fast becoming extinct.
The Theromorphs ceased to exist towards the end of the Triassic
formation.

The so-called Mesozoic formations, the Triassic, Jurassic, and
Cretaceous, are the typical periods of the reptiles. The seas
are inhabited by the Ichthyosaurus, Plesiosaurus, and many other
weird forms. In its appearance the Ichthyosaurus, which
probably reached a length of 12 metres, resembled a dolphin.

Reptiles of the Triassic and Jurassic Period.

THE EVOLUTION THEOKY 109

Its entire body was adapted to a life in the water ; its extremi-
ties, consisting of an enormous number of single bones, were
transformed into an ideal rowing-apparatus. The long tail ended
in a large perpendicular fin, whilst the back carried another
powerful fin. Like the Plesiosaurus, the Ichthyosaurus was a
voracious animal of prey whose food consisted chiefly of fishes
and molluscs. As was proved by some interesting discoveries of
female individuals, the Ichthyosauri were viviparous. The
Plesiosauridse are remarkable for their long swanlike neck which
carries a relatively small head.

When we leave the sea and. turn to the coast and the river
courses we meet large crocodile-like reptiles of which the
Teliosaurus and Belodon may be mentioned. On the land lived
the members of the giant Atlantosaurus family, of which the
enormous Brontosaurus is the best-known representative.

If previous exaggerated statements which gave the length
of these animals as about 36 metres and their height as 8 to
10 metres have been reduced by modern researches to 22 and
5 metres, these giants must, nevertheless, have created the
impression of walking houses, and even our largest elephants
would have been completely dwarfed by them (see coloured
plate). Nevertheless^ there is no ground for believing that pre-
historic animals were much larger than living forms. The
Greenland whale (Balcena mysticetus) equals in size the largest
Brontosaurus ; our modern land mammals, the elephant, giraffe,
rhinoceros and hippopotamus are as large as any Saurian, and
there are few animals in prehistoric time which equalled, let
alone exceeded, our Cachalot (Physeter macrocephalus) , with its
length of 18 to 20 metres. That fossil reptiles, nevertheless,
appear to have been possessed of enormous dimensions is due to
a subjective reason.

A horse 3 ft. high looks small, and a dog of the same
height would look gigantic, because we are accustomed to seeing
larger horses and smaller dogs. Prehistoric animals do not in
fact exceed the present forms in size, but we find that individuals
of certain fossil classes possess far larger dimensions than their
modern congeners. This is especially true as regards reptiles.
Because to-day lizards about a foot in length, and snakes 10 ft.
long seem large to us, it is not surprising that the relatively

110 LECTURES ON BIOLOGY

gigantic forms of extinct Saurians never fail to impress us. But
whilst reptiles reached the zenith of their development during
the Mesozoic and particularly the Jurassic Period, mammals are
flourishing and developing gigantic forms in our time.

In spite of its terrifying appearance the Brontosaurus must
have been very harmless. It was herbivorous. Remarkable is
the head, the smallness of which is ludicrously disproportionate
to the mighty body. It was probably a slow, awkward animal,
of a low mental development. We may also mention among the
land animals the Stegosaurus, distinguished by a dorsal series of
large erect bone-plates, the gigantic Iguanodon, and individuals
of the Ceratopsidse whose skull was crowned with horns of an
enormous size.

Not only land and water but also the air has been subjugated
by the Saurians of the Mesozoic Period. Most famous among the
Pterosaurians are the Pterodactyls. Their size varies between
that of a sparrow and an eagle. Their bones are hollow, like
those of birds, the structure being generally adapted to a life in
the air. As in the bat, the flying organ is formed by the anterior
extremities, the outer of the four digits being enormously elon-
gated. Its object was to extend the flying membrane proceeding
from the trunk and posterior extremities (see Lecture I.). A
gigantic flying-saurian, the Pteranodon, has been found in North
America. It measured 8 metres from tip to tip of the extended
wings.

In the Jurassic Period we encounter for the first time a bird-
like organism, the Archseopteryx. It was discovered nearly fifty
years ago by workmen in the lithographic stones near Solenhofen,
the scene of many other important finds. Though only a few
fragments had been preserved of this strange feathered animal,
it was purchased by the British Museum in London for the large
sum of £600. Owing to the incompleteness of the skeleton,
opinions were long divided concerning the nature of this unique
being ; some declared it to be a bird, others a reptile. Several
years afterwards another much better preserved specimen was
found not far from the scene of the first discovery and deposited
in the Berlin Museum of Natural History (fig. 27), forming one
of its most valuable possessions. With this discovery the position
of the Archseopteryx in the system was definitely recognized.

' :%

I
I

*

FIG. 27. — AKCH^OPTERYX.

Photograph of the specimen in the Natural History Museum at Berlin.

THE EVOLUTION THEORY

111

When we observe some of the living forms of reptiles and birds
they appear so widely differentiated that no one would suspect
them of being closely related. In Archaeopteryx, however, we
have an animal which is neither bird nor reptile, but a transition
form, uniting in its body the characteristics of both classes. It
was probably as large as a rook. The plumage, which covers the
whole body and develops in the wings into long flying-feathers,
the merry-thought (furculum\ and the structure of the bones

FIG. 28. — PEEIPATUS CAPENSIS.

prove beyond doubt that the Archaeopteryx belongs to the birds.
On the other hand, the shape of the vertebrae, the presence of
teeth, the three-fingered reptile-hand armed with claws, the long
lizard-like tail containing twenty vertebrae unmistakably indicate
that it is closely related to the reptiles.

Such transition forms exist among the living animals. The
Peripatus, a little animal measuring about 10 cm. and living
in South Africa and New Zealand, resembles in appearance a
myriapod (fig. 28). Its discovery gave rise to fierce contro-
versies, for the Peripatus unites in its body a remarkable mixture

112 LECTURES ON BIOLOGY

of characteristics of annelids and arthropods. Like the annelids,
it possesses typical segmented organs, but like the arthropods,
in particular the insects, it has trachaea-like respiratory organs,
and simple yet well-defined articulated extremities, resembling
in their structure the parapodia of the marine chsetopods. We
must, therefore, assume that the ancestors of the Peripatus were
evolved from the annelids, and that we have to regard its family
as the progenitors of the myriapods and insects.

To mention a palseontological proof, Paludina, a snail with
an almost complete series of forms showing its phylogeny, is
found in the strata of the Tertiary Period on the Island of Kos,
and also in Slavonia, Croatia, South Hungary, and Roumania.
They are distinguished by a great variability. In the earliest
deposits we find specimens with smooth shells and simple whorls.
But the higher we ascend in the succession of strata, the higher
is the development of the snails ; their ornamentation increases
in richness, differentiating them widely from the ancestral forms.
If we knew only the first and last link of their phylogenetic
chain, the stem- form, Paludina neumayri, and the most recent
discovery, P. hoernesi, we should certainly take them to be very
distant species. It is only by discovering the intermediate links
that it was possible to prove their gradual change from one
species into another.

Similar instances are offered by other snails. Thus Planorbis
multiformis begins, in the oldest strata, with perfectly flat shells,
but evolves, in subsequent strata, more and more drawn-out
tower-like structures.

The horse, rhinoceros, and tapir constitute the Sub-order of
Perissodactyla. Whilst the tapirs possess in their anterior ex-
tremities four, in the posterior, three well-developed toes, the
rhinoceros has anteriorly and posteriorly three toes. In the
horse the number has become limited to one, for it walks exclu-
sively on the strongly developed and elongated middle-finger,
while all other fingers or toes of hand and foot have degener-
ated, only the thin so-called ' splint-bones ' having remained to
prove the existence of the second and fourth digit, or, if we
wish to compare them with our hand, the index and ring finger.
It is chiefly these great differences in the formation of the
extremities which compel us to-day to regard the horse, rhino-
ceros, and tapir as members of three distinct families.

THE EVOLUTION THEORY

113

As proved by many fortunate finds made in the Tertiary
strata of Europe, supplemented afterwards by more complete
discoveries made in North America, the horse has descended
from ancestral forms which possessed a much more complete
hand and foot skeleton and closely resembled the tapir. The
oldest ancestors of our horse may be traced to the deepest
Eocene strata. Here we find the genus Hyracotkcrium, little
animals about the size of a fox. They probably lived together
in large herds. The anterior extremities of Hyracotherium had

PIG. 29. — DEVELOPMENT OF THE FOOT OF THE HORSE.

(1) Anterior foot of Hyracotherium (Eocene) ; (2) of Mesohippus (Lower Miocene) ;
(3) of Anchitherium (Middle and Upper Miocene) ; (4) of Hipparion (Pliocene) ;
(5) of Horse (Eqiius) (modern). (After v. Zittel and Kayser.)

still four, the posterior three well-developed toes. In the later
strata this form is replaced by the three-toed Palacotherium, an
animal partly tapir, partly rhinoceros, about the size of a pony.
In the Miocene strata we find at the bottom the so-called
Mesohippus of the genus Palceotherium, and in the higher strata
the Anchitherium. These animals are still three-toed, but the
middle digit is already much larger than the other two and
almost exclusively supports the weight of the body.

The retrogressive process becomes still more complete in the
Hipparion in the Pliocene strata, the youngest of the Tertiary
Period. In these animals the equine type is complete, the only
8

114

LECTURES ON BIOLOGY

difference being their more delicate and graceful structure, their
smaller size, which is much less than that of the zebra, and finally
the vestiges of the lateral digits which do not now touch the
ground. Living together in large herds, these animals were widely
distributed, remains having been found in the Tertiary strata of
Europe, Asia, Africa, and even the New World (North America).
The more we approach the present day, the more rudimentary
the toes become, finally to disappear completely in the modern
horse (fig. 29). But not only the legs of the Equidae have been
modified ; the teeth, too, have undergone a great change, the
original short root teeth having developed into long prismatic
teeth (fig. 30).

3 -^ The palseontological proof, though

sufficiently strong in itself to convince the
profoundest sceptic, is corroborated by
other evidence. Occasionally animals are
born with one or two supernumerary toes
on the anterior or posterior feet. Such
reversions to a previous phylogenetic
stage are described as atavism. We
shall hear of similar instances later on.

While we found in the Jurassic
period the first bird-like animals, we find
in the Cretaceous strata typical birds,
as, for instance, the Hesperornis, a bird
of the size of a stork and the shape of
a penguin. Mammals, too, begin to play
an important part and to resemble in
their development the present forms.

If we now draw our conclusions, it is above all remarkable
that the very first animals observed in the oldest stratum in
which life existed, the Cambrian Period, are comparatively highly
organized Trilobites and Brachiopods. But, on the other hand,
we perceive a gradual continuous development from the simple
to highly complex forms which is quite incontestable. We are,
therefore, forced to assume that the Cambrian is not the first
period during which life existed, but that low forms of life must
have existed in preceding periods. Indeed, some of the most
eminent palaeontologists say that the preceding period of more

FIG 30. — TOOTH-DEVELOP-
MENT OF THE HORSE.

(1) Tooth of Anchitherium ;

(2) of Hipparion ; (3) of
Equus (half natural size).

(After Kayser.)

THE EVOLUTION THEORY 115

primitive life far exceeded in length the time from the Cambrian
to the present day.

That no certain traces of life from these earliest strata are
known to us need no longer surprise us. The longer the periods
elapsed, the greater the probability that these documents of
Nature perished, especially as we have to conceive the lowest
forms of life as ' lumps of slime ' without any skeleton parts, like
the existing microscopically minute Amoebae.

The best instance of the inaccuracy of the older leaves of the
earth's chronicle is supplied by a comparison of the wealth of
forms found in the Silurian with that of the Cambrian Period.
From the latter we know only about six hundred or seven
hundred species, whilst from the Silurian strata over ten thou-
sand have been described.

But if the earth's history does not elucidate the mystery of
the origin of life, it furnishes incontrovertible evidence of the
gradual development of the higher and highest organisms from
simple forms and for the transition of one species into another.
We need only once more recall the Archseopteryx, the evolution
of the horse, and finally the whole tendency expressed in the
evolution of the organic world.

An equally convincing proof of the truth of the Evolution
Theory is supplied by Ontogeny, the history of the evolution of
the individual. Indeed, the development of most multicellular
animals proceeds during the earliest embryonic stages in such
remarkably uniform manner that we are only able to compre-
hend it on the hypothesis of a descent from a common ancestor.
For every animal passes in its development essentially through
all the stages which we observe permanently preserved in the
lower representatives of the same stem. Haeckel formulated this
fundamental law thus : " The evolution history of each animal,
its ontogeny, is a short recapitulation of its phylogeny ; i.e., the
principal stages of organization through which its ancestors
passed in remote ages are reproduced, though in a somewhat
changed form, in the development of each individual."

But however clearly this law operates in numerous cases,
there are instances in which its application seems vague and
indistinct. The reason is that not only the adult individuals
exhibit new characters but that also all other stages are able to

116 LECTURES ON BIOLOGY

acquire such new adaptations to special conditions of life. It
behoves us, therefore, to proceed with the utmost caution in
drawing phylogenetic conclusions on the basis of embryological
evidence. It would, for instance, be erroneous to assume,
because in the larva of the Libellulse the second pair of maxillae
(labium) is modified into a pair of long nippers for deftly seizing
the prey, that the dragon-flies had descended from a similarly
constructed ancestor.

All multicellular organisms consist at the starting point
of their development of a single cell, the ovum, or, more
accurately, the fertilized egg-cell. For with the exception of
a few cases it is necessary that two cells, the male spermatozoon
and the female ovum, unite in order to make the origin of a

FIG. 31. — OVA OF DIFFERENT ANIMALS.

(1) Egg of a fowl ; (2) egg of a sea-cucumber (Holothuria tubulosa) ; (3) egg of
fresh- water polyp Hydra. (I, Less than natural size ; 2 and 3, greatly magnified.)

new multicellular organism possible. The process during which
unification takes place is called fertilization ; the result is the
fertilized egg-cell. We shall have an opportunity later on to
deal with this subject in greater detail.

The similarity, in their minute structure, of ova of even the
most distant species is remarkably great. Indeed, the resem-
blance is often so complete that in many cases even an expert
finds it difficult to distinguish between them. This difference in
appearance, however, is due to secondary factors conditioned
apparently by specific conditions of life ; nevertheless the general
structure is always essentially alike. The ovum of whichever
animal we may examine, we shall always find it to be a simple
normal cell, consisting of the cell-body — protoplasm — and the cell-

THE EVOLUTION THEORY 117

nucleus (fig. 31). That the egg of a mammal, which develops in
the shelter of the maternal body and is fed directly by it, must
differ from a bird's egg, which is laid at an early stage and
exposed to unfavourable external influences, needs no specific
mention.

The first important feature of the egg of a fowl is its
enormous size compared with the egg-cells of most other animals
of the size of a fowl. The ovum of Man measures but the fraction
of a millimetre, and other animal ova are still smaller. But when
we examine the structure of a bird's egg we see that its size is due
to a secondary circumstance, the enormous accumulation of dead
food material, the yellow and white yolk and the albumen. As
the growing bird-embryo is dependent entirely upon itself and
cannot be fed by its mother, it becomes necessary that it is
supplied with a sufficient quantity of food. That this is the real
cause of the difference in size is further proved by the fact that
the only mammals which are not viviparous, the Duckmole
(Ornithorhynchus) and the Porcupine Ant-eater (Echidna), quaint
inhabitants of the Australian Archipelago, betraying in the
organization of their bodies their descent from the reptiles, lay
large eggs rich in yolk.

The shell of the bird-egg is not a part of the germ-cell but
an organ of protection secreted by the maternal oviduct. The
real cell is the minute light spot which observation reveals in
the yolk. This consists, like the egg-cells of other animals, of
protoplasm and the nucleus.

This remarkable uniformity does not only apply to the
structure of the ova, but extends also to the developmental
stages, though there are of course certain individual differences
due to different conditions of existence. The type, however,
always remains.

Before fertilization can take place, ova and spermatozoa must
pass a maturing process which will be described on another
occasion. When fertilization is complete the ovum always
divides first into two, then into four, eight, &c., cells, forming the
morula or cell-heap. By further cell-multiplication is formed
the blastula, a minute hollow sphere the walls of which consist
of one layer of firmly united embryonic cells. Whether we have
so far observed a sponge, an echinoderm, a tunicate, or even

118

LECTUEES ON BIOLOGY

the archetypal fish Branchiostoma, the process is in every case
exactly similar.

The one-layer germ forms the two-layer germ, the gastrula,
by folding one wall of the cell-vesicle against the other. It must
be understood that we are considering here only the most fre-
quent form of development, as observed in the most distantly
related species. In numerous other cases the process of gastru-
lation shows considerable differences, and very frequently the
gastrula stage can only be demonstrated by accurate comparative
examination. It seems obvious that this should be so, for an
egg with -a large accumulation of yolk cannot divide in the same
manner as an egg which, having no yolk, offers less resistance to
the process of division.

FIG. 32. — EMBRYOS OF VERTEBRATES.

(1) Budgerigar (Melopsittacus undulatus) (after Parker) ; (2) Cercopithecus
cynomolgus (after Lalenka) ; (3) Man (after His).

The gastrula stage is of particular importance to the Evolution
Theory, for not only do we meet with it in the embryonic stages
of the most widely differentiated stems, but there even exists
a fully developed animal which permanently remains at this
stage, the Protohydra, belonging to the Coalenterates. Its whole
organization consists of a stomach cavity surrounded by a double
cell-mantle, having an orifice at the thinner end which serves
both as mouth and anus.

Similar resemblances are observed in the higher stages of the
embryonic development, thus impelling us to the hypothesis
of a common ancestor. Even superficial comparison of the
embryos of different classes of the vertebrates prove this striking

THE EVOLUTION THEORY 119

uniformity of structure (fig. 32). It is, however, not so great
that an embryologist would be unable, as was formerly thought,
to differentiate between embryos, for there are always more
or less prominent distinctions present. But on examining
the different organs accurately we cannot fail to notice the
uniformity.

All vertebrates, for instance, form the central nervous system
as a dorsal gutter-like depression of the ectoderm, extending
from the anterior to the posterior end of the young embryo.
In the course of the development this gutter becomes deeper and
deeper, finally separating completely from the ectoderm and
forming the neural canal or spinal cord. In the lowest verte-
brate, the archetypal fish Branchiostoma the nervous system
retains this form permanently. Moreover, the lumen of the
neural canal is here uniform in thickness, except at the anterior
end where we can perceive in a slight vesicular enlargement the
first stage of the brain.

In the other vertebrates the nervous system does not stand
still on this primitive stage, but makes further progress in the
formation of the anterior part of the brain. The neural canal
continues to distend, the enlargement forming three separate
parts, the primary cerebral vesicles. By a further division we
obtain the five parts of the brain, which are to-day found in all
vertebrates from the fishes upwards. That the brain of the
different classes is, nevertheless, widely different in appearance is
explained by the different size and formation of the five cerebral
vesicles. Together with the growth of the mental faculties we
observe in the cerebrum and cerebellum a growth as well as a
differentiation ; both gradually assume in birds, but especially in
mammals and man, a size far exceeding all other parts of the
brain.

The common origin may further be perceived in the develop-
ment of the vascular system and respiratory organs. As it is
one of the principal functions of the blood to distribute the
oxygen received from the respiratory organs throughout the
whole body and to remove the carbon dioxide resulting from
the vital processes these two systems are strictly correlated.
If, therefore, on an animal leaving the water, lung-breathing
replaces breathing through gills the circulatory system must

120 LECTURES ON BIOLOGY

similarly adapt itself to the altered conditions and undergo
suitable modifications.

Fishes breathe through gills ; hence we find in them a specific
circulatory system, the branchial arches, which carry the blood
to and from the gills. In the majority of fishes four pairs of
such arches are developed. In the amphibians, at any rate
in those that live in the water and are therefore dependent on
breathing through gills, the gill-arteries and gill-veins are formed
in a similar manner. But many amphibians — for instance, frogs
— leave the water at an adult stage and become air-breathing land-
animals. The gills, now of no further use, become degenerate
and are replaced by the lungs.

When the gills disappear the blood-vessels which fed them
similarly degenerate, and thus we observe a far-reaching modifica-
tion. One pair of the arches is completely lost, another moves to
the lungs, forming the pulmonary circulation, &c. The details do
not concern us here, but it is important to note that even in the
higher and highest vertebrates — the reptiles, birds, and mammals,
which at no stage of their development live in the water — the
branchial circulation is present in the embryonic stage exactly as
in fishes and amphibians. Moreover, if we examine a human
embryo of an early stage we do not only find rudimentary
branchial arches, but also externally visible typical gill-clefts
(fig. 32,3). We are even able to distinguish in the human heart
successively the fish-heart with single ventricle, auricle, and
venous sinus ; and the stage of the amphibian heart in which the
single ventricle has been divided into two sections. Later a
median wall is formed representing the stage which is per-
manently present in the reptiles. Finally, by a complete separa-
tion of both ventricles and other modifications, the type of the
mammalian heart is reached.

We have seen that amphibians at an early stage live in the
water and have gills, but at an adult stage take to a terrestrial
life and develop lungs. In the mountainous districts of Europe
the little black salamander (Salamandra atrd) is very common. It
is distinguished from related species by the fact that its young
are born as air-breathing animals. Nevertheless, its embryos
possess w7ell-developed gills which, however, do not normally
fulfil any functions. But if we kill a pregnant female, remove

THE EVOLUTION THEORY

121

the embryos at this stage from the oviduct, and place them into
water we shall observe that in spite of the abnormal conditions
the gills are still able to discharge their proper functions. Here
it is clear that the black salamander descended from ancestors
whose larvae were aquatic and breathed through gills, and that
the habit to bear the young up to the complete development can
only be a recently acquired character.

FIG. 33. — Ambly&toma tigrinum.

On the land, the adult form ; in the water, the larval form, Axolotl, formerly
called Siredon axolotl or S. pisciformis.

An opposite instance is furnished by the well-known Mexican
Axolotl, for it is able, under ordinary circumstances, to pass its
entire life and to reach maturity and power of reproduction in
the gill-breathing larval stage. Formerly it was even thought
that the larval form was its permanent form, and it was conse-

122 LECTUEES ON BIOLOGY

quently long thought that they belonged to a type with per-
sistent gills. But great was the surprise when a few individuals,
born in the aquarium of the Botanical Gardens at Paris, were
observed to lose their gills, leave the water, and become typical
terrestrial salamanders.

There are, however, two explanations possible : it may be
that we have here either a tenacious clinging to a low state of
development, or a reverting to an earlier stage, consequent upon
changed conditions of life. This latter view is upheld by "Weis-
mann who thinks that ancestors of the Axolotl which inhabits
to-day the Mexican lakes were already in the diluvial period
fully developed salamanders. As a result of deforestation and
consequent decrease of moisture of the air the animals would
have been doomed to extinction if they had not been able to live
once more in the water by reverting to the gill- stage. However
that may be, they offer in any case a proof of the variability of
organic forms.

It is a rule almost without exception that the vertebrates
possess a shape which is strictly bilateral-symmetrical, i.e., we
can divide their bodies longitudinally into two equal parts. This
applies to fishes as well as to Man. If we know one side of the
body we shall be able to reconstruct the other. In particular,
the extremities and the most important sense organs — eyes and
ears — are distinguished by their uniform distribution upon the
right and left half of the body.

A remarkable exception is furnished by the flat-fishes — plaice,
flounders, turbot, and soles — which form one of our staple foods
and are therefore universally known. Their structure seems to
controvert all our conceptions concerning the construction of
the vertebrates, but this deviation from the normal state is due
to an unusually strong lateral flattening of the whole body. As
a result of this process the distance between the dorsal and
ventral surface becomes enlarged while that between the sides
is shortened. Because the formation of the right and left half
is widely different in appearance, and because the flat-fishes,
moreover, swim laterally — due to the strong lateral compression
of the body — most observers would unhesitatingly describe one
side as the back and the other as the abdomen.

The underside of the flat-fishes, corresponding to the right or

Flat-fishes.
Plaice (Pleuronecta platessa), Sole (Solea vulgar*), Turbot (Rhombus maximus).

THE EVOLUTION THEORY 123

left half, according to the species, is distinguished by a colour
ranging from light to white, and by the absence of the eye.
Both eyes are fixed close together on the upper side which is
distinguished by varied design, and frequently by bright colours.
Yet in spite of the pronounced asymmetry the existing flat-
fishes have doubtless descended from symmetrical ancestors,
for each individual must to-day pass in its own body through all
the stages of its phylogeny.

The little flat-fish is hatched, having a lateral-symmetrical
structure, being in nowise distinguished from the embryos of
other fishes, excepting its destiny. But with increasing growth
it exhibits increasing differences. Of the eyes, which were
originally fixed one on each side, one leaves its normal position
and wanders to the other side. Simultaneously the flattening
of the body proceeds until at last the final stage of the adult
animal is reached (fig. 34).

To their structure corre-
sponds their mode of life. Their
domicile is the sea-floor. Here fjssMl fV
they lie with the light under-
side on the sand, often buried
in the sand. They are not 1 2

easily detected, for the resem- FIG. 34.— SHIFTING OF THE EYES DUR-
blance of the upper side, which ING THE DEVEp^™ °F A YOUNG

is alone visible on the Ocean- (i} Original position ; (2) second stage ;
bed, equally well protects them (3) final position (after Leunis-Ludwig).

against their numerous enemies

and conceals them from their unsuspecting prey.

One of the best instances of adaptation to special conditions
of life is furnished by the Baleen whales, the giants of modern
times. Only the ocean with its inexhaustible wealth of life is
able to produce and maintain such monsters. It is impossible
to conceive the volume of food which must be taken every day
by an animal weighing more than 50,000 kilogrammes in order
to exist. The problem becomes all the more difficult when we
learn that owing to the construction of the pharynx they are
dependent for food on small animals, minute transparent snails,
cuttle-fishes, shrimps and prawns, and small fishes not larger
than a herring. If these giants had to capture their prey

124

LECTUBES ON BIOLOGY

singly they would soon die from starvation, but Nature has
furnished them with a most ingenious straining-apparatus. The
mouth-cavity is of an enormous size ; from each side of the
upper jaw a double series of more than 200 horny plates hangs
down into the mouth. " Each plate is shaped like a sword or
scythe, reaching often a length of 5 metres. Both apex and

FIG. 35. — THE COMMON RORQUAL (Balcenoptera musculus).

In the corner, head of embryo of Rorqual, with teeth of the upper jaw exposed,
also five single teeth.

inner edge are frayed into hairy shreds. The mouth wide open,
the whale swims through shoals of pelagic animals. Having
secured a good mouthful, it closes its jaw, raises its tongue,
allows the water to strain out at the sides through the baleen
plates on the edge of which the food animals are caught ; thence
they are swallowed."

THE EVOLUTION THEOKY 125

It would be difficult to imagine an organization better
adapted to their mode of life than this enormous sieve. Yet
embryology tells us that the whales were not always furnished
with baleen, but that they developed by gradual adaptation from
dolphin-like ancestors possessing teeth. This fact was already
known to St.-Hilaire, one of the pioneers of the Evolution
Theory. Recently the development of the baleen whales has
been thoroughly studied by Kiikenthal, upon whose drawings our
illustration is based. According to him the embryos develop
early both in the upper and under jaw complete rows of teeth,
but these teeth become degenerate before birth without having
functionated (fig. 35).

We may even affirm that the ancestors of the mammals had
a regular change of teeth ; for. behind this first row of teeth, the
milk teeth, stands a second which also remains rudimentary.
Similar conditions are observed in the tortoises, whose toothless
jaws are armed with sharp horn-plates similar to the bill of a
bird ; their embryos, however, possess degenerate teeth.

It is these rudimentary organs which no longer reach the
functional stage that are of utmost importance to the Evolu-
tion Theory. Or is there any other logical explanation of the
presence of rudimentary organs than that these now useless
formations once discharged certain useful functions in the
ancestors ? Is there a stronger argument against a divine
creative act in the Biblical sense ? Is it not an impossible con-
ception that an omnipotent God had created his creatures thus
imperfectly? But the Evolution Theory solves this problem
without difficulty. We know that climatic conditions on this
planet, air pressure, and the moisture in the atmosphere, have in
the course of the earth-periods undergone many changes. Where
to-day the land stands there the oceans roared in ages long ago ;
vast stretches of water have been replaced by continents. As
the gills of the amphibians and the swim-bladders of the
fishes lose or change their functions when the aquatic life is
exchanged for a life on land, so all other organs change with a
change in the conditions to which they were adapted. We have
seen that all organs which fall into disuse become degenerate
until at last they disappear altogether. As a result of the con-
servative element which is present in each organism and finds

126 LECTURES ON BIOLOGY

its most eloquent expression in heredity, such parts of the body
reappear more or less distinctly during the embryonic stage,
long after they have been lost in the adult animal. We have
already considered the evolution of the human circulatory system ;
further instances are that the human embryo, like that of an
anthropoid Ape, possesses thirteen pairs of ribs, whilst adult man
has only twelve ; that at an early stage the coccyx is compara-
tively large and protrudes beyond the lower extremities ; and that
in the carpus of the embryo an Os cent rale is rudimentary, just
as in amphibians and reptiles. In view of these and many other
facts that can be adduced every unbiassed student will admit
that the Evolution Theory is proved by abundant evidence, and
applies to the animal kingdom as well as to the human species.

In adult man we are able to demonstrate the presence of
numerous organs and parts of organs which are either quite
rudimentary and useless, or in an obvious state of degeneration.
Like the males of all mammals, Man possesses rudimentary
teats. There are even certain cases known in which milk was
secreted by the male mammary gland. On the eyes of birds,
and in many fishes, amphibians, and reptiles is found a thin skin,
the so-called nictitating membrane or third eyelid. In the lower
mammals this membrane is still fully developed, but in the
higher mammals and in man it has been reduced to a tiny
rudiment, a crescent-shaped fold which lies in the inner angle
close to the lachrymal gland.

The muscles of the ear are apparently in a similar state of
retrogressive development. True, there are still the same muscles
as in mammals, the attollens, attrahens, and retrahens aurem,
but they are only faintly developed and generally without
functions. This is so much true that in a person who is still able
to move his auricle this atavistic ability is regarded as an amusing
freak.

Other rudimentary organs are the wisdom tooth, which in the
European is sometimes entirely absent ; the coracoid process of
the scapula, a bone which is strongly developed in reptiles, birds,
and the lowest mammals, the Ornithorhynchus, or duck-mole,
but rudimentary in man ; and, finally, the greatly degenerate tail-
muscles.

The best known of all rudimentary organs is doubtless the

6a

FIG. 36. — RUDIMENTARY ORGANS.

(1) Indian child with tail. (2) " Tailed " Bongo woman (after Kanke). (3) Rudi-
mentary ear muscles. (4) Polymasty in a young man (after Ammon). (5) Caecum,
with vermiform appendix. (6) Eye of an owl with nictitating membrane. (6a)
Human eye with crescent-shaped skin-fold (after Romanes).

THE EVOLUTION THEORY 1'27

caecum, with the vermiform appendix. In herbivorous mammals
a structure of considerable size, and of great importance in the
assimilative act, it is much smaller, and often absent, in carnivorous
animals. In man the caecum is small and insignificant, playing
probably a small part in the absorption of the digested food. But
its appendix, the notorious Processus vermiformis, has no function
or value whatever, its object being rather, apparently, to create
suffering. Its extremely variable length, ranging, in an adult,
from 25 cm. to a mere vestige, seems to support this con-
ception, which is further proved by the fact that it may be
completely removed by a surgical operation without any
subsequent injury to the body.

Many travellers in Africa and Australia have reported the
existence of human beings with long hairy tails, but further
investigations have always proved these reports to be due to errors
or deception. Many savage tribes, the Njam-Njam, the Bongas,
&c., are accustomed to wrear as girdles the skins and tails of wild
animals, and this habit may have given rise to such fables which
should have been discredited at the first report if it had been
remembered that even the highest apes have only rudimentary
tails. Nevertheless, these reports are not entirely without
foundation. We heard before that each human embryo possesses
a rudimentary tail, consisting of a few vertebrae. In most cases
this vestigial tail soon becomes degenerate, nor is it even dis-
cernible in adult man. Yet now and then we hear of the forma,
tion of a typical tail-stump, as shown in the picture of a small
Indian child (fig. 36). There are even cases known in which
the tail attained a length of about 10 centimetres.

Among the invertebrates we find numerous facts supporting
the Doctrine of Descent. Everyone knows the extremely varying
structure of the different classes of the Echinodermata, the sea-
urchins and starfishes, the elegant ophiuroids, and the ungainly
holothurians. Who would think that all these forms are closely
related? Yet in spite of the numerous external differences
their organization is surprisingly uniform. This is best shown
by their remarkable organs of locomotion which distinguish them
from all other animals.

Those who have seen Echinoderms in an aquarium must
have been impressed by their beautiful radially-symmetrical

128 LECTUBES ON BIOLOGY

shape. This structure is probably an echo of ages long ago
when they were fixed animals, as the beautiful feather-stars
Pentacrinus and Khizocrinus are to-day. For it is a pheno-
menon universally observed in the animal kingdom that a
sedentary mode of life usually leads to a modification into, or
at any rate an approach to, the radial type. I need only mention
corals, Chsetopod worms (Serpulidse) and the Bryozoa.

In any large aquarium we may see lying on the sand awk-
ward looking sea-cucumbers ; sea-urchins menacingly project
their spines, whilst a star-fish is about to force its proboscis-like
stomach between the valves of a mussel and slowly devour the
living contents (fig. 37). The superficial observer cannot under-
stand how these awkward, cumbersome animals, clad in their
calcareous armour, can reach the highest point and crag of the
artificial rocks, and even ascend, slowly but surely, the smooth
glass wall. But observation will supply the solution. The
starfish has finished its repast which seems to have stimulated
its energy. Slowly it pushes itself along the bottom until it
reaches the glass wall. "As it erects its arms, preparatory to
ascending the side, we see that scores of soft tube-feet are pro-
truded from the central groove of each arm. These become long
and tense and their sucker-like terminal discs are pressed against
the glass. These are fixed, and towards the attachment the
starfish gently lifts itself. By repeating this process it gradually
ascends the glass. The protrusion is effected by the internal
injection of fluid into the tube-feet ; the fixing is due to the fact
that the contained fluid, flowing back again from the tube-feet
to the internal reservoirs, produces a vacuum between the end of
the tube-feet and the surface of the glass.

" On the dorsal surface, between the bases of the arms, there
is a complex calcareous sieve through which the sea-water is
taken in. Its pores converge into a fine tube, the ' stone-canal '
which, like a complex calcareous filter, extends vertically through
the body, and leads into a ring round the mouth. This circum-
oral ring gives off nine transparent vesicles and five radial tubes,
one for each arm. Each radial vessel lies in the ventral groove
of an arm, roofed by the rafter-like vesicles, and gives off
internally reservoir-like bladders or ampullae, and externally the
tube-feet. The fluid in the system seems to pass from the radial
vessels into the tube-feet, and back again into the ampullae.

FIG. 37. — ECHINODERMATA.

In the foreground, Starfish (Asterias glacialis] sucking a mussel ; on the rock,
Sea-urchins (Sphcerechinus granularis] ; in the background, Sea-cucumber (Holo-
thuria tubulosa).

THE EVOLUTION THEOKY

129

" In the other classes of the Echinodermata the water-vascular
system is similarly formed, though with interesting deviations.
That of the sea-urchin does not differ greatly from that of the
starfish, but in the Holothurians we observe greater diver-
gencies. Of the five radial tubes (ambulacralia) only three are
developed into organs of loco-
motion and furnished with suc-
tional discs, while the tube-feet
of the other two have become
modified into pointed tentacle-
like formations. The ' stone-
canal ' does not extend to the
exterior, but opens into the
body - cavity. In the Ophiu-
roidea and Crinoidea degenera-
tion has gone still further ;
for being well capable of loco-
motion, owing to the greater
mobility of their arms, all the
ambulacralia have become modi-
fied into ampulla-less organs of
touch."

The other organ-systems show a similar uniformity of
structure, but the larva-form, finally, furnishes the most con-
vincing proof of the phylogenetic relation of all Echinodermata.
They differ from the adult animals by their pronounced bilateral
symmetry as well as by their soft gelatinous body. In the sea-
urchins and Ophiuroids we find the Pluteus-larva form, in the
Holothurians the Auricularia form, and in the Crinoids the
Bipinnaria forms. All these diverse larvae may be traced to a
common ancestral form which had a trigeminous intestine and
a simple circumoral cilia- wreath (fig. 39).

These various developmental stages of the Echinodermata
are of particular interest to the Evolution Theory, because they
form the connecting link with another stem, the Vermes. A more
highly organized marine form, the famous Balanoglossus, passes
through a larval stage which, owing to its extraordinary simi-
larity, was once held to be an evolution form of an Echinoderm.
The adult worm possesses in its ' proboscis ' an organ similar to

9

FIG. 38. — WATER-VASCULAR SYSTEM OF A
STARFISH.

130

LECTUEES ON BIOLOGY

the water-vascular system of the Echinodermata. Above all the
mode of formation of these two organs is strikingly similar, for
they develop as protusions of the larval intestinal canal (fig. 40).
One class of the Echinoderms, the Crinoidea, differs greatly
from the rest in the course of its development. Let us choose
for illustration a small form which frequently occurs in the
Mediterranean, Antedon rosacea. By means of their thin ' arms/
which are furnished with lateral pinnules and processes, they skil-
fully climb about in the seaweed. They possess great power of

FIG. 39. — LARVAE OF ECHINODERMATA.

(1) Common ancestral form ; (2) Pluteus-larva of Sea-urchin ; (3 and 4) Bi-
pinnaria- larva of Starfish ; (5 and 6) Auricularia-larva of Sea-cucumber. (After
J. Midler.)

resistance, are easily 'kept ' in an aquarium, and often proceed to
reproduction. The enormous tenacity of Antedon is proved by
its insensibility to injuries ; one may break off all their ' arms '
without causing them any serious injury; under favourable
circumstances they will repair the damage in a short time by
regenerating them.

THE EVOLUTION THEORY

131

In their development the Crinoids pass two distinct larval
stages. First the ovum produces a minute, free-swimming form,
resembling a barrel bound with five ciliated hoops. These and
a wisp of cilia at the most anterior end effect
locomotion. But soon the larvae give up
their free life, anchor themselves by means
of a ventral groove to Algse or a stone, and
change into a stalked larval form, entering
the so-called Pentacrinus stage. From this
proceeds finally the adult Echinoderm which
once more returns to a free life (fig. 41).
The appearance of a typical fixed stage in

FIG. 40.

the evolution of a free-living animal is

FOBNABIA-LABVA OF A
MAKINE-WOBM, Bdla-

noglossus. (After E.
Metchnikoff.)

remarkable. Does not perhaps the bio-
genetic law apply here, and is not the free
mode of life in Antedon a recent
requisition ? Turning back the leaves
of the earth's history we find that
the Crinoids are a very ancient and
widely distributed group, and that
the species existing to-day are evi-
dently only the remainder of a vast
class which is rapidly approaching
extinction. The Palaeozoic period
represents the zenith of their develop-
ment, and though the various species
of Crinoids occurred in narrowly
defined regions, they are found in
such masses that frequently deposits
of a thickness of several metres
almost exclusively consist of their
limy armour. If we examine these
extinct forms accurately, we see that
they were in fact stalked, fixed
forms. Such forms still exist to-day
on the ocean bed, where conditions

remain eternally alike — the Ehizocrinus and Pentacrinus, which
I have already mentioned.

Larval forms similar to those with which we met in the

FIG. 41. — LABVAL FOBM OF A
CBINOID.

(1) Free-swimming larva, with
skeleton of adult forming in-
side ; (2) fixed stalked young
Pentacrinus stage. (After
Clauss.)

132 LECTURES ON BIOLOGY

star-fishes, holothurians, sea-urchins, ophiuroids, and finally in
the Balanoglossus, are found in other vermes and also in many
molluscs. Interesting conditions are observed in the annelids.
Among the marine forms of this class we find a free-swimming
pelagic larval form, the celebrated Trochophora. By gradual
metamorphoses, growing longitudinally, this larva is trans-
formed into an adult worm. In distinction to the pelagic annelids
the freshwater forms have no intermediate larval stage ; the ovum
produces the fully developed worm. Nevertheless, there is no
doubt that the freshwater annelids formerly passed through a
larval stage, for in the growing embryo we find indubitable
characteristics of Trochophora.

Finally let us consider an instance taken from the crustaceans.
Who has not heard of the duck-mussel (Lepas anatifera), which
in the old German legend was the young
stage of the barnacle (or barnicle) goose.
According to old ideas, the distinctive feature
between the animal and vegetable kingdom
was the ability of locomotion. Hence, corals
and sea-anemones, sponges and tunicates
were even until recent times counted among
the flora. Duck-mussels, being fixed, were
regarded in mythical zoology as plants, and
A MARINE ANNELID. the monks in mediaeval times were not slow
in arguing that as duck-mussels were plants,
so the geese and ducks that developed from them must also
be 'plants,' and were not therefore included in the rule which
forbade on certain days the eating of flesh.

The position in the system of zoology of the Cirripedes, of
which I have mentioned the duck-mussel as the best known type,
was long obscure. Almost universally these animals were regarded
as mussels. The calcareous plates which encase their body and
greatly resemble mussel-shells not unnaturally gave rise to this
error. But now we know that the Cirripedes are crustaceans,
though when we fish a piece of driftwood or pumice-stone from
the sea and observe on the underside ' mussels ' anchored by a
strong leather- stalk, and projecting from the partly open valves
numerous cirral threads, we see a picture widely differing from
our usual conception of crustaceans. But a knowledge of the
phylogeny of Cirripedes leaves no room for doubt.

THE EVOLUTION THEORY

133

In the evolution of all lower crustaceans, the so-called Ento-
mostraca, we meet invariably (however differentiated the adult
animals may be) with' a. peculiar larva, the Nauplius. It is a
lively little animal which swims gaily about by means of its three
pairs of appendages, of which the first is unforked, the other
pairs doubly-branched. The median eye is also a distinctive
feature in the Nauplius larva. By
a more or less complicated meta-
morphosis the Nauplius changes
into an extremely great variety of
crustacean forms. And as ' duck-
mussels' also originate from Nauplius
larvae their position in the system
becomes clearly defined.

It is here necessary to modify to
some extent the preceding statement
that all crustaceans pass through a
Nauplius stage, for an exception is
made by the water-flea (Daphnia),
the favourite food of aquarium
fishes. This little crustacean passes

the stage of the free-swimming Nauplius in the ovum and
emerges as an adult Daphnia.

It is a well-known fact that the growing larvae and young
crustaceans moult several times. The object of the moulting is
chiefly to enable the animal to grow, for the rigid chitinous
cuticle wrould make development impossible. Before the moulting
the old shell becomes virtually dead, a new shell begins to form
within the old, and finally the shell is cast with considerable
effort. Almost at the moment of casting the anirnal, hitherto in
a compressed state, increases in size. The new shell is at first
soft and requires several hours to acquire firmness. When it
has become rigid the growth of the crustacean is once more
postponed till the next moult.

However strange it may sound, the Nauplius in the ovum
performs a similar act of moulting, a process which is all the
more illusory as the rigid egg-shell effectually precludes every
increase in size. Is not the only explanation this, that the
Daphnia, like other crustaceans, had once a free-swimming

PIG. 43. — NAUPLIUS STAGE OF

Pen&us potimirum.

134 LECTURES ON BIOLOGY

Nauplius stage ? Can this aimless moulting in the ovum
indicate anything else but a tenacious retention of a once vitally
important process?

The following instance will convince even the greatest sceptic.
If we take from any lake in Europe or North America a
tumblerful of water and pour it through a fine sieve we see a
number of minute ' transparent animals, each formed round a
minute brown point. By means of a magnifying glass we are able
to distinguish them as crustaceans, about 1 centimetre in length,
whose scientific name is Leptodora hyalina, a species of the
Daphnidse. No better case of adaptation of an animal to its
environment is known. The transparent body makes the
crustacean practically invisible in the water and thus enables
it to escape its enemies, but approach its prey undetected. The
tiny brown point, the only part which is visible of this re-
markable creature, is the food in the stomach.

Like the related forms, the Leptodora produces two different
kinds of female reproductive cells, described as summer and
winter ova. Whilst from the former the growing animals
emerge under normal conditions fully developed, having passed
the larval stage in the ovum, the winter ovum produces a free-
swimming Nauplius. It seems difficult to find a more convincing
proof. The cause of this differentiation is probably to be found
in the fact that the supply of food furnished to the winter ova,
which leave the maternal body prematurely, is insufficient for
the complete course of their development. Unless it is therefore
to die in the egg from hunger, the young crustacean is compelled
to seek its own livelihood.

The higher crustaceans, the so-called Malacostraca, begin their
free life with the more advanced Zosea stage (fig. 44). Forma-
tions of a Nauplius larva occur now only rarely ; our common
crayfish, indeed, emerges from the egg fully developed, differing
from the adult in size but having all organs properly formed.
In contrast to it, some of its nearest relations, for instance the
beautiful Brazilian Penseus (P. potimirum) has a very complete
ontogeny (fig. 43). It comes into existence as a dainty little
Nauplius. Gradually the larva increases in size, the body be-
comes segmented and the anterior half protected by a shell.
Thus cephalothorax and abdomen are formed. Simultaneously

THE EVOLUTION THEORY 135

other changes take place, transforming the Nauplius into a Zosea.
The median eye is replaced by a pair of eyes, and the three pairs
of appendages become six.

Continued growth and moulting produces
the second Zosea stage, which approximates
more and more that of the adult animal.
The number of appendages is again in-
creased, but whilst formerly all served the
purpose of locomotion, they now gradually
assume separate definite functions. The
two first appendages become antennae, the
next develop into jaws, whilst the other be-
come rowing-legs. The abdomen is better FIG- 4
developed and the eyes become typical lateral
outgrowths of the head.

Another moult brings the Zosea to the Mysis stage. It has
now thirteen pairs of appendages, of which the third, fourth, and
fifth are used as masticatory organs. The legs are assisted in
locomotion by the abdomen, the most posterior segment of which
has become transformed into a frond. This larval form is
especially interesting because we find in the order Schizopoda
a family of the Mysidse the members of which permanently
remain at the Mysis stage. A last moult concludes the com-
plicated metamorphosis, giving the Penseus its final shape.

Though the opinion is no longer held by modern naturalists
that adult animals of the Nauplius type existed among the oldest
ancestors of the crustaceans, and though it is now doubted for
several reasons whether a mature Nauplius ever existed, their
regular appearance in the evolution of all crustaceans can only be
explained in the sense that the Nauplius is the primary larval
form from which the living crustaceans have descended.

There is no better test for the accuracy of a theory than that
it enables us to make certain predictions which are subsequently
corroborated by observation. One of the most brilliant triumphs
of science is the discovery of the planet Neptune. As early as
1828 Bessel contended, on the basis of certain slight irregularities
in the path of Uranus, that they were probably caused by some as
yet far-distant planet. Independently of each other, yet almost
simultaneously, the famous English astronomer Adams, and

136 LECTUEES ON BIOLOGY

Leverrier, the Director of the Paris Observatory, determined the
place in which this planet would have to be looked for ; and when
Galle in 1846 directed his telescope to this spot he actually dis-
covered the planet Neptune. The Evolution Theory enables us
to make similar prophecies. The discovery by Goethe of the
human intermaxillary bone is a case in point. I have already
mentioned the ues centrale of the carpus which is present in
reptiles and amphibians but absent in man. If our view concern-
ing the descent of man from lower animal ancestors is correct
we are justified in assuming that a rudimentary ues centrale exists
in an early evolution stage of man. In point of fact it was dis-
covered in a human embryo by Kosenberger.

Investigations made into various species of Hawk-moth
caterpillars (Sphingida) led Weismann to believe that the simple
longitudinal stripes in the caterpillars of the Humming-bird
hawk-moth (Macroglossa stellatorum) are phylogenetically older
than the more general spots or oblique stripes, because cater-
pillars thus marked developed from earlier forms with longi-
tudinal stripes. If this assumption was correct it was equally
applicable to other Sphingidae caterpillars with oblique stripes,
and Weismann predicted that the then still unknown earlier
forms of the Privet Hawk-moth (S. ligustri) possessed longi-
tudinal stripes. A decade afterwards the English zoologist
Poulton succeeded in demonstrating that the young caterpillars
actually possessed the postulated longitudinal stripes.

[Recently the advances made in biochemistry have supplied
valuable corroborative evidence of the theory of phylogenetic
relations. It is a remarkable fact concerning the blood of two
different animals that the red blood corpuscles of one individual
are destroyed by the blood plasm of the other. Decomposition
becomes more difficult in proportion as the relation of the species
becomes more distant, whilst in closely related animals or in
individuals of the same species no injurious effects are observed.
After numerous experiments Friedenthal proved that the blood-
serum of man destroys the red corpuscles of the baboons and
mandrills, but not those of the anthropoid apes. Conversely, the
red corpuscles of man were in most cases decomposed by the
serum of the lower monkeys, but remained entirely unaffected by
the blood of the higher apes.

THE EVOLUTION THEORY 137

We have already become acquainted with the valuable faculty
of the living organisms to generate for each toxin introduced an
antitoxin which combats the dangerous effects of the former.
It is owing to this faculty that the organism is able, by a gradual
increase in the doses, to become immune against the most
dangerous poisons. When we inject the blood of a distant species
certain elements are formed in the body, called ' precipitins,'
which under a continued systematic treatment with a certain
kind of blood are capable of accumulating in the blood of the
animal under treatment. These ' precipitins ' possess the
faculty of causing in the blood- serum of the species from which
the blood for the injection was taken remarkable precipitates,
whilst leaving unaffected the blood of all other animals.

If, for instance, we inject at regular intervals of two to
six days ten ccm. of blood from a fowl — the dose may be
increased after the rabbit has become accustomed to the foreign
blood — we shall obtain at last a rabbit serum (rabbit-fowl-
antiserurn) which mixed with fowl's blood causes in it strong
turbid precipitates. But the fact chiefly of interest to us is that
the rabbit serum has this action not only upon fowl's blood, but
also, though in a lesser degree, upon the blood of related species.

This faculty has been made by Nuttall and others the starting-
point of a series of comprehensive biochemical experiments, in
order to ascertain the degree of relationship of various different
species. These tests generally corroborate the results which
have been obtained by a study of comparative anatomy and
phylogeny. It must, however, be understood that it is in no case
possible to say how closely the examined animals are related ;
it is only possible to ascertain from the strength of the precipitate
that an animal A is more nearly related to an animal B than to
an animal C.

By an ingenious method Nuttall was able to compare the
degree of relationship simply by measuring the volume of the
precipitates. Thus an anti-sheep serum, obtained from a rabbit
by systematic injections of sheep's blood, when mixed with
sheep's blood yielded a strong precipitate the volume of which
was assumed to equal 100 per cent. Mixed with bullock's blood,
the precipitates now equalled only 80 per cent., with that of the
antelope 50 per cent., reindeer 30 per cent., pig 20 per cent.,

138 LECTURES ON BIOLOGY

horse 16 per cent., cow 12 per cent., dog 7 per cent., and kan-
garoo only 5 per cent. Correspondingly, anti-pig serum indicated
in the pig 100 per cent., in the para 14 per cent., cat 14 per cent.,
dog 13 per cent., sheep 13 per cent , and kangaroo 5 per cent.
An anti-cattle serum produced reactions in other species of cattle
and, to a lesser degree, in sheep, goats, antelopes, and gnus.
We are, therefore, probably justified in assuming that the phylo-
genetic degree of relationship of these animals corresponds to the
volume of the precipitates.

In like manner Nuttall endeavoured to shed light upon man's
position in the zoological system, and it is remarkable that these
experiments not only led to the same result as those of Frieden-
thal, but also agreed with Haeckel's genealogical tree of the
princates. For it was shown, as was indeed expected, that an
anti-man serum produced the greatest reaction in individuals
of different races, weaker results in the anthropoid apes, still
less results in guenons and baboons, and least of all in the
monkeys of the New World, the Platyrhini. Finally, in the
lemurs a very faint turbidity occurred only very rarely and
under employment of very strong serum. These experiments
seem to indicate that the biochemical differences between
man and the chimpanzees and gorillas are less than those
between these anthropoid apes and their lower relatives. Though
the blood-reaction alone does not supply an incontestable proof
of the phylogenetic relation of the various animal species, it
forms a most valuable link in the chain of evidence.

The biochemical method, it may be mentioned in passing, is
of the utmost practical value in the detection of crime, for it is
often necessary to prove whether blood-stains on a floor or the
clothes of an accused person are those of a human being or animal,
a question upon the answer to which often depends life and death.
To-day such proof is easily furnished, for all that is necessary is
to dissolve the blood-stain in water and add a small quantity of
anti-man serum. If the blood is that of man, a turbid, flaky
precipitate appears after a few minutes, if that of an animal, no
reaction takes place. So accurate, indeed, is the effect of the
serum that it is even possible to demonstrate with it the origin
of blood which is already in a state of putrefaction, or has been
in a state of desiccation even for a period of several years. This

THE EVOLUTION THEORY 139

method was therefore adopted many years ago in forensic
medicine in Germany and most other civilized countries. As
it is further possible to detect in sausages the presence of
horse and dog flesh, biochemistry is a powerful adjunct in the
prevention of adulteration of this kind of food.

Instead of blood other albumens may be used. Thus
Uhlenhut showed that the blood-serum of a rabbit which had
received at regular intervals in the abdominal cavity injections of
solutions of albumen from a fowl's egg caused a strong precipi-
tate if mixed with a solution of such albumen, but no precipitate
if mixed with a solution of any other albumen. Thus it is now
possible to distinguish between the albumen of the various
species of birds. These precipitates may still be distinctly
observed in dilutions of 1 gramme of albumen to 100 litres of
water. Ascoli states that he has observed reaction in dilutions of
1 in 1,000,000. An interesting fact in this connection is that von
Hansemann succeeded by the biochemical method in determin-
ing the origin of mummies more than five thousand years old.

I have now completed my task of giving in large outlines
the facts that prove the gradual evolution of the organic world.
We have seen that palaeontology, comparative anatomy, the
history of evolution, and finally, the investigations of bio-
chemistry, all form a chain of evidence for the truth of the
Evolution Theory in which every link is firmly forged.

140

CHAPTER VII.
THE FACTOKS OF EVOLUTION.

WE may now take the Evolution Theory as proved. Our
opinions may alter in detail concerning the phylogenetic relation-
ship of the organic world and suffer modifications, but the hypo-
thesis that the simpler preceded the more complicated, the
lower the higher, chronologically and causally, must now be
regarded as an established fact of scientific research.

It is, however, a different question when we ask what causes
have effected these modifications and development of the species.
The most comprehensive and thorough attempt at explanation
is made by the so-called Darwinian Theory. In many circles,
in particular in those who are hostile to the evolution theory
— 'the monkey hypothesis,' as it is sarcastically called — the
Darwinian theory and the doctrine of evolution are regarded as
identical, and it is assumed that if the ingenious hypothesis of
the great British naturalist, the fundamental principle of which
is the theory of natural selection and the survival of the fittest,
can be refuted such refutation would at the same time dispose
finally of the theory of descent.

We cannot protest too strongly against the confusion of the
teachings of Darwin with the doctrine of evolution. They are
strictly separate, for the hypothesis of natural selection takes the
evolution of organic life as proved, and merely claims to give an
explanation of the forces which were active in the changes of
the organic world, and of the methods by which the origin of the
higher organisms from simpler ancestors may have taken place.
Even if the doctrine of this great naturalist should be proved
to be erroneous, such proof would in no way affect the accuracy
of the evolution theory. All that would be attained in that event
would be the admission that science has so far been unable to

THE FACTORS OF EVOLUTION 141

give a satisfactory uniform explanation of the manner and method
by which the changes and development in the organic world
have taken place. But the doctrine of Darwin in its main points
still holds the field unrefuted, and honest criticism has found
itself restricted to pointing out the many weak parts of the
hypothesis of natural selection and to indicate that Darwin, and
in particular his successors, greatly over-estimated the importance
of natural selection, and that there are other causes which have
played a significant part in the development of the organic
world. But whatever may be the verdict of coming genera-
tions concerning the doctrine of this great man, it cannot detract
from his merits ; no one can rob him of the fame that it was
he who by his great and comprehensive work carried the Theory
of Descent to universal recognition and final victory.

What are animal species? Are they really immutable, final,
natural units, as Cuvier states, or are they more or less arbitrary
definitions made by man in order to facilitate the study of
nature. Just as easy as it is strictly to define any species so
long as only a few specimens of it are known, so difficult and
yet even impossible becomes the task of an accurate definition
when we have to deal with numerous individuals, especially if
they belong to different geographical districts. It will always
happen that some will fit easily into the system whilst others
will diverge on many points.

We see everywhere in Nature that the descendants, the
children, are more or less like their parents, and exhibit
generally the same specific features as the latter. On the other
hand, there is no doubt that no child is entirely and in all details
like its parents ; it will always differ from either to a greater or
lesser degree. In addition to the character of the species, it
bears its own individual character. This seems natural enough
when we remember that every higher organism owes its
existence to two different individuals and inherits charac-
teristics both from its father and mother.

These individual fluctuations and deviations are called varia-
tions. Speaking generally, they move within very narrow limits,
revolving, as it were, round a fixed centre, the type of the
species. But under certain circumstances, when by accident or
owing to external unknown influences such variations accumulate

142 LECTUEES ON BIOLOGY

through various generations towards a certain direction, there
may arise individuals which differ from the rest of the species
and their parents in so many points and so widely that we may
no longer regard them as sub-species or variations, but must
give them the rank of a separate species. If this assumption is
correct, variations must claim our utmost interest as the starting
point of the genesis of new species, and it would mean an
enormous progress towards the solution of the mystery of species-
formation if we succeeded in discovering the causes that lead
to it.

Nowhere in the whole of Nature do we find such obvious
inclination to variations and race-formation as in domesticated
animals and plants. Long neglected by science, because to
deal with these 'unnatural' products of human self-interest
or fancy was almost regarded as a desecration of pure science,
they were chosen by Darwin as the basis of his epoch-making
investigations.

The cause of the contempt felt by older investigators was of
a very different nature. Dogmatic supporters of the theory of
the immutability of the species did not know what to do with
these ' degenerate ' creatures which would in nowise fit into
their system. As happens very frequently, their mask of
contempt concealed a very real fear. No one denies to-day that
domesticated animals and plants are equally with their free-
living relatives entitled to our fullest interest, for the history of
domestication is the record of one of the most marvellous,
though unintentional, scientific experiments ever conducted.
Where is there another experiment undertaken with such a
wealth of material, and carried on during thousands of years ?

There is no doubt that the first individuals of the human
species had no domesticated animals. The animal kingdom was
to the primitive man of interest only in so far as he had to
defend himself against the attacks of the larger carnivorous
animals, or as some supplied him with food and warm coverings.
Even throughout the Stone Age domestic animals appear to
have been unknown in Europe, for the most ancient and lowest
form of the descriptive arts, that of the cave-dwellers, shows
us only pictures of beasts of the chase scratched upon bones
with the point of a flint. Domestication is therefore obviously

Plate o

Domesticated Pigeons.

THE FACTORS OF EVOLUTION 143

preceded by the chase and capture of animals in the wild state.
At first they were probably captured without any idea of domesti-
cating them for the purpose of deriving certain advantages, and
only gradually primitive man began to perceive the uses to which
animals might be put. Thus there developed out of an originally
hostile relationship in the course of time an alliance from which
both parts derived advantages. Man gave to certain animals
shelter and food and derived in return certain services. In true
perception of the high value of domestic animals to civilization,
their treatment, with a few deplorable exceptions, has always
been humane ; in many countries they have even been regarded
with feelings of veneration and worship.

The majority of the species of our domestic animals are very
old, many thousand years having elapsed since they began to
1 live with man/ We are, therefore, unable to judge of the pro-
cess of domestication by direct observation and shall be in most
cases referred to tradition, hypotheses, and experiments in order
to obtain a clear view of their history. A familiar exception is
the African ostrich, which has recently been thoroughly domes-
ticated, whilst gradually becoming extinct as a free-living bird.
It is, therefore, no matter of surprise that of a few domesticated
animals we know only their wild prototypes. Nevertheless, it
would be a mistake to assume that as a result of a long period
of domestication the wild ancestors have either become extinct
or have been gradually gathered up into man's stock of domesti-
cated animals, for research has been able to prove the existence
of wild ancestors in many cases where they were thought to
have become extinct, and it was also shown that the different
races of many domesticated animals, as for instance dogs, do
not descend from only one wild ancestor but from several. The
exact number of such ancestors investigation has not been able
to ascertain. In other cases the numerous breeds of domestic
animals have been traced back to one wild ancestor.

Darwin has proved that, for instance, our domesticated
pigeons have all descended from the rock-pigeon (Columbia
livia). If we glance at the coloured plate (Plate V.) and notice
the sharply differentiated birds, we find it difficult to agree with
this statement. Nevertheless, it has been clearly proved by the
fact that in the crossing of widely differentiated races of pigeons,

144 LECTUEES ON BIOLOGY

each of which is again widely differentiated from the rock-dove,
some of the descendants revert to the wild stem-form, exhibiting
distinct characteristics of the species Columbia lima.

The rock-dove is found throughout the greater part of
Europe, North Africa, and wide stretches of Asia. Its colour
resembles that of the ordinary bluish-grey field pigeon, but
is distinguished from it by the bright metallic sheen on the
feathers of the neck and breast, and a more delicate and elegant
shape. It lives socially in large communities, like all other
animals suitable for domestication, builds its simple nest in
inaccessible crevices of rocks out of dry twigs, straw, and fibres,
and, like its domesticated descendants, lays regularly two eggs,
and has a strong inclination against settling on trees. It shows
the same peculiar nodding of the head and clapping of the
wings, and delights in large swarms to circle round and round
high up in the air. By what magic has man been able to
produce these widely divergent birds ? We need only think
of a carrier, fantail, pouter and turbit to admit that the dif-
ferences between these various breeds are extraordinary, more
striking than the differences of many natural species — for instance,
hare and rabbit, the common nightingale (Daulias luscinia) and
"sprosser" (Daulias philomela), &c. The distinctive features
are indeed so great that if he met them in the free state no
systematist would hesitate to describe each of the mentioned breeds
as well-defined, distinct species If, nevertheless, we continue
to describe the different forms of our domestic animals only as
varieties and not as species it is merely due to force of habit,
for there is no valid reason why they should not be regarded
as distinct species. If by nothing else, they prove their right
to the rank of species by the fact that as long as the protective
conditions remain under which they came into existence they
will continue to breed straight like any other natural species.
The magic which has effected this apparent miracle is called
artificial selection. (Compare Plate V.)

The variability and plasticity of the pigeon is most remark-
able. A skilled and persistent breeder may in the course of a
few generations produce almost any desired variation, and there
is hardly a part of the body which cannot be altered at will.
In the carrier-pigeon the bill is very long and straight, and

THE FACTOKS OF EVOLUTION 145

covered by a thick red growth of the cere ; in the bagadottes
it is crooked, like an awl. The beak of the turbit becomes, in
extreme cases, so short that the birds are no longer able to
feed themselves, but must be fed artificially. The pouter carries
its body upright. Its throat is of an enormous size, and the
bird is able to blow it out with air. The fantails possess, instead
of the normal number of twelve tail-feathers, thirty to forty,
which are carried upright spread out like a fan. Some pigeons
have smooth plumage, others rough ; some have a feather-
mantle or a cap, others a ' white beard ' or a ' shirt-collar.'
Further, the size of their body, the number and size of ribs and
tail-vertebrae, the formation of internal organs, differ widely
ia the different breeds. In the home of pigeon culture, England,
many breeders have attained such a state of skill that they are
able to inbreed to order, in a very short time, any desired feature.
It is even possible by breeding to alter mental qualities, i.e., the
delicate structure of the brain, as is proved, to mention only one
instance out of many, by the well-known tumblers. Fanciers
have succeeded to breed into this pigeon the habit of ascending
to a considerable height and then turning a complete somer-
sault or several somersaults in the course of their descent. A
similar phenomenon is observed in the Japanese dancing mice,
which will sometimes turn round for several minutes exactly like
a live spinning top.

Let us now consider the question of the methods to be
adopted by a breeder in order to obtain a certain feature. Let
us suppose that he intends to breed a carrier-pigeon, that is, a
bird with an extreme growth of wattle round the bill and eyes.
A prize monstrosity such as this is pictured on the coloured
plate. First of all, the breeder will look around his stock
of pigeons, pick out those birds which either possess these
characteristics, or at least one of them, in a more pronounced
form than their companions and breed from them. As inborn
features are inherited, as the descendants are in a sense the
' sum of both parents,' there will in most cases be found among
the young pigeons some birds which possess more strongly
developed wattles than their parents. These birds will now be
selected for further operations, and by carrying on this process
of selection for several generations the breeder will frequently

10

146 LECTUKES ON BIOLOGY

succeed in a comparatively short time in producing, more or less
exactly, the desired results.

Suppose, now, that the breeder desires to impart to his
carrier-pigeons a certain colour. He will, therefore, proceed in
precisely the same manner, selecting among his carriers, for
breeding purposes, only such as exhibit the desired colour in the
most pronounced manner. Continuing along this line, a skilful
breeder will gradually produce pigeons which are widely differen-
tiated from their parents on almost every point. The production
of new races is, therefore, not, as is generally assumed, the result
of the crossing of different animal species, though crossing may
lead to the formation of new races, but by the patient, deter-
mined accumulation through numerous generations of slight
variations.

What is said here of pigeons applies to all other domestic
species. We need only mention the breeds of dogs, sheep bred
for wool or flesh, rabbits with long lop-ears or short erect
ears, domestic cattle and pigs, and many other products of
domestication.

It must, of course, be understood that only those parts and
characteristics can be subject to conscious variation that may
be controlled from outside. It is, therefore, frequently found
that domestic species exhibit, together with widely divergent
external characteristics, a surprising similarity of internal
structure. It is obvious that by artificial selection generally
only those parts are transformed the transformation of which is
desired, whilst the rest of the body retains its original qualities.
For instance, in the various breeds of strawberries and goose-
berries flowers and leaves are alike, but the fruits differ both in
appearance and taste. In the numerous species of cabbage, the
flowers, which are of no account to man, are uniform, but the
edible leaves show a great variety. In tulips and other garden
flowers the process is reversed ; flowers are widely differentiated
in form, size, and colour, whilst there is no difference between
the leaves. Finally, in the various breeds of silk-worms, cater-
pillar, pupa, and imago are surprisingly alike, but the cocoon —
that is, the part which is of value to man — has been greatly
transformed by breeding.

How enormously artificial selection may increase certain

THE FACTORS OF EVOLUTION 147

features may be illustrated by a few instances. The tail-feathers
of a Japanese "phoenix" attain a length of 4 metres; good
laying breeds like Wyandottes and Hamburgs may reach an
annual production of 200 eggs. Toulouse fattening-geese fre-
quently reach a weight of 15 kilogrammes; and finally the
merino sheep, in particular the so-called electorals, produce wool
at the rate of 6,000 hairs per square centimetre, whilst ordinary
breeds have only about 1,000.

In most cases the breeder will be unable to modify, by means
of artificial selection, any one organ without affecting, even
though it be undesirable, other parts of the body. If, for
instance, it were desired to breed goats with such enormous horns
as are carried by the Alpine Steinbock (Capra ibex] the process
would inevitably lead to a modification of the entire organization
of the affected animals. The skull would become thicker and
stronger to be able to carry the heavy burden of the horns ;
similarly the muscles of the neck and back would increase in
strength, and the neck tendon would become thicker ; the spinous
processes of the cervical and dorsal vertebrae would, therefore,
become longer and stronger in order to meet the greater demands
made upon the muscles of the neck. Further, the skeleton parts
as well as the muscles of the anterior legs would adapt themselves
to the greater burden, and this, again, would react upon the
nervous and vascular systems. We see, therefore, that even the
transformation of a comparatively unimportant part of the body*
the horn, necessitates a corresponding transformation of the rest
of the body.

Every organism is, as it were, in a state of fluctuating
equilibrium. Its single parts are mutually dependent, so that
each modification of one organ disturbs the harmony of the
whole, and produces compensating modifications in other parts.
This remarkable interdependence of different systems and parts
is called the correlation of the organs, an important factor in the
formation of new species and breeds.

A remarkable case, in which the correlation of the various
parts may be clearly perceived, was recently observed in a deer
park. Shortly after a large red deer had thrown his antlers
he fractured his left anterior leg. It was set with every possible
precaution, and the healing progressed favourably. But whilst

148 LECTUKES ON BIOLOGY

a normally formed antler developed on the right, the sound side,
no formation whatever took place on the left, the side of the
fractured leg. It was plain that the organism had used the
material originally destined for the left horn for the more
necessary object of repairing the injury to the leg. In the
following year the deer grew once more two well-formed antlers.

In many other cases the correlation of the different parts is
easily understood. Thus we find that a strong elongation of the
vertebral column is accompanied by a shortening of the extremi-
ties, whilst, on the other hand, in animals with well-developed
extremities we observe a shortening of the vertebral column. It
is only necessary to mention here newts and frogs, lizards and
blind-worms or snakes, and carp or tench and eels. A strong
formation of the anterior extremities, as it is observed in the
frog, ostrich, kangaroo, jerboa, and man is accompanied by a
weaker development of the anterior extremities. In all these
cases use and disuse can be held responsible for the compensation.

Of the remarkable correlation of the sexual organs and the
so-called secondary sexual characters no explanation has so far
been forthcoming. The best known instance is that of the
famous papal singers whose marvellously pure and powerful
voices attract numberless strangers to Rome ; indeed, the im-
pression caused by these wonderful sounds is unforgetable.
These singers are men whose generative glands were in their
infancy removed by a surgical operation. As a result the throat
retains its small boyish formation ; there is no breaking of voice,
and the eunuchs retain into their ripe age the clear, high timbre
of a boy's voice, which excels in power and beauty of tone every
woman's voice. Though the castration of boys for this purpose
was forbidden by more than one papal bull it has been practised
until quite recent times. The destruction of the sex-organs does
not only affect the formation of the voice but also of the entire
body. The body of the eunuchs has a feminine form, they
incline to be corpulent, do not grow a beard, but have very
pronounced mammals. More striking still is the effect of castra-
tion upon the mind, for eunuchs are generally indolent, stupid,
and treacherous. Castration is frequently practised in domesti-
cated animals, in horses, to take the ' fire ' out of them, and in
rams, bulls, boars, and cocks, to prepare them for fattening.

THE FACTORS OF EVOLUTION 149

Other strange phenomena in this problem of the correlation of
the parts are that dogs and cats with white coats and blue eyes
are always deaf, and cats with yellow, black, and white stripes are
always females. It is a subject in which there is much room for
investigation.

But let us return to artificial selection. We have already
seen that three factors must co-operate in the production of a
new breed : (1) Variability ; (2) conscious selection by the breeder
of such variations towards a certain direction, and (3) the ability
of the parental organisms to transmit their physical and mental
qualities in a more or less perfect degree to their descendants.
These three factors are the sine qua non of artificial selection : if
•only one of these conditions is absent the formation of a new race
is not possible.

Darwin's daring doctrine which he formulated on the basis
of his experiments in artificial selection was this : Exactly the
same causes which we see at work in the formation of new
•species and breeds of domestic animals effect in free Nature, too,
the transformation of the organic world and the formation of new
species. But instead of the artificial breeder there is another,
natural, mode of selection, the battle of the organisms for
the means of livelihood, the struggle for existence. But whilst
in artificial selection animals are frequently bred possessing
characteristics which are of advantage to man in one way or
another, but of injury to the animals themselves, sometimes to
such an extent that many domestic animals are to-day unable to
live in a free state without the protection of man, the struggle
for existence tends to select those individuals which are best
fitted for the conditions of their existence ; the gradual develop-
ment, adaptation, and perfection of the ^organic world must,
therefore, be a necessary consequence of natural selection.

That heredity is as highly developed in animals living in a
free state as in domesticated animals requires no proof, for it is
notorious that the various animal parents produce offspring
which, with the exception of slight individual variations, are their
very images.

Less obvious is the second essential condition, variability,
though accurate investigation will always succeed in demon-

150 LECTURES ON BIOLOGY

strating its presence. Whilst we are able to observe at the first
glance the individual characteristics of those belonging to our own
race, and even recognize after a lapse of years persons whom we
met perhaps only once before, all persons who go to a country
peopled with a different race find it very difficult to distinguish
between the natives. At first it seems to them as if all people look
exactly alike, at any rate those of the same age and sex, or those
that are not accidentally marked by some distinguishing feature.
Only long habit and practice enable the eye to discern the
personal distinctions of these foreign races, and wre become
gradually accustomed to discriminate as easily between those
individuals as between our own countrymen. Still more difficult
it is of course to discern individual distinctions in animals which
are so much more differentiated from us in their organization.
Here the individual disappears, as it were, entirely behind the
type, and only a trained eye is able to perceive individual varia-
tions, especially in the lower animals. If, for instance, we
observe with an untrained eye a large flock of sheep we shall
only be able, at the best, to distinguish the bucks, the ewes, and
the lambs, but all the other sheep would seem exactly alike,
But a good shepherd knows each single individual of his flock,
though it may often number thousands. The large sheep-
breeders of Australia possess this skill to a remarkable degree,
and the rapidity and certainty with which they pick out a
certain animal, though it does not appear to be distinguished by
any special mark, is almost incredible. In the same manner
many gamekeepers are able to recognize every head of game in
their preserves.

Thanks to the careful investigations and laborious measure-
ments which are now undertaken on our biological stations
we possess conclusive proof that even important organs may be
subject to great modifications in different individuals living
under identical conditions. Professor Heincke, of the biological
station in Heligoland, made such investigations with the herring
and was able to demonstrate a most extraordinary deviation in
the structure of fishes inhabiting the same district. To men-
tion only one instance, he showed that the number of vertebrae
fluctuated in different fishes between fifty-three and fifty-eight.
That certain parts in man are subject to similar fluctuations we

THE FACTORS OF EVOLUTION 151

have already heard on a previous occasion. I will here only recall
the great differences in the formation of the caecum with its
vermiform appendix. Whilst the appendix possesses in many
persons a length of 10 to 20 cm., it is in others entirely absent.

Let us now consider the third factor, the natural breeder, the
struggle for existence. It is a rule without exception in Nature
that both animals and vegetables produce more germs than grow
to maturity or become capable of reproduction. If all the germs
of only one species of animal remained alive and reached maturity*
the world would soon be too small, and the quantity of food
insufficient for supporting this one species, for organisms multi-
ply in geometrical progression and thus would soon reach count-
less numbers. The well-known illustration of the chess-board
will best explain this rapid mode of multiplication. If we place
upon the sixty-four squares of the chess-board grains of corn in
the manner that we put one grain on the first, 2 on the second,
4 on the third, 8 on the fourth, and so on, we shall have to place
1,024 on the eleventh square, and on the twenty-first more than
a million. The thirty-ninth would require one billion grains, and
the last square a number of such length that is impossible to
pronounce it. It has been calculated that its volume would be
sufficient to cover the entire earth surface with a thin layer of
corn. If all the germs produced attained maturity and the power
of reproduction the multiplication of organisms would proceed in
the same terrifying manner. The elephant is the best known
example of an animal with a low ratio of increase. According to
Darwin, its power of reproduction begins with the thirtieth year
and continues to the ninetieth. As the elephant-parents have to
care for their young a considerable time before these reach the
state of independence, they produce in this long period only
about six young. If we assume the average life of an elephant
to be a hundred years we should have, if all the young elephants
grew up and reached the power of reproduction, after 740 to
750 years about nineteen million elephants as descendants of the
first pair.

Among the minute organisms, the bacteria, the comma
bacillus, the dreaded cause of the Asiatic cholera, multiplies
under the most favourable conditions about every twenty minutes
by fission ; this would at the end of one day give the enormous

152 LECTURES ON BIOLOGY

figure of 1,600 trillion descendants of one bacillus. According
to Fischer, this mass of bacilli would, in spite of the extreme
minuteness of the individuals, yield in the dry state a weight
of 100,000 kilogrammes. It would therefore require a gigantic
experiment to let only one of these minute bacilli grow and
multiply during twenty-four hours to the fullest of its ability.

But long experience has shown that the number of the
individuals of a species within a certain definite area maintains
itself during the course of great periods usually on the same level,
however great may be the temporary fluctuations. The supposi-
tion is, of course, that the conditions of life remain the same.
Devastating epidemics, famine, great floods, long periods of frost
or drought may, of course, reduce the species in a short time to
partial or actual extinction. On the other hand, we know from
numerous instances that animals as well as plants which are
suddenly introduced into new and favourable conditions multiply
often in a most extraordinary manner. No one imagined when
the first rabbits were brought from Europe to Australia what
a terrible scourge they would some day become to that country.
Threatened by no natural enemy, and finding food in super-
abundance, the animals increased at such enormous ratio that
they soon became a veritable pest, for the extermination of which
the Government in vain promises large premiums.

Cattle and horses belong to the animals with a low ratio of
reproduction. If it had not been proved beyond doubt we should
not believe it possible that the enormous herds that to-day
inhabit the wide pampas of South America are the descendants
of European races. The vegetable kingdom offers many other
similar instances. The plains of La Plata are for miles covered
with artichokes and another large species of thistle, yet it is
not so long ago that the first plants were brought from Europe.

As with the increase in the numbers of the species there is
observed a corresponding increase in the desire for reproduction,
it might be thought that multiplication can proceed without
limitation. That is, however, not so ; for we find that very soon
an average number is reached which constitutes the maximum
even for the changed favourable conditions ; increase beyond that
point is rigidly prohibited by the conditions of life. If now this
normal number of a species is not to be exceeded there must not

THE FACTOKS OF EVOLUTION 153

remain alive more than two descendants of each pair of parents.
The oak or yew, the age of which is said to reach a thousand
years, need even produce only one germ in order to maintain
their species. As the rate of fertility of a species is generally
a fixed factor, it is easy to conclude from these two data how
many germs of life must be doomed to destruction if the normal
number is to be maintained.

Of the six young of a pair of elephants it is therefore neces-
sary that four should die before they have reached maturity.
Considerably higher is the destruction in a pair of crows, for
supposing that crows hatch each year on an average twice, each
time five young, and continue to do so for twenty years, then
of these 200 descendants it is necessary that 198 die a premature
death. An even more terrible destruction we see in organisms
with high fertility. A carp, for instance, produces every year
about 250,000 eggs, and remains capable of reproduction for at
least fifty years. Of the 12,500,000 descendants, therefore,
12,499,998 perish as eggs, or embryos, or immature fish, for
only two must reach maturity to reproduce their species. If out
of the 100 million eggs produced by a sturgeon only one million
developed into mature females, and if this increase proceeded in
a similar manner through four generations, the fourth generation
would produce an amount of caviare greater than the entire
volume of the earth. But this wonderful event will, unfortu-
nately, never take place, for it is possible only for a very limited
number of individuals to exist ; all others are doomed to perish.

Still more remarkable is the waste of life-germs in many para-
sitic worms, such as tape- worms, liver-flukes, &c., which produce
at one time up to 100 million eggs. In view of this fact it seems
almost inconceivable why parasites with such tremendous fertility
do not molest man and animals whom they infest to a far higher
degree, and why it is possible that such innumerable germs should
be destroyed almost as soon as they are born. But if we consider
the perilous life-cycle with its hundreds of dangers that beset
these parasites from the egg to the mature worm we shall under-
stand why among the many millions only a few favoured ones
are able to escape unscathed. The liver-fluke, Fasciola hepatica
(fig. 45), lives in the bile-ducts of sheep, cattle, pigs, &c., and
occurs sometimes in man. By stopping up the bile-ducts and

154

LECTUEES ON BIOLOGY

preventing the flow of bile these parasites cause a disease,
called distomiasis, which gradually brings about the death of
the host. In order that the eggs of the liver fluke may develop
they must first of all reach the intestinal canal of their host,
whence they are liberated with the faeces. But only those

I

FIG. 45. — LIPE-HISTOEY OF THE LIVER-FLUKE (Fasciold (Distomum) Jiepatica.)

(1) Ovum ; (2) free- swimming ciliated larva ; (3) sporocyst with young redise
(second larval form) ; (4) redia with cercarise (third larval form) ; (5) adult free-
swimming cercaria ; (6) adult sexual fluke.

which chance carries to a ditch or pool will reach maturity.
For having arrived in the water there emerges from the egg
a larva which by means of its cilia swims about until it happens
to meet a certain water-snail. Forthwith it bores its way into
the snail, sheds its cilia-dress, and changes in the interior of

THE FACTORS OF EVOLUTION 155

the mollusc into a so-called sporocyst. By budding, the sporo-
cyst produces a second larval form, the redia, and this again
a third, the cercaria, a little trematode furnished with a strong
paddle-tail.

The task now before the cercaria is to pass from the snail
into the open. Having succeeded in this, it swims about in the
water searching for a new intermediate host, adopting as such
usually small crustaceans or insect larvae. If chance should
carry the cercaria into the body of a suitable animal it throws
off the tail which has now become useless, surrounds itself with
a firm shell, and changes in the cyst into the young, but as yet
sexless, liver-fluke. Finally, it is necessary that the second inter-
mediate host containing the parasite is eaten by the final host,
a vertebrate. This happens if sheep and cattle graze on wet
meadows. The arthropod is digested by the juices of the
stomach, the cyst is dissolved, and the young liver-fluke is
liberated from its prison.' All that now remains to complete its
metamorphosis into a mature trematode is to invade the bile-
ducts : with that it has reached the end of its journey and can
proceed to the reproduction of its species. We can now readily
understand that the fertility of species must stand in strict pro-
portion to the dangers to which the development of its individuals
is exposed.

In addition to the vicissitudes of the life-cycle, the question of
food and of natural enemies play an important part. For instance,
the butterflies and caterpillars of a district are chiefly kept down
by the number of insectivorous birds, in particular the tits, by
ichneumon flies, and numerous other parasites which prey upon
them and their brood. A severe winter and want of food, causing
destruction among their natural enemies, is usually followed by
an enormous increase in the caterpillar pest. Thus a plague of
one of the most noxious insects of our forests, the ' nun '
(Psilura monacha), is usually due to a previous reduction in the
number of its natural enemies. What enormous dimensions this
pest may under favourable conditions assume was shown in
1890-1891 when, in the kingdom of Bavaria alone, about 12,000
hectares of State forests were destroyed, and over £125,000
spent on exterminating this pest. Fortunately, an increase in
the pest of caterpillars always brings about, by the super-

156 LECTUEES ON BIOLOGY

abundance of food, a more rapid increase in the number of
their natural enemies. Then the reverse process begins. The
caterpillars and butterflies, threatened by numerous enemies, are
rapidly reduced in number ; their enemies, consequently, suffer
want of food, and their increase becomes in turn restricted ;
thus the pendulum swings to and fro.

But it is not always easy to determine the factors that main-
tain the balance, as is proved by Darwin's classic instance,
which showed that the fertility of the daisy and red clover in
any district is determined by the number of cats in that district.
It is well known that insects play an important part in the
fertilization of many flowers as carriers of pollen. Indeed, many
flowers have so much adapted themselves to certain species of
insects and their visits that only these can effect fertilization.
Without fertilization there can be no seed. According to
Darwin the pollination of red clover and daisies is exclusively
effected by the humble-bee, because only these insects are able,
thanks to their long proboscis, to reach the object of their
desire, the nectar. Thus a hundred clover plants yielded under
normal conditions two thousand seven hundred seeds, whilst a
like number, which had been protected against the visits of the
humble-bee, did not yield a single seed. The number of the
humble-bees of a district depends chiefly upon the number of
field-mice which on their subterranean wanderings rob and
destroy many nests. Finally, cats are probably the most zealous
persecutors of field-mice, and their number is, therefore, the
determining factor in the number of field-mice. There are, of
course, other enemies, the mouse-buzzards, owls, storks, &c.,
who play a not unimportant part, but in this way the fertility
of clover can, nevertheless, be traced directly to the number
of cats.

Though these instances show that the various species of
animals exercise a controlling effect upon each other, this form
of the struggle for existence is not that to which Darwin ascribes
the rdle of the natural breeder. Natural selection can only take
place if among the numerous descendants of one species
generally those individuals survive, reach maturity, and transmit
their individual characteristics to their offspring, which in the
structure of their body are best fitted to the conditions of life,

THE FACTORS OF EVOLUTION 157

i.e., if always those are selected which have proved most
successful in the fight for existence. The result of such natural
selection must be a constant improvement of the organic world.
That this survival of the fittest is actually the rule in Nature
Darwin endeavoured to prove by an enormous mass of evidence.

Those who are accustomed to look around with open eyes
will always be surprised anew at the high degree of fitness
observable in every organism. Most distinctly does this far-
reaching adaptation and fitness of organisms appear in the
many cases of protective coloration and mimicry which have
become known during recent years.

Formerly it was thought that such adaptation could only be
understood on the basis of a teleological principle. Like an
engineer about to construct an engine, who first calculates and
draws on paper the single parts and their proportions to each
other, and then proceeds to transform the theory into reality,
thus the world-creating spirit, according to teleology, first
designed the plan of his mighty work and then created the
world. But there is no doubt whatever, nor need of specific
proof, that the means employed by Nature in the evolu-
tion and, speaking figuratively, in the construction of its
organisms, when compared with human planning, can only be
described as the result of blind chance. Wherever we may
look, we shall find in Nature an extraordinary waste of life.
Thousands, even millions, of descendants are produced ;
thousands and millions must prematurely perish, and only a few
reach full maturity. Indeed, the destruction of life-germs, the
failure is the rule, and the attainment of the aim, the natural
development, the rare exception. "If," says Friedrich Albert
Lange, " a man standing on a large heath were to fire millions
of barrels in all directions in order to shoot one hare ; if in
order to enter a locked room he were to buy ten thousand
different keys and try them all one after the other ; if in order
to procure shelter for himself he were to build a town, choose
one house, and leave the rest to become the play of wind and
weather, no one would call his action rational ; still less must
we look in his procedure for hidden reasons and a superior
wisdom."

There is another point : the different species of animals and

158 LECTUEES ON BIOLOGY

vegetables when observed individually exhibit undoubtedly a
surprising fitness, but we see a different picture when we regard
them as a whole. The large majority of animals are herbivorous ;
day after day they destroy enormous quantities of nourishing
plant life in order to exist. To the plant kingdom, therefore, the
existence of herbivorous animals represents the very extreme
of unfitness. The vegetarian animals again serve as food for
the great number of carnivorous animals, and from their point
of view the existence of the flesh-eaters is most undesirable.
Even if we take up a purely anthropocentric position and regard
man as the measure of all things and the final aim of creation,
and assume that everything has been created for his purposes
we cannot, even in that case, escape from contradictions. For
what fitness is there in the fact that intestinal parasites, trichinae,
fleas, bugs, and mites, as well as the enormous armies of bacteria
and parasitic protozoa, are so perfectly adapted for their parasitic
mode of life that they scorn all medical art, torment the ' lord
of creation,' and exact each year a toll of millions of human
lives ? Is it not rather blasphemy to assume that an all-loving,
all-powerful Creator had designed the animal and vegetable
plant world so perfectly only to let them loose against each
other in a fierce relentless struggle ?

Finally, whilst, generally speaking, the structure of organisms
is well adapted to the conditions of life, it is far from perfect, for
we have already met in man organs which actually endanger
his life.

It is Darwin's great merit that he for the first time directed
the attention of the scientific world to the enormous waste of
life and discovered in this apparently blind play of chance a
deeper reason. The theory of the struggle for existence and
the doctrine of a natural selection of the fittest in this struggle
enables us to explain the origin of numerous adaptations in
nature mechanically, that is, naturally, without being driven
to the assumption of a supernatural Creator. Whether Dar-
win's hypothesis suffices in all cases is another question which
we shall have to examine later. But whatever the decision
may be, it must be clear that the validity of the theory of
selection can only affect a relatively unimportant problem of
organic life, for to the first and foremost question, the riddle

THE FACTORS OF EVOLUTION 159

of life, it supplies no answer. Indeed, it naively takes the life-
process as granted. But must not even the most primitive
animal, the simplest plant, be already organized, and able to
react suitably to external stimuli in order to be capable of
existence and reproduction ? Of this fundamental fitness of
all life this doctrine supplies no explanation.

Those who have ever tried when shooting without a dog to
stir a hare know how difficult it is unless lucky chance guides
our steps. We may time after time pass within a few steps of
a hare, cowering down in his nest and trusting for safety to the
colour of his fur, which with its brown, black, yellow and white
hairs harmonizes so completely with the colour of the soil or
dried leaves as to be almost indistinguishable. Thanks to this
protective coloration the hare has been able to preserve his
species in spite of the most merciless persecution both by man
and animals. But it is only in the summer that he enjoys this
protection. When in the winter the land is thickly covered with
snow there begins for the hare a hard time, for only too fre-
quently is he seen by his enemies even at great distances.
But suppose that the temperature in Europe should sink from
year to year and that the snow which now covers the earth
not longer than two months should remain three, five, or six
months. It is obvious that in that event the conditions of life
for hares would be completely changed and rendered unfavour-
able. According to the laws of probability, under this doctrine
only those animals would in that event have the best chances
of escape from their enemies which had the largest mixture
of white in their fur and were therefore less easily distin-
guished from the snow-landscape than other hares. Of course
even these animals would sometimes fall victims to their
enemies, but their average prospects of life would be more
favourable. Thus the light-coloured hares would survive, reach
maturity, and transmit their distinctive feature to their offspring.
Continuing this natural selection through many generations, all
dark-coloured hares would eventually become extinct, and there
would remain only the light-coloured variety. But natural
selection would again make a choice among these white-
coloured hares, for the best chances of escape would be to those
most nearly like the colour of the snow. Thus we should

160 LECTURES ON BIOLOGY

gradually reach a time when there would be only snow-white
hares. That this is no mere imagination is proved by experi-
ence, for we actually find in the Arctic regions and high
mountains that our common hare is there represented by a
white variety.

The soft, white fur of the Polar fox has in recent years become
fashionable with ladies. How can the origin of white foxes be
explained ? Apart from man the fox has not so many enemies
that he should stand in need of protective coloration. Here the
theory of selection provides an easy explanation. Dark foxes
may be distinguished on the white snow at great distances, and
their colour thus becomes a warning to their prey to escape to
safety. A red fox would therefore be severely handicapped in
searching for food, an all-important reason for putting into
motion the forces of natural selection. From century to century
increasingly less numbers of red foxes would be able to
reproduce their kind, whilst the light-coloured foxes would
steadily increase in number and transmit their useful charac-
teristics to subsequent generations ; thus the process would go
on until natural selection had left only pure white foxes.

Whilst the human breeder must usually content himself with
being able to breed or change one definite feature, natural selec-
tion begins at the most different points. To resume our illus-
tration, Nature would not be satisfied with selecting only
light-coloured animals, but among these again the best runners,
the most intelligent, the strongest, and those who possess any
other useful quality in a higher degree. Natural selection
employs the same methods as man, but in its effects leaves man's
work far behind.

But there are other animals in the Arctic regions which are
distinguished by a white colour. The brown bear would on the
vast snowfields of the Far North be doomed to die from hunger,
whilst the Polar bear is easily able to stalk his prey, the seals,
and obtain abundant food. All our owls are brown, grey, or
spotted, but their Polar relative, the great snowy owl, has
white plumage. The sable of the Far North forms the only
exception, for it has dark fur. A look at its life-habits will
explain this seeming inconsistency. It lives and seeks its prey
on trees. A white colour would therefore not only be no ad-

THE FACTOKS OF EVOLUTION 161

vantage to it but a distinct disadvantage. A great white lump
cowering on the dark branches would be at once detected,
while its dark fur is but with difficulty distinguished from the
trunk and branches.

Eemarkable instances of colour-adaptation are offered by
many animals of the temperate zones, whose covering assumes
an appearance which differs with the different seasons of the
year. We need only recall the case of the ermine, whose winter
fur is much sought after ; the mountain hare, and the ptarmigan.
Even the Arctic fox and the great white snowy owl assume in
summer a darker hue. Our common weasel, in the summer
of a simple brown colour, changes its fur in the autumn and
becomes gradually snow-white. It is remarkable that in the
southern parts of Europe, where there is little snow, the weasel
retains its reddish-brown summer fur throughout the winter.

Just as in the Arctic regions and on the mountain tops the
white colour predominates among animals, so inhabitants of
deserts and jungles have chiefly a yellow or brownish-yellow
colour. The king of the animals is yellow, like the little desert
fox or its prey, the jerboa. The ground-colour of the tiger
is a yellow-brown, and the black stripes make it still more
difficult for him to be perceived in the jungle. This colour-
arrangement of dark stripes or spots on a yellow ground is found
in numerous animals of the Tropics, for instance, the zebra,
giraffe, okapi, leopard, panther, and gepard. In all these cases
we are confronted, according to the opinion of most investigators,
with an effective protective colouring.

Numerous instances are found among the lower animals,
among others, the sand-coloured vipers, the cobras, the geckos,
lizards and numerous species of insects, in particular the many
locusts to be found in desert regions. It might be objected that
the locust cannot be regarded as a well chosen instance because
of its scarlet under-wings, as there could scarcely be anything
more unsuitable or unsafe than these red wings which actually
attract the eyes of birds afar to the welcome food. Nevertheless
this arrangement grants to the locust ample protection. It is
true that the birds are attracted by the striking colour and at once
give chase, but no sooner do they approach the locust than the
insect folds its wings and disappears as if the earth had swallowed
11

162 LECTUEES ON BIOLOGY

it up. The bird which is looking for a fiery-red insect is all the
more apt to overlook the insignificant sand-coloured animal which
cowers motionless on the ground. For when the locust is in a
resting attitude the underwings are completely covered by the
upperwings. It seems, therefore, that the striking colour is only
one more protective measure for these insects. It might be
possible to explain similarly the striking colour of the under-
wings of the death's-head moth, some 'owls,' 'red underwings/
and many other butterflies.

It is further known that the inhabitants of grass and
leaves, such as grass-locusts, many caterpillars and larvae,
frogs, lizards, etc., are green. Only birds appear to supply the
exception to this rule. There are no green species of birds in
Europe, with the exception of the green woodpecker, greenfinch,
siskin, serin, some tomtits, the golden-crested wren, and a few
others. But if we consider that in our climate trees and shrubs
are without leaves during a great part of the year, we shall
understand that here a green colour gives to birds only a very
doubtful advantage. On the other hand, in the Tropics, or in
countries with evergreen forests, we meet very frequently with
green birds.

As regards the denizens of the water, we became acquainted
in the last lecture with the dainty Leptodora hyalina, a little
freshwater crustacean, whose transparent body is almost
invisible. But the phenomenon observed in Leptodora is not
an isolated one. Numerous other inhabitants of fresh water and
the sea have acquired a similar transparency. Fixed to the
ocean-floor live the " glass-rope sponges." Who would regard
these beautiful forms as animals ? Do they not rather seem
like artistically executed Venetian crystal ornaments woven
from the finest glass threads ? Floating about the ocean are
immense numbers of salpae, medusas, jelly-fishes, snails and
crustaceans, swimming in close formation through the water,
yet being hardly visible. Even the eggs and free-swimming
larvae of many of the lower marine forms are distinguished by
perfect transparency (see fig. 46).

A remarkable phenomenon of adaptation must be briefly
mentioned. It is well known that light can only penetrate to
relatively shallow depths of the sea, perhaps 400 to 600 fathoms ;

31
Is

I

I

^^
ft,

THE FACTORS OF EVOLUTION 163

deeper regions of the ocean are hidden in eternal night. It is,
therefore, remarkable that in spite of this fact the greatest depths,
abysses of 7,000 to 8,000 metres, are inhabited by fishes, cephalo-
pods, crustaceans, etc., which have well-developed, highly organized
eyes. This seems to contradict all previous experience, which
shows that organs which have ceased to fulfil their functions
become degenerate. The Proteus is blind, the mole has diminu-
tive eyes. How is it possible that in the absolute darkness of the
ocean-bed there exist animals with well-developed eyes ? The
reason is, however, simple : though it is true that deep down in
the ocean there was originally darkness, the denizens of the deep
sea have entirely changed these uncomfortable conditions by
creating an artificial illumination, for the large majority of the
animals which are known to us as inhabiting the ocean-bed are
furnished with luminous organs.

The ability to produce light is widely distributed among
organisms, but we are still in the dark concerning its biological
importance. Among the vegetable organisms it is in particular
certain bacteria and fungi which show this remarkable property.
Among animals we know phosphorescent species among fishes,
tunicates, insects, crustaceans, vermes, molluscs, echinoderms,
coelenterates and protozoans.

Among the terrestrial arthropods it is above all the glow~
worm, Lampyris noctiluca, which attracts our attention. It is
distributed throughout the whole of Europe. During warm June
nights we may frequently observe the males in swarms of hun-
dreds flying about like floating drops of fire, searching for the
wingless females who sit on the ground in the grass of meadows
or under shrubs, pointing the way by means of their natural
lanterns. One may keep glow-worms easily in a terrarium or
large preserve-glasses planted with grass, and observe night after
night their scintillating movements, provided that they are always
well supplied with moisture. But these ' beauties of the night '
look most disappointing when seen in the daylight. The female
is an ugly ' worm/ 2 centimetres long ; nor can the male, which
is about half as long, lay claim to good looks. The luminous
organs of these insects lie on the ventral surface of two of the last
abdominal segments, which even during the day may be distin-
guished by their yellow colour. The larger American relatives of

164 LECTUEES ON BIOLOGY

our glow-worm are frequently worn by ladies as living ornaments
in the hair, or enclosed in little medallions, as brooches, for
their brilliant sparkle vies with the costliest diamonds.

Glow-worms possess phosphorescence for purposes of repro-
duction, for it helps the sexes to find each other. In other
animals it may serve to attract the prey or to deter enemies;
but there are also numerous other organisms to which it gives no
demonstrable advantage.

The most magnificent instance of organic light-production is
observed on the ocean. Never shall I forget the wonderful im-
pression which I received when noticing for the first time this
wonderful spectacle on the Algerian coast. A mild spring even-
ing had kept us on the sea till darkness. When at last we were
making for the shore there rose every now and again, here and
there, sparks giving a bluish light. Gradually the phosphor-
escence became more and more frequent and intensive, each wave
bursting into a rain of fire. On our bow silver drops glittered
and glistened, and each stroke of the oars called forth myriads
of lights. It was a veritable fairyland of light and splendour.
The magicians which performed this miracle were little uni-
cellular organisms belonging to the Infusorians. Their names
are Noctiluca and Leptodiscus. They are barely 1 millimetre in
diameter, and their organization can only be observed with a
microscope. If we examine them we see that specific luminous
organs are absent, and that it is the whole cell-body which lights
up at the slightest stimulus of a mechanical or chemical nature-
Of their mode of life only very little can be said : they float on the
surface of the water, drifting with the current ; if storms, rain,
or cold approach they sink down to a depth where there is eternal
calm. Only during favourable weather they rise to the surface,
often in such vast swarms that during the day the sea for
long distances appears to be covered as with a reddish film.
The rising and sinking of the Infusorians in the water is
probably effected by taking in and ejecting sea water, a process
by means of which they alter their specific gravity. The little
creatures do not appear to derive any advantage from their
phosphorescence.

If different instances of colour-adaptation showed us many
strange things, the phenomena of mimicry cause still greater

THE FACTORS OF EVOLUTION 165

surprise. We can here distinguish two forms of imitation.
Some animals possess an astonishing likeness to dead objects —
dried twigs, leaves, etc. ; others again assume the exact shape
and appearance of a distantly related species. Phyllopteryx
eques, a fish of the Australian seas, strikingly resembles, with its
bizarre, ragged, ribbon-like body-appendices, the seaweed in
which it lives and with which its colour harmonizes. Many
looper-caterpillars (Geometridce} look like dry twigs, several
little beetles and loopers like the dejecta of birds. Xylina
vetusta in a resting position looks like a piece of dried wood-
Striking instances of protective mimicry are found among
the Phasmids. Every one knows the so-called Walking-leaf
(Phyllium siccifolium), which, in its colour as well as the form
of its body, legs and wings, looks exactly like a green leaf.
We frequently find in butterflies — for instance, in the Death's-
head moth, the ' red underwings,' etc. — that in a resting position
they mimic the dry bark or leaves of trees, while during
flight they exhibit brilliant colours. Because the day butterflies
fold both pairs of wings over the back, but the ' owls ' place the
upper wings over the underwings, in the former the whole lower
side, in the others only the upper side of the wings is furnished
with protective colours and designs. Probably the most beau-
tiful instance is the famous Kallima, a tropical butterfly, which
in a resting position looks exactly like a dried leaf.

Equally general is mimicry proper, that is, the mimicking of
one species of animal by another species. In these cases the
mimicked species is usually one which has little to fear
from natural enemies by reason of its venom, its objectionable
taste, etc., while the mimicker is unarmed and exposed to
numerous attacks. Thus the hornet with its dreaded poison
dart is mimicked by the harmless hornet-moth ; humble-bees
and honey-bees serve as a ' model ' for various species of flies ;
wasps are mimicked by beetles, and ants by spiders. In the
forests of Guatemala lives the coral-snake (Elaps corallinus),
dreaded on account of its fatal venom, a strikingly beautiful
animal whose brilliant red body with black transverse bands
is conspicuous afar. In social union with it lives a harmless
snake of another genus, Erythrolampsus, which resembles the
coral-snake in all details so much that only an expert is able to

166 LECTUEES ON BIOLOGY

distinguish between them. In other districts Elaps corallinus
is represented by a related species which has black transverse
bands bordered with yellow seams. This characteristic is again
faithfully repeated by the non-venomous mimicker.

Such cases of mimicry appear more remarkable still when
the animals concerned belong to widely distant classes. In
New Pomerania was recently found a sea-snake, Pla turns
colubrinus, a venomous, strikingly coloured animal of 2 to 3
metres. This is mimicked by a fish, Ophickthys colubrinus.

Let us conclude with an instance in the life of butterflies.
On the banks of the Amazon one may frequently observe gaily
coloured butterflies (Heliconiidse), which lazily flutter about in
large swarms. Though these butterflies are visible at great
distances and even appear to exhibit a desire to be noticed, they
are almost never molested by enemies ; it may sometimes happen
that a young, inexperienced bird attacks one of them, but it will
never repeat the mistake, for these butterflies possess such an
objectionable smell and taste that they are avoided by all
animals. Many years ago Bates observed that these swarms
of Heliconiidse were regularly invaded by other butterflies which
belong to a widely different genus, the Pieridse or " Whites," and
furnish tasty food. But as the Pieridse copy the Heliconiidse, not
only in shape and colour, but also in their awkward manner of
flight, and live in their society they, too, escape persecution. Who
would, after hearing of such instances, of which many more
could be adduced, doubt the great importance of protective
colouring and mimicry? Nevertheless, we are confronted, on
accurate examination of the facts, by many objections, and will be
driven to admit that in the consideration of cases of mimicry
many gross mistakes have been made. Because an animal has
the colour of sand or looks like a leaf it does not follow that it
derives advantage from this characteristic. Because one species
of butterfly — for instance, the Yellow Underwing — mimics in
colour and design dark, lichen-covered parts of a wall, this is con-
sidered a splendid case of adaptation, causing much astonishment
in every Natural History Museum. But unfortunately for the
theory this butterfly settles in nature by preference on well-
lighted places, where it is easily distinguished from its sur-
roundings.

THE FACTOKS OF EVOLUTION 167

We have already heard that the large feline carnivores of the
Tropics — the tiger, lion, jaguar, leopard, etc. — possess a ' sym-
pathetic coloration' which is thought to facilitate their approach
to their prey, the gazelle, antelope, giraffe, zebra, etc., which on
their part appear to be protected by means of a similar colour-
adaptation. This sounds very plausible, but it may be objected
that all cats hunt their prey chiefly in the dark, during the night
when not only cats but all other animals are grey, and even
the most perfect protective coloration becomes useless both to
them and to their prey. Moreover, it is generally the sense of
hearing which is better developed in cats than the sense of sight,
and they rely, therefore, in the search for prey, chiefly upon their
acute hearing. Other carnivores make still less use of their eyes,
and track their prey by the scent. Here, too, protective colouring
is of no advantage.

As regards the protective resemblance of insects, it cannot be
denied that there is a ' purpose ' in their coloration, but
here, as elsewhere, we must be on our guard against an exaggera-
tion of its biological importance. If, as is generally believed,
the adaptation really granted such enormous advantage over
the species not so furnished, how is it that the latter have
not been exterminated in the struggle for existence ? We should
at least expect to see the ' protected ' species far exceeding
in distribution and number of individuals the ' unprotected '
species, an assumption which is flatly contradicted by everyday
facts. We need only think of the Pieridse and Vanessidse at
home. Further, everyone who has collected caterpillars, butter-
flies, and beetles knows how soon the eye becomes accustomed
to discover even the most perfectly adapted insects in their
protective surroundings, and we may be sure that their animal
enemies, whose senses are usually much keener than ours,
cannot be deceived by a superficial resemblance. Moreover,
how can we explain these cases in which one species mimics
a species living in a far-distant land? A convincing instance
is related by Aigner-Abafian concerning the Brazilian butterfly
Semnia auritalis, which is a perfect copy of Caryatis viridis,
which lives in the Cameroons, and is only very distantly related.
It is plain that we have here to do with an accidental similarity ;
not by any means an improbable explanation, considering the

168 LECTUEES ON BIOLOGY

enormous number of different species. Animals must have
some kind of colour an,d marking — why should there not be
some who closely resemble other insects or dead objects ?

Even proofs apparently so convincing as the mimicry between
venomous and non-venomous snakes, or the mimicking of the
sea-snake (Platurus) by a fish (Ophichthys) create considerable
doubt. As regards the snake and the fish, the latter has in its
powerful teeth such an excellent weapon that most enemies
would be chary of attacking it. It does not, therefore, stand
in need of specific protection, and it seems rather that the
inconvenient similarity would render it more difficult for it
to gain a livelihood. Probably we find here, as in many other
cases, ' the phenomenon of convergence/ of the cause of which
we know very little, but of the existence of which there are
many proofs. Numerous fishes of the Southern Seas, in par-
ticular the inhabitants of the coral reefs, have a uniformly
striking appearance. Cats, goats, sheep, cattle and dogs which
inhabit Angora and Thibet are distinguished by long, silky hair.
We might even regard the light colour of the Polar animals as
a case of convergence, for there exists no fundamental difficulty
in understanding that similar external influences may produce
similarity of appearance. That the white colour of many animals
is actually subject to influences of temperature is proved by the
lemmings. In their Northern home these animals assume, at
the approach of autumn, a white coat, but when in captivity if
they are kept in a warm room they retain their grey fur through-
out the winter. No sooner, however, are they exposed to cold
than they will again grow a white fur.

As regards the venomous snakes and their non-venomous
mimickers, there is first of all, as far as is known, no snake-eating
animal which distinguishes between venomous and harmless
snakes. The common hedgehog devours with equal gusto the
harmless ringed snake and the poisonous adder, the digestion of
the poison glands causing it apparently not the least discomfort ;
even to the bite of the viper the hedgehog appears to be quite
insusceptible : as we have already seen, this is due to the fact
that there are immunizing substances in the blood of the hedgehog
which neutralize the effect of the poison. The guinea-pig dies
quickly from the bite of the adder, but an injection of blood-serum

FIG. 47. — COLOUR- ADAPTATION AND MIMICRY.

In the foreground, Calocala fraxini, flying and resting. Above, a looper-caterpillar.
In the centre, flying, below, a hornet (Vespa crabro) ; above, on the right, a hawk-moth
(Trochilium).

THE FACTORS OF EVOLUTION 169

from the hedgehog imparts to it temporary immunity. What is
true of the hedgehog is true of all other snake-hunters, so that
mimicry fails so far as these enemies of snakes are concerned.

The only remaining enemy, man, makes no difference what-
ever in the destruction of venomous and harmless snakes, except
perhaps that the destruction of the former is carried on more
energetically. Probably snakes possessed their coloration long
before the species man appeared on earth.

As a last interesting instance let us examine the little trees
frog, which is said to adapt itself voluntarily and consciously to
its surroundings. Like many other amphibians, the tree frog
possesses to a high degree the faculty of changing its colour. In
its skin there are found two kinds of contractile pigment-cells, or
chromatophores, of which one contains granules of a yellowish-
green, the other a bluish-black pigment. In proportion as the
light or dark chromatophores contract or expand, the colour of
the frog changes from a light green to a deep bluish-grey. It
was at one time generally believed that the frog was able to effect
this striking change as the result of an eye impression, and adapt
itself accordingly to its surroundings. But this statement if
incorrect, quite apart from the fact that it ascribes to the animal
an exalted degree of intelligence. This change of colour is nothing
but an unconscious reflex-action resulting not from an eye im-
pression but from the reaction to the stimulus of touch. For, as
Biedermann has shown, if we place the frog on a smooth surface,
whether light or dark, yellow or grey, it will invariably assume a
greenish hue, whilst on a rough surface it assumes a dark colour.
Whenever the animal, therefore, harmonizes with its surroundings,
it is merely due to an accident which must very frequently happen
in Nature, because numerous leaves have a smooth surface,
whilst the earth's surface is rough. In many cases, however,
the tree frogs are completely deserted by this characteristic.
After Biedermann's experiments we can therefore no longer speak
of conscious adaptation.

What, after these observations, is now our position towards
the hypothesis of protective colouring and mimicry? Though
we do not agree with Eimer and other investigators, who flatly
deny the usefulness of such characteristics, there is no doubt that
the biological importance of colour-adaptation and mimicry has

170 LECTUEES ON BIOLOGY

hitherto been greatly exaggerated. But this conviction does
not detract from the fact that the organic world is infinitely
rich in adaptive arrangements. Almost inexhaustible seem the
means with which Nature furnishes her children, in order to
make them better fitted for the struggle for existence ; but since
to mention such organs of adaptation by name only would require
volumes, we must be content with a few of the most characteristic
instances.

We read frequently in the newspapers reports of a conscript
having cut off a finger or a toe in order to escape military
service. Few people know that similar cases of self-mutilation or
autotomy are common in the animal kingdom. It is not easy
to give a strict definition of this remarkable process, because
autotomy represents in one direction a transition into another
biological process — asexual reproduction by fission, or budding;
on the other hand, it is questionable whether, for instance, the
disintegration of many protozoa, in consequence of certain
stimuli, may be regarded as self-mutilation, because in this
process the whole animal is destroyed. The best definition is
probably that autotomy is a physiological process due to the
effects of stimuli of varying kinds, whereby one part of the body
is sacrificed in order to save the whole. It is an adaptation of
the organism to definite demands of the conditions of life.

I well remember the painful disappointment which I felt
as a boy when, after a tiring hunt for lizards, I had at last been
successful in catching one by the tail, only to be left in possession
of the tail, whilst the lizard itself had long disappeared in the
grass. It is remarkable with what rapidity the dismemberment
of the tail takes place : a few vigorous striking and twisting move-
ments of the lizard, and the operation is over. The amputation
does not take place, as one would be inclined to assume, between
two vertebrae, but right in the middle of one of the caudal
vertebrae. From the seventh caudal vertebra, counting from the
base, the vertebrae are enlarged to double their normal size and
formed in the shape of an hour-glass. At the thinnest point there
is a small non-osseous partition-wall, and it is here where the
fracture takes place.

A simple experiment will show that the separation of the
tail takes place quite independently of the animal's volition,

THE FACTORS OF EVOLUTION 171

being purely a reflex-action. This independence is so complete
that lizards which have been beheaded or cut up, throw off
the tail on receiving a stimulus at the tail-end, exactly like
healthy animals. Investigations further proved that the reflex
centre lies in the spine between the posterior extremities in the
so-called lumbar region, for if we cut a lizard in two below the
tail-extremities self-amputation does not take place.

Mutilation is chiefly performed by the tail-muscles, and it
requires a vigorous effort on the part of the animal to rid itself
of its tail. Thus it is that only healthy and strong lizards are
capable of self-mutilation, while those that are exhausted by
hunger or frost have not the power to do so. It is interesting to
note that after death it is as difficult to break off the tail as it
is to break off one of the legs, and it is further remarkable that
in that case the tail usually breaks in any other but the normal
place.

Immediately upon self-mutilation the muscles of the stump
contract to prevent haemorrhage. After a few days the wound
is healed, a new tail-end begins to grow, and in a few months
the damage has been repaired. Sometimes during the healing
process peculiar malformations occur, and in the place of one
tail two or three are formed. Aldrovandus even recorded a case
of a four-tailed lizard. Though a mutilated lizard is no doubt
at first harassed in its movements, the advantage which it derives
from this faculty of autotomy is obvious, since it is probably in
most cases the tail by which the lizard-hunter succeeds in grasp-
ing the agile animal.

In crustaceans and other arthropods self-mutilation is equally
general, but here it is chiefly legs and claws which are freely
sacrificed. The females of termites and ants at the end of their
nuptial flight break off their wings which in their future domestic
life would only be a hindrance to them. It has frequently been
observed how these intelligent animals help one another in
performing this operation.

The most peculiar behaviour is shown by many species of
locusts, of which it is exceedingly difficult to obtain uninjured
specimens. No sooner are they caught than they push the
captured leg or wing between their powerful jaws and bite it
off. Though ineffective with man, this self-mutilation probably

172 LECTURES ON BIOLOGY

frequently helps them to escape from their natural enemies.
Locusts in captivity will slowly and with the utmost indifference
chew off legs, the ovipositor, and even the abdomen. One
might be inclined to think that the locusts are cleaning and
polishing their legs, whilst in truth they are busily engaged in
eating them up. What induces the insects to perform this
incomprehensible operation we do not know, but it has been
observed that if they have once tasted their own flesh it is
impossible to induce them to take any other food.

Another kind of self-mutilation observed in locusts and some
beetles is the ejection of blood, a process which is chiefly employed
by them as a means of defence against enemies. By the violent
contraction of the abdominal muscles the blood-pressure in the
body is increased to such an extent that the epidermis ruptures
in certain places and permits a thin stream of blood to be ejected
against the attacker. The most perfect instance of this is
supplied by the Algerian locust (Eugaster) which is able to
eject its blood with wonderful accuracy up to a distance of 50
centimetres. As the blood contains substances with a pungent
smell, their enemies, usually lizards, as a rule refrain from making
a second attack.

If we finally recall the fact that many worms fall into pieces,
each of which may once more grow into a fully developed indi-
vidual, that starfishes can sever their arms by constriction, and
sea-cucumbers on receiving a stimulus expectorate their intestines
without apparently receiving any fatal injury from this horrible
mutilation, no one will doubt the great biological importance of
this adaptation.

We have already seen in what manner Darwin's theory of
selection attempted to explain the origin of useful adaptations
and the formation of new organic species. It now only remains
for us to examine whether this theory is able to fulfil all that it
promises. Keceived first with astonishment, then with bound-
less enthusiasm, the doctrine of this great naturalist made in
a short time a veritable triumphal journey round the world,
and there was a time when the great majority of prominent
zoologists were all convinced Darwinists. The doctrine of natural
selection seems, indeed, exceedingly clear, nor is there any other
theory which vouchsafes such uniformity of observation. It is

THE FACTORS OF EVOLUTION 173

only quite recently that we have arrived at a just and critical
appreciation of the theory of selection, and naturalists, while
recognizing its strong points, point out to-day its weaknesses to
an increasing degree.

The theory of natural selection is the doctrine of chance.
According to it the individual possesses no formative power in
the development of its body. By chance it receives at birth,
through an accidental variation in one or another part of its
organism, some useful character which the other innividuals of its
species do not receive. But however useful this new acquisition
may be to the animal, it is not able to develop it. Only by
pairing with another individual which by chance possesses the
same useful modification, may this new character be improved
in the next generation. The only participation of the living
body in the development of its useful characters lies, therefore,
in reproduction. Of the causes which lead to the origin of such
variations this theory does not supply any explanation. It takes
variability as granted ; and since chance is the determining
factor it is obvious that there will arise equally numerous cases
of useful, indifferent and harmful variations. We can therefore
already see that selection, in order to be at all effective, must
work with an immense number of variations which form the
material from which the struggle for existence makes its selection
on the ground of usefulness.

That variability is universal in Nature is well known, but the
necessarily large number is said to be due to the excess of births,
because each species produces far more descendants than will
reach maturity. If we consider that many fishes annually pro-
duce hundreds of thousands of young, and that parasitic worms
produce millions of germs, this demand made by natural selection
seems justified, for with such a wealth of descendants it is perfectly
intelligible that chance would be able to produce a sufficient
number of variations to give rise to all the useful characters
found in the organisms. But we also know animals — for instance,
the elephant, and, in fact, most mammals, as well as numerous
species of birds — which possess a very low ratio of increase. Nor
must it be forgotten that in the animals with an immense progeny
most germs are destroyed as ova or immature individuals, and
thus the material for the stage at which the fiercest competition

174 LECTUKES ON BIOLOGY

for fitness must take place, i.e., that of the adult animal, becomes
in consequence very limited. In addition to the immense num-
ber, the accidental play of natural selection requires equally
immense periods of time. The question arises therefore whether
the geological periods are actually long enough to make the
genesis of all the useful characteristics within that time conceiv-
able. It is true that at the first glance the geological period
appears enormously great. Some geologists estimate that since
the Cambrian period up to modern times 200 millions of years
have elapsed ; yet what an enormous labour is Nature expected to
have performed since then ! Mayer-Eymar estimates the Tertiary
period to have lasted 325,000 years, while others estimate it at
2,350,000 years. But when we consider that within this time all
the higher mammals and all their numerous adaptations are said
to have been originated, even this immense period seems much
too short for a pure play of accident.

While natural selection excels artificial selection in that it
is able to commence in many points at the same time, it is
decidedly inferior to it in one important respect. If in an
enormous number of domesticated animals there appears only in
two or three individuals a variation which seems useful to man,
he is able to select those animals among thousands, cross them,
and thus maintain and increase that particular variation. But
let us suppose that something similar happens in Nature. Let us
assume that of the hares in a certain district two are by the
colour of their fur better adapted to their surroundings than the
rest. Is it safe to assume, or is it even probable, that because of
this little advantage these two, and only these two, animals escape
the persecution of their enemies ? Even supposing a lucky acci-
dent so to ordain it that they do remain alive, would it not be in
the nature of an extraordinary miracle if these two animals with
the useful variations found each other and paired with each
other ? Must we not rather assume that the pairing would take
place with one of the other hares, and that thus the distinctive
feature would after a few generations disappear again? Assuredly
we cannot assume that all the hares not thus distinguished would
be suddenly exterminated. For if the hares were previously able
to maintain their species without this protective character, it is
unintelligible why it should be difficult now, so long as conditions

THE FACTOES OF EVOLUTION 175

of life remain the same, merely because in some individuals of
the species there has appeared a new favourable characteristic.
It seems clear that as long as useful variations appear as isolated
phenomena they undoubtedly disappear again at a later stage,
and are in consequence without importance to natural selection.

Wagner's theory of migration, or the theory of the formation
of species by geographical isolation, attempts in a certain sense to
master these difficulties. The most important condition of the
phenomenon that isolated variations are not again lost by
repeated crossing with individuals which do not possess them
is found in the isolation of the relevant animals from the rest
of the species. If, therefore, some individuals of a species are
separated from the rest by emigration or geological causes and
placed among new conditions of life, this fact may give rise to
a new species. An interesting instance is supplied by the rabbits
which were liberated in the beginning of the sixteenth century
by Portuguese sailors on a small island near Madeira, Porto Santo.
They soon became acclimatized and have assumed a number of
quite distinct characteristics : they have grown much smaller, are
distinctly ' vicious,' have a reddish fur, and are indeed so widely
differentiated from their European parents that it is impossible
to pair them.

It is easily conceived that changed external influences which
affect in like manner all individuals of one species produce
simultaneously in a large number of animals corresponding
useful variations. If a large number of variations exist,
tending in the same direction, natural selection would, in fact,
be able to accelerate the change. Only with this supposition is
it, in my opinion, intelligible that in a comparatively short time
there can be formed so many adaptations and new characters
that natural selection is able to act at all. Chance, of course,
becomes thereby limited, and part of the formative power is trans-
ferred to the organism. It is the admission that the organism
is capable of suitably reacting from inner causes to external influ-
ences, or, as Pauly expresses it, ' the awakened need itself produces
the means for its satisfaction.' Selection in that case would only
occupy the position of a secondary factor in species-formation.
The mechanistic school may call this mysticism, but it is far less
mystic than the attempt to ascribe every rational performance to

176 LECTURES ON BIOLOGY

the blind play of chance ; it is, moreover, the only way to com-
prehend the wonder of organic formation. The assumption of
inner factors of evolution could only then be justly described
as mysticism if we represented these inner forces as something
absolutely incomprehensible ; but there is not the least cause to
do this. To these inner forces — the psychic energy — there applies
the same demand, as to all other forms of energy, that they must
be capable of being determined quantitatively. That it is still
beyond the powers of science to do so is no proof to the contrary.
One must agree with Pauly when he raises the objection against
the theory of selection that logically it turns matters upside down
by making reproduction the causa causans of the acquisition of
fitness ; as a matter of fact the organism must first have acquired
fitness by fulfilling certain conditions of its existence before it
can proceed to reproduction.

That species will remain unchanged for unlimited periods if
the conditions of their life remain unchanged is conclusively
proved by the inhabitants of the deep seas. At vast depths
in which no change has taken place since times immemorial, we
still find the same forms which we also know as petrefacts
from the oldest geological formations. A brachiopod, Lingula,
has to-day the same appearance as its ancestors in the Silurian
strata. The crinoids, or 'feather-stars,' which live in shallow
water near the surface, differ from their geological ancestor
Pentacrinus, or sea-lily, in numerous points ; above all, they
are free-moving, whilst sea-lilies are fixed on the ocean bed.
But the history of their evolution proves their origin beyond
doubt, for the crinoids have a fixed Pentacrinus stage. The
venerable sea-lilies were long thought to be extinct. The
surprise was, therefore, all the greater when recently there were
found on the ocean bed crinoids which exactly resembled their
ancient ancestors. (Compare coloured plate II.)

Another difficulty of the theory of selection is that generally
the variations are far too slight and of far too little advantage
to the animals which exhibit them to be regarded as being of
real value in breeding. To mention again the instance of the
hare, snow-white varieties do not appear among the brown
animals all at once, but according to the theory of selection
we are to assume that first there appear a few individuals

THE FACTORS OF EVOLUTION 177

with a fur of a somewhat lighter colour. It seems, however, very
doubtful whether this slight difference in coloration would grant
a sufficient advantage to enable just these animals to survive.
The same observation applies to numerous cases of protective
resemblance and mimicry, of which it is equally difficult to
understand at the first faint stage why they could be of use to
their possessor. But if they were without importance at their
first appearance, how was natural selection able to increase
them?

Owing to these objections an attempt has been made, with
very little success, to refer the origin of colour-adaptation and
protective marking to the direct influence of light and surround-
ings. It was thought that, as in colour-photography certain
substances present in photographic plates undergo under the
influence of coloured light a corresponding change, so we have
to regard in the numerous insects this phenomenon as being due
to a ' photographic sensitiveness of the skin.' But the colour of
the butterfly originates while it is still hidden in the pupa-case.
Further, how could the leaf-designs of the Kallima be produced
by colour-photography, for, as Plate has pointed out, it would
be necessary in that case for, each butterfly to assume for that
purpose a certain definite attitude and retain it until the design
had been photographed upon its wings?

' More difficult still is it to explain by the doctrine of selection
the appearance of large and comprehensive organs, such as the
lungs or wings of the flying saurians and bats. In many cases
these are not new formations — that is my reason for quoting them
— but represent a change of functions which the organs have
undergone. The lung appears first as a swim-bladder in the fishes,
then undertakes in the lung-fishes temporarily, and as it were as
an additional office, the duties of a respiratory organ, and finally
serves exclusively in that capacity from the amphibians upwards.
The origin of the wings of the bat and pterodactylus may perhaps
be explained in this manner that at first there appeared in some
animals lateral skin-folds which in jumping acted as a
parachute. A similar state we find in the flying squirrels
(Pteromys alborafus), the taguan (P. petaurista) the assapan (P.
volucella) and in a lesser degree in the common squirrel (Sciurus
mdgaris). But if such a parachute was already in existence,

12

178

LECTURES ON BIOLOGY

and natural selection may very likely have gradually increased its
usefulness until it acquired the value of a wing (fig 48).

In other cases an attempt at explaining certain pheno-
mena in the terms of the doctrine of natural selection fails com-
pletely. How, for instance, shall we explain by natural selection
the origin of such complete and highly differentiated formations
as the electric organs in the tail of the Bay, which are apparently

FIG. 48. — THE TAGUAN (Pteromys petaurista).

of no advantage whatever to its owner in spite of their perfect
development^ The electric current is so weak that we are only
able to demonstrate it by means of an apparatus. Efficiency
as a weapon of defence, like the electric organ of the Electric
Eel, it does not appear to possess. The assumption that it is
a rudimentary organ is clearly contradicted by its phylogenetic
history.

The long-winged seeds of the Dipterocarpaceans (or Diptera-

THE FACTORS OF EVOLUTION 179

ceans), trees that grow in Asia and Africa, important to
man as the producers of dammar-resin, Gurjun balsam, copal,
camphor, etc., were always admired as a case of perfect
adaptation for the distribution of the fruits. But according
to Kidley's observations the use of these long wings is of very
problematic value. If this arrangement in its modern perfect
form does not grant any noticeable advantage how can it
have originated by natural selection ? It is a fact, unfortunately
not to be denied, that there occur in organic nature many pur-
poseless and even entirely unsuitable arrangements, a phenomenon
which testifies as much against an intelligent Creator as against
the validity of the theory of selection.

The weakest position of the Darwinian doctrine, a redoubt
which has now been abandoned by numerous supporters, is the
hypothesis of sexual selection. It is a familiar phenomenon that
the male and female individuals of a species are greatly differen-
tiated in colour as well as structure. We have had several oppor-
tunities to refer to these remarkable secondary sex-characters.
In many cases the males are furnished with powerful weapons,
while the females have none, or possess them in a less developed
form. It is only necessary to mention the horns of the roebuck
and of cattle, the mighty antlers of the stag, the spurs of the
cock, etc. We find, further, that in numerous animals the male
is distinguished by a greater beauty, magnificent colour, or striking
scent. Who has not, for instance, admired the plumage of the
bird-of-paradise, pheasant, and peacock ; the delicate colours and
markings on the wings of butterflies ; the remarkable ornamenta-
tion on the armour of the rhinoceros-beetle ; the mandibles of the
stag-beetle ; the song of the birds and the formation of scent
scales in numerous insects? (See fig. 49.)

As many animals, such as the stag, roedeer, and bull, fight
fierce battles for the possession of the females, it might be
possible to explain by natural selection, the origin of specific
weapons in the male, for they are undoubtedly an advantage
and help them to acquire a mate. We may therefore assume
that generally the strongest and best-armed males have the
best chance to reach maturity and transmit their characters to
their progeny. But we are helpless in the face of such
phenomena as, for instance, ornamental coloration, which so

180 LECTURES ON BIOLOGY

far from bestowing any advantage on their possessors actually
constitutes a distinct impediment in the struggle for existence.
Origination by natural selection appears in such cases precluded.
Darwin thought that selection was here affected by an intelligent
factor : that the females would always select the best and
strongest males. This assumption seemed intelligible because
in many instances occurring among fishes, amphibians, reptiles,
etc., these ornamental colours, the so-called nuptial dress, appear
only at pairing time.

It is also well known that numerous birds, among them
bustards capercaillie, grouse, and pheasants, perform during
courtship before the hens a regular dance, during which they
seek to display their beauty as much as possible. But if we look
at the wonderful symmetry and design of the wings of a butterfly
or of the peacock's tail, would it not be a gross exaggeration of
the intelligence of these animals were we to ascribe the origin
of such works of art to their critical selection '? Moreover,
we have no right to assume that that which we regard as
beautiful is equally so regarded by animals. The fact is that
so far observation has supplied no proofs that the more beauti-
ful males are really on that ground selected by the females.
As with so many phenomena in Nature, we must here again
admit that no satisfactory explanation of this phenomenon exists,

What happens to every eminent man who comes forward
with new, fruitful ideas happened to Darwin : he gained enthu-
siastic supporters who extended his doctrine but also frequently
misunderstood him and fell into gross exaggerations. His
method was thought to be the Philosopher's Stone which
would reveal all the mysteries of life. The struggle foi
existence became a phrase ; the deep meaning which Darwir
had given to it became gradually obscured ; competition became
a real sanguinary battle ; Hobbes' ' bellum omnium contra
omnes ' was accepted as the ' leitmotiv ' of the whole of organic
Nature : and in this sanguinary strife was to be found the driving
force of all development and progress. Even Huxley said in ar
article on the struggle for existence and its importance to man thai
from the standpoint of the moralist the animal world presentee
a picture resembling that of a battle among gladiators — the
fighters were first well fed and then incited to merciless slaughtei

THE FACTORS OF EVOLUTION 181

in which the strongest, quickest, and most cunning survived,
only to be compelled to fight again on the morrow. No need
for the spectators to ' turn down the thumb,' for in this battle no
pardon was given.

Kessler, and recently Kropotkin in his work on ' Mutual Aid
in Evolution,' have shown that a brutal fight for existence is
not the only factor in evolution, for in addition to strife and
rivalry we find numerous cases of alliance and mutual support-
This friendly living together has undoubtedly had a far-reaching
influence upon the development of physical characteristics as
well as upon the improvement of the mental qualities. Time for-
bids my dealing in detail with animal colonies and communities,
but I will mention a few instances of symbiosis, because they
produce sometimes almost the same kind of change which we
saw as having taken place in consequence of the domestication
of animals. When several years ago I was at the zoological
station at Naples I derived much pleasure from spending a
considerable part of my leisure time in the magnificent aquarium.
The inhabitants of one of the tanks consisted of several individuals
of the well-known hermit-crab (Pagurus calidus), and of some
Actiniae, the beautiful 'sea-roses' or ' sea-anemones.' They are
children of warm climes, and are found in large swarms on the
ocean-bed, attached to rocks or empty shells, waiting with
spreading tentacles for their prey. Near Capri one may see with
each wave many thousands of brilliant-red ' sea-anemones ' which
surround the entire coast of the island as with a girdle of fire.
But their magnificent and alluring exterior is protected by a large
number of offensive elements from which fine lassoes, bathed in
poison, are thrown out on the least provocation, numbing or
killing both prey and enemies, and causing a violent burning
pain. Other tentacles are concealed in the body of the sea-
anemone, which, in case of need, are thrown forward through
the mouth aperture. On account of these poison-lassoes the
Actiniae are greatly feared by all animals, while there are hardly
any of which they themselves need be afraid.

The hermit-crab is a near relative of our crayfish and of
about the same size. It owes its name to its mode of life, for
it has assumed the peculiar habit of making its domicile in the
shell of snails, the legal owner of which it effectively disposed of

182 LECTUEES ON BIOLOGY

by eating it up. Only the head with the powerful claws protrude
from the shell, the rest of the body is concealed inside. As
result of this mode of life the abdomen of the hermit-crab has los
its natural armour which has been rendered unnecessary by th
shell of the snail, and has become soft, offering a favourable poir
of attack to the crab's numerous enemies. But the animal is we
aware of its weak side, and does not abandon its artificial armon
except for the most urgent reasons. Only when it has outgrow
the dimensions of its house it leaves it, to exchange it mime
diately for a bigger domicile. Thus the hermit-crab, clad i
armour, moves along on the ocean-bed in search of its prey.

Attached to the shell are always found one or more sea
anemones (Adamsia rondeletii], which are thus carried throug
life by the hermit-crab. It is not difficult to see the advantag
which the Actiniae derive from this remarkable arrangemem
They use the legs of the crab as a means of locomotion for thei
own voyages in search of prey, and take at the same time
moderate toll of the repasts of their hosts. It is more dimcul
to understand the advantage which the sea-anemones grant t
their host in return for these services, for unless the hermit
crab derived some advantage for itself from this alliance it woul
probably leave its house with the anemones on it and see'
another domicile. But I was soon to be shown this side of th
1 contract.'

One day, by the mistake of an attendant, a large octopu
(Octopus vulgaris), one of the most dangerous enemies of th
hermit-crab, had been put into the tank, and at once commence"
to attack the crab, seizing it with its strong tentacles covere<
with powerful suckers, and endeavouring to pull it out of th
snail-shell. Resistance seemed quite hopeless, but the octopu
had reckoned without the ' sea-anemone,' for no sooner had i
seen the danger which threatened its host than it threw it
numerous poison-lassoes on the naked arms of the attacker
The octopus at once let go its prey and took to flight with ever
sign of pain and terror.

How well the hermit-crab understands the security grante<
by the presence of its friend is easily demonstrated. If w<
forcibly remove the crab from its house, a process during whicl
we are exposed to a painful cannonade from the batteries of th<

Symbiosis.

Hermit-crab (Eupagurus Bernhardus) with Actinia (Adamsia Eoudeletii). On the rock

Octopus vulgaris.

THE FACTORS OF EVOLUTION 183

sea-anemones, and close up the entrance to the shell, the crab
will make the most desperate efforts to remove the obstacle.
Only when it is convinced of the futility of its endeavours it will
go in search of another domicile. Having found a suitable shell,
it examines it carefully with its claws in order to find out
whether any enemy may be hidden within. When the examina-
tion has proved satisfactory the hermit-crab settles down within
its new armour with every possible speed. Then begins a most
attractive spectacle. Quickly it returns to its old domicile and
attempts by means of its claws gently and carefully to remove
the sea-anemone from the shell and induce it to settle down
on the new house. Very soon its endeavours are crowned with
success. Sometimes it happens that for some reason the new
home does not please the sea-anemone. In such case it has
been observed that the crustacean will untiringly change its
domicile until it meets at last with a shell which pleases its
friend the sea-anemone.

With many hermit-crabs the settling on their houses of sea-
anemones has become a perfect mania. I have myself seen
hermit-crabs which carried about with them three and even four
Actinias, all of them of the same size as the crustacean itself,
and their combined weight so heavy as to make it almost
impossible for the crab to drag its dwelling along (see coloured
plate 6).

A similar alliance was observed by Sluiter between a ' sea-
anemone ' and a fish. On the coral-reefs near Batavia are found
numerous large yellow Actiniae with long tentacles thickly
covered with stinging-cells. Between these weapons, which are
carefully avoided by other animals, there swims calmly a little
defenceless fish which is thus completely protected against all
attacks. In return for the protection the fish faithfully shares
its food with its friend by placing pieces into the mouth of the
Actinia. The ' sea-anemones/ which as a rule reply to even the
slightest stimulus from outside with a fierce attack, quietly
submit even to being hustled and pushed by these little fishes.

The symbiosis between Algae and Actiniae I have already
mentioned. We saw that in that case adaptation had advanced
to such a degree that the animals had lost their ability to feed
themselves.

184 LECTURES ON BIOLOGY

Among ants we find instances comparable to the domesti-
cation by man of animals and vegetables. Familiar among these
are the fungus-gardens of the notorious leaf-cutting ants of South
America, which raid plantations in armies of thousands, cut the
leaves into pieces, and carry them to their nests. There the
leaves are chewed to a dough which is built up in a loosely-
packed heap. In the interior of this the ants form numerous
chambers in which they place their young. Soon after this
peculiar nest has been formed we are able to observe the appear-
ance in the heap of threads of a certain kind of fungus, which
form in places little 'heads,' the so-called 'kohlrabi.' These
remarkable formations are undoubtedly a cultivation-product of
the ants, for if the fungus is withdrawn from the care of the
ants the formation of these ' heads ' does not take place. They
form the staple food of the ants and are cultivated by them for
this purpose.

The numerous animals which live in the nests of ants and
termites have further, as a result of that mode of life, under-
gone remarkable changes. In the ' hills ' of termites in the East
Indies, South Africa, and the Soudan is found a small species of fly
(Termitoxenia), from 1 to 2 millimetres long. If accurate investi-
gation had not proved beyond doubt that these little animals are
really flies, no one would judge from their appearance that they
belong to the Muscidse. In place of the two wings these flies
carry small club or hook-like formations which owing to their
horny structure are totally unsuitable for flying, and are, in fact,
used for very different functions. According to Wasmann, they
serve as balancing poles to keep the body in equilibrium. They
act further as breathing-tubes, organs of touch, and of exudation
which secrete from the blood a volatile substance greatly liked by
their hosts. Finally, these transformed wings serve as organs of
transport by which they are lifted up by the termites and carried
about. These little flies are, in fact, walking contradictions.
They are not only diptera without wings, but flies without a
larva or pupa stage, and insects without males and females.
They are hermaphrodites, both sexes being united in one body.
Apart from these flies we know numerous other beetles, etc.,
which are kept by ants or termites as domesticated animals, fed
by their hosts, and have greatly changed in consequence of this
altered mode of life.

THE FACTORS OF EVOLUTION 185

But let us return to Darwin's theory of selection. We have
seen that natural selection alone can supply no satisfactory
explanation of the evolution of the organic world, that its value
in the first heat of enthusiasm has been greatly exaggerated, and
that it is in no case possible to prove that the struggle for exist-
ence does perform the role of the breeder. Hence many natural-
ists have now arrived at the conclusion that the part played by
the struggle for existence is not so much one of selection, but
rather tends to exterminate inferior individuals, and to exclude
them from the reproductive function. But here, again, we must
be careful not to exaggerate the value of elimination, for
numerous cases are known of cripples with serious pathological
defects who have been able to live many years in spite of this
supposed fierce struggle. For instance, Pauly describes a pike,
about 50 cm. long, with a malformation of the upper jaw, giving
the appearance as if this had been cut off 2 or 3 cm. from the
point. But though the fish was greatly hampered in its chase
after prey, it was yet well able to maintain itself until chance
brought it into the fisher's net. Hofer mentions a case of a
two-year-old carp whose mouth-aperture was grown together in
such a manner that the fish was forced to take its food through
the gill-clefts.

But if the theory of selection does not offer a satisfactory
explanation, how are we to conceive the process of the origin of
species? We have already heard how Lamarck attempted to
overcome these difficulties. His explanation seems clear, even
if it does not suffice for all phenomena, in particular for the
acquisition of passive adaptations. It is, however, based to some
extent upon a factor which has not yet been proved, — the heredity
of acquired characters.

If we consult the scientific literature upon this point we find
widely divergent views. While some writers flatly deny the
possibility of acquired characters, others equally emphatically
support it. Let us, therefore, consider the facts so that we may
form our own conclusions.

In order to demonstrate the impossibility of such transmission
some people cut off the tails of mice, repeating this process
during several succceeding generations ; and as the descendants
of these tailless mice were nevertheless always born with fully

186 LECTUEES ON BIOLOGY

developed tails, this experiment was thought to have furnished
a conclusive proof against the transmission of acquired characters.
It is, however, hardly possible to imagine a more naive experi-
ment, for there can scarcely anyone be found to believe that
the loss of an organ takes place in Nature in such a rough and
ready manner. That external injuries of parents are not trans-
mitted to the children should be accepted a priori without any
such experiments. Moreover, we can on this point refer for a
proof to a far greater experiment. Though circumcision has
been practised among the Jews on an enormous scale. for several
thousand years, nothing has ever been heard of the foreskin
having, as a result of the operation, begun to atrophy or become
degenerate. In another case, however, it seemed as if under
certain circumstances injuries, or at least their consequences,
are transmissible. Thus Brown-Sequard contended that epilepsy
caused in guinea-pigs by severing the Nervus sciaticus had
actually been transmitted to the descendants. Later experiments
made by Sommer have, however, cast some doubt upon Brown-
Sequard's observations, so that in the absence of corroborative
evidence we are compelled to disregard them.

The classic instance of the transmission of acquired charac-
ter are the important experiments made by Weismann, Fischer,
Standfusz, and others, with butterflies. Fischer selected the Tiger
moth (Arctica caja), and fed a large number of young cater-
pillars on white dead-nettle and dandelion, their ordinary food-
plants. One hundred and two caterpillars reached the pupa stage.
For purposes of control, some of these pupae were kept under
normal conditions and produced normal butterflies ; the rest,
were exposed intermittently to a temperature of — 8° C. The
result of these unusual conditions, which were well sustained
by the pupae, was most remarkable. The emerging butterflies,
forty-one in all, were considerably darker than their normal rela-
tives ; in particular the brown or black spots or stripes on the
anterior and posterior wings were much larger. It seems plain
that these changes were the direct consequence of the effects of
the cold to which they were exposed during the pupa stage.

Fischer now selected among these butterflies a male possessing
this variation in a strongly pronounced form, and paired it with a
female exhibiting the characteristic in a similarly pronounced

THE FACTORS OF EVOLUTION

187

manner. Their progeny were kept at the ordinary room tem-
perature of about 20° C. Of the resulting 173 butterflies most had
the normal appearance, that is, they had reverted to their original
form ; but seventeen of those that had emerged last exhibited the
characteristics of their parents. Other experiments undertaken
later with various species of Vanessidse produced similar results
(fig. 50).

Here we have a
clear proof that
changed conditions
of life can produce

changes that may be mm , \

transmitted to sub- B

sequent generations.
If Weismann objects
that we are here not
dealing with a trans-
mission of acquired
characters, because
the cold did not only
change the body-
cells, but simultane-
ously also the germ-
plasm, we cannot
admit this objection
as relevant. The
qjuestion is not
whether changes
affecting the body-
cells are transmiss-
ible, but whether it is
possible for a change
in the conditions of
life to cause reactions
and changes in the
organism that can be
transmitted to the descendants. It is obvious that nothing
can be transmitted which is not deposited in the heredi-
tary substance of the body. But if the individual is able to

FIG. 50. — INFLUENCE OF TEMPERATURE DURING THE
PUPA STAGE UPON COLOUR AND DESIGN OF BUTTERFLIES.

(1) Camberwell beauty (Vanessa antiopa L.}.
(2) V. antiopa L. v. hygicea Hdrch., obtained after
exposing pupa two hours twice a day for four days
to a temperature of — 10° C. (after Lampert).

188 LECTURES ON BIOLOGY

acquire changes in its germ-plasm, and if these changes appear
afterwards in the body of the descendants, it is none the less a
transmission of acquired characteristics. It is only essential
that the acquired change is permanent, and is not lost with
the disappearance of the cause. Hertwig justly says that 'a
possession is what one has acquired ; an inheritance, that which
passes to the descendants. How the possession was acquired
can in nowise affect the question whether inheritance shall be
called apparent or real.' Hertwig further says that the quoted
' instance of the temperature-experiments with butterflies leaves
no doubt whatever that the acquired characters of the
parents were transmitted to the descendants.' The possibility of
a transformation of the species in the sense of Lamarck receives
strong support from these experiments. Selection plays, of
course, a certain part, because among the numerous normally
coloured butterflies he selected those that exhibited modifications.

Engelmann has further shown that it is possible to alter
the natural colour of certain Algae (Oscillatorid) by exposing
them some time to coloured light. Very soon the little plant
assumes a complementary colour. If, for instance, we expose
an Alga for several weeks to green light it becomes red, and
retains this colour even when it is kept once more in ordinary
light. As during this process cell-division, i.e., reproduction,
continues, and as the daughter-cells similarly retain the red
colour, it is clear that in this case, too, the change has become
hereditary.

It is known that firs and larches on mountains grow more
slowly and form thinner rings than those that grow on the
plains. If we sow the seed of Alpine forms in districts slightly
above the level of the sea, it will be found that they retain their
acquired characters, that is to say, they will differ from the
plant, the seed of which was ripened in the plain, by slower
growth and thinner rings.

For the origination of new species by external influence
it is not even necessary that the newly-acquired characters
are really transmitted. It is sufficient for the purpose that a
change in the conditions of life is able to cause changes which
remain constant so long as the changed conditions remain.
If we, for instance, assume that our climate had become colder,

THE FACTORS OF EVOLUTION 189

and that the pupae of the butterflies would be exposed during their
development to such a cold temperature as that to which they
were subjected in Fischer's experiments, then the same causes
would, according to our knowledge, produce the same results in
nature. We should see species of butterflies exhibiting modifi-
cation in their colour towards the dark shade. As the descen-
dants of these modified butterflies would again be exposed to
the same conditions, they would possess the same charac-
teristics as their parents, no matter whether these newly-
acquired characteristics had been transmitted by their parents
or not.

It is clear that the change in the conditions of life
exercises an influence not only upon colour, but also upon size,
scale formation, etc. In course of time, therefore, animals would
come into existence so widely differentiated from their ancestors
that we should regard them as new species. The original
species, however, would in the meantime have become extinct.
Whether the characters of these new species would be
really constant and able to maintain themselves in the event
of the reconstruction of the original conditions does not affect
the question which could, indeed, only arise if some one would
be able artificially to reproduce the old conditions. If it were
possible to sow seeds of different plants of our earth on any
suitable star in the Universe, the plants that grew from those
seeds, would probably be so changed that we should be unable
to recognize them.

How radical the changes can be which are produced in an
organism by an artificial modification of the normal conditions
of life will require further attention. It is not saying too
much when we describe animals and plants as compulsory forms
dependent in appearance and structure upon the stimuli which
have influenced them during their life. There exists, as it were, a
fluctuating balance between the organisms and the external con-
ditions of life : a change in the stimuli calls forth corresponding
changes in the organization. The simpler an organism, the less
resistance it seems generally able to offer to changes. From
numerous facts one gains further the impression that the length
of the action of a stimulus has a considerable influence upon
the constancy or inconstancy of a certain modification.

190 LECTURES ON BIOLOGY

On starch-containing substrata — moist bread, potatoes, etc. —
appear sometimes little red spots which look like drops of
blood. This strange phenomenon gave rise to the legend of
the ' bleeding host/ and was frequently employed for the pur-
poses of the Church. The cause of the ' bleeding ' of the bread
is a minute organism, the so-called miracle-monad (Micrococcus
prodigiosus) , whose colonies are distinguished by a bright blood-
red colour. But if we take this little bacterium from its ordinary
nutrient medium and place it in an alkaline agar-agar solution
its red colour becomes less and less distinct and finally dis-
appears altogether. The vitality, however, of the micrococcus
is by this experiment in nowise injured, for if we return it
after a short time to its normal condition the red colour
reappears. But if we cultivate it on agar-agar for a considerable
time it grows afterwards white on bread and potatoes, and will
regain its normal colour only after many generations. We see
here quite clearly that changed conditions do produce a change
in the organisms, and that these are more permanent the longer
the new conditions have been in operation.

We have already seen that irritability is one of the funda-
mental properties of the protoplasm. We saw, for instance,
that the little Amoeba Umax may be influenced both in its
vital functions and in its shape by mere changes of temperature.
If the temperature was reduced the animalcule became rigid and
assumed the shape of a sphere ; if the temperature was increased
it revived, unrolled itself, and crawled about, only at a further
increase in the temperature, once more to lapse into heat coma
and contract into a sphere. Still more interesting and im-
portant are those changes which we are able to produce in the
same animalcule by chemical influences. It is known that the
form of the pseudopodia in the various species of Amoebae is
the principal means of distinguishing between the different
species. Thus Amoeba Umax forms usually only one long
pseudopodium. A. proteus is distinguished by numerous shape-
less pseudopodia, and in A. radiosa, finally, the pseudopodia
radiate from the body in all directions in the form of long spines.
The last mentioned species, in particular, with its peculiar con-
struction, could not be mistaken for any other Amoeba. But let
us now observe A. Umax under the microscope. At first the

THE FACTORS OF EVOLUTION 191

animalcule lies quietly, rolled up into a ball, but soon it pro-
trudes in all directions short amorphous pseudopodia, and we see
before us a form which reminds us of a young A. proteus. Then
the whole protoplasmic body expands longitudinally, and the
typical A. Umax crawls about beneath our eye. If we now leave
the animalcules undisturbed they will permanently retain this
form, but if we make the wrater slightly alkaline by adding a trace
of liquor potassae the Amoebae suddenly roll themselves up
as if frightened, but gradually become accustomed to these
changed conditions and project once more their pseudopodia.
But remarkable to relate, these are now no longer broad and
shapeless, but pointed and spiny : the Amczba Umax has become
a typical radiosa. This form remains permanent as long as the
water remains slightly alkaline; but when they are once more
placed in ordinary water the compulsory form disappears and the
normal Amoeba Umax reappears.

Among higher animals and vegetables numerous instances
are known which form an eloquent proof of the dependence of
the organisms upon their environment. We have already seen
that the temperature exercises a far-reaching influence upon the
colour of the butterflies. Phenomena such as have been obtained
artificially in the Tiger-moth and other butterflies may be also
observed in Nature and are then described as seasonal dimorphism.

A little butterfly (Vanessa levana) occurs with us in two
forms which differ so greatly from each other that for a long
time they were regarded as two distinct species. As soon as
the sun sends down its first warm rays the little spring-form
emerges from the pupa and begins its flight. Each collector
knows the pretty brown-coloured butterfly with the coloured
design on its wings. From the eggs of the spring-form deve-
lops about July the summer-form, which is not only considerably
larger but possesses a black ground colour with light spots and
stripes. As the pupae of the spring-form must pass through the
cold of winter, but as the summer form is never exposed to cold
the attempt was made to seek an explanation of the difference
in appearance in the temperature. Experiments confirmed the
hypothesis : we may at any time produce the spring-form from
pupae which under normal conditions would have been developed
in the summer-form simply by exposing them to cold. Con-

192 LECTURES ON BIOLOGY

versely, in warmth the eggs of the summer-form will duly
produce once more a summer-form. Analogous results were
produced by experiments made with other butterflies in which
seasonal dimorphism occurs. We have, therefore, good grounds
for believing that the different races of the various species of
butterflies which live in the Northern zones were produced by
climatic influences. This is in particular proved by a case
mentioned by Eimer. ' The common Lycseniad, Polyommatus
phlceas, which occurs from Lapland to Sicily, has in Lapland
one generation, in Germany two. But only in South Germany
are these two generations distinct ; in North Germany they are
still identical.'

' Another Blue, Lyccena agestis, exhibits a double seasonal
dimorphism, for it occurs in three forms. A and B in Ger-
many alternate as winter and summer forms. B and C in Italy
occur successively as winter and summer forms. Form B,
therefore, occurs in both climates, but in Germany as a summer
form, in Italy as a winter form. The German winter form A
is entirely absent in Italy, whilst the Italian summer form does
not occur in Germany. We have here a distinct if small
chain of changes, obviously caused by climatic conditions.'

As in the higher animals, so in the butterflies the sexes
frequently differ widely in appearance. The male ' lemon-
butterfly ' (Rhodocera rhamni) is distinguished by a brilliant
yellow colour, but the wings of the insignificant female are
pale yellow, almost of a whitish hue. If we keep the pupae at
room temperature the seasonal dimorphism disappears, and the
female butterflies, too, rejoice in the decorative colours of the
male. Standfusz has further succeeded with the magnificent
Parnassius apollo to give by warmth to the female the colour
of the male, by cold, to the male the colour of the female. When
we see this correlation of temperature and colour and design
in animals is it a wonder that doubts should arise whether so-
called protective coloration and colour-adaptation can be regarded
as a result of natural selection? Is it not rather more likely
that the colouring, etc., is the effect of different stimuli which
so far we have been unable to analyse ?

Like heat and cold, light has a far-reaching influence upon the
shape and colour of the organisms. Everyone knows that the

THE FACTORS OF EVOLUTION 193

inhabitants of dark caverns are disposed to lose the pigment
of their body surface. The famous denizen of the Adelsberg
grotto, Proteus, is quite colourless. But if it is kept for some
time in a well-lighted aquarium the skin begins to form pigment
and its colour becomes darker.

Light may further have considerable influence upon the
course of the processes of regeneration. On this subject Loeb,
whose valuable labours have elucidated many difficult points, has
made a series of remarkable experiments. In every marine
aquarium is found a pretty polyp-stock, Eudendrium ramosum,
a relative of our fresh-water polyp Cordylophora lacustris.
These organisms are highly sensitive, and after the first few
days of their captivity, in consequence of having been handled,
throw off all ' heads,' only the bare trunk remaining. If the
aquarium has plenty of daylight the damage is quickly repaired,
for in a few days the trunk has regenerated numerous polyps. In
the dark, however, not a single head appears even after many
weeks. A similar result is obtained if we expose them to
different-coloured light ; blue rays favour regeneration, while
red rays have the same effect as complete darkness. Finally,
if we expose to red light the polyps that were formed in blue
light they inevitably perish.

Plants are to a still higher degree affected by light. Light is
not only an essential condition of the nutrition of all green plants,
but controls also in a high measure their growth, formation of
organs, and generally the whole external appearance. What
an enormous difference is there for instance between the Black
and the Lombardy Poplar. The cause of this dissimilarity is
the light. In the Black Poplar generally the upper side of
the branches is exposed to the daylight, and it is only here
that the leaf-buds are able to develop. Exactly the reverse is the
case with the high-rising branches of the Lombardy Poplar,
which are mainly illuminated on the under side and therefore
develop the leaves below.

Most striking is the correlation of form and the effects of
light in the organs of plants which carry the chlorophyll and
carry on assimilation. In the normal state the stems of the
Prickly Pear (Opuntia), which is very common in the countries
of the Mediterranean, are greatly flattened and therefore often
13

194 LECTURES ON BIOLOGY

regarded as the leaves of the cactus. However, the leaves
proper of Opuntia have degenerated into spines. If we keep
this plant for some time in a cellar or in a darkened room the
stems continue their growth cylindrically, i.e., in the original
form, and of a flattening there is no longer a trace. Again, in
many orchids it is the roots which carry the chlorophyll and per-
form the functions of leaves. As long as the roots remain hidden
in the moss-cushions of the branches of the trees on which these
orchids are parasitic they retain their cylindrical form. Only
when they come forth into the light, i.e., when they can take
up their new functions of assimilating carbon dioxide, they
proceed to flatten.

A still better proof is supplied by one of the liverworts,
Marckantia, which flourishes in many districts in large colonies
in moist sandpits, on stones, or in flower-pots. This little plant
possesses a rather large dorso-ventrally flattened thallus, the
upper side of which is strikingly differentiated in structure from
the under side. Nevertheless it is uncertain, in the spores as
well as in the germ-discs originating from the spores, which of
the parts will afterwards develop into the upper side and which
into the under side. But if we expose the germ to daylight only
for two or three days it becomes definitely settled, though
neither a flattening nor differentiation is visible, that the illu-
minated side is to be the upper side. Whatever we may now
do with the young plants will not alter this fact : a stimulus
extending only over a short time was sufficient permanently
to fix its structure.

Different is the behaviour of the prothallus of the ferns. In
his the non-illuminated underside which lies on the ground carries
the rhizomes and the male and female sex-organs. But if we
make a newly-formed prothallus pass its development swimming
on a nutrient medium we may, at will, change the upper
into the under side, and back again into the upper side, merely by
altering the lights. In order to fix the structure definitely it is
in this case necessary to let the stimulus remain active through-
out the entire period of development.

If I now mention that the ivy forms its aerial roots invariably
on the shady side, that the anatomical structure of the leaves of
foliage trees differs in proportion as they are well or indifferently

THE FACTORS OF EVOLUTION 195

illuminated or grow in the dark, I hope to have adduced a suffi-
cient number of facts to prove the formative power of the light.

In botany an apparatus is frequently used in which a hori-
zontal axis is made to rotate slowly by clockwork. This clinostat,
as it is called, enables us to withdraw objects fastened to the
rotating drum from the influences of the law of gravitation.
The results obtained are in many cases very wonderful. For
instance, if we place climbing plants on the clinostat, the
turning movements of their stem cease, the younger growing
parts unravel, and the entire plant extends longitudinally.

A still better instance of the influence of gravitation is given
in experiments by Massart, recently published by Lotsy. In the
manner mentioned Massart withdrew a tropical plant, Alloplectus
sanguinea, which is distinguished by its asymmetrical leaves,
from the influence of gravitation, without bringing about any
particular change ; even newly-formed leaves exhibited a similar
asymmetrical form. But if the seed was permitted to germinate
on the clinostat, Massart obtained plants with symmetrical
leaves.

Interesting observations concerning the effects of altered
gravitation on the development of the eggs of a water-beetle,
Hydrophilus aterimus, were recently made by Megusar. Like
its better known relative the Great Water-beetle, H. piceus, this
insect builds peculiar boat-like cocoons which float on the surface
of the water. In the interior of the firmly closed fabric the eggs
stand in close vertical formation. The lower part of the eggs

-k OO

becomes afterwards the anterior side of the embryo. This
position remains unaltered during the normal course of develop-
ment, but when Megusar turned the whole cocoon upside down
and fixed it in this unusual position, thereby changing at the
same time the poles of the eggs themselves, he first of all retarded
their development, for the first larvae appeared several days later
than those which had come from a cocoon which had been left in
its normal condition. Furthermore, the size of these larvae was
much less than that of normal larvae ; they possessed plumper
bodies ; their movements in the water were awkward and clumsy ;
they endeavoured in vain to seize their prey, and after a few
days were found dead.

There are in nature only very few cases in which the formative

196 LECTURES ON BIOLOGY

influence of environment is so distinct as in Polygonum amphi-
bium, a relative of the well-known knot-grass, which was recently
made the subject of important experiments by Massart. This
one instance alone would suffice to refute the doctrine of the
constancy of organic species and to show that organisms remain
alike only under like conditions and change with a change in
circumstances. This little plant is exceedingly modest in its
demands; it flourishes equally well on marshy land and in the
water, and is even found on the hot sand of dunes. But it
changes its appearance according to its habitation. At a first
glance we are only able to distinguish between the land-form and
the water-form, and between this and the dune-form. On the tend
Polygonum amphibium is distinguished by an erect stem covered
with knots. The small lancet-shaped leaves are, particularly on
the under side, furnished with cilia and attached to the stem by
very short foot-stalks. In the water-form, however, the stems are
much longer, float on the water surface, and grow horizontally.
The number of the leaves is increased, they have lost their cilia,
changed their form, developed longer foot-stalks, and formed in
addition below each knot large rudimentary roots. The difference
does not only extend to external appearance but is seen also
in the internal structure, for while the stem of the land-form
is solid that of the water-form is hollow. In the dune-form,
finally, the branches crawl on the sand, the knots are greatly
enlarged, the foot-stalks short, the leaves small, thickly covered
with cilia, and of a viscid nature. As Massart showed, all
these radical differences are the direct result of the influence
of environment, for if we lead the branches of the land-form
into water the newly-formed leaves and stems assume at once the
shape of the typical water-form ; if we transplant the water-
form on wet land the stems become erect and form ciliated leaves
with short foot-stalks. Finally, if we place the dune-form into
water all existing leaves rapidly perish, the stems become longer,
and the new leaves are the typical long-stalked leaves of the
water-form.

Let us now consider for a moment the effects of chemical
stimuli. It is generally known that the salts and other sub-
stances contained in the soil are able to affect the growth of the
plants and the production of fruits favourably or unfavourably-

THE FACTORS OF EVOLUTION 197

On this knowledge is based the system of artificial manuring
and cultivation of the soil, which through the exhaustive investi-
gations and experiments of the last two decades has been raised
to a science to which agriculture principally owes its increased
productivity.

In order to obtain information concerning the nutritive value
and effects of a substance, water-cultures are employed, i.e.,
the seeds are made to grow in vessels containing distilled water
to which the salts under examination are added in a chemically
pure form and in definite quantities. In a solution which
contains all the necessary salts land-plants flourish as well as
under natural conditions, but if we omit one or another of the
substances we observe immediately a retarding of the growth
and other changes. If, for instance, we omit to add iron the
young leaves become pale, assume a yellowish colour, and are
no longer able to decompose carbon dioxide. The plant is
therefore doomed to die, but so long as there are signs of life we
may again restore the green in the leaves by adding soluble
iron salts. Other substances are able to produce in the plants
entirely new characteristics. If, for instance, we grow maize
in a water-culture containing hypo-sulphuric magnesium the
plant produces flowers which differ so much from the normal
that we are unable to regard them any longer as maize-flowers.

Chemical stimuli may greatly affect the life and development
of animals. For obvious reasons such experiments are mostly
conducted with aquatic animals. Several years ago Loeb
discovered the interesting fact that slight traces of potash when
added to the water accelerate development, while the adding of
a small quantity of an acid has a retarding effect. If we transfer,
during their developmental stage, the fertile eggs of echinoderms
into sea-water completely free from lime, the embryo-cells
continue to live, but they gradually fall asunder, and in place of
an organism we see ultimately a chaotic heap of individual cells.
If we decrease the amount of lime in the sea- water by only one-
tenth the process is likely to produce remarkable disturbances in
many animals. The Pluteus-larvae of the sea-urchin are normally
distinguished by the possession of long tentacle-like processes
which are supported by a special lime-skeleton, but in water with
a diminished amount of lime the formation of a lime- skeleton

198 LECTUEES ON BIOLOGY

is suspended, the arms become rudimentary, the larvae assume
the shape of a hemisphere, and recall only distantly the normal
Platens.

Considerable sensation was caused not long ago by an observa-
tion made by Schmankewitsch in some of the lower crustaceans.
In the brine-pools on the coast of Istria is found a small
crustacean, the Brine-shrimp (Artemia salina). It is distin-
guished from its northern relative Branchipus, which occurs in
many districts of Germany in ponds and ditches, chiefly by the
smaller number of abdominal segments and certain other
characteristics. Schmankewitsch contended that by a gradual
sweetening of the water it is possible to transform Artemia into a
typical Branchipus, and further, that gradual increase in the
salinity transforms Artemia salina into a different species, Artemia
milhauseni, which is distinguished by a very short, thick, spineless
tail-lobe. Kecently the statements made by Schmankewitsch
have been contradicted, at any rate in this form, by the investi-
gations of Samter and Heymons. Nevertheless, it is certain — and
this is the essential point — that the change in the salinity of the
water undoubtedly brings about a radical change in Artemia
salina, even if it cannot transform it into a typical Branchipus
or Artemia milhauseni.

Numerous observations of apiculturists and observers of ants
have made it clear that the variety of form in ants, bees, and
termites is mainly the result of an external factor, fcod. Thus,
if the queen of a bee-colony is dead and there is no royal brood,
the bees are able to transform ordinary worker-larvae into queens
by giving them suitable food. In bees we also seem to obtain
a deeper insight into the causes which determine the future sex
of the animals, for it is believed that the drones, i.e., male bees,
always develop from the unfertilized eggs of the queen, while
queens and workers always develop from the fertilized eggs.

Numerous instances are known of the influence of food upon
higher animals. In tadpoles which are fed on vegetable matter
the intestines reach a length which exceeds that of the body
about seven times ; but if we feed the larvae exclusively on flesh
food the intestines become only half as long. Canaries fed with
cayenne pepper assume a reddish colour ; a course of hemp-seed
diet gives to bullfinches a black colour. The natives of the

THE FACTORS OF EVOLUTION 199

region of the Amazon Kiver feed the common green parrot with
the fat of certain cat-fishes and thus produce birds with brilliant
red and yellow feathers. Finally, it is well known that horse-
dealers frequently dose horses before a sale with small quantities
of arsenic, as a result of which the body fills out and the hair
assumes a smooth and silky appearance.

I must not omit to mention here that a change of external
conditions does by no means affect 'all organisms in the same
manner, but that rather many stimuli which effect radical changes
in one animal species frequently pass by others for considerable
periods without producing the slightest effect. An instance will
make this clear. Towards the middle of last century the
delicate, branched trees of a polyp, Cordylophora lacustris, were
only known on the European and North American coastlands-
Suddenly, however, the little polyp began to wander up the river
into the interior, and now we find them in many parts of Germany
inhabiting rivers and inland lakes. In many towns Cordylophora
makes itself most unpleasantly noticeable by invading the water-
pipes, where it proceeds to grow luxuriantly, and thereby finally
stops them up. The life-history of this polyp forms one of the
few instructive cases of a typical marine species changing
within a short time and almost under our very eyes into a fresh-
wateranimal. But in spite of the greatly changed conditions of life
the organism of the animal has hitherto been in nowise modified.

If we observe the cycle of evolution of an animal or plant
as a whole we see how numerous different stages follow upon
one another in law-governed succession. As the hand of a clock
must traverse one hour after another, and can only indicate the
twelfth hour after having previously indicated the eleventh, so
in the organic world the succession of stages appears to be
unalterable. The embryo develops into a child, the child
becomes a boy, the boy a man, and only he may reproduce a
similar offspring which in his turn must travel the same cycle.
This regularity is manifest throughout the whole of Nature,
and it does not, therefore, cause surprise that the mind of man
conceives it as an iron necessity. According to a famous botanist
' each stage of development generates the next in succession ;
it operates continuously ; there is no single stage which may be
omitted. Each differentiation is the condition of the next.

200 LECTURES ON BIOLOGY

With a machine-like precision and inevitable necessity one
phase follows another until the * inherited form has become
complete.'

If there were no progress on earth, if the conditions remained
always alike, this statement would be correct, but under present
circumstances it is impossible to agree with it in this rigid form.
The organism is more than a machine, for it is able to adapt
itself. The course of development, too, is, to a certain degree,
subject to external influences : certain stages may, under certain
circumstances, be suppressed, not only for a short time, but also
permanently ; even the succession of developmental stages may
be altered. A few observations will prove that this statement is
incontrovertible.

We distinguish among the mosses two generations : an asexual,
which reproduces by means of unicellular spores ; and a sexual,
which originates from the spores and produces male and female
germs. From the fertilized egg-cell develops then once more the
asexual generation which is represented by the stalked capsules.
These two generations alternate in a regular cycle. In the
ferns we observe a little deviation from this scheme, in that there
proceeds from the spore at first the so-called protonema from
which the moss-plant proper grows with its sex-organs. But, as
Klebs has shown in the minute Funaria hygrometrica, it is
possible by withdrawing the light to suppress the sexual genera-
tion. While normally the life of the protonema was but brief,
and was extinguished soon after the development of the moss-
plant, it grew in the dark almost continuously for two years with-
out producing germ-cells. The early form remained therefore
alive far beyond its ' destined span.'

Like the mosses, the ferns have an alternation of generations.
Normally the spore is followed by the prothalltis which carries
the sex-organs, then impregnation takes place, and from the
impregnated egg develops the asexual generation, the fern
proper, which later proceeds to spore-formation. This succession
is as definite and regular as the course of development in the
higher animals and in man. But under special conditions we
may succeed in shortening the development, and in omitting not
only the most comprehensive and important stage, the fern-
plant itself, but in suppressing at the same time the development

THE FACTORS OF EVOLUTION 201

of the sex-products. It is true that the spore produces the
prothallus as before, but this does no longer produce sex-organs,
but proceeds, contrary to all rules, at once to the formation of
spores.

It seems now clear that the evolution of the organism is
strictly dependent upon numerous external factors. We have
seen that organisms are not unalterable quantities, but that
there exists correlation between them and surrounding nature.
Alterations of the conditions of life immediately call forth more
or less radical alteration in the characters of the species.

It is a question of another kind whether the change thus
produced may be regarded as an adaptation, or, in other words,
whether the organism responds to each stimulus by a suitable
modification. If we recall the different changes that took place
in Polygonum amphibium in water, on wet land, and on the
dunes it would seem as if we had here to deal with conscious
reactions. Many other facts may be interpreted in this sense.
But if, for instance, a bullfinch assumes a black colour on a diet
of hemp-seed, or an oyster, transplanted from the North Sea into
the Mediterranean, forms on its shell rich ornaments and long
spines ; or if, finally, the egg of the Sea-urchin, forced to develop
in water poor in lime, produces a Pluteus which has no support-
ing skeleton or arms, we are unable to perceive the purpose
in such changes. Indeed, many changes are obviously injurious
and weaken the vitality of the organism concerned. We see
therefore that direct influence produces useful as well as
indifferent and injurious alterations.

Each organism maintains, as we have already seen, an ever-
changing balance with its environment. If the equilibrium
is disturbed by a change in the external conditions in life, the
organism either perishes or continues to modify until a new
state of equilibrium has been created. But this new state need
not necessarily be an advance upon the former, but only a more
or less successful ' squaring ' with the changed conditions of
life. It is exactly upon this that the constancy of the new
form depends. If the acquired modifications are useless or
indifferent, the individual concerned may be able to maintain
itself; but if they are injurious we are justified in assuming
that natural selection steps in and extinguishes the useless
organism.

202 _ LECTURES ON BIOLOGY

It might be asked here whether new species may not also
be produced by crossing different species. This question is
justified, but as I shall have to refer later on, when dealing
with the theory of heredity, to the famous crossing-experiments
of Mendel, I need here allude to it only very briefly.

As long as the dogma of the constancy of the species was
omnipotent the possibility of a crossing of species was flatly
denied. This denial is all the less comprehensible as the
bastards of the ass and the horse were known in Egypt, Persia
and India thousands of years ago. It is, however, possible that
as both mule and hinny are sterile this instance was not regarded
as possessing any value. But to-day we know a large number
of results of crossing not only different species, but even different
genera. I need only mention the bastards of sheep and goat,
dog and wolf, hare and rabbit, canary and different finches,
salmon and trout. It is true that in most cases the bastards
possess degenerate sex-organs and are sterile, but we know,
nevertheless, already quite a large number of crosses which are
capable of reproduction. The bastards of rabbits and hares
have remained fertile through many generations, and bastards
of the domestic goat and the steinbok, of different species of
butterfly, etc., have produced several generations of descendants.

It is remarkable that it is very easy to cross the male of one
species with the female of another, but not vice versa. Thus
the egg of salmon may be fertilized with the seed of trout and
developed, but every attempt to cause the development of trout-
ova with the spermatozoa of salmon has ended in failure.

Many interesting results were derived from experiments
which Standfusz made with butterflies. Here it appeared that
the products of crossing generally followed in appearance the
phylogenetically earlier form. If, for instance, we cross the
female of the Poplar Hawk-moth (Smerinthus populi} with a
male of the Eyed Hawk-moth (Smerinthus ocellatus'), the
descendants of the two species resemble more the phylo-
genetically older S. populi than S. ocellatus. The same result
was obtained in crossing a male Dilina tilice with S. ocellatus.
In this case, too, the characters of the phylogenetically earlier
kind predominated in the descendants.

It seems not open to any doubt that by crossing we can obtain

THE FACTOES OF EVOLUTION 203

animals with new characters. But as animals generally
only pair with like and entertain a distinct antipathy to unlike,
we Ccannot ascribe to this factor an important role in the origin
of species. The race feeling is in most animals so strongly
developed that very highly differentiated individuals are fre-
quently avoided by their own species and even persecuted.

It only remains to examine one more point. We heard that
variability is one of the fundamental conditions of natural selection,
but we have not yet examined the question whether the selection
of such variations really can produce new species, and whether
such selection is not quickly obstructed by natural impediments.
This objection might to many seem irrelevant, because the facts
of artificial selection and the history of domesticated plants and
animals have already decided this question affirmatively. Never-
theless, the doubt is fully justified. Many naturalists even go so
far as to regard the results of artificial selection as strong evidence
against the possibility of selection in Darwin's sense. In an
important (though one-sided) paper1 G. Wolff sums up his doubts
thus : * It is true that the theory of selection proceeded from an
empirical fact, the results of an artificial breeding of plants and
animals, but from these facts it could never deduce an empirical
justification of the evolution theory. The facts of artificial breed-
ing are rather to be interpreted as an empirical proof of the
constancy of the species, because a change in form can only be
produced by it within certain well-defined limits that may not be
transgressed. The very fact that in many cases we are able by
artificial selection to produce comparatively considerable modifi-
cations, often within a few generations, is an empirical argument
not for, but against Darwin's doctrine of natural selection, for
the manifest limitation of the sphere of effectiveness of artificial
selection speaks against one of the first suppositions of the
theory of selection, which demands such a plasticity of the
living material as to make a constant unlimited alteration of
organisms in every possible direction possible by the process of
selection. It is precisely because artificial selection performs so
much that it proves so little. It would be so much more
advantageous for the tactical position of the theory of selection

1 * Die.Begriindung der Abstamnmngslehre.'

204 LECTURES ON BIOLOGY

if artificial selection could not point to any successes, i.e., if their
supporters could employ here, too, their only strategic method of
defending themselves behind the fortifications of the impossibility
of controlling their hypotheses. But in no case can the facts
of artificial selection be employed for proving empirically the
doctrine of natural selection, for all that they can do is to render
possible the idea of natural selection and its explanation.'

' Never was there a theory so completely without proofs taken
from experience. All hypotheses of the doctrine are put for-
ward as self-evident, and are thus so much without an empirical
foundation that even the first hypothesis of the theory, the
survival of the fittest in the struggle for existence, is an assump-
tion which is supported by no observation and the justification of
which is confuted by everything that experience may ascertain.'

These are grave accusations made by a serious thinker who
certainly cannot be suspected of dogmatic prejudice. Let us now
consider what are the grounds upon which Wolff takes up his
hostile attitude.

If we recall the many results of artificial selection, if we think
of the various races of pigeon, or of the pheasant in which
Japanese breeders have produced tail-feathers 4 metres long, it
would seem as if a natural limit did not exist ; as if, in fact, the
organism were ' like clay in the hands of the potter,' to which
selection can impart any desired form. But if we examine with-
out prejudice the various factors we shall observe everywhere
lines of demarcation beyond which selection is not able to pass.

When, in 1850, the systematic cultivation of the sugar-beet
was taken up in this country, the sugar in the beet-root
(Beta vulgaris) from which the sugar-beet had been produced by
artificial selection was at that time not more than 7 to 8 per
cent. To-day it is 14 to 16 per cent. Beyond this it has been
found impossible to increase the percentage of sugar, even in
spite of determined efforts and the most careful selection.

The weight of a gooseberry was towards the end of the
eighteenth century about 15 grammes. In the course of seventy
years this was increased to 60 grammes, but since then the weight
has remained at that point, though theoretically there is no
reason why gooseberries should not reach the size of apples or
pears. A similar limitation is apparent in other cases, but are

THE FACTORS OF EVOLUTION 205

we thereby justified in exploiting this fact as a proof of the
ineffectiveness of selection ? In my opinion this would prove
an entirely wrong conclusion.

By artificial selection we are generally able to increase at any
rate one defined characteristic. In consequence there arises in
the organism a discord, a disturbance of the equilibrium of
the correlated parts. The more extreme the development of a
characteristic, the greater the disturbance, until finally Nature
imposes a strict halt to any further advance. Man has no regard
for the well-being of the affected animals, but solely seeks to
increase those qualities which are of advantage to him. It is,
therefore, entirely wrong if someone — as for instance, H. St.
Chamberlain — regards our thoroughbreds as specific samples of
perfection, and seeks to derive from their genesis hints for the
improvement (Veredlung) of the human race. But we should
rather regard the animals with a one-sided development as cripples
which are barely capable of independent life. Would it not
sound ridiculous to speak, for instance, of a dachshund as
'improved' (Veredelt)? It is only the uniform, harmonious
development of all characters which we regard generally, and
in particular with regard to man, as improved (edel).

But let us return to our example. If it should be possible
to increase and improve, for instance, in the gooseberry, simul-
taneously with the size of the fruit, the strength of the branches,
the formation of the sieve vessels, tracheae, roots, circulatory
system, etc., selection would in all probability be able to obtain
still further success, and would thus remain for ever dissatisfied.

Many accurate investigations into the variability and the
power of increasing it have been made, in particular by botanists.
But, as is the case in the experiments with sugar-beet and
gooseberries, they have not hitherto led to incontrovertible
results; the conclusions, therefore, which have been drawn
from these experiments by the different investigators are widely
divergent.

If we examine a number of peas or beans, or the leaves of a
shrub, with reference to size we observe a remarkable law :
all the sizes vary around a mean. Most numerous are the
leaves of a medium size, and the variations become rarer in
proportion as they are larger or smaller than the medium size.

206 LECTURES ON BIOLOGY

This universal rule — called, after its discoverer, Quetelet's law-
does not only apply to differences in size but apparently also
to all other variations. We have already heard of the case of
the vertebrae of the herring, which also follow Quetelet's law.

While the variability remains generally unchanged as long as
circumstances remain unchanged, it is possible to alter the average
by a selection of greatly differing variations. But the longer the
selection operates the less progress is made, until finally a point
is reached which apparently cannot be passed by selection. Let
us consider in this light the breeding experiments made by
Miiller with maize. His attention was directed to the number
of rows in the cobs of Zea mays, which in the beginning of the
experiment varied between eight and eighteen. Twelve occurred
most frequently. If he sowed corns taken from a cob with
sixteen rows he obtained descendants having from ten to twenty-
two rows ; the average, therefore, had been shifted to fourteen.
In proportion as the experiment was continued the average was
gradually shifted to eighteen. At this point it came to a stand-
still. Strict selection was able to maintain this average but
unable to increase it, while in the absence of selection the
descendants returned to the original average of twelve. The
characteristics remained, therefore, constant only so long as
selection was maintained, and could only be increased up to a
certain point, beyond which selection was powerless. Whether,
in the event of the experiments being continued for very long
periods, a fixing of the characters obtained by selection and
a further shifting of the average is possible must at present
be regarded as an open question.

In addition to the fluctuating or individual variability we
may occasionally observe a jump in variations, described as
' mutation.' The most important difference between these two
forms is that the qualities produced by mutation are strictly
hereditary. But mutation frequently affects not only one or
another character ; it changes also more or less suddenly the
entire character of the affected organism. While in former
years mutations were not thought to be of any importance to
the origin of new species, De Vries made them the foundation
of a new theory of evolution. The classic object with which this
great investigator conducted his experiments was (Enothera

THE FACTORS OF EVOLUTION 207

famarckiana. As long as one does not know the conditions
under which new species are produced it will always remain
difficult, according to De Vries, to find suitable material for
an experiment of this kind, and here chance must help to
smooth the way.

In the 'eighties De Vries commenced to investigate the flora
in the environments of Amsterdam with the object of dis-
covering, if possible, a plant in the mutation stage. He was
fortunate enough to find what he wanted in 0. lamarckiana,
on a fallow potato-field near Hilversum. This plant had in
the course of ten years taken possession of more than half the
field in question. Among the many hundreds of individual plants
he observed in particular two forms which differed in so
many characters that they had to be regarded as new species.
They were afterwards described as 0. brevistylis and 0. Icevifolia,
and proved constant in cultivation.

In order to obtain more accurate information concerning its
mutability, De Vries transplanted in 1886 a certain number of
roots and seeds from the potato-field into his experimental garden
at Amsterdam. In the course of several generations the seeds
of these first individuals produced, in addition to normal plants,
suddenly, by mutation, seven new species, which were described
as 0. lata, mbrinervis, gigas, albida, oblonga, nanella and
scintillans. With the exception of lata and scintillans all proved
to be constant under self-fertilization. The other forms do not
interest us, as they either produced no seed or could not be
sharply differentiated, but the history of one of these new species
is most instructive.

In 1895, the fourth generation of the 0. lamarckiana , 14,000
seeds had been sown, of which the ' mutating ' individuals were
cultivated, but all others recognized as lamarckiana pulled up
to make room. In August about a thousand of these plants
were in bloom, but many had remained rosettes. Of these, thirty-
two of the most vigorous individuals were planted in a special
bed. In the following year they commenced to flower, and one
specimen at once attracted attention by its thicker stem, closer
inflorescence, and considerably larger flowers. Enclosed in a
parchment bag, it was artificially fertilized with its own pollen.
It produced a wealth of seed and formed the origin of a new

208 LECTURES ON BIOLOGY

species, 0. gigas. It is absolutely certain that its ancestors, at
least in the last three generations, were the ordinary 0. lamarck-
iana. The self-fertilized seeds of this TJrpflanze were in the
following year sown separately and produced more than 450
plants, which all exhibited the pure type of 0. gigas, there being
not one reversion to lamarckiana. Even in later generations
0. gigas remained constant. It seems therefore proved that a
new elementary species can appear suddenly in a single indi-
vidual and be constant from the beginning.

While 0. gigas remained constant in its characters, there
were other mutations in which this fact was not observed. Such
an instance is 0. scintillans, from the seed of which, in spite of
self-fertilization, originated always three forms, 0. scintillans,
0. oblonga, and finally the original 0. lamarckiana. In addition
the seed frequently produced elementary species of a second
order which united in themselves the characters of two species.
Thus an intermediate form, for instance, between 0. scintillans
and 0. nanella, has not unfrequently been observed.

Concerning the causes which effect the mutation, we can only
make guesses. We gain, however, the impression from the many
facts before us that mutability occurs periodically and is replaced
by periods of apparent constancy. As a rule the new ' elementary
species ' appear at once in a large number of individuals, an excep-
tion being formed by 0. gigas, of which only one individual was
observed. Like all organisms, 0. lamarckiana and its mutations
are subject to fluctuating variability, and one is able to produce
among them new races by rational selection. But they always
remain forms which are in ' need of selection ' ; they differ very
little from the type, probably only in one essential, and must not
be mistaken for the ' elementary species.' Of mutations, too, it
is therefore true that they can be either useful, or indifferent,
or injurious. Numerous mutations, like the sterile 0. lata, the
weakly nanella and albida, and the fragile rubrinervis, would
hardly be able to maintain themselves in nature. On the other
hand, 0. Icevifolia appears to be at least equal to 0. lamarckiana,
whilst 0. gigas certainly excels it. From these experiments
De Vries concludes that ' the mutations have no tendency ; some
of the new types perish without progeny. Among the rest of
fche newly formed and fully developed species natural selection
must later make its decision.'

THE FACTORS OF EVOLUTION 209

Since naturalists have begun during recent years to observe
more accurately the occurrence of jumps in variations many
instances have been made known both from the plant and
animal kingdom. The occurrence of ' single variations ' had
even been noticed by Darwin himself, though in view of the
individual or fluctuating variations he did not ascribe to them any
particular importance in the formation of species. Nevertheless
we are indebted to him for a record of several significant instances.

In 1791, in Massachusetts, a sheep was born in an ordinary
flock having a long back and short bow-legs like a dachshund.
Because of its strange, unnatural appearance the animal was bred
from in the hope of obtaining a race unable to jump over the
hedges and fences which separate the homesteads. As only one
individual of this variety existed it was necessary to cross it with
an ordinary sheep. The first generation consisted of both bow-
legged and straight-legged individuals. When two bow-legged
sheep were crossed the character proved constant in the de-
scendants. Thus originated the Ancon race. Afterwards the
Ancon sheep were replaced by the more profitable Merinos, and
are probably extinct to-day. It is interesting to note that the
crossing of an Ancon with another sheep did not produce a
mixed race, but that the lambs were either bow-legged or
straight-legged. There are even cases on record of twins, of
which one exhibited the Ancon type while the other was normal.

It is probable that our dachshunds originated by mutation
from greyhounds, but as the race of dachshunds is very old —
they were kept as long as two thousand years ago in the Nile
delta — it is impossible to obtain accurate information concerning
their origin.

According to the investigation of Keller and others, it may
be taken as proved that the absence of horns in different races
of cattle, sheep, and goats, the races of goats with four horns,
the three-toed pigs of the Crimea, the famous tailless Manx cats,
black panthers, black-shouldered peacocks, the ' telescope ' and
multiple-tailed gold-fishes and many other animal forms owe
their existence to mutation. Most of the species mentioned are
distinguished by the fact that in in-breeding all their characters
remain constant.

It seems now proved that non-continuous variability can lead

14

210 LECTUEES ON BIOLOGY

to the formation of new ' elementary species.' Nevertheless, the
theory of mutation cannot satisfy us as being the sole factor of
development, for it leaves unexplained above all the origin of
numberless adaptations. As mutations do not tend in any direc-
tion it would mean confiding too much in chance if we were to
regard it as the creator of organic fitness.

If, in conclusion, we review the results at which we have
slowly arrived we shall be compelled to confess that research
and speculation has hitherto not been able to formulate a
universal law of evolution. We are still at the beginning of
a long, tedious journey. The deeper we seek to penetrate into
the mysteries of life the greater become the difficulties. All
theories that have been promulgated are only able to explain a
strictly limited part of phenomena, leaving by far the greater
number unexplained. Darwinism yields no information con-
cerning the causes of variability ; the theory of Lamarck fails in
front of passive adaptations ; the theory of mutation plays with
chance even more than selection ; and vitalism finally brings
forces into play, of the nature of which we are not even able to
form an idea.

It is not pleasing to admit this insecurity, but a hypothesis
is nevertheless much to be preferred to wearing dogmatic blinkers
and resting in delusive security. It is true that there are many
investigators who regard all questions as solved, and' with the
aid of a few well-sounding phrases furnish an explanation of
every phenomenon ; but truth is not found with the dogmatist.

We have, however, attained one result : no longer will it be
possible to shout : " Hie theory of selection, hie Lamarckism,
hie hypothesis of mutation ! " The forces which these different
theories regarded as the foundation of organic genesis do not
exclude each other, but we must rather assume, on the grounds
of our experience, that selection and mutation, direct influence
of environment, and internal forces of the organism, as yet
unknown but for all that not unknowable, all participate in the
evolution of organic life.

•211

CHAPTER VIII.
THE CONSERVATION OF LIFE.

THE veil which covers the genesis of the first germs of life
on earth seems impenetrable. Wherever we may search we find
no answer to the question of the origin of life. On this subject
exists no accurate knowledge, and we must be content with
hypotheses and probabilities. Nor are we able to obtain informa-
tion about the nature and appearance of the first primitive
organisms. The runes of the earth's history engraven in
geological formations have mostly perished. The book of Nature
is defective and illegible, and the further we attempt to go back
the less clear, the more confused and strange the writings become,
the more pages are missing. Though palaeontologists have
achieved wonders in deciphering many of these mysterious signs
they have failed to answer the most important question of all.

If we now, in spite of the knowledge that we move on
unstable, hypothetical ground, were to accept as true the doctrine
of spontaneous generation, how are we to conceive the first repre-
sentatives of the organic world ? Organisms generated from
inorganic matter could manifestly be but extremely primitive.
Whether they were little structureless lumps of albumin, as
Haeckel thinks, or vast masses of protoplasm that were as yet
unindividualized and covered large tracts of the primordial ocean-
bed we are unable to say, and all speculating after details will
be futile. However low and simple we may imagine the ances-
tors of the present animal and plant world to have been they
must have held in common with the present species two funda-
mental properties : metabolism, and the faculty of maintaining
the species by means of reproduction. That this latter ability
must be ascribed also to the problematic primordial organism is
proved by the different plant and animal species and by our own

212 LECTUEES ON BIOLOGY

existence. But as the children are never an exact replica of the
parents but differ from them in one or another characteristic,
there must already have been inherent in the ' primordial slime '
the possibility of variation. If that were not the case evolution
of the organic world could not have taken place, and it would
bear to-day the same primitive appearance as in the ages in which
it came into existence.

Most zoological text-books divide the animal kingdom into two
great divisions. On one side are the higher multicellular animals,
among which we find all animal organisms, from the little polyp
whose body consists merely of a cavity surrounded by a double
cell-layer, up to man with his body complex with numerous
tissues and organs. On the other side are the unicellular
protozoa.

The name protozoon always seems to create in the lay mind
erroneous ideas. It came into use at a time when our knowledge
of the structure of the protozoa was of rather primitive nature,
and when serious men still believed that the Amoebae and Infuso-
rians owed their existence to spontaneous generation, developing
everywhere in decaying substances. To-day we know that the
unicellular organisms are no more original forms than are
mammals or insects. Like the latter they bear unmistakable
signs proving that their species have undergone a development
extending over vast periods. But all vital functions go on in
the protozoa in a much clearer and simpler manner than in the
multicellular animals, and a study of their reproduction throws
a clearer light upon the conditions obtaining in the higher
animals. It is one of the greatest joys of biological research to
penetrate with the aid of the microscope into this rich world of
invisible life and to discover its secrets.

In a drop of water taken from any ditch or pond we are able
to observe with the naked eye a minute white spot. This little
insignificant thing holds within itself all the wonders of life.
All the numerous functions of respiration, nutrition, locomotion,
and reproduction which are performed in man by highly complex
organ- systems, are going on in this minute drop of transparent
slime in a different yet none the less perfect manner.

The little organism before us is a member of the so-called
Khizopods, an Anloeba. It owes its name ' change ' to the fact

THE CONSERVATION OF LIFE

213

that it does not retain the same shape for Jong. Now it rolls
itself up into a ball, the next moment extends the body longi-
tudinally, or thrusts out sac-like processes. Only sudden stimuli
or unfavourable external conditions render the animalcule tem-
porarily motionless (fig 51).

Like all other typical cells, the Amoeba consists of protoplasm
and a nucleus. Specific organs have not yet been evolved, but
the protoplasm has become separated into a granulated endo-
plasm and a more trans-
parent hyaline ecto-
plasm which surrounds
the whole body of the
animalcule as with a
thin mantle. In the
endoplasm take place
the digestive functions,
while the light-coloured
protoplasm - mantle
serves the purposes of
locomotion. Apart from
this the cell-body ap-
pears uniform, except
that in the interior we
observe a little vesicle,
filled with a clear liquid,
which expands at rhyth-
mic intervals, then col-
lapses suddenly, and ejects its contents into the surrounding
water, when the process commences anew. This is the l pulsat-
ing vacuole.' Its task is the removal of the water which has
been taken in and used up in respiration, and the ejection of
liquid excreta.

If we let the Amceba remain undisturbed we shall see that
the ectoplasm begins to protrude in a certain spot, and forms a
pseudopodion. The rest of the body-substance flows lazily after
the protrusion, and by repeating this process the Amceba floats
slowly along the surface of the rock or plant. If any particle of
food, such as a bacterium or alga or any other similar protozoon,
is encountered by the Amceba it is engulfed by the pseudopodia.

FIG. 51. — Amoeba proteus IN VARIOUS STAGES OF

MOTION. BELOW, DIVISION OF THE PABENT-AMCEBA
INTO TWO DAUGHTER-AMCEBJE.

214 LECTURES ON BIOLOGY

The digestible parts are decomposed and assimilated, and the
refuse expelled in the same manner in which it was taken.
Thus the Amoeba floats along with an awkward movement, eats,
and grows. But the growth is not unlimited. When a certain
size has been reached the nucleus of the Amoeba begins to
constrict in the middle and changes from the globular form
into the shape of dumb-bells. Gradually the constriction
becomes more distinct and deeper, and finally we get two cell-
nuclei which wander to the two opposite cell-poles. Now the proto-
plasm of the cell-body also begins to draw itself out until finally
the two cell-halves of which each contains one of the two nuclei
remain connected only by a thin bridge of protoplasm (compare
fig. 51). In the end, this last connecting link breaks and instead
of the parent-Amoeba we have two daughter- Amoebae which are
exactly like the mother excepting in size. Like her they crawl
about in search of prey, quickly grow, and divide in their turn,
after some time, into two little Amoebae. How simple and yet
how wonderful ! This primitive process of reproduction, which
is not inaptly called a ' growth beyond the individual measure,'
is repeated through innumerable generations, for so far as our
experience goes there is no natural limit. In these lowest
organisms that which we call development is as yet absent, for
the young are at ' birth ' exactly like the mother, excepting in
volume. But growth without a change in the organism cannot
be described as development. Nor do these primitive organisms
know death. They possess, as Weismann expresses it, potential
immortality, because, as we have seen, the mother divides into
two daughter-individuals and continues to live in them. But the
Amoebae are not immortal for all that, for like other organisms
they can be destroyed by poison, heat, or serious mechanical dis-
turbances. Potential immortality only expresses the idea that
a physiological death — the normal decay, which is a fundamental
necessity of the organism — does not take place in the Amoeba
and many of its relatives among the protozoans.

It is a difficult idea to grasp, for that death follows life and
must of necessity follow it is an idea so deeply rooted in the
human mind which is reminded of it daily and hourly that
it will not accept any exception of this law. The doctrine of
potential immortality of the unicellular organism was therefore

THE CONSEKVATION OF LIFE 215

fiercely contested and declared to be false. But if we examine
the question itself on the basis of the facts we shall see that the
assumption of, as it were, an eternal life for numerous protozoa
is neither sophism nor playing with words, but a logical necessity,
unless indeed we wish to rob the conception of ' death ' of its
natural meaning.

Let us first of all define the problem and agree upon the
meaning of the words. ' Death ' is distinguished from ' life ' by
two factors, one a physiological, the other a morphological.
Death is indicated by the cessation of the individual life-pheno-
mena and by the appearance of a corpse : only when these
conditions are fulfilled we are able to speak of death having
taken place. Let us apply this to the Amoeba. That there is
no appearance of a corpse or of anything that can be regarded
as equal to it we have already seen, for the mother divides into
two daughter-individuals without leaving a residue. Thus it
would seem that the question is already decided in our favour, but
the opponents of this doctrine of immortality cling in this specific
case to the physiological factors and contend that in reproduc-
tion by fission the mother ceases to exist as an individual, and
that the individual life-phenomena are thereby discontinued. To
a certain degree and with many modifications this conception
might be defended, but if we examine the question more
thoroughly it seems to me that we shall reach a different result.

Like many higher animals and most protozoa, our Amoeba
possesses the important faculty of replacing lost parts of the
body by regeneration. If I cut from the body of the Amoeba
a particle of protoplasm it does not perish from this injury. On
the contrary, the wound closes up, the Amoeba takes in food as
before, and the damage is soon completely repaired. How does
this affect the individuality of the Amoeba ? Are we still dealing
with the same individual? I took away a part of its body,
a part of its individuality, which afterwards was replaced by
regeneration. No one will contend, because part of the whole
body was cut off, that we have no longer before us the same
animalcule, that because of the operation we now have to regard
the old Amoeba as dead, and that the Khizopod which now
crawls about under the microscope is an individual other than
the first. With the same right it might be contended that the

216 LECTURES ON BIOLOGY

man whose arm or legs have been amputated is no longer
himself: that the healthy man is dead, and that the cripple is
a new individual which did not exist before. No one would
seriously maintain such an absurdity, and the patient himself
would most energetically protest against such suggestions.

How large the part of the body is which I cut away from the
Amoeba or any other organism cannot in any way affect this con-
sideration. It does not matter whether I take away a minute
particle or half of the Amoeba cell-body, so long as the animal-
cule is able to survive the operation. Equally so it is without
importance which part of the body is lost, whether it consists
only of protoplasm or of nuclear substance, or whether it was
an eye, or an arm, or leg that was destroyed.

Let us now assume that I divide an Amosba in such a manner
that both halves are equally large and that both contain proto-
plasm as well as a share of the nucleus. Let us call these two
pieces A and B, and trace their future fate. A closes its wound
in a short time, rolls itself up, and begins to act like a healthy
animalcule. It crawls about, eats, and grows. Before long the
damage has been repaired and everything is exactly as it was
before. As regards A, therefore, we stand towards it in the same
position as we stood towards the wounded Amoeba, i.e., A is no
newly-generated creature, but the old Amoeba cut in two, which
has not died but continues to live in spite of the terrible mutila-
tion, and continues its individuality uninterrupted in A. We
cannot, therefore, speak here of death and dying.

But what has in the meantime ^become of the other piece ?
Has it perished like the smaller particles of protoplasm which we
cut off before ? Here, too, we find that it has developed precisely
like A, and become a complete Amoeba. But because B also
remained alive can we conclude post festum that A is dead, in
spite of all observation to the contrary ? No, the only conclusion
which we are justified in drawing is that the original Amoeba is
not only not dead but has doubled its individuality and continues
to exist undiminished in A and B.

What I have done here by an artificial operation does not
differ fundamentally from the natural manner of reproduction by
fission. We must therefore acknowledge that the natural act of
reproduction is not bound up with a process of dying, but only

THE CONSERVATION OF LIFE 217

leads to a doubling of the individuality. We must also admit
that the unicellular organisms — or at any rate the Amoebae which
have been the subject of this examination — have this advantage
before us and all multicellar organisms that they possess potential
immortality.

As we shall see later, many protozoa divide during reproduc-
tion not only into two, but three, four, and even more parts
which are all able to develop into complete individuals. But
does that justify us to speak of death ? Assuredly not, for the
sole difference which we are able to observe is that in these the
mother-individual has not only doubled, but trebled, or quadrupled,
or multiplied x times. The facts are so clear and incontestable
that one cannot see why anyone should find it difficult to accept
them.

Reproduction in such a simple manner can take place only
in very few protozoans, because only very few have such low
organization. In the closely related Foraminifera it requires
already rather elaborate preparations before the mother may
proceed to fission. The Foraminifera differ from the Amoebae
only by the possession of a shell. We may describe them as
Amoebae which have made for themselves a solid shell as a
protection against enemies (see fig. 52). As great as are the
differences in shape and appearance of the shells of the individual
species, as different is the material which has been used in their
construction. Sometimes the shell is merely a delicate mem-
brane, a secretion of the outer protoplasm. Sometimes the
animalcules absorb little particles of sand, or of diatom-shells,
which they afterwards deposit on the surface of the body and
join into a firm armour by means of a chitinous glue. In others
the shell consists of numberless minute silica-plates, while the
vast numbers of marine Foraminifera make their shells by pre-
ference of carbonate of lime which they take from the sea
water and secrete, often in the form of most delicately shaped
shells. These marine species are of particular importance
to man and the history of the earth because in many parts of
the earth they have been the origin of enormous mountains and
rocks. Just as the Foraminifera are to-day found swimming in
the ocean in vast masses, driven by current and wind hither and
thither, so they must have peopled the ocean in remote ages.

218 LECTURES ON BIOLOGY

In spite of their potential immortality numberless individuals con-
tinually sank to the bottom, their bodies decaying, but their tiny
shells accumulating. Generation followed generation, and higher
and higher rose these mountains of corpses until they almost
reached the surface. Now follows a period of enormous revolu-
tions of the earth's interior or contractions of the rigid crust : the
sea floor rises and from the floods ascend the mountains of dead
Foraminifera, an enormous continent of chalk. But what Nature
builds to-day she destroys to-morrow, for nothing in this world
is eternally constant except alone change.

Keines verbleibt in der gleichen Gestalt, denn Veranderung liebend,
Schafft die Natur stets neu aus anderen andere Formen,
Doch in der Weite der Welt geht iiichts — das glaubt mir — verloren.
Wechsel und Tausch ist nur in der Form, Entstehen und Werden,
Heisst nur anders als sonst anfangen zu sein, und Vergehen,
Nicht mehr sein wie zuvor. — Goethe.

Kain and wind beat incessantly against these rocky masses,
and cut deep valleys into the land ; the sea follows, completing
the work of destruction, but what it tears away in one place it
deposits in another. Such alternations of destruction and con-
struction created the island of Riigen in its rugged shape as
we know it to-day. But still the waves thunder against the
chalk cliffs, and rain and storm continually carry away to the
sea earth and fragments of stone. Many thousands of years
may yet have to pass, but the day will come when Kiigen will
be no more, just as there was once a time when it was not yet.
But the ' becoming ' is eternal. Billions of Foraminifera still
people the ocean — the descendants of those whose shells built
the island of Riigen — and they will make new lands and new
islands arise. But it takes longer than the short life of man to
observe Nature in its giant task. The enormous numbers of
lives which must be born and die before one single chalk-rock
can be built will be understood when we hear that one gramme
of chalk contains more than fifty thousand shells of Foraminifera.

Foraminifera played an important role in other parts of the
earth's history. In the Carboniferous Formations we know in
many districts extended strata which consist almost exclusively
of shells of Fusulina and Schioagerina. In the Tertiary Period
we encounter, especially in Southern Europe, in the Carpathians

THE CONSERVATION OB' LIFE

219

and Pyrenees, deposits of the familiar nummulites. These are
probably the largest unicellular organisms which ever lived, for
we know shells of the size of a florin.

But let us return to our subject : it is not the protozoans
as such but the natural phenomena of reproduction which are
of interest to us here. Some Foraminifera, with a compara-
tively soft shell, are, like Amoebae, able to reproduce by a simple
drawing-out process (constriction), whereupon each of the
daughter-animalcules completes the inherited shell-half by
regeneration. Others leave
their house when the time of
reproduction comes and
divide, whereupon each half
builds for itself a new house.
Other species exhibit more
remarkable deviations. Let
us examine these in the little
Euglypha alveolata which
lives in fresh water. This
pretty animalcule possesses a
shell shaped like a pear or
egg, artistically built of
minute silica-plates. At the
pointed end is a round open-
ing through which Euglypha
protudes its protoplasmic
pseudopodia and takes in
food. When it is about to
divide it secretes first of all
proximity of the nucleus,

FIG. 52. — NUMMULITES.

in the cell-body, in immediate
numerous minute silica-plates.
Then all pseudopodia are retracted, and in their place appears
a thick, round lump of protoplasm, representing about half
of the entire body substance. Now the nucleus begins to
change : in a highly complicated manner it divides into two
parts which are exactly alike. While one half remains in the
shell the other wanders into the protruded protoplasm. In the
meantime the silica-plates have also passed on to the protruded
protoplasm and formed themselves into a connected shell which
is an exact replica of the old house. A chitinous glue joins the

220

LECTURES ON BIOLOGY

single plates firmly together, thus giving to the whole the neces-
sary solidity. For some time the two animalcules remain in
the position of shell-opening pressed against shell-opening, con-
nected only by a bridge of protoplasm, but suddenly the connect-
ing cord breaks, the two animalcules crawl apart, and the
division is complete. Very soon afterwards the living contents
of the two shells have by growth regained the normal size.

FIG. 53. — A MICRO-AQUARIUM. FORAMINIFERA AND CILIATES (MAGNIFIED).

On the left, below, on the branches of an Alga, a Foraminiferon (Euglyplia
alveolata) ; crawling on the bottom a shell-animalcule (Stylonycliia mytilus) ;
above, swimming, two slipper-animalcules (Paramcecium caudattim) ; a third
attacked by a ciliate infusorian (Didinium nasutum}. Below, hanging from the
branch of an Alga, three bell-animalcules (Vorticella nebulifera) ; on the stone, a
trumpet-animalcule (Stentor roeselii).

Among all protozoans the Ciliates occupy undoubtedly the
highest position. Indeed it seems no exaggeration to say that
there is as great a distance between an Amoaba and a Ciliate, as
between the sponge or polyp and a mammal. We are continually
filled with wonder when we observe the differentiations of which
a simple cell is capable. In the Ciliate we find as perfect a

THE CONSEEVATION OF LIFE 221

division of labour of the single parts of the cell-body as we would
only expect to find in the highest multicellular organisms.
Indeed many of these Infusorians — I will only mention the
trumpet-animalcule Stentor, the dainty bell-animalcules Vorticella
and Epistalis, the so-called shell-animacule Stylonycliia — appear
to be so highly organized that no unprejudiced observer will
wonder at the founder of scientific protozoology, Ehrenberg, con-
tending that these animalcules possessed, like the multicellular
animals, an intestinal canal, nervous system, sexual glands,
muscles, kidneys, etc., and it is only quite recently that the
opinions of Dujardin and von Siebold, which perceived the
protozoa to be simple cells, were universally accepted (fig. 53).

If we fill a glass with water and decaying leaves we shall
find after a few days a magnificent culture of Infusorians. This
will in most cases contain the slipper-animalcule (Paramcecmni),
one of the most widely distributed species. Under favourable
conditions of temperature and food the water becomes in a short
time filled with innumerable little white points.

The Paramcecium reaches a length of about 0*2 to 0*3 mm. ;
we are therefore just able to see it with the naked eye. Its
shape is an asymmetrical oval, slightly flattened dorso-ventrally,
in which we can distinguish an anterior and posterior end.
While the former appears somewhat rounded the posterior end
is more or less pointed. The whole body-surface of Para-
mcecium is covered with an enormous number of delicate cilia
arranged in longitudinal rows, which during life almost inces-
santly carry out lashing movements and enable the animals
to dart skilfully through the water. We can best compare the
movements of the cilia to the uniform waving of a cornfield,
gently stroked by the wind, rising following falling; but just as
the ears are not always bent by the wind in -the same direction
so the play of the cilia changes from time to time in order to
drive the animalcule in another direction.

The ciliation of the body is not quite regular in Paramcecium,
though more uniform than in many other Infusorians. Thus
we observe in the anterior end a flat groove, the so-called
' peristomical area,' on which we find stronger cilia of somewhat
different shape. The posterior end is distinguished by some-
what longer cilia.

222 LECTURES ON BIOLOGY

In the other orders of the Infusorians the differentiation incilia-
tion has made very considerable progress. We see that the cilia
are here used for the most diverse functions — swimming, crawling,
jumping, wafting-in food, etc., and are transformed according
to their work. Perhaps the most remarkable transformation
which the cilia have undergone is observed in the shell-animalcule
(Stylonychia mytilus) which I have already mentioned. Here some
of the cilia of the ventral surface — i.e., the surface which during
locomotion is opposite the object on which they crawl, and where
is found the mouth-aperture — have assumed a strong, rigid ' slate
pencil' form, while others have become cirrhous. These simple
' legs ' are skilfully used by Stylonycliia for running along the
ground when in search of prey.

But let us return to the Paramceciiun. At the bottom of the
peristome- field we observe a small round orifice, the ' mouth '
of the cell. Food, which chiefly consists of bacteria and similar
minute organisms, is wafted into this mouth by means of the cilia,
and reaches through the orifice the pharynx of the animalcule,
a short canal bent in the shape of an S, which leads direct into
the body-plasm. The indigestible parts are passed out through
a special aperture. The very complex contractile vacuoles
serve as organs of excretion. They are no longer the primitive
vesicles of the Amoeba, for we can here distinguish between an
inner vesicle and a canal - system, extending from it to the
protoplasm. The canals collect the used-up fluid from the entire
body, conduct it to the central vesicle which, in consequence,
swells gradually until it reaches a considerable size. Suddenly
it collapses by passing its contents to the outside through a
minute pore. This process is repeated at regular intervals.

If the organella mentioned before demonstrated already an
enormous progress in the development as compared with the
Rhizopods this distance will be still further increased when we
consider the other differentiations of the protoplasmic body. In
Param&cium we are able to distinguish an ectoplasm from the
endoplasm. The first, again, is divided into a more solid cover,
the pellicle, under which we can distinctly perceive a thin
layer of a honeycombed structure. Below this lies the so-called
1 cortical plasm,' a fine hyaline mantle which is sharply separated
from the endoplasm and can, moreover, be distinguished from it

THE CONSERVATION OF LIFE

223

by the fact that it never takes in particles of food. The cortical
layer is specially marked as the seat of the ' trichocysts,' minute
organs of defence which may be compared to the familiar poison-
cells of the corals, sea-anemones, and jelly-fishes.

The trichocysts are small, spindle-shaped formations, filled
with a watery fluid. Touch or other stimuli of a mechanical
or chemical nature void them. When brought in contact with
water the contents congeal into a thin, hard thread which
eventually bores into the body of the attacker or prey, and. there
probably causes a burning pain or paralysis. I say, probably,
because no satisfactory observation has so far been recorded.

The alveolate layer, which I mentioned before, is partly trans-
formed into contractile fibrils, a kind of primitive muscle surround-
ing the body longitudinally, thus imparting to it great motility.

As regards the
nuclear apparatus
of the Ciliates, we
find in this class
two different kinds
of nucleus, the so-
called macro-nuc-
leus, which is often
distinguished by a
very considerable
size, and the micro-
nucleus. While
the macro-nucleus
controls and regu-
lates all ' vegetal

functions ' of cell-life — digestion, respiration, locomotion, etc.,
the micro-nucleus becomes active only at times of sexual
reproduction. The changes which it then undergoes are so
striking that it has been described as the sex-nucleus.

Like Amoebae, the Ciliates reproduce by simple fission, but
while in the Amoebae the two daughter-individuals are at once
equivalent it is not so here, owing to the great differentiation of
the body. Let us observe, as an example, a trumpet-animalcule
(fig. 54).

The anterior end, which has been enlarged like a trumpet,

FIG. 54. — TRUMPET-ANIMALCULE (StentOT CCerilleUS) IN
THREE SUCCESSIVE STAGES OF DIVISION.

224

LECTURES ON BIOLOGY

bears the mouth-aperture to which leads a long ciliated wreath, the
' adoral cilia-zone ' ; the pointed posterior end is furnished with a
suctorial disc.

Let us now suppose that Stentor begins to draw itself out.
What would happen ? We should have two halves, of which
one would be without a mouth-aperture and the * adoral ciliated
spiral,' whilst the other would be without the suctorial foot.
But Nature could not be so unjust as to endow both individuals
with unequal proportion. When a Ciliate therefore feels that it has
become too large for one animalcule it begins to lay down its most
important organella in duplicate. It is no longer content to double
only its nucleus but forms, as, for instance, in Stentor, a second
ciliated wreath and a new mouth-aperture below the old. Division
only takes place after this has been done, and now the two young
Stentors resemble the mother in everything except in size and
unimportant details.

It is possible to multiply the Ciliates
artificially by dissection. As long as
these individual sections have obtained
their share of nuclear substance they
are always able to survive the operation,
and shortly to regenerate the absent
organella. Thus a Stentor may easily
be cut into two, three, four, or more
parts, and each section will grow again
into an adult animalcule if the condi-
tions are at all favourable (see fig. 55).

The fertility of most unicellular
organisms is astonishing. After a few
months we would find in our Para-
weeciww-culture a succession of hun-
dreds of generations. If we discon-
tinued our observations at that period

we should gather the impression that reproduction by fission
proceeds here until all eternity, apparently as undisturbed and
uniform as in the Arnosba. But further observation teaches us
that this does not apply to the Ciliates without considerable
modifications.

After multiplication has proceeded for some time normally

FIG. 55. — Stentor cceruleus.

(1) Cut into three nucleated

parts ; (2) Three Stentores

produced by regeneration.

THE CONSERVATION OF LIFE 225

we observe all at once a remarkable change. All the animal-
cules which we may take out and observe under the microscope
will be found to be in pairs, joined ' mouth to mouth.' It seems
as if the entire culture suffered from a kind of conjugation
mania. When this intimate union has continued for some time
the pairs separate and the individuals live as heretofore, swim
hither and thither, and divide, until after a number of genera-
tions they enter into a fresh union of pairs.

What is the meaning of this mysterious process ? It is doubt-
less one of great importance to the destiny of the Infusorians, for
if it were otherwise how would it have become so universally
established? A light is shed upon the mystery by the classic
experiments made by Maupas. As conjugation usually takes
place simultaneously in a very large number of individuals it
seemed probable that its cause was to be found in the influence
of external conditions, an assumption which to a certain extent
proved accurate. It appeared that this conjugation-epidemic
always took place when food became scarce, but that abundance
of food always postponed it. It is no doubt necessary that other
internal conditions have to be fulfilled before conjugation can
take place, but we will disregard details.

The results of preventing such conjugation are curious. At
first no injurious changes of any kind are perceptible and repro-
duction takes place undiminished : the mother-animalcules divide
into children, these into grandchildren, and thus generation
follows generation. But very soon the descendants do not seem
to possess any longer the same vigour as their ancestors : the
cilia-covering becomes incomplete : the size decreases ; all signs
of ' senile degeneration ' appear ; and in a short time the entire
colony has become extinct. If, however, the Paramcecia are
permitted to proceed to conjugation at the right moment all signs
of degeneration are absent, and the power of fission is inherited
undiminished from one generation to another.

It seems, therefore, that Infusorians already die a natural
death, though a death which does not take place as an inexorable
necessity, as in the higher animals, but may be ' switched off' by
the mysterious process of conjugation. Maupas and other investi-
gators, on the basis of these observations, conceived therefore
conjugation to be a life-renewing process, a means of unlocking
15

226 LECTURES ON BIOLOGY

new sources of energy without which the existing life-machine
would inevitably perish.

But though this explanation sounds clear we must not forget
that in test-tubes we offer to the ParamcBcium very different con-
ditions from those that obtain in free nature. Moreover, in the
last-mentioned case we deliberately continually overfed the
animalcules in order to prevent the union. It is, therefore, at
least as probable tbat the gradual degeneration and the final
extinction of the colony can be referred to these factors, as that
the cause is to be found in our having prevented conjugation.
I am rather inclined to agree with Weismann, who says that the
importance of conjugation lies in the amphimixis, in the exchange
of the characters of two different individuals, and the increase
in variability and adaptability conditioned by it.

Let us now endeavour to find out with the aid of the micro-
scope what conjugation really is, and what happens during this
process (fig. 56). Soon after the animals have taken up a
' mouth to mouth ' position their nuclear apparatus shows
characteristic changes. The macronucleus which, as we have
already seen, controls tbe ' vegetal ' functions of cell-life is
not in any way affected, but in the micronucleus we notice
far-reaching alterations. At first it divides indirectly into two
daughter-nuclei which immediately divide again. It must be
remembered that all these processes take part in both con-
jugated animalcules in the same manner. We see therefore in
each in place of one uniform micronucleus four daughter-
nuclei, but very soon three of these once more perish and are
disintegrated in the protoplasm, while the fourth once more
divides into two nuclei. But though identical in appearance,
their destiny is very different. We distinguish them by calling
one the ' female/ the other the ' male ' nucleus.

Though our observation has already taken up several hours
we must not lose patience, because the most important process
is yet to come. In the region of the mouth-aperture a small
plasm-bridge has formed between the two Paramcecia, and now
we see that the male nucleus of A wanders into the proto-
plasm of B, and vice versa. The two united individuals therefore
exchange their nuclear substance. When that has been done the
Paramcecia once more separate and each travels its own road, in

THE CONSERVATION OF LIFE

227

appearance still the same, yet differing from what it was before.
The whole process of conjugation, which mostly begins in the
early hours of the morning, lasted about twelve hours,

But the process is still incomplete, and is continued in the
separated animalcules until a complete renovation of the entire
nuclear apparatus has taken place. The first step towards this

FIG. 56. — CONJUGATION OF PARAM^CIUM IN SIX STAGES.

(1) Two slipper animalcules, 'mouth to mouth': the micro-nucleus of the
individual on the right begins to divide ; (2) division of the micro-nuclei in both
individuals is complete, and the parts are about to divide again ; (3) in each of the
conjugating individuals four daughter-nuclei have been formed ; (4) three of these
in each individual perish, the fourth divides again into two nuclei, which are
described as ' male ' (shown as black or white) and ' female ' (striped) nuclei ; (5)
exchange of the two 'male' nuclei; (6) separated, male and female nuclei have
become fused into one conjugation-nucleus, from which a new macro-nucleus and
micro-nucleus originates, while the old macro-nucleus disintegrates.

end is that the ' male ' and ' female ' nucleus fuse into one.
Then the old macronucleus disintegrates with every sign of
degeneration ; we may see its remains lie for some time in the
cell-body until they have gradually been eaten up. But from the
conjugation-nucleus a new macronucleus and a new micro-
nucleus originate by fission, and thus the animalcules are ready

228 LECTUEES ON BIOLOGY

once more for a new period of reproduction. The opinion which
I expressed briefly in the beginning that in the mingling of the
characters of two different individuals we have to look for the
most important meaning of conjugation is hereby corroborated.

We have already heard (in the fourth lecture) that cogent
proofs force us to regard the nucleus as the carrier of heritable
characters — i.e., as that organ or that part in which all heritable
characters of the whole individual are potentially contained in
the form of minute ' primary constituents.' In exchanging and
mingling their nuclear substance each of the conjugating infusoria
transmits to its ' pair ' its own characters. After the fusion both
Paramcecia have, therefore, in their body the primary constituents
(Anlagen) of their old characters augmented by those of the
second animalcule. What increase of variability, or, at least
what possibility ? Let us assume, for instance, that both Para-
mcecia about to unite differ from each other in the formation
of their cilia-dress, which consists in the one of short and strong,
in the other of long and delicate cilia, and that the correspond-
ing Anlagen were fixed in the nuclear substance. In that case
each ' pair ' would obviously have at their disposal after the nucleus-
exchange both these different * dispositions/ and we can conceive
that in them and their descendants those which reach develop-
ment would prove to be best adapted to the existing conditions
of life. And what applies to the cilia-dress applies to all other
organella and divergencies, so that the two sets of characters
of both animals may find in their descendants a development of
the most varied combinations.

But while amphimixis increases on one side the extent of
variation it limits on the other side the danger that might
threaten from one-sided variation. If, for instance, one part of
an animal ' varies ' in the descendants, the variations may either
become weaker or, with the same probability, it may tend to
* vary ' in a forward direction and become more and more pro-
nounced. If now such variations accumulate through several
generations we can imagine that the form of the affected part may
assume such extreme dimensions that it would constitute a serious
danger to its owner. Such a one-sided tendency of variation is
entirely prevented by conjugation, because an exchange of
characters implies, generally, a simultaneous correction of too
pronounced deviations.

THE CONSERVATION OF LIFE 229

That this is, in fact, the meaning of amphimixis is shown by the
observation that only those animals proceed to conjugation which
are most distantly related. Further, a large number of accurate
investigations made with different species of infusorians have shown
that individuals descended from the same mother-animalcule did
not conjugate even when all other conditions of union have been
fulfilled. In fact, conjugation between closely related individuals
greatly resembling each other would lose its greatest object ; in
many cases it would even do greater harm than good by
accumulating like characters. It is the same cause which forbids
inbreeding of higher animals and matrimony between blood-
relatives. Just as Nature generally prevents self-fertilization
and inbreeding, so the human race, in true perception of the
dangers, created in early times laws and manners which limit or
entirely forbid marriage between parents and children, between
brothers and sisters, and even between cousins.

The great resemblance of the conjugation-process of the
unicellular organisms to the fertilizing act of the higher animals
is remarkable. Fundamentally they are processes of the same
kind, and we are probably entitled to assume that conjugation
is the early phylogenetic stage from which the process of
fecundation was, in the course of phylogenetic development,
gradually evolved. We shall see later on that there are in the
organic world of to-day almost imperceptible transitions from the
' union ' of Paramcecium to the phenomena of sexual reproduction
of the multi-cellular animals. We shall also learn when first the
phenomenon of death became an inexorable law, and we shall
further see how with progressive differentiations its power became
more and more established and extended until it appears finally
as a necessary stage of organic development.

Though the conjugation-process of the infusorians and the fer-
tilization of multi-cellular organisms are of equal importance
they differ greatly in their external appearance. While in Para-
macium and its class the entire animalcules fuse in the ' sexual
act,' to separate again after the exchange of nuclear substance,
in the higher and highest organisms it is only the germ-cells,
definite minute particles separated from the body, containing the
heritable characters, which unite during the act of fertilization
and remain in a permanently close union. From this insignificant

230 LECTURES ON BIOLOGY

rudiment (Anlage) slowly originate, by division, differentiation, and
growth, the embryo, and finally the mature organism. The reason
why in the multi-cellular organisms a conjugation of the entire
animals can no longer take place is that only naked cells can
become fused together. A further important difference is that the
two copulating sex-cells, which for that reason we describe as male
and female germ-cells or as spermatozoon and ovum, differ greatly
in size, form, and appearance, while the Paramcecia which proceed
to a union are perfectly alike. On the other hand we know
already among infusorians a separation into 'male ' and ' female '
sexes, and the similarity in the phenomena in the highest animals
is increased by the further fact that the union of conjugating
individuals is permanent.

If we examine the numerous minute crustaceans which
are among the most familiar inhabitants of the fresh-water
aquarium we shall find their shells sometimes covered with a
dense, woolly fungus which under a magnifying-glass are dis-
covered to be colony-forming infusorians, members of the genus
Vorticella. Dainty, many-branched little trees carry at the ends
of their branches minute bells, the animalcules proper. In con-
trast to Param&cium, the cilia in Vorticella are greatly reduced
in number; the entire stalk is naked, only one well-developed
spiral fringe leading to the mouth-opening at the anterior end.
Its function is to waft, by its lashing movement, the food to the
mouth. In some species we observe, in addition, a simple circle
of cilia round the body. As Vorticella generally does not change
its habitat a complete cilia-dress is no longer of any use to them.

Like all other infusorians, no bell-animalcule is able to repro-
duce by simple fission, but as the drawing-out or constricting
process i& in numerous forms incomplete, and as the daughter-
individuals remain connected with the mother-body by means of
a thin stalk proceeding from the posterior end, this frequently
gives rise to the formation of large colonies. When the time of
conjugation arrives the individual citizens of the Vorticella colony
exhibit remarkable differences. While one part remains un-
changed, others in quick succession divide each into four indi-
viduals, without waiting for growth to make up the resulting
decrease in size. After a short time these little ' male ' germs
separate from the mother-stem and swim freely about by means

THE CONSEBVATION OF LIFE 231

of a specially formed cilia- wreath. How long this free life lasts
depends upon many accidents. If the little ' microgamete,' as it is
called, encounters on its travels a normal individual of the same
species, a so-called macrogamete, it attaches itself at once to its
side and fuses with it into one uniform whole. And as the cell-
body, so their two nuclei unite. Only the discarded pellicle of
the microgamete, which is not included in the union, is the external
sign that conjugation has taken place.

At first the fertilized Vorticella differs from an ordinary macro-
gamete by the formation of its nuclear apparatus, but by repeated
division descendants are produced which have once more a normal
appearance and are ready for a new conjugation — or we must
now, perhaps, more accurately use the term fertilization.

If we cannot know with certainty what caused in the bell-
animalcule this division of labour — i.e., the formation of male
' wander-spores,' — it is, nevertheless, perhaps not quite futile to
make at any rate an attempt to explain the probable causes of
these differentiations. I have already pointed to the great
importance of the conjugating-process, and we saw that only an
exchange of nuclei between non-related individuals can guarantee
the intended result. The large majority of Vorticella are fixed to
the ground. Is it not then obvious that in most cases conjugation
is practically impossible, or at any rate most difficult? It is true
that in the colony-forming species numerous individuals are pro-
duced together in a small space, but since they are all blood-
relatives union between them cannot take place. Thus the
formation of free-moving germs which separate from the mother-
stem and are able independently to seek another strange stem,
seems the only possible means to bring about conjugation, in
spite of a sedentary mode of life.

Why the * wander-spores ' are so differently shaped, why
there is a differentiation into two sexes, and, in the act of fecun-
dation, why the union of both sexes becomes permanent instead
of the animalcules contenting themselves as heretofore with a
mere exchange of their nuclear substance our hypothesis has so
far left unexplained. Yet the reason for this new phenomenon
is not difficult to perceive : it is the same circumstance which
conditions the difference in the formation of ovum and sperma-
tozoon in the higher animals.

232 LECTURES ON BIOLOGY

As the ' wander-germs ' which separate from the mother- stem
must often seek their partner in a comparatively enormous space
it is clear that numerous individuals must perish before they
reach the desired goal. It was therefore necessary to produce a
very large number of free-moving germs. The best way to pro-
duce such large numbers was at the expense of the body. Here
we have a natural explanation of the fact that the free-moving
individuals were developed as microgametes, or, as we may
also call them, male germs, while the others, the macrogametes,
or females, which remained fixed to the stem and had to await
the coming of the microgamete, could retain their original cell-
size. And as the small quantity of protoplasm made it probably
difficult for the microgametes to reproduce from themselves
alone normal individuals the permanent complete fusion with
a inacrogamete seemed most advantageous for their further
development.

If this assumption sounds at first somewhat forced it finds,
nevertheless, corroboration in many facts derived from the life-
history of the higher animals. It is known that in these the
difference in size between male and female germ-cells has become
still more considerable than in the unicellular organisms, for the
ova belong to the largest, the spermatozoa to the smallest,
animal-cells known to us. The ovum of the sea-urchin exceeds
in volume the male germ by more than 200,000 times, and in
animals with unusually large eggs, in particular reptiles and
birds, the proportion becomes still more unfavourable to the
spermatozoon. The simple reason is that ovum-cells are pro-
duced in much less numbers than the spermatozoa, consequently
they are able to develop their protoplasm very strongly and load
themselves, in addition, with abundant quantities of foodstuff
for the maintenance of the growing embryo during the later
stages of its development.

Spermatozoa on the other hand, being minute, are produced
in enormous quantities. Because it is necessary that they
possess the utmost motility to seek and find the ovum we
observe how during growth the spermatozoa reject almost their
entire protoplasm, so that a fully developed spermatozoon
consists only of nuclear substance.

Although the spermatozoon contains, like the ovum, in its

THE CONSEBVATION OF LIFE 233

nucleus, the ' primary constituents ' of a complete organism,
it cannot develop one out of itself because of the absence of a
sufficiency of food. If, therefore, the spermatozoon is not to
perish before it has fulfilled its mission it must join with an
ovum. Only in an ovum will it be possible for it to develop its
individual faculties. The male germ is, therefore, of necessity
dependent upon the fertilization process.

It is different with the ovum. Though we know that it has
been built for fertilization, this process is far from being an
absolute necessity, for there are numerous cases known of
natural and artificial parthenogenesis in which eggs develop
into complete individuals without preceding fertilization.

When I said before that the male spermatozoon is of necessity
dependent upon the fertilization process the statement was only
conditionally true, for we know a case in Nature, that of an
Alga (Ectocarpus siliculosus), in which not only the female germ-
cells are able to develop parthenogenetically, but that also the
male sex-cells can, under certain circumstances, reach inde-
pendent development. How is this unique behaviour to be
explained ? In Ectocarpus siliculosus the difference in size
between the germ-cells is not considerable, for the spermatozoa
possess a proportionately large amount of protoplasmic substance,
and this is no doubt the reason for a certain independent
development. Nevertheless, the independent formation of the
separated germ-cells is only an act of necessity, and the little
male plant always grows into a poorly-developed organism,
corresponding to the smaller size of the male-germ.

The most important lesson to be learnt from this observation
is that ovum and spermatozoon are formations fundamentally of
equal value, normal cells of which each carries within itself the
' primary constituents ' of a complete organism and is able,
under certain circumstances, to develop them independently.
Generally, however, we find that the sex-cells are dependent
upon conjugation. Nature, indeed, makes all preparations, if
I may use such anthropopathic expression to describe its blind
work, for forcing fertilization and mixing the qualities. Thus
we see that in progressing evolution the independent develop-
ment is made more and more difficult and even impossible to
both spermatozoon and ovum. This is effected in the male

234 LECTUEES ON BIOLOGY

germ-cells chiefly by a great reduction of their protoplasm, in
the egg-cells by a very ingenious means with which I will deal
in the next lecture.

We see, further, that the separation of the organic world into
two sexes, the existence of male and female sex-cells is no original
state but a secondary acquisition which has been developed very
gradually by adaptation to changed external conditions of life,
and hand in hand with the differentiations in organization.
Division of labour, this omnipotent factor which rules the entire
organic world, is here again the motive power to which we are
able to trace all phenomena.

Apart from reproduction by fission we know in many
infusorian s and in numerous other protozoans another kind of
propagation, budding. The most important difference is that in
fission the mother-animalcule divides itself completely into its
two descendants, while in budding it remains complete and
undivided, and merely separates from its body one or more
minute nucleated particles which gradually develop into complete
organisms.

The highest perfection and the most complete conformity
with the phenomena of reproduction in the multicellular animals
is without question exhibited by the sporozoa. Their name
sounds strange even to-day, when science has succeeded in
exposing them as the most dangerous enemies of the human
race, for though they are microscopically minute they are capable
of causing the most terrible devastations in the human body.
Among these sporozoa are found the organisms which are the
dreaded cause of malaria. Their presence or absence decides
whether vast tracts of fertile land shall be inhabited by man or
not. In addition to the malarial parasite we find among the
sporozoa the cause of Texas fever, horse-sickness, and many
other diseases. Though all sporozoa do not cause serious illness
they all lead a parasitic life and there is hardly a class of animals
on which they do not prey. But as the malaria parasite is
undoubtedly the most important to man we will examine it
here in detail.

It is more than a quarter of a century ago since Laveran, who
was then an army surgeon in Constantine, Algeria, discovered
for the first time the malaria parasite in the blood of a fever

THE CONSERVATION OF LIFE

235

patient. Later, by the co-operation of English, Italian, German,
and French investigators, its relation to malaria and its strange
life-history were clearly ascertained.

These species of malaria parasite are distinguished : Plasmo-
dium (Hamamceba) prcecox, vivax, and malarice. Whether these
three forms are really three separate species, or only varieties
of the same species appears doubtful, especially as all three

10

PIG. 57. — LIFE-CYCLE OF THE MALARIA PARASITE OF MAN.

(1-5) Stages of development in the human blood. (6-11) Stages of development
in the Anopheles. (1) Sporozoites. (2) Sporozoite has invaded a red blood-corpuscle.
(3) Sporozoite becomes a monont. (4) Nucleus multiplication. (5) Spore forma-
tion. (6) Female germ (oogony). (6") Male germ (antheridium). (7) Mature
female germ. (7') Formation of microgametes. (8) Fertilization. (9) Fertilized
oogony (amphiont). (11) Development of gymnospores in the interior of the
amphiont.

are dependent upon the same hosts, man and a mosquito
(Anopheles). These three parasites are the cause of the three kinds
of human malaria which are distinguished as perniciosa, tertiana,
and quartana. I shall deal here with the cause of tertiana,
Plasmodium vivax. The other two closely conform in their mode
of life to this .species.

If we examine the blood of a fever patient we find in the

236 LECTUEES ON BIOLOGY

blood-corpuscles minute amoeboid germs which quickly grow by
feeding upon the blood-corpuscles. The red pigment of the
attacked blood-corpuscles is decomposed by the protoplasm of
the parasite and deposited in the interior of the Plasmodium in
the form of dark brown, almost black, pigment. The little
parasite continues to grow and finally completely fills the con-
siderably expanded blood-corpuscle which surrounds it now only
as with a thin membrane.

By multiple division, first the nucleus and then the proto-
plasm of the Plasmodium form a number of spores, usually
twenty, at first held together by an undivided central plasrnic
body which, owing to the large accumulation of pigment in
its interior, appears almost black. Finally the blood-corpuscle
disintegrates, the spores are liberated, reach the blood and soon
infest another blood-corpuscle in which this process of growth
into a 'monont' and of spore formation begins anew (fig. 57,
1 — 5). Owing to the distribution in the blood of the pigment of
the disintegrated (residual) plasmic body the blood of a malaria
patient assumes a dark appearance.

The entire process of the growth of the spore and its repro-
duction comprises in Plasmodium vivax and prcecox a space of
forty-eight hours, in Plasmodium malarice seventy-two hours.

Externally the division of the monont into spores and their
invasion of healthy blood-corpuscles is perceptible by a rise in
the temperature — a fever which, according to the species of the
cause of the disease, recurs regularly every forty-eight or seventy-
two hours.

From the simple reproduction by fission as we saw it in the
Amoebae, Foraminifera and Infusoria the division of the malaria
parasite differs in many ways. In the first place the mother-
sporozoon disintegrates at once into a large number of descend-
ants, and, secondly, its body is not used up entirely for generating
the bodies of its descendants but a considerable part is doomed to
decay as a residuum. Does it not seem that we have here what
is called the morphologic side of the death problem, the first
appearance of a corpse ? Indeed, the process is so interpreted by
many investigators and the residual body described by them as a
corpse. But here, again, confusion arises from an inaccurate
definition of terms.

THE CONSERVATION OF LIFE 237

What is a corpse ? The question sounds so simple and is yet
full of difficulties. May we describe any rejected part of a living
organism being doomed to decay as a corpse? Can this term
be applied in this specific case to the residual body, and am
I justified by the appearance of this residual body in saying
that the Plasmodium dies a natural death? In my opinion
I would be equally justified in contending that the act of
birth in the higher mammals and man is accompanied by an
act of death, because during birth a part of the material body,
the placenta, is rejected and perishes. But no one thinks
of describing the placenta as a corpse in this sense, as no one
would think of describing an amputated arm or leg, a destroyed
eye or a drop of shed blood as a corpse. Why should we then
make an exception here in the unicellular organisms, limit the
contents of the term, and give it another meaning? It seems
clear that we can only describe as a corpse the sum-total of all
parts of an organism that has perished, but not any one part
of a body which did not before possess autonomous life. In the
unicellular organism it is the entire cell, in the multicellular and
the higher animals it is the entire organism which becomes
a corpse.

While the asexual form of reproduction serves as a means of
rapid distribution of the Plasmodium in the body of the host
and leads to the flooding of the host with disease germs, the
transmission of the parasite from man to the second host, the
gnat, is effected by an act of reproduction. In speaking of these
processes I shall confine myself to Plasmodium (Hcemamoeba)
prcecox, the cause of the dangerous perniciosa.

If a mosquito of the genus Anopheles sucks the blood of
a fever patient it happens that with the healthy blood-corpuscles
some that have been attacked by parasites reach the intestines
of the gnat. While in the body of this intermediate host the
disease germs which were predestined to asexual reproduction
soon perish, we observe that some of the spores undergo
a remarkable transformation. For a number of the parasites
grow vigorously, assuming at first the shape of a half-moon, but
gradually a globular form. We describe them as female germs or
oogonies. Others, the male germs or the antheridia, pass at first
through a similar transformation and are then only with difficulty

238 LECTUEES ON BIOLOGY

distinguished from the oogonies. In these the nucleus had
already in the human body commenced to divide into several
particles. These little nuclei vigorously extend longitudinally
and each surrounds itself with a small quantity of protoplasm
which undertakes the function of nagellse. In this manner
there originate from each antheridium five to six microgametes
which we may describe as male spermatozoa, but the main mass
of the protoplasm of the antheridium becomes residual matter
and thus ceases to play any further part.

In the intestines of the Anopheles fertilization takes place,
i.e., the fusion of an oogony with a microgamete. The fertilized
oogony now loses its globular form and changes into a sickle-
shaped motile form which begins to wander, perforates the intes-
tinal wall, and seeks a fresh place for its further development.
Immediately below the elastic intestinal membrane the little
parasite settles down. Protected by the membrane which sur-
rounds them as if they were encysted, they rapidly grow into
buds which may be clearly observed with the naked eye. The
oogonies are now described as amphionts.

In the meantime the originally homogeneous nucleus has
directly divided by constriction into an enormous number of
small nuclei which, as the result of further division, become
more and more minute. Around each of these minute nuclear
particles the protoplasm aggregates in the form of long, thin
threads. Thus originate in the cyst the gymnospores. How
enormous the multiplication of the parasites can be is shown
by a calculation of Grassi, according to which one amphiont is
able to produce up to ten thousand gymnospores. As on the
stomach-wall of an infected Anopheles there are frequently found
more than a hundred such cysts, a simple calculation shows
that one gnat may harbour simultaneously more than one
million spores.

When the formation and maturing of the gymnospores is
complete they leave the arnphiont-capsules, invade the body-
cavity of the A noplieles, wander through the fat-tissue, and finally
accumulate in the salivary gland of the host, probably attracted
by chemotactic stimuli. Here they may be frequently observed
in astounding numbers. If now a mosquito in this state seeks
the blood of a healthy man, there enter the human blood, simul-

THE CONSERVATION OF LIFE

239

taneously with the drop of saliva which the Anopheles lets flow
into the wound which it bores with its proboscis, numerous
gymnospores. Here the germs probably invade at once the red
blood corpuscles — direct observations are as yet absent — develop
into mononts, and these divide again into spores. With this act
infection is complete and the cycle may commence anew. A
fever fit announces the presence of malaria.

The reproduction-cycle of the malaria parasite represents a
change of generations, that is, a change and a regular succession
of sexual and asexual reproduction. We shall hear of similar
cases among the metazoans in the next lecture.

Though all this sounds very simple it required an enormous
amount of labour before the life-habit of the parasite had been
accurately determined, for it must be clear to every one that the
disentangling of the threads entailed much more than merely to
observe under the microscope the
blood of a malaria fever patient,
the contents of his intestines, and
the salivary gland of a mosquito.
But man does not penetrate
Nature's secrets quite so easily.
Perhaps it was only chance which
first indicated in the blood of a
malaria patient one or another
stage of the parasite. This dis-
covery is followed up, the blood
of healthy and infected persons
is examined, and it is found that
only the latter contains similar
germs. Gradually the conclu-
sion is arrived at that these
minute organisms stand in some relation to the sickness. One
searches for the way by which the parasites can have reached
the human blood. There are so many possibilities. The most
diverse hypotheses are constructed, examined, and rejected until
finally the right way is found and the carrier discovered in blood-
sucking insects : in this particular case, the Anopheles.

Again, in the interior of the Anopheles are found parasites
of very different appearance, gymnospores, amphionts, oocytes,

FIG. 58. — THE MA.LARIA MOSQUITO

(Anopheles clavig&r) .

240 LECTUEES ON BIOLOGY

and male germs. Only gradually is it perceived that these
many-shaped organisms are but different developmental stages
of one organism ; but even with that discovery little progress
has been made, for these numerous stages have apparently no
connection. One knows neither beginning nor end of the chain.
Yet patient research never flags ; what remains hidden to one
is discovered by another investigator, and gradually untiring
observation and combination forge link to link in right succes-
sion, until at last after long laborious work and many errors the
cycle of evolution is perfect.

I have so far omitted one important fact in the process
of the reproduction of the protozoa. It happens frequently that
multiplication commences with a period of rest. In such cases
the organs of locomotion in the protozoa usually become de-
generate, the organization becomes more simple, the plasmic
body assumes a globular form and secretes a protective cover —
a cyst. They remain in this state often for a considerable time
without exhibiting any perceptible signs of life. Protected by
the cyst they are almost unsusceptible to all injurious external
influences, and in many cases, when the water in which they live
dries up, when winter comes, or food becomes scarce, it is only
this ability to enter into a stage of rest which can keep death
away. Reproduction takes place usually before the encysted
protozoans leave their shells.

The faculty of proceeding under certain circumstances to
cyst-formation we find developed already in the lowest proto-
zoans. The beautiful Amoeba proteus is found in our ponds,
clinging to water plants or floating in the water. With its
life-history we are already acquainted, because it is identical
with that of other aquatic amoebse to which I referred in the
beginning of this lecture. But at certain times we can observe
striking changes in the body of Amoeba proteus. The pseudo-
podia are retracted, the difference between the hyaline ectoplasm
and the granulated endoplasm disappears, and the amoeba
assumes the shape of a ball which soon commences to rotate
slowly around its axis. In the course of its movements the
amoeba secretes a covering; at the end of five days this has
become a complete cyst, and the revolving movement ceases.
In the meantime the nucleus has undergone certain changes,

THE CONSERVATION OF LIFE 241

having become divided into numerous parts which still further
divide into smaller particles until from 500 to 600 nuclei have
been formed. Around each of these minute nuclei a part of the
protoplasm becomes aggregated, and now the cyst is inhabited,
instead of one mother-amoeba, by many thousands of minute
amoebae of about O'Ol to 0'015 millimetres. Very soon the entire
company leaves the protecting shell, enters the outer world, and
grows gradually into normal amoebae.

It will be noticed that here, as in other cases mentioned, the
mother-amoeba does not become entirely dissolved into her
descendants, for a considerable part of the maternal protoplasmic
body remains as residue in the cyst ; but though doomed to rapid
decay we cannot call it a ' corpse.' The entire process as
described has taken about three months.

If we have not been able to become here acquainted with
all the details of the evolution of the protozoans we have at any
rate learned the most essential factors. It is now necessary
to answer one important question : How did the higher animals
develop from the unicellular organisms ? Which causes brought
about a combination of numerous cells into a higher organic unit,
a multicellular organism ? Are there any connecting links be-
tween the protozoa and the strange world of metazoa ? These
are questions which force themselves upon us and demand an
answer.

Like protozoa and the lowest plants, all multicellular or-
ganisms consist at the starting point of their individual existence
of one cell, the fertilized ovum. Like a protozoon, the egg-cell
multiplies by simple fission, and thus gives existence to number-
less descendants. And yet what an enormous difference in their
future fate ! While both daughter-individuals of a protozoon
completely separate one from the other, and after fission exert
no influence whatsoever upon their mutual development but
develop into individuals which are equivalent to the mother-
organism in every detail of organization and able to proceed
after a short time to the reproduction of similar descendants, the
future destiny and appearance of the young offspring of the ovum
is very different.

In contrast to the daughter-cells of the protozoon, the
descendants of the ovum remain, even after fission, permanently
connected, and retain a far-reaching influence upon each other.

16

242 LECTUEES ON BIOLOGY

Let us imagine the earliest developmental stage of an echino-
derm : for instance, the Sea-urchin. The egg-cell, which has
been liberated into the sea-water and there been impregnated,
divides at first into two cells, and these again into four, eight,
sixteen cells, and so forth. In this -manner a large " cell-
heap " is produced, the morula. The individual citizens of
this primitive cell-state are as yet equally well constructed,
no one cell being favoured to the disadvantage of another cell,
but this state does not remain long. As development progresses
the embryonic cells multiply at an increasing ratio, the individual
parts of the cell-heap become more closely united, and from the
morula originates the germinal vesicle, the blastula, a hollow
sphere the wall of which consists of a single layer of firmly
compressed cells.

That the single parts of the cells are developed differently is
clear from the fact that the outer side of the cells, coming into
immediate contact with the surrounding sea- water, is exposed to
influences very different from those which affect the inner side,
which is opposite the ' groove-cavity.' So far the conditions were
the same for all parts of the blastula, but now a change takes
place. Some cells separate from the rest and wander into the
groove-cavity. Then the little hollow sphere invaginates in
the manner of an india-rubber ball the wall of which is being
pressed at one point towards the other side, and the one-layer
germ, the blastula, has become a two-layer germ, the gastrula.

The necessary consequence of this change is a different
development of the cells, corresponding to the influences that
act upon them, and to the differences of their functions. But
I can only deal with it here in large outlines. The external
cells undertake the task of locomotion and form cilia ; the inner
cells opposite the cavity, the primary archenteron, undertake
the work of nutrition and are suitably transformed ; finally those
cells which we saw enter the groove or segmentation cavity
produce by continued division and differentiation the connective
tissue, the supporting matter, and muscles. In the growing
organism division of labour becomes more and more perfect.
The individual citizen of the cell-state does not, like the
protozoon, care only for his own well-being, but places a part
of his strength and work at the disposal of his comrades, the
community, which in their turn undertake many of his cares.

THE CONSERVATION OF LIFE 243

But we must not anticipate. "Let us therefore return to the
unicellular organisms. Among the Vorticellae we saw numerous
forms whose mode of reproduction in so far resembled the re-
production of the ovum, as with them division did not lead to
a complete separation of the newly formed individuals. We saw
that from one animal gradually proceeded many-headed polyp-
stocks, the single individuals of which remained somewhat loosely
connected by means of plasmic stalks. But though this con-
nection in the Vorticellae is slight and only the individual cells
are independent of each other, it represents nevertheless the
first step which leads from the protozoon to the multicellular
organisms. It is true that the most important difference between
them and the higher organisms, the division of labour between
the single parts of the community, is as yet absent, unless we
conceive the formation of two kinds of sexual cells to be its
beginning. For this reason the advantage of this close associa-
tion is difficult to understand, and yet a reason must exist, for
how else can the habit of making only an incomplete division
have been fixed by heredity?

It is interesting to observe that other species of protozoans,
which generally lead a lonely life, occasionally proceed to the
formation of a colony. This is another proof that such union,
even without division of labour, must be useful, though we are
not always able to perceive its advantage. Thus the parasitic
gregarines, which have long been known as intestinal parasites
of lower animals, and the Peridinia which live in enormous
swarms in European ponds and waters, are frequently observed
to exist as long 'chains of dozens of individuals.

Most striking, however, because apparently giving proof of
intelligence, is the system of commensalism, as observed in
particular in the brilliant-coloured Heliozoa. If two, three, or
more of these animalcules accidentally meet near a prey which a
single Heliozoon would be unable to overpower we notice that
their pseudopodia suddenly fuse on contact. Like hunters who
surround a head of game the Heliozoa form round their prey a
circle which gradually closes. The rigid central filaments which
ordinarily give to the pseudopodia their firm shape become
rudimentary, the pseudopodia shorter, and the circle more and
more narrow until finally all the bodies have become fused into

244 LECTUBES ON BIOLOGY

one uniform mass : and in the midst of this common protoplasm
the prey lies defenceless. When the meal is finished the animals
separate once more, and each goes its own way until want of
food may chance to bring them together in another spot.
Thanks to this union, the Heliozoa are able to overpower even
animalcules which are far superior to them in strength and
ability.

A still higher stage of development than the Vorticella
colonies has been reached by Pandorina morum, the small
flagellate belonging to the family of the Volvocidae, an organism
which stands on the borderland between the plant and the animal
world.

In many parts of Europe the Pandorina may be observed in
any little pond. Sixteen cells have formed themselves into a ball
and produced by common labour a gelatinous cover which pro-
tects their bodies. In Pandorina all parts of the body are as yet
equivalent ; each cell is furnished with nucleus, protoplasm, con-
tractile vacuoles, light-sensitive pigment, chlorophyll, and two
flagellae which protrude from the gelatine-cover and serve as
organs of locomotion. Equal is also the work which each cell
performs for itself and the community, but in the fact that
the members of the colony already do common labour, undertake
the nutrition in common, and build the gelatine-cover in common,
lies the great progress as compared with the union of the
Vorticellse. What binds the Pandorina-cells so firmly together
that they no longer live as individuals is difficult to see, for
each cell is capable of performing all essential life-functions,
and would therefore undoubtedly be able to look after itself, yet
they are always found united in this manner. Strictly speak-
ing, Pandorina can no longer be regarded as a protozoon or
protophyton, for it represents already a higher multicellular
organism.

As in Pandorina, in contrast to the multicellular organisms,
the single body-cells are as yet entirely equivalent, each indi-
vidual cell possesses therefore the faculty of reproduction and
of forming a new colony of sixteen cells. Moreover, repro-
duction proceeds exactly as in a typical protozoon. When a
Pandorina colony has been floating about for a certain time in
the water and the cells have become fully developed as the result

THE CONSERVATION OF LIFE 245

of abundant food they commence almost simultaneously to
multiply by successive fission. We have now a colony of sixteen
Pandorinae, each consisting of sixteen cells. For some time the
young animalculse are held together by the old common gelatine-
cover, but very soon the daughter-colonies leave the maternal
home and go into the outer world.

They are delicate creatures whose existence is only too often
threatened by injurious external influences. How are they to
preserve their kind when the waters in which they live dry up,
or severe cold freezes them to ice ? Like the amoebae and many
other protozoans, the Pandorina is assisted through the hard
times by a rest stage. Sometimes the body-walls of a colony
divide each into eight flagellate parts, the so-called gametes.
When these have reached the water, gametes of different descent
proceed to pair. The products of these fusions live free only
for a short time ; then they encyst and their flagellae become
degenerate. Whatever may happen now, the young germ pro-
tected by the cyst is able to withstand all perils, calmly awaiting
better times. As soon as conditions are once more favourable
it crawls forth from its prison and by fission develops once more
a young colony of sixteen heads. Thus we find in the Pandorina,
in addition to asexual, a sexual reproduction, but the latter is
still of a very simple character, for a differentiation of male and
female sex has not yet been reached.

A near relation of Pandorina is Volvox which lives in close union
with it. Every lover of Nature knows the little greenish balls,
easily noticeable with the naked eye, which slowly rotate through
the water. While the body of the Pandorina consists only of
sixteen cells, a Volvox colony may consist of more than ten
thousand cell-individuals which form the one-layer wall of a large
hollow sphere filled with a thick fluid. The union of the indi-
vidual citizens of this cell-state has become most intimate. Here,
too, the entire colony is covered with a uniform gelatine shell,
but the different individuals are in addition connected with each
other by thin bridges of protoplasm. As in Pandorina, so the
single cells possess here still great independence. Like every
other cell, they consist of protoplasm and nucleus, but in addition
we find some which are furnished with contractile vacuoles,
chlorophyll, pigment, flagellae, and a special close-fitting gelatine

246

LECTURES ON BIOLOGY

mantle. Apart from the far greater individual number of the
Volvox colony, which is joined together in one organic unit, a
fundamental difference does not appear to exist between them
and Pandorina. But while there every single body-cell is able
to reproduce independently and form a new colony of Pandorina,
the cell-individuals of a Volvox colony have lost this old inborn
faculty. They are still able to divide, but the parts are no longer
able to grow into a young Volvox. Only a few favoured indi-
viduals of the cell- state have retained the faculty to preserve the
species. We see, therefore, in Volvox a beginning of what we
may describe as the essential characteristic of a higher multi-
cellular organism : the separation of the whole organism into
body-cells and germ-cells (fig. 59).

If we observe a Volvox colony,
slightly magnified we distinguish
at once among the numerous cells
a few which are differentiated
from their neighbours in shape and
size, and among these differently
constructed cells we discern again
two different kinds. They are the
rudiments of male and female sex-
cells. The microgametes or sperm-
atozoa develop first. The semen-
globator. forming cells grow rapidly, producing
by continual fission within the
colony a large heap of a hundred or
more spermatozoa about 0'005 milli-
metres long. Two long flagelliform processes serve as organs
of locomotion. In the meantime the female germ-rudiments
(Anlagen] have become more distinct. The absence of flagella3
differentiates them from all other members of the colony.
Without fission having taken place every female germ, of which
the colony contains about twenty to thirty, develops into a large
cell of a round shape and rich in plasm, the macrogamete or ovum.
When the female and the male sex-cells are mature they separate
from the rest of the cells and fertilization takes place in the
water. From the fertilized ovum a new Volvox is then gradually
developed.

FIG. 59. — Volvox

SEXUAL HERMAPHRODITIC COLONY
WITH OVA AND SPERMATOZOA IN
PROCESS OF FORMATION.

THE CONSERVATION OF LIFE 247

Bat what becomes of the old maternal organism ? When all
germ-cells have seceded it continues to exist for some time but
is unable to form new germ-cells, for body-cells can only produce
body-cells. Soon symptoms of old age become perceptible ; one
function after the other ceases, and finally the colony reaches
the inevitable end and dies. For the first time in the organic
world we see here that death appears as a physiological necessity,
as the inexorable fate which is henceforward in store for all
organisms after a shorter or longer existence. Of what benefit
would continued life be to the unfertile Volvox ? It would be
useless for the preservation of the species, and at the best
handicap the course of life for the young colonies. But Nature,
as we have seen on various occasions, has no regard whatever
for the individual and will waste billions of life germs, to achieve
its only object : the preservation of the species.

But why must the Volvox die? Why was it necessary for
death to come into the organic world ?

We are confronted here for the first time with a new difficulty.
How is it possible for the germ-cells to develop into a complete
Volvox colony? Owing to which qualities are they able to
reproduce not only their like, but also body-cells which are widely
differentiated in structure ? It is the great problem of heredity
which here unrolls itself before us. That both parts of the
amoeba develop into organisms which are like the mother we
seem to understand without any difficulty, for here no develop-
ment is required, only simple growth, because progeny and
progenitor are morphologically equivalent from ' birth.' But
it is already different in the Infusorians. Here both descendants
must by new-formation generate the organella of which they
were deprived during fission. It is therefore clear that the
infusorium must have inherited from the mother the faculty to
develop all the individual parts of its body out of itself. But I
must leave the problem of heredity for a later occasion when we
come to consider the various theories of heredity.

If a colonist, a second Eobinson Crusoe, should reach a
strange, uninhabited land, fertile as the Garden of Eden, and find
everywhere identical favourable conditions of existence he will
not be long in doubt where to choose his future homestead.
Wherever he happens to be he will erect his hut. As Nature

248 LECTURES ON BIOLOGY

offers to his body and mind everywhere the same wealth and
beauty there is no cause for cautious hesitation and careful
selection. His life in the midst of abundance might seem
to us an enviable lot, and yet it will be but a poor life spent
in laborious work. Being alone, all labour rests on his two
shoulders, for there is no one to lend a hand to help him lift the
burden. In spite of the generosity of Nature his days will be
spent in labouring for the satisfaction of the simplest needs of
life. He must cook and bake, he must be his own tailor and
shoemaker, he must keep house, tend the field, and harvest the
fruit ; almost defenceless he is exposed to attacks by beasts of
prey, nor will the night bring him complete rest, for there is no
one who will keep watch over him while he is asleep. He will
drag along a miserable existence, and in the end his mental
faculties, wanting almost every stimulus, will atrophy.

But if perchance a second colonist should reach by the same
island the position of his domicile is predetermined. Where else
would he erect his hut but in the neighbourhood of the first ? If
their mutual condition be still far from being splendid they are
now able to make more of their lives than if each were dependent
only upon himself. They can exchange their ideas and take
counsel with each other and, above all, they will proceed to a
division of labour. The burden which was far too heavy for
one is easily carried by two pairs of shoulders. While one
cultivates the garden, tends the field, and carries wood and water,
the other undertakes the care of the household and prepares the
food ; when one rests the other keeps guard, and their united
forces grant an increased security against hostile attacks. The
more persons immigrate as time goes on, the more favourable
the conditions of existence become for all ; for in proportion as
the division of labour becomes more complete, more time and
leisure is gained by each individual for play, pleasure, and the
culture of the mind and body. But not only is the burden of
labour decreased : the output of labour is increased. Who must
do something in all branches of work will never do something
really great in any. The waste of energy resulting from the
splitting of his force, from which the first colonist suffered
severely, does no longer exist with the citizens of this colony.
The agriculturist is only an agriculturist and need not trouble

THE CONSERVATION OF LIFE 249

about anything else ; the shoemaker makes only foot-wear, the
tailor only clothes, and in return their other needs of life are
satisfied by the community. But such union and civilization
holds many perils. Whilst the individual does splendid work
in his own narrow sphere, he becomes far too highly specialized.
Left to himself he would be unable to exist ; in a certain sense
he ceases, and must cease, to be an individual and becomes a
part of the state.

The development of the higher organisms from the protozoa
is an exact parallel to this example. The individual cell, how-
ever highly it may be organized, will always remain on a very
low stage. Its forces are split in the satisfaction of all the many
necessities of life, for it must be intestine, lung, kidney, muscles,
and sexual organ, all at the same time, and therefore remain
amateurish in all its performances. But by way of compensa-
tion it is free, dependent upon no one and, as it were, immortal —
yet it lives but a life barren of the possibilities of development to
which Nature for ever tends. Several cells join together in a
higher union for purposes of common labour, without as yet
bringing about a real division of labour. As a result all con-
serve their original characteristics and faculties, no one-sided
differentiation of the individual taking place. Thus we obtain an
organism such as the Pandorina.

The formation of the cell-state makes it possible that not all
the members need breathe and eat in order to exist. It is suffi-
cient that some of them render this service. Thus in order to
carry out these vegetal life-functions better and more intensively
the affected cells devote themselves exclusively to this different
activity and participate no longer in the preservation of the
species. That care is now left exclusively to other citizens which
are maintained and nourished by a community. The separation
of the vegetal functions from reproduction, the division of the
organism into body-cells and germ-cells is the first result of the
division of labour. On this stage of development stands the
Volvox.

But Nature knows no standstill. The Volvox stage is over-
taken, the division of labour becomes more and more perfect,
and the labour of the organism correspondingly increased. In a
higher animal, a sponge, worm, insect, or even vertebrate, the

250 LECTURES ON BIOLOGY

body is not only divided into body-cells and germ-cells, but the
body-cells again have become differentiated and changed, accord-
ing to the functions which they perform, into muscle-, nerve-,
blood-, intestinal, kidney-, connective tissue-, and bone-cells.
Indeed, the degree of division of labour is a measure of the
degree of development of an organism : the higher the organism
the more completely does the individual cell give up its indepen-
dence and individuality and become a serving link of the whole
without any self-purposes. In many cases the cell must even die
in order to fulfil its task in the organism. The epidermis cells of
man, at first living cells full of plasm, become harder and harder
and finally die, thus forming the cover under which the con-
nective tissue and other delicate organs are protected against
injuries and other noxious external influences. The bone-
forming cells become osseous, and as dead bone-corpuscles help
to build up the skeleton.

Is it not now clear why the Volvox body and the body of
all higher multicellular organisms must die, and why only the
germ-cells are able to reproduce a species ? Having through
division of labour undergone a one-sided transformation, the body-
cells are only able to perform certain definite functions that have
been given to them. But as the parts of every engine deteriorate
by wear and tear and finally disintegrate, so life uses up the
body-cells, and, being deprived of the faculty to regenerate by
reproduction, they are doomed to early decay. The continuity
of life rests now only on the germ-cells. With the production
of germ-cells the most important mission of each organism is
fulfilled, for its death no longer imperils the preservation of the
species, and thus we see that death and reproduction frequently
synchronize.

251

CHAPTER IX.
KEPEODUCTION AND HEREDITY.

THE last chapter gave us an insight into the marvels of
reproduction of the lowest forms of animal life. Though the
conditions were simpler the questions were essentially the same
that confronted us in the reproduction of the higher and highest
animals. Death, which only makes a shy attempt to establish
its dominion over the lowest forms, now never ceases to hold
its victim in a firm grasp. Individuals come and go; only the
germ-cells guarantee the continuity of life and make the charac-
ters of the parents reappear in their descendants. If sexual
union played a comparatively inferior role in the unicellular
organisms it now rules the entire life. Love now becomes
an omnipotent factor which brings together the two sexes
with elemental force. In addition we observe a new factor :
the care of the parent for the offspring. There are, of
course, many multicellular animals which, having deposited
the ova, do not take any further interest in them, but the
higher we ascend in the animal world, the greater the solicitude
demanded by the young brood, the longer are the parents
compelled to look after the young, jointly or separately, until
they have at last acquired the necessary strength and skill to
join in the struggle for existence. In many of the higher
animals and man, this union of parent and offspring may even
become so complete that it will end only with the death of
the progenitor.

The jump from the protozoa to the higher animals is not so
great as would at a first glance appear, for the two great kingdoms
are connected by gradual transitions : organisms of more than
one cell. Farther, the lowest multicellular animals as yet
occupy in their whole organization a very low stage, enormous
though the progress they represent, compared with the organisms

252 LECTUEES ON BIOLOGY

consisting of one and more cells. Of a division of their bodies
into sharply-defined sections — head, thorax, abdomen, and ex-
tremities— so characteristic of insects and vertebrates, there is as
yet no vestige. The single body-parts are similar to each other,
the whole being not so much sub-ordinate as co-ordinate. In
addition, the differentiation and specialization of the individual
cells has not made much progress : they have rather retained
their embryonic character. Thus it is that injuries or mutilations
are easily sustained by these animals, while more highly differ-
entiated forms would perish from them. A Gothic dome, a Doric
temple, partially destroyed, is a useless ruin, no longer capable of
serving its purpose; but a simple beam we may, without dis-
advantage, cut up into several parts, and each section, though
smaller than the whole, will still retain the qualities of the beam.
The same holds true in the case of the higher, differentiated
animals, as compared with the lower, more homogeneous forms.
A vertebrate or an insect that has been cut into two parts is no
longer able to live. Both halves are too closely correlated : the
life-functions of one half are conditioned and supplemented by
those of the other, and they can only fulfil their numerous tasks
when joined in a close union, but are doomed to perish when
separated. But an animal whose body has not yet reached such
a high state of differentiation and whose structure is as yet more
uniform retains its power to live even after mutilation, and
is even able quickly to replace the lost parts by regeneration.
Hence numerous lower multicellular species still retain the
asexual form of reproduction by fission or budding, in addition
to reproduction by germ-cells, while representatives of the
highest classes of the animal kingdom are exclusively dependent
upon sexual preservation of their species.

Fission, as well as bud-formation, is closely, associated with
the power of regeneration, for in order to retain life the animals
that have been cut into parts must of course be able to replace
the lost parts. For this reason it is clear that this faculty is far
greater in the lower organisms.

The celebrated experiments made in 1740 by Trembley with
the green freshwater polyp Hydra viridis proved that even the
smallest particle of the body of the polyp is able to develop into a
complete organism. Jager was further able to observe in Hydra

REPRODUCTION AND HEREDITY 253

grisea that all body-cells became completely severed, lived inde-
pendently for several months, and crawled about like amoebae.
Jager suspected that the individual cells hibernate in this state
and develop into complete polypes in the following spring.

In the other relatives of Hydra, the sea-anemones and corals,
this regenerative faculty is still equally well developed. In
numerous species of higher multicellular organisms, most of the
worms, echinoderms and tunicates, the power of regeneration is
frequently very strong. One may often observe, for instance, how
an earth-worm, attacked by a myriapod, separates in the middle,
thus saving its life by sacrificing part of its body. The anterior
end is in that case able to reproduce a new posterior end, while
the latter can reproduce a new anterior part.

The enormous tenacity of many Turbellarians is astounding.
For example, we may cut up a Planarian into longitudinal and
transverse sections and yet see how each part once more develops
under favourable conditions into a complete organism. In this
almost indestructible creature one may easily reproduce the
strangest malformations, for the slightest injuries are sufficient
to stimulate the body-cells into regeneration. If, for instance,
we make several incisions into the body of a Planarian new
anterior or posterior parts will be formed in these spots, according
to the direction of the incision.

We heard on a previous occasion that the starfishes are able,
by constriction, to shed their arms, and that each arm is able to
re-grow a complete star. The sea-cucumbers are able at touch,
or after prolonged captivity, to eject, by means of powerful con-
traction of all the muscles of the body, their intestines through
mouth and anus, without suffering any vital injuries from this
horrible mutilation. In the molluscs and arthropods the re-
generative faculty has considerably decreased, being restricted
to the re-growing of tentacles and syphons, or to the supplement-
ing of lost legs, antennae, or claws. In a few rare cases more
important organs may be repaired : for instance, eyes in crabs. I
must not omit here to point out a remarkable experiment made
by Herbst. If he removed the eye of one of the higher crusta-
ceans, together with the eye ganglion, an antenna was developed
in place of an eye ; if, however, only an eye was removed and
the nerve-centre left intact regeneration of an eye took place
with perfect regularity.

254 LECTURES ON BIOLOGY

Among the vertebrates the tailed amphibians possess the
most highly developed regenerative power. Axolotls and Tritons
are not only able to regenerate their tails but also their legs.
That lizards are able to regenerate a new tail we have already
heard, but they are not able to regenerate the legs. Among
fishes, birds, and mammals it is no longer possible to speak of
regeneration, for with them re-growth is restricted to restoring
the skin and healing slight wounds. We perceive, therefore, that
the faculty of regeneration decreases in proportion as the differ-
entiation of the animal body increases. The fact that embryos,
including those of the highest vertebrates possess, generally
speaking, a considerable regenerating power well accords, there-
fore, with our hypothesis concerning the causes of regeneration.

We have so far seen that regeneration of different body-parts
appears usually as the result of pathological changes. But it
may also be observed in the normal course of life of numerous
species of animals, when it is closely associated, as I have already
mentioned, with asexual reproduction. As among the protozoans
individuals sometimes ' grow beyond their individual measure,'
and then divide into two or more individuals, a similar process
may be observed in the higher organisms. But as the sections
are naturally deficient in certain parts and organs, these must be
supplemented either before or after constriction. The more
frequent and more practical course, however, is when division —
as in Infusorians — is preceded by regeneration, and the most
important organs are at a certain time ' laid down ' in several
sets. Only when that has been done separation takes place.
The young in that case are like their mother in every detail,
excepting alone in size,

This method of reproduction may be very distinctly observed
in a little Turbellarian, the Microstoma, When it has reached a
certain size a little circular sulcus becomes visible in the centre
of its body. Below this dividing line there appear a new
mouth-aperture, primary optic spots, and other important organs
of the region of the head. The animal is now ready to divide
into two individuals. In Microstoma the asexual reproduction
frequently proceeds so rapidly that new dividing lines may be
observed in both halves long before the constriction has become
complete, and this generation of grandchildren sometimes bears

KEPRODUCTION AND HEREDITY 255

the rudiments (Anlagen) of the organs of the great-grand-
children.

This division with preceding re-growth of the organs occurs,
in addition to the Turbellarians, among quite a large number of
the individuals of other classes ; for instance, various tunicates,
numerous jelly-fishes and sponges reproduce in a similar manner,
when fission may take place either transversely or longitudinally.
But as in these organisms division is frequently incomplete the
result is the formation of larger and smaller stocks. Sponges,
in particular, are as a rule composed of a large number of
individuals whose relations are very frequently so close that they
appear to us as one individual. A division followed by regenera-
tion is also observed in several worms and also many starfishes.

Very remarkable are those cases in which the young do not
await their full development, but proceed to sexual reproduction
even at an early embryonic stage. Kleinberg states that the
embryos of Lumbricus trapezoides, a relative of our earthworm,
divide shortly after the gastrula stage. In this manner two
individuals regularly originate from each ovum. In exceptional
cases a longitudinal division will take place in embryos of verte-
brates. This usually gives rise to strange duplicate malforma-
tions, but leads sometimes to the formation of normal, healthy
twins.

These conditions are most striking in the case of Encystus,
one of the Ichneurnonidse. This little robber searches usually for
butterfly-eggs into which it lays one egg by means of its ovipositor.
Before this proceeds to develop it first divides into a considerable
number of cells from each of which, if conditions be favourable,
there arises a larva and later a young ichneumonid.

While during reproduction by fission the mother-individual
is completely divided into the two young, and therefore ceases to
exist as an independent individual, it continues in the budding
process to exist as an independent individual even after the
separation of the bud. Generally speaking, fission generates
equivalent products, budding non-equivalent products. But this
must not be taken too literally, for as Nature is always an enemy
of sharp contrasts so a gradual transition exists between these
two forms of asexual reproduction, and in numerous cases it is
left to our choice to describe the process either as budding or as
fission.

256

LECTURES ON BIOLOGY

In sponges and jelly-fishes bud-formation can proceed from
any part of the body, but the higher animal-forms which are still
able to produce by budding, above all numerous tunicates and
the moss-animalcules, possess a special generative gland, the
Stolo prolifer, from which the formation of the young proceeds.

A beautiful illustration of
the first method of budding is
offered by the green fresh-water
polyp, Hydra viridis. The
structure of this delicate animal
is still exceedingly simple and
corresponds in its essentials to
the organization of a gastrula.
The mouth aperture, situated
at the distal extremity of the
body, and surrounded by filiform
tentacles, leads directly into the
body-cavity, which is covered
on all sides by a double cell-
layer, endoderm and ectoderm.
Here the mouth has still a
double task to fulfil : to take
in the food - particles and to
eject the digested food. It
serves, therefore, at the same
time as mouth and anus. The

proximal end of the Hydra is closed by an adhesive disc, whereby
the animal is commonly attached to some water-weed.

If the polyps are kept in an aquarium and are liberally
supplied with their favourite food, minute crustaceans, which
they seize skilfully with their tentacles, one is soon able to
observe remarkable changes in most of the individuals. While
the body was at first slender and uniform, small prominences
now appear on the body-wall, increasing rapidly in size. The
microscope shows that both endoderrn and ectoderm jointly
participate in this new formation and that the maternal body-
cavity is continued into the bud. Gradually these prominences
assume the form of a polyp, with mouth and tentacles being
formed at its distal extremity. The hydra has become a polyp-

FIG. 60. — GREEN FRESH-WATER POLYP

(Hydra viridis).

REPRODUCTION AND HEREDITY 257

stock which, however, mostly consists of only two or three
individuals. Only very rarely have I succeeded in breeding large
stocks of four and sometimes five individuals. This state, how-
ever, does not last long, for the buds soon separate from the
maternal body and commence an independent existence.

Numerous species of marine jelly fishes form, by gemmation,
enormous animal states which frequently consist of hundreds
of thousands of individuals remaining permanently united. But
as stocks of such immense size and with such enormous power
of reproduction could not exist long without the necessary
mechanical support, these animalcules jointly construct a firm
skeleton of carbonate of lime, or, in some species, of a horn-
like substance.

The most magnificent instance of such animal colonies is
doubtless that of the corals whose restless labour makes new
islands and reefs rise from the ocean-bed and played a most
important part in the early life of the earth.

It seems that asexiial reproduction cannot proceed uninter-
ruptedly in any of the higher species, for after a series of genera-
tions it is always once more replaced by sexual reproduction. We
are probably right in assuming that the faculty of gemmation and
fission was only a very late acquisition of the higher multicellular
animals, and that it is an adaptation to the special conditions
of their life. The original method however is reproduction by
germ-cells. In the metazoans, therefore^ conditions would on this
assumption be opposite to those in the protozoans, in whom we
saw asexual reproduction to be the original method, while sexual
union was doubtless only evolved at a much later period of their
phylogeny. That with an assumption of this kind we are only
dealing with probabilities and guesses and that we are completely
deserted by experience needs no particular mention.

The inclination to asexual reproduction is usually increased
by favourable conditions of life and abundant food ; want of food,
on the other side, and other influences which threaten their
existence induce organisms to proceed to sexual reproduction.
It is only necessary to restrict the amount of food of the Hydra
in order to observe soon afterwards the formation of sex-
products.

Whether exclusive asexual reproduction is not possible in
17

258 LECTUEES ON BIOLOGY

higher multicellular organisms, but must in a short time lead
to degeneration ; on that point opinions are greatly divided. But if
we consider that the pyramid-poplar and numerous other ancient
domesticated plants, such as the vine, fig-tree, date-palm, banana-
tree and many other useful plants, have propagated their species
since times immemorial by an exclusively asexual method, (i.e., by
layers), without having suffered the least decrease in strength
or the quality of their fruits, it would seem that a natural limit
does not exist and that the asexual propagation can in many
cases replace sexual reproduction. But the pyramid-poplar is
frequently mentioned as a case against this hypothesis, because
it has lately everywhere commenced to sicken and die out. The
cause of this phenomenon is, however, probably not so much to
be found in their mode of reproduction as in an undue increase in
parasites and other unfavourable external conditions, against
which the seed would be equally defenceless.

As a kind of inner gemmation we may describe a method of
reproduction which has hitherto been observed with certainty
only in fresh-water sponges. At the beginning of the inclement
season there are formed in the interior of the sponge-body globular
accumulations of cells which all originate from the central ger-
minal layer. The external cells form themselves into a mantle
consisting of one cell-layer, and secrete a solid cover which is
supported by numerous silica-needles, the amphidiscs. When
severe frost sets in the sponge-body perishes, but these gemmulae
are, thanks to their power of resistance, able to live through the
winter. They sink to the bottom of the water and remain there
apparently lifeless, but when spring returns the encysted sponge-
particles leave their protecting membrane, commence to grow,
and proceed to reproduction.

In the moss animalcules found in fresh water we observe
similar multicellular interior buds, called statoblasts, which after
a long period of rest may develop into a new colony of Bryozoa.
The principal task of these buds is to help the animalcules
through the perilous times of drought and frost, and later, when
the warmer weather has returned, to bring about rapid multipli-
cation of the species. In order to be able to discharge these
functions the statoblasts are enclosed in a hard chitinous shell
which effectually protects the young germ against all the dangers

REPEODUCTION AND HEREDITY 259

of life. In the flattened margin of the shell are numerous small
cavities which during the dry period are filled with air and act
like swim-bladders, keeping the buds suspended on the surface,
when they once more reach the water. Winds and currents then
drive the statoblasts over vast stretches of water and finally each
produces a young Bryozoon.

One may frequently see that asexual reproduction regularly
alternates with sexual, so that one asexual generation, or a
limited number of asexual generations, is always succeeded by a
sexual generation. In that case we speak of an alternation of
generations. As long ago as 1815 this phenomenon was dis-
covered in one of the locomotor tunicates, the Salpa, by the poet-
naturalist Chamisso. Fig. 61 shows in the foreground a large speci-
men of the asexual generation of Salpa democratica, the ' nurse,'
which always leads a hermit life. At the posterior end we see the
Stolo prolifer from which sprout successively several colonies of
Salpae. As soon as these buds have reached a certain size they
separate from the body of the ' nurse ' and float about, joined
together in long chains. These chain-Salpae are sex-animalcules
which possess sexual organs, but no stolon. Soon after its separa-
tion from the stolon each individual of this chain produces an
ovum, and afterwards male spermotozoa. They are therefore
hermaphrodites. Fertilization and embryonic development take
place in the interior of the maternal body, and from each ovum
there develops finally a solitary Salpa, the ' nurse,' which once
more proceeds to the formation of chain- Salpse. This completes
the cycle of development. As the body of most Salpae possesses
a high degree of phosphorescence, it is a wonderful sight to see,
from a seat in a boat, these long glowing chains move like fiery
serpents below the surface of the water.

The alternation of generations is most interesting in numerous
members of the jelly-fishes. Here it is frequently connected with
a strict division of labour and a resulting great diversity of forms.
While the fresh-water polyp Hydra is a sexual animalcule and an
asexual generation in one individual, numbers of its near and
distant relatives have abandoned the faculty of producing repro-
ductive cells, and proceed to the formation of special sexual
individuals, the Medusae.

During prolonged north winds the ports of the Baltic Sea

260

LECTURES ON BIOLOGY

are often littered with the brilliant blue-tinted jelly-fish, Aurelia
aurita. From the ovum of these animals originates a little ciliated
free-swimming larva, the Planula. After a short life of freedom
it attaches itself to the sea-floor and changes into a polyp, the
sixteen-armed Scyphostoma. On the body of this polyp soon
appear numerous circular grooves which gradually increase in
depth, resembling a pile of tea-cups. This is the beginning of

FIG. 62.— DEVELOPMENT OF Aurelia aurita.

On the left, above, the free-swimming larva form a Planula ; below, on the
the rock, a Planula about to become a Scyphostoma. Three other fixed forms show
the process of strobilation and the formation of the Ephyra-form ; on the left,
swimming, an Ephyra soon after separation from the Strobila ; on the right, an
adult Aurelia aurita.

sexual reproduction by means of terminal gemmation. The
Scyphostoma has thereby become a Strobila. Now the polyp-
tentacle in the most extreme bud begins to degenerate, and the
bud separates. The others quickly follow, and in a short time
numerous so-called Ephyrae are swimming about. But there is
still a long road to travel. Only a slow and complicated meta-
morphosis, during which the larvae must pass many different

REPRODUCTION AND HEREDITY 261

stages, transforms the Ephyra into the sexually mature Aurelia.
(See fig. 62.)

Apart from terminal gemmation, the scyphostomae are able
like Hydra, to reproduce by lateral sprouting, but in that case
only polyps are produced. In many Medusae, as for instance the
brilliant Pelagia noctiluca, the original asexual polyp generation
is often entirely suppressed. In this case the ova produce at
once Epbyrae which change into Medusae.

Somewhat different is the' cycle of development in the marine
types of polyps, the Hydrozoa. These animalcules usually form
small, but sometimes very extensive stocks which adhere to the
stones and shells by means of* many-branched roots, the Hydro-
rhiza. From this root rises the Hydrocaulus, the stem which
carries at the end of its branches the different citizens of its
colony, polyps and medusae. Hydrorhiza, as well as Hydrocaulus,
possess essentially the same structure as the polyps : they consist
of ectoderm and endoderm and an interposed supporting lamella.
As the stomach of the individuals is continued directly into
the stem and roots, all the food which is captured by any one
individual is distributed among the entire community. The
union of this little colony is so intimate that it has almost become
one individual of a higher order, and the single individuals
have sunk to the level of organs. In a still more distinct form this
is the case in the magnificent Siphonophora, which are nothing
but free-swimming polyp-stocks. Very few people would believe,
having fished one of these delicate structures from the sea, that
they hold in their hand not one organism, but a complete animal-
state. (See fig. 46.)

These hydroid-stocks derive the requisite support from a
horn-like cover, a secretion of the outer cell-layer, which surrounds
the entire stock. In some species this armour ends at the base
of the polyps, but in others it widens here into a bell of con-
siderable size, into which the polyp withdraws, like a snail into
its shell, at the least sign of danger.

As long as the little animal-stock is growing, new buds con-
tinue to sprout laterally from stem and branches, and develop
either into sexual polyps or into medusae. The polyps remain
throughout their lives fixed to the stem, but the adult medusae
separate from the stem to carry their sex-products through the

'262 LECTUKES ON BIOLOGY

ocean and develop new colonies far away from home. They
serve, therefore, prominently as a means of distributing the
species and opening up new regions for the Hydrazoa to colonize.
The great importance of this fact to the preservation of the
species, the advantage which they possess as a result of this
arrangement in the struggle for existence over other fixed
organisms which have no free-living sex-animals or similar
arrangements for the distribution of their germ-cells and are
therefore restricted to their narrowly defined domicile, is obvious.
We are probably right in assuming that the medusae owe their
origin to this very need. Phylogenetically the polyps represent,
therefore, the older and lower stage of development from which
the free-moving sexual form was gradually evolved. For how-
ever much the medusa may differ in its appearance from the
polyp, accurate comparison shows that it is nothing else but a
more highly differentiated polyp which has become adapted to
a free mode of life.

It must strike us as very remarkable that some species of
Hydrozoa have again abandoned this advantage. It is true that the
PlumularidcB and Sertularidce still produce a medusa-generation,
but the sex-animals are incomplete and no longer adapted
to a free-swimming life. Like the polyps, they are fixed to the
stem. The sex-animals have thus become sex-organs.

The degree of degeneration can vary greatly. In some species
only the mouth-aperture, tentacles, and the strong muscular
velum is lost, but the protective bell with its most important
organs and the mouth-stalk remain. In others the form of the
sex-organs hardly permits us even to suspect that they have
originated from medusae, and that once they were independent
animals. Only the degenerate stomach and the sexual organs
have remained, being surrounded by the sparse remainders of the
bell as by a thin mantle.

It is necessary to mention here the alternation of generations
in some of the tapeworms. While the larvae of most tape-
worms produce only one sex-animal, in the cysticerci of Tcenia
ccenurus, the cause of the deadly "gid " of sheep, and of T. echino-
coccus, one of the smallest, but probably also one of the most
dangerous tapeworms, there are produced by inner budding
several, sometimes numerous, tapeworm-heads from which

REPRODUCTION AND HEREDITY 263

afterwards the proglottides begin to sprout as soon as the
heads have reached a suitable animal-host. The fertility
particular of T. echinococcus is enormous. The sex-form, the
'worm ' proper, lives in the intestines of the domestic dog. Its
length rarely exceeds -J- centimetre, but it is all the more dan-
gerous for being so easily overlooked. If many lovers of dogs
knew to what dangers they expose themselves and their family
by petting their dogs or even kissing them they would exercise
greater caution, for if mature proglottides or ova of Echinococcus
reach the intestinal canal of man they rapidly develop into
embryos which penetrate the tissues and settle in the brain, lung,
or other organs. What terrible pains they are able to cause
will be understood when we hear that these cysticerci often
reach the size of a human head and a weight of 20 Ibs. On the
large original cysticercus-bladder are produced by invagination
many thousands of smaller bladders, and from each of these
vesicles originate the minute cysts in which the tapeworm-heads
are ' rudimentary.' We observe, therefore, a regular alternation
between a sexual generation, the tapeworm, and an asexual, the
cysticercus, which reproduces by budding. In this case it was
probably the extremely favourable conditions of nutrition which
gradually induced the larvae to proceed to asexual reproduction.

Those who have kept a fresh- water-aquarium are well
acquainted with the agile water-flea, Daphnia. In tine weather
our ditches, ponds, and lakes are often so peopled with them
that it is only necessary to draw a fine net through the water
to capture numberless thousands. Many observers will have
noticed that all water-fleas which are caught in the sea during
summer are females. No matter how much we may search, it
seems impossible to discover a male. Nevertheless, the animals
are not doomed to sterility, for they are able to produce numerous
young and these can proceed to reproduction in their turn, but
still no male water-fleas are to be found. To take these tiny
crustaceans for hermaphrodites able to produce male spermatozoa
in addition to ova would be wrong, for they are typical females.

After reproduction has in this manner proceeded through
numerous generations until the approach of the autumn, all at
once male individuals appear, easily noticeable to the trained
eye by their smaller size. The females again produce ova, and

264 LECTUEES ON BIOLOGY

finally a union and fertilization of the sexes takes place. But
while formerly the egg-cells passed their embryonic development
in the roomy brood-chamber of the mother, situated between the
chitinous armour and the dorsal surface of the body, and come
into life as developed crustaceans, the fertilized ova enter the
brood-chamber only for the purpose of being furnished with
a solid chitin-shell, the ephippium. After that they are left to
their fate : the death of the mother, which soon after intervenes,
would in most cases render further care impossible.

What is the meaning of this maleless reproduction which,
apart from the daphnids, we find in numerous other animals ?
How is it possible for eggs to develop without fertilization ?
The answer to the latter question must be postponed till later,
but I will here recall what we have already heard on a previous
occasion. No matter how well the germ-cells may be adapted
for fertilization, it happens not infrequently that ova, and in one
case even male sex-cells, proceed to parthenogenetic development.
The value of parthenogenesis lies principally in the fact that it
increases the fertility of the species and produces in a short time
an enormous number of individuals. We meet, therefore, with it
chiefly in animals whose conditions of life are favourable only
for a brief time, and which are afterwards threatened with
destruction from numerous causes. However important ferti-
lization and the resulting mixing of qualities may be for the
development and adaptation of organisms, parthenogenesis un-
doubtedly confers in such cases a greater advantage. It becomes,
however, never so much the exclusive method of reproduction
that sexual union is entirely suppressed.

A simple calculation will show the degree to which partheno-
genesis is able to increase reproduction. In the first genera-
tion, when all ova develop into female individuals,- the number
of descendants is doubled, but with each further generation of
females the number increases rapidly, because the number of
the ova increases in geometrical progression. We need only
think of our illustration of the chess-board to understand the
extent of this extraordinary multiplication.

The strict dependence of their reproduction upon external con-
ditions of life in the water-fleas has been proved by most careful
experiments made by Weismann, and recently by Issakowitsch.

REPRODUCTION AND HEKEDITY 265

It is clear from what I have already said that the partheno-
genetic ' summer eggs ' serve chiefly the purpose of rapid
increase and distribution, but the fertilized ' winter eggs,'
able to resist equally well drought and frost and to develop
even after a rest period extending over years, serve to
preserve the species in times of necessity. It appears that
in the daphnids the number of the successive asexual genera-
tions is the lower the more frequently the colony of the
species concerned is visited by periods of destruction. Thus the
species which are threatened by severe frost only once in a year
have the largest number of successive parthenogenetic genera-
tions. These are, of course, above all those daphnids which
live in the large waters. On the other hand, those species of
water-fleas which have their domicile in shallow, easily drying
pools or ditches, have only very few, and sometimes only one,
generation of parthenogenesis which is immediately followed
by the appearance of males and, resulting from that, asexual
generation.

According to the most recent investigation by Issakowitsch,
the causes that govern the question of sex lie exclusively in the
external conditions, i.e., in the changes of temperature and
the conditions of nutrition. In order to decide the question,
this investigator chose the method of experiment which naturally
•offers the greatest prospect of success if a question is rationally
put. He started several cultures, one at a temperature of
+ 24° C., one at room temperature, i.e., + 16° C., and one at
-h 8° C. The result of these experiments was that with decreas-
ing warmth the tendency towards the formation of sex-animals
increased, while with an increasing temperature the ova developed
into an asexual generation of females.

In accordance with these results were the experiments made
by Issakowitsch with a view to determining the influences of
nutrition. Here, too, it was found that hunger-cultures produced
only sex-animals, quite independent of the number of the
asexual generations that had preceded them. Issakowitsch sums
up the results of his investigations in these words : "When the
nutrition of the maternal organism has become so low that it
is no longer able to provide the ovum with the food sufficient
for its development into a female, a less exacting male develops.

266 LECTUKES ON BIOLOGY

If the nutrition becomes still lower so that it is not even able
to develop from the egg a male individual a large number of
primary egg-cells join together in order to form at the expense
of the whole mass one fertilizable winter egg." This explains
why at the beginning of the inclement weather, or before the
drying up of the home-water, suddenly the male daphnids appear
and ' winter eggs ' are formed.

This result becomes still more clear -when we observe the
process of egg-forming in the water-fleas. Let us choose for
this purpose a well-known daphnid, Sida crystalline/,. The ovary
of this animal has the form of a long tube closed at one end.
At the blind end lies the germ-layer in which the young egg-
cells are formed, always four at a time, which are at first
equivalent, stored one behind the other, and filling the entire
lumen of the egg-tube.

But the fate of these four young egg-cells is very different :
only one of them, no doubt the strongest, develops into a normal
ovum, while the other three are used as food. As new groups
of similar cells continue to proceed from the germinal layer,
there is a succession in the ovary at regular intervals of one
egg- cell with three food-cells. Afterwards egg-cells and food-
cells surround themselves with a yolk-membrane. With that
the ' summer egg ' is ready and may be voided into the brood-
cavity. In the ' winter eggs/ which are very rich in yolk, the
ovum is not satisfied with the ordinary food material, and thus
we observe that as many as twelve groups, that is, forty-eight
cells, are used for its construction.

But the daphnids possess yet another arrangement for the
nutrition of their ' summer eggs.' The epithelial cover of the
ovary-tube consists in the adult female Daphnia, whose ovary
is filled with germ-cells, of cells so flat that they have been
overlooked by investigators until quite recent days. But in
the new-born females these epithelial cells are expanded so
much that they fill almost the entire lumen of the tube, and
make the whole ovary look like a solid chain of bladder-cells.
If now the young cells begin to advance from the germinal
layer they impress themselves deeply into these epithelial
vesicles which under the influence of pressure give the fluid
which they contain to the egg-cell and thereby resume their

REPRODUCTION AND HEREDITY 267

insignificant normal size. There is no doubt that we observe
here a process of nutrition in which the epithelial cells only act,
as it were, as intermediaries which receive food-material from
the maternal body, store it within themselves, and transmit it,
as required, to the advancing egg-cells for their support during
development. When the mature eggs have left the ovary the
epithelial cells once more suck themselves full with the nutrient
fluid, fill once more the whole lumen, and once more give up
their contents to the next advancing group of eggs. It is sig-
nificant that this feeding process is only observed during the
production of the ' summer eggs,' but is entirely at rest when
the ' winter eggs ' are formed. This makes it also clear why
' winter eggs ' require such a large number of egg-groups for their
development, and why their formation is only undertaken in
case of necessity.

Such succession of sexual and parthenogenetic generations
is described as heterogony, to distinguish it from the typical
alternation of generation, the metagenesis. In numerous cases
heterogony is connected with a very prominent polymorphism ;
indeed, the members of the various generations are often so
divergently constructed that only after closely observing their
cycle of development it becomes possible to recognize them as
belonging to one species.

Everyone has seen on oak-leaves the pretty red-cheeked gall-
apples, but less known are the little gall-flies whose sting produces
these growths. These gall-flies form a very numerous family
(Cynipida), those that live on the oak numbering alone about
a hundred species, each of which produces a different gall.
From the root to the fruit there is hardly a part which is not
exposed to the stings of the flies and cannot be compelled to
form such diseased growths. Though the gall-flies have a
special predilection for oaks, they do not on that ground despise
other plants, but attack the maple, fig-tree, wild rose, blackberry,
and many herbs. The bedeguar of wild roses, which is caused by
the sting of Ehodites rosce, and often assumes the size of
a fist, is known everywhere. But above all the Aleppo-galls of
the gall-flies of Asia Minor have attained a certain celebrity,
because they were formerly much employed in the prepar-
ation of very good ink. If we cut an oak-apple with a knife

268 LECTURES ON BIOLOGY

the blade becomes black, for chemical union has taken place
between the iron and the tannic acid of the gall, the result being
ferrous gallate or, in other words, ink. Apart from gall-flies,
several beetles, leaf-wasps, plant-lice, and gall-midges also
produce galls.

Galls are nothing else but the brood-chambers in which the
ova pass their development. Thus 'two birds are killed with
one stone ' : the growing larvae enjoy a perfect protection against
most animals, and have at the same time in the juices of their
host-plant a rich source of food. Sometimes each gall contains
only one larva, sometimes several.

One of the most interesting kinds is the little oak gall-fly,
JBiorhiza renalis, whose strange life-cycle was first discovered by
Adler. Before the beginning of spring, the first generation of
small wingless creatures leave their brood-chambers. They differ
so much in appearance from the gall-flies that it is hard to believe
they belong to the species. All individuals are females. As
soon as they enter the outer world they set about their most
important and indeed only task, egg-laying. The oak-trees, like
all nature, are still in their winter sleep ; young and tender
leaves are, therefore, not available for the brood of these wasps,
and they must undertake the laborious task of sinking their eggs
into the firmly closed winter buds. For this purpose the females
are splendidly equipped, their strong, dagger-like ovipositor ap-
pearing expressly created for perforating the firm cover-leaves.
Nevertheless it requires many hours before the wasp has pene-
trated to the interior of the bud, bored numerous fine tubes
and deposited into each one egg. In the course of the spring
these buds develop into large galls containing numerous larvae.
In July the second generation emerges, this time males and
females, typical gall-flies, distinguished, apart from their wings,
by their larger size, longer legs, and different formation of their
ovipositor. After fertilization the females lay their eggs singly
into the parenchyma of the under-side of the young oak-leaves,
and around each leaf is formed a small, longish gall from which
the parthenogenetic generation of females is hatched before, the
coming of the next spring.

How ingeniously has Nature cared here again for the pre-
servation of the species? In the inclement season a maleless

KEPKODUCTION AND HEREDITY 269

generation has more hope of surviving, and the task of egg-laying
can be much more rapidly discharged when there is no need for
the two sexes to seek each other. But rapidity is in this case an
inestimable advantage, as only a few bright days are available,
and the coming of the frost soon terminates their life.

Plant-lice, too, have two generations. In spring and summer
we find parthenogenetic females, usually furnished with two pairs
of wings. They are viviparous, and their young again develop
into females. This is repeated for several generations, until
finally ir> autumn the winged males and the wingless generations
of fertilizable females appear ; these lay eggs which hibernate
and produce in the next year a viviparous generation of females.

Still more complicated is the reproductive process in the
notorious Phylloxera vastatdx, the destroyer of vineyards. Its
economic importance alone justifies its mention.

Until the beginning of the 'sixties Phylloxera was unknown
in Europe. It was imported from America and became only too
quickly acclimatized. The devastations caused by these tiny
animals are enormous. In the course of eight years they destroyed
in France alone 75,000 hectares of vineyards and caused a loss to
the country of about £300,000,000, costing France more than
the unfortunate war of 1871. In Germany, too, the damage done
by Phylloxera in the districts of the Khine and other vineyards
is very considerable. For years the Government has bravely
endeavoured to proceed against this pernicious pest with all the
means provided by science, but has not so far succeeded in
exterminating it, though it has been found possible to prevent
Phylloxera from spreading further afield.

But more interesting than its economic importance is to us
its mode of life. From the winter eggs that have been laid
under the bark, small wingless insects emerge in spring, which
ascend the stem and deposit on the leaves numerous heaps of
unfertilized eggs. The eggs of this first ' nurse '-generation
develop again into wingless females, and as this process is in
the course of the summer repeated about seven or eight times
we can imagine how enormous the multiplication of this terrible
plague must be. They pierce the vines with their sharp pro-
boscis and suck the sap. Beginning with August, there appear
in addition to these females other more slender and winged females

270 LECTUEES ON BIOLOGY

which fly from vine to vine, laying eggs here and there, partly
large, partly small.

This generation, too, reproduces parthenogenetically, but
from their eggs the sex-animals are developed, from the large
eggs, the females; from the small, the males. They are most
remarkable creatures, and can only be properly described as
walking sex-organs. They have not only no wings, but are also
without proboscis and intestinal canal. They are 'ephemeral,'
unable to feed themselves, and their only object appears to be
to alternate the succession of asexual generations by fertilization.
Each female deposits under the bark only one egg and with that
its life-task and its life is ended. These are the ' winter eggs '
from which the first rays of the spring sun call forth the young
parthenogenetic generation of females.

It is only in a few cases where it may be so clearly observed
that fertilization and reproduction, though closely related, are
yet processes of a different kind. In Phylloxera fertiliza-
tion does not lead to multiplication, for as each female produces
only one fertilizable egg, the appearance of a sex-generation
reduces the number of individuals in fact by half. The im-
portance of fertilization does not consist in reproduction, as
was formerly believed, but in bringing about amphimixis — a
mixing of the qualities. Development can, therefore, take
place without previous fertilization, and, on the other hand,
fertilization need not necessarily cause development or repro-
duction. But fertilization is of the utmost importance to organ-
isms, as it imparts to them a far greater adaptability, and, on
the other hand, suppresses, or at any rate hinders, an injurious
one-sided tendency to variation. Arrangements have therefore
subsequently arisen which as a rule prevent the independent
development of the germ-cells. Thus the inability of sperm and
ovum to proceed separately to development is an adaptation
to the demands of life.

Heterogony proceeds largely in a like manner in the parasitic
Trematodes of which we have heard before. As may be
remembered, in the larval forms of the Liver Fluke sporocysts
and rediae produce asexually numerous larvae which are all able
to develop into sexual mature animals. This process is not to be
conceived as interior gemmation, as was formerly believed, but

REPRODUCTION AND HEREDITY 271

the larvae develop from unfertilized ova which originate in
the interior of the sporocyst. As the ova are not generated
by an adult form but by the undeveloped larval form, this
process is described as psedogenesis, as distinguished from
parthenogenesis.

Before we enter upon the internal phenomena of sexual
reproduction, and in particular the structure and genesis of
germ-cells, let us cast a brief glance at one of the most attrac-
tive and beautiful parts of zoology, the care of the parents
for their offspring.

In the lowest classes of Metazoa, the sponges and jelly-fishes,
a proper care of offspring does not exist. As the majority of
these forms are fixed to the spot but lead in their earlier stages
a free-swimming life, the parents are for this reason alone pre-
vented from watching over the well-being of their offspring.
But in sponges and most jelly-fishes the ova pass at least the
first stages of their development in the maternal body, and leave
it only as independent larvae when they are nearly capable to
struggle through life without assistance.

Most unfavourable among all animals is doubtless the condi-
tion of the brood of many intestinal parasites whose progenitors
remain behind in the interior of their host while the descendants
are ejected with the faeces as ova, and handed over defenceless to
all the perils of a protracted development. Countless individuals
meet with an early death, and the preservation of the species
is only secured by the immense fertility of these parasites.

Many other higher animals, in particular numerous insects*
are prevented by their death, which often follows upon egg-
laying, to watch over the fate of their young, but they endeavour
to compensate them by depositing the germ-cells in safe places
or surrounding them with protective covers, and at the same time
providing that the young on being hatched shall find sufficient food.
We saw a practical illustration of this kind in the gall-flies
which deposit their eggs in the interior of plant-parts, grant-
ing at once protection and abundant food. Other insects seek
out certain animals whom they compel to shelter and feed their
brood. Such a case is shown in the illustration. We see here
a caterpillar whose entire surface is covered with cocoons of an
ichneumon-fly (fig. 63).

272

LECTURES ON BIOLOGY

FIG. 63. — CATERPILLAR COVERED WITH COCOONS OF

Microgaster glomeratus.

The females of the Ichneumonidae lay their eggs into the
eggs, larvae, pupae, and adults of other insects, but by prefer-
ence into caterpillars of different butterflies ; from the eggs

emerge the grub-
like larvae without
feet and anus,
which forthwith
commence to eat
up their unwilling
host, first devour-
ing those parts of
the body which
are not of vital
importance, the
large accumulations of fat made by the caterpillar to serve for
its own support during the long pupa rest-stage. Owing
to this cunning procedure the host remains apparently quite
healthy and may even proceed to the pupa stage, but then its
fate is sealed, for the larvae, having now devoured the food in
the reserve store, mercilessly attack the vital organs. When
all the food has been eaten up and the caterpillar is dead, the
development of the parasite is usually complete. The method
adopted by the ichneumon-fly in depositing her eggs shows a
remarkably fine instinct, for large caterpillars receive a consider-
able share of eggs, while only a few are deposited into cater-
pillars whose small size does mot guarantee a sufficiency of food.

Year after year the Ichneumonidae
take immense toll in the insect world,
greatly to the advantage of man, for
without their help the insect plague
would soon become overwhelming.

Many wasps place their brood-
cells in the soil or decaying wood,
but before proceeding to egg-laying
they fill the cavity with numerous
caterpillars, insects; and spiders,
which they paralyze by stinging
them (fig. 64). Though alive, these unfortunate animals are
unable to escape «and are later eaten up literally alive. A

FIG. 64. — Ammophila hirsnta

PARALYSING A CATERPILLAR.

REPRODUCTION AND HEREDITY

273

similar phenomenon is observed in the mole, in whose run,
especially in winter time, large heaps of living earth-worms
are found which the mole has paralyzed by a bite in the cephalic
lappet. This method secures the victim as safely as if it had
been killed, and has the additional advantage that the food-
supply remains always fresh.

Numerous weevils make really touching provision for their
offspring. The so-called leaf-rollers cut with their proboscis
deep into the leaves of the host-plant and by means of their legs
skilfully roll the section up. Having hidden a few eggs in this
roll, they fold the open ends down and firmly seal them with
a viscid fluid secreted through the anus. Untiringly the abdomen
moves up and down until the minutest hole has been closed.
The inner parts of the leaf afterwards serve as food for the young
larvae (fig. 65).

Still greater pre-
cautions are taken
for the safety of its
offspring by the nut-
weevil. In default
of an ovipositor it
bores with its long,
thin proboscis a little
circular hole into the
soft nutshell and
pushes, also with its
proboscis, into each
nut one egg. By the
beginning of October
the larva has mostly
consumed the kernel

and now cuts with its sharp mandibles a way through the nut-
shell, which in the meantime has become hard, in order to pass
its development deep down in the earth.

Equally remarkable is the behaviour of the so-called grave-
diggers which are not content, like numerous other carrion-
beetles, simply to deposit their eggs on the carcases, but first bury
the corpse by joint labour. Most celebrated is doubtless the
Sacred Pill-beetle (Sisiphus sacer), which was in ancient times
18

FIG. 65.— (1) Rhynchites populi, WITH LEAF-BOLL.
(2) LEAF-ROLLS OF Rhynchites betulce.

274 LECTUKES ON BIOLOGY

worshipped as the emblem of a world-creating Divinity. Even
to-day in many of the Southern countries a scarabseus cut in
precious stone and worn on the breast is regarded as an unfailing
remedy against the sterility of women. These beetles are, like
their near relative the horse-dung beetle, very partial to this
useful but not very appetizing material. By joint labour male
and female shape the dung into large balls which they drag
and roll to some hole. Here the ball is pulled to pieces and
carefully cleared of all parasites ; the female then lays her eggs
and the ball is reformed.

In many species of the Libellulidse both sexes participate in
egg-laying and the care for the further development of the eggs.
According to Tiimpel, in Libellula cancellata the male seizes
the fertilized female, drags her to the nearest water, and compels
her, by rapidly moving his abdomen up and down towards the
water, to make also with her abdomen whipping movements, as
the result of which the gelatine-covered eggs fall into the water.
In another species of Libellulida, Lestes sponsa, the care of the
parent goes still further. After fertilization both sexes united fly
to a rush; to this the female clings, pushing by means of her
ovipostor one egg after the other under the surface of the stem.
During this process they descend slowly downwards to the
bottom of the pond, a thin layer of air, which covers the body
of the thoughtful parents, protecting them against drowning.

Many Ephemeridse proceed in a similar manner, in particular
Beatis gemellus, and other related species, but here the perilous
dive is undertaken by the female alone. As soon as the
period of egg-laying approaches she compresses her wings and
surrounded by an air-bubble as by a diving-bell, she descends
to the bottom of the pond where she carefully conceals the eggs
under stones. As soon as the work is done she attempts to rise
once more to the surface, but only too frequently strength fails
and an early death is the reward of her sacrifice and courage.

Numerous other insects, as for instance most butterflies, are
content to deposit their eggs in the neighbourhood of a suitable
food-supply. They are guided by an unerring instinct, so that
even when during the adult stage they have passed to a widely
divergent mode of life and acquired other needs they yet never
make a mistake. This astonishing instinct is one of the most

REPRODUCTION AND HEREDITY 275

mysterious phenomena in the life of animals. To us it is simply
incomprehensible why many mud-flies which themselves feed on
honey but deposit their eggs in mud or even in manure-pits are
able to know that the conditions of life which are necessary and
pleasant for them are entirely unsuitable
for their offspring (fig. 66). That the
animals during the egg-laying process are
reminded of their own youth we are not
justified in assuming, for it would mean
giving too much credit to their intelli-
gence and to their memory. Phrases
such as ' innate impulse,' ' habit bred by
selection/ explain nothing and merely
serve as a cloak for ignorance. But the
case quoted here is only one out of

innumerable instances of this mysterious FIG. 66.— MUD-FLY (Eris-

, tails tenax) WITH ITS LARV^:

pbenomenon. WHICH LIVE IN MANURE-PITS.

If there existed a degree in the incom-
prehensible I might quote as still less comprehensible the fact
that in preparing its cocoon a caterpillar does not make it to fit its
own size and form, but to fit the form of the emerging pupa, of
which it is impossible for the caterpillar to know anything from
experience. Similar unsolved questions are raised by the ingenious
and artistic construction of many of the nests of birds, and their
migration. How do the young birds find their long way to their
southern winter-quarters ? Who informs them that the autumnal
storms in the land of their birth announce a period of short rations
and famine ? Formerly it was believed that the inexperienced
young simply followed the parents. That may be true for many
species, but it has been demonstrated in others that the young
commence their flight before the old. We must, however, not lose
ourselves in such theoretical questions. All that I intended was
to show how many problems are still awaiting their solution, and
how unfounded is the fear that the rapid progress of science will
leave to future generations nothing to investigate.

Species with a longer span of life are still better able to care
for their offspring. Among the most careless parents are most of
the Echinoderms which liberate their germ-cells into the sea,
where afterward impregnation and development take place.

276 LECTUKES ON BIOLOGY

But even among these lazy animals we find a great number
which for a considerable time carry their brood with them.
Some of the starfishes protect their growing young under their
bent spines. In the Ophiuroidea the eggs pass a large part of
their development in the bursa, a thin-walled air-sac ; several
species of starfishes form a kind of breast-pocket by bending
back their arms, and in many Holothurians the young develop
either in the body-cavity or under calcareous spines which
have been specially formed for that purpose on the dorsal
surface.

Most highly developed among all vertebrates is the care of the
offspring among the social insects, in particular, ants, bees, and
termites. It is remarkable that among these animals the nursing
is not done by the parents but by a specific asexual worker
caste, or more accurately, a caste in which the sex-organs have
become degenerate. These facts, however, are so universally
known that I need not deal with them in detail. The familiar
spider, which is such a notoriously bad wife that after impregna-
tion she kills and eats the male without ceremony, exhibits a
touching care for her eggs : there are, in fact, only very few
classes of animals in which the care of the offspring is regarded
as such an important function as among spiders. Many spiders
construct for their eggs ingeniously spun bags which they attach
to leaves and branches of different plants. These webs are often
so constructed that they closely resemble in shape and appear-
ance the fruit of the plants to which they have been fixed, a fact
which enables them, of course, more easily to escape their natural
enemies. Among other species of spiders the females are not
satisfied with this passive care, but fasten the egg-sac to their
abdomen and continually carry it about with them.

Crabs exhibit a similar care for their young. Most people
have probably seen a female crayfish, at any rate when boiled,
firmly grasping with its little abdominal feet a large number of
minute round eggs.

Fishes are regarded by most men as unintelligent and stupid,
but when we see the truly touching and devoted care which
many species bestow upon their offspring, and if in particular we
observe the common stickleback (Gasterosteus aculeatus) during
spawning time, we shall not be inclined to agree with this

REPRODUCTION AND HEREDITY

277

opinion. In April or May the male stickleback builds out of
fibres a little tube-like nest, sometimes supported by, and hidden
between, the stems of water plants, but more frequently, for the
sake of greater security, buried deeply in the sand at the bottom
so that only one opening is visible. Marital faithfulness exists
neither in male nor female ; numerous females are fertilized and
compelled to lay a few eggs, until at last several hundred eggs

FIG. 67. — Geophagus brachyurus. MALE WITH YOUNG.

have been collected. While the work of the female ceases with
the egg-laying, for the male begins now a period of much
sacrifice and hard work. Untiringly he keeps guard over the
safety of the nest and the well-being of the growing young.
Frequently the male stands for hours over the entrance to the
nest, and by gently moving his pectoral fins fans towards the
young a continuous stream of fresh water rich in oxygen. The
courage of this little fearless fish is astounding, and even larger

278 LECTURES ON BIOLOGY

animals will frequently withdraw before its fierce onslaught and
the threateningly erected spines. The same care which the
stickleback bestows upon the nest and the eggs he gives after-
wards to the young brood. Only when they are no longer in
need of his protection his interest decreases, and the grown-up
sticklebacks will act wisely in not venturing on their father's
' beat/ for he will now attack them as fiercely as other fishes.

The care for its offspring of another fish, Geophagus
brachyurus, is illustrated in fig. 67. Here, again, it is the male
who leads his young, after having previously watched over their
incubation. Like a hen with her chicks, he follows the little
company devotedly about, but if one of the young should

FIG. 68. — Hippocampus antiquorum. MALE WITH BREAST-POCKET.

venture too far in purposeless daring the male seizes it with the
mouth and — often not very gently — flings it back among the rest
of the swarms.

It is remarkable that among fishes, as well as among
numerous amphibians, the male, as a rule, attends to the care
of the offspring, while the female takes no interest of any kind.
Only in very rare cases are females observed to care for their

KEPKODUCTION AND HEEEDITY 279

offspring after egg-laying. Such notable exception is that of
Aspredo lavis, a species of shad. Here the female glues the
egg to her body.

The female of Solenostoma, a fish belonging to the small
group of the Lophobranchii, carries the egg in a bag which has
been specially formed by the ventral skin and fins. In other
Lophobranchii, however (the grotesque Hippocampus antiquorum
and the Syngnathus acus) in defiance of all rules it is the male
who becomes ' pregnant, ' for he possesses on his ventral
surface a roomy sac in which the eggs pass their embryonic
development (fig. 68). With the growing of the young brood
this breast-pocket enlarges enormously, and thus gives the
strange appearance of ' pregnancy.' Also against all rules of the
game, it is the female which puts on the ' nuptial dress.'

Some other fishes which do not possess a special arrangement
for the care of the young carry the eggs in the mouth-cavity.
Thus it is reported of an inhabitant of Lake Tiberias, Chromis
paterfamilias, that the males at times carry up to two hundred
embryos in their mouth.

The life-habits of the Brazilian frog, Pipa americana, remind
us of the behaviour of Aspredo which I have already mentioned.
In these animals both sexes share in the care of the offspring,
the larger and heavier share being taken by the female. The
male places with its anterior legs the impregnated eggs upon
the back of the female. The stimulus exerted by the eggs upon
the skin causes peculiar growths, and around these are formed
cells, like those of a honeycomb, in which later the young frogs
are found (fig. 69).

The female of Nototrema, a frog which inhabits tropical
America, also carries the fertilized eggs in a pocket on her back.
If I further mention that the male Alytes obstetricrans winds the
long egg-chain round his posterior legs, and then lives for some time
in concealment on the land, finally to bring the young frogs back
into the water when they are ready to emerge, I hope to have
given a sufficient number of instances to show the important part
which is played by parental care and devotion even among the
lower animals.

How closely the union becomes between the young of birds
and mammals and their parents requires no particular mention.

280 LECTUEES ON BIOLOGY

Both males and females share with extraordinary devotion in
the task of hatching and obtaining food, and it seems to us
quite unnatural that the cuckoo, which is a very faithless wife,
giving herself to any male, leaves the hatching of her eggs to
other smaller birds. More frequently it is the males who do not
trouble about their young or are even hostile to them, so that the
female is barely able to protect the young against the attacks of
their brutal sire.

FIG. 69.— BRAZILIAN FROG, Pipa americana, WITH YOUNG.

The female rhinoceros-bird, to mention only one instance,
exhibits a particularly touching devotion to her young. It
almost seems as if these birds do not trust to their own
patience and endurance, and in order to prevent themselves from
leaving their eggs they actually close up the nest-hole in the tree
with their faeces, letting only a small aperture remain through
which the female receives food from the male.

By way of contrast, other birds know how to make hatching
an easy task. The Australian Talegalla (Catheturus lathame)
and some related species gather heaps of leaves and decaying

REPRODUCTION AND HEREDITY 281

plant matter, and hatch their eggs by the heat produced in the
heaps by fermentation. In order to regulate the temperature in
the interior of the heap they actually build several ventilating
shafts. Other birds even understand how to make use of the
volcanic activity of their own countries, and bury their eggs in
the warm ashes of volcanoes.

The highest stage of perfection in the care of the offspring is
reached by the mammals, among whom the parental care forms
such striking characteristics that the whole group has derived
from it its name. In most mammals the young pass their
entire embryonic development in the interior of the maternal
body, and are fed immediately from the blood-circulation of the
mother. They are born developed but helpless creatures, and
must be suckled for a considerable time with the secretions from
the maternal milk-glands until they are able to obtain their food
independently. Only the Monotremata of Australia lay eggs like
reptiles and birds. These eggs are either hatched in the nest, as
in the case of Ornithorhynchus paradoxus, or in a pouch on the
ventral surface of the mother, as in the case of Echidna aculeata.

In the marsupials the young are born in a very incomplete,
helpless state. The Giant Kangaroo, for instance, bears barely
forty days, and produces a naked, blind embryo, the size of a
walnut, without extremities. Carefully the mother seizes it with
the lips, and places it into the pouch, where it at once commences
to suckle. After about seven months the embryonic development
is complete, and for the first time the young looks shyly out into
the world. Soon it leaves the pouch, first for a short time, but
gradually for longer periods. But even fairly large young
kangaroos dive, at the least sign of danger, with a bold jump into
the maternal pouch until they are finally forcibly prevented in the
interests of a new generation.

I could mention many more attractive features of the life-
habits of other mammals, but time forbids, and I will, there-
fore, now turn to the formation of germ-cells.

The problem of fertilization and reproduction has occupied
the human mind for many ages. Until the discovery of the
microscope this was therefore a region which the mind in vain
endeavoured to invade. All that was possible was tentatively to
build up hypotheses. As the act of impregnation itself was

282 LECTURES ON BIOLOGY

hidden to the eye, it can be only too easily understood that
earlier investigators regarded the act of impregnation as the life-
creating element. Nevertheless, Aristotle was already on the
track of truth when he assumed that the mother supplies the
formative matter for the new creature, but that the father
causes the development.

Owing to the difference in the size, animal ova were known
to investigators long before spermatozoa were discovered. The
scientists of the seventeenth and eighteenth centuries believed
almost universally that each ovum contained the developed
animal with all its different parts and organs, as it were, en
miniature, and that impregnation stimulated this animalcule
into growth and 'metamorphosis.' The celebrated Italian
physiologist and physicist Spallanzani thought, on the ground
of his investigations concerning the reproduction and develop-
ment of the frog, that as the frog developed by metamorphosis
from the tadpole, and this again directly from the ovum, the
egg itself must already contain a minute frog. Moreover, as the
fertilized egg is in appearance exactly like the unfertilized egg
which still adheres to the maternal ovary, he thought that the
embryos of frogs must be in existence in the maternal body
long before impregnation had taken place. In pursuing this
idea, therefore, investigators arrived at the conclusion that these
germs again contained the primary constituents (Anlagen) of
subsequent generations, so that according to them in the case of
man the ovary of Eve contained all the human beings that ever
lived and ever will be born.

Difficulties arose for the supporters of this doctrine of ' pre-
formation,' when a disciple of the great Dutch physiologist,
Leeuwenhoek succeeded, in 1667, in discovering in the sperm
of a man suffering from gonorrhoea minute motile formations,
resembling microscopic tadpoles. At first these ' semen- worms'
were regarded as parasitic organisms, living in the seminal fluid
and being of no particular importance. But very soon Leeuwen-
hoek himself succeeded in demonstrating the presence of exactly
similar ' worms ' in the semen of many other animals, mammals,
amphibians, birds, fishes, molluscs, and so on. Finally, continued
investigations led him at last to the conviction that these sper-
matozoa were not the ova but the actual germs which merely

FIG. 70. — SPERMATOZOA OF DIFFERENT ANIMALS.

(1) Man. (2) Eaja clavata. (3) Torpedo marmorata. (4) MILS decumanus. (5)
Perca fluviatilis. (6) Molge marmorata. (la) Vermiform; (76) piliform spermato-
zoon of Paludina vivipara. (8-11) Various crustaceans. (8) Latona seiifera. (9)
Moina rectirostris. (10) Homants vulgaris. (11) Polyphemus pediculus. (12)
Ascaris megalocepliala.

REPRODUCTION AND HEREDITY 283

found in the maternal body the most suitable conditions for
their further development. This hypothesis reduced the ovum
to the level of mere food-material, at the expense of which the
spermatozoa live and grow. Supporters of Leeuwenhoek's
doctrine even went so far as to contend that they had under the
microscope actually seen a minute ' homunculus ' in the sper-
matozoon, and observed its development. Though this fantastic
contention soon met with the liveliest contradiction, the real
nature of the spermatozoon remained completely unknown until
the nineteenth century ; only with the establishment of the cell-
theory and with the proof that ovum and spermatozoon are
simple cells, and that fertilization consists in the union of the
male with the female cell, the veil began to lift from this
mysterious phenomenon.

We have already become acquainted with the structure of
different animal- ova and seen how their cell-form becomes quite
distinct. Indeed, the eggs of sponges, and of some jelly-fishes
bear in their appearance a striking resemblance to simple
Amoebae, and have by their protoplasmic pseudopodia retained a
certain independence of motion (compare fig. 31, 3). In those
cases, however, in which the cell-character of the ova appears
hidden the differences lie either in a large accumulation of food-
material for the growing embryo, or in other secondary protective
arrangements demanded by the conditions of their later fate.

But quite differently formed are the spermatozoa (fig. 70).
No one would take these thread-like motile formations to be
simple cells. However, we are here aided by evolution, which
shows how these thin threads are transformed, step by step,
from normal cells. We need not follow this process of develop-
ment in all its highly complicated phases, but it will suffice for our
purpose to remember that the head of the spermatozoon corre-
sponds to the cell-nucleus, thejmiddle part to the centrosome, and
the long, thin tail essentially to the protoplasm.

By far the largest part of the cell-plasm is during the develop-
ment of the spermatozoon rejected as useless ballast. The sper-
mazoon acquires thereby greater motility and is at the same time
prevented from developing independently before impregnation.
This is one of the methods of Nature to enforce amphimixis.

Why do ova not divide, though they are liberally supplied

284 LECTURES ON BIOLOGY

with protoplasm? What prevents them from producing an embryo
independently ? When we were dealing with cell-reproduction
we saw that the centrosome underwent important transforma-
tions, that it appeared to control all processes, and that most
investigators saw in the centrosome the real organ of cell-division.
In the maturing of the egg-cells this small but important
form becomes apparently degenerate and, in this circumstance,
according to Boveri, whose theory is gradually gaining universal
recognition, is to be sought the cause of the inability of the ova to
divide. For in the act of impregnation the spermatozoon brings
into the egg-plasm a new organ of division, a new centrosome,
and with that the impediment which hitherto stood in the way of
division is removed and development can begin. It must, how-
ever, be remembered that this is only an hypothesis with which it
is very difficult to reconcile many observations ; still it is the best
attempt so far made at explaining this phenomenon.

In some of the lower crustaceans, in which copulation leads
with a high degree of certainty to fertilization, and spermatozoa
need only be produced in small numbers, these have retained
their typical cell-form. The extraordinary, even unique import-
ance of the germ-cells to the preservation of the organic world
makes us understand that Nature has made the most compre-
hensive preparations to secure their origination and growth.

The most primitive method of germ-cells is found in the
sponges, in which ova and spermatozoa, being distributed through-
out the entire body, may originate in the mesodermic tissue. In
this case we speak of a diffuse formation of germ-cells. But even
in these primitive forms we find special preparations made for
the better nutrition of the sex-cells, for each ovum is during its
period of growth closely surrounded by a wreath of other cells
whose one purpose appears to be to feed their protege and guard
it against external influences. In similar manner the young
semen-forming cells are cared for in all animals.

In most of the higher classes of animals, and from the vermes
upwards, in all classes, the formation of germ-cells goes on in
specific sexual glands, the testes and ovaries. We cannot consider
here all the numerous preparations that have been made for the
care of the germ-cells, but the final results are in all of them, the
same : the formation of healthy, vigorous germ-cells.

REPRODUCTION AND HEREDITY 285

Two fundamentally distinct methods may be discerned in
their nutrition : either the food-cells themselves serve as food and
are simply eaten up by the growing sex-cells, or they draw a
continual stream of food-material from the maternal body and
pass it on to the ova and spermatozoa. They act therefore
as intermediaries.

That the food-cells, as a rule, are nothing but degenerate
germ-cells we have already heard on a previous occasion, where
I also referred to the silent but fierce struggle which takes place
in the sexual glands, as the result of which competition only the
best and strongest germ-cells are able to develop. But even the
complete germ-cells are sometimes subjected to a severe test of
their fitness.

To give only one instance, the spermatozoa of the higher
animals possess the peculiar habit of taking up their position in
the fluid with the head opposite to the direction of the current.
If we now gradually increase the strength of the current we see
under the microscope that numerous spermatozoa become ex-
hausted and are carried away by the stream, but that the
strongest pluckily work their way up stream. Though this
seems insignificant, it has a very important meaning. In the
female uterus and oviduct there is a slight current to the outside,
which indicates the direction to the invading spermatozoa and
must be overcome by them before they can penetrate to the
ovum and complete the union. In this simple manner once
more selection takes place, by which weakly elements are generally
eliminated.

In order that the germ-cells may fulfil the purpose of their
existence it is above everything necessary that there should be
suitable preparations for bringing ova and spermatozoa together.
The first condition is a permanent, or at any rate periodical,
coming .together of the two sexes. Most favourably situated
appear to be in this regard the hermaphrodites, those animals
in which both kinds of germ-cells originate in one and the same
individual. Especially in the lowest classes of multicellular
animals, hermaphroditism is still fairly general, and in the
numerous species which are either fixed or slow-moving it
becomes the general rule. We know hermaphrodites among
sponges, jelly-fishes, vermes, echinoderms, molluscs, crustaceans,

286 LECTUEES ON BIOLOGY

and tunicates ; and even among the highly developed vertebrates
cases of hermaphroditism exist in the Myxine glutinosa and
Serranus scriba.

We are therefore compelled to assume that the hermaphroditic
development of animals represents the original stage from which
in the course of their phylogenesis a separation of the sexes
was gradually evolved. This assumption is supported in parti-
cular by the fact that even in vertebrates and insects, in which
normally the sexes are always separate, hermaphroditism occurs
occasionally abnormally. As these animals frequently possess
a prominent sexual dimorphism, it sometimes happens that
striking malformations appear. Thus we know of ants, bees,
butterflies, beetles, &c., in which the right body-half is purely
male and the left purely female, or vice versa. In the Berlin
Natural History Museum is preserved the skin of a bull-finch
in which the dividing line of the body is strictly vertical, so
that one breast half exhibits a bright red male plumage while
the other half shows the insignificant grey of the female. As
unfortunately only the skin was handed in, it was impossible
to examine the inner organization in this bird. This is all the
more regrettable as hermaphroditism frequently exists only
externally, the sexual glands of animals having in spite of it
retained their purely female or purely male character.

In some cases it seems clear that the bi-sexual forms have
acquired their hermaphroditism only secondarily, and that separa-
tion of sexes had already taken place in their ancestors. The
cause of this lies in a change of the life-habits, and rests either,
as in the fly Termitoxenia and numerous other animals, on
adaptation to a parasitic mode of life, or, as in the Cirripedia,
in the loss of free motility.

The degree of hermaphroditism frequently varies. As a rule
ovaries and testes are found in the same individual, but in
different places, possessing separate outlets for the sex-products.
Only in very few cases — as, for instance, in some vermes,
echinoderms and mussels, as well as in numerous snails — male
and female germ-cells originate generally in the same gland,
which is in that case described as the hermaphroditic gland.
But even in this extreme case Nature seeks to prevent self-
fertilization, permitting it only in cases of extreme necessity.

KEPKODUCTION AND HEREDITY 287

Frequently self-fertilization is made impossible by the fact that
both kinds of sex-cells mature at different times, so that these
animals either function first as males and then as females, or
first as females and then as males. The advantage of herma-
phroditism is, however, evident, in spite of the avoidance of
self-fertilization, especially as regards slow-moving animals, for
whenever two individuals meet it is always male and female that
meet. The great value placed by Nature on cross-fertilization
becomes quite clear when we find that even in many herma-
phrodites— as, for instance, the duck-mussels — there appear at
certain times dwarf adjunct males, in the event of mutual impreg-
nation of the hermaphrodites being for some reason or other
rendered impossible.

With the numerous other arrangements which aim at a
union of the sexes for the purposes of impregnation I can deal
here only in very brief outlines. Only in the higher vertebrates
do we observe the kind of union which may be described as a
marital association, but in the lower forms the union of the sexes
occupies only a short period.

Very remarkable conditions are observable in a species of
vermes, Bonellia viridis, found in the Mediterranean. The
female lives concealing its sac-like body among the rocks, only
the cephalic lappet, which is about 20 to 30 centimetres long,
being visible ; the minute male, which is only about 1 millimetre
long, and was only quite recently discovered, leads an unnoticed
existence in the sexual organs of the female. Similar arrange-
ments are found in some of the parasitic crustaceans.

In Diplozoon paradoxum, a trematode which is parasitic on
the gills of carps, both sexes are found grown together like the
celebrated Siamese twins. In another species, Schistosomum
(Bilharzia) hcematobium, a parasite of man dreaded in Southern
countries, the male by rolling up the lateral margins of its body
forms a partly closed tube, the canalis gyncecophorus, in which
it carries the more slender female through life. These cases are,
however, as much exceptions as the symbiosis of many bark-
beetles and land crabs, for generally the sexes separate
immediately after fertilization if, as in the majority of insects,
they do not die shortly after having fulfilled this all-important
task.

288 LECTURES ON BIOLOGY

In order to increase the sexual stimulus and make both parts
more eager for copulation, various arrangements have been made
which frequently condition a considerable difference in the
appearance of the two sexes. As a general rule the female
animals are the more passive and reserved part. In order to
overcome their objection, the males possess numerous means,
while conversely many characteristics of the female serve to
excite the sexual instinct of the male. Among these stimuli
are the scent- scales of the male butterflies, the musk-glands of
alligators, beavers, and civets, the anal glands of lizards and
snakes, and the strong broody odour of many mammals. Sounds,
too, play an important part in attracting the sexes and conquer-
ing female shyness. The autumnal forest reverberates with the
calling of the stag ; at the awakening of springs the songs of the
nightingale and finches sound from the bushes ; from the bare
tops of the firs the blackbird untiringly sounds its sibilant note ;
in the meadows locusts and crickets play their monotonous
tune ; and from the near pond comes in thousand voices the
concert of the frogs, interspersed from time to time with the
melancholic call of the toad.

When dealing with sexual selection, we saw that striking
colours and other ornaments may serve as sexual allurements.
Indeed, many males assume for the ' wedding ' a special festive
garb and perform in addition before the females strange dances,
as if they hoped to charm them either by their beauty, skill, or
strength. During copulation snails bombard one another with
' love-darts,' little daggers of carbonate of lime which are driven
into the male genital passage to increase the sexual desire. The
hook-like spicula at the posterior end of the male thread-worm
serves the same purpose.

But just as the instinct of copulation produces love and affec-
tion between two sexes, so it causes simultaneously hatred
against rivals, and many animals, from the gentle dove to the
truculent stag, fight in the pairing period fierce and sanguinary
battles which sometimes end only with the death or at least
expulsion of the rival. The male sex is consequently furnished
with special weapons which are absent in females. We remem-
ber at once the mighty mandibles of the stag-beetle, the spurs of
the cock and Ornithorhynchus — in this latter species a poison

REPRODUCTION AND HEREDITY '289

gland opens directly into the spur — the horns of deer, the antlers
of stags, manes of horses and lions, and tusks of boars. In
addition we find that the males are furnished with better or
even new sense-organs for seeking, and with other preparations
for seizing and holding the females.

The male Leptodora, a little transparent fresh- water crus-
tacean, which I have mentioned several times, possess greatly
prolonged antennae.

The males of numerous beetles, butterflies, etc., are
also often distinguished by their strongly developed and highly
differentiated antennae. In the male of the honey-bee almost the
entire head is occupied by the greatly enlarged, faceted eyes,
while the queen and the workers possess considerably smaller
eyes. This extraordinary development of the organs of sight is
absolutely necessary for male bees if they do not wish to lose
sight of the queen during the rapid nuptial flight. In some
male flies of the genus Cloe and Potamanthus there are found
special ' turban eyes ' which are absent in the females flies.
Finally, we have already seen that the faculty of producing light
is of great assistance among many animals in the finding of the
sexes.

Among the instruments for seizing and holding the females
I need only mention the gripping-antennae of several of the lower
crustaceans, the swelling on the ' thumb ' of the male frog, and
the spurs of the Ornithorhynchus.

Fertilization may either be external, daring which the germ-
cells are simply voided into the water, or internal, with preceding
copulation. The first form is widely found, particularly in the
jelly-fishes, echinoderms, numerous vermes, mussels, tunicates,
fishes, and, finally, many amphibians. Though it may sound im-
probable, interior fertilization can take place without preceding
copulation, while, on the other hand, in a few animals with
external fertilization there is also a kind of copulative act per-
formed.

While generally the male semen is secreted as a fluid, in
some animals so-called sperinatophores or ' sperm-cartridges ' are
formed. These are delicate capsules of many shapes and often
highly complex construction, which contain a large number of
mature spermatozoa. During pairing time we may often notice
a very attractive spectacle in many of the tritons. The male has
19

290 LECTURES ON BIOLOGY

then put on its most brilliant nuptial dress, and performs before its
mate the queerest contortions. Finally his efforts are rewarded
with success, for the sexual desire is aroused in the female. The
triton now deposits on the ground, before the eyes of his chosen
mate, several spermatophores. The female crawls upon them
and takes them up with her sexual organs, in which they open
and liberate the spermatozoa, when fertilization takes place.
We find a similar process in our native myriapods, the Scolo-
pendra. Here the male attaches his spermatophores with self-
spun threads to the earth ; then the female receives them in her
sexual organs.

Frogs unite for the purpose of fertilization. The male sits on
the back of the female, holding her with his fore-legs and wait-
ing until she discharges the ova, when he pours the sperm upon
them. Fertilization, therefore, is here external.

In animals which have no external sex-organs, copulation
and transmission of the semen is performed by pressing together
the sex-apertures. In such case it happens sometimes, as, for
instance, in the earth-worm, that both copulating animals become
firmly joined together by elastic bands, secreted by a glandular
organ, the clitellum. More frequently it happens that the male
sexual passage ends in a protruding or protrusible penis which,
during copulation, is introduced in the female vaginal orifice, and
thus transmits the male sperm-cells with much greater certainty.
Sometimes other parts of the body may be employed for this
purpose, when they become suitably transformed.

In the crayfish, for instance, it is the two most anterior
pairs of abdominal feet that serve as instruments of copulation.

In the male spiders, the palpus has as its point a pear-shaped,
sac-like process, furnished with a spiral thread, the ' palpal
organ.' Before the spider approaches his mate he fills this
sac from the germinal gland with sperm, and thrusts it at
the moment of copulation into the vaginal orifice. He must,
however, proceed most cautiously, for the female is in many
species considerably larger and stronger. With the strangest
contortions the male endeavours to approach the female, but only
too often it happens that she rushes upon him and kills him.
Even if he has successfully braved all dangers and reached the
goal of his desire, he must, after completed copulation, quickly

EEPRODUCTION AND HEKEDITY 291

run away, unless he wants to serve as a meal for the ravenous
female.

Among the cuttle-fishes, too, fertilization takes place by means
of spermatophores. These have such' a remarkable structure and,
owing to the swelling, perform in the water such strange move-
ments that they have been described as independent parasitic
worms. The transference of the spermatophores to the other
sex is performed by the ' hectocotylus,' one of the tentacles of the
male which has been specially transformed for this purpose. In
Argonauta this hectocotylus separates during copulation and
enters the mantle-cavity of the female where it continues to live
for some time. It was therefore formerly looked upon as the
rudimentary male of the Cephalopods.

In dealing with the interesting processes which take place
during the maturing of the sex-cells and their preparation for the
copulative act and copulation itself, we must recall what we have
heard about indirect nucleus-division or mitosis. We saw that
each individual body-cell of a certain animal-species always
contains the same number of chromosomes. All cells, for
instance, of Ascaris megalocephala bivalens, have only four chromo-
somes, all human body-cells sixteen. This normal number is
maintained in all descendants by the dividing mechanism which
works with mathematical precision. It will be remembered that
the chromosomes were regularly doubled before cell-division by
longitudinal fission, and that each of the two young daughter-
cells received half, that is, once more the normal number of
chromosomes. These behave in their reproduction almost like
independent organisms which reproduce by division.

The act of fertilization consists in the fusion of a male with
a female sex-cell. As the ovum as well as the spermatozoon are
typical complete cells, we should nearly expect the fertilized
ovum to contain double the normal number of chromosomes. But
if that were so, the animal originating from the fertilized egg by
cell- division would, in its turn, have twice as many chromosomes
in each body-cell as its progenitors. The act of copulation
would therefore plainly lead to an accumulation of the chromatic
elements which would be doubled in each succeeding generation.
But that must not be and is, indeed, contrary to all observations.
The cell-nuclei of sexually-begotten children and grandchildren
have always exactly as many chromosomes as the nuclei of their

•292

LECTURES ON BIOLOGY

ancestors. Hence it follows of necessity that at a certain time,
before the germ-cells unite in the copulative act, there must take
place a reduction of the chromosomes to half their normal num-
ber. Investigation shows, in fact, that the mature spermatozoa
and ova contain only half as many chromosomes as the body-
cells, and that only by copulation the normal number is once
more restored. If, for instance, an animal species has in its-
body- cells four chromosomes, their mature sex-cells contain only
two, and if the human body-cells contain sixteen, we can say
a priori that the mature spermatozoa and ova have only eight
chromosomes.

M

Sch

\

FIG. 71. — CHANGE OF A SPERMATID IN A SCUTTLE-FISH (Octopus Defilippi)
INTO A SPERMATOZOON.

(1) Spermatid. P, protoplasm ; N, nucleus ; C, centrosome ; S, sphere. (2) The
sphere has wandered to one nucleus-pole, the centrosome to the other. (3-8) Gradual
transformation. The sphere forms the pointed .'end Sp, the nucleus the
head K, the centrosome partly the neck M, and, together with part of the proto-
plasm the tail-thread Sch. the rest of the protoplasm is rejected and perishes.

How is the reduction of the chromosomes in the germ-cells
affected ?

In the testes of numerous animals may sometimes be distin-
guished several sharply denned segments in which the gradual
formation of the spermatozoa takes place. Usually in the pos-
terior section of the testicle lies the so-called germ-zone,
in which by division of the original germ-cells new cells are

BEPRODUCTION AND HEREDITY 293

continually formed, the so-called sperrnatogoriies (or spermato-
blasts). Adjoining the germ-zone is the zone of growth. In
this the spermatogonies develop and become spermatocytes of
the first order. These are, according to Yves Delage, the grand-
fathers of the spermatozoa proper, for from each spermatocyte
of the first order originate, by two successive divisions of four
cells each, the spermatids, which change into the mature sper-
matozoa. In the course of this transformation takes place the
reduction of the chromatosomes to half the normal number.

I shall describe only the most frequent case, omitting all
deviations from this typical process, and choose for our illustra-
tion Ascaris megalocephala (the thread- worm of the horse)
which, as a result of the experiments of 0. Hertwig and
A. Brauer, is one of the best-known organisms. Like the body-
cells of this nematode, so its spermatocytes of the first order
contain as yet each four chromosomes. Before they proceed
to division, a doubling of the chromosomes takes place and
each two of these double-chromosomes approach each other, as
if impelled by a mysterious force. In this manner are formed
two groups of four, each of which consists of two double-chromo-
somes. This mutual seeking and uniting of two chromosomes
which were separated during the entire previous life of the
organism recalls, as Heider justly says, the process of conju-
gation in unicellulars. Now the whole complicated dividing
apparatus of the cell comes again into activity, the centro-
proceeds the characteristic radiating figure), move to opposite
some divides, and the two resulting parts (from each of which
nucleus-poles. Now the nucleus-membrane disappears, and
before our eyes is formed the ' equatorial plane.' A longitudinal
fission of the chromosomes need no longer take place, as the
chromosomes have already been doubled.

The remaining course is normal. Two cells are formed, the
two spermatocytes of the second order, of which each has again
received four, that is, the normal number of chromosomes. But
while after each mitosis the nucleus is reconstructed and
returns to the rest stage, here the first cell-division is imme-
diately followed by a second. Again the four chromosomes
arrange themselves in each of the two spermatocytes of the
second order into a group of four, the centrosomes divide and
wander to the poles, and again we see the equatorial plane. But

294

LECTURES ON BIOLOGY

this time there is no longitudinal fission of the chromosomes ;
division proceeds forthwith, and into each of the cells forming
only two chromosomes are introduced. With that the maturing
of the germ-cells is complete, and each of the four spermatids
is transformed into a complete spermatozoon which has in its
head-nucleus only half the chromatin of the body-cells of the
nematode.

In much the same manner proceeds the maturing of the
egg-cells, only that here instead of four equivalent cells, one large
cell, the ovum proper, and four small cells are formed. The

FIG. 72. — DIAGRAM OF A MATURATION DIVISION.

(1) Spermatocyte of first order. 2-4 first maturation division ; (5) second
maturation division ; (6) the resulting four germ-forming cells, with the nuclear
loops reduced by one-half, and changed immediately into spermatozoa. (The
maternal and paternal nuclear loops are indicated by white and black squares
respectively.

latter are the so-called 'directive-bodies' which soon perish.
We begin again with the grandmother of the matured ovum, the
oocyte of the first order. Here, too, the chromosomes have
doubled before the division began, and arranged themselves in
two groups of four. Now the nucleus moves to the margin of
the cell, the dividing apparatus begins to act, and four chromo-
somes remain in the ovum, while a small quantity of protoplasm

EEPBODUCTION AND HEREDITY '295

becomes aggregated around the others. The result of the first
division is therefore again two cells, one large and one small,
but each with the normal number of four chromosomes. The
second division, which follows without an intervening rest-period,
effects again a reduction in chromatin, removing two of the
chromosomes, each of which is surrounded by a minute quantity
of protoplasm from the ovum. In the meantime, the first small
cell has once more divided, and with that there have again been
formed from the oocyte of the first order four cells, the mature
ovum, and the three ' directive-cells ' which soon perish without
having performed any particular role.

It may be asked, Why does Nature act in this case so un-
justly? Why do not four normal ova develop from the oocyte?
But the reason seems plain : we see here once more manifested
the great adaptive faculty of organic genesis. As the egg-cell
is to supply the formative matter for the growing embyro, is
it not above everything necessary that it is liberally provided
with food-material so that it shall be able to fulfil its task?
Thus it seems more advantageous that fewer ova are created and
that one cell enriches itself at the expense of the other three
sister-cells, than that more but weaker germs are produced.

When both kinds of sex-cells have been prepared as described,
only then can the process take place at which all these prepara-
tions aim, fertilization. It must have been a wonderful im-
pression which Oscar Hertwig received in 1875 when he, first of
all men, was able to observe with a microscope these mysterious
processes in the ova and spermatozoa of sea-urchins, and thus
open a path to a region which the human mind had during
thousands of years in vain endeavoured to invade. Even to-day,
when anyone with a good magnifying glass can easily observe
the spectacle, it never fails to fascinate us. We feel that we
stand here on the threshold of one of the profoundest riddles
of life.

The process of fertilization is exceeding!)*- simple. As soon
as we mix in a drop of sea-water a few mature ova of the sea-
urchin with spermatozoa, we see that immediately all the sperma-
tozoa rush towards the ova, each one endeavouring to reach
an ovum before the others. When a spermatozoon is close to an
egg-cell the egg-plasm raises a minute prominence into which

296 LECTUKES ON BIOLOGY

the spermatozoon penetrates, head forward. Almost at the same
moment the egg-plasm becomes rigid, forming a firm cover, the
yolk-membrane, which offers an insuperable obstacle to the
entrance of other spermatozoa. This is Nature's way of prevent-
ing the injurious consequences of over-fertilization, or polyspermy.

Let us now observe, with the aid of the illustrations, the
subsequent stages of the fertilization act in Ascaris megalo-
cephaia. As soon as the spermatozoon has entered the ovum it
turns round so that its central part points to the centre of the
ovum. Presently there appears in this central part, which, as
we know, corresponds to the centrosome, the familiar radiating
figure. Nearer and nearer approach the sperm-head and the
egg-nucleus. Consisting at first only of a compact mass of
chromatin, the head of the spermatozoon meanwhile becomes
enlarged by absorbing fluid, and changes into a typical resting
nucleus in which the chromatin lies distributed in the form of a
fine net. In the meantime the paternal centrosome has divided
and the two new centrosomes have taken up a position on the
two opposite points in the egg-plasm. Between them the
maternal and the paternal nucleus lie now close together.

Fertilization is now practically at an end, and the second act
of the drama commences, the division of the impregnated egg-
cell, or, in other words, its development. A direct fusion of egg-
nucleus and sperm-nucleus into the segmentation-nucleus can
take place in many cases, but there is no necessity. As a rule
we may assume that the processes take place as shown in our
illustration. We see there that now the egg proceeds to fission.
In the maternal and paternal nucleus there appear again two
chromosomes each. The nucleus membranes disappear and the
four nuclear loops arrange themselves in a uniform ' equatorial
plane.' Now takes place the familiar longitudinal fission of the
chromosomes and the formation of the daughter-planes. At last
the division is complete, and each of the two young embryonic
cells has once more received four nucleus-loops, of which one half
comes from the father, the other from the mother. During the
later stages of development division always takes place in exactly
the same manner, so that the ' hereditary substance ' — for as
such we regard the chromatin — due to each body-cell of the
developed animal is derived from both parents in the same

\

\

t

1

2

\

II
H

6

FIG. 73. — FERTILIZATION.

(1) The spermatozoon invades the ovum. (2) The head of the spermatozoon
becomes the male nucleus ; radiation appears in the centrosome. (3) and (4) The
centrosome has divided ; both nuclei are close together in the centre of the ovum.
(5) The chromatin of the nuclei has closed up and become the chromosome.
6) Beginning of the development.

REPRODUCTION AND HEREDITY 297

proportions. The nucleus of the body-cells has therefore essen-
tially the same structure as the first ' segmentation-nucleus.'

Before we pass 011 let us shortly sum up the facts which
justify us in regarding the chromosomes as the material carriers
of heritable characters. In the first place we notice the extreme
care with which the ' nuclear-loops ' are distributed during cell-
multiplication among the daughter-cells. There is no other sub-
stance either in the protoplasm or in the nucleus the accurate
distribution of which is watched over with the same solicitude.

Again, the 'maturing division' clearly shows that the
chromatin possesses a specific vital importance, and the examina-
tion of the fertilization-process finally removes all doubt on
that point. It is clear that the germ-cells must contain the
' hereditary substance,' for they produce from out of themselves
a new organism equal to that by which they were produced.
As further the children inherit characteristics from the father
and from the mother in equal proportions, the tiny insignificant
male germ must of necessity be equivalent to the ovum which
far exceeds it in size, and also contain the ' hereditary substance '
of a complete individual. But egg-cell and spermatozoon agree
only in the composition of the chromatin elements and in the
number of their ' nuclear loops.' If we now add the fact, already
known to us, of the strong individual independence of the indi-
vidual ' nuclear loops,' we cannot escape the conclusion that the
chromosomes are the real carriers of the heritable characters.

Moreover, it is possible to demonstrate experimentally that
the nucleus and not the protoplasm of the germ-cells determines
the development of the growing organisms. By shaking we are
able to break the mature eggs of a sea-urchin into several parts
which are either nucleated or non-nucleated lumps of protoplasm.
If we now bring together, in a drop of sea-water, such non-
nucleated fragments with male spermatozoa, the latter behave
exactly as if they were dealing with the complete ova : they
invade the plasm, or, in other words, perform fertilization.

As the most important part of the egg-cell, its nucleus, is
absent we should expect the act of fertilization to remain with-
out results ; but, contrary to expectation, this is not the case, for
we see that the egg-plasm, which is here only furnished with the
sperm-nucleus, proceeds, nevertheless, to division and growth.

298 LECTUBES ON BIOLOGY

Indeed, Boveri and other investigators succeeded in obtaining
from such non-nucleated the fertilized fragments of sea-urchin
ova fully developed Pluteus-larvse which were distinguished from
the normal larvae only by their smaller size, corresponding to the
smaller quantity of food material. As in the parthenogenesis of
the egg-nuclei, so in cases of fertilization of non-nucleated egg-
fragments, which we will describe with Kawitz as ephebogenesis,
the sperm-nucleus alone suffices to produce a complete organism.
These experiments corroborate, therefore, our assumption that
only want of protoplasm prevents the spermatozoon from pro-
ceeding independently to division and development.

But Boveri carried his experiments still further, and was able
to demonstrate that animals bred from non-nucleated fragments
possessed only purely paternal characters, and that therefore
only the nucleus transmits heritable characters. He proved
this by the method of crossing. In the two species of sea-
urchins, Echinus and Sphcer echinus, the Pluteus-larvse differ
greatly in the shape of their body and the structure of the lime-
skeleton. The larva of Echinus is slender, that of Sphserechinus
plump. Boveri convinced himself first of all that by a normal
process of bastardization, i.e., impregnation of complete ova, or
at any rate nucleated egg-fragments, of Sphserechinus with the
spermatozoa of Echinus he obtained mixed larvse which ex-
hibited paternal as well as maternal characteristics, and held
the mean between the two extreme types. B.ut if spermatozoa
of Echinus invaded non-nucleated fragments of Sphserechinus,
the result were larvse of the pure Echinus type, i.e., larvse pos-
sessing only paternal characteristics. Unfortunately Boveri was
unable, owing to technical difficulties, to carry out these impreg-
nating experiments with isolated, non-nucleated egg-fragments,
so that his observations are open to objections. But as the
nuclei of the larvse of the pure Echinus type were considerably
smaller than the typical bastard-larvse of the same size — a pheno-
menon that can only be understood on the assumption that they
received only half the normal chromatin quantity, i.e., only one
of the two paternal nuclei — it may nevertheless be accepted as
proved that they originated from fertilized non-nucleated egg
fragments.

However necessary the fertilizing act may be for the begin-

REPRODUCTION AND HEREDITY 299

ning of the development of most animal ova, we are yet able to
force many of them by artificial means, often of a very crude
kind, to proceed to fission and parthenogenetic development.

Considerable sensation was created in the 'eighties by the
discovery made by Tichomirow, showing that it is possible to
bring about a development of the ova of the silkworm, which
otherwise stand regularly in need of impregnation, by brushing
them, or rubbing them between cloth. The same result was
produced by a brief immersion of the ova in concentrated sul-
phuric or hydrochloric acid. Chemicals, therefore, which under
other conditions destroy life are here used to awaken life, a
mystery which even to-day has been only incompletely solved.
Others made experiments with other animal ova and different
means, and obtained similar result. The most important suc-
cesses in these experiments were achieved by J. Loeb who
* succeeded in producing complete larvae out of unfertilized ova
of different Echinoderms.

How can we explain this process of artificial parthenogenesis,
and how can we bring it in harmony with our hypothesis of the
action of the centrosomes during cell-division ? As Morgan and
afterwards others have shown, the development of the ova
taking place during artificial parthenogenesis exhibits the same
phenomena as during normal fertilization. In the egg-plasm
appear minute granules resembling centrosomes which become
the starting point of a radiating figure, arrange themselves in the
manner known to us, and apparently regulate the division. We
may, therefore, assume that the centrosome which during fertili-
zation is introduced into the ovum by the spermatozoon, forms
itself here independently under the influence of certain chemical
substances and thus gives the impetus to division. It must
however, be understood that this is more a description of what
we see than an actual explanation of how the centrosome acts :
on that point we are only able to form a very imperfect
conception.

As the ova, which were forced into artificial parthenogenesis,
had already passed a ' maturing-division,' their nucleus, as well
as that of all their descendants, contains only the reduced number
of chromosomes. Thus this process is at once perceived as
something unnatural, and is at the same time a new proof that

300 LECTURES ON BIOLOGY

the egg-nucleus, as well as the sperm-nucleus, each contain the
' primary constituents ' of a complete organism. The fertilized
egg-cell contains, therefore, within itself potentially the ' rudi-
ments ' or ' primary constituents ' of two individuals, of which
one possesses the paternal, the other the maternal characters.
The growing embryo has, therefore, at its disposal each of its
characters in duplicate, a fact which, as we shall see later,
has an important bearing upon the doctrine of heredity.

While individuals that were produced by artificial partheno-
genesis possess in their body-cells only one half of the chromo-
somes that are possessed by those which originate sexually, we
find in those animals that were produced by natural partheno-
genesis always the normal number of chromosomes. According
to numerous investigators this arises from the fact that in the
maturing of the egg-cells only one * directive-body ' is formed,
but that the second division, which, as we know, conditions the
reduction of the number in the chromosomes, is omitted. In
some cases formation of a second ' directive-body ' can take
place even in ova which develop parthenogenetically, but it
remains in the ovum and afterwards fuses again with the egg-
nucleus, so that here, too, we find the normal number of nuclear
loops.

This circumstantial and apparently purposeless procedure
may perhaps be explained by assuming that the relevant animals,
speaking phylogenetically, have reached the stage of partheno-
genetic reproduction since comparatively recent time ; they
prepare, therefore, their ova for fertilization atavistically and
' cancel ' these preparations only at the last moment.

We are now sufficiently prepared for turning our attention to
the theory of heredity. When we have freed this problem of all
mystic accretions it resolves itself into this question, How is it
that the children resemble their parents? Each single higher
organism consists of an enormous number of different cells and
cell-products which join together to form tissues and organs, and
perform all life-functions. Only one cell-species, the germ-cells,
have no share in the labours and cares for the well-being of the
individual, for theirs is the higher duty of caring for the
preservation of the species. We see that again and again there
proceed from these germ-cells all the different kinds of cell-

KEPKODUCTION AND HEREDITY 301

forms and become co-ordinate, until finally a new organism has
been produced resembling that from which the germ-cells origi-
nated. How can we explain this mysterious process ?

As far back as 1868 Darwin promulgated his ' Provisional
Hypothesis of Pangenesis ' to which he was led merely by
speculative considerations and entirely without a knowledge of
all the delicate processes of which we now know. In the opinion
of Darwin it is incorrect to say that the whole organism repro-
duces ; it is rather each individual cell or cell-species which repro-
duces itself : the nerve-cells reproduce nerve-cells, the muscle-
cells produce muscle-cells, the cells of the blood, blood-cells,
and so forth. The reproduction of the cells consists in secreting
minute germs which become distributed throughout the entire
body, finally to settle down in the germ-cells which are them-
selves devoid of character and undifferentiated. These minute
germs possess great independence and are able to multiply by
fission, producing continuously equivalent descendants. If now
such germ reaches an embryonic non-differentiated cell it may
force it into a definite developmental direction. If, for instance,
the little germ of a nerve-cell invades an embryonic cell this
becomes a nerve-cell. The more heterogeneous germs an embryo-
cell contains, the richer are its developmental possibilities. As
the reproductive cells are able to produce out of themselves
the complete organism, they must of necessity contain at least
one minute germ of each kind of body-cell. These minute germs
are able to remain in a state of inactivity for a considerable
time, sometimes throughout several generations, suddenly to
proceed once more to reproduction. This explains the appearance
of atavism. If the development is to take place normally, it is,
of course, necessary that all these minute germs awaken into
activity in proper succession.

We see, therefore, that Darwin had already reached the
opinion that each heritable characteristic of the paternal
organism is bound to a specific material carrier, and must have
been deposited in the germ-cells. This essential point in
Darwin's doctrine is valid to-day.

Our observations concerning mitotic nucleus-division,
maturation-division, and fertilization have led us to the con-
clusion that the chromatin of the nucleus in its totality is the

302 LECTURES ON BIOLOGY

carrier and transmitter of the heritable qualities, and this won-
derful substance must therefore contain a corresponding ' rudi-
ment ' (Anlage) for each heritable characteristic of the parents.
Without this assumption the very elaborate processes in the
course of mitosis, which manifestly aim only at the most
accurate distribution of the chromosome substance, are without
any reason ; and only with it can we obtain a teleological con-
ception of these mysterious phenomena.

Investigations of parthenogenesis and ephebogenesis further
showed incontestably that the nucleus of the mature ovum, as
well as the head-nucleus of the spermatozoon, must contain
the complete ' rudiments ' (Anlagen) of at least one complete
individual. Fertilized egg-cells would, therefore, virtually con-
tain the ' rudiments ' of at least two perfect organisms, one of
paternal, the other of maternal descent. We may therefore
assume that during the embryonic development a struggle for
predominance takes place between paternal and maternal ' pairs
of characters,' and that upon its result depends the question
whether afterwards the child shall exhibit a corresponding quality
of its father or its mother. The ' rudiment ' of the characteristic
of the defeated part is of course also contained in its cells, but
it remains latent.

Upon the more delicate structure of the * nuclear loops,' and
the distribution and arrangements of the various * primary con-
stituents ' (Anlagen) in them, we have not yet expressed an
opinion, as the observations made so far afford no foundation
for satisfactory conclusion.

Several years ago Weismann constructed an ingenious theory
of heredity in which he undertakes to analyse the organization
of the chromosome into the minutest details ; but though he
seeks to defend his theory with the utmost sagacity, and though
he marshals numerous reasons in support, his arguments enter
far too much into the region of pure hypothesis and find but
little corroboration • in the facts of observation. We may there-
fore content ourselves with a brief sketch of his theory of
germ-plasm.

If we examine greatly magnified ' nuclear-loops ' of different
animals at various stages of development, we see that the
apparently uniform ribbons are formed of a large number of

EEPRODUCTION AND HEREDITY 303

lamellae arranged successively one behind the other. According
to Weismann, this structure is the rule. He classes each of these
tiny plates as an id, and each id is, according to him, not only the
carrier of certain heritable characteristics, but contains already
the entire hereditary substance, the ' rudiments ' of a complete
organism. Most chromosomes or idants contain therefore the
entire hereditary substance several times because they are con-
structed of several ids. Only in animals with minute globular
* nuclear-loops ' can they become identical with an id.

Each id is said to possess a very complex structure and to
be composed of smaller units, the determinants. These deter-
minants are ' those parts of the germ-substance which determine
a * heritable part ' of the body, i.e., it depends upon their presence
in the germ whether a certain part of the body, be it cell-group,
single cell, or cell-part, is formed specifically, and the variations
of which induce only these definite parts to vary.'

The regions which are singly determinable by the germ are
apparently differing in extent, ' according to whether we have
to do with large or small, simple or complex organisms.' The
unicellular Infusorians probably possess special determinants
for a number of their cell-organella ; in the lower uniformly con-
structed multicellular animals the determinants govern probably
larger cell-groups and are therefore present in comparatively
small numbers, whilst in the higher animals, in particular, insects
and vertebrates, the number of determinants must be enormous,,
and certainly exceed thousands or even hundreds of thousands,
because each independent heritable variation of the organism,
no matter how minute, is controlled by one of these determinants.

We see, for instance, in many families that there occurs in
the skin in front of the ear a tiny dimple, barely the size of the
head of a pin. Weismann was able to observe the transmission of
such dimple from grandmother to son and several grandchildren.
In his opinion there must, therefore, be present in the germ-plasm
of the relevant persons a corresponding minute heritable part,
a determinant which effects the unusual development of this
little spot. Similarly, a white tuft in a dark head of hair, which
is transmissible by heredity, is said to possess a corresponding
determinant in the germ-plasm.

These determinants, however, are not by any means the

304 LECTURES ON BIOLOGY

smallest units, but in their turn contain an enormous number
of biophores which control still smaller parts of the body. Con-
cerning their size it can only be said that they lie far beyond
the limit of visibility, and that the minute granules revealed to
us in the germ-plasm by the strongest microscope represent
always groups of biophores. But these last ' heritable units '
must be larger than any chemical molecule because they are
themselves constructed of groups of molecules.

In the protozoa the determinants may probably consist of
single biophores, in which case biophores and determinants become
identical ; but in the higher organisms the determinants, as has
already been said, are composed of groups of biophores " which
joined together by inner forces form a higher life-unit. The
determinant must be able to live as a whole, i.e., be able to
assimilate, grow, and reproduce by fission, as every other life-unit,
and the biophores must be able to vary singly, so that also those
individual parts which they control are hereditarily variable."
But determinants and biophores are not distributed arbitrarily
but each id has a definite architecture. The hypothetical forces
which bind the determinants to their definite position in correla-
tion to the rest Weismann describes as ' vital affinities.' By means
of these different hypotheses Weismann now seeks to answer the
question as to the causes of the different formation of the cells
in the course of evolution. This brings us to the second and
more important part of his theory.

Our observations of the process of mitosis impelled us to
the assumption that all cell-divisions lead to an equal division of
the hereditary substance among the descendants, so that each
body-cell contains an equal number of paternal and maternal
chromosomes and possesses the ' rudiments ' for all heritable
characteristics. But Weismann advances a diametrically opposed
opinion. He distinguishes sharply between two different kinds
of cell-division, homoeokinesis and heterokinesis (erbgleiche und
erbungleiche Zellteilung), which lead to very different results,
though, to judge from external appearances, they seem to
proceed on uniform lines.

The first kind of reproduction is the usual, occurring in all
those cases where unicellular organisms divide by fission into
two equal daughter-organisms, or where the cells of the higher

REPRODUCTION AND HEREDITY 305

organisms produce by fission cells of their own kind. In homoeo-
kinesis it is said that there takes place a doubling of the
determinants and an equal distribution among the separated
halves, respectively among the daughter-chromosomes ; but in
heterokinesis the determinants group themselves in the id
unequally, so that one id-h&lt contains determinants of one,
the other, determinants of another kind. This inequality of the
daughter-cells cannot be directly observed, as the determinants
lie beyond the limits of visibility, but * can only be concluded
from the different role which both daughter-cells play in the
farther development of the organism.' ' If, for instance, of two
sister-cells of the embryo one supplies the cells of the intestinal
canal, the other those of the skin and nervous system/ Weis-
mann concludes from that ' that the mother-cell has divided
its nuclear substance unequally among the two daughter-cells,
so that one received the determinants of the endoderm, the
other, those of the ectoderm ; or, if on a butterfly-wing a red
and black spot lie close together/ it follows according to Weis-
mann " that the stem-cells of these two spots divided unequally, so
that one received the 'red/ the other the 'black/ determinants.'
It appears from this that according to Weismann hetero-
kinesis plays the principal part in the development and trans-
formation of the ovum into the complete organism. Through
each division of the embryonic cells their germ-plasm suffers a
diminution in the number of determinants because the nuclei
of the daughter-cells receive always only those groups of
determinants which shall afterwards govern their own and their
descendants' formation and differentiation. In other words,
the germ-plasm of the fertilized egg-cell is divided during the
course of its ontogeny into more and more minute groups of
determinants, until finally the body-cells of the mature animal
possess only their own determinants. Nerve-cells possess,
therefore, only nerve-cell determinants, muscle-cells only muscle-
cell determinants ; hence they are still able to produce by fission
like, but no longer unlike, cells. In order that the embryonic
development may proceed normally, forces must be assumed in
the organism which watch with the greatest care that the deter-
minants always reproduce at the right moment, and always reach
at the right time the right place in the complete body.
20

306 LECTURES ON BIOLOGY

But how is it that the organism can produce germ-cells, i.e.,
cells which contain the complete germ-plasm with all the deter-
minants? Just as a cell cannot originate otherwise than by
fission of an already existing cell, so specific determinants are
unable to create themselves anew. Tf we regard abiogenesis
of the simplest ' biophores ' conceivable we may with the same
right believe that the lowest organisms owed their existence to
abiogenesis. It follows that the reproductive cells can only be
formed where as yet all determinants of the relevant kind
are present, co-ordinated in ids. That is to say : in addition
to heterokinesis of the egg-cell there must also take place
homoeokinesis. This also is said to be actually the case, and
certain cells resulting from a division of the ovum are said to
receive a small part of the unaltered germ-plasm which they
transmit in a similar manner to their descendants. These cells
then become the stem-mothers of the sex-cells. In other words,
there are certain ' germ-paths ' along which the direct trans-
mission of the total hereditary substance proceeds from genera-
tion to generation. Thus, in spite of heterokinesis, the continuity
of the germ-plasm remains preserved ; the descendants possess
in their reproductive cells the same number of determinants
which accumulated from the parents in the egg-cell during
fertilization, and transmit it afterwards to subsequent generations.
Tn Weismann's opinion we must therefore distinguish between
the germ-plasm on the one side, and the body or somatic cells
on the other.

It cannot be denied that on the basis of Weismann's germ-
plasm theory the marvellous processes of differentiation during
the evolution of the egg-cell to the complete organism fini a
simple and satisfactory explanation. By distributing the deter-
minants of the hereditary substance by heterokinesis among
the different descendants of the ovum, and thus determining the
characters which the individual cells are to assume, the process
of mitotic nucleus-division would represent ihe means of determin-
ing the form. But though a solution would be most acceptable,
this hypothesis is confronted with serious objections : for without
taking refuge in supernatural, uncontrollable forces, it is incon-
ceivable how the same process of indirect nucleus-division can
now lead to an absolutely just and equal division of the heredi-

REPRODUCTION AND HEREDITY 307

tary substance, and then with the same precision to an
amazingly complex, unequal distribution of the germ-plasm.

In his earlier writings Weismann stated that the first division
of the egg-cell is heterokinetic (erbungleich) , and leads to a
separation of germ-plasm from soma. If this were true it would
naturally follow that only one of the first two segmentation-
grooves, the stem-mother of the later sex-cells, contains the
entire hereditary substance, while the other, the somatic
cells, has already suffered a decrease in the original store of
determinants. If it were possible to separate these first two
blastomeres artificially without injuring the vitality of the cells,
and force them into a separate independent development, the
' primordial cell ' might perhaps have prospects of developing
into a complete organism, but the somatic cell would at the
best be only able to produce partial formations. But numerous
experiments made with ova of medusae, echinoderms, tunicates,
the primitive fish Branchiostoma, and higher vertebrates show
that isolated segmentation-cells produce only small embryos,
proportionate to the smaller quantity of nutritive material.
Moreover, the first two blastomeres produce under favourable
circumstances not only two complete larvae, but in many
cases, for instance, in medusae and others of the lower organ-
isms, each of the four, eight, sixteen, and even thirty-two
first segmentation-cells is able to develop into a complete
embryo. This proves that, at any rate up to this stage, the
division of the hereditary substance has been equal, and that all
embryo-cells possess the entire hereditary substance.

Apparently contradictory results are shown by certain experi-
ments made with the ova of annelids, snails, mussels, and jelly-
fishes. In particular in the Ctenophores, complete larvae are
easily distinguished from part-formations by the number of
ciliated ribs. In the normal larvae which have been bred from a
complete ovum the number of ciliated ribs is eight. If the first
two blastomeres are isolated they continue to develop into
embryos, but these possess only four ciliated ribs : and if we
finally succeed in inducing blastomeres of the eight-cell stage to
develop separately the larvae possess always only eight ciliated
ribs between them.

Weismann regards this result as a proof of heterokinesis,

308 LECTURES ON BIOLOGY

and thinks it impossible to escape the conclusion that the rib
' determinants ' are only distributed to certain daughter-cells.
But we cannot accept this conclusion, for the ova of the Cteno-
phores are distinguished by a highly differentiated structure, and
this may well be the reason why after an experimental opera-
tion regulative transformations no longer take place, but part-
formations develop instead. Again, it is observed in the ova of
Echinoderms that the isolated blastomeres divide at first
exactly as if they were still connected with the others ; in other
words, they produce at first part-formations. It might, there-
fore, seem as if here, too, heterokinesis had taken place, but after
some time we observe that regulative transformation-processes
set in and that after all normal larvae develop from the part-
formations.

This opinion is supported by experiments made with ova
of amphibians by Eoux, Hertwig, Morgan, and Herlitzka. If
Roux killed one of the first pair of segmentation-globules of a
frog-ovum by puncturing it with a hot needle, the sound half
continued to develop as if no operation had taken place, i.e.,
segmentation stages were developed which corresponded to the
lateral half of a corresponding normal developmental stage. It
even happened sometimes that such section of the ovum developed
into a complete lateral half-embryo (Hemi-embryo lateralis). By
subsequent regeneration, the so-called post-generation, these
half-embryos were sometimes afterwards able to supplement the
deficient body-half.

Hertwig repeated Roux's experiments with frog eggs, but
reached entirely different results. According to him the sur-
viving blastomeres developed at once complete embryos, which
exhibited but slight defects, and even then only in unimportant
body parts.

Morgan succeeded in explaining the apparently contradictory
results obtained by these two investigators, and demonstrated
that the position of the uninjured segmentation-cell after the
operation greatly influences its further fate. Even with the
naked eye wre are able to recognize in the frog-egg a dark side
rich in plasm, and a light side rich in yolk; they are usually
described as the animal and vegetal poles. During the normal
course of development the light-coloured vegetal pole, being

REPRODUCTION AND HEREDITY 309

burdened with the heavy food, is, according to the laws of gravi-
tation, always pointing downwards, whilst the dark animal
pole, containing the light formative yolk, is pointing upwards.
The entire yolk substance is, therefore, distributed in the ovum
bilatero-symmetrically, and, according to all appearances, this
arrangement regulates the course of the ' division planes.'

As long as this semi-lateral distribution of the yolk is pre-
served in the surviving segmentation-cell by the pressure of the
egg-half which has been killed, it seems that only a half-forma-
tion can develop from it. But if we turn the segmentation-cell
in such a manner that the yolk-substances become arranged
as they were in the normal ovum, it develops into a complete
embryo. Herlitzka's experiments with ova of the salamander
point to the fact that in the semi-lateral distribution of the yolk
is found the real reason for the imperfect development of the
embryo. By means of an ingeniously constructed apparatus
Herlitzka succeeded at the two-cell stage in constricting the
ovum in the ' division plane,' by means of a fine cocoon thread,
and making both blastomeres completely independent of one
another. They were afterwards able to round off, the yolk dis-
tributed itself in them according to its specific gravity exactly as
in the complete ovum, and the two isolated blastomeres developed
consequently into two normal embryos.

These experiments proved unambiguously that at least in
these cases it is not a deficiency in the nuclear substance but the
structure of the egg-plasm which must be held responsible for
the appearance of part*formations. That an analogous process
goes on in the case of the ova of the Ctenophores is shown by
experiments made by Driesch and Morgan, in which larvae with
a diminished number of ribs were obtained from an unsegmented
ovum merely by removing a considerable part of the outer plasm.
Although, therefore, the ova possessed their full nuclear material,
and consequently the ' determinants ' for the full number of ribs,
these ' rudiments ' (Anlagen) were unable to develop owing to the
deficiency in plasm.

Weismann himself has recently admitted that in many animals
the first divisions of the ovum are homoeokinetic, and that only
later the ' germ-paths ' become separated from the soma.

A certain amount of corroboration of the doctrine of hetero-

310 LECTUKES ON BIOLOGY

kinesis of the hereditary substance was found by many in Boveri's
observations of a ' chromatin-reduction ' of the nuclei during
the development of the ovum of Ascaris megaloceplialus, for in
these nematodes it is possible to distinguish already at the stage of
the two-cell segmentation the cells of the ' germ-path ' from the
somatic cells : only the stem-mother of the future reproductive
cells retains her nuclear rods unchanged and with her full com-
plement of chromatin, but in the body-cells both ends of the
chromosomes are regularly rejected and perish, only the central
sections being transmitted to subsequent cell-generations. We
cannot pursue this process in its details and must content our
selves with the admission that a ' germ-path ' can here actually
be distinguished from the body-cells, and that the body-cells
have suffered a diminution in their ' hereditary substance.' But
it seems to me that this fact speaks rather against than for
the occurrence of heterokinesis, for if mitosis can already effect a
qualitative separation of the ' complement of determinants ' and
transmit with this hypothetical precision to the body-cells only
those ' primary constituents ' that belong to them, there would
seem to be no need for the subsequent chromatin-reduction.
That a qualitative diminution of ' the hereditary substance ' does
take place subsequent to the division is a sign that simple nuclear
division is unable to effect it, but can only always lead to homoeo-
kinesis. It can certainly not be denied that direct observation
supplies no proof whatsoever of the occurrence of heterokinesis.
On the other hand, many arguments may be adduced against
this assumption.

Among modern investigators it is in particular 0. Hertwig
who submitted the doctrine of inequivalent nuclear division to
sharp criticism. His statement sums up the case so tersely that
I will quote his own words : ' What purpose in the life of the
cell is served by the division in which nuclear segmentation
plays the leading part? Assuredly only the purpose of propa-
gation and reproduction — and these are the means employed by
Nature for effecting the preservation of the organism as species.
The organism which is perishable as a single individual is
multiplied in its characteristics by reproduction and preserved as
species.

* Of plants and animals we know from experience that each

REPRODUCTION AND HEREDITY 311

individual of a species is only able to reproduce new individuals
of the same species. The theory of heterogeneous generation
was soon abandoned as a gross error wherever it was promul-
gated. Thus it is a universally accepted principle of biology
that ' like produces only like,' or better, * species always produces
its species.' Among all unicellular organisms homoeokinesis
of their cell-organisms is the only method of division that occurs
and can occur. On it rests the constancy of the species. If
it were possible in any unicellular organism to divide the heredi-
tary substance (idioplasm) into unequal component parts and
transmit them unequally to the daughter-cells we should have
a case of heterogeneous generation — the origin of two new
species from one species. But, as all observations teach us, the
characters of the species are even in the unicellular organisms
transmitted by division with such minute care that unicellular
fungi, algae and infusorians, even in the millionth generation, are
exactly like their ancestors. The process of division, as such,
appears therefore, never as a means of generating new species,
not even in the unicellular organisms.

' For these reasons it seems to me inadmissible to regard
cell-division in the ovum as a means of obtaining diametrically
opposed objects, producing now like, now unlike. Here,
too, each cell-division can in its very nature be only a
homoeokinetic one, therefore all the cells that develop from the
ovum by reproduction must be carriers of the entire idioplasm,
and of equal species.'

Weismann's theory is further met by almost insuperable
difficulties in the phenomena of regeneration and asexual repro-
duction by division and gemmation. As the germ-plasm in the
course of the development of the ovum is, according to him,
divided into its determinants and, as it were, used up, it can
naturally only be capable of producing the single body-parts and
organs once. But, as we have seen, numerous lower animals
may be divided into several parts, and yet each part may develop
once more into a complete organism ; other animals are at least
able to repair lost extremities; the single cell of a begonia leaf is
able to reproduce the entire plant ; and finally the ability, under
favourable conditions of reproducing the species must, in the
opinion of most botanists, be ascribed to every plant-cell : the

312 LECTURES ON BIOLOGY

cells must therefore contain the complete idioplasm in spite
of heterokinesis. If Weismann, in order to overcome this diffi-
culty, assumes that in cases of this kind the cells in question
have been invaded during the division by the active, isolated
determinants, and also by complete ids containing the entire
idioplasm (which remained, however, ineffective under normal
conditions, and only entered into the development and brought
their primary constituents to development under certain con-
ditions), he contradicts with that assumption his own hypothesis.
For in that case we should have to reckon, apart from homoeo-
kinetic and heterokinetic division, with a third mode of division
in which only one part of the ids is divided, while another part
is transmitted to the daughter-cells uncurtailed, as a ' reserve
army,' so to speak. In order to satisfy all possibilities, we should
have to conceive almost every cell furnished with such latent
additional germ-plasm. That the cells contain more * rudiments '
than can usually reach development is proved by many patho-
logical formations, as, for instance, the occurrence of hair or
teeth in cysts.

Further, as Hertwig and Driesch have shown, it is possible,
by artificially changing the shape of ova by slight pressure and
other means, greatly to influence the course of the division
process, ' or to throw about the first nucleus-generations in the
ovum-cavity like balls,' and yet see ova which have been thus
treated yield a normal developmental result. If now the single
nuclei were already ' determined ' in their developmental tendency
as the result of unequal division of the idioplasm, the ova that
have been submitted to pressure would necessarily develop into
the queerest malformations. We may therefore assume that
the nucleus still possesses all qualities, for only in this manner
can we explain its apparently illimitable mutability.

In order to explain the normal course of development we
may well dispense with this very circumstantial mechanism
which would be necessary for the division of the germ-plasm into
its complement of determinants and their distribution. Even
when we conceive all cells to be supplied with the full hereditary
substance we can comprehend the cause of the processes of
transformation which take place during the development of the
embryo. It is true that the question why embryonic cells,

REPRODUCTION AND HEREDITY 313

though containing the same ' primary constituents,' will yet
assume such widely divergent appearances, forms at first a for-
midable obstacle. But we must consider that though the
nucleus is the real carrier of the heritable qualities the structure
of the ovum nevertheless regulates the course of segmentation
and the shape of the embryonic cells.

Let us take a simple case. The ovum of the sea-urchin
possesses only a small quantity of food-yolk which is almost
uniformly distributed throughout the entire plasm. As a result
the egg-body meets a division in all parts with approximately
equal resistance, and the young segmentation-cells are therefore
of approximately equal size. In the frog-egg the vegetal pole
we know to be strongly loaded with the solid food-yolk which
hinders segmentation, and we see, therefore, that in the animal
pole which is poor in yolk the process of division takes place
far more easily and rapidly. Thus numerous small plasmic cells
are formed here, while at the vegetal end originate large cells
full of yolk. In spite of the equivalence of the nuclei, which at
this stage we are still able to demonstrate experimentally, the
fate of the individual embryonic cells is widely divergent, simply
owing to the unequal distribution of the yolk in the ovum. It
seems further clear that with progressing division young cells are
continually changing their relation to one another and to the grow-
ing embryo ; that the individual elementary organisms obtain a
continually decreasing share in the entire life-work, in proportion
as their multiplication proceeds ; that, further, in space as well
as in time, they meet unequal conditions ; and that, therefore, the
external factors exercise their influences upon the different cells
in a different manner. It is true that we are not in a position
exactly to define the stimuli which determine the shape of
cells and their special task in the complete organism, and cause
only a well-defined part of their inherited ' rudiments ' to develop,
but we may at least regard it as probable that in the conditions
mentioned are given the essential causes for the future deter-
mination of the form. The assumption of a differentiating effect
of external factors is far more in accord with our observations of
the far-reaching influence of environment upon appearance and
structure of the developed organisms, and our mind finds in this
conception greater satisfaction ; for the mysterious process of

•314 LECTURES ON BIOLOGY

organic development is thereby driven from its isolated position
and brought into intimate correlation with the entire genesis of
life. The development of the ovum becomes a little wheel in the
whole wonderful engine of Nature.

We heard before that during fertilization only one spermato-
zoon invades the egg, the entrance being immediately closed to
the others. In immediate connection with the centrosome,
which has been carried into the ovum by the spermatozoon,
is formed the normal bi-polar ' spindle-figure ' which divides the
nucleus with the accuracy of the most perfect apparatus into
two daughter-nuclei equal in volume and value. As Boveri
showed, this accurately working apparatus of division suffers
serious disturbances when accidentally two spermatozoa simul-
taneously arrive within the ovum. For each of the two centro-
somes behaves as if it were alone in the ovum, and in place of
the normal ' spindle ' we observe a figure of four poles. Accord-
ingly the egg divides immediately into four cells, but as in such
a case the ' nuclear loops ' of three nuclei must be distributed
among four daughter-nuclei, none of the nuclei obtains the
number due to it. Though the vitality is not thereby destroyed,
and though the ova continue to develop, the larvae which emerge
exhibit varying defects corresponding to the incomplete store of
chromosomes. This phenomenon suggests that the individual
nuclear loops are not equivalent to each other, but that to each
chromosome is due only part of the ' rudiments ' of a complete
organism, and that a well-defined selection of nuclear loops is
necessary for a normal development. The facts of partheno-
genesis and ephebogenesis, when considered with this observation,
permit us, therefore, to conclude that the mature ovum as well
as the spermatozoon each contain exactly the material for one
complete organism.

But while generally no distinction can be made between the
individual chromosomes of the nucleus their unequal character
is in certain cases distinctly shown by their appearance. Thus
the nucleus of the male primary germ-cells of a species of locust,
Brachystola magna, contains, according to Button's investigations,
twenty-four nuclear loops which may be easily distinguished in
form and size : eighteen large and six small chromosomes. These
may be still further subdivided into twelve degrees of size, so

[REPRODUCTION AND HEEEDITY 315

that we get in all twelve equal pairs. It is possible to prove
that of each of these chromosome-pairs one part is of maternal,
the other of paternal origin. We have already assumed that the
paternal nuclear loops which became united by fertilization
become once more separated during the course of the maturing
of the ovum. In this case, owing to the different appearance
of the chromosomes the process may be directly observed, for
of each pair of nuclear loops only one reaches, during division,
the nucleus of the semen-forming cell.

Apart from the investigations of the cells and observations
of the processes of fertilization, there is another way which
leads us into the mysterious region of heredity : rational breeding
experiments. Both ways lead towards the same goal, and if
they meet or run parallel, if only for short distances, we can
see in that fact a proof that we are on the track of truth.

Half a century ago this second path was chosen by Mendel,
an Augustinian priest, in the Queen's cloister at Briinn. Earely
has a great success met with a stranger fate. Buried in a
little unknown journal, the record of these remarkable ex-
periments slept unnoticed and forgotten, until in the first year
of this century the facts which they proved were re-discovered
independently and almost simultaneously by three investigators,
de Vries, Correns, and Tschermak, and confirmed by new com-
prehensive experiments. As Correns justly remarked, this
long period of oblivion was a heavy loss to science, but fortu-
nate for Mendel's posthumous fame, ' for if his work had
been immediately understood we should not now speak of
4 Mendel's law ' and ' Mendelism.' The rapid advance of science
would have soon overtaken the labours of this solitary man and
left them far behind.

For his important experiments Mendel employed chiefly peas.
From seed merchants he bought thirty-four varieties of peas
which differed in various characteristics : the shape of the mature
seed, the position of the leaves, the colour of the seeds, etc. By
cultivating each kind for two years he determined whether the
character of each variety remained constant, and then chose for
his experiments only those which had stood the test successfully.
For many years Mendel continued his experiments, and only after
having made more than ten thousand ' crosses ' did he decide

316 LECTURES ON BIOLOGY

to publish his results. We shall here consider only some of the
most important results at the hands of a few simple instances,
and follow not only the experiments of Mendel but also those of
more recent investigators.

The case is simplest when both parents used for crossing
differ in only one point. In that case we are dealing with two
characters, one from the father and one from the mother. Both
together are described as 'character-pair.' If, for instance,
we cross a red-flowering and a white-flowering pea the colours
'red' and 'white' forms the 'character-pair.' The bastards or
hybrids which develop from such cross are all red-flowering ; they
all follow one of the parents and cannot be distinguished from
them. The white colour seems to be entirely suppressed. But if
we breed from these red-flowering hybrids and fertilize them with
their own pollen we see a remarkable phenomenon. Though in
this case the ' father ' as well as the ' mother ' had red flowers
the second hybrid generation produces only 75 per cent, of red-
flowering descendants, while in the rest the white flower of
the one 'parent,' which in the first generation had entirely
disappeared in favour of red, reappears here suddenly anew.
If the breeding is continued we see that the white-flowering
hybrid form under self-fertilization remains from now perma-
nently constant, producing always white-flowering descendants.
In the red-flowering variety, however, we find that under equal
conditions in all subsequent generations always the same division
takes place, i.e., we always obtain 75 per cent, red-flowering
' dividing ' and 25 per cent, white-flowering ' constant ' peas.

The same regularity may be observed in crossing two species
of nettle, of which one, Urtica patulifera, has dentate, the other,
Urtica dodartii, entire leaves. Here in the first generation the
' character-pair ' of the parents, dentate and entire leaf, produces
exclusively bastards with dentate leaves, indistinguishable from
those of the one ' parent.' In the second generation the ' split '
takes place, only three-quarters of all descendants having a
dentate leaf, while in the rest the suppressed character, the
entire leaf, reappears. The further course is identical with the
behaviour of the two varieties of peas. From this and other
facts Mendel concluded that for each independently inheritable
character of the parents there must be present in their idio-

REPRODUCTION AND HEREDITY 317

plasm a corresponding independent rudiment (Anlage) : that
during crossing and fertilization these ' rudiments ' together
form in the egg-cell a pair ; and that finally during the further
development the characteristic feature of one 'parent' conceals
that of the other. The prevalent character is usually described
as the ' dominant,' the other as the ' recessive ' character, and
the phenomenon itself as the 'rule of prevalence.' 'If,' says
Correns, ' the phylogenetic condition of both parents can be
determined it is almost always clear that the phylogenetically
higher character, i.e., the younger ' rudiment,' dominates.

But this ' rule of prevalence ' finds no universal application,
for there are numerous cases in which the hybrids of the first
generation represent a mixed form and exhibit the characters
of their parents jointly. These two extremes are then connected
by every conceivable transition form. The following is an in-
stance of the second case. Every one knows the beautiful
Mirabilis Jalapa, for its large red, white, yellow, or variegated
flowers has made it a favourite garden flower. If we cross a
red-flowering with a white-flowering form, the hybrid shows
flowers of a light pink shade. If we now breed from these
hybrids under strict self-fertilization "we shall again observe a
remarkable ' Mendelian split,' but this time in somewhat different
proportions. The second hybrid generation supplies three dif-
ferent kinds of individuals, half of all descendants showing the
intermediate light pink type, 25 per cent, the red, and 25 per
cent, the white type. But while the intermediate hybrids con-
tinue to l mendel,' i.e., to produce 50 per cent, of differentiated
descendants, the hybrids which had reverted to the form of their
' grandparents ' prove themselves perfectly constant. This case
illustrates Mendel's rule far more clearly than the instance of
pea and nettle, because here the ' splitting' is not obscured by
the ' domination ' of one character. Is it not strange that when
in the crossing of two varieties, differing only in one point, the
character of one parent is completely lost in the hybrid genera-
tion, or that, as in the Mirabilis Jalapa, only intermediate hybrids
originate, in the second generations the characters which were
apparently extinct, or at least intermixed, suddenly reappear pure
and in fixed proportions ? Can cell-investigations or speculation
offer a satisfactory explanation of this phenomenon ?

318

LECTURES ON BIOLOGY

Iii my opinion the conclusion at which Mendel arrived on the
basis of his facts is a simple and natural straightening of all the
tangled skeins. Only when we assume that the two ' rudiments '
of the paternal organisms which fused during fertilization in the
ovum to one pair, once more separate during the formation of

FIG. 74. — MENDEL'S LAW.

(1 and 2) Germ-cells of the parents. (3) Their union in the fertilization-act ;
also diagram of the structure of the body-cells of the first hybrid-generation. (4-6)
Germ-cell formation of the hybrids. The germ-cells formed are ' pure.' (7 to 10)
Fertilization-process during continuation of the breeding ; the four possibilities.

the germ-cells of the hybrids, so that one half of the sex-cells
receives the ' rudiments ' of the paternal character, but the
other that of the mother, we can gain a proper perception of
the cause of this remarkable phenomenon. The germ-cells of the

REPRODUCTION AND HEUEDITY 319

hybrids, therefore, do not possess a hybrid character, like the
egg-cell from which they developed, but the germ-cells that are
formed are pure, at any rate, with regard to the ' character-pair.

It will already have been observed how fully this assumption
agrees with our observations concerning the origin of sex-
cells, for this separation o£ the paternal and maternal rudi-
ments takes, of course, place daring both ( maturing-divisions '
through which all germ-cells have to pass in the course of
their development. Let us endeavour to make this clear with
an assumed case (compare figs. 72 and 74).

Let us assume that the ripe germ-cells of Mirabilis Jalapa
contain only two chromosomes, and that one entire nuclear loop
is the carrier of the 'rudiments' (Anlagen) of the colour of the
flowers. Whether we think of the mother as red-flowering and
the father as white-flowering, or vice versa, is of no importance to
the course of the process. In our illustrations the nuclear loops
are indicated in the red-flowering parent by a little black, in the
white-flowering by a white circle. During the act of fertilization
these ' rudiments ' of both parents are united in the fertilized egg-
cell, and all body-cells of the hybrid have the same structure as the
ovum. But as in Mirabilis Jalapa both ' rudiments ' endeavour
in an equal manner to reach development, the hybrid which
develops from the egg-cell must take up a middle position
between its parents and produce pink flowers. If now these
hybrids proceed to the formation of germ-cells, the before-
mentioned reduction-process sets in, by which the complement
of chromosomes is reduced by half and the idioplasm inherited
from the mother and father separated. The result of this double
division is represented in fig. 74, G. Each original germ-cell has
produced four mature sex-cells of which two have received the
' rudiments ' of the red, two those of the white colour. If we
now cross the hybrids there are manifestly four possibilities
in the union of germ-cells all having the same prospect of
realization (fig. 74, 7-10) : either ovum as well as sperm possess
only the ' red ' chromosome, or of the conjugating sex-cells one
is furnished with a ' red,' the other with a ' white rudiment ' — this
case will naturally occur twice as often as the other — or finally
there is the possibility that both germ-cells possess only the
white ' rudiment/ Corresponding to the different ' outfit ' of

320 LECTURES ON BIOLOGY

the fertilized egg-cell, the plants developed from them must
naturally vary greatly. If a plant contains only the red, or
only the white nuclear loops, it can naturally only produce red
or white flowers and must under self-fertilization keep pure in
all future generations ; but in the case in which the two united
germ-cells were differently supplied, where one possessed the
paternal, the other the maternal characters, intermediate hybrids
with pink flowers will originate, and these will always again
produce two kinds of germ-cells, and, therefore, even under self-
fertilization, always in exactly the same manner ' split ' into three
kinds of descendants exactly as the hybrids of the first generation.
A simple calculation shows that the ' split ' — in perfect accord
with Mendel's results — must occur in the proportion of 1 : 1 : 2,
in other words, the hybrids will always produce 25 per cent, red-
flowering constant plants, 25 per cent, white-flowering constant
plants, and 50 per cent, intermediate ' mendeling ' plants.

These results are corroborated by numerous crossing experi-
ments made with animals. The well-known Helix hortensis
and Helix nemoralis occur in two varieties, of which one has
a shell with a delicate dark ribbon design, while the shell
of the other variety is without this ornament. If we pair a
' ribbon snail ' with a ' ribbonless snail ' we shall obtain only
ribbonless descendants. But as Arnold Lang was able to show
by the most painstaking experiments extending over several
years, the suppressed ribbon design reappears in the second
hybrid generation in about 20 per cent, of all the descendants,
and remains 'thence constant in all subsequent generations.
The behaviour of the snail is therefore exactly analogous to that
of peas with red and white flowers, or the nettles with dentate
and entire leaves, in which one character dominates the other.

Poultry-breeders have been puzzled for many years by the
fact that the Blue Andalusians, a splendid variety of poultry, the
breeding of which is with particular success carried on in England,
never breed pure. Even in the best strains only about one-half of
the young are Blue Andalusians, wThile the remainder are spotted
or black. The most surprising fact, however, is that while the
black and the spotted birds prove constant and continue to breed
constant, the. blue without exception always produce different-
coloured descendants ; if, however, spotted birds are paired with

REPRODUCTION AND HEREDITY 321

black, the first hybrid generation contains only blue birds, which
on continued breeding regularly split once more into blue, black,
and spotted. We have here doubtless a typical case of Mendelism
which may be compared to the behaviour of the two Mirabilis
Jalapa races.

As early as 1895 Haacke reported that the crossing of the
spotted Japanese dancing mouse with our ordinary grey mouse
in the first generation produced only normal young. In sub-
sequent generations the remarkable abnormity reappeared in a
number of individuals in the definite proportion demanded by
the rule.

Finally, let us consider two interesting instances taken from
human life. The first case was observed by Castle on the
marriage of a negro-albino with a normal negress, Here albinism
proved to be ' recessive.' More remarkable is the second case.
In the year 1710, in Brandon, in Suffolk, a child was born of
healthy parents, whose entire body, with the exception of the
face and inner hand and feet surfaces, began a few months after
birth to be covered with a black horny skin. All the numerous
brothers and sisters of this strange child, which appeared to feel
quite comfortable in its horny skin and possessed no other defects,
were normally formed. When the boy became a man he married
a young Irish woman, and all six children of this marriage
uniformly exhibited this characteristic of their father. With
remarkable tenacity the horny skin was transmitted from genera-
tion to generation, but of the three following generations only
a part of the descendants had this natural armour, while the
others were normal. A descendant of this strange race was the
1 porcupine man,' J. Lampert, made famous by the investiga-
tions of Tilesius and Darwin, who at the beginning of the last
century, in company with his similarly affected younger brother,
travelled through England, Germany and France, exhibiting
themselves for money. The seven sisters of these two brothers
had perfectly pure soft skins. In view of the small number of
descendants we are naturally unable to prove a complete agree-
ment with the figures which we should expect under the
Mendelian law, but we may, nevertheless, regard this case with
a high degree of probability as an instance of the 'rule of
prevalence.' The only unexplained feature in this case was that
21

322 LECTURES ON BIOLOGY

only the male children possessed the characteristic horn}7 skin.
This was probably only accidental, but may on the other hand
provide an opportunit}' for obtaining information concerning the
cause of sex-determination.

As a rule the species employed for hybridization will differ
from each other in more than one point, so that we have to
direct our attention to two, three, or even more 'character-pairs.'
Such cases are particularly instructive because they let us clearly
perceive the independence and arbitrary mutability of the
different ' rudiments' (Anlagen], and because they show that the
various parental qualities may reappear in the hybrids in any
combination.

Let us make this clear by choosing the simplest possibility,
the difference of only two points, a pea variety with a yellow-
wrinkled and one with a green-smooth seed. The yellow colour of
the seed of one parent and the smooth surface of the seed-shell of
the other prove dominant, so that the hybrids of the first generation
produce only ' yellow-smooth ' peas. As now the two character-
pairs, 'yellow-green' and ' smooth-wrinkled,' split independently
of each other, the hybrids form in this case four different kinds
of germ-cells with the ' rudiments ' yellow-wrinkled, yellow-
smooth, or green-wrinkled, green-smooth. It is not necessary to
mention that here again the egg-cells as well as the male germs
are each furnished in perfectly equal proportions with one of
these four combinations. If the breeding is continued under
strict self-fertilization these four different germ-cells may meet
in nine new combinations, which, however, as a result of the
prevalence of yellow and smooth produce only four differentiated
plants bearing either yellow-smooth, yellow-wrinkled, or green-
smooth and green- wrinkled seeds. We see, therefore, that the
different parental ' rudiments ' can, in fact, come together in
•every possible combination. If we now consider the frequency
•of the single characters we obtain again a result which agrees
accurately with our calculations, for we find three smooth to
each wrinkled and three yellow to each green pea. It is unneces-
sary to carry this instance further : I would only mention that
already in this second generation a certain percentage of each
•of the four different plants proves constant and that therefore
the characters yellow-smooth and green-wrinkled have met in
a fixed combination.

REPRODUCTION AND HEREDITY 323

The veil which has so long covered the problem of heredity
begins to lift and at least for a short distance we are able clearly
to perceive the tangled paths leading into this labyrinth of the
numerous cases of the plant and animal kingdom which follow
Mendel's law and can be explained rationally, thanks to the results
of cell-investigation.

We must, however, never forget tha't Mendel's law applies
only in the case of a proportionally small number of ' characters,'
now that there are more instances which do not follow the rule,
and in which the result of crossing is essentially different from what
the law would lead us to expect. Externally it is impossible to
observe in a character whether it will ' mendel ' or not ; breed-
ing experiments alone can supply the proof. But, as Julius Gross
points out, all cases of Mendelism have this in common, in spite
of their heterogeneousness, that the ' splitting ' characters
originated suddenly, by abrupt variation. Wherever we re-
investigate a case it will always be found that t the Mendelian
race represented by one or more was suddenly observed on a
flower-bed or in a field of the stem-form, without the presence
of the transition-forms between the two types. Indeed it is re-
garded as a rule for experiments with 'prevalence' and 'splitting*
that it is necessary to select two forms which in the first place
breed pure, and secondly form sharp contrasts to each other with-
out transition forms. But as for this reason old and good species
are excluded there remain on this ground alone only varieties that
have suddenly come into being.' ' This empirical result obtained
from the facts could also be obtained per deductionem. For varie-
ties that originated gradually and are still connected with the
type of the species by transition-forms must in crossing with the
species undoubtedly produce intermediate hybrids simply on
account of their own variability and their tendency towards the
type. Therefore, only forms that have suddenly come into
existence can exhibit the phenomena of Mendel's law.' In other
words, cases of Mendelism are mutants in the sense of De Vries.
Even in animals or plants in which one or the other character
follows the rule of prevalence and splitting we can observe that
the other characters of both are found in the hybrids in any
proportion. In order to put forward an explanation for this
phenomenon we are compelled to leave experiments and venture
on the ground of pure speculation ; the result will therefore only

324 LECTURES ON BIOLOGY

be able to claim a heuristic value. I am following here again
essentially the arguments of Gross, because I regard his attempt
as comprehensive and intelligible, but disagree with him when
he, in accordance with Weismann, conceives the single nuclear
loops to be composed of several equivalent ids. I rather believe
the simple hypothesis that the different ' primary constituents ;
are capable of independent reproduction to be an entirely satis-
factory explanation. It is also far better supported by the facts of
observation.

The fundamental condition of Mendelian ' splitting,' the
purity of the germ-cells of the hybrids, is, as we have seen,
guaranteed by the process that takes place during the maturing
of the ovum and sperm, and it only remains now to explain why,
in spite of this purity of the sexual products, most crosses produce
mixed forms, and why not all ' character-pairs ' behave alike.

Our observations compelled us to assume that each cell of the
body of an organism originated bi-sexually contains the complete
hereditary substance, as it were, in two ' editions/ from the father
and from the mother. While during the nucleus-division the
idioplasm becomes visible in the shape of individualized chromo-
somes, it lies in the meantime during the so-called nucleus-
rest distributed throughout the entire interior of the nucleus.
Only during the preparation for a new reproduction the
chromatin fragments collect and arrange themselves once more
into the characteristic nuclear rods or loops. We may now
contend with some degree of certainty that the chromosomes
are not uniform formations, but that they consist of an enormous
mass of different 'primary constituents' — (about the nature of
which we know nothing) — of which each corresponds to a certain
body-part. If the two parents from which each half of the chromo-
some complement came resembled each other closely the conclu-
sion is not far-fetched that the paternal and maternal rudiments
resemble each other; indeed, according to Gross, they are so
much alike that they may be arbitrarily exchanged. They are of
course always corresponding ' rudiments,' as otherwise the young
would exhibit most remarkable defects. When therefore the
nucleus awakens from the rest-stage into activity the different
pairs of chromosomes reassume once more their definite number
and form, but they are no longer the same chromosomes as
heretofore. They are no longer purely paternal or purely

REPRODUCTION AND HEREDITY 325

maternal nucleus-loops, but each carries the 4 rudiments ' of the
parents in any conceivable combination. Though now the
parental chromosomes are in the case of the germ-cell formations
divided by maturity-division each of them will still be composed
of * primary constituents ' of a double lineage. Accordingly, the
descendants must exhibit the character of both parents in an
entirely different combination.

In the majority of cases the paternal and maternal qualities
will on an average balance each other ; in many individuals the
influence of the father will be prevalent, in others that of the
mother, and sometimes it may even occur that the type of one
of the parents is almost entirely suppressed.

In the cases of Mendelism the proportions are somewhat
different ; there we may conceive that the ' rudiments ' of the
' splitting characters ' have gradually become differentiated so
much that they may no longer be exchanged one for another, but
that all ' rudiments ' of the relevant character always meet
again in their definite nucleus-loops. However different the
combinations of the other rudiments may be, as regards the
' Mendelian rudiments,' always pure sex-cells are formed.
Formally this hypothesis suffices for explaining all the manifold
phenomena of heredity, but how far it will be possible to support
it by facts, or what modifications it will have to undergo, only
the future can say.

Our excursions into the region of biology have led us along
many sinuous paths, across luxuriant fields and dry deserts, to
the high summits of mountains, and through narrow ravines.
We have seen what the human mind has achieved in restless
labour, and how Nature has been forced to yield up many profound
secrets, but our wanderings have also led us to many locked
doors at which we knocked in vain for admission. However
great the treasures of knowledge raised in the past and the
present, still greater treasures await discovery by future genera-
tions. Moreover, who will say that much which is to-day valued
as gold may not be rejected as dross by posterity, and much that
is regarded to-day as a secure possession of science may not
have crumbled away in a few generations ?

Der steht wohl niedrig in der Toren Mitte,
Wer blindlings hat das Ja und Nein gefunden,
Nicht unterscheidet, ob es Deutung litte.

320

INDEX.

ABIOGENKSIS, 89, 306

Actinia, 181

Adams, 135

Adanisia rondeletii, 182

Adaptation, 157

Adelsberg grotto, 193

Adler, 268

Aigner-Abafian, 167

Air-pressure, 32

Albinism, 321

Alca impennis, 23

Aldrovandus, 171

Alga, 64

Alloplectus, 195

Alpine trees, 188

Alteration of generations, 259

Alytes obstetricans, 279

Amblystoma tigrinum, 121

Amceba, 72 ; Umax, 31 ; protcus, 212,

240 ; radiosa, 190
Amphimixis, 226, 283
Amphiont, 238
Amphiuma, 16
Anaerobic bacteria, 31
Anal glands, 288
Anaximander, 4
Anchitlierium, 113
Ancon sheep, '209
Annelidse, 132
Anoplieles, 237
Ant-eater, 117
Antedon rosacea, 130
Antlieridium, 237
Anthracosaurus, 108
Antitoxin, 48
Ants, 184, 276
Arch&opteryx, 14, 110
Arctica caja, 186
Arctic animals, 160

Argoniuta, 291

Aristotle, 6, 87, 282

Armadillidiiim, 106

Artemia, 81, 198

Ascaris megalocephalus, 81, 291, 293,

296, 310
Ascoli, 139
Aspredo Icevis, 279
Assapan, 177
Atlantoswirus, 109
Atomistic theory, 86
Auk, great, 23
Aurelia aurita, 260
Auricular ia, 129
Autotomy, 170
Axolotl, 121

Bacillus tetani, 48

Balana, 24, 109

Balcenoptera musculus, 124

Balanoglossus, 129

Barnacle, 65

Barnicle-goose, 132

Barramunda, 18

Bates, ! 166

Bathybius, 98

Bats, 43

Beatis gemellus, 274

Bedeguar, 267

Bees, 198

Beetroot, 204

Belladonna atropa, 49

Bell- animalcules, 230

Belodon, 109

Belt, 46

Bert, 33

Bessel, 135

Beta vulgaris, 204

Biedermann, 169

INDEX

327

Bilharzia, 287

Biochemistry, 13G

Biophores, 304

Biorhiza renalis, 268

Bipinnaria, 129

Blastomere, 307

Blastula, 117

' Bleeding host,' 190

Blue Andalusian variety of poultry, 320

Bonellia viridis, 287

Boveri, 284, 298, 310, 314

BracJiystola magna, 314

Branchiostoma, 12

Branchipus, 198

Brauer, 293

Brine-shrimp, 198

Brontosaurus, 109

Brown-Sequard, 186

Bryozoa, 128, 258

Budding, 255

Bullfinch, 198

Burke, 98

Bursa, 276

Biitschli, 69

CACHALOT, 109

Cambrian system, 104

Cameroons, 167

Canalis gyncecophorus, 287

Canaries, 198

Capra ibex, 147

Carboniferous system, 108, 218

Carp, 153, 185

Caryatis viridis, 167

Castration, 148

Cataplexy, 45

Catastrophism, doctrine of, 8

Catheturus lathame, 280

Cats and fertilization of red clover, 156

Caulerpa, 70

Caviare, 153

Cell, discovery of the, 64 ; cell-nucleus,

74, 283; cell-state, 249; cell-theory,

79 ; food-cells, 285
Centrosome, 75, 283
Cephalopoda, 104, 291
Ceratopsidae, 110
Cercaria, 154
Chaetopoda, 128
Chamberlain, H. St., 205

Chameleontidae, 19
Chamisso, 259
Chemical stimuli, 190, 196
Chill-coma, 31

Chlorophyll, 30

Chromatio, 74, 295; chromatin reduc-
tion, 310

Chromatophores, 169

Chromis paterfamilias, 279

Chromosome, 291

Ciliata, 220

Circumcision, 186

Clinostat, 195

Clitellum, 290

Cloe, 289

Cobitis, 42

Cold, effect of, 186

Colour- adaptation, 164

Columbia livia, 143

Conjugation, 225

Constriction, 214

Convergence, phenomenon of, 168

Copernicus, 7

Coral-snake, 165

Corals, 257 ; corals and sea-anemones,
62

Cordylophora, 193, 199

Coregonus, 35

Corethra, 46

Correlation of parts, 147

Cortical plasm, 222

Cosmozoic theory, 91

Crayfish, 45

Cretaceous period, 108

Crinoidea, 129

Crossbreeds, 202

Cuckoo, 280

Cuttle-fish, 291

Cuvier, 7

Cyclostoma, 12

Cynipidae, 267

Cysticercus, 262

Cytodes, 69

DACHSHUND, 209
Daphnia, 133, 263
Darwin, 301, 321
Darwin, Erasmus, 9
Darwinian Theory, 140
Deadly nightshade, 49

328

LECTURES ON BIOLOGY

Death's-head moth, 162, 165

Death, what is, 214

Deluge, 84, 293

Desert animals, 161

Determinant, 303

Devonian system, 107

Dilina tilice, 202

Dimorphism, sexual, 286

Dioncea, 66

Diplozoon paradoxum, 287

Dipnoi, 38, 107

Dipterocarpaceans, 178

Directive body, 294, 300

Dog, host of Tcenia, 263 ; ecliinococcus,

262.

Domestication of animals, 143
Dominant character (in crossing), 317
Driesch, 309, 312
Driver-ants, 46
Drosera, 65
Dry-coma, 36
Du Bois Reymond, 59
Duckmole, 117
Duck-mussel, 132, 287
Dujardin, 221
Dusch, 96

Echidna, 117 ; aculeata, 281

Echinodermata, 127, 276

Echinus, 298

Ectocarpus siliculosis, 233

Ectoplasm, 213

Eel, 178

Egg-cells, 116

Ehrenberg, 95, 221

Eimer, 169, 192

Elaps corallinus, 165

Electric organs of ray and eel, 178

Elementary species, 208

Elephant, 151

Embryo, development of, 119

Empedocles, 5

Encystus, 255

Endoplasm, 213

Engelmann, 188

Entomostraca, 133

Eocene strata, 113

Ephebogenesis, 298

Ephemeridae, 274

Ephippium, 264

Ephyra, 260
Epilepsy, 186
Epistalis, 221
Equatorial plane, 82, 293
Equidae, 114
Erber, 41

Erythrolampsus, 165
Eudendrium ramosum, 193
Eugaster, 172
Euglypha alveolata, 219
Eunuchs, 148
Eyed hawk-moth, 202

Fasciola hepatica, 154

Fechner, 91

Ferns, 194, 200

Fertility, 151, 153, 224

Fertilization, 117, 156, 229, 270, 289, -295

Feuerbach, 2

Fischer, Emil, 51, 186

Fish-heart, 120

Fittonia, 67

Flat-fishes, 122

Flying-fishes, 15

Flying-membrane. 25

Flying- squirrel, 177

Food, influence of, 198

Foraniinifera, 217

Friedenthal, 136

Frog, 290

Funaria hygromctrica, 200

Fungi, 65

Fungus-gardens of ants, 184

Fusulina, 218

Gadus (cglefinus, 15

Galen, 7

Galilei, 7

Gall-apples, 267 ; gall-insects, 267

Galle, 136

Gardening-ants, 184

Gasterosteus acnleatus, 276

Gastrula, 118

Gecko, 19

Gemmation, 257

Geometridse, 165

Geophagus brachyttrus, 278

Germ- cells, 246 ; formation of, 282 ;

germ-path, 306 ; germ-zone, 292
' Gid,' 262

INDEX

329

Gills, 120

Giraffe, 11

Glands, anal, 288; poison, 289; sexual,

284

' Glass-rope ' sponge, 162
Glow-worm, 163
Goethe, 8, 10, 60, 88
Gooseberry, 204
Grassi, 238
' Gravediggers,' 273
Green birds in Europe, 162
Grew, 68

Gross, Julius, 323
Gryllotalpa, 24
Guillemot, 23
Guinea-pig, 169
Gurnard, 14
' Gutter-fauna,' 38
Gymnophiona, 16
Gymnospore, 238

HABERLANDT, 67

Haeckel, 36, 97, 115, 138, 211, 321

Hcemamc&ba, 235

Haltica atropce, 49 . -

Hansemann, 139

Harvey, 87

Hatter ia punctata, 108

Hawk-moths, 136

Heat-coma, 32

Hectocotylus, 291

Hedgehog, 49, 168

Heider, 293

Heincke, 150

Heliconiidse, 166

Heliozoa, 72, 243

Helix pomatia, 81 ; hortensis, 320 ; nemo-
ralis, 320

Helmholtz, 91

Hemi-embryo lateralis, 308

Herbst, 253

Heredity, 185, 247; hereditary sub-
stance, 83, 296

Herlitzka, 308

Hermaphroditism, 285

Hermit-crab, 181

Hertwig, 188, 293, 308, 310, 312

Hesperornis, 114

Heterogony, 267, 270

Heterokinesis, 304

Heymons, 198

Hibernation, 43

Hipparion, 113

Hippocampus antiquorum, 279

Hobbes, 180

Hofer, 185

Hoff, 9

Holothurians, 129

Homoeokinesis, 304

Homunculus, 88, 283

Hooke, Robert, 68

Hornet and hornet-moth, 165

Horse, 113

Horse-dung beetle, 274

Hot springs, fauna of, 40

Hunger- culture, 265

Hurley, 180

Hydra viridis, 63, 252 ; grisea, 253

Hydrocaulus, 261

Hydrophilus piceus, 195

Hydrorhiza, 261

Hydrozoa, 241

Hypnosis, 44

Hyracotherium, 113

ICHNEUMON fly, 272
Ichneumonidae, 255, 272
Ichthyopterigium, 13
Ichthyosaurus, 108
'Id,' 'idant,' 303
Idioplasm, 311
Iguanodon, 110
Immortality, potential, 214
Immunity, 49
Infusoria, 221
Insectivorous plants, 65
Issakowitsch, 264
Ivy, 194

JAGER, 252
Jelly-fishes, 162
Jurassic period

Kallima, 165
Kangaroo, 281
Kant-Laplace Theory, 85
Kelvin, Lord, 91
Kepler, 7
Kessler, 181
Kircher, 44

330

LECTURES ON BIOLOGY

Kiwi, 22
Klebs, 200
Kleinberg, 255
Knot-grass, 196
Koch, W., 37
Korschelt, 76
Kropotkin, 181
Kiikenthal, 125

LAMARCK, 10, 185

Lamprey, 12

Lampyris twctiluca, 163

Lancelot, 13

Lang, Arnold, 320

Lange, 157

Laveran, 234

Law of causality, 85 ; of evolution, 85

of gravitation, 7, 195
' Leaf -rollers,' 273
Leeuwenhoek, 38, 282
Lemming, 168
4 Lemon ' butterfly, 192
Lepas anatifera, 132
Lepidosiren, 107
Leptodora hyalina, 134, 162, 289
Lestes sponsa, 274
Leucocyte, 73
Leverrier, 136
Libellula cancellata, 274
Libellulidse, 274
Liebig, 91

Light, effect of, 177, 188, 192
Lingula, 176
Linnaeus, 65
Liver fluke, 153, 270
Liverwort, 194
Lizards, 19
Lockjaw, 48
Locusts, 161
Loeb, 197, 299
' Loopers,' 165
Lophobranchii, 279
Lotsy, 195
« Love-darts,' 288
Lumbricus trapezoides, 255
Lungs, evolution of, 120
Lyccena agestis, 192
Lyell, 9

MACFADYBN, 41

Macrobiotus hufelandi, 38

Macrogamete, 231

Macroglossus, 136

Macronucleus,!l223

Maize, 206

Malacostraca, 134

Malaria, 234

Manuring, artificial, 197

Marchantia, 194

Massart, 195

Mastodon, 108

Maturing-division, 299

Maupas, 225

Mayer-Eymar, 174

Medusa, 162

Megusar, 195

Mendel, 202, 315 ; Mendelism, 315 ;
Mendel's law, 315

Mesohippus, 113

Mesozoic formations, 109

Metabolism, 28, 62

Metagenesis, 267

Mice, experiment with, 186 ; Japanese
dancing, 145, 321

Micrococcus prodigiosus, 190

Microgamete, 231

Microgaster, 272

Micronucleus, 223

Microstoma, 254

Migration, Theory of, 175

Mimicry, 165

Mimosa pudica, 65

Miocene strata, 113

Mirabilis Jalapa, 317

Miracle-flower, 40 ; miracle -monad, 190

Mithridates, 47

Mitosis, 80, 293

Mohl, Von, 68

Mole, 24, 163

Mole-cricket, 24

Monera, 36

Monont, 236

Morgan, 299, 308

Morula, 117

Moses, 45

Mosquito, 235

Mosses, 200

Moulting of crustaceans, 133

Mud-fishes, 39 ; mud-fly, 275 ; mud-
jumper, 17

INDEX

Muller, 6, 206

Mummy-wheat, 36

Musk-glands, 288

' Mutant,' 323

Mutation, Theory of, 206

Mysidse, 135

Myxine glutitiosa, 286

Nauplius, 133

Nebular Theory, 85

Neoccratodus, 18

Neoscopelus macrolepidotus, 35

Neptune, discovery of planet, 135

Newt, 41

Newton's law of gravitation, 7

Nototrema, 279

Nucleus, 74, 223; nuclear loops, 296

nucleolus, 79; nucleus- corpuscle, 75

nucleus-division, 81
'Nun moth,' 155
Nuptial dress, 180
Nurse-generation, 269
Nut tall, 137

Octopus, 182

Oedendorf, 108

(Enothera, 206-207

Ontogeny, 115

Oocyte, 239

Oogony, 237

Ophichthys colubrinus, 16G

Ophiuroidea, 129, 276

Opuntia, 193

Orchids, 194

Organella, 77

Oi-nitliorhynchus paradox us, 117, 281

Os centrale, 126

Oscillatoria, 188

Ostrich, 22

Over-fertilization, 296

Oyster, 201

PJEDOGENESIS, 271
Pagurus calidus, 181
Palceotlierium, 113
Palpal organs, 290
Paliidina, 112
Pandorina morum, 244
Paradoxides, 104
Param&cium, 221

Parnassius apollo, 192

Parrot, 199

Parthenogenesis, 264, 299

Pasteur, 30

Pauly, 175, 185

Pavy, 47

Pelagia notiluca, 261

Pelomyx, 69

Penaus, 134

Penguin, 22

Pentacrinus, 128, 176,

Peridinium, 243

Periophtlialmus, 17

Peripatus, 111

Peristome-field, 222

Peruvian system, 108

Peysonnel, 62

Pfliiger, 101

Phagocyte, 73

Phasmidse, 165

Phosphorescence, 163

Phyllium siccifolium, 165

Phyllopteryx eques, 165

Phylloxera vastatrix, 269

Physeter macrocephalus, 109

Physiologus, 7

Pictet, 42

Pieridse, 166

Pigeons, races of, 143

Pigment cells, 169

Pike, 185

Pill-beetle, sacred, 273

Pinnipedia, 24

Pipa americana, 279

Placenta, 237

Planorbis multiformis, 112

Planula, 260

Plant-lice, 269

Plasmodium, 235

Platurus, 166, 168

Platyrhini, 138

Plesiosaurm, 109

Pliny, 6

Pliocene strata, 113

Plumularidae, 262

Pluteus, 129, 197, 298

Poison gland, 289

Polar bear, 160 ; fox, 160

Polygonum amphibium, 19G

Polymorphism, 267

332

LECTURES ON BIOLOGY

Polyommatus, 192

Polyspermy, 296

Poplar, 193 ; hawk-moth, 202

Porcupine-man, 321

Potamanthus, 289

Pouchet, 94

Poulton, 136

Power, 42

Prehistoric animals, 109

Preyer, 37, 89

Prickly pear, 193 *

Primary constituents, 228

Privet hawk-moth, 136

Processus vermiformis, 127

Proglottis, 263

Protective coloration, 159 ; resemblance,

165

Proteus, 163, 193
Prothallus, 200
Protohydra, 118
Protonema, 200
Protophyta, 244
Protopterus annectens, 107
Protozoa, 212
Protozoology, 221
Pseudopodia, 72
Pteranodon, 110
Pterodactylus, 25
Pteropus edulis, 25
Pteromys alborufus, 177
Pterosaurians, 110
Ptolemaic system, 86

QUETELET'S law, 206

RABBITS, 175 ; rabbit pest, 152
Eadiobes, 99
Ramsay, 99

Ratio of reproduction, 151
Rawitz, 298
Ray, 178
Reaumur, 62
Recessive character, 317
Redia, 270

' Red underwings,' 162, 165
Regeneration, 193, £83
Regnard, 35
Remack, 79

Reproduction, 214 ; in freshwater
sponges, 258 ; by layers, 258

Rhinoceros-bird, 280
Rhizocrinus, 128
Rhodites rosce, 267
Rhodocera rhamni, 192
Rhynchocephala, 108
Rock-dove, 143
Rosenberger, 136
Rose of Jericho, 40
Rossia macrosoma, 54
Roux, 52, 308

Rudimentary organs, 12, 126
Riigen, 218
Rule of prevalence, 317

SABLE, 160

St. Hilaire, 8

Salamander, 120

Salpa, 162, 259

Samter, 198

Saurians, 26

Sea-anemones, 181 ; sea-cucumbers, 253 ;

sea-gull, 20
Seal, 24
Selaginella, 40
Selection, artificial, 142, 203; natural,

141 ; natural selection a doctrine of

chance, 173 ; sexual, 179, 288
Self-mutilation, 170
Semnia auritalis, 167
Serpulidse, 128
Serranus scriba, 286
Sertularidse, 262
Serum experiments, 136
Sex-nucleus, 223 ; sex-organs, 262 ; sexual

glands, 284
ScarabfBus, 274
Scent-scales, 288
Schistosomum, 287
Schizopoda, 135
Schleiden, 68
Schmankewitsch, 198
Schopenhauer, 95
Schroder, 96
Schultze, Ferdinand, 95

Max, 68

Schwagerina, 218
Schwann, 68
Sciurus vulgaris, 177
Scolopendra, 290
Scyphostoma, 260

INDEX

333

Shell-animalcule, 222

Sida cri/stallina, 266

Siebold, Von, 35, 221

Silk-worm, 299

Silurian system, 106

Siphonophora, 261

Sisiphus sacer, 273

Skinks, 17

Slipper animalcule, 221

Sluiter, 183

Smerinthus, 202

Solenhofen, 104

Solenostoma, 279

Sommer, 186

Spallanzani, 282

Spennatid, 293

Spermatoblast, 54, 293

Spermatocyte, 54, 293

Spermatogony, 293

Spermatophore, 289

Spermatozoa, 232

Sperm-cartridge, 289

Sphcer echinus, 298

Sphingidse, 136

Sphinx ligustri, 136

Spiders, 276, 290

Spindle-figure, 81, 314

Spontaneous generation, 79

Sporocyst, 155, 270

Sporozoa, 234

Squirrel, 177

Standfusz, 186, 192, 202

Starfish, 128, 253

Statoblast, 258

Stegocephala, 108

Stegosaurus, 110

Steinbock, 147

Stentor, 224

Stickleback, 276

Stolo prolifer, 256

Strobila, 260

Struggle for existence, 151

Struggle of the Parts, Theory of, 52

Sturgeon, 153

Sti/lonychia, 221

Sugar-beet, 204

Sulphuric acid, Verworn's experiment

with, 29

Summer-eggs of insects, 265
Sun-animalcules, 72

Sun -dew, 65

Survival of the fittest, 156, 204

Button, 314

Symbiosis, 63, 181

Sympathetic coloration, 167

Syngnathus acus, 279

TADPOLES, 198

Tfsnia ccemirus and T. echinococcus, 262

Taguan, 177

Talegalla, 280

Talpa, 24

Tarentola, 19

Teju, 19

Teliosaurns, 109

Termites, 184

Termitoxenia, 184, 286

Tertiary period, 112

Thalassicola, 76

Thales, 3

Thallus, 194

Theromorph, 108

Thibet, animals of, 168

Tichomirow, 299

Tiger-moth, 191

Tilesius, 321

Tissandier, 33

Transparency of animals, 162

Tree-frog, 169

Trembley, 252

Triassic period, 108

Trichocysts, 223

Trigla hirundo, 15

Trilobites, 104

Triton, 289

Trochophora, 132

Trumpet-animalcule, 223

Tuatara, 108

Tupinambis, 19

Turban -eyes, 289

Turbellaria, 253

Tiirn pel, 274

UHLENHUT, 139
Uria troile, 23

VACCINATION, 48
Vacuole, 213, 222
Vanessa levana, 191
Vanessidse, 167, 187

334

LECTURES ON BIOLOGY

Variability of species. 141
Variations, 142
Venus's fly-trap, 66
Verworn, 28, 76
Vinegar- eel worm, 46
Virchow, 79
Vis vitalis, 47, 50
Vital affinity, 304
Vitalism, 51
Volvocidal, 244
Volvox, 245
Vorticella, 221, 230
Vries, De, 206

WAGNER, 69, 175
Walking-leaf, 165
Wander-spores, 231
Wasmann, 184
Waste of life in nature, 157
Water-beetle, great, 195

Water- cultures, 197

Water-flea, 133, 263

Water pressure, 34

Weasel, 161

Weismann, 3, 122, 136, 186, 187, 214,

226, 264, 302, 324
Whales, 23, 123
Winter-eggs of insects, 265
Wohler, 51
Wolff, G. 203
Wolff, J., 52
Wundt, 57

Xylina vetusta, 165
'YELLOW underwings,' 166

Zea mays, 206
Zocea, 134

JOHN BALE, SONS & DAXIKLSSON, Ltd., 83-91, Great Titchtield Street, W.

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