Life and death, heredity and evolution in unicellular organisms

LIFE AND DEATH

HEREDITY AND EVOLUTION IN UNICELLULAR
ORGANISMS

LECTURES DELIVERED
UNDER THE

RICHARD B. WESTBROOK FREE LECTURESHIP FOUNDATION

AT THE

WAGNER FREE INSTITUTE OF SCIENCE

PHILADELPHIA

BADGER'S
STUDIES IN SCIENCE

THE HIGHER USEFULNESS OF SCIENCE.
By William Emerson Ritter.
THE UNTTT OF THE ORGANISM, OK THE
ORGAXISMAL CONCEPTION OF LIFE. Two
volumes. IlliLitrated. By William Emer-
ton Ritter.

THE BEGINNINGS OF SCIENCE. By
Edward J. Menge.

THE PROBABLE INFINITY OF NATURE
AND LIFE. By William Emerson Ritter.
AN ORGANISMAL CONCEPTION OF CON-
SCIOUSNESS. By William Emerson Ritter.
INSECT BEHAVIOR. Illustrated. By
Paul G. Howes.

LIFE AND DEATH, HEREDITY AND EVOLU-
TION IN UNICELLULAR ORGANISMS. Illus-
trated. By H. <S. Jennings.
THE ORNITHOLOGY OF CHESTER COUNTY,
PENNSYLVANIA. Illustrated. By Frank
L. Burns.

FAMILIAR STUDIES OF WILD BIRDS. Illus-
trated. By F. N. Whitman.
QUESTIONS AND OUTLINES IN GENERAL
CHEMISTRY. By W. S. Haldeman.

SANITARY ENTOMOLOGY. By W. Dwight
Pierce.

RICHARD G. BADGER, PUBLISHER, BOSTON

/.ool.

cT

LIFE AND DEATH

HEREDITY AND EVOLUTION
IN UNICELLULAR ORGANISMS

BY

H. S. JENNINGS

BOSTON

RICHARD G. BADGER

THE GORHAM PRESS

COPYRIGHT, 1920, BY H. S. JENNINGS

All Rights Reserved

Made in the United States of America
The Gorham Press, Boston, U. S. A.

PREFACE

THE present work is not a book on Protozoology, but on
Genetics, employing Protozoa as material, and compar-
ing the conditions there found with those in higher organ-
• isms.

To select from a great mass of varied material, not yet
reduced by science to unity, those features that appear
most significant for certain general questions is a task of
difficulty, not unattended with the danger of justifying
the critic. I cannot hope to entirely escape that danger.
Much that is of extreme interest must be omitted, if any
clearness of outline is to be preserved. A certain one-
sidedness appears inevitable, unless an encyclopedic work
is attempted. The relatively great prominence given to
the infusoria in these lectures is an example; the hetero-
geneous and still more imperfectly known genetic phe-
nomena in the other Protozoa lend themselves less readily to
a unified presentation. I can only hope that the limita-
tions of the work aid in defining certain large problems.

Technical terms have been avoided. This is not alone
because the lectures were for an audience not composed
of specialists. Technical terms, in spite of their con-
venience, bring many disadvantages, even in strict scientific
work. They seem to give to phenomena a distinctness and
uniqueness which does not exist in nature. They create
separate entities for things that are mere variations on a
general theme. Any phenomenon has many-sided relations
to the others ; to bring these out we have not hesitated even

3

4 Preface

to employ in different passages diverse designations for the
same thing. What we observe in the Protozoa are combina-
tions of chemicals; of matter and energy, with their char-
acteristic activities. Technical terms tend to set these apart
and render them unintelligible; what we need is to render
them intelligible by showing their community with the world
of every-day experience.

The book deals with heredity, variation, evolution, as
present physiology, not as past history. It discusses what
now occurs, not what may have occurred in the past. Hence
discussion of the origin of the conditions now existing will
hardly be found.

A cknoidedgments

To the Editor and Publishers of Genetics I am indebted
for the use of the blocks for Figures 12 and 23, taken from
Genetics, volume 1. To my wife, Louise Burridge Jennings,
I am indebted for the drawing of the remainder of the
figures in the book.

Johns Hopkins University,
Baltimore, Md.

CONTENTS

HUH

I. GENERAL SURVEY OF THE LIFE HISTORY IN THE PROTOZOA, WITH
THE QUESTIONS IT RAISES. LIFE, DEATH, REPRODUCTION,
MATING, REJUVENESCENCE, "POTENTIAL IMMORTALITY" ... 13

II. HEREDITY AND VARIATION IN PROTOZOA, IN REPRODUCTION FROM
A SINGLE PARENT. "THE INHERITANCE OF ACQUIRED CHARAC-
TERS." THE EXISTENCE OF MANY DIVERSE STOCKS IN A SINGLE
SPECIES. CONSTANCY OF THE STOCKS. "PURE LINE INHERIT-
ANCE" AND THE RESULTS OF SELECTION. DIFFICULTIES FOR THE
THEORY OF EVOLUTION 38

III. RESULTS OF INTENSE AND LONG CONTINUED STUDY OF CHANGES
IN A STOCK. INHERITED VARIATIONS IN THE PURE RACE. VISIBLE
EVOLUTION 67

IV. CAN WE EXPERIMENTALLY CHANGE THE HEREDITARY CHARAC-
TERS? HEREDITY OF ENVIRONMENTAL EFFECTS. HEREDITY AND
VARIATION IN BACTERIA AND SIMILAR ORGANISMS .... 85

V. THE NATURAL HISTORY OF MATING. SEX, ITS NATURE AND CON-
SEQUENCES. SEX IN THE PROTOZOA. Is SEX COEXTENSIVE WITH
LIFE AND NECESSARY TO ITS CONTINUANCE? 106

VI. WHAT ARE THE RESULTS OF MATING? REJUVENESCENCE AND

MATING. HEREDITY AND VARIATION, AND MATINO .... 141

VII. How DOES MATING BRING ABOUT BOTH BIPARENTAL INHERIT-
ANCE, AND DIVERSITY IN HEREDITARY CHARACTERS? WHAT
EFFECT HAS MATING ON THE STOCK AS A WHOLE? DOES IT
INCREASE VARIATION? DOES IT DECREASE VARIATION? WHAT
Is ITS RELATION TO EVOLUTION? 170

VIII. COMPARISON OF THE GENETIC PHENOMENA IN THE PROTOZOA WITH
THOSE IN HIGHER ORGANISMS. GENERAL VIEW OF DEVELOP-
MENT, MATING AND EVOLUTION 198

LIST OF ILLUSTRATIONS

1. SIMPLER FORMS OF PROTISTA 15

2. SOME OP THE MORE COMPLEX PROTOZOA 16

S. STRUCTURE OF ONE OF THE MOST COMPLEX OF THE PROTOZOA . 17

4. DIVISION OF AN INFUSORIAN, PARAMECIUM 20

5. DIAGRAM OF THE DESCENT OF GENERATIONS IN PROTOZOA . . 21

6. CONJUGATION OF PARAMECIUM 22

7. DIAGRAM OF THE PROCESS OF DIVISION IN PARAMECIUM ... 25

8. DIAGRAM OF THE INTERNAL PROCESSES IN THE CONJUGATION OF
PARAMECIUM CAUDATUM 26

9. DIAGRAM SHOWING WHAT OCCURS IN ENDOMIXIS 31

10. REPRODUCTION IN AMCKBA 89

11. REPRODUCTION IN DIFFLUGIA 40

12. Two PARENTS, WITH THEIB OFFSPRING, JUST BEFORE SEPARATION

IN DIFFLUGIA CORONA 41

13. REPRODUCTION IN STYLONTCHIA 43

14. INHERITANCE FROM MUTILATED PARENTS, IN DIFFLUGIA CORONA 45

15. REPRODUCTION IN AN INFUSORIAN (PARAMECIUM) IN WHICH THE
ANTERIOR END HAS BEEN CUT OFF . 46

16. REPRODUCTION FOR SEVERAL GENERATIONS OF A PARAMECIUM . 47

17. REPRODUCTION IN A PARAMECIUM BEARING A SMALL ABNORMAL
PROJECTION NEAR THE POSTERIOR END 47

18. REPRODUCTION IN A DEFORMED INDIVIDUAL OF PARAMECIUM . 48

19. DIFFLUGIA CORONA 51

20a. DIFFLUGIA CORONA 52

20b. DIFFLUGIA CORONA 53

21. DIFFLUGIA CORONA; PORTIONS OF FOUR FAMILIES 55

22. EIGHT DIVERSE FAMILIES OF PARAMECIUM, SHOWING VARIATIONS 57

23. DIFFLUGIA CORONA, TO SHOW THE CHARACTERS STUDIED IN THE
NEW WORK ON INHERITANCE 71

7

8 List of Illustrations

F10CRB PA<SB

24a. PARTS OF FIVE HEREDITARILY DIVERSE BRANCHES OF A SINGLE

FAMILY IN DIFFLUGIA CORONA 74

24b. PARTS OF FIVE HEREDITARILY DIVERSE BRANCHES OF A SINGLE

FAMILY IN DIFFLUGIA CORONA . 75

25. THE INFUSORIAN STYLONYCHIA 81

26. ARCELLA VULGARIS 83

27. TRYPANOSOMA BRUCEI 97

28. ORGANISMS USED IN DALLJNGER'S EXPERIMENTS ON THE EFFECTS

OF HIGH TEMPERATURES 98

29. CHROMOSOMES AND THEIR MATING 108

80. DIAGRAM SHOWING THE CHIEF PROCESSES IN THE CONJUGATION OF

PARAMECIUM CAUDATUM Ill

31. DIFFERENCES BETWEEN THE CHROMOSOMES OF THE NUCLEI IN THE
Two SEXES 115

32. THE ACTIVE AND PASSIVE PORTIONS OF CELL AND NUCLEUS IN THE
EGG OF THE WHITEFISH 117

33. BEGINNING OF CONJUGATION IN EPISTYLIS 121

34. SUCCESSIVE STEPS IN THE PROCESS OF CONJUGATION IN VORTICELLA
NEBULJFERA 121

35. DIAGRAMS OF THE MICRONUCLEAR PROCESSES IN CONJUGATION, IN
PAHAMECIUM AND VORTICELLA 122

36. MATING IN THE MOULD, MUCOR 123

37. MATING OF SIMILAR CELLS, FROM THE ALGA STEPHANOSPH.ERA . 126

38. CHILODON 129

39. CONJUGATION OF CHILODON 129

40. THE EXCHANGE OF THE HALF NUCLEI m PARAMECIUM CAUDATUM 131

41. MATING OF Two HALVES OF THE NUCLEUS OF SAME CELL . . 134

42. Two METHODS OF CONJUGATION m SPIROGYRA 135

43. PROCESS OF CONJUGATION OF Two BRANCHES OF THE SAME PLANT. 136
43a. BlPARENTAL INHERITANCE AND THE PRODUCTION OF DIVERSITY BY

CONJUGATION, IN CHLAMYDOMONAS 156

44. A FAMILY OF PARAMECIUM CAUDATUM, DESCENDED FROM AN Ex-

CONJUGANT, AND SHOWING HEREDITARY ABNORMALITIES . . . 160

45. THE SEPARATION OF THE Two GROUPS OF PAIRED CHROMOSOMES
INTO DIFFERENT GERM CELLS, IN THE INSECT NEZARA HILARIS. . 172

46. THB CHROMOSOMES, AND THEIR SEPARATION INTO Two NUCLEI
WITH REDUCED NUMBERS IN THE PROTOZOAN MONOCYSTIS ROSTRATA 176

List of Illustrations 9

FIGCBB PAGE

47. REDUCTION OF THE NUMBER OF CHROMOSOMES AT CONJUGATION IN
DIDINIUM NASUTUM 178

48. CONJUGATION AND REDUCTION IN THE NUMBER OF CHROMOSOMES

IN THE INFUSORIAN ANOPLOPHRTA BRANCHIARUM 180

49. THE CHROMOSOMES AND THE DIVISIONS PREPARATORY TO MATING

IN PARAMECIUM CAUDATUM 182

50. REDUCTION OF THE NUMBER OF CHROMOSOMES BEFORE MATING IN
THE RHIZOPOD PELOMTXA 183

51. DIAGRAM TO ILLUSTRATE How MENDELIAN INHERITANCE WOULD
OCCUR IN AN INFUSORIAN (PARAMECIUM) 189

52. CoNJUGANTS AND NoN-CONJUGANTS FROM A CULTURE COMPOSED OF
A MIXTURE OF Two RACES OF DIFFERENT SIZE, OF PARAMECIUM

AURELIA. 191

.33. PAIRS FROM A SINGLE RACE OF PARAMECIUM AURELIA, ILLUSTRATING

ASSORTATTVE MATING 192

LIFE AND DEATH

HEREDITY AND EVOLUTION IN UNICELLULAR
ORGANISMS

LIFE AND DEATH

HEREDITY AND EVOLUTION IN
UNICELLULAR ORGANISMS

General Survey of the Life History in the Protozoa, with
the Questions it Raises. Life, Death, Reproduction, Matmg,
Rejuvenescence, "Potential Immortality"

LONG ago, when Latin was the common language of
science, there was current a saying that nature is
greatest in the things that are smallest: Natura maxima in
minimis. Radl, in his History of Biological Theories, cites
this as a maxim which led to mere superficiality ; to the
degeneration of biology toward an amusement, with the
microscope as its instrument ; to neglect of the really great
problems of life. The naive literature of the old fashioned
microscopical societies, with their talk of the "golden tube,"
and of the "Oh, my! objects," passed from one member to
another for the consecutive delectation of their eyes, lends
some plausibility to this indictment; even yet the drama
of life seen in a drop of water has its purely aesthetic fasci-
nation. But such fascination is not inevitably inconsistent
with a serious value for studies of these creatures ; there
still exist students who, in spite of Radl's sarcasm, believe
that the simpler creatures have something to teach us on
even the deepest problems of life. I am going to try to

13

14 Life and Death, Heredity and Evolution

present what we have thus far learned from them on the
great questions of life and death; of heredity and evolution.
I hope that this may serve as an introduction and founda-
tion to a general understanding of these things ; and that we
may even find that the simplest organisms have a distinctive
and important contribution to make toward such under-
standing. Study of what actually occurs even to its smallest
details does indeed hamper uncomfortably the sweep of un-
trammeled theory, but if our aim is to attain truths that
are verifiable rather than theories that are magnificently
free, we shall welcome this result.

Let us set forth clearly at the beginning, that in these
lectures our interest will not be primarily in Protozoology,
but in Genetics : in the problems of life, its continuance and
reproduction; and that we deal with the lower organisms
only for the light they throw on these matters. With many
of the technical problems which most interest Protozoologists
we shall therefore have no dealings; on the other hand, the
facts which we use will be such as do form constituent parts
of the structure of Protozoology.

The traditional ground for hoping that the Protozoa
may aid greatly in understanding the foundations of life and
reproduction is this: As we pass from the complex organ-
isms to the simpler ones, we must find that life retains its
essential nature — for otherwise it would not be life — while
stripping off all merely adventitious details. The highest
organisms are of interest to us because they show the heights
to which life may rise; the lowest because they show the
fundamentals of life relatively unconfused. These lowest
organisms are commonly said to consist of a single cell;
whereas higher ones consist of an almost infinite number
of cells of diverse kinds.

This point of view has been challenged in recent times,

General Survey

15

as well as in earlier years. It is admitted that some of these
lower creatures appear to be extraordinarily simple as
compared with trees, birds and men: such are evidently the
bacteria among plants ; such are aina-bu and its relatives
among animals (see Figure 1). But in some of the Proto-

Figure 1. Simpler forms of Protista, a to g, Diverse kinds of bac-
teria, after Zopf, Engelmann and Fischer, h, Amoeba radiosa, after
Leidy.

zoa we find an astonishing complexity of structure (Figure 2,
Figure 3) and of function, so that some of the students of
the group, like Dobell (1911), maintain that there is no
ground for calling these organisms "simple" or "lower" or
even "unicellular" : that they are merely small organisms,
diverse in their plan of structure from the larger animals
and plants. But it appears evident that amoebse and bac-
teria are structurally simpler than vertebrates, in that they
consists of fewer kinds of differentiated parts; and this
seems to me true even for the more complex Protozoa, when
compared with the Metazoa. They may therefore be
properly called simpler or lower organisms, meaning thereby
that they have fewer differentiated parts than the higher

16 Life and Death, Heredity and Evolution

Figure 2. Some of the more complex Protozoa. A, Caenomorpha
tnedusula Perty, after Biitschli. B, Didinium capturing prey, after
Balbiani. C, Vorticella nebulifera O. F. M., after Biitschli. D, Fol-
liculina, after Mobius. E, Stentor roeselii Ehr., after Stein. F, Lacry-
maria olor, after Verworn. G, Stylonychia mytilus Ehr., after Engel-
mann.

animals. The interest of their study lies partly in dis-
covering what life can do with few differentiated structures.
Moreover, it seems clear that in their general plan of struc-
ture they are comparable to single cells of higher animals,
though, like such single cells elsewhere, they may be very

17

Figure 3. Structure of one of the most complex of the Protozoa,
Diplodinium ecaudatum Fiorentini, after Sharp (1914). A, mouth; B,
oral cilia; C, adoral membranelles ; D, oesophagus; E, oesophageal re-
tractor strands; F, ventral skeletal lamellae; G, entoplasm; H, ecto-
plasmic boundary layer; I, cuticle; J, caecum; K, rectum; L, rectal
fibers; M, ectoplasm; N, posterior contractile vacuole; O, micronucleus ;
P, suspensory fibers; Q, macronucleus ; R, anterior contractile vacuole;
S, posterior ciliary roots; T, dorsal membranellae ; U, operculum; V,
motor mass; W, circumoesophageal ring.

complex; they may properly therefore be called unicellular
organisms.

But whatever the facts as to their simplicity or complex-

18 '•Life and Death, Heredity and Evolution

ity, they do show us life and the passage of generations
following a very different course from that which we see in
ourselves; a course which opens to us many new vistas.
And they do present us a world of life in miniature in which
structures are condensed into small space, in which activities
are condensed into brief time. In a watch-glass on our
table we may in a week see generations come and go; may
observe the rise of whole faunas and floras ; their decline and
replacement by others; we may follow in successive genera-
tions the struggle for existence; may see natural selection
at work and discover its results. In a few days we may
see the birth, babyhood, youth and age of individuals, and
their replacement by descendants ; we may study the inherit-
ance of parental traits by the new generation, or the ap-
pearance of new traits ; we may observe how the population
changes with the passage of ages, — and all while we wait
for one of the changes of the moon.

This seems to present a wonderful opportunity for solving
in a short time some of the problems at which men have been
at work for ages, so that students have taken up this work
with enthusiasm. But unhappily, when we cease to regard
it as a mere fascinating spectacle, and desire to establish,
in the rigid and detailed manner required by science, just
what is happening in these teeming populations, and just
what are the laws that the events follow, we find the diffi-
culties very great, and our success perhaps less rapid than
we might hope. For to do this we have to come to know
these creatures individually; we have to work with them as
we would with guinea pigs or with calves. The distance
of size between them and ourselves is almost as difficult to
overpass as are the spatial distances between us and the
stars, so that studying them individually is a little like try-
ing to get acquainted with the inhabitants of Mars. To

General Survey 19

become personally intimate with particular amabae or in-
fusoria; to control their goings out and comings in; their
diet and personal habits; to interfere with their social and
domestic relations; to feed them and mate them; to make
them do and live as we want them to live, — this is what
we have to do if we are to really understand their lives,
their behavior, their growth, their matings, their heredity,
their evolution.

Some years ago I devoted myself to obtaining an intimate
personal acquaintance with the life and behavior of indi-
viduals among these creatures ; to study of their individual
biography and perhaps psychology. I had the honor to
present this to the public in a book on the Behavior of
Lower Organisms. Building on the experience thus gained,
I have since devoted myself to what happens in the passage
of generations in these creatures ; to a study of the biology
of races rather than of individuals; to life, death, mating,
generation, heredity, variation and evolution in the Protista.
I am going to attempt to present a picture of these matters
so far as our present knowledge makes possible. We shall
find that there are still many questions which are not yet
answered, but unsolved problems after all have their fascina-
tion ; and much has been learned ; this I shall try to present
and compare with what is known for higher organisms.

When we follow the lives of particular individuals in the
miniature jungle which a protozoan culture presents, we
come upon a fact that is most astonishing to one who knows
only the higher organisms. The creatures seem never to
die, save by accident. If we follow a single individual, we
find that after a time he divides into two (Figure 4).
Which is the parent, which the offspring? Each of these
again divides into two, and this continues for generation
after generation. Nowhere does a corpse appear; nothing

20 Life and Death, Heredity and Evolution

dead is found in the entire history of a race, save by accident.
The present living generation transforms directly into the
next one. Figure 5 illustrates the course of life and the
passage of generations in these creatures as compared with
that of higher organisms.

All this is so different from what we ourselves experience

Figure I. Division of an infusorian, Paramecium. Successive stages.

and from what we see around us in the animals that we know,
that it has always aroused the greatest interest and led to
many questions. What ! are these creatures really im-
mortal? Is there no decline, no old age, no natural death
in their history? Does their life realize the dream of
perpetual youth? Such questions, as you know, were put
long ago by the early students of these creatures, and for

General Survey

fl

almost a century the answers have been sought, through
speculation and reasoning, through observation and experi-
ment. After much conflict of opinion and many changes

B

Figure 5. Diagram of the descent of generations in Protozoa or
other organisms reproducing from a single parent; as compared with
that in organisms reproducing from two parents. The lines represent
the lives of individuals or germ cells, beginning at the left and passing
to the right. A, uniparental reproduction by fission. The line of
ancestry traced back from any individual at the right is always single;
and there is no corpse found at any point, the present body transform-
ing directly into the bodies of the next generation. B, biparental repro-
duction by union of germ cells, as in the higher organisms. The tri-
angular structures are the bodies, the lines the germ cells. The line of
ancestry from any individual traced back from the right forks at each
generation, becoming in a few generations multiplex. The bodies of
any generation are not continuous with the bodies of the previous gen-
eration— the latter dying, while the new bodies are produced by germ
cells from two diverse lines.

22 Life and Death, Heredity and Evolution

in the prevalent views, I believe that the last few years have
brought us the facts which answer these questions. It is
of these that I wish first to speak; they form a foundation
for all our knowledge of these matters in Protozoa.

Although it appears that these creatures do not naturally
die, it is equally apparent that they do reproduce. Every
day or two, or even more frequently, each individual divides,
and we have two where there was but one; in a few days
their number has multiplied enormously. And if we watch
them long and closely enough, we make another discovery
of great interest: after the passage of many generations
these creatures mate, as do higher animals and plants. Two
individuals unite and exchange parts of their bodies (figure
6), then separate and both continue to live and to reproduce
as before.

Figure 6. Conjugation of Paramecium.

Now if these creatures do not die, why should they mate
and why should they reproduce? In ourselves and in the
animals we know intimately, mating and reproduction are
the prelude to death; reproduction seems a method of re-
placing the individuals that are to die. What is the use
of these processes if those now alive are to continue to

Theory of Rejuvenescence 28

exist? Suppose that there were no death in man; there
would be no need for reproduction; and if reproduction
occurred it must soon result in overcrowding and in violent
killing off on even a greater scale than occurred in the
great war. And in these simple creatures the same over-
crowding must result, for reproduction every twenty-four
hours, without death, would in a few months pack all the
waters of the earth with a solid mass of these creatures.

Reflecting thus on these things, many biologists came to
the conclusion that these creatures must get old and die
after all ; that otherwise they would not mate and reproduce.
We see indeed that not every individual gets old, for they
continue to live and reproduce for generation after genera-
tion. So it was thought that it must be the passage of
many generations that brings on age. The creatures, it
was held, begin young and strong; they divide again and
again, gradually getting old and weak; it is only the indi-
viduals of these later generations that show the weakness of
age.

Now, since in higher animals mating and reproduction
bring it about that an old and worn individual is replaced by
one young and unworn, so, it was reasoned, must mating in
these lower creatures cause an old individual to be replaced
by a new one. But when we observe the process, we do not
see a new individual replace an old one after mating; ap-
parently the old one continues to live and multiply as before.
To avoid this difficulty it was concluded that in these crea-
tures mating must rejuvenate the old individual; must in
some way get rid of the age and wear, leaving the same
individual, but physically young.

This was the famous theory of rejuvenescence through
conjugation. It held that the young animal may live and
reproduce for many generations without mating, till thou-

24 Life and Death, Heredity and Evolution

sands of specimens have been produced from a single one.
But as this goes on, they gradually lose their vitality, get
old and decrepit, and must all in the course of time die,
unless mating intervenes to save them. On the question of
just what form this aging and decrepitude takes and just
what are its symptoms, there was much difference of opinion.
Biitschli and Baibiani believed that the power of reproduc-
tion gradually became less and less, so that division into
two became less frequent; this idea was accepted by many,
and has been maintained in recent years by Calkins; it is
perhaps the prevalent form of the theory. On this view the
occurrence of conjugation restored the reproductive power,
so that fission now continued as rapidly as it did before the
animals grew old. On the other hand, though the fact is
not generally realized, some of the chief upholders of the
theory of rejuvenescence emphatically rejected this idea
that aging showed itself in a decline of the rate of repro-
duction ; this is notably the case with Maupas, who asserted
positively that no such decline occurred before conjugation,
and that after conjugation reproduction was no more ener-
getic than before; and Richard Hertwig discovered experi-
mentally that Maupas' statement is correct. They believed
however that in other ways the animals become decrepit and
that they must die if conjugation did not occur.

Now, just what is it that happens in mating which might
be conceived to bring about rejuvenescence? There are two
main things that occur in conjugation; to get these clearly
in mind we must look for a moment at the structure of an
infusorian, and at the process of conjugation.

An infusorian resembles in the plan of its structure a
single cell of a higher organism, but has its nucleus in two
parts (Figure 7, a). One of these parts is large and seems
to be the active portion ; the other is very minute and appears

Conjugation 25

to be a sort of reserve of nuclear material. The large part,
or macronucleus, seems to take a part in the physiological
processes of the daily life of the cell. The reserve nucleus,
or micronucleus, seems to lie inactive, save at the time when
the creature divides ; then both the large, active nucleus and
the small reserve nucleus divide, so that both the new indi-

— c v

ma.

-c v

cv

Figure 7. Diagram of the process of division in Paramecium; suc-
cessive stages, c. v., Contractile vacuoles; m., Mouth; ma., Macro-
nucleus; mi., Micronucleus.

viduals produced contain half of each (Figure 7, c, d).
Sometimes there are several of the small reserve nuclei.

Now at the time of conjugation (Figure 8), in each of the
mated individuals the old active nucleus breaks up and
gradually disappears, being apparently absorbed like so
much food, by the rest of the body. The reserve nucleus
(mi., Figure 8), on the other hand, divides several times;
in Paramecium caudatum, for example, each divides twice

26 Life and Death, Heredity and Evolution

(A to C), so that four are produced. Three of these dis-
appear, as did the old active nucleus, and the fourth divides
into two halves (D). One of these halves passes over into

Figure 8. Diagram of the internal processes in the conjugation of
Paramecium caudatum; successive stages, ma., Macronucleus; mi.,
Micronucleus. The striated and spindle-shaped bodies are the micro-
nucleus in the process of division. In D the three smaller dotted clouds
are three degenerating micronuclei.

the other individual, with which mating is occurring (Figure
8, E), where it unites with the corresponding remaining half
of the reserve nucleus of that individual (F, G). That is,
the two conjugating animals exchange halves of their reserve

Conjugation 27

nuclei, so that each has after mating a new reserve nucleus,
composed half of reserve nuclear material from its own
body, half of reserve nuclear material from the body of its
mate (Figure 8, G.). Later this new reserve nucleus divides
into parts, some of which become new, large, active macro-
nuclei, while the others remain as minute micronuclei (see
Figure 30, Lecture 5).

So there are two main things in conjugation: (1) The
old active nucleus is replaced by parts of the reserve nuclei ;
(2) the two mating individuals exchange parts of their
nuclear material.

Now, evidently the replacement of the old active nucleus
by part of the reserve nucleus is just the sort of thing that
one would expect if there is to be .rejuvenescence; indeed, it
is rejuvenescence of the macronucleus. It looks very much
as if the old active nucleus might have gotten worn out or
used up in its activity, so that it has to be replaced by
reserve material which has not been used.

But what has all this to do with mating? If the point
is merely the replacement of worn nuclear material by fresh
material from the reserve store that each animal carries,
why need there be this complicated process of exchange of
nuclei? Rejuvenescence should occur just as well without
this exchange, without conjugation, as with it.

The occurrence of these two distinct things at conjuga-
tion,— replacement from a reserve, and exchange, — has al-
ways kept the theory of rejuvenescence ambiguous. Is it
the replacement from reserve material, or the exchange, that
makes the organism young again? Authors have as a rule
either not ventured to answer this question, or have not
clearly analyzed the process into its two elements, — speaking
merely of conjugation as a whole. Arguments based on the
replacement process have been used or accepted as argu-

28 Life and Death, Heredity and Evolution

ments referring to the process of exchange, and vice versa.
What was needed was some method of separating the two
processes, to see what effect each has by itself.

In the meantime, taking conjugation as it occurs, without
this analysis, men attempted to find out: first, whether the
organisms do grow old and die if conjugation does not
occur; second, whether conjugation does save them, does
make them young, does cause them to reproduce more ener-
getically.

The first question, as to whether without conjugation the
creatures degenerate and die, has cost an infinite amount
of labor to generations of investigators. It appeared that
as a matter of fact, if the animals were kept without con-
jugation they do die out in the course of time; such was
the result of the long continued labors of Maupas (1888,
1889). After some hundreds of generations without con-
jugation the animals weakened, became abnormal, sickened
and died. It is worth while to note Maupas' exact results
on this point. We may give them in the words of his own
summary : —

"I have kept six cultures until their final extinction by
senile exhaustion. The first (Stylonychia pustulata) be-
came extinct after 215 fissions; the second (Stylonychia
pustulata) after 316; the third (Stylonychia mytilus) after
319; the fourth (Onychodromus grandis) after 320 to 330;
the fifth (Oxytricha) after 320 to 330; the sixth finally
(Leucophrys patula) after 660." (Maupas 1888, p. 260). 1

But as other investigators took up the same sort of work,
a curious fact was found. As men began to be able to

' * It is worth noting that different infusoria showed great diversities
as to length of life; Leucophrys patula lived in Maupas' experiments
for twice as many generations as any of the other species. It should
also be observed that Maupas did not carry out such experiments on
Parameoium, the organism that has been most used for such work since
his time.

„ Conjugation 89

take better care of the animals ; to give them proper food,
and to vary their food, the organisms were found to live
longer and longer without conjugation, and to give less
indication of old age. Calkins (1904) kept the infusorian
Paramecium caudatum for 742 generations without conjuga-
tion, but they finally weakened and died. Enriques (1903)
kept Glaucoma scintillans for 683 generations without con-
jugation and with no sign of harmful effects; and he has
recently (1916) kept Glaucoma pyriformis for 2701 genera-
tions with no sign of degeneration. And finally Woodruff
(1917) has kept Paramecium aurelia for more than 6000
generations without conjugation; the stock at last accounts
was still flourishing, with no sign of aging, of degeneration.

Now the fact that one can by proper methods of culture
keep these creatures healthy for more than 6000 generations
without conjugation, shows that the degeneration which
came, under other methods of culture, in a few hundred
generations, was not evidence that conjugation is required,
but only that the culture methods were not good. It has
been found that some species of infusoria cannot stand, save
for a short time, the method of culture necessary if the
separate generations are to be accurately counted; others
can exist under these conditions for a greater number of
generations; other indefinitely; and that without conjuga-
tion.

Thus the result of the work so far done has been to con-
firm the view that the infusoria may live indefinitely without
mating. I believe that we may look upon this as one of
the secure results of science. There are many of the uni-
cellular creatures, particularly the bacteria, in which
nothing like mating is known. It is sometimes held that
such processes must yet be found in these creatures. But
the fact that infusoria, which do mate, may nevertheless live

80 Life and Death, Heredity and Evolution

indefinitely without mating, makes it probable that the ap-
pearances are correct, and that such organisms as bacteria
actually never mate.

Does this confirm the theory of the "potential immortal-
ity" of these creatures? It apparently does, if we are to
take that expression in its broadest meaning. But the real
question underlying that phrase is this: Does the exercise
of the functions of life itself necessarily result in deteriora-
tion, in senility, in final death? This was the question that
Maupas believed his experiments to answer affirmatively.
He says of his experiments, "They demonstrate that indeed
in the ciliate infusoria, as in so many, if not all, other living
things, the organism deteriorates, uses itself up, simply by
the prolonged exercise of its functions" (1888, p. 261).

The experiments we have described above show that con-
jugation is not required to remedy the wearing away of the
organism through the exercise of its vital functions. But
is this the complete story ? Is there after all no significance
in the fact that these creatures keep a reserve nucleus, along
with the active one? Does the active nucleus never require
replacement from the reserve?

This question too has recently been answered, mainly by
the work of Woodruff and Eidmann (1914). And in an-
swering the question they have succeeded in observing that
separation of the two processes in conjugation, of which
we said above there was so much need. These two processes
are: first, the replacement of the active nucleus from the
reserve; second, the exchange of parts of the nuclei in
mating. Woodruff and Erdmann observed the replacement
process occurring without mating and exchange. And in
doing this they have uncovered what is evidently one of the
most important phenomena that occur in the life of these
Protozoa; something that must form a background for all

Endomi&is

31

study of life and reproduction in these creatures. We must
therefore examine it with care.

In their race of Paramecium that lived indefinitely with-
out conjugation, Woodruff and Erdmann found that the
active macronucleus is replaced at rather short intervals

B

Figure 9. Diagram showing what occurs in endomixis, or the replace-
ment of the old active macronucleus by a part of the reserve nucleus
(micronucleus) in Paramecium aurelia. Constructed from the figures
and description of Woodruff and Erdmann.

The earliest stage (condition before the process begins) is shown at
the top; successive later stages from above downward. The larger black
bodies represent the macronuclei; the smaller ones the micronuclei. The
clear circles are the micronuclei that degenerate.

82 Life and Death, Heredity and Evolution

by parts of the reserve nucleus. Every forty to fifty gen-
erations the macronucleus breaks up and is absorbed and
disappears, just as happens when conjugation is to occur.
Then each reserve nucleus divides (in Paramecium aurelia)
so as to produce eight (see Figure 9). Six (or seven)
of these disappear by absorption, like the active nucleus.
The remaining one later produces by division the new active
nucleus (macronucleus), and the reserve nucleus or nuclei.
That is, the active nucleus is replaced every forty or fifty
generations by material from the reserve store. This whole
process Woodruff and Erdmann call endomixis.

This process has been found to occur, not only in the
single race in which Woodruff and Erdmann first found it,
but in many other races, and in another species of Para-
mecium. And in another infusorian, Stylonychia, the Rus-
sian investigator, Fermor (1913), states that a similar proc-
ess occurs at the time of encystment. After multiplying
for five or six weeks the animals lose all their appendages
and other organs, gather into a sphere and form a sort
of thin shell about them. Then the two active nuclei disap-
pear and are replaced from the reserve nuclei, which have
united to form one. The very brief account of this process
by Fermor was published earlier than the work of Woodruff
and Erdmann. It has been stated by Calkins that a similar
process occurs in the encystment of Didinium (1915 a), and
in Uroleptus (1919). It seems probable that it will be found
generally in the infusoria.

The discovery of this process of replacement of the active
nucleus by the reserve nucleus evidently puts a new face
on the matter. To the question whether the living sub-
stance uses itself up in functioning, so as to require replace-
ment, it seems to answer "yes !" even more definitely than any
discovery that conjugation was necessary would have done.

Endomixis 83

The question becomes of extreme interest whether such a
process as this is of general occurrence ; whether it is neces-
sary in order that aging and death shall not occur. Many
investigations are therefore at this time directed upon
endomixis. Certain questions must occur to everyone ; some
of these have already been answered.

First, is this replacement perhaps merely something
brought about by unfavorable conditions of the environment,
and not necessary if all conditions continue good? Jollos
(1916), Young (1917), and Woodruff (1917) find that
under unfavorable conditions the replacement of the active
nucleus from the reserve is brought about more quickly than
would otherwise occur. But under the most favorable con-
ditions the process does not cease, and under uniform con-
ditions it takes place at uniform intervals. Woodruff found
that whenever the process of replacement ceases entirely, the
race dies out. This evidence therefore indicates that the
process is a necessity for continued life.

Enriques (1916) attempted to test the matter by studying
an organism in which there were no periods of slow fission,
such as are found while the replacement is occurring. He
found such an infusorian in Glaucoma pyriformis, which
under the conditions he used may produce 10 to 13 genera-
tions a day, and continue this without interruption. He
kept this organism for 2701 generations, or for more than
8 months, during which there were not less than 9 genera-
tions every single day. The culture was still in progress
at last accounts; there was no sign of degeneration, and
no periods of slow fission. Enriques therefore holds that
endomixis has not occurred. He concludes that endomixis
is not necessary to continued life, just as conjugation is not.

It is evident however that this conclusion is insecure; it
is not impossible that endomixis occurred without interrupt-

84 Life and Death, Heredity and Evolution

ing the regular series of fissions. The matter requires actual
cytological study before we can know whether or not any-
thing of the sort occurs in Glaucoma.

In Parainecium, Woodruff and Erdmann (1914) found
that while the replacement is going on, and for a little before
and after, the rate of reproduction is slower than at other
times. Does this mean a waning of vitality, which is cor-
rected by the replacement? Jollos (1916) found that under
favorable and uniform conditions, the slowing of the fission
rate is limited sharply to the period in which the processes
of replacement are occurring. This slowing of cell division
would therefore appear to be merely a natural result of the
fact that while the complicated changes of endomixis are
occurring, the organism does not so readily • undergo at
the same time the involved processes of fission. It is there-
fore not clear that there is any indication of loss of rigor,
of senile changes, setting in before each replacement occurs.
The process seems to be carried on, like many others in
nature, with such a "margin of safety" that there is no
indication of exhaustion before it occurs; it takes place be-
fore there is a pressing need for it.

Thus it appears that in these organisms nature has em-
ployed the method of keeping on hand a reserve stock of a
material essential to life ; by replacing at intervals the worn
out material with this reserve, the animals are kept in a state
of perpetual vigor ; not, as individuals, growing old or dying
a natural death. Nevertheless, a wearing out process, such
as might be called getting old, does occur in the structures
employed in the active functions of life, and these must be
replaced after a time of service. So far as the conditions
in these organisms are typical, deterioration and death do
appear to be a consequence of full and active life ; life carries
within itself the seeds of death. It is not mating with an-

Continuity of Life 35

other individual that avoids this end ; but replacement of the
worn material by a reserve.

It results that the continuity of life in the infusoria is in
principle much like that in ourselves, though with differences
in details. As individuals, the infusoria do not die, save by
accident. Those that we now see under our microscopes
have been living ever since the beginnings of life ; they come
from division of previously existing individuals. But in just
the same sense, it is true for ourselves that everyone that
is alive now has been alive since the beginning of life. This
truth applies at least to our bodies that are alive now ; every
cell of all our bodies is a piece of one or more cells that
existed earlier, and thus our entire body can be traced in
an unbroken chain as far back into time as life goes (see
the diagram, Figure 5). The difference is that in man
and other higher organisms there have been left all along
the way great masses of cells that did not continue to live.
These masses that wore out and died are what we call the
bodies of the persons of earlier generations; but our own
bodies are not descended by cell division from these; they
are the continuation of cells that have kept on living and
multiplying from the earliest times, just as have the existing
infusoria. From our own personal point of view it seems
unfortunate that the mass of cells which is next to wear out
and be left behind in the chain of life is that with which our
own selves seem to be bound up ; but certain samples of our-
selves may continue to live indefinitely,! like the infusorian.
The great mass of cells subject to death in the higher
animals dwindles in the infusorian to the macronucleus ; this
alone represents a corpse. But the dissolution of this corpse
occurs within the living body. It resembles much such a
process as the wasting away and destruction of minute parts
of our own bodies, which we know is taking place at all

36 Life and Death, Heredity and Evolution

times and which does not interrupt the life of the individual.

What now are we to think of conjugation in the light of
these new facts? In conjugation there occurs the same re-
placement of the old active nucleus by a part of the reserve,
which we have seen to take place also without conjugation;
but with the additional fact of an exchange of pieces of the
new nucleus between the two conjugating individuals. If
this replacement means rejuvenescence when it occurs with-
out conjugation, there is no reason to deny it that meaning
in conjugation; conjugation too should result in rejuvenes-
cence. But what is the significance of the additional feature
— the exchange of nuclear parts between the two conjugat-
ing animals? This seems not necessary to rejuvenescence,
since the replacement of the old by the reserve may occur
without it. Why then does the exchange take place?

In many higher organisms we observe that the mating
and the union of parts of two different cells which we call
fertilization are necessary in order that reproduction shall
occur; the egg does not develop unless fertilized. But in
these Protozoa we observe that reproduction occurs for gen-
eration after generation without this mixture of two diverse
cells; and we have just seen that rejuvenescence likewise
occurs without such a mixture. In these lower creatures we
find separated therefore two processes which in many higher
animals are so closely bound together that we get the im-
pression that they are inseparable; that development neces-
sarily depends on fertilization. But even in many higher
animals development may take place without fertilization;
these, taken with the facts in the Protozoa, show that there
is no generally necessary relation between the two things.
It appears, as we shall see later, that the close association
of reproduction with a union of two cells is only a special
peculiarity of certain organisms: something that might be
called a special adaptation.

What is the Result of Mating 37

What ground then can we possibly give for the inter-
change of parts of the two individuals that conjugate? We
shall take up this question in all its aspects later; here I
wish to bring up but one of the possibilities. We observe
that after the two individuals have conjugated and sep-
arated, they are no longer just what they were before. Each
is now formed of parts of two individuals, — a body and half
the nucleus from one; half the nucleus from the other.
Will the two individuals therefore now be diverse in other
respects from what they were before? Will their general
characteristics be changed? Will they behave differently;
will they develop differently; will they produce young of a
different sort ? In other words, are the other characteristics
of the two individuals mixed as well as their nuclei?

This we know is what happens as a result of fertilization
in higher organisms ; the young produced inherit from both
parents. Does it happen also in the Protozoa? If so, it
will give us an understanding of the exchange of parts in
mating.

This raises for us the problem of heredity in the Protozoa.
Do the young produced by any given parent inherit also
from the individual with which that parent has mated?

But this is heredity in its most complex form. In the
Protozoa, as we have seen, we have, for generation after
generation, reproduction from a single parent by simple
division. This presents the problem of heredity, and also
those of variation and evolution, in the simplest possible
form, and we shall do well to study the problems here
before we take them up in cases where two parents are
involved. We shall therefore examine this matter in our
next lecture, and later take up the entire natural history of
mating.

II

Heredity and Variation in Protozoa, in Reproduction from
a Single Parent. "The Inheritance of Acquired Characters."
The Existence of Many Diverse Stocks in a Single Species.
Constancy of the Stocks. "Pure Line Inheritance" and the
Results of Selection. Difficulties for the Theory of Evolu-
tion.

first chapter led us to the question of heredity in
the Protozoa. In the present chapter we take up
the study of heredity and variation in the simplest kind of
reproduction, where the offspring1 are produced by the divi-
sion of a single individual into two. By way of introduction,
let us bring this into relation with the problem of life and
death, which we have already considered. We have seen
that these organisms are so constituted that they live in-
definitely, with no intervention of natural death of the in-
dividuals ; yet we have seen also that they continue to
reproduce. The inevitable result is that more individuals
are produced in each species than nature can provide space
and opportunity for, so that most of them are condemned
to violent and unnatural death. Is there anything changed
by this continual over-production, with destruction of the
majority? Are those produced exactly like those that
existed before? Or do the animals change as generations
pass, so that some are better fitted for the conditions that
they meet, and therefore continue to exist, while others are
killed off? That is, can we see evolution occur as we watch
these creatures through generation after generation?

38

Heredity m Protozoa

39

Turning now to the facts, we know that when one of
these animals reproduces, it merely divides into two; the
parent simply transforms into two offspring. Are the two
just like the parent from which they came? When we

Figure 10. Reproduction in Amoeba; successive stages. After F. E.
Schulze.

examine the facts in amaba (Figure 10), we can see no
reason why they should not be ; they merely are the parent,
but now in two parts; the difference in size between the
formerly existing parent and its two progeny is quickly
remedied by a little growth. So at first view the problem
of heredity seems in these creatures absolutely simple; the
progeny are the parents, merely divided; so they must be
like the parents.

But if we don't stop at amoeba, but examine other Proto-
zoa also, we find that the matter is not so entirely simple
after all; indeed, we find that heredity presents the same
problems that it does in higher animals. Take for example

40 Life and Death, Heredity and Evolution

a close relative of amoaba, — Difflugia (Figure 11), which
is merely an amreba with a shell. In reproduction the two
products do not receive half the parental shell ; if they did,
as you can readily see, they would indeed not be like the

Figure 11. Reproduction in Difflugia. After Verworn (1888).

parent. What happens is this : The shell consists of sand
grains, embedded in a hard chitinous substance. As the
parent creeps about in its daily life, it takes up sand grains
and stores them in the interior of its body. At reproduction
the protoplasm of the parent swells and projects from the
mouth of its shell (Figure 11, A). This projecting mass
takes a form similar to that of the parent (B). The sand
grains within the parent body pass out into the projecting
mass, come to its surface, and spread over it (C, D). They
are embedded in a fluid secretion which now turns hard, so
that they form a shell like that of the parent. The two
shells are in contact at their mouths (D). Now the common
mass of protoplasms divides into two, and the two individuals
separate — one retaining the old shell; the other with the
new one.

Now you see that it is by no means a simple matter of

Heredity in Protozoa

41

course that the new individual should have a shell just like
that of the parent. Its shell is produced anew. The parent
shell may have a peculiar form or structure; in some cases

Figure 12. Two parents, with their offspring, just before separation,
in Difflugia corona. The parents are above, the offspring (slightly
lighter in shade) below. After Jennings, 1916.

for example, as you see in Figure 12, it bears spines of a
certain length and form and in a certain number. There is
no simple evident reason why the progeny should produce
a shell of the same form, with spines of the same length and
number. These things are not by any means merely handed
on bodily from parent to offspring. On the contrary, just

42 Life and Death, Heredity and Evolution

as in higher animals and ourselves, the progeny produce
anew their peculiar characteristics. The problem of heredity
is : — Why should they produce the same sort of characteris-
tics that their parents produced? The problem is of just
the same sort in the Protozoa that it is in ourselves. In such
a case as Difflugia the answer which suggests itself takes
something of this form: the offspring produce the same sort
of shell that the parents did, because they are made of the
same sort of material. This answer is indeed little but a
form, but it is a form into which a more complete answer
will have to fit.

Before inquiring further as to just how closely the off-
spring do produce the same characteristics as the parents,
let us look at another example or two, showing in a still
more marked way the nature of inheritance. In some of the
more complex Protozoa, such as the hypotrichous infusoria,
we find that the body bears a great number of organs of
definite form and number, arranged in a precise manner.
Observe in this Stylonychia (Figure 13), the numerous leg-
like or fin-like appendages, for creeping or swimming. When
the animal reproduces, it divides cross-wise, and if it merely
divided without any rearrangement of parts, you can see
that the two progeny would be most unlike the parents. As
a matter of fact, during reproduction all these organs dis-
appear; they are apparently gradually absorbed into the
body. Then on each half of the body there appears (even
before the disappearance of the old organs) a new group
of minute projections (Figure 13, B), all close together;
not arranged at all as were the appendages of the parent,
and not showing the differences of size and structure that
were found in the parent. These small projections now
proceed to change place, scatter themselves, and take up
positions corresponding to those of the appendages of the

Heredity in Protozoa

43

parent (Figure 13, C) ; at the same time they transform in
various ways, till they have produced anew a set of organs
like those of the parent, of the same number, form, size and
arrangement.

Figure 13. Reproduction in Stylonychia, after Wallengren (1901).
A, Parent before reproduction. B, Appearance of the two groups of
small projections that are to form the appendages of the two offspring.
C, Division is occurring; the two groups of embryonic appendages are
scattering out to take up their final positions. The old appendages
have not yet disappeared.

All this illustrates the general nature of the problem of
heredity in both lower and higher organisms; the features
which the offspring are said to inherit from the parents they
really produce anew for themselves, and the problem is as to
why and to what extent they produce the same things that
the parents did.

There are certain exceptions to this general rule that the
progeny have to produce anew what they inherit; a few

44 Life and Death, Heredity and Evolution

things are directly handed on from parent to offspring.
These few things are of great importance, for it is these
that in some way provide the foundation for the new pro-
duction of the other structures. In the Protozoa these are
as follows: (1) The halves of the nuclei; in cases where
there exist separate active nuclei and reserve nuclei (macro-
and micro-nuclei), a half of each of these is passed on to
each of the progeny (see Figure 7). (2) Secondly, a half of
the general protoplasmic mass, or cytoplasm, goes to each
of the progeny. As we have seen, the particular organs
borne by this mass usually are not handed on bodily, but
first disappear and are then produced anew by the offspring.
Some few definite organs are in particular cases passed on
bodily, but even in these cases at least half of them must
be produced anew, — else of course they would occur only
in half as great number in each of the two offspring. All the
precision of the process of division and handing on is seen
in the nucleus (see Figure 49, page 182), so that it seems
probable that its different parts, each so accurately divided,
have special and diverse functions to perform in the
production of the new organs of the progeny. Just what
part is played by each of these things that are directly
handed on from parent to offspring, in producing the final
characters of the progeny, is one of the chief questions of
heredity. .

When we have gotten to our present point in the examina-
tion of reproduction and heredity in these creatures, we shall
not make the mistake of some of the earlier writers, as to
the inheritance of acquired characters in the Protozoa. By
acquired characters we mean characters that an individual
did not inherit from its parents, but which were produced
by special conditions during its life. Since the parents mere-
ly divide and become the offspring, it was set forth as a

Heredity of Acquired Characters

45

matter of course that in the Protozoa the offspring would
receive the acquired characters as well as the other char-
acters of the parent. But as we have seen, it does not
receive even the other characters of the parent ; but produces
them anew. Is there any reason why the offspring should

Figure 14. Inheritance from mutilated parents, in Difflugia corona.
The three parents, A, B and C, have had their shells and spines broken.
The offspring of each is seen below it, with normal, unmutilated shells
and spines. The mutilations of the parents are not inherited by the
offspring. (From observations by the author.)

produce anew characters that the parent has acquired merely
accidentally, owing to special circumstances? Let us look
at a few of these. There is no place so favorable for getting
in a simple way a clear idea of the problems and difficulties
involved in the "inheritance of acquired characters" as in
the Protozoa. To many persons who have not examined the

46

Life and Death, Heredity and Evolution

details it seems extraordinary that anyone should doubt that
acquired characters are inherited.

Let us examine first the simplest possible case, of an ac-
quired injury or mutilation. Suppose that in Difflugia one
or more of the spines is broken off, or a hole is broken in the
shell (Figure 14). Is there any reason why when this
animal reproduces, the progeny should have a corresponding
broken spine or broken shell? The offspring produces anew

Figure 15. Reproduction in an infusorian (Paramecium) in which
the anterior end has been cut off. At the first division the posterior
offspring is quite normal, and by the third division even the anterior
offspring has regained its normal anterior tip. Only the divisions of
the mutilated individual are represented, as all others produce normal
offspring. After Jennings, 1908.

its own shell and spines; what sort it shall produce under
given outer conditions depends on the nature of its proto-
plasm and nucleus. These are not altered by the breaking
of the shell or spine of the parent, and as a matter of fact
we find that the offspring produce entire shells and spines,
just as their parents did. The injuries acquired by the
parent are not inherited. Again, look at an infusorian with
its anterior tip cut off (Figure 15). Is there any reason
why the offspring produced from the hinder half should have
its anterior tip cut off? As a matter of fact, it has not.

Heredity of Acquired Characters

Figure 16. Reproduction for several generations of a Paramecium
bearing a large projection on its aboral surface. At each division only
one individual receives the projection; all other offspring and descend-
ants are normal. After Jennings, 1908.

Figure 17. Reproduction in a Paramecium bearing a small abnormal
projection near the posterior end. All offspring but one are normal
from the beginning, and even the abnormal individual becomes normal
after two divisions. After Jennings, 1908.

48 Life and Death, Heredity and Evolution

Or observe an infusorian in which a new structure — a pro-
jection for example — has been produced through some ac-
cident (Figure 16). Will this thing be inherited at repro-
duction? It will naturally be carried on mechanically by a
single one of the individuals at reproduction, but the rest
do not produce it. After a thousand new individuals have
been produced, one of them may still have the new structure,
but no more than one. As a matter of fact, such a new
structure is usually gotten rid of completely in the making
over that accompanies reproduction (Figure 17). I have

Figure 18. Reproduction in a deformed individual of Paramecium.
After the second division all descendants are normally formed. After
Jennings, 1908.

examined a large number of such cases in the infusorian
Paramecium; the acquired peculiarities are not inherited
(see Figures 15 to 18).

So there is no simple direct inheritance of acquired char-
acters in the Protozoa, any more than there is in the higher
organisms. The progeny have to produce the characters
that they get, just as the parents did, — and they usually
produce what the parent produced when it developed, — not
what the parent may accidentally happen to have when it
divides. The progeny start where the parent did, as a
rule.

Of course if we could get the fundamental constitution of

Heredity in Protozoa 49

the organism to change — say the chemical nature of its
nucleus and cytoplasm — we might expect it to develop in a
new way, and produce new structures ; and as this would be
repeated in later generations, we should have the new char-
acters inherited. But this is a deep and difficult matter;
we shall take it up later. What we wish to bring out now
is the fact that the mere existence of an acquired character
in the parent presents no reason for expecting the progeny
to produce that character anew, and that as a matter of
fact they do not ; such acquired characters are not inherited,
either in Protozoa or in more complex organisms.

We have seen thus that the progeny do not receive their
organs ready made from the parent; that on the contrary
they start from just the same condition the parents did —
an undifferentiated condition without organs, on the whole
— and produce their characteristics anew. The ground for
their producing the same characteristics as the parents
lies precisely in this fact, that they start just as the parents
did.

Having thus examined the groundwork for inheritance, we
wish to look at the matter more minutely. Is it true that
the offspring start just where the parents did, and that they
produce just what the parents did? Or is there a gradual
change as generations pass, so that evolution occurs?

If the progeny begin just where the parents did and de-
velop in the same way, then if we begin with a single parent
and obtain from it great numbers of progeny in successive
generations, we shall find that they are the same at the
end as they were at the beginning; and that all are alike
(save in so far environmental differences have made diver-
sities). There will be no opportunity for some to be pre-
served because they are better fitted to live, while others
die because ill fitted. Is this the situation, or do inherited
variations come on as generations pass?

50 Life and Death, Heredity and Evolution

This problem of the origin of Inherited variations, and of
the nature and grounds of evolution, meets us in these
animals in a peculiarly simple form. For in their repro-
duction from a single parent it is not complicated by the
continual mixing of diverse lines of descent, which enormous-
ly confuses the matter in higher organisms. In creatures
in which reproduction is always from two parents, the de-
scent of generations takes the form of a network, such as
illustrated at B in Figure 5. If we attempt to trace back
the ancestry of any one of the individuals indicated by the
lines at the right, we find that it is a mixture of many lines
of descent, with diverse hereditary characters ; in any given
past generation many ancestors of the present stock appear.
It becomes extremely difficult or impossible to predict what
hereditary characters it should show, or whence it has de-
rived those that do appear; and it is hardly possible to
distinguish an actually new character from one resulting
from the mixture of earlier stocks. In the Protozoa while
descent from a single parent is in progress, the passage of
generations takes the form indicated at A in Figure 5.
There is no mixing of diverse lines, and in any past genera-
tion there is but one ancestor for the existing stock ; descent
can be traced backward in a single line. On account of this
relatively simple state of affairs, the origin of variations and
the course of evolution has been much studied in the Proto-
zoa. We will illustrate the conditions found by means of a
series of figures of Difflugia corona (Figures 19-21).

When we examine a single species, be it a bacterium, a
rhizopod (as Difflugia corona) or a ciliate infusorian (as
Paramecium aurelia), we find a great diversity in the in-
dividuals, in their form, their structure (Figure 19), and
their physiology. Part of this diversity is, in some of these
creatures, due to the different conditions under which they
are living. But if we bring them all into the same conditions

Variation m Protozoa

51

Figure 19. Difflugia corona; collection of individuals to show the
variations in size and form; in number, length and shape of the spines,
and the like. All drawn to the same scale. (The numbers are the
designations of the families to which the individuals belonged.) After
Jennings, 1916.

52 Life and Death, Heredity and Evolution

FIG. 20o.

Inheritance in Difflugia

53

FIG. 206.

Figure 20. Difflugia corona. Parent and immediate offspring in 18
diverse families, all drawn to the same scale, to show the variation and
the inheritance of the parental characteristics by the progeny. In each
pair the parent is above, its offspring below, the two connected by a
line. If each family is bred for many generations, it continues to
remain true to the type shown. (The numbers are the designations of
the families to which the individuals belonged.) After Jennings, 1916.

54 Life and Death, Heredity and Evolution

and allow them to reproduce, we find that the offspring re-
peat in large measure the peculiarities of their parents
(Figure 20). That is, the particular characteristics of the
parents are inherited, — quite independently of diversity in
conditions. If we allow each of the diverse parents to re-
produce for generation after generation, we find that in each
case the peculiarities of the original stock are retained
(Figure 21). That is, each single species, such as this one
of Difflugia corona, consists of a large number of hered-
itarily diverse strains or families ; of strains remaining di-
verse for generation after generation.

This is one of the facts of capital importance in the
biology of these organisms ; something that has to be kept
continuously in mind in all attempts to work with them or
to understand them. It forms the key and explanation for
many remarkable phenomena in their lives. We shall there-
fore look at the concrete facts for a number of typical cases,
and examine their results in relation to heredity and varia-
tion.

In Difflugia corona the number of hereditarily diverse
strains that have been found is indefinitely great. They
differ in size and form, in the number of spines, in the length
of the spines, in the number of teeth surrounding the mouth.
Different strains have hereditarily different combinations of
these characters; some have large shells with few spines;
others have large shells with many spines, and so on for
other combinations.

Besides these marked structural peculiarities, the strains
of Difflugia differ in many other ways not apparent to the
eye. Some of the strains multiply rapidly, others slowly.
Some are very hardy and easily cultivated in the laboratory ;
others are delicate, dying out under artificial conditions.
Some are very active, others quiet. Some are adapted to

Inheritance in Difflugia

55

Figure 21. Difflugia corona; portions of four families, to show the
inheritance of the diverse combinations of characters. All individuals
of any column are descendants of the one at the top. The num-
bers at the top are the designations of the different families. Ob-
serve that in family 198 all are small, but with numerous spines; in
197, they are larger, but with few spines. In 324> the individuals are
large, with large spines; in 323 they are of about the same size, but
with small spines. The families remain true to such peculiarities, no

56 Life and Death, Heredity and Evolution

one set of conditions, others to other conditions. The exist-
ence of these strains presents an enormous diversity within
the species ; anything that we learn about one strain cannot
be transferred directly to another. A similar condition of
affairs has since been found to occur in other Rhizopods.
Root,1 (1918) shows that many diverse stocks exist in
Centropyxis aculeata; and Hegner, 2 (1918) shows the same
to be true in Arcella dentata.

In many other Protozoa most of the differences between
the stocks are in respect to the characters not readily de-
tectible by the eye. In Paramecium caudatum, or Para-
mecium aurelia, for example, the visible differences between
the strains are mainly in respect to size, and since the size
is changed greatly during growth, it requires thorough study
to detect the differences of strain. But such study shows
that strains of different size do exist (Figure 22). Within
each strain there is great variation of size among the dif-
ferent individuals, owing to differences of growth and of
environment. But each strain or family has its own char-
acteristic average size. If we pick out any individual of a
given strain and allow it to produce many offspring, we shall
find that their average size will correspond to that of the
family from which they came; an individual from a larger
strain will produce larger progeny ; one from a smaller strain
will produce smaller progeny. Thus a "population" of
Paramecium as we find it in nature is made up in the way
shown in Figure 22. There are many strains, diverse in
mean size, but each with many individuals of diverse size.

In Paramecium, as in Difflugia, these strains differ in other
respects also. I found that some multiply rapidly, others
slowly; that some conjugate frequently, others rarely.

'Root, F. M., Genetics, March, 1918.

' Hegner, R. W., Proceedings of National Academy, September, 1918.

Diverse Strains in Protozoa

57

Figure 22. Eight diverse families of Paramecium, showing varia-
tions. Each row represents a single family, showing the maximum,
minimum and intermediate sizes of individuals of the given family.
The differences in size within the family are due to differences in
growth and environment. The differences in average size between the
families are hereditary.

The numbers show the lengths in microns. The mean length for the
entire set together is given by the perpendicular line at 155 microns.
The mean size for each family is that of the individual above which is
placed a + sign. After Jennings, 1909.

Hutchison (1913) found that they differ in their resistance
to heat, — some strains standing a higher temperature than
others; the same thing was observed by Jollos (1913).

58 Life and Death, Heredity and Evolution

Jollos observed also that some strains are more resistant
to poisons than others. Hance (1917) found that a cer-
tain race of Paramecium caudatum has a tendency to pro-
duce one to three extra contractile vacuoles, so that its
members may have three to five of these, in place of the two
found in most races. Powers and Mitchell (1910) found
a race of Paramecium that had several micronuclei, in place
of the single one, or the pair, commonly found.

All together, it is clear that the different races of Par-
amecium present the greatest diversities in all sorts of
structural, and particularly physiological, characters, — so
that from our knowledge of the biology of one race we cannot
be certain as to that of the others.

A similar condition of affairs is known to exist in many
species of bacteria. Diverse families exist, differing in their
nutritive peculiarities, in their resistance to chemical agents,
in their virulence as producers of disease; in the way they
grow on artificial media, and the like.

The condition of affairs has been found to exist in all
the Protista that have been thoroughly studied from this
point of view. It is probable that it will be found in all
species ; certainly it would be of much interest to examine
thoroughly any species that seems to be uniform, in order
to discover if there is such a thing as a species that does
not consist of hereditarily diverse stocks.

The same condition of things is likewise found in higher
organisms. It is worth while to recall the name and work
of the man who first recognized that the so-called species of
animals and plants are really made up of a great number of
hereditarily diverse stocks, which remain quite distinct so
long as they are reproduced without crossing; for this work
was done long ago, and, like the work of Mendel, remained
for many years quite without influence on the world. In

Diverse Strains in Higher Organisms 59

recent years it has been rediscovered, and some believe that
it is almost worthy to rank with the great work of Mendel,
whose obscure fate it long shared. The discoverer, Alexis
Jordan, was, like Mendel, a devout Catholic ; and was guided
in his experimentation (unlike Mendel, apparently) by his
theological beliefs. He did not believe in the variability of
organisms, as taught by many prevailing doctrines, but
maintained that the differences within a species that were
commonly cited as variations were in reality permanent dif-
ferences between races. So as early as 1854 he undertook
the culture in his garden of certain common plants, notably
Draba verna, the common little weed called whitlow grass.
In ten years he was able to show that this contained ten
diverse races ; after twenty years' culture he announced that
he had now found 53 races ; and after culture for thirty
years he could show that there were 200 permanently diverse
stocks of Draba verna. He discovered the same thing to
be true for a number of other plants, and maintained there-
fore that his faith had been verified; the differences found
within a species were not variations in the sense of actual
changes which occurred, but merely permanent diversities,
which Jordan believed had existed ever since the organisms
were created; instead of variations that occurred, there was
multiplicity that existed.1

Now whatever we may think of Jordan's line of argument,
the facts which he set forth have been confirmed in recent
years for a great number of organisms. And his direct
conclusion from those facts likewise stands fast. The dif-
ferences that we observe among the members of a species are
in the overwhelming majority of cases not "variations" in
the sense of being due to recent actual changes in the hered-

irThe facts as to the work of Jordan are taken mainly from Lotsy,
1916 and 1916 a.

60 Life and Death, Heredity and Evolution

itary constitution. On the contrary they are merely lasting
hereditary diversities between stocks whose origin has not
been observed. The situation is very greatly confused when
these diverse stocks continually intercross, as happens in so
many higher organisms, but it is not thereby essentially
changed.

The situation in these lower organisms is very much what
it would be in man if in man new individuals were regularly
produced by the division of those already existing. We
have reason to believe that this practically does occur in the
case of identical twins; they are produced by the division
of a single egg, which if it had not divided would have pro-
duced but one individual (see Newman, 1917). If this
occurred in man regularly and frequently, as it does in most
Protozoa, we should find that the human population con-
tained great numbers of individuals as precisely alike as are
identical twins. Each of us would meet his precise counter-
part at every turn. All these closely similar individuals of
one type, taken together, would correspond to a single one
of the stocks or races of the Protozoa. And as in the
Protozoa, there would exist great numbers of such diverse
stocks ; in man as many as there now exist hereditarily dif-
ferent individuals. That is, each person present in this
room would represent a diverse stock or race; for each
person has a constitution hereditarily diverse from every
other (save in the case of identical twins).

Now this fact that a species consists of a great number
of hereditarily diverse stocks or races, often differing in
only minute particulars, throws a most unexpected, and to
many persons unwelcome, flood of light on many supposed
studies of evolution, and particularly on the effects of
selection in the breeding of organisms, — giving to such
studies a significance quite diverse from that which they

Selection Among Diverse Strains 61

were supposed to have. A great school of biologists, the
immediate followers of Darwin, forming the so called English
biometrical school, set themselves the problem of measuring
variation, inheritance, the effects of selection, and from
these the rate of evolution. In so doing they assumed these
existing differences as variations, and based their calcula-
tions upon these; they found of course a high degree of in-
heritance from diverse parents, and they found that by
selection rapid progress could be made in changing the
species. But if we examine a species made up of a lot of
hereditarily diverse strains (for example, Difflugia corona),
it is evidently easy by selection of a particular character to
obtain a set of animals that differ from the average in that
respect; one merely picks out representatives of the races
that have the character for which we are selecting. Thus,
in Difflugia corona, if one desires to increase the average
numbers of spines, he will pick out parents with many
spines. These belong to races in which a large number
of spines is hereditary, so that after selection the progeny
produced will have a higher number of spines than was the
average for the species before selection. By continuing
the process of selection, we gradually exclude more and more
completely the races with few spines, and so by selection we
make steady progress in increasing the number of spines in
this animal. Similar results follow from selecting for any
other inherited character; and in any species composed of
diverse races, the same sort of results are reached.

Thus in a population of Paramecium we can easily obtain
by selection all sorts of apparent hereditary alterations in
size ; in nutritive peculiarities ; in resistance to heat or to
chemicals; in reproduction; in all sorts of characters. All
these and many other results have been produced again and
again, in this and in other species. But what we really do

62 Life and Death, Heredity and Evolution

is to pick out races that already have these peculiarities.
This is unquestionably the explanation of the effects of
selection in by far the greater number of experiments or
observations where it is found to have an effect.

Such results were long interpreted as showing actual steps
in evolution; by selection changes in hereditary characters
were thought to be produced, and new hereditary characters
obtained. On the basis of such interpretations the rate of
change through selection, the rate of evolution, was
measured.

But what really occurs in all such cases is a gradual
picking out of existing races with certain characteristics
(in our example, with numerous spines), and discarding the
rest. So far as the selection is based on these differences
between preexisting stocks, no evolutionary change has been
produced or measured.

But can selection do nothing more than this? Are there
no actual changes in hereditary characters? Are these
diverse races really permanent in their hereditary char-
acters? Or do changes gradually occur even within such
stocks? How do these diverse races happen to exist? Can
several diverse races be produced from a single one ?

On this question a great deal of work has been done in
the unicellular forms ; we shall examine the results.

One of the animals most studied from this point of view is
Paramecium. If we take a wild set of these animals, it is
easy, as we have seen, to bring about differences by selection,
for all we have to do is to pick out such as we please of the
different races that exist. But suppose we take but a single
race ; suppose we begin with a single individual and get our
entire population from this one parent. Shall we then be
able by selection to bring about hereditary differences? In
other words, do any hereditary changes occur in such a
single race? This question is evidently the fundamental one

Effect of Selection 68

for evolution, for if no such changes occur, there would
appear to be no such thing as evolution.

I made long continued attempts to change the size in
such a race of Paramecium, by selecting on the one hand
large individuals; on the other hand small ones (Jennings,
1908). No effect was produced. Large parents and small
parents, if they belonged to the same race, produced progeny
of the same size. Ackert (1916) has recently repeated these
experiments, with the same results.

I tried also to produce from a single race stocks differing
in their rate of reproduction, but these attempts, like those
to change the size, met with no success. Each race seemed
permanent in its size and rate of reproduction.

Jollos (1913) attempted by selection in Paramecium to
obtain stocks that differed in their resistance to heat and
to chemicals. He found this very easy when he began with
a wild population containing many diverse stocks; all he
had to do was to isolate the differing stocks. But when he
began with a single stock, he found that he could not by
selection get stocks diverse in their resistance. Throughout
the ordinary multiplication, all the individuals, like identical
twins, remained just alike.

Similar results have followed many other attempts to
change by selection the inherited characteristics of a single
stock. Barber (1908) made long continued attempts to
change in this way the characteristics of pure stocks of
bacteria and of yeasts. He found a very few cases of sud-
den mutation, such as occur with similar rarity in higher
organisms ; of these we shall speak later. But, as a rule, any
differences found within a single race were not inherited;
the races were permanent. Wolf (1909) made repeated and
long continued attempts to change through selection the
colors in pure races of the red bacterium, Bacillus pro-
digiosus ; but he could produce no change in this way.

64 Life and Death, Heredity and Evolution

Thus in general, until very recently at least, the experience
of investigators has been such as to confirm what was said
many years ago by that greatest of investigators of the
Protozoa, Emile Maupas. In 1888 Maupas, after long con-
tinued study of Protozoa, said:

"In long and numerous experiments on fifteen to twenty
species, I have never observed anything which permits belief
in the existence of morphological and physiological differ-
ences between, not merely the products of a given fission,
but even among all those which have descended from such a
fission by regular and continuous generations." 2

The same sort of results have been reached from the
study of higher organisms when they reproduce without mix-
ture,— the progeny arising from a single parent instead of
two. The most famous work of this sort is the study of
self -fertilizing beans, made by Johannsen (1903). Diverse
races existed, but in seven years of selection no effect was
produced on the characters studied, so long as the selection
occurred within the limits of a single race. Lashley (1915)
studied Hydra in the same way ; no effect was produced from
selection continued for many generations. Ewing (1916)
attempted to modify by selection plant lice multiplying
parthenogenetically. The work extended over eighty-seven
generations, and many different characters were investigated
for longer or shorter periods. In no case could he change
the hereditary characteristics by selection. Agar (1914)
studied in a similar manner certain of the lower Crustacea
multiplying by parthenogenesis, and reached the same re-
sults. A great number of investigations could be cited, all
ending in the same way. The organisms studied contained
many diverse races. But when a single race or "line" is
studied, not mixing with other races, the differences between
» Maupas, 1888, p. 176.

Constancy of Strains 65

individuals were not inherited, and long continued selection
was without effect. This sort of study has come to be
known as the "pure line'* work, and the general result of
it all has been that "selection has no effect within a pure
line."

Such results have profoundly modified the theories of
heredity and of evolution. The Danish investigator Jo-
hannsen (1913) has based on them and similar results a
general system or doctrine of heredity. According to Jo-
hannsen's views, the hereditary constitution of a given organ-
ism is a perfectly definite thing, not subject to gradual or
indefinite fluctuation. This constant hereditary constitution
he calls the genotype. The genotype is comparable to a
definite chemical compound. It may become altered some-
times, as a definite chemical compound may by certain reac-
tions be transformed into another and diverse compound;
such complete transformations are called mutations; they
are extremely rare. The genotype is not subject to slight
and gradual alterations, any more than is the nature of the
chemical compound NaCl. When diverse genotypes are
mingled, as in reproduction from two parents, the results
give us what is called Mendelian inheritance. We cannot
take up the details of this here, but the same principles hold.
Selection in such cases, according to this view, merely brings
about varied combinations of things already existing; the
nature of its effects is therefore the same as when it is ap-
plied to organisms descending from but a single parent.
This view of the matter may perhaps be said to have dom-
inated recent work in heredity.

This constancy of races in organisms descending from a
single parent, and the application, which appears unavoid-
able, of the same point of view to the cases where there are
two parents, presents very great difficulties for the theory

66 Life and Death, Heredity and Evolution

of evolution. It could indeed be plausibly maintained that
all these results are opposed to the theory of evolution ; that
the logical conclusion to be drawn is that which was main-
tained by Jordan, the originator of this sort of work —
namely, that there is no variation; no change from genera-
tion to generation ; no evolution.

Is that indeed the conclusion to which we are driven? Is
that the upshot of the modern attempts to study evolution
experimentally; to see evolution occur? We shall take up
this question in our next chapter.

Ill

Results of Intense and Long Continued Study of Changes
in a Stock. Inherited Variations in the Pure Race. Visible
Evolution.

I N our last chapter I tried to picture the first results of
* the attempts to study evolution experimentally — to ac-
tually see evolution occurring — to see variations take place
and to see their inheritance.

These first results were:

That any kind of organism is really composed of a great
number of diverse stocks or races, whose differences are
hereditary, lasting from generation to generation;

That the supposed effect of selection in modifying organ-
isms consists in isolating certain pre-existing races, having
the characteristics that one is selecting; or in making,
through biparental inheritance, new combinations of charac-
ters that already exist ;

That in general, the apparent variations in organisms
are not real changes in their hereditary constitution, but
are merely these static diversities persisting from genera-
tion to generation ;

That when one takes a single one of these races and tries
to discover in it hereditary variations, or to modify it by
selection, he finds it extraordinarily constant, and his efforts
are without result. That seemingly the only variations
which appear are sudden large mutations; that gradual
alterations do not show themselves.

67

68 Life and Death, Heredity and Evolution

I wish to emphasize that this is a picture of the present
situation of affairs; that it gives the prevailing theory of
these matters. If you will read the address of President
Pearl of the American Society of Naturalists, published in
the American Naturalist (Feb. 1917), you will find that this
is the theory which he presents. I have no doubt that if
there are any experimental workers in heredity in this audi-
ence, this is the theory which they maintain.

Now, as I remarked at the end of the last chapter, this
situation of affairs presents great difficulties for the theory
of evolution, — leading indeed logically to the conclusion
that evolution does not occur. And even when we add to
these results the observed cases of sudden change of charac-
ters, which are called mutations, it becomes extremely diffi-
cult to see how evolution can occur. For most if not all of
these mutations, as is well known, consist in defects and
losses ; and it is difficult to believe that evolution has occurred
by repeated losses, — although attempts have been made to
maintain even that paradoxical theory (Bateson, 1914;
Davenport, 1916).

Moreover, there are certain facts about organisms which
it seems impossible to explain by the appearance of sudden
extensive mutations. In the organisms that we have been
describing we find that the hereditary differences between
the races are as minute as can possibly be detected by the
most refined methods; they run down to the very limits of
visibility with the microscope. Such are the differences be-
tween the diverse races of Paramecium (Figure 22) ; such
those between the races of Difflugia (Figures 19 to 21). It
is clear that such differences cannot have been produced by
"saltations" — by mutations of large extent. And the same
condition of affairs is found in higher organisms ; the differ-
ences between Jordan's 200 races of Draba verna were so

Inheritance of Variations 69

slight that it took years to detect them with certainty. If
such minutely differing races have been produced from a
single one, the steps in the change have been at least as
minute as these differences. If evolution really occurs, why
should we not see these minute changes?

Again, the existence of complex adaptive structures, such
as the eye or the ear, presents difficulties for the theory
of origin by extensive mutations perhaps fully as great as
does the existence of races differing only by minute grada*
tions. The difficulty here is not so readily presented in a
simple way, but to me it appears that thorough analysis
would reveal it as an insuperable one.

Furthermore, paleontologists maintain, with practical
unanimity, that the study of extinct organisms shows that
the change in the characteristics of animals of a given stock,
as we pass from one geological period to another, has 710*
been by leaps, but by gradual alterations. The study of
paleontology is the most direct study possible of past evolu-
tion ; we cannot neglect its conclusions.

On the whole then it is difficult to rest content with the
results of what we may call this first examination of the
diversities in such organisms. Shall we yield to the argu-
ment of Jordan, that evolution is not occurring? Or shall
we rather proceed to more refined studies ; to what might be
called investigations in the second degree? In our first
studies have we not possibly been overwhelmed and halted
by the great discovery that most of what we had thought
were real variations and real effects of selection were de-
ceptive,— were mere consequences of the existence of heredi-
tarily diverse races, — so that we have stopped before the
end? Shall we not next merely accept all this that has been
learned in the last fifteen years as a background, take a
new hold; select the most favorable organism possible; avoid

70 Life and Death, Heredity and Evolution

all sources of confusion met in the earlier studies, limit our-
selves to one single race, pursue its history with more minute
and unwearying steadiness, for longer periods than has
before been done, — to such a degree that we may properly
call our studies investigations in the second degree, — as
compared with the earlier ones?

This is what I decided four years ago to attempt ; with a
number of associates to set on foot such "second degree"
investigations. They have now been carried far enough to
show results. The main difficulties in the previous work
along these lines have been the following:

(1) In these simple organisms it is difficult to find
definite distinctive characters, such as color of eyes or hair,
that are inherited. In Paramecium, for example, all the in-
dividuals are very similar, the diversities being mainly slight
differences in size or shape; or in indefinite physiological
traits. Such characters are not favorable for work in
heredity because they are hard to distinguish; yet in prac-
tically all the earlier work such characters were employed, —
for the good reason that these seemed the only characters
available.

(2) Further, the characters studied have been such as
were continually changed by growth during the life of the
animals ; and likewise greatly modified by changes in environ-
mental conditions.

The first thing to do therefore was to find if possible an
organism in which these difficulties did not exist. From this
point of view an ideal animal was found in Difflugia corona,
— the creature that I have already employed in these lec-
tures to illustrate a number of points. This animal is an
amoeba with a shell, and the shell presents a number of
definite characters that can be easily counted and measured
(Figure 23). These characters are: the number of spines;

Inheritance of Variations

Figure 23. Difflugia corona, to show the characters studied in the
work on inheritance. A. Side view. B. Oral view, showing the mouth
and the teeth surrounding it. After Jennings, 1916.

the length of the spines ; the number of the teeth about the
mouth (Figure 23, B); and the size of the shell, as meas-
ured by its diameter. All these characters are formed when
the individual is produced (Figures 11 and 12), and are not
subject to change by growth, nor are they altered by changes
in the environment during the life of the individual. No
more favorable combination of characters for the study of
variation and heredity could possibly be found.

Difflugia is extremely fastidious as to just how it shall
live and what sort of food it shall be furnished, so that it
is not easy to keep pedigreed stock for generation after
generation, as is necessary for ah1 work in heredity. When
these difficulties are overcome, we follow for long periods the
history of a given race. As we have seen, when we compare
different races, the peculiarities of the parents reappear in
the offspring in a high degree (Figures 19 to 21). But di-
versities do arise even within a single race. Parents with
many spines have as a rule progeny with many spines, but

72 Life and Death, Heredity and Evolution

often the number is not the same in the progeny, and the suc-
cessive progeny of the same parent may have different num-
bers of spines. As we pass from parent to offspring, similar
variations arise as to length of spines, number of teeth, and
size of the shell. These facts are illustrated in Figures 19
and 20.

Now the question of interest is, whether these differences
within the single race (all derived by fission from a single
parent) show any tendency to be inherited. When a single
parent produces one offspring with few spines and another
with many spines, does the former tend then to produce a
set of progeny with few spines, the latter a set with many?
If so, we have the beginning of the origin of two races from
one. Or will there be mere chance variations, — with no tend-
ency on the part of the later descendant to reproduce its
parent's peculiarities ?

Study shows that there is certainly no complete or even
very marked tendency for the progeny within a race to
reproduce the diverse peculiarities of the parents. If a
parent has many spines, some of its offspring have many,
some few ; if the parent has few spines, some of its offspring
have many spines, some few ; and so of all the other charac-
ters (see Figures 20 and 21). To get any results whose
meaning is clear, we have to resort to averages, and to
mathematical measures of correspondence, for very large
numbers of parents and progeny belonging to a single race.
What we have to do is to determine whether, within a single
stock, on the average and in the long run, parents with many
spines produce offspring with a greater number of spines
than do parents with few spines.

When we do this we find in Difflugia indications that there
is some correspondence between parents and progeny. In
some cases the indications are very slight; in others more

Inheritance of Variations 78

marked. Measuring by the coefficient of correlation, we find
that when the parent differs from the average, the progeny
tend to inherit somewhere from one-tenth to three-tenths of
this peculiarity. That is, the race shows a slight tendency
to break up into several races, hereditarily distinct. (For
details see Jennings, 1916.)

For the investigator who has searched in vain for years
to find in uniparental reproduction any tendency for a single
race to evolve into several, such faint indications are excit-
ing ; here we begin to get hold of the beginnings of evolution.
Most of the grounds on which we believe that evolution
occurs are inferential ; we believe that it must have occurred,
— in order to account for the diversities that we find now
existing. But can we actually see it occur?

To carry our work farther, we begin to exercise selection
within the single family. On the one hand we select all the
long-spined individuals and place them together; on the
other hand we select all the short-spined ones and place them
together. In the long-spined group we continue to save for
generation after generation only the individuals that are
long-spined; in the short-spined group only the offspring
with short spines. In the same way we select other sets for
numerous spines and for few spines; for large shells and
for small shells ; for many teeth and for fewer teeth.

And now as we keep this up for generation after genera-
tion we find that the correspondence between parent and
progeny becomes more and more marked. We find that
our single family is breaking up into many different groups,
which differ from one another hereditarily. We get finally
what appear to be two diverse races, — one with long spines,
the other with short spines (Figures 24, M and F), —
the difference continuing for generation after generation.
A third set (L) has constantly large shells, while others

74 Life and Death, Heredity and Evolution

Inheritance of Variations

76 Life and Death, Heredity and Evolution

(M and N) consistently produce small shells. We also get
stocks hereditarily different for numbers of spines; and for
numbers of teeth. Our single stock, derived by fission from
a single parent, has gradually diversified itself into many
stocks that are hereditarily different. If this is what we
mean by evolution, we have here seen evolution occur.

In Figure 24 we see a number of the hereditarily diverse
stocks that arose by fission from a single parent, in one of
my experiments. It will be worth while to summarize the
main facts as to the appearance of hereditary variations in
this animal.

(1) Hereditary variations arose in some few cases by
rather large steps, which might be called mutations, or
saltations.

(2) But the immense majority of the hereditary varia-
tions were minute gradations. Variations are as continu-
ous as can be detected.

(3) Hereditary variations occurred in many different
ways, on many diverse characters: the number of spines;
the length of the spines ; the size of the body ; the num-
ber of teeth. There was no single line of variation that was
followed exclusively, or by the great majority of cases.

(4) Any set of characters gave rise to variations in-
dependently of the other characters. Thus many diverse
combinations of characters arose; large animals with long
spines ; small animals with long spines ; large animals with
short spines ; short animals with short spines, and so on
for other sorts of combinations.

(5) The hereditary variations which arose were of just
such a nature as to produce from a single strain the heredi-
tarily different strains that are found in nature.

I judge that if the intermediate strains were killed off, the
two most diverse strains found in nature might well be

Inheritance of Variations 77

classed as different species, — although the question of what
constitutes a species must be left to the judgment or fancy
of the individual.

Since the work on Difflugia was done, the same methods of
work have been applied in our laboratory to two of its rela-
tives, Centropyxis aculeata and Arcella dentata. In the
former, Root (1918) found considerable evidence that cer-
tain variations within the single stock were inherited, al-
though the work was not carried so far as in Difflugia. In
Arcella, Hegner (1918) found that heritably diverse stocks
could be isolated by selection from a single stock multiplying
by fission.

All together then, our "second degree" study of the mat-
ter has been rewarded by the discovery that for these animals
at least the situation that I sketched in my last lecture is
not final. In these animals we do find the diverse races present
under natural conditions, just as in other organisms (see
Figures 20 and 21); and by mere selection among these
diverse races we can get all sorts of apparent changes —
which are not real changes; which are not evolution. But
when we take a single race and devote all our attention to
that alone for years, then we find that real changes do
occur; that the race differentiates into many races in the
way I have described ; that evolution visibly does occur.

Now I told you that the other theory was the prevailing
one; so much is this the case that some of my readers
will not accept unreservedly these cases as actual changes
in hereditary constitution; as actual steps in evolution; on
the contrary they are trying to devise various possible
schemes by which it could be made to seem that even here
in Difflugia we are getting nothing but new combinations of
what was before present. Many such schemes have been
devised for explaining apparent effects of selection in higher

78 Life and Death, Heredity and Evolution

organisms. None of them can be applied directly to Dif-
flugia, since here we have uniparental reproduction, and most
of these schemes depend upon the mixing of two stocks. But
other schemes can be devised, which might apply to Dif-
flugia; we shall mention some of these. At present I wish
to ask your patience for a few moments for a closer analysis
of just what has happened in such a case as this. Such an
analysis will bring out the main questions and difficulties
that can be raised.

What then is it that has actually occurred in such a case ?
We began with a single individual; it consisted of a shell
filled with a mass of protoplasm, containing one or more
nuclear bodies. This mass had behaved in such a way as to
produce a shell of definite size, form, number of spines, and
the like. We found that when this mass of protoplasm gives
off one-half of itself to the outside of the old shell, this half
is made up, — chemically or otherwise, — in the same way
as was the original parent mass; for it does just the same
things that the parent mass did. That is, it produces a
shell essentiailly like that of the parent, — of a similar size,
shape, number of spines, and the like (see Figure 12). This
is particularly striking when we compare this individual with
others of different race, or of different species, as in Figure
20; it is extraordinary to see these tiny masses of proto-
plasm, each conducting itself in a manner different from any
other ; each holding true to type. There must be very defi-
nite, and at the same time very delicate, chemical differences
between them.

But as we follow for a long time our original individual
and its progeny, we find that the chemical nature of the
protoplasm very gradually changes as divisions occur, for
the behavior begins to slightly change. Although all under
the same conditions, some of the masses commence to produce

Inheritance of Variations 79

longer spines ; others shorter ; others more numerous spines ;
others fewer (Figure 24). Different masses change in dif-
ferent ways ; the number of kinds of diversity that we get is
large; apparently indefinitely large. The protoplasm cer-
tainly gradually becomes diversified as it continues to exist
and increase.

A number of important questions at once arise. What
part of the protoplasm is it that thus changes? Is it the
cytoplasm, or the nucleus, or is it both? And how does
the change occur? Through irregularities in the division
of certain substances or parts? Or through chemical or
physical changes produced by the environment, — by changes
in temperature, chemical composition of the water, by food,
or the like? And why do we find these changes to occur in
Difflugia when we could not find them in Paramecium ; when
in almost all the other organisms studied in this way such
changes have not been discovered?

All these questions are bound up together. We shall
perhaps deal best with them by taking up the last question
first. Why do we find such a difference in this respect be-
tween Difflugia and other organisms, — for example, Para-
mecium?

As was set forth earlier, Difflugia was selected for study
precisely because it was much more favorable for such work
than Paramecium or most other organisms investigated. In
Difflugia there are many well defined distinctive characters,
which are not modified by growth, nor changed by the con-
ditions under which the animals live. In Paramecium and
most other forms studied, on the other hand, the characters
are continually changing through growth and environmental
action. Such changes are well known not to be inherited.
This makes a tremendous difference as to accomplishing
anything by selecting any particular character. Thus in

80 Life and Death, Heredity and Evolution

Figure 22, showing the diverse races of Paramecium, we see
that in any single race there are individuals of many different
sizes. But almost all these differences are matters of age,
nutrition, and the like. So when we select and separate
large and small individuals, we are likely to get merely well-
grown, well-fed individuals in one set; young, ill-nourished
ones in the other. Even if there are arising really hereditary
differences in size, we cannot distinguish these from the much
more numerous transitory changes, so that our process of
selection may be rendered quite without hereditary effect.
Similar difficulties beset any attempt to select for other
characters that are dependent on growth and present en-
vironmental conditions.

It appears possible therefore that the difference between
the results of selection in Difflugia and in other organisms is
due to these facts ; that there is no real difference as to the
sort of thing that happens, but only as to whether one can
detect the hereditary changes that actually occur. Are we
to believe that the hereditary constitution of parent and
progeny is actually identical in these other forms ? Or shall
we find changes in it, if we study with sufficient minuteness ?

Now on this point we have a certain amount of evidence,
based again on what I have ventured to call our "second
degree" investigation of these matters. If we watch the
divisions of Paramecium or of any of its relatives, we find
that the two individuals produced by the division of one
do not always behave exactly alike; after they have grown
to adult size, one of them often divides before the other
does (se*e Figure 25). Are such differences due to some
change in the fundamental and hereditary constitution, or
only to some slight difference in nutrition or the like ? Here
was an opportunity for minute study of the matter; it was
undertaken in our laboratory by Middleton (1915). He

Inheritance of Variations

81

investigated from this point of view the infusorian Stylony-
chia. Beginning with a single individual, he selected on the
one hand the offspring that divided first ; on the other those
that divided last. Continuing to select for rapid fission rate
in one line, for slow fission rate in another, and keeping this
up for hundreds of generations, he found after many gen-
erations that there were real inherited differences. Two sets

Figure 25. The infusorian Stylonychia; diagram to show the differ-
ences in time of fission in offspring of the same parent, with the method
of selection for rapid fission and slow fission. Constructed on the basis
of a figure of Middleton, 1915.

A, the parent divides into two offspring (B), one of which divides
into two while the other still remains undivided (C). Similar condi-
tions appear in D and E. Thus in E, the large individual at the right
is the result of but two divisions from A, while the small individuals
to the left are the result of four.

were produced from among the descendants of a single par-
ent,— one set that divided more rapidly than the other.
The difference persisted for long after selection had stopped.

Thus it was shown that in this case hereditary diversities
are arising, even with respect to a character so readily
modified by the environment as is the fission rate.

This, so far as it goes, of course tends to raise the pre-
sumption that other characters of such organisms will be
found to show similar hereditary changes when studied with

82 Life and Death, Heredity and Evolution

sufficient thoroughness, — so that there would be no real dif-
ference in this respect between them and Difflugia. I should
like to say, however, that from experience with Difflugia
and other organisms, and from the work of other investi-
gators, I arn personally convinced that there is a difference
between organisms as to the frequency with which hereditary
variations occur. They occur on the whole relatively fre-
quently in Difflugia. In many other organisms the germinal
material is apparently so protected, and so precisely divided
at reproduction, that such changes are rare.

And this brings us to the question as to just what in the
organism it is that is altered when the hereditary characters
change. A number of possibilities are open here. Hegner *
(1919) discovered that in Arcella the hereditary size varies
with the number of nuclei or the amount of chromatin pres-
ent, and that these change at times as a result of irregulari-
ties in division. The number of spines further was found to
be related to the size; larger individuals have more spines
than smaller ones. Hence hereditary diversities in the num-
ber of spines also were brought about by alterations in the
number of nuclei or volume of chromatin. Some of the heredi-
tary changes in the characters of Difflugia may have been
brought about in the same way, but it is clear that most of
them were not. For as we have seen, some of the new lines
produced were small with large spines, some large with large
spines, — the different characters being independent in their
hereditary diversities, so as to give stocks with different com-
binations of characters. These cannot be accounted for by
quantitative alterations in the amount of the nuclear material.
Another possibility lies in certain peculiarities of the nu-
cleus in such animals as Difflugia. There is in addition to one
or more very sharply defined nuclei a cloud-like mass of nu-
1Hegner, R. W., Proceedings of National Academy, January, 1919.

Nature of Heritable Variations 88

clear material spread through the cytoplasm; this is known
as a chromidium. Unfortunately these things are not yet
thoroughly known for Difflugia itself, but in some apparently
close relatives, such as Arcella (Figure 26), they have
been much studied. In the division of these organisms the
nuclei divide with the same minute precision that is evident
in higher creatures. But the chromidial masses merely sep-
arate loosely into halves, with no indication of precision. So
possibly the offspring may get different parts of the
chromidium in different cases, and it has been suggested that

Figure 26. Arcella vulgaris, to show the two nuclei (N), and the
chromidium (C), or loose cloud of nuclear material. After Hertwig.

the differences that arise in the hereditary characters are
due to this inexactness of division. But this is all specula-
tion as yet, without much foundation of probability.

But in any case it appears to me that these details do not
affect the main fact, which is that in these organisms gradual
inherited variations are occurring, so that in the course of
time many hereditarily diverse families arise from one. In
other words, if we study these organisms with sufficient mi-
nuteness and perseverance, we see evolution occurring.

We are now in position to sum up the facts as to heredity
and variation in these animals when they are reproducing
from a single parent. Any species consists of a great num-

84 Life and Death, Heredity and Evolution

ber of hereditarily diverse families or races, whose charac-
teristics show a high degree of permanence from generation
to generation. The offspring inherit in a high degree the
characteristics of the parent. But this inheritance is not
through an actual handing on of the parent's characteris-
tics; on the contrary the offspring have to produce anew
the same kind of characters that the parent had. For this
reason any peculiarities acquired by the parent during its
life time are not inherited.

Inheritance is very exact, but when we study a family for
many generations, we find that it is not absolutely precise,
for minute hereditary variations gradually appear, and the
single race separates into many hereditarily diverse races.
The process of evolution becomes visible.

IV

Can We Experimentally Change the Hereditary Charac-
ters? Heredity of Environmental Effects. Heredity and
Variation m Bacteria and Similar Organisms.

have seen that in Difflugia hereditary variations
arise even when the organisms are all kept as nearly
as possible under the same conditions. Thus from a single
strain, all derived by fission from one ancestor, many strains
arise, diverse in their hereditary characters.

Can such changes be brought about by the action of
special conditions in the environment? Can we experimen-
tally produce such hereditary changes? Have some of the
diverse strains existing in nature been produced by action
of the environment? We saw in our introductory lecture
that structural characters produced in the body of Protozoa
during the lifetime are not inherited any more directly than
are such characters in higher organisms. To be inherited,
the acquired structures must be produced anew by the off-
spring, and for most acquired characters this does not
occur; the offspring are produced in the same state that
the parents were. But there still remains the question
whether the organisms cannot be so altered that in the
succeeding generations characters will be produced that are
different from those produced in the earlier generations.

1. Bacteria

Most of the significant work bearing on this question has
been done on the bacteria, and on other organisms that

85

86 Life and Death, Heredity and Evolution

have to do with the production of disease. In bacteria
the knowledge of variation and heredity has passed through
the same series of stages that we noted in other organisms.
At first there seemed to be a mere chaos of diverse forms,
with no constancy or order; any kind of bacterium seemed
producible from any other, or from other organic or inor-
ganic sources. Then came a period of thorough study, with
development of precise technical methods. It was discov-
ered that there are a very great number of kinds of bacteria,
but that each remains true to its characters; each is pro-
duced only by pre-existing individuals of the same race.
The differences between the races are often minute, mere
matters of a slight diversity in the chemical processes, in
the kind of sugar that is fermented by the particular race,
or the like. But each race remained true to its type, even
in these minute physiological details. This stage of knowl-
edge is the same as that on which is based the theory of
the constancy of genotypes in all sorts of organisms, — a
theory that we sketched in Lecture 2. The constancy of
the races of bacteria has been set forth as one of the facts
opposed to the theory that organisms are undergoing evolu-
tionary changes.

But in recent years a still more intensive study has
brought to light, in this group as in others, the actual
occurrence of hereditary changes, the production, from a
given race, of other races whose hereditary characters are
diverse from those of the parent race. In the bacteria, more
than in any other group of organisms, something has been
learned of the conditions which bring about these changes,
though knowledge on this point is still scanty.

The bacteria present extreme difficulties for the critical
study of heredity and variation, owing to their minuteness.
To be certain of what the results mean it is necessary to

Inheritance in Bacteria 87

work with races all members of which are derived from a
single original individual. It was long impossible to ful-
fill this requirement ; the so-called pure cultures of bacteria
were derived from a large number of individuals. There
might be slight racial differences between these original indi-
viduals, the different strains being adapted to different con-
ditions. Then under given conditions one strain multiplied
until the entire population seemed to take on its characteris-
tics, the other strains remaining without activity or mul-
tiplication. But on a change of conditions this prevalent
strain ceased its multiplication, while some other strain
became active, multiplying until the population showed the
characteristics of this second strain. It appeared as if the
changed environment had altered the hereditary characteris-
tics of the organisms, but this appearance would be illusory.
It seems probable that such impurity of the original stock
accounts for some of the apparent transformations that
have been described.

But in recent years a number of methods have been de-
vised for isolating a single bacterium, so that an entire
stock can be derived from this.1 From such pure races
dependable results can be obtained.

One of the first to isolate pure stocks was Barber (1907) ;
he worked both with bacteria and with yeasts. Barber did
not attempt to modify the organisms, but merely to deter-
mine whether the variations in size and form often observed
are inherited. From a pure race he picked out large indi-
viduals, long individuals, individuals of peculiar form, and

Apparently the simplest and most effective method is that of
mixing a very little of the fluid containing bacteria with a large quan-
tity of India ink, then producing a thin layer of this ink between two
cover glasses. Single bacteria, owing to the fact that they are sur-
rounded by a cloud of gelatinous material, are visible as minute clear
specks in the dark ink. A cover glass preparation containing but a
single individual is taken as the beginning of a culture. This is known
as Burri's method.

88 Life and Death, Heredity and Evolution

the like, and determined whether their descendants inherited
their peculiarities.

Barber discovered the same thing that others have found
in other organisms ; in the very great majority of cases such
peculiarities within a race are not inherited. Great num-
bers of indivduals selected for certain peculiarities gave
offspring of the usual types. Nevertheless a few heritable
variations were discovered. In Bacillus coli, 140 individuals
that were longer than usual were isolated. All but one
gave descendants of the usual size, but this one gave a race
having bodies longer than usual. The race was permanent ;
selection of longer and shorter specimens within it was
without further effect. Two other long-bodied races were
obtained in later extensive selections. Similarly, from among
a great number of selections of peculiarly shaped yeast cells,
a number of new races were obtained in which the cells
were narrow and elongated, as compared with the more
nearly spherical cells of the parents.

What caused the production of these new races is not
known, but they demonstrate that in bacteria and yeasts at
times the inherited characteristics of a race become altered.

More definite results have been reached in the study of
color changes in bacteria. The organism known as Bacillus
prodigiosus produces a bright red color; it is supposed to
be the cause of the "miracles" in which the bread of the host
appears to become bloody. Wolf (1909) attempted by
various means to obtain from this organism races that give
a different color or that are colorless. A series of fifty
successive selections of the lightest parts of the colonies
produced no inherited effects; the descendants were still of
the typical color.

Wolf further tried cultivating the colored bacilli on media
containing chemicals of various sorts. He employed in dif-
ferent cases copper sulphate, potassium bichromate, carbolio

Production of Heritable Variations in Bacteria 89

acid, corrosive sublimate, and other metallic salts. It was
not at all difficult to cause the organisms to lose color when
cultivated with these chemicals; keeping them at a high
temperature had the same effect. But in most cases, as soon
as the organisms were returned to natural conditions the
normal production of color was resumed; the "acquired
character" was not inherited. Similarly transitory modi-
fications of the color in other directions were produced.
Such results were reached with infinite pains in a great num-
ber of experiments, with this organism and with other
colored bacteria.

But in cultures in which the nutritive medium contained
potassium bichromate, certain white colonies appeared which,
when transferred to media without the chemical, continued
to remain white, though there appeared also red spots amid
the white. A long series of selections was carried on, choos-
ing always the whitest parts of the colonies, but the tend-
ency to return partly to the red condition could not be
gotten rid of by selection. It was found that the longer the
organisms were cultivated with potassium bichromate, the
more firmly was the white established. When it first ap-
peared the white color disappeared again as soon as the
organisms were restored to normal surroundings; later the
white became hereditary, though there was always a tendency
for some part of the colonies to produce the red color.
Some similar results were reached also with other chem-
icals.

In these cases therefore we have a most interesting tran-
sitional condition. The hereditary character of the race
has been changed, for now the colonies are largely white
under the same conditions in which they were formerly
red. But they still show a tendency to return to the original
character.

But with another substance, corrosive sublimate, a white

90 Life and Death, Heredity and Evolution

color was produced that was permanent. When the bac-
teria were restored to their natural conditions they re-
mained white, no matter how long the culture was continued.
And with certain other chemicals the bacterial color became
permanently a darker red; although restored to normal
conditions and kept there for hundreds of generations, the
acquired dark color persisted.

This work proves therefore that in bacteria by the action
of the environment definite changes that are hereditary can
be produced. From a single race, by subjecting parts of it
to these diverse agents, a number of hereditarily diverse
races are obtained.

In this case the alteration is evidently a change in the
chemical processes of the organisms. The red color of this
bacterium is not in the body of the creature, but is due to
some substance produced by it, which colors the material on
which the organisms live. In the experiments the effective
substances changed the chemical processes so that the
bacteria no longer produced this substance, or produced
one of another color.

Similar in the fact that they deal with peculiarities that
are visible to the eye are certain experiments of Toenniessen
(1915). He investigated a certain strain of the bacillus
which produces pneumonia. This organism produces a quan-
tity of mucus, which forms a thick envelope in which the
cell is imbedded; the volume of this envelope is perhaps
several hundred times that of the cell itself. When the
organisms are cultivated for a long time in dense colonies on
agar, the products of their nutritive processes collect, until
they decrease the organisms' power to produce the envelope
of mucus. After a time some of the bacteria are found with
only a thin envelope, others with none at all.

If these modified bacteria are transferred to normal con-

Production of Heritable Variations in Bacteria 91

ditions, in which the products of metabolism are not allowed
to gather, they usually at once produce the normal amount
of mucus ; the change was not a hereditary one. But if the
organisms are kept for a long time under the unfavorable
conditions (four weeks or more), some produce no mucus
at all; and if these are restored to normal conditions, they
and their descendants continue to be without the mucous
envelope. The change has become hereditary. But it is still
not permanent, for by special means the organisms can be
caused to begin anew to produce the normal amount of
mucus. This is most completely brought about through
allowing the organisms to live for a time in a living animal,
by infecting a white mouse. After passage through the
animal's body the bacteria have regained their normal
powers of producing the mucous envelope.

By long continued cultivation with the products of metab-
olism, using special methods, Toenniessen produced other
changes that were permanently hereditary. The organisms
gradually produced less and less mucus, so that successive
gradations could be distinguished. At least three of the
grades were independently hereditary; one had a mucous
envelope a little smaller than normal; a second had a very
small envelope; the third had no envelope whatever. Long
continued cultivation under normal conditions left each of
these three grades unchanged; even passage through the
body of animals did not restore the organisms to the normal
condition. The alterations produced were permanently in-
herited.

In this case, as in the former, we observe the striking fact
that what seems outwardly the same modification may appear
sometimes without being hereditary; sometimes as heredi-
tary for a number of generations ; sometimes as permanently
inherited. The difference appears to depend on the length

9& Life and Death, Heredity and Evolution

of time that the modifying factors have acted; the longer
they act, the more decidedly hereditary become the changes
they produce.

Many of the hereditary changes that have been produced
in bacteria manifest themselves only in altered physiological
activities. Bacteria break up many sorts of organic com-
pounds, obtaining by the recombination of their components
the energy necessary for their own vital activities. Diverse
species or races thus decompose different compounds. In a
number of cases it has been found that if bacteria of a
particular sort are cultivated in the presence of a com-
pound which they do not decompose, but which is not too
unlike some compound on which they can live, in the course
of time some of the individuals acquire the power of de-
composing and living upon this unaccustomed substance.
This power then remains hereditary, so that the descend-
ants have it also, — even though they may be cultivated
under conditions in which it is not exercised.

For example, Massini (1907) found that a certain bac-
terium which belongs to the group of which the typhoid
bacillus is a member, had not the power of decomposing
lactose. But if they are grown on a culture medium that
contains lactose, after a few days certain parts of the
colonies begin to grow rapidly, forming small nodules ; and
tests show that these are now decomposing the lactose. If
these are removed to other media and cultivated for many
generations without lactose, their descendants still retain
the power of decomposing this substance, as is shown by
replacing them on a medium with lactose.

This fact has been confirmed by many observers, and sim-
ilar changes have been observed in other cases. Bacteria
have been caused to acquire the power of splitting up lactose,
dulcite, rhamnose and various other carbohydrates, though

Production of Heritable Variations in Bacteria 93

when first cultivated on these substances they had not this
power. In many cases the cultures so tested have been de-
rived originally from a single individual, so that there is no
question but that there has been an actual change in the
hereditary capabilities of a single race.

In most cases the change thus brought about is per-
manent ; the descendants never lose the capability that they
have acquired. But in some cases it has been found that
long cultivation under other conditions causes the descend-
ants to lose the power which their ancestors had acquired.2

The slightness and delicacy of the hereditary changes so
induced, and the fact that they increase by gradations, is
shown in certain other experiments of Wolf (1909). Cer-
tain peculiar organisms known as Myxobacteria form dense
swarms on decaying substances. A species known as Myxo-
coccus rubescens thus forms circular red patches on culture
media. If from a single patch of these, two distinct cultures
are made, and the two swarms are later allowed to come in
contact, they flow together and form a single swarm. But
if the swarms come from diverse but related species they will
not unite, but remain sharply separate. Even within the
single species named above it was found that swarms from
diverse sources refuse to unite, so that there are diversities
of race showing in this behavior. A large number of races,
diverse according to this test, were isolated from the single
species.

The possibility naturally suggests itself that such differ-
ences can be produced within a single race. This was at-
tempted by Quehl (1906), and later by Wolf (1909). A
single swarm was divided into a number of parts, which
were kept under different conditions, on diverse culture

* An excellent summary and review of all such cases up to 1912, with
a helpful account of methods, and important details, is given in the
paper of Dobell (1912).

94 Life and Death, Heredity and Evolution

media. Later the parts were brought together again, under
uniform conditions, to see whether they would still unite,
or whether they had become sufficiently diverse to remain
distinct.

It was found that the parts might be cultivated for a long
time under diverse conditions, without becoming so changed
as to refuse to unite. The first investigator who studied
the matter did not succeed in getting a single race to
divide into two that were diverse.

Wolf continued the experiments, keeping the parts sep-
arated a longer time, and using many diverse cultural con-
ditions ; in particular he added various sorts of chemicals to
the different cultures. In this way after long periods dif-
ferences were produced within a single race. An example
will make clear the important facts. In one experiment a
single original race was divided into eight parts, which were
cultivated on diverse media. At intervals it is necessary
to transfer each stock to a new lot of its medium, in order
to keep the organisms healthy, and it is convenient to use
the number of transfers made as a measure of the relative
time required for changes to occur. After a few transfers,
each of the eight divisions was tried with all the others, and
it was found that all would unite readily. This still oc-
curred after fifteen transfers. After twenty-five transfers,
it was found that a few of the parts refused to unite with
some of the others. After thirty transfers there were re-
fusals in more than half of the combinations, and after
thirty-five transfers each division refused to unite with any
of the others. Each of the eight parts of the original race
had now become diverse from each of the others ; eight dif-
ferent stocks had been produced from one. Before the
change was complete there were many transitional condi-
tions, in which there was a reluctance to unite, without a

Production of Heritable Variations in Bacteria 95

complete refusal; conditions in which union was incomplete,
and the like.

After the differences had been brought about, the colonies
were all restored to the same culture medium and to the
same other conditions, and cultivated thus for a long time.
They still refused to unite when brought in contact. The
change produced was hereditary and permanent. After
fifteen transfers under uniform conditions, — representing
hundreds of generations of the organisms, — the diversities
still existed.

Even when the organisms were cultivated separately for a
very long time, but without diverse chemicals in the culture
media, they ultimately became diverse. In an experiment
of this sort, it required fifty-six transfers, occupying a year
and a half, to bring about hereditary diversities within a
single stock.

When the diverse stocks thus produced were examined
under the microscope, no differences could be detected. The
change was evidently in the intimate chemical processes of
the organisms, not showing in any visible way. The case is
of particular interest because it shows that hereditary
changes may arise in most delicate shadings which gradually
become more and more marked.

Besides the work which we have just described, there has
been much experimentation upon induced changes in heredi-
tary characters of bacteria, with special relation to virul-
ence, to immunity, and the like. The production of "attenu-
ated" strains of bacteria, weaker in their injurious effects on
other organisms, is a not uncommon practice. But most of
this work has been done without the precautions necessary
for establishing the results, from a genetic standpoint, as ac-
tual cases of the inheritance of induced modifications. But
the recent critical work of Wolf and others, described above,

96 Life and Death, Heredity and Evolution

largely validates this large mass of material; it shows that
hereditary changes of the kind which appear to occur in
much bacteriological work, actually do take place when
the matter is studied with all the required precautions.
Summaries of much of this work, with references to the orig-
inal papers, will be found in the publications of Dobell
(1912), Jollos (1914) and Pringsheim (1910).

#. Modifications of Inherited Characters in Higher
Protozoa

A relatively small amount of work has of late been done
on the modification of inherited characters in the larger and
more complex forms of Protozoa; some of the results here
perhaps throw light on the nature of the processes occur-
ring.

In the parasitic flagellates Trypanosoma, facts similar to
some of those above described for bacteria have been dem-
onstrated. A good review of the facts has been given by
Dobell (1912). One case introduces a new element, which
possibly throws light on certain general relations. The
trypanosome possesses, besides a typical nucleus, a small
body known as the kinetonucleus (see Figure 27, 2). This
structure is placed close to the inner end of the motile
flagellum, and may have some relation to the activity of
the latter. In reproduction, the kinetonuclei of the two
progeny are formed by division of the kinetonucleus of the
parent. In Trypanosoma brucei cultivated in mice, it was
found that when certain chemicals were injected into the
mice, the kinetonucleus of the trypanosomes disappears
(Figure 27, 1). The animals now multiply as usual, but
remain without kinetonuclei; this continues indefinitely.
Thus by the action of the chemicals a stock has been ob-
tained which differs structurally from the original race;

Production of Heritable Variations m Protozoa 97

and this diversity is inherited in ordinary reproduction by
fission. The same result has been produced wth several
other species of Trypanosoma.

In Paramecium, as well as in some other infusoria, many
attempts have been made to so modify the organisms that
they will live under conditions which normally kill them.

1 2

Figure 27. Trypanosoma brucei. 1. Individual from which the
kinetonucleus has been removed, by treatment with chemicals. 2.
Normal form (the kinetonucleus is the dark body near the lower end).
After Werbitzki, from Dobell, 1912.

All such attempts, if they are successful, involve a change
in the animals and the inheritance of this change by the
progeny. For since they reproduce every twenty-four hours
or oftener, the acclimatization would not last longer than
that period, if the progeny returned at once to the original
condition.

Most experiments in acclimatization attempt to accustom
the organisms to high temperatures ; or to poisons of various
kinds. One sometimes gets, from reading, the impression
that it is easy to do this. But most persons who try it are
greatly disappointed. The organisms appear quite un-
changing; if the experiments are not carried on for a very
long time, and the change of conditions made with extreme

98 Life and Death, Heredity and Evolution

slowness and gradualness, the animals usually show no ac-
climatization ; they die as soon as the temperature or the
poison reaches the intensity which was destructive to them
at the beginning. But with extreme patience and perse-
verance, a change gradually appears. Perhaps the most
thorough experiment of this sort ever made was carried out
long ago by Dallinger (1887); he continued the process of
acclimatizing the animals to Higher temperature for seven
years, and reached more striking results than anyone else
has attained.

Figure 28. Organisms used in Dallinger's experiment on the effects
of high temperatures. 1, Monas Dallingeri; 2, Dallingeria Diysdali;
3, Tetramitus rostratus. After Dallinger.

Dallinger worked with three minute flagellates that live
in putrefying infusions : Tetramitus rostratus, Monas Dal-
lingeri, and Dallingeria Drysdali (see Figure 28). The
temperature at which they flourished was 60° F. (16° C.);
they were killed at once by a temperature of 142° F. (61°
C.). But at least one of them, Dallingeria, formed spores
which could resist (in fluids) a temperature of 220° F.
(104° C.).

Dallinger undertook to accustom the animals to higher
temperatures. He found that up to 70° F. little difference
was observable in the life and growth, although the animals
lived better under the later increases if the change from 60°
to 70° was made very slowly. Above 70° it became neces-
sary to proceed with extreme slowness ; Dallinger raised the

Production of Heritable Variations m Protozoa 99

temperature by only two degrees each month. At 73°
many died, but as the temperature remained at this point
for two months, the remainder recovered their vigor. At
78° a critical point was reached; as many of the animals
died, the temperature was lowered to 77° till there was
recovery, and by repeatedly alternating the temperature
between these two points, in eight months the animals lived
healthily at 78°. They now underwent a most interesting
visible change; the protoplasm became filled with small
vacuoles. These continued for a month or two, then the
vacuoles disappeared. Now the temperature could be far-
ther increased; in three months it was raised to 80° F.

By a continuation of this slow and painful process the
animals were finally brought to live vigorously at a tem-
perature of 158° F. (70° C.). There were repeated critical
points, at which the animals had to be kept for months
before further advance could be made. In several of these
there was a renewed formation of vacuoles in the protoplasm,
the vacuoles finally disappearing. After these periods the
raising of the temperature could continue more 'rapidly.
To bring the animals to 158 degrees, seven years were re-
quired. The experiment was then most unfortunately ended
by an accident.

No such long continued experiment has ever been carried
through since this work of Dallinger. It is clear that not
only had the organisms of a given generation been changed,
but they transmitted the change to their offspring. For at
the end of any period of 24 hours a totally new generation
was present. At the beginning of the experiment all the
individuals were destroyed by a temperature of 78° ; while
at the end they lived and flourished at a temperature above
150°.

Yet it is to be remembered that even at the beginning

100 Life and Death, Heredity and Evolution

the animals could form spores which resisted a much higher
temperature (242°) than that to which the active animals
were finally accustomed. In transforming from active ani-
mals to spores, the protoplasm must go through some
proces's which makes it more resistant to heat. It seems
probable that during the acclimatization the protoplasm
of the active animals went through a similar process. It
has been suggested that the essential point in both cases is
the getting rid of a certain proportion of the water in the
protoplasm, leaving it denser, for protoplasm containing lit-
tle water is as a rule less injured by heat than when it
contains much water. This new physical condition of the
protoplasm must then have persisted through reproduction,
and so been handed on to the offspring.

Some effects of temperature in altering a different mani-
festation of the hereditary constitution have recently been
studied by Middleton (1918). Progeny of a given indi-
vidual of the infusorian Stylonychia pustulata were divided
into two sets; one set was kept at a high temperature, the
other at* a low temperature. Those at the high tempera-
ture (about 30° C.) divided more rapidly than those at
the low temperature (about 10° C.). After various inter-
vals, members of the two sets were brought to a common
intermediate temperature, and their rates of fission com-
pared.

It was found that the stay in diverse temperatures had
altered the hereditary constitution so as to give diverse
rates of fission in the two stocks. After about thirty days
in the different temperatures, the set that had been kept
at high temperatures continued to divide more rapidly than
the others, even though both were now at the same tempera-
ture. But after longer periods in the diverse temperatures,
— after two or three months or more, — there was a change

Production of Heritable Variations in Protozoa 101

in the inherited effects. Now when both sets were placed at
intermediate temperatures, the set that had been .kept at the
higher temperature divided less rapidly than the set that
had been kept at low temperature. This difference per-
sisted for as long a period as the stocks were retained,—'
about two months.

Other evidence showed that the high temperature grad-
ually injured the stock, so that in the course of time the
high temperature set came to divide less rapidly even while
subjected to high temperature. At the end of six months
those kept at the high temperature all died out, while the
other set was still vigorous. The persistent low fission rate
of the high temperature set when restored to normal tem-
perature was apparently a manifestation of this injury.
The latter, whatever its nature, was inherited in the vege-
tative reproduction.

Dallinger did not determine how long the acquired re-
sistance to heat would have lasted if his animals had been
restored gradually to lower temperatures ; and no study for
long periods of the permanence of the effects observed by
him was made by Middleton. But this matter has been
studied particularly in Paramecium, by Jollos (1913 a,
1914). He attempted to accustom Paramecium caudatum
to higher temperatures, and to increased concentration of
certain compounds of arsenic. Some races resisted acclima-
tization completely. In others after long periods the ani-
mals could stand somewhat higher temperatures or higher
concentrations than before. But when they were returned to
the normal conditions, they lost their acquired immunity al-
most at once.

In other cases the animals acquired a resistance to poisons
which was retained by their descendants for many genera-
tions. Thus, in a certain race B the animals were killed

102 Life and Death, Heredity and Evolution

when 1.1 parts of a standard solution of arsenic was added
to 100 parts of the water. By a gradual process they were
rendered resistant to 5 parts of this same solution to 100
of water. When they were again cultivated in fluid without
arsenic, they retained their resistance unchanged for seven
months, or at least 200 generations. But in the eighth
month it was found that the resistance was partly lost;
they could now stand only 4 parts of the arsenical solution
in 100. The resistance continued to decrease gradually,
until at the end of ten and a half months they had entirely
lost the resistance to arsenic that they had acquired, so
that they were killed by the same weak doses that had
been destructive at the beginning. In many other cases
Jollos thus produced modifications of the power of resisting
chemicals, which thus lasted for months, but finally disap-
peared. He found that if the animals were kept under con-
stant conditions their resistance lasted much longer ihan
was the case if they were subjected to many changes of
temperature and food.

One particular phenomenon did away immediately with
the acquired resistance ; this was conjugation. Jollos found
that after the animals had acquired resistance to a consid-
erable concentration of arsenic, this resistance was com-
pletely lost as soon as they conjugated. To this fact Jollos
attributes a deep significance. He believes that it shows that
the modifications thus produced and for a long time passed
from parent to offspring by fission are in reality very dif-
ferent things from the permanently inherited characteristics
of the species. These characters — the typical form, struc-
ture, and physiology — are inherited not only in fission, but
also in the changes which follow upon conjugation. A
change in these characters, — a permanent change in the
inheritance, — Jollos would call a mutation, while these

Nature of Heritable Variations 108

changes that arc handed on only through vegetative repro-
duction he calls modifications; the two he believes to be of
essentially diverse nature. Such a "mutation" Jollos be-
lieves that he saw in a single instance in Paramecium. In
one of his cultures kept at high temperatures there ap-
peared individuals which were much more resistant to heat
than most of the animals; they could be cultivated at 39°
C., which soon killed the others. These individuals retained
their high resistance even after conjugation; it had become
a permanently inherited character. In no other case was a
modification retained through conjugation. Jollos holds
that practically all the changes in bacteria and other Pro-
tista, which we have described above, are merely instances
of these temporary modifications.

That there is such a difference in principle between the
two things, — between modifications that are passed on only
in fission, but disappear as soon as there is sexual reproduc-
tion, so that they cannot be said to form part of the really
hereditary characters of the stock, — between these and the
really hereditary characters, — cannot yet be considered es-
tablished. If there is such a difference, one can hardly re-
frain from bringing it in some way into relation with the
two nuclei. Since in fission the new active nuclei are pro-
duced by division of the active nucleus of the parent, one
might naturally assume that the seat of the temporary
modifications is in the active or macronucleus, while the re-
serve nucleus (micronucleus) has not been affected. Thus
would be accounted for the fact that at conjugation, when
the macronucleus disappears and is replaced by the mi-
cronucleus, the modifications also disappear; they go with
the macronucleus. But we now know from the work of
Woodruff and Erdmann that the macronucleus disappears
and is replaced from the micronucleus at intervals even

104 Life and Death, Heredity and Evolution

without conjugation; the modifications should therefore dis-
appear at such times. The fact that they do not indicates
that the distinction is not one depending on whether the seat
of the modifications is in the macronucleus or the micronu-
cleus ; it leaves the distinction indeed with no very intelligible
foundation. It appears possible that modifications which
are novr known to last for months might endure still longer,
and become as permanent as any character, if the conditions
producing them lasted for much longer periods. The fact
that the modifications sometimes disappear at conjugation
may be due to the fact that variations of many sorts occur
as a result of conjugation, as will be set forth in our account
of that matter. The number of cases in which these phe-
nomena have been studied is very small ; too small for basing
positive conclusions on these points.

All together, the studies of the effects of external agents
on heredity in the Protozoa show that changes in the
hereditary characters are in this way produced only most
slowly and rarely. The organisms are found most resistant
to such changes ; any alteration produced in a given genera-
tion is usually compensated for in the next generation. Al-
most every investigator of the matter passes through a long
stage in which he can hardly resist the conviction that no
hereditary changes can be brought about in this manner.
But if work is continued for very long periods of time, the
hereditary constitution of the stock is seen to gradually
yield; at first only in a slight degree and with results that
are transitory. The differences finally become so fixed that
they are transmitted in the ordinary reproduction by fission.
After conjugation, with its extreme physiological altera-
tions, and production of new combinations of inherited char-
acters, the inherited environmental effects are frequently no

Nature of Heritable Variation* 105

longer clearly in evidence ; possibly they are masked by the
new combinations occurring; possibly actually lost.

In general the results of the work suggest that the many
slightly differing stocks found in any one of these lower or-
ganisms may owe their origin partly to the inherited effects
of long continued environmental diversities.

The Natural History of Mating. Sex, Its Nature and
Consequences. Sex in the Protozoa. Is Sex Coextensive
with Life and Necessary to Its Continuance?

\\ 7E have dealt with heredity and other genetic problems
* in the cases where there is but a single parent; we
now turn to reproduction where there are two parents in-
stead of one. The mating of two individuals that occurs at
times in almost all organisms is one of the most extraor-
dinary processes in nature; it has the effect of complicat-
ing tremendously all biological questions. Volumes have
been written as to its purpose and meaning.

Some tell us that it is unscientific to ask as to the "pur-
pose" or "object" of any process; Dobell (1914) has made
this point with relation to all such discussions of mating.
The criticism is justified, so far as the method of expression
goes, and the literal implications of that method of expres-
sion; science cannot deal with purposes or ends, save in the
case of conscious human purposes. Nevertheless, a really
scientific question is often hidden under this form of expres-
sion. What it really means is : What difference does it
make whether this process occurs or not? Any question
as to "purpose" or "object" that can be put in this form is
a scientific question in spite of its teleological clothing; any
teleological question that cannot be put in this form is no
affair of science. To ask what difference this phenomenon
makes, leads at once to experiment; a question that could

106

Effect of Mating 107

not be settled by any conceivable experiment is not part of
science.

So the question in which men have been interested in re-
lation to mating and fertilization is: What difference does
it make whether this occurs or not? This is strictly a ques-
tion of observation and experiment, on the same footing as
the question : What difference does it make whether animals
take food or not?

When we ask this question regarding the union of two
individuals or parts of individuals which we call mating and
fertilization, we find that there is hardly another phenomenon
in biology that so alters the whole face of things. Biology
would be a relatively simple subject if there were no periodic
unions of diverse individuals, with the accompanying proc-
esses. This union has results of so many different kinds,
some immediate and obvious, others remote and hidden, that
we find little agreement in the accounts of its fundamental
features given by different investigators.

A picture of what happens in the higher organisms that
we are familiar with will bring the question sharply before
us. Mating here involves two diverse individuals that we
call male and female, and two diverse germ cells, which we
likewise call male and female. But this is not the end;
the final mating is between certain parts of the cell, after
the two germ cells have joined to form one. The cell
now contains a set of pairs of small visible packets of
chemicals, the chromosomes (Figure 29). These mate in
pairs (Figure 29, D, E, F) and again separate. This is the
final and elementary action of mating; the union of these
chromosomes contains the secret of sex and of mating.
The union of the two diverse germ cells forms the starting
point for the development of the new individual.

Several questions of general interest come into view in

108 Life and Death, Heredity and Evolution

) I K I < 14 Jf j..
Hit

60

Figure 29. Chromosomes and their mating. A. Nucleus containing
the chromosomes, from the salamander. B. The 23 chromosomes in a
single cell of a male grasshopper, as seen under the microscope. C.
The chromosomes of B drawn separately and arranged so as to
show that the group consists of a series of 12, the two members of
each pair being of the same size and form. One chromosome (fifth
from the left in the upper row) is without a mate in the male; in the
cell of a female it has a mate. D. The members of the pairs after
mating. Each of the 12 structures (save one) is formed by the union
of the two members of a pair. E and F. Details of the mating of the
chromosomes in the cells of another species of grasshopper. In E only
two chromosome pairs are seen; in the pair to the right mating side
by side has begun, but is not complete. In F several pairs are shown,
fully mated. Each of the granules of which the chromosome is com-
posed mates with a granule of corresponding size and position in the
other chromosome. B to D, after Robertson, 1908. E and F, after
Wenrich, 1916.

Nature of Sex 109

regard to this strange process. First, is this union a neces-
sity of life? That is, could and would life continue with-
out its occurrence? Could and would development occur
without it? Is there some single general result produced
by mating, — something as general as the production of
energy through the taking of food ? If there is, what is this
single general effect of mating? This is the real point
underlying the much-discussed question: What is the pur-
pose of mating?

Certain other questions arise from the differences between
the two sexes. What is the nature of this difference? That
is, is there a single kind of chemical or physiological dif-
ference between the sexes, wherever sex differences occur; a
fundamental diversity of which all other sex diversities are
consequences? And does this diversity exist whenever there
is union of individuals or cells or parts of cells? Are the
two chromosomes that unite diverse in this manner? And
are all the unions that occur a consequence of such diver-
sity? Or may two precisely similar individuals or cells or
chromosomes unite at mating? Again, is this sex differ-
ence coextensive with life, so that all living things are com-
posed of two classes of substance, male and female, as
some have asserted? Or is sex difference something that
pertains only to certain kinds of organisms; perhaps some-
thing that has arisen during evolution, like the difference
between two species of plants or animals?

We shall examine the facts in the Protozoa in their bear-
ing on these questions, and shall try to determine with which
answers these facts agree best. We will take up the process
of union first in what is perhaps the best known and most
instructive case in these lower organisms, in the infusorian
Paramecium. Then we shall compare what happens in this
animal with what occurs in others, keeping in mind through-
out the fundamental questions that we have set forth.

110 Life and Death, Heredity and Evolution

As you recall, Paramecium multiplies for many genera-
tions in single lines, the offspring having but one parent.
Then mating occurs ; the animals place themselves with their
oral sides together and become partly united (Figure 6).
The surfaces of the two adhere, and at a certain spot the
interior protoplasm of the two comes into actual union. The
main things that then occur are those shown in Figure 8
(page 26), and in Figure 30. The old active nucleus (mac-
ronucleus) breaks in pieces and is gradually absorbed, like
so much food; it disappears completely. This really re-
quires a long time, so that fragments of it are still found
in later stages, but as this has no importance, these frag-
ments are omitted from the figures in order not to confuse
them. The single small reserve nucleus (micronucleus)
divides twice, into four (Figure 30, A, B, C), and three of
these are absorbed and disappear (C) like the macrcnu-
cleus. Then the remaining (fourth) one divides into two
parts (see Figure 40). Of these two parts, one lies a little
nearer the surface of union of the two conjugants, and is a
little smaller than the other. This one begins to move
toward the opposite conjugant; it is therefore commonly
spoken of as the "migratory" half nucleus. In the other
conjugant the same thing happens, so that the two migra-
tory half nuclei meet and pass each other at the boundary
between the two conjugating individuals (Figure 40, B, C,
D). Each of the two stationary halves remains in place
till the migratory half nucleus from the opposite individual
reaches it; then the migratory and stationary half nuclei
unite (Figure 30, E; Figure 40, E, F).

Thus the general upshot of the process is that the two
mating animals exchange halves of their micronuclci, and
at the end each has a micronucleus composed of substance
partly from one mate, partly from the other. This is evi-
dently the central point in the conjugation.

Conjugation of Paramecvum Caudatum 111

B

Figure 30. Diagram showing the chief processes in the conjugation
of Paramecium caudatum. The larger black bodies are the macro-
nuclei; the smaller ones the micronuclei. The clear circles are the
micronuclei that disappear. The connecting lines show the origin by
division of the various structures. After the separation of the two
members of the pair, at G, only one of them is followed farther; the
other goes through the same processes (H to L).

Life and Death, Heredity and Evolution

After the union of the two half nuclei the two mates sep-
arate; we may now call each an ex-conjugant. Now a set of
peculiar processes occurs, with the result of restoring,
through two divisions of each ex-conjugant, individuals hav-
ing the same structure as did the mates before conjugation.
Some of the details of the process may turn out of great
significance, though at present their meaning is not clear.

At separation each ex-conjugant has a single nucleus
formed by the union of the two half nuclei (Figure 30, F).
This single nucleus divides, producing two ; these divide, pro-
ducing four; these again divide, producing eight. These
eight are all present in the single ex-conjugant (Figure
30, J). Now of these eight three dissolve and disappear,
leaving five in the single individual. One of these, as it
later turns out, is the nucleus from which all the micronuclei
of later generations arise by division; the other four later
form four macronuclei of four individuals of later genera-
tions,— each increasing greatly in size.

Now the single ex-conjugant divides, producing two indi-
viduals. At the same time the single micronucleus divides
into two halves, each passing to one of the offspring. Each
of the two offspring also receives two of the four macronu-
clei, which are now enlarging (Figure 30, K). Next each
of these two offspring divides anew with repetition of the
division of the micronucleus, while of the four progeny each
receives one of the four macronuclei. Thus after two divi-
sions of the ex-conjugant, offspring are produced with a sin-
gle macronucleus and a single micronucleus (Figure 30, L),
— like the individuals before conjugation. Each of the four
individuals arising from an ex-conjugant has one of the
four macronuclei that were present in the ex-conjugant.
But the micronucleus of each of the four is derived by divi-
sion from a single micronucleus present in the ex-conjugant.

The Processes in Mating 11$

What significance is to be attached to the dissolution of
three of the eight nuclei originally present in the ex-con-
jugant is not clear; nor is it clear what is meant by the
diverse method of origin and distribution of the new mac-
ronuclei as compared with the new micronuclei ; to this point
we return later.

The many details in these processes that occur after sep-
aration of the ex-con jugant must not be allowed to obscure
the essential features. These are simply that the new
micronucleus formed by union of the half nuclei divides so
as to produce new active macronuclei and new reserve mi-
cronuclei (Figure 30, G to L) ; and the animals continue to
divide by fission, as they did before conjugation, — each of
the offspring getting a single active nucleus and a single
reserve nucleus.

In this process of conjugation are involved all the prob-
lems of sex and of mating. It is of interest to examine it
in connection with the general questions which we proposed
in our introduction to this lecture, and with some of the
commoner answers to these questions. To this we turn.

What are the results of this mating? What difference
does it make to the organisms or to the race which they
make up?

As we saw in our first lecture, the best known theory as
to the effect of such conjugation is that it rejuvenates the
organism; that it gets rid of the effects of age. But in
what way can it have this effect? There are two possible
answers to this question. One is that the rejuvenescence is
a result of the replacement of the old active nucleus by the
reserve nucleus. But for this no union of two individuals
is necessary, either logically or in fact; we know now that
such replacement occurs without union. This answer there-
fore gives no explanation of the fact of union ; what we are

114 Life and Death, Heredity and Evolution

interested in now is precisely the question as to the effect
of the mating as distinguished from the process of replace-
ment. A second answer to the question as to how rejuvenes-
cence is brought about holds that it is due primarily to this
union of two individuals or of two nuclei. In the minds of
many that hold it the grounds for this belief are undefined;
Maupas (1889, p. 486) in his great work on rejuvenescence,
in which he maintains this theory, avows distinctly that he
cannot see how the union of nuclei should produce re-
juvenescence, though he believes that it does. But there has
grown up a theory as to how the rejuvenescence is brought
about by union ; a theory that is rather generally held. This
theory depends upon the farther theory that there is a
fundamental difference between the male and female sexes,
and that this essential sex difference is always present in
mating, and is the basis of rejuvenescence. We know indeed
that in many organisms the differences between the sexes are
not mere superficial diversities, but that the two sexes differ
in every cell of their bodies, and the observable difference
lies precisely in the nuclei, which we know to be funda-
mentally important parts of the organism. In the com-
moner cases, including man, the nuclei of the female have one
more chromosome than have those of the male (Figure 31,
A and B); in other cases one chromosome of the female
differs from the corresponding one in the male (Figure
31, C and D). Such a visible structural difference neces-
sarily means further a diversity in the most fundamental
and intimate physiological processes of the two sexes; in
the chemical changes that determine the nature of life.
These chemical changes we observe to occur in the physio-
logical interaction between the nucleus and the rest of the
protoplasm.

What are the fundamental physiological differences be-

Nature of Sexual Diversity 115

tween the sexes? In a general way the male in most organ-
isms is the more active and the less inclined to store up re-
serve food in its tissues; the female less active and given
to storing up more reserve nutrition. These differences are
of course seen most clearly in the germ cells of the two
sexes; the typical male germ cells are minute and actively
motile, while the female germ cells (eggs) are large and
inert, with much food material stored in them. Less marked

Figure 31. Differences between the chromosomes of the nuclei In
the two sexes. A and B, male and female chromosome groups, re-
spectively, of the hemipterous insect Protenor, after Wilson (1910).
The nucleus of the female (B) has two of the large chromosomes x,
while the male (A) has but one.

C and D, male and female chromosome groups respectively, in the
nuclei of the fruit fly Drosophila, after Morgan (1916). The male"
group (C) has one bent chromosome (Y) in place of one of the straight
ones (X) of the female (D).

differences of the same general character distinguish the
male and female individuals that produce these germ cells.

Many attempts have been made to express such differences
in general terms. Geddes and Thompson (1880) say that in
the female the preponderating process is that of anabolism,

116 Life and Death, Heredity and Evolution

or the building up of organic material, with the storing up
of energy, while in the male the prevailing process is catabo-
lism, or the breaking down process by which energy is set
free, — this energy showing itself in greater movement. Or
the male has been called "kinetic," the female "trophic";
or the male progressive, the female conservative ; or it is said
that in the male the animal functions prevail, in the female
the vegetative functions.

In recent times emphasis has been laid on a parallel
diversity observable between certain parts of the cell or
parts of nuclei. When the cell divides, certain parts seem
to actively initiate and carry on the movements that bring
about division, while other parts are passively moved by
these active portions. In many organisms a structure called
the aster, which in some cases itself appears to be derived
from a small body called the centrosome, sets up at the time
of division a great activity in the protoplasm, forming the
spindle, and taking the lead in the activities of cell division
(see Figure 32). Another portion of the cell, comprising
the part commonly called the nucleus, and particularly the
chromosomes, is apparently passively moved by these active
portions. In some Protozoa there are two nuclei, one of
which, the so-called kinetonucleus or blepharoplast (see Fig-
ure 27), is connected with the organs of motion, and is sup-
posed by some to correspond to the centrosome. The other,
the so-called trophonucleus, appears more passive, and is
held to have functions connected primarily with metabolism.
The materials of which these two parts are composed are
called respectively kinetoplasm and trophoplasm, and it is
held by the upholders of this doctrine that these two kinds
of material are present, either mingled or separate, in every
cell that can divide.1

1 A r6sum6 of the facts on which this notion is based is found in the
paper of Dobell (1909).

Nature of Sexual Diversity

117

nu.-

Figure 32. The active and passive portions of cell and nucleus in
the egg of the whitefish. A. Egg just before division; with centrosome
(c), aster (a), nucleus (nu.) and cytoplasm (cy.). A set of radiations
have been formed about the aster as a center, indicating great activity.
B. Later stage; the aster has begun to divide into two. C. Still later;
the aster has separated into two with the nucleus between them. The
activities of the asters have begun to invade the nucleus. D. The
.nucleus has been transformed by the activities of the asters into a
spindle-like body, with the small chromosomes (chr.) ranged across its
middle.

These two kinds of material, with their contrasted ways
of behaving, are held by many to form the basis for maleness
and femaleness, so that sex diversity is coextensive with life,

118 Life and Death, Heredity and Evolution

although both sexes may be present in a single nucleus. But
individuals or cells in which the active material or kine-
toplasm prevails are male, while those in which the nutritive
material or trophoplasm is preponderant are female.

According to such theories, for vigorous and successful
life, an organism must have both these substances or proper-
ties,— both maleness and femaleness, — in fitting proportions.
And it is the gradual dislocation of the required proportions
that gives rise to the necessity of periodical unions of di-
verse cells or nuclei, and to the rejuvenescence that results
from such unions. It is commonly held that as life goes on,
certain individuals or cells or parts of cells develop, perhaps
by accidental inequalities in division, farther and farther in
the direction of maleness, of activity, of energy-production;
that is, the kinetoplasm exceeds greatly the trophoplasm.
Others develop farther and farther in the female or vegeta-
tive direction ; toward a decrease in activity ; toward a stor-
ing up of food and energy; toward a great preponderance
of trophoplasm.

After this divergent development has gone on for a time,
in each case, according to such theories, a stage of too great
specialization is reached; a stage of lack of balance; the
male has developed too far in one direction, the female in
the other. Life can therefore no longer continue vigorously
and normally; the chemical changes go awry; the process
that we call aging comes on ; reproduction ceases ; and life
must end unless compensatory changes occur. Such com-
pensatory changes are brought about by a reunion of the
separated male and female substances or tendencies ; the*
balance is thereupon restored. In other words, rejuvenes-
cence is produced by mating ; reproduction can now go on as
before ; the cycle of youthful life begins anew.

According to this way of looking at the matter, there is

Nature of Sexual Diversity 119

an actual attraction of some sort between the two kinds of
substance when separated, and this it is which causes them
to unite. In the case of entire individuals, and particularly
in higher organisms, this attraction must of course work
through many indirect means, but in the actual union of
male and female nuclei or of chromosomes (Figure 29) it
must show itself in some direct and simple manner, as when
two chemicals unite (compare Doflein, 1911, page 260).
The cause of the union is the diversity. Thus, according to
this view, the attraction of the sexes has its basis in the
foundations of life.

To be consistent, this theory has to maintain that in
the union of the chromosomes in pairs, which appears to
be the elementary act in mating, there is this same diversity
between the two uniting members, and that this is the cause
of their union.

Further, it would appear that according to this view,
growing old is fundamentally a different thing in the two
sexes ; in the female it would be the consequence of an excess
in the vegetative functions ; in the male a consequence of ex-
cess in the kinetic functions. This consequence I have not
seen drawn, but it appears an unavoidable one if the theory
is held. Doubtless in each sex it could be held that some other
processes essential to life are interfered with through these
changes, in such a way as to give similar outward signs of
age, such as we gee in the two sexes of higher organisms.

Let us now turn back to the conjugation of Paramecium
and see its relation to such ideas.

First, of the two individuals that mate, each plays the
part of both male and female. Each furnishes a smaller,
active half nucleus, which moves over to join the larger in-
active half nucleus of the other individual; this active half
nucleus evidently takes the role of the male germ cell. But

120 Life and Death, Heredity and Evolution

each individual of the pair likewise produces a larger inac-
tive half nucleus, which remains passive and is sought out
by the smaller one; this inactive nucleus plays the role
of the egg in ordinary fertilization. If there is a funda-
mental sex difference between the two uniting half nuclei, we
must call the active one male, the passive one female. But
the two individuals that unite both produce male and female
nuclei; each would have to .be characterized either as both
male and female, or as neutral. There appears a difficulty
here, for why, if the two individuals are alike, should they
be drawn together and mate? According to the theory it is
unlikeness of sex that brings about mating.

Attempts have been made to show that the two individuals
which conjugate really are of preponderatingly different
sexes; that one exceeds the other in maleness, the other in
femaleness, if we may so speak; this would avoid the diffi-
culty we have mentioned. Calkins (1902) pointed out that
often only one of the two individuals that have mated repro-
duces freely, the other reproducing but weakly or dying with-
out reproduction; Miss Cull (1907) showed that this hap-
pens in a great number of cases. The individual that pro-
duces young after mating would be mainly female, while the
other would play the role of male. Of course the differentia-
tion of sexes was not held to have gone far in such a case ;
all that is needed is that one should be, as it were, more
female than the other; then the diversity would operate to
produce mating and consequent rejuvenescence.

This idea is rendered plausible by the fact that in some
other infusoria, such as Vorticella and its relatives, there is
an observable difference between the two individuals that
conjugate. One of the two mates is small and active, taking
the usual role of the male, while the other is larger and
remains quiet, taking the part of the female (Figures 33

Sexual Diversity m Protozoa

121

Figure 33. Beginning of conjugation in Epistylis, after Wallengren,
i99. The small active male is attaching itself to the side of the larjre
inactive female.

Figure 34. Successive steps in the process of conjugation in Vorti-
cella nebulifera, after Maupas, 1889. The small male unites with the
base of the female; its protoplasm entering the body of the latter, so
that finally only the outer layer remains outside (at F), finally drop-
ping off.

122 Life and Death, Heredity and Evolution

and 34). In these cases too the "male" completely unites
with the female, the two merging into one (Figure 34), just
as happens with the two germ cells of higher organisms;
they do not separate and continue their individual lives, as
the two mates do in Paramecium. Each of the mates in

Figure 35. Diagrams of the micronuclear processes in conjugation,
in Paramecium (P) and Vorticella (V), up to the formation of the
new micronucleus .by the union of the migratory and stationary half
nuclei. In each case the original micronuclei of the two mates are
shown below, and successive stages and divisions are shown in passing
upward. The micronuclei that dissolve and disappear are shown as
clear circles. In Vorticella (V) the divisions in the small free indi-
vidual (microgamete) are shown at the right; those of the large stalked
individual (macrogamete) at the left, Based on diagrams by Maupas,

Sexual Diversity in Protozoa

Vorticolla produces just before the fertilizing process two
half nuclei, as Paramecium does, but in the large "female"
one is absorbed and disappears, while in the small "male"
likewise one is- absorbed and disappears. The remaining
half nucleus of the "male'* then passes into the "female,"
and unites with its remaining half nucleus. (See the dia-
gram of the process in comparison with that in Paramecium,
in Figure 35).

Figure 36. Mating in the mould, Mucor, after De Bary. A to E,
successive stages. The large black body at E formed by the union of
the ends of the two branches, is the zygospore, which later separates
off, and produces a new plant.

In Paramecium, it was held by Cull (1907) that there is a
less advanced stage of a similar process ; though the two in-
dividuals look and act alike, they still are different, since
after mating one reproduces more vigorously than the other.

Cases are known in which the individuals of a species are
really physiologically diverse as to sex, although in appear-
ance they are alike. This has been worked out fully by
Blakeslee (1904) in the common moulds, Mucor. Here the

Life and Death, Heredity and Evolution

filaments produce small side branches, which are separated
off by a partition as club-shaped germ cells or reproductive
bodies (see Figure 36). These reproductive bodies unite
with corresponding bodies from another individual plant.
By test it is found that these reproductive bodies are not
indifferent as to their mates ; with reproductive bodies from
certain other plants they will not unite. Thorough study
shows that there are two classes ; that those of one class will
not unite with those of the same class, but will mate with
those of the other class. The characteristic behavior of the
two sexes occurs, although the two show no external differ-
ences (save that in some cases the "female" plants grow
somewhat more luxuriantly than the "male" ones).

Such cases show that the physiological distinction of two
sexes may exist without any structural difference. Is this so
in Paramecium? The case here turns out not to be so prob-
able a one for sex difference as it at first seemed. It is true
that sometimes after mating one individual reproduces freely,
while the other does not. But on the other hand, sometimes
both individuals reproduce freely ; sometimes both reproduce
feebly; sometimes neither reproduces at all. That is, there
are great differences among the individuals that have con-
jugated, in respect to these (and other) matters. The ques-
tion therefore reduces itself to this : Are the two that have
mated any more unlike in these respects than are any two
ordinary individuals that have not mated together? For
example, if one multiplies strongly, is its mate more or
less likely than the average individual to multiply weakly
or not at all?

This led Lashley and myself to investigate the matter
(Jennings and Lashley, 1913). If we have a large number
of cases it becomes a mathematical problem to determine
whether the two members of a pair are more unlike than

Sexual Diversity m Protozoa

usual ; there are definite methods for solving such a problem.
Using a large number of experiments, we found, rather un-
expectedly, that the two individuals that have mated are
more alike in all these respects, not less alike. If one mem-
ber of a pair dies, its mate is more likely to die than are
the mates of individuals that live. If one member multi-
plies feebly, its mate is likely to multiply feebly, too. If one
member multiplies vigorously, its mate is likely to multiply
vigorously also. All these relations are quite the opposite
of what would be expected if there are pronounced sex dif-
ferences between the members of pairs. But from an en-
tirely different point of view, they are what might well
be expected. The two mates have exchanged parts, so that
after conjugation each is, as it were, half composed of
material from the other (this is strictly true of the nuclei),
so that it is natural that they should become alike. This
brings into vie^ another result of mating; one of the first
importance. Mating causes the progeny of the two indi-
viduals that mate to be alike. This point we shall take up
later; here we shall look farther into the question of sex
differences.

We have therefore no positive evidence of a difference of
sex between the two members of a pair of Paramecia. On
the other hand, perhaps it can hardly be maintained that
such differences are disproved. If all we hold is that some
difference in degree of "maleness" or "femaleness" is what
produces mating, it becomes extremely difficult to test
whether this holds or not. In this connection there are a
number of facts, some of them very curious, that require
consideration.

In many lower organisms, germ cells are formed by the
division of a single cell, and so far as anyone can see, these
are all alike. Further, so far as anyone can determine, any

126 Life and Death, Heredity and Evolution

two of these can mate, — just as apparently any two indi-
viduals can mate in Paramecium. This is particularly the
case with many lower plants (Figure 37). It is true even
in many of the moulds, in some species of which, as we have
before seen, physiological sex differences exist, even though
there is no visible difference between the sexes. In other
species of moulds, no such physiological sex difference exists ;
a given individual can mate with any other individual of the
species.

Figure 37. Mating of similar cells, from the alga Stephanosphaera;
successive stages from left to right. The two unite to form a single
reproductive body or zygote (at the right). After Hieronymus, from
Doflein, 1911.

From such cases, common in lower organisms, it is natural
to draw the conclusion, which many have drawn, that in
these lowest creatures there is no difference of sex ; that sex
has arisen as evolution progressed; that therefore sex is
not coextensive with and fundamental to life.

On the other hand, some have held that the very fact that
two unite is sufficient proof that they are sexually diverse;
this for example is the view taken by Minchin (1912) in
his Introduction to the Study of the Protozoa and by Coul-
ter in his interesting work on the Development of Sex in
Plants (1914).2 If this argument is advanced as one of

'See Minchin, p. 160: "The fact that gametes and pronuclei tend to
unite proves that in all cases there must be intrinsic differences be-
tween them which stimulate them to do so." Coulter says on page 26:

"The gametes are alike in appearance, but that they are not alike in
fact is evidenced by their pairing and mutual attraction." A similar
idea is expressed by Collin (1909): "But this invisible difference be-

Nature of Sexual Diversity 127

general validity, it of course requires one to hold that the
two chromosomes which unite at their mating within the cell
are sexually diverse.

But it hardly appears possible to argue on general
physical grounds that attraction and union imply diversity ;
two masses of substance of identical physical and chemical
constitution show the attraction called gravitation; they
tend to come together and unite. Adhesion occurs between
bodies of like character, and in general it is not clear that
all attraction of bodies must be due to chemical or physical
diversities between them.8 It appears therefore that we
should look for further evidence before holding that union
is itself an evidence of sex diversity. If the two things
that unite are characteristically diverse, the difference must
show itself in other ways, so that we must rely on other
tests to determine whether the diversity exists. Is there
any evidence that the two chromosomes that unite in mat-
ing are sexually diverse?

In Vorticella and its relatives, as we have seen, the two
individuals that mate are diverse (Figures 33 and 34). But
curiously, these two — the "male" and "female" — are formed,
in some species at least, by the division of a single ordinary
fixed individual into a male and a female individual. The
ordinary fixed individuals before this division do not mate, —

tween the two mating cells is necessary o priori, and theoretically re-
quired. For it would not be intelligible that two identical cells coming
in contact should affect one another, stimulate one another out of the
state of repose and cause the complicated sexual processes to begin"
(p. 376).

1 It is doubtless possible that all physical attraction between bodies is
due to their make-up of the two opposite kinds of electrons, so that
in this sense attraction may be due to diversity. But this is diversity
within the bodies: the two masses, as such, may have the same consti-
tution (including these diversities) and still attract. That is, the attrac-
tion of the two cells or individuals that mate does not a priori imply
any greater diversity between them than that between two leaden bullets
that gravitate toward one another.

128 Life and Death, Heredity and Evolution

neither with one another nor with any males that may be
present. So the ordinary individual is neither male nor
female; it is apparently non-sexual or neutral. After it
has divided to produce male and female, the female resembles
externally the neutral individual, but differs from it in
that it may mate. If it Jias no opportunity to mate, it
may return to the neutral condition, and may then again
divide to produce males and females. These facts have
been worked out in Opercularia by Enriques (1907). In
some other Vorticellidse a single ordinary individual may
divide into four or eight males. It is asserted that some-
times the small motile "males" mate together; and that
similarly sometimes two fixed "females" mate; but these
things are uncertain. There is still much to be learned
as to sex diversity in this most interesting group of the
Vorticellidae.

In some infusoria the two individuals that mate are alike
before mating, but become changed and diverse during the
mating process. This is inevitable in some infusoria, since
the organisms are unsymmetrical, and if their mouth sur-
faces are to be brought together, either the two must take
different positions, or one or both must become altered
in structure. This will be evident from Figure 38.

Enriques (1908) studied this matter thoroughly in
Chilodon. In this animal the mouth lies to the left on the
ventral surface, so that when the two individuals are placed
side by side, the mouth of the right-hand individual lies at
the surface of contact, but the mouth of the left-hand in-
dividual is directed away from the surface of contact (Fig-
ure 38). But in the early stages of the process of mating,
the mouth of this left-hand individual moves along the ventral
surface over to the right, — so as to meet the mouth of the
other individual which does not change its position (Figure
39). The anterior end of the left individual likewise be-

Nature of Sexual Diversity

129

Figure 88. Chilodon. Two individuals placed side by side, showing
that the mouths, lying at the left side, will not come in contact in
conjugation without some alteration of position. Based on a fijrure
by Enriques, 1908.

Figure 39. Conjugation of Chilodon. The mouth of the left-hand
individual has moved over to the right, to meet that of the right-hand
individual. After Enriques, 1908 (but reversed in position).

comes bent, in such a way as to shorten the animal, so that
when the two members of a pair are measured, the left
member is shorter than the right-hand one. The result of
all this is that the two mates have become diverse. Enriques
holds this to be the first stage in the production of sex differ-
ences; that the right-hand individual corresponds to a fe-
male, the left-hand one to a male. He believes that in other
species the difference thus produced in the mating process

130 Life and Death, Heredity and Evolution

lasts and is inherited, so that the two sexes are distinguish-
able even before mating occurs. There is no evidence that
such inheritance occurs. This notion of the origin of sex
of course implies that there is no underlying general
physiological difference that makes sex ; sex would be a mere
matter of the external differences. If we believe that there
is such a general physiological difference, then we could
hardly hold that these differences produced during mating
are sex diversities, unless we believe that they are guided
by and coincide with the previously existing physiological
differences. It appears probable that the diversities be-
tween the two mating individuals in Chilodon are mere
transient alterations, with no lasting consequences.

So far we have dealt with the differences, or lack of
differences, between the two individuals that mate. But the
sex diversities reach to the more intimate structures ; to the
nuclei. In Paramecium (Figure 40), each individual pro-
duces a larger half nucleus that is passive, like the female;
a smaller half nucleus, which is active, seeking out and
uniting with the passive half nucleus of the other mate,
and thus playing the part of the male. What is the differ-
ence between these two half nuclei produced by a single
individual, — the "male" and "female" half nuclei? Here we
have the problem of the nature of sex diversity brought to
a point; if we could answer this question for these two
half nuclei, we should know the nature of sex.

The differences that we observe are the following (see
Figure 40 and Figure 49, H). The "male" half nucelus is
a little smaller; it is usually nearer to the surface of union
of the two mates ; it separates from the "female" half nucleus
of the same individual ; it moves toward and across the sur-
face of union ; it refuses to unite with the other "male" half
nucleus which it meets and passes on its way; but it does

Nature of Sexual Diversity

131

Figure 40. The exchange of the half nuclei in Paramecium caudatum,
after Maupas, 1889. The spindle-shaped bodies are the half nuclei.
A. The micronucleus of each member of the pair is dividing into two
halves, which are still connected by a band. The upper half in each
case is the "male" or "migratory" half nucleus. B. The two migratory
half nuclei are passing one another at the surface of separation of the
two individuals. (Each is still connected by a long band with the
other half (stationary) of the nucleus from which it came.) C. The
two migratory half nuclei are close together, but have now completely
lost their connection with the stationary halves. D. The migratory
halves have nearly separated, each passing into the body of the opposite
individual. E. The migratory and stationary half nuclei have come in
contact. F. The migratory and stationary half nuclei have almost
united.

Life and Death, Heredity and Evolution

unite with the "female" half nucleus of the other individual.
The "female" half nucleus is a little larger; it simply re-
mains quiet, finally uniting with the "male" half nucleus which
migrates to it.

The problem of sex comes in perhaps its sharpest form
in the question: What makes the two "male" half nuclei
refuse to unite when they come in contact, while the "male"
and "female" do unite?

One answer to all these questions is that given by Maupas
(1889). According to this, there is no physiological differ-
ence between the two half nuclei ; their difference of behavior
is a mere result of their accidental difference of position.
The migratory half does not move through any peculiarity
of its own ; the fact is merely that the half nearest the surface
of separation is seized by the movements of the surrounding
cytoplasm and carried over into the other mate. The fact
that the two migratory halves do not unite is again merely
due to their being pulled along by the cytoplasm. Save
for this activity of the cytoplasm, the two migratory half
nuclei could just as well unite with each other as with the
inactive half nuclei. According to this way of looking at
the matter, there is no sex difference between these two
half nuclei; indeed, sex has no meaning, save as a name
for certain external peculiarities.

This is one possible opinion on the matter ; it rests upon,
or results in, the general view that there is no general
underlying peculiarity that constitutes sex diversity; and
that sexuality is not a general characteristic of living things.

The other possible view is that the difference in appearance
and behavior of the two half-nuclei is a consequence of an
underlying physiological difference that makes sex. In sup-
port of this view there are urged certain facts observed by a
number of later investigators. One is that it is not true

Nature of Sexual Diversity 1SS

that the half nucleus nearest the surface of separation of
the two mates is always the one that moves over into the
other mate; on the contrary, sometimes one of the nuclei
farther from this surface is the one that thus becomes the
migratory half nucleus.1 Further, the actual smaller size of
the migratory half nucleus, observed in Didinium by Prandtl
(1906), and in Paramecium by Calkins and Cull (1907), is
held to imply an intrinsic difference between the two. Ac-
cording to this way of looking at the matter, one of the half
nuclei is male, the other female, in virtue of their diverse
chemical make-up. Such a view goes with the general theory
that sex in a matter of fundamental physiological diversity,
not a mere name for certain external peculiarities.

Keeping these two contrasted opinions in mind, certain
facts as to what occurs in these organisms are of much
interest. The theory of the need for periodic unions; the
theory of rejuvenescence through such unions, is based,
as we have seen, on the notion that the two nuclei have
been developing in opposite directions, — one toward "male-
ness," the other toward "femaleness," till an unbalanced con-
dition is reached ; union is then required to restore the bal-
ance.

The situation in Paramecium and other infusoria seems
almost to reduce this idea to an absurdity as an explanation
of a necessity for periodic mating. For here the two nuclei
that are assumed to have been developing in opposite direc-
tions,— the "male" and "female" half nuclei, — have in fact
been developing continuously together, in the same micro-
nucleus of the same individual! It is only immediately be-
fore the union of "male" and "female" parts in the mating
that the united "male" and "female" parts have separated.

1 Calkins and Cull, 1907, p. 393; Prandtl, 1906, p. 246; Collin, 1909,
p. 359, etc.

Life and Death, Heredity and Evolution

There is no point to the male element's separating from
the female element, to immediately unite with another female
element, if the whole rationale of the process is merely to
bring maleness and femaleness together; they needed but
to remain where they were!

From the fact that "male" and "female" of one individual
separate, and immediately unite with "female" and "male"
respectively of another individual, it might, seem that the
essential point is not the mere union of male and female,
but the union of parts of diverse individuals, and many
have held this to be the case. But here again we meet a
set of extraordinary facts which make this way of looking
at the matter as difficult as the other. In many organisms
the unions that occur are between half nuclei of the same
individual, or even of the same cell; this is the process that
is becoming familiar under the name autogamy.

A typical striking case of this union of half nuclei from
the same cell is seen in the small parasitic flagellate in-

Figure 41. Mating of two halves of the nucleus of same cell (auto-
gamy) in the cyst of Trichomastix. A, division of the nucleus; B, the
two nuclei separated by a large mass of reserve food matter; C, first
"reduction" division of each nucleus; D, second reduction division; E,
the two nuclei approach each other; F, they unite into one. After
Prowazek from Hartmann, 1909.

Mating of Similar Parts 185

fusorian Trichomastix, found in lizards. The flagellate
forms a small round cyst (Figure 41), in which the single
nucleus divides into two (A), the two nuclei coming to lie
one on each side of a large mass of reserve nutritive material.
Then each nucleus divides twice unequally, giving off two
small nuclei (Figure 41, C and D). [These two small nuclei,
commonly known as reduction nuclei, are absorbed and dis-
appear. The remaining two nuclei, which we can now, for
reasons to be clearly seen later, call half nuclei, move
toward each other (E), and finally unite (F). The process
of mating or of fertilization is now finished.

Here again we have a case in which the two half nuclei
that unite have been developing continuously together, in
a single nucleus; they separate, then (after two more divi-
sions) reunite. And in this case, it is the same two nuclei
that have separated that come together again, — save for the
fact that each has divided off two small nuclei.

The same sort of thing occurs in many lower organisms.

Figure 42. Two methods of conjugation in Spirogyra, after Walton,
1915. A, conjugation between adjoining cells of the same filament.
The contents of the cell to the right have passed into the other cell and
united with its contents. B, conjugation between the cells of different
filaments. From the filament above, the cell contents have passed into
the cells of the filament below.

136 Life and Death, Heredity and Evolution

Sometimes, as in the case just described, it is two nuclei of
the same cell that reunite. In other cases it is half nuclei
from two adjoining cells of the same organism; this hap-
pens in the alga Spirogyra (Figure 42). Or again, two
branches of the same plant may unite, with union of nuclei
(Figure 43). Or again, two cells that have been formed by
the division of a single cell may mate ; this happens in Para-
mecium aurelia at times, as well as in other infusoria, and
in algae. We find every possible transition, from the union
of two half nuclei of a single cell (Figure 41) to the fertili-
zation of one individual by another quite unrelated to it.

In all these cases in which the two half nuclei that unite
have been developing together in a single nucleus, evidently

Figure 43. Process of conjugation of two branches of the same
plant, in the mould Zygorhynchus. A to H, successive stages, leading
to the formation of the dark zygospore in H. After Blakeslee, 1913.

Mating of Similar Parts 187

we cannot explain the process as due to the gradually diver-
gent development of two nuclei, one in the male direction,
the other in the female direction, till they have become so
diverse as to be unbalanced, and so to require reunion. And
there is no ground on this basis for any rejuvenescence to be
produced by the union ; for the male and female parts that
become united were already in union before the separation
and reunion occurred.

A modified form of this notion is held by some students, —
a form so modified as to be almost if not quite empty. In
such a case of reunion of two half nuclei of a single cell
as we see in Figure 41, as well as in all other cases, it is
maintained that the two half nuclei have become diverse, in
the divisions, that have just occurred, — one retaining more
of the kinetic or male characteristics, the other more of the
vegetative or female characteristics; and that this is the
reason why the two now unite. That is, after the two half
nuclei have separated, this theory if correct gives a ground
for their reuniting. But it gives no ground at all for the
fact that the organism periodically goes through this whole
process, of separating off two half nuclei, which then again
unite, — since what is accomplished by their union was al-
ready existent before the process occurred. And it gives
no ground for expecting any rejuvenescence or other marked
physiological result from mating. That is, it gives no ex-
planation for periodic mating such as is given for periodic
taking of food, when we show that it is the taking of food
that makes possible the activities and growth of organisms.
It is therefore not surprising that in a paper maintaining
this theory, Hartmann (1909) concludes with the statement
that he thinks it most improbable that this will turn out to
be the full explanation of the matter.

On the whole, it appears that in Paramecium and many

138 Life and Death, Heredity and Evolution

other infusoria, both mates of a pair play the part of both
"male" and "female," so that diversity of sex cannot be
given as ground for the union of the two individuals. Fur-
ther, there is no clear evidence that the two uniting half
nuclei are diverse in any generally characteristic way ; indeed
on the whole the facts perhaps agree best with the view that
there is no underlying sexual difference in the uniting half
nuclei; that indeed in many of these lower organisms there
is no such thing as sex diversity.

And this too appears the natural conclusion with rela-
tion to that ultimate act of mating, the union of the chromo-
somes. In many organisms this does not occur at once after
the mating of the individuals or the germ cells (though in
some, particularly in the flies, it does). Usually after the
germ cells unite, the chromosomes remain without mating for
many cell generations, through which the fertilized egg de-
velops into the body of a new individual. It is only when
the germ cells of this new individual are ripening for their
next mating that the chromosomes within each cell mate.
At this time they form a set of structures that in some or-
ganisms show many diversities of size and form (Figure 29).
But the mating is not between the most diverse individuals.
On the contrary, we find as a rule that each chromosome has
found as its mate one precisely like itself, — so that the
group was really composed of sets of two individuals of the
same size and form, and it is these two that mate. Excep-
tionally two chromosomes of diverse size or form may mate,
but this is only when a similar mate is lacking, and there is
evidence that such mating between chromosomes of diverse
size or form is not so intimate as that between chromosomes
of similar structure. All the indications are that in this ul-
timate act of mating the union is between structures that are

Nature of Sexual Diversity 139

alike, not structures that are characteristically diverse.4
The theory of two essentially diverse substances, male and
female, cannot be applied in any form to the mating of the
chromosomes.

But if similarity between the parts that mate is the orig-
inal and elementary condition, how does it happen that in
other organisms, and indeed in most, we do find a diversity of
sex? Why do we find that as a rule in higher organisms a
small germ cell unites with a larger one?

This has often been conceived as a special case of division
of labor. It is necessary for movement to take place in
order that union shall occur ; it is also necessary that there
shall be a certain amount of food or stored up energy for
the beginning of development in the new organism. Hence
it appears conceivable that variations should arise among
the originally equivalent cells, — such that one set would
become more active, while the other would store up more
food. This of course could not occur without a correspond-
ing change in the underlying chemical processes, but the
difference in sex would according to this view not be coex-
tensive with life. It would be a difference that has arisen
in evolution, just as the difference between two races of
Difflugia has arisen. In Paramecium and related organisms,
according to this view, the difference has not arisen.

What chiefly raise doubt as to the correctness of this idea
are the cases in which there is a physiological sox diversity
where no structural diversity can be seen. But there exist

4 The chromosomes are structures that perpetuate themselves from
generation to generation, by division, as Protozoa do. It is not difficult
to so arrange breeding experiments that two offspring of the same
chromosome shall in a later generation be found in different sexes and
mate together, just as we may cause two offspring of the same Para-
mecium to breed together. This again would be difficult to reconcile
with the idea that the two mating chromosomes must be sexually di-
verse.

140 Life and Death, Heredity and Evolution

equally cases in which by any physiological test that can
be applied, there is no diversity of sex.

The only doctrine of sex diversity that appears to offer
possibilities of general application is one that holds, not
that there is always a definite positive characteristic for
each sex, but only that there is some relative difference be-
tween the two mates, one of them being somewhat more
developed in a certain direction than the other. It would
not be inconsistent, on this view, if a given individual or
cell or chromosome could play the part of male in some
matings ; of female in others, — depending on the relative de-
velopment of the two mates. But such a doctrine is cer-
tainly not established; and for the mating of the chromo-
somes it has not a particle of evidence.

It is most important not to commit the error common in
evolutionary thought, of identifying the question of the
primitive nature of a phenomenon with that of its physio-
logical significance where it is developed in a marked degree.
The question whether all organisms have sex has little to do
with the question as to what the nature and results of sex
are, in organisms that are divided into two sexes. In such
organisms — in man for example — the fundamental chemical
processes that make up life are in the two sexes diverse in
every cell of the body, for the nuclei on which these processes
depend are diverse. There is no ground therefore for sup-
posing that the characteristic differences in the activities of
the two sexes are mere matters of different education or en-
vironment.

VI

What are the Results of Mating? Rejuvenescence and
Mating. Heredity and Variation, and Mating.

WE have so far tried to judge of the nature and effects
of mating chiefly by examining the organisms or
parts that unite, and the processes that occur in union.
Now we turn to the study by observation and experiment of
the effects of mating. Does mating actually cause re-
juvenescence, in the sense of an increased vigor and vital-
ity? The chief definite criterion for such increased vigor
that has been proposed is, that after mating, reproduction
should take place more energetically than before. In the
Protozoa, for example, old age is held to show itself by a
decrease in the rate of multiplication; rejuvenescence by
an increase. This is the theory which Calkins in recent
times has made peculiarly his own. "As with the metazoon
so with the aggregate of Protozoa cells, we note a period
of youth characterized by active cell proliferation; this in
both groups of organisms is followed by the gradual loss
of the division energy accompanied by morphological
changes in type of the cells preliminary to conjugation and
fertilization and to the renewal of vitality by this means"
(Calkins, 1906, p. 232). This appears to be what is com-
monly understood by the theory of rejuvenescence.

How can we find out whether this theory is correct? Evi-
dently the direct method is to take a stock of infusoria, such
as Paramecium, and allow it to multiply till it gets to the

141

143 Life and Death, Heredity and Evolution

stage in which it is ready to conjugate. Then permit part
to conjugate, while the rest are not allowed to do so, and
compare the rate of reproduction in the two sets, under
identical conditions. Will those that have been allowed to
conjugate reproduce more rapidly than the others?

This experiment was first performed by Richard Hertwig
(1889). At the beginning of conjugation in Paramecium
he separated some of the pairs before the process had been
accomplished, while others he allowed to complete the mat-
ing processes; he then compared the rate of fission in the
two sets. Calkins in 1901 similarly compared the rate of
fission in a single line derived from an ex-con jugant with
the non-conjugant stock from which it came (the experi-
ment is described in full by Calkins and Gregory, 1913).
I carried out the same experiment on a very large scale
in 1913, employing large numbers of lines of both non-con-
jugant and conjugant origin, and repeating the experiment
many times. Mast (1917) has recently repeated the ex-
periment with a number of lines of Didinium, and Calkins
(1919) has still more recently carried it out with Uroleptus.

Furthermore, Maupas (1888 and 1889) made extensive
and thorough researches on the same question in many kinds
of infusoria, by somewhat different methods. He did not
separate pairs that were beginning to mate, as was done
in the other experiments, but compared the rate of fission
soon after conjugation with the rate after many genera-
tions had elapsed since conjugation.

All these experiments except those which have been re-
ported by Calkins, gave concordant results, unfavorable to
the idea that conjugation increases the rate of fission. Mau-
pas found that there was in none of the cases he studied an in-
crease of fission rate after conjugation; nor a decrease as

Conjugation and Rate of Reproduction 143

the time elapsed since conjugation became greater. Hertwig
found that the descendants of those that had conjugated
actually multiplied more slowly than the descendants of their
companions that were not allowed to conjugate. Mast found
that in Didinium the rate of fission after conjugation is a
little slower for a short time ; but soon the rate becomes equal
in those that have and those that have not lately conjugated.
In my own extensive experiments, with large numbers of
lines in both classes, the average fission rate was uniformly
less in the lines that had recently conjugated than in those
that had not. Conjugation, as we shall see, caused much
variation between the diverse lines, in fission rate as well as in
other hereditary characters. At the upper extreme of the
variation sometimes a conjugant line exceeded in its fission
rate the non-con jugant lines. But in most of the lines the
rate after conjugation was decidedly less than before; uni-
formly the average of the conjugant lines was much below
that of the non-con jugant ones.1

In Calkins' experiments, on the other hand, the lines which
had conjugated showed a more rapid fission rate than those
which had not. In his experiment on Paramecium in 1901, in
which a single line derived from an ex-con jugant was com-
pared with the non-con jugant stock from which it had come,
the ex-con jugant line divided 376 times in nine months,
while the non-con jugant lines produced but 277 generations.
It may be noted that another conjugant line, derived from
the mate of the line that produced 376 generations, died out
completely after the production of but 11 generations. This
illustrates the fact shown on a targe scale in my own ex-
periments, that conjugation brings about great variation in

irThe actual comparative rates in certain cases will be given later
(page 158).

144 Life and Death, Heredity and Evolution

vitality and reproductive power. The single line retained by
Calkins was clearly one of the rare extreme variants, with a
high fission rate.

In Calkins' recent work on Uroleptus,2 it is set forth that
in this organism the effect of conjugation is uniformly to in-
crease the rate of fission. The fission rate, however, is given
in terms of averages of sets of five lines ; such averages were
always higher for the sets that had recently conjugated than
for those that had not. But it is not possible to discover
from them whether the fission rate was higher in all the lines
of conjugants or whether as in my own experiments the rate
was lower in some of the conjugant lines, higher in the oth-
ers. Obviously it was higher in the majority at least of the
conjugated lines.

The fact that some of the lines derived from ex-con jugants
may exceed in their fission rate the non-conjugant stocks,
whether this is rare, as in Paramecium and apparently in
most other infusoria, or common, as in Uroleptus, is unques-
tionably a fact of great significance ; this renders it possible
to still maintain, as Calkins does, that mating at least some-
times produces rejuvenescence, in the sense of increased
vigor of reproduction. We shall discuss this matter farther
in dealing with the question of the production of inherited
variation by conjugation. But if we base our conclusions on
the usual or the average effect of conjugation, we are forced
to conclude that in most cases it does not increase the rate
of multiplication, but rather it decreases this rate.

It is not generally realized that the work of Maupas
and of Richard Hertwig, on whose authority the theories of
rejuvenescence by conjugation largely rest, was squarely
opposed to the idea that this rejuvenescence manifests itself
by an increase of reproductive vigor. It is worth while to
a Calkins, G. N., Proc. Nat. Acad., AprU, 1919.

Conjugation and Rate of Reproduction 145

emphasize this point, for upon it an extraordinary error has
"become widespread even among expert biologists.8

Maupas rejected emphatically the idea that aging shows
itself in a decrease of the rate of reproduction and that
conjugation restores this declining rate. He asserted posi-
tively that no such decline occurred before conjugation and
that after conjugation reproduction was not more rapid
than before, and he based these statements on the results of
careful experimentation. Richard Hertwig's experiments
led to the same result and induced him to subscribe com-
pletely to Maupas' assertions on this point. The matter is
important, since the doctrine that conjugation restores the
declining fission rate depends largely on their supposed sup-
port of it. I therefore give in their own words statements
of the results of their experiments, and of the conclusions
they drew from these results.

Maupas sums up the prevailing mistaken doctrine of his
time on this matter in words which well fit the present con-
ditions :

"It has been affirmed that the faculty of reproducing by
fission in the ciliates was modified by conjugation, and that
the principal effect of this sexual act was to reenforce and
accelerate this reproduction. The ciliates, it was held, im-
mediately after conjugation multiply more rapidly than
they do at a later stage. This opinion has become current,
and one finds it reproduced in Memoirs and in general trea-
tises, as if it were a definitely established truth" (1888, p.
254-255).

Maupas then proceeds to show that the evidence on which

•Thus, investigators so experienced as Woodruff and Erdmann in a
recent paper (1914) contrast my own results on this point wit
view of Butschli, Maupas, Hertwig and others," asserting that i
ing to the latter, after mating "all the processes of the cell, including
reproduction, proceed with greater vigor."

146 Life and Death, Heredity and Evolution

this theory had been based amounted to nothing. Next he
sets forth that in his own records of fissions in different in-
fusoria, beginning in a number of cases with animals that
had just conjugated, there is no indication of a greater
rate of fission after conjugation. He says of the fis-
sions :

"They succeed one another uniformly, modified only by
the changes of temperature. I did not remain content with
this one experiment. I isolated other ex-con jugants of
StylonycJiia pustulata, of Onychodromus grandis, of
Euplotes patella, of Paramecium aurelia, of Leucophrys
patula. I followed day by day the successive generations of
their descendants, during periods of from fifteen days to two
months. In none of these species did I see any difference in
the succession of fissions. Whether they had conjugated
lately or a long time ago, all the individuals acted in the
same way" (1888, p. 255-256).

Maupas sums up the matter in a later paper as follows:

"I have asserted, besides, that this power of multiplica-
tion is maintained regularly and uniformly during the entire
life cycle; that there is no gradual weakening of this power
from the first generation after conjugation up to the re-
turn of a new period of maturity. In other words, I deny
that the infusoria after conjugation have a more energetic
reproduction than they have at a later period" (1889, p.
504).

Certainly this is sufficiently explicit not to be misunder-
stood ! It is because Maupas' papers, with their hundreds of
pages of text filled with observational and experimental de-
tails, make hard reading, that it is possible for mistaken
ideas of his results to become prevalent.

Richard Hertwig (1889) found in his experiment, to his
surprise (as will anyone that tries it), not only that the

Theories of Rejuvenescence 147

Paramecia which had conjugated did not reproduce faster,
but that, on the contrary, it was those that had not been
allowed to conjugate that multiplied more rapidly. Speak-
ing of the theory that conjugation increases the energy
of reproduction, Hertwig says :

"The grounds on which this theory is based have already
been combated by Maupas; he showed by extended experi-
ments that the power of multiplication of an infusorian is
neither decreased before conjugation nor increased after it.
... I am compelled to say that Maupas was entirely right"
(1889, p. 222).

What then did Maupas and Hertwig mean by holding that
conjugation does nevertheless rejuvenate?

Maupas' theory is not easy to seize or to state in experi-
mental terms. He believed that without conjugation the
organisms became deformed and structurally degenerate,
although their power of reproduction remained unimpaired;
at last they died. Conjugation, he believed, if it occurs
before degeneration has become evident, prevents the process
of degeneration. His own experiments showed him, as we
shall see, that after observable degeneration has begun con-
jugation does not remedy it. The only test for Maupas'
theory is formed by such experiments as those of Woodruff,
in which it is shown that even without conjugation the ani-
mals live indefinitely and do not degenerate.

Hertwig (1889), taking the bull by the horns, held with
relation to Paramecium, that an increase in the rate of re-
production was one of the symptoms of degeneration; that
conjugation in restoring the balance decreased the rapidity
of fission. It is extraordinary that two such contradictory
views can be held of what conjugation does, as are these two
— Hertwig on the one hand maintaining that conjugation
decreased the rate of reproduction, Calkins on the other
that it increases it — both under the name of rejuvenescence!

148 Life and Death, Heredity and Evolution

Hertwig has developed later a general theory of mating,
rejuvenescence and kindred matters; a theory which is at
once extremely special and extremely indefinite. It is based
upon ideas of a necessary proportion in quantity between the
mass of the nucleus and that of the cytoplasm. As growth
occurs the cytoplasm increases faster than the nucleus, so
that after a time the nucleus is too small in proportion to the
amount of cytoplasm. This brings about in some way a sud-
den increase in the growth of the nucleus, and this in turn
causes division of the cell. In the course of many cell
generations, through irregularities in division, and through
the action of various agents, the nucleus may become too
large in proportion to the cytoplasm. Enlargement of the
nucleus has been found to occur in cold; in hunger; when
the animals are overfed; and in various other conditions.
In such conditions the animals do not divide frequently,
and this is attributed by Hertwig's theory to the dispropor-
tionate size of the nuclei. Such conditions of large nuclei
and slow reproduction, with other pathological symptoms,
are spoken of as states of "depression." Such depression
can be remedied, if it has not gone too far, in various ways,
— by change of temperature, of food, and the like ; the essen-
tial thing that then happens, according to this view, is that
the nucleus reduces its size, by throwing out substance into
the cytoplasm, or otherwise. But if depression has gone
too far for this, it can be overcome only by a deep-seated
reorganization of the nucleus. Such a deep-seated reor-
ganization occurs in the processes connected with mating;
hence, it is held, these processes restore the normal balance
of nucleus and cytoplasm; the depression disappears, and
the organisms continue to live and multiply.

Such a theory appears artificial and based upon super-
ficial features. When one considers the complex chemical

Theories of Rejuvenescence 149

processes that lie at the foundation of life, it is difficult
to believe that mere relations of proportionate quantity of
two things so complex as nucleus and cytoplasm, both having
the power of assimilation and growth, is at the bottom of
these matters of life, death and reproduction. As Dobell
(1909) expresses it, one would almost "as soon argue that
grey hairs are the cause of old age in man" as to hold
that increase in size of the nuclei is the cause of degeneration
and death in cells. Furthermore, this theory gives absolutely
no ground for the chief feature of mating, — the fact that it
i* a mating of two nuclei, not a mere reorganization. And
like the other theories of rejuvenescence, its validity depends
finally on whether mating actually does restore vigor, vital-
ity and reproductive power to depressed organisms. Most
of such theories have proceeded on the ground that if one
can show that degeneration occurs without conjugation, then
it must follow that mating remedies the degeneration. The
real test lies in observing whether mating actually does rem-
edy the degeneration. If it does not, the probability be-
comes strong that the degeneration is simply a pathological
result of bad conditions. The evidence is becoming over-
whelming that this is the case; that to avoid degeneration,
it is merely necessary to avoid the bad conditions. The work
of Woodruff, set forth in our first lecture, goes far to dem-
onstrate this.

Hertwig's own test of the matter in Paramecium was, as
we have seen, opposed to the idea that mating increases
the vigor of reproduction; Maupas5 extensive work gave the
same result, and my own still more extensive experiments
led to the same conclusion. Only Calkins' experiments with
Uroleptus stand in the way of asserting this to be the general
rule in infusoria.

Certainly no clear case has been made out for rejuvenes-

150 Life and Death, Heredity and Evolution

cence as a general result of mating. Indeed it is only fair to
say that most of the positive evidence is against rejuvenes-
cence of any sort. In Paramecium, not only do those that
have conjugated multiply less rapidly than before, but a
great many of them die after conjugation, — although with-
out conjugation they live vigorously. Many others are weak
and sickly, multiplying but little. Still further, conjugation
produces a great number of abnormalities and monstrosities,
such as do not occur without mating (see Figure 44). In
many other infusoria all or nearly all those that have con-
jugated die, or completely cease multiplying. Maupas made
many investigations giving such results. He allowed two
unrelated cultures of Stylonychia to become degenerated,
then let them conjugate together; according to the theory
this ought to have rejuvenated them. But it did not; they
all died. He found that even in wild Stylonychias conjuga-
tion rather usually results in death ; later students have con-
firmed this. In his experiments, Maupas found that the
individuals that have conjugated all die in Spirostomum,
Climacostomum and Didinium. In Leucophrys he found
that a large proportion of those that mate die. It used to
be supposed that such results were due to the fact that the
animals had become so degenerate that even conjugation
could no longer save them. But we now know that under
proper conditions, without conjugation they would (in most
cases at least) live and multiply vigorously, while after con-
jugation they are weak or die.

Such facts are most extraordinary; they are difficult to
explain on any theory. A priori it appears that the re-
placement of the old active nucleus by the reserve nucleus,
must tend to give rejuvenescence in conjugation, just as
when it occurs without conjugation. But this tendency is in
the majority of cases completely overwhelmed and done away
with by other features of mating; by the physiological diffi-

Distinctive Results of Mating 151

cultics of the complex process; and as we shall try to show,
by frequent incompatibilities between the parts united.

In rare lines in Paramecium, and other infusoria, and
commonly in Uroleptus, according to Calkins, all these diffi-
culties are overcome, so that the replacement of the worn
nucleus by the reserve has the same effect in renewing vitality
that it has when its occurs without mating. There is thus
far no evidence either from Calkins' experiment with Urolep-
tus, or from any others, that the mating, as distinguished
from the replacement of tLc macronucleus by the micronuc-
leus, produces rejuvenescence. Calkins finds that in Ui'olep-
tus this replacement without conjugation restores vitality
as it does with conjugation.

What then are the distinctive results of mating, as com-
pared with that replacement which occurs without mating?
On this the theory of rejuvenescence has not thus far cast
light. The thing to do under such circumstances is to drop
for the time any definite preconceived theory, and examine
with care the facts ; sometimes these carry a theory of their
own! Just what differences can we find between individuals
that have mated and those that have not?

In our examination of the question of diversity of sex in
Paramecium (Lecture 5), we mentioned a difference that was
found in that organism. After mating, the two individuals
and their offspring have become more alike than the two
stocks were before mating. This, as before remarked, is not
unnatural, for each individual after mating is, if it may
be so expressed, partly made up from the other individual.
In all of the offspring of each of the mates, half the nucleus
has come from the other mate. It is therefore not surprising
that the two sets of offspring are much alike; they are all
children of the same family; they are as closely related as
are the brothers and sisters of a human family.

Here then we have in these lower organisms a distinctive

Life and Death, Heredity and Evolution

visible difference made by mating, and it is the same sort of
result that is produced by mating in higher organisms. In
the latter in consequence of mating, offspring are produced
which are more alike than are individuals taken at random;
this is likewise what happens in Paramecium.

In higher organisms we say that this result is an example
of heredity. As we are accustomed to think of it, the off-
spring in a family inherit from both their parents ; they
resemble both the parents ; and this appeals to us as a reason
why they should resemble each other. Now, whether the off-
spring of the two members of a pair of Paramecia resemble
also their parents has not been directly shown; it is a dif-
ficult matter to get at; what we know is that they do
show a marked resemblance to each other. But this matter
of resemblance between the external features of parent and
offspring is in reality a totally inadequate basis for a con-
ception of heredity. In the study of heredity in higher
organisms, we often discover that the offspring inherit char-
acteristics from the parents which the parents do not show,
— as when rabbits with white hair are born from parents
with brown hair; or when two white fowls produce black
chicks. In such cases the black color of the chicks is in-
herited from the white parents just as truly and in just the
same way as occurs when black parents produce black
chicks. The only consistent meaning we can give to heredity
is this : heredity is the appearance of peculiarities in the off-
spring that are due to the peculiarities of the germinal
material they obtained from their parents. And we know
that white fowls do frequently transmit to their offspring
germ cells of such a character that the offspring must be
black; such cases are recognized as clear examples of Men-
delian inheritance. By biparental inheritance we mean mere-
ly this: the peculiarities of the given individual, taken as a

Biparental Inheritance and Conjugation

whole, are due to its origin from the united germinal ma-
terial from two parents; if the individual had been derived
from but one parent, it would have shown other peculiarities.
Thus when two kinds of white flowers produce by their union
plants with red flowers, this is an example of biparental in-
heritance (and one of not uncommon kind), — for the red
color would not have occurred if the two parents had not
united, as for example in reproduction through cuttings.

Now, the resemblance between the offspring of the two
members of a pair of infusoria is due to the fact that they
have received germinal material (the two halves of their
micronuclei) from both parents. It is therefore an example
of biparental inheritance.

I have given this brief exposition of what heredity means,
and particularly of what biparental inheritance means, be-
cause to persons not familiar with experimental work in
heredity, it appears surprising that we should speak of bi-
parental inheritance when we haven't proved that the off-
spring resemble their two parents; this objection has been
made by Dobell (1914). But as a matter of fact we speak
of inheritance equally when we know that the offspring do
not resemble their parents, — provided their peculiarities are
due to the germinal material derived from their parents.
The other significance for heredity would exclude a large
proportion of the best known cases of Mendelian inherit-
ance.

We find then that in these Protozoa, as in higher organ-
isms, mating results in inheritance from both parents. In
what respects does such inheritance occur? Our knowledge
on this point is scanty ; until recently it was confined to cer-
tain characteristics of Paramecium; to this has now been
added important knowledge of these matters in Chlamydo-
monas. This is one of the lines of work in which there is

154 Life and Death, Heredity and Evolution

most room for acquiring valuable knowledge; but it is ex-
tremely difficult to carry out the necessary experiments.
The facts for Paramecium are as follows :

[1^ In Paramecium, biparental inheritance occurs with
reference to size; after conjugation the two sets of progeny
are more alike in size than before (See Jennings and Lash-
ley, 1913a).

(2) It occurs also with relation to rate of fission. Here
it is often very striking. If the descendants of one member
of a given pair multiply rapidly, the descendants of the other
member are likely to multiply rapidly also ; if one set multi-
ply slowly, so also as a rule does the other set. One or two
examples (from Jennings and Lashley, 1913) of this are
perhaps worth while. Calling the two members of a given
pair a and b, the numbers of fissions for four successive
periods of ten days each were for several pairs, all under the
same conditions :

Pair

Total

/ii

fa

10

11

10

12 = 43

•i

I b

10

10

12

15 = 47

/ a

5

5

5

4 = 19

47

5

7

4

5 = 21

9

{I

9

8

7
6

8
6

8 = 32
9 = 29

108

{b

11
11

11
13

13
12

13 = 48
13 = 49

As you see, both members of pair 41 divide rapidly; of
pair 47 slowly ; of 9, at an intermediate rate ; while in pair
108 both members divide still more rapidly than in pair
41. This condition of affairs is typical.

(3) There is a similar resemblance between the offspring
of the two members in respect to vigor and vitality. Some-
times the descendants of one pair are weak and the stock
gradually dies out. When this happens, the family derived

Biparental Inheritance and Conjugation 155

from the other member is likely to be weak and die out also.
This was studied with very great thoroughness, and a strong
tendency to likeness in this respect was found (Jennings and
Lashley, 1913).

(4) A fourth respect in which the offspring of the two
that have mated tend to be alike is in certain structural ab-
normalities that often occur (Stocking, 1915). Such abnor-
malities are shown in Figure 44. Frequently these abnor-
malities are hereditary in the family derived from one mem-
ber of a pair; when this is the case, the family derived from
the other member often shows such abnormalities also, —
much more frequently than is the case when its mate is nor-
mal.

In all respects in which the matter has been examined,
therefore, mating tends to make the families derived from
the two members of a pair more alike than they would have
been without it. Conjugation produces biparental inherit-
ance, just as fertilization does in higher organisms.

In Chlamydomonas, Pascher 4 has recently succeeded in
studying the results of conjugation with respect to charac-
ters of a more tangible sort than those that we were forced to
use in Paramecium. Two species of Chlamydomonas differed
in form and structure in the way shown in Figure 43a, A and
B. In conjugation these organisms unite completely, form-
ing a cyst. The cyst formed by the conjugation of two in-
dividuals like A has no membrane and is covered with pyra-
midal elevations, as shown at AA, while that formed by two
like B is smooth and surrounded by thick membranes (BB).
When A conjugates with B, the resulting cyst is intermediate
between the two pure forms, as shown at AB ; there is a thin
layer of membranes, and the surface is covered with low
rounded elevations.
4 Pascher, A., Ber. d. Deutsch. Bot. Ges., 1916 and 1918.

156 Life and Death, Heredity and Evolution

Figure 43a. Biparental inheritance and the production of diversity
by conjugation, in Chlamydomonas, after Pascher (1916), from Hart-
mann.1 A and B, the two parental species. AA, cyst formed by con-
jugation of two individuals such as A. BB, cyst formed by conjugation
of B. AB, cyst formed by conjugation of A with B. a, b, c, d, the
four types of free living individuals resulting from the conjugation of
A and B.

In this cyst the reducing divisions occur (Lecture VII),
and it divides into four free-swimming individuals which
when the conjugation was between two similar parents, are
like those parents. These free individuals now reproduce by
fission in the usual way.

When the cyst had been formed by conjugation of the two
diverse kinds of parents A and B, the progeny show a num-
'Hartmann, M., Zeittchr, f. Ind. Abst., September, 1918.

Diversity Produced by Conjugation 157

bcr of diverse combinations of the characters of the two par-
ents, as shown in Figure 43a, a, b, c, d. Pascher (1918)
gives a table of the various combinations found in the four
sets of individuals in comparison with the characteristics of
the parents A and B, as follows:

Form Membrane Papilla Chromatophore Eyetpot
Parent A pear-shaped delicate none lateral linear
Parent B spherical coarse present basal broad

Offspring o

pear-shaped

delicate

none

lateral

linear

b

pear-shaped

delicate

none

basal

broad

" c

ellipsoidal

coarse

present

lateral

linear

d

spherical

coarse

present

basal

broad

The offspring a and d resemble very closely the two par-
ents respectively, while the types b and c show characters of
both the parental forms. This is the same sort of result
which we find in the Mendelian inheritance of higher organ-
isms. It is a demonstrated fact, therefore, that in the
Protozoa, as in higher forms, conjugation results in bi-
parental inheritance.

Observe also that besides producing resemblance of par-
ents to progeny or of progeny to one another, conjugation
causes stocks or races or families to arise which are diverse
in their hereditary characters from either of those from
which they took origin. This is evident in Chlamydomonas ;
6 and c show diverse combinations from those found in
either of the parents, and these diversities are perpetuated
in their propagation by fission. In Paramecium, from a
single race — all having the same hereditary characters —
there may arise by conjugation many races with diverse
hereditary characters.

What this means will best be illustrated from an actual
experiment (from Jennings, 1913). We begin with a set of
individuals all alike, all derived by fission from a single
parent. We study their rate of fission ; taking 174 separate

158 Life and Death, Heredity and Evolution

lines of descent, we find it to be extremely uniform. For
periods of 21 days we find the number of fissions of the
first 24 lines to be those given under "Non-con jugants,"
a and b, in the following table. (The two individuals, a
and 6, of the non-conjugants, had begun to mate, but were
separated before the mating process occurred.) In these 24
lines the number of fissions in 21 days varied only from 21
to 26; it is on the whole very uniform. One finds little or
no indication of inherited differences between the families;
they run very evenly.

Now we allow a large number of the individuals to con-
jugate, then follow the rate of fission for 88 lines derived
from these mates. We find that there are now great dif-
ferences between the separate families with respect to fission
rate. In the table the two last columns, headed "Con-
jugants," give the number of fissions for 24 lines descended
from pairs of con jugants, for the same 21 days as for
those that have not conjugated. The two sets (Non-
conjugants and Con jugants) were kept throughout under
the same conditions.

Table

Comparative fission numbers in 21 days of Con jugants and Non-
conjugants derived from a single race E (from Tables 34 and 35;
Jennings, 1913). In the non-conjugants, a and b represent two indi-
viduals that had begun to mate, but were separated before mating was
accomplished. In the con jugants, a and b are the two mates from one
pair.

Non-conjugants Con jugants

Pair a b a b

1 25 23 30 29

2 24 23 25 24

3 24 24 29 28

4 24 25 10 9

5 23 24 8 10

6 22 25 24 24

7 22 26 29 28

8 21 25 25 24

9 24 25 30 27

10 22 26 26 26

11 23 24 28 27

12 25 24 11 9

Diversity Produced by Conjugation 159

In the descendants of those that have conjugated the
number of fissions in 21 days varies from 8 to 30, as
compared with 21 to 26 in the others. There are now
strongly marked differences between the different families
that have descended from those that have conjugated. It is
interesting to compare the records of fissions by two-day
periods for a considerable time in such families derived
from different ex-con jugants. Typical examples are as
follows :

Family

•

1 ax

0

2 2

2

3

4

4

4

3

3

3

3 = 33

6 ax.

0

2 1

1

1

1

1

2

2

2

1

2 = 16

9 ax

1

2 2

3

4

4

3

3

3

3

3

4 = 35

2T ax

1

3 1

1

2

2

2

2

2

2

3

1=22

15 ax

2

2 2

2

1

2

0

1

0

1 -

0

0 = 13

All these families were kept under the same conditions,
yet they consistently differ in their fission rates throughout
the entire period. They have become hereditarily diverse
in this respect, as a result of conjugation, for all were
derived, before conjugation, from the same original par-
ent, and all had the same rates of fission. Conjugation
has produced from a single stock a number of hereditarily
diverse stocks. The same thing was demonstrated in a
number of extensive experiments.

Another respect in which conjugation increases the
hereditary differentiation is in the matter of abnormalities.
If we begin with a set of the animals that are all quite
normal, showing no inherited abnormalities, and allow them
to conjugate, we often find that many of the families pro-
duced have hereditary abnormalities, while others have none.
Such hereditary abnormalities, produced as a result of con-
jugation, are shown in Figure 44. In work by Dr. Stock-
ing (1915), of 450 families descended from animals that
have conjugated, 262, or more than half, showed hereditary

160 Life and Death, Heredity and Evolution

ABNORMAL
ABNORMAL

ABNORMA

ABNORMAL ABNORMALABNORMAL ABNORMAL

ABNORMAL;

DEAD

Figure 44. A family of Paramecium caudatum, descended from an
ex-con jugant, and showing hereditary abnormalities. Besides the indi-
viduals figured, other abnormal branches of the family are indicated
by the lines with the word "abnormal." After Stocking, 1915.

abnormalities, — although none were present before conjuga-
tion.

Diversity Produced by Conjugation 161

There is evidence also that conjugation causes hered-
itary differentations to appear with respect to size, but
this matter has not been so precisely studied as it deserves
to be.

Calkins and Gregory (1913) observed similar hereditary
variations in fission rate and vitality arising after con-
jugating. They give some evidence that hereditarily differ-
ent families may arise from the first two divisions of one
of the members of a pair. During conjugation, as we have
seen, new macronuclei are produced from the reserve micro-
nucleus (see Figure 30). The first four of these are pro-
duced within the individual that has conjugated, and be-
fore reproduction takes place. Then by two divisions these
are distributed to the four progeny. According to the re-
sults of Calkins and Gregory, the four individuals receiving
the four macronuclei thus produced may become diverse;
probably, it would seem, through the diversity of these four
nuclei. In later divisions such diversities do not appear,
according to their account ; at least they say that each of
the lines produced remains "true to its type for many
months at least." I believe that this matter requires
further study, but according to the results of Calkins and
Gregory, these inherited differences are strictly the result of
conjugation, just as are those shown in my own work.

We find therefore that in all the hereditary characters in
which the matter has been studied, conjugation gives rise to
inherited differences; in other words, diverse stocks arise
as a result of conjugation. Such work needs to be greatly
extended; all we know on the matter is based on Chlamydo-
monas and Paramecium, mainly the latter, and its characters
are not so favorable for such work as might perhaps be
found. But such' work is extremely difficult.5

'Certain work of Mast (1917), bearing on this point, in the infu-
sorian Didinium, will be taken up in our next lecture.

162 Life and Death, Heredity and Evolution

We must now revert for a moment to the relation of this
production of diverse stocks through mating to the problem
of rejuvenescence. Among the characters in which mat-
ing induces hereditary diversities is the rate of fission.
This has been illustrated on page 158.

Now, as we have seen, if we take all the lines that have
conjugated and average them, we find that their average
fission rate is somewhat less than the average rate of those
that have not conjugated. Thus in a very extensive ex-
periment in which 69 lines derived from conjugants were
compared for a period of three weeks with 145 lines derived
from non-conjugants, the daily fission rate of the con-
jugants averaged 1.097, that of the non-conjugants 1.144
(Jennings, 1913, p. 349). For the period of 21 days the
average number of fissions in each line was for the
descendants of the conjugants 23.041 ; for the descendants
of the non-conjugants, 24.034. When we study the separate
lines we find that those descended from the non-conjugants
all show nearly the same number of fissions ; the slowest line
had 18 fissions, the fastest 28, in 21 days. But the lines
descended from the conjugants differ greatly among them-
selves; the slowest line has for the twenty-one days only
9 fissions, while the most rapid one has 31. That is,
mating has caused much hereditary diversity in the fission
rate of the descendants of the conjugants, and those at one
extreme of the variation have a higher fission rate than
those descended from the non-conjugants. Out of the de-
scendants of 56 ex-con jugants, in this experiment, 9 pro-
duced families with a higher fission rate than any of the
families descended from the 130 non-conjugants. On the
other hand, 9 of the families produced from the ex-
con jugants had a lower fission rate than the low family (with
18 fissions) produced from the non-conjugants.

Diversity Produced by Conjugation 163

The descendants of the ex-con jugants do not by any
means always show some families that exceed the fission
rate in the descendants of the non-con jugants. In nine ex-
tensive cultures (described in my paper of 1913, page 364)
in which this matter was studied, in but three did the
maximum for the con jugants exceed that for the non-con-
jugants, while in all cases the minimum and the average for
the con jugants were below those for the non-con jugants.

We have set forth the relations found in Paramecium, the
organism most thoroughly studied from this point of view.
In Uroleptus, according to Calkins, those that have conju-
gated usually show an increased rate of reproduction. The
production among the combinations resulting from mating,
of families with an increased vigor of reproduction (usual
in some infusoria, occasional or rare in others) of course
justifies the statement that in those cases conjugation has
caused rejuvenescence, as maintained by Calkins. Consis-
tency with the facts compels one, however, to say that while
mating sometimes produces rejuvenescence, in most of the
individuals that mate, it produces, in the majority of the
known species, not rejuvenescence, but its opposite, the ani-
mals being less vigorous and multiplying less rapidly than
would have been the case if they had not mated.

But why should mating produce rejuvenescence in some
cases and not in others? In other words, why has mating
such diverse hereditary results in different cases ? This ques-
tion we shall take up in our next lecture ; here we may indi-
cate merely the general nature of the answer which we shall
reach. We should expect conjugation to give rejuvenescence
just as endomixis does, since in it too the old macronucleus
is replaced by the unused micronucleus. But in conjugation
there are likewise produced new combinations of the parental
characters. Some of these new combinations are vigorous;

164 Life and Death, Heredity and Evolution

others are^weak. In the former rejuvenescence appears; in
the latter it does not.

What conditions bring about mating in these organisms?
It is of interest to bring the facts observed on this point
into relation with those we have above set forth as to the
results of mating. Many generations pass in which the in-
dividuals do not mate; then at a certain time some or many
or all of them mate. Is this the result of the internal
changes that have been in slow progress, so that conjugation
occurs when a condition of ripeness or of need for it has
been reached? This was the idea held by many who believed
the life of these creatures to go in cycles of youth and age;
mating occurs, it was held, in a certain period of the cycle.
Or, on the other hand, is mating rather brought on by certain
external conditions? Much study has been devoted to these
questions.

In Paramecium and many other infusoria it has been ob-
served that an epidemic of mating usually occurs when a
period of high nutrition, resulting in rapid multiplication,
is followed by a period of scarcity of food. Artificial cul-
tures of hay or other vegetation in the laboratory often go
through such a cycle; at first bacteria are abundant and
the infusoria flourish on them ; then fermentative changes go
so far that the appropriate bacteria are scarce; the in-
fusoria become thin, and begin to mate. It is easy to fur-
nish these conditions if from a flourishing hay culture we
remove a watch glass of the water with many of the infusoria
and allow it to stand, without any of the vegetable material,
for 24 hours. As the bacteria become scarce the infusoria
conjugate. This method has been used practically by many
investigators in order to obtain matings for study.

If part of the animals are kept supplied with abundant

Conditions Inducmg Conjugation 165

food, while the ofhers are subjected to a scarcity, the latter
conjugate, while the former do not. This is true even when
all the individuals are derived from the same single original
parent. It is clear therefore that there is no imperious ne-
cessity for conjugation at a particular period in the life
history ; and that a period of scarcity following a period of
abundance will induce conjugation when it would otherwise
not occur. In some of the writer's experiments the offspring
of a single individual were divided into two sets ; one set was
caused in this way to go through conjugation four times
in succession (the mates at any conjugation being the off-
spring of the mates at the preceding conjugation) ; while the
other set during the entire period did not conjugate at all.
In some races of Paramecium aurelia, after a pair had mated
their descendants in the fourth generation were thus caused
to mate again ; while in other members of the same stock
hundreds of generations passed without conjugation.

Clearly therefore the occurrence of conjugation is in large
measure the result of special external conditions. This mat-
ter has been much studied of late by Enriques and by his
pupil Zweibaum. They have found that conjugation is
favored by special conditions in particular species of in-
fusoria; thus in Colpoda steinii conjugation occurs when the
layer of water in which they are has become a thin film
(Enriques 1907), — as happens just before a pool is dried
up by evaporation. In Cryptochilum Enriques (1910) dis-
covered that certain salts greatly favor conjugation. In
Paramecium caudatum Zweibaum (1912) has determined
with great precision the conditions that induce mating. He
finds that after the infusoria have been subjected to a period
of scarcity of food for 6ve to six weeks, if the nutritive
conditions are suddenly changed for the worse, and at the
same time certain salts are present in proper concentration,

166 Life and Death, Heredity and Evolution

the animals will always conjugate. The salts found to favor
conjugation were the compounds of sodium and other metals
with chlorine, bromine and other halogens. Aluminium
chloride was found to be the most favorable of those studied.

Thus Zweibaum and Enriques hold that the environmental
conditions, past or present, fully determine whether conjuga-
tion shall occur. It is true that of two stocks side by side
under the same present conditions, one may conjugate, the
other not ; but this in their opinion is due to the fact that
one has been subjected to a long period of scarcity of food,
while the other has not. That is, while the two stocks may
indeed at a given time differ in their internal conditions, this
difference is not a matter of diversity in the life cycle, com-
parable to youth, maturity and age, but is merely a result of
the different external conditions under which they have been
living. The occurrence of conjugation is, they hold, in last
analysis, determined by external conditions.

There is certainly a large measure of truth in this conclu-
sion, though it is perhaps not yet completely established in
its absolute form. The question may be asked why it is
necessary that the period of scarcity of food should last so
long as five to six weeks before it induces conjugation? Does
this perhaps indicate that a certain number of generations
after a foregoing conjugation are necessary before a new
mating can occur? * Zweibaum's experiments need to be
repeated in such a way that after one conjugation a new
culture is produced from an ex-con jugant, and the period of
time determined (or if possible the number of generations)
that must necessarily elapse before a new conjugation can
be induced. Zweibaum did not determine whether an inter-
vening conjugation does away with the accumulated effects
of continued scarcity of food, so that the organisms must

irThis question has already been raised by Erdmann (1913).

Conditions Inducing Conjugation 167

again be subjected to five or six weeks' scarcity before they
will again conjugate. This is really the essential point; for
if this turns out to be the case, then evidently the length of
time since a previous conjugation is one of the things that
determine whether conjugation shall now occur.

But independently of this doubtful point, it is clear that
mating at a particular period is not required independently
of the outer conditions, for Paramecium aurelia will live in-
definitely (over 6000 generations) without conjugation
(Woodruff), yet may be induced to conjugate if the required
outer conditions are supplied (Woodruff, 1914) ; and in some
stocks of this species a second conjugation may be induced
in the fourth generation after a previous conjugation (Jen-
nings, 1910, p. 286). Certainly by far the largest part is
played by external conditions (past or present) in producing
conjugation.

The conditions under which mating occurs (sudden
scarcity of food and the like) are conditions which are dis-
tinctly unfavorable to the life of the organisms. Some
species of infusoria respond to such conditions by becoming
encysted ; they transform into a small sphere, protected by
an outer coating; and in this state they can withstand con-
ditions that would otherwise destroy them. Some other
Protozoa respond by first conjugating, then encysting. In
others, such as Paramecium, there is only conjugation, with-
out encystment. But as we have seen, conjugation results
in the production of many diverse stocks, some of which are
more resistant to given conditions than others. It appears
that some of the stocks so produced may be able to survive
the unfavorable conditions which induced conjugation, al-
though (as observation shows) most of them die out if the
conditions are not altered for the better. Later generations
would therefore all be derived from the most vigorous and

168 Life and Death, Heredity and Evolution

resistant stocks resulting from the new combinations formed
in mating. There is ground for believing that in nature
this process occurs on a large scale.

Looking back over what has been found out as to the
effects of mating, the general picture is as follows : It has
been shown that infusoria may live and multiply indefinitely
without conjugation (Woodruff, Enriques). It has been
shown that at intervals the old active macronucleus is re-
placed by a part of the reserve micronucleus. These things
demonstrate that the mating process (as distinguished from
the replacement process) is not necessary for continued life
and vigor. They appear to disprove any theory of sexuality
that maintains that there must for continued life be a
periodic reunion of two substances, male and female, which
inevitably become separated as a result of life and develop-
ment. Rejuvenescence is through the replacement of used
parts by unused ones, and this occurs without mating, al-
though it may occur at mating also. The distinctive con-
tribution of the mating itself is something else.

Investigation shows that mating produces two very strik-
ing results: (1) It causes the offspring of the two individu-
als that have conjugated to become more alike; it produces
biparental inheritance. (2) It causes the different families
produced by different pairs to be hereditarily diverse in many
respects; and this even when all the parents come from a
single ancestor and are hereditarily alike.

Do we find anything of this sort elsewhere in organisms?
Consideration brings to light the fact that this is precisely
what results from mating in higher organisms; we call the
detailed working out of these results Mendelian heredity.
In heredity in higher organisms, the offspring produced by
any pair resemble each other more than they do other in-
dividuals; they show biparental inheritance. Furthermore,

Results of Mating 169

the offspring produced by the different germ cells of even
the same pair of parents are hereditarily diverse; a single
pair of parents may, in plants or certain animals, produce
in Mendelian inheritance hundreds of hereditarily different
kinds of offspring. The only reason why this may not
occur in the highest animals and man is that in these cases
relatively few of the possible combinations develop, since
but few offspring are produced.

In these respects, therefore, mating does the same thing
in the Protozoa that it does in the higher organisms. In
both it brings biparental inheritance and the production of
hereditarily diverse stocks.

VII

How Does Mating Bring About Both Biparental Inherit-
ance and Diversity in Hereditary Characters? What Effect
Has Mating on the Stock as a Whole? Does It Increase
Variation? Does It Decrease Variation? What Is Its
Relation to Evolution?

I N our last chapter we showed that mating produces bi-
parental inheritance, as well as diversity of inherited
characteristics, in lower as well as in higher organisms.
How are these results brought about? How does it happen
that the offspring of the two members of a pair on the whole
resemble each other, yet are hereditarily diverse?

The main outlines of the way this is brought about are
well known. Each parent hands on bodily to the offspring,
through the germ cells, certain packets of chemicals. Since
these are directly transmitted from parent to offspring,
while the later characters are secondarily derived from
them, we may call these packets of chemicals the primary
hereditary characters. These packets are present in each
animal in a certain definite number, stored within the
nucleus; they are called chromosomes (see Figures 29, 31»
32). Individuals which get different sets of packets from
their parents develop differently even under the same outer
conditions; that is, they show different hereditary char-
acteristics.

This arrangement of the primary hereditary characters —
the chemicals that determine the hereditary peculiarities —

170

The Primary Hereditary Characters 171

into packets is fundamental for an understanding of how
heredity occurs; it is this that directly brings about all the
peculiar phenomena that are called Mendelian inheritance.
And this is a typical illustration of the part played by
structure and arrangement in organisms ; it demonstrates
that a chemical study alone, omitting arrangement of the
chemicals, can never lead to comprehension of what occurs.
The point is that when two or more chemicals are in a cer-
tain space, it makes all the difference in the world as to
what happens, whether the two substances are in separate
bottles, or merely poured together. To neglect this fact
in organisms is as fatal to understanding them as it would
be to try to comprehend what occurs in a chemical labor-
atory without realizing that the different chemicals are
kept in separate containers. The nucleus of the cell is a
chemical laboratory containing diverse chemicals in separate
packets. At times substances come out of these packets,
intermingle, and therefore react. It is their reactions with
each other, and with external conditions, in an orderly way,
that bring about growth and the development into a struc-
ture with diverse organs. The packets are shifted about and
distributed in certain ways at the time of mating and fer-
tilization, and it is the rules of their distribution that are
what we call the rules or laws of inheritance.

We know that in any organism these packets of chemicals
are present in a definite arrangement. We know that each
larger packet or chromosome is a chain or group of con-
nected small packets (Figure 29, E, F), and that the dif-
ferent chromosomes present in a nucleus are diverse. Their
number is definite in any individual, and they are so con-
stituted as to form a set of diverse pairs (Figure 29; Figure
45 B); (though sometimes there is a single package or
chromosome that is not paired with another).

172 Life and Death, Heredity and Evolution

When mating is to occur we know that these paired
packages of each nucleus separate into two groups, each
group containing one member of each pair (Figure 45, B,
C, D). These two groups are then separated by cell divi-
sion into different cells. Each of these cells therefore con-
tains a group with half the number of packages that were
present in the parent nucleus. It is such cells with half
the original number of packages or chromosomes in their
nuclei that form the germ cells, — the two cells that are to

Figure 45. The separation of the two groups of paired chromosomes
into different germ cells, in the insect Nezara hilaris, after Wilson,
1911. A, the 14 chromosomes in a single cell, before the germ cells
are formed. B, the 14 gathered into 7 pairs. C, the members of the
seven pairs separating as the division to form the germ cells occurs.
D, the two groups of 7 chromosomes each, in different germ cells,
formed by the separation of the 14 shown in A and B.

unite in mating (Figure 45, D). After the two half nuclei
have united, of course the original number of chromosomes
is restored.

As before remarked, we know that the different chromo-
somal packages present in a nucleus are diverse. The
evidence for this is complete, but cannot be given here.
Now, when these diverse packages separate into two half
groups, different half groups are formed in different cases,
depending on which member of any given pair goes into a
given group. In this way a great number of diverse com-

Distribution of the Primary Hereditary Characters 173

binations are formed in the half-groups derived from dif-
ferent nuclei, even of the same parent.

This diversity of the combinations of the primary hered-
itary characters in the different half nuclei is the essential
point in understanding the way the later hereditary char-
acters are distributed, so that it will be best to illustrate
how it comes about. Suppose we represent the chromosomal
packages in the nuclei of a particular animal by letters of
the alphabet. We will indicate the diversities by giving
a different letter to each chromosomal packet, and to the
two members of a given pair we will give a capital letter
and a small letter respectively. To illustrate the principles
in simple form, we will suppose that there are but four pairs
of chromosomal packages in each nucleus. That is, the
chromosomes of each nucleus would be represented as fol-
lows:

A B C D
abed

Now in each parental nucleus of this kind the chromosomes
separate into two groups, one member of each pair in each
group. But either member of any pair can go into either
group. That is, from one nucleus' the two 'groups formed
may be A B C D and abed; from another nucleus they
are A B c D and a b C d ; from another A b C d and a B c D,
and so on. The number of different combinations from
four pairs is 16, and each occurs as frequently as any other.1

So from a number of parental nuclei, all having the same
combination of packages, a large number of different com-
binations will be formed in the half nuclei.2

1 These facts, fundamental for the understanding of the rules of in-
heritance, have recently been directly demonstrated for certain higher
organisms, by Carothers (1917).

' The number of diverse combinations possible in the half nuclei
formed from nuclei all of the same kind is 2", if n is the number of
diverse pairs of chromosomes present in the original nuclei.

174 Life and Death, Heredity and Evolution

The offspring are finally produced by the mating of two
of these half cells (or germ cells), each containing a half
nucleus. A nucleus with any combination of the chromo-
somal packages may unite with any other. Thus we shall
get in the case imagined such combinations as

ABcD aBcd

aBCd or aBCd and the like.

The total number of different combinations produced when
these were originally four pairs of different chromosomal
packets is 81.3

If the number of chromosomal pairs is greater, the number
of combinations produced by mating is greatly increased.
For each additional pair of chromosomes the number of
possible combinations is multiplied by 3. Where there
are 24 pairs of chromosomes, as apparently in man, the
number of possible diverse combinations mounts far up into
the trillions.

Each one of these combinations of chromosomal packets
gives a different result in heredity; a different set of
hereditary characters. The result is that the progeny pro-
duced by the different germ cells differ from each other, and
it becomes impossible to predict from the characteristics of
the parent what will be the characteristics of particular off-
spring, for many diverse kinds of offspring can be produced
by the same pair of parents.

These processes are best known in higher organisms. But
study shows that the same things occur in the Protozoa.
These matters are extremely difficult to work out in these
minute creatures, and an immense amount of work remains
to be done before we shall know with full details what happens

•If there were n pairs of chromosomes present in the original nuclei,
then by the formation and mating of germ cells in the way described,
the number of different combinations producible is 3*.

Distribution of the Primary Hereditary Characters 175

in these organisms. But we find that in these, as in higher
animals, there are diverse packets of chemicals, which are
directly transmitted from parent to offspring, so that they
constitute the primary hereditary characters. In some of
the Protozoa these chromosomes are extremely minute and
numerous ; this is the case in the infusorian Paramecium
caudatum (Figure 49). In others they are larger and
present in smaller numbers, appearing much as they do in
higher organisms (Figure 46). The primary hereditary
characters or chromosomes are shown for a number of
Protozoa in figures 46 to 50.

In preparation for mating, and in mating itself, these
chromosomes undergo the same process of reduction in
number and recombination into new groups that occurs in
higher organisms. In figures 46 to 50 is shown what occurs
in some of the groups of Protozoa.

Examine for example figure 46, which shows the process in
a protozoan belonging to the Gregarinidse, and parasitic in
the earthworm, — as described by Mulsow (1911). The
chromosomal packets are in the form of eight long threads
(Figure 46, A) much resembling the chromosomes of many
higher animals. In ordinary multiplication by fission each
of these chromosomes splits lengthwise (Figure 46, B), and
half of each goes to each of the two offspring (C). In the
two parents before conjugation there are as usual eight of
these chromosomes, which become arranged side by side in
pairs, as occurs in higher organisms. This grouping into
four pairs is partly seen in figure 46, D and E. Now in
the early stages of mating, a division of the nucleus occurs
at which one member of each pair goes to one of the daughter
nuclei, one to the other (F, G, H). That is, each of the
two nuclei produced now receives four entire chromosomes, in
place of eight. Then in the mating, two such nuclei, each

176 Life and Death, Heredity and Evolution

S " -~ O.'M c £ S E
Bd-l^^l

i

Reduction in the Protozoa 177

with four chromosomes, unite, forming a new nucleus with
eight chromosomes. In this entire process of reducing the
number to four and then, by mating, restoring it to eight,
of course many different combinations of the chromosomes
may arise in the different resulting individuals, in the way
already set forth. The number of possible diverse com-
binations in this case with four pairs of chromosomes, is,
as we have seen, 81.

Reduction is better known in the ciliate infusoria than
in any other group of Protozoa. To understand what hap-
pens, one must recall the fact that at the beginning of mat-
ing there are three successive divisions of the micronucleus,
the third one producing the migratory and stationary half
nuclei. These are indicated in Figure 35. These three
divisions are commonly spoken of as the first, second and
third maturation divisions; we shall employ these designa-
tions.

In the infusorian Didinium nasutum (Figure 47), accord-
ing to Prandtl (1906), there are 16 minute chromosomes
(A). In the first of the three maturation divisions each of
these 16 chromosomes divides into 2, so that the resulting
two micronuclei still have 16 chromosomes (Figure 47, B).
But in the second division, the 16 chromosomes merely sep-
arate into two groups of 8, one group going to each of the
two resulting micronuclei (C, D, E, F). In the third
division (G, H, I) each of the 8 chromosomes present divides
into two, so that each of the two half nuclei now has 8
chromosomes. Now the migratory half nucleus from one
mate passes over and unites with the stationary half nucleus
of the other (Figure 47, J, K, L), so that the resulting
complete nucleus now has 16 chromosomes. In the later
divisions of this complete nucleus, each of the 16 chromo-
somes divides (M), so that all the nuclei of later generationa

178 Life and Death, Heredity and Evolution

FIG. 47.
(For description tee opposite page)

Conjugation in Didinium Nasutum 179

Figure 47. Reduction of the number of chromosomes, and other
processes in the nuclei, at conjugation in Didinium nasutum. After
Prandtl, 1906.

A and B, the first of the three divisions of the micronuclei ("first
maturation division"). In A, a spindle has been formed with 16 chromo-
somes; in B, each chromosome has divided, so that two groups of 16
are present; one group to go to each of the two resulting micronuclei.

C, D, E and F, the second division (the "reducing" division). C, 16
chromosomes; spindle forming for division. D, the 16 chromosomes
beginning to separate into 2 groups. E, the 16 chromosomes have sep-
arated into 2 groups of eight each, each going to one of the two result-
ing micronuclei. F, the two resulting micronuclei, each with 8 chromo-
somes; still united by a connecting strand from the spindle.

G, H and I, the third division, which forms from a single nucleus
the migratory and the stationary nucleus. G, each of the 8 chromosomes
dividing. H, the two groups of 8 chromosomes widely separated, to
pass into the two resulting nuclei. I, the migratory nucleus (above)
and the stationary nucleus (below) still joined by a connecting strand.
The stationary nucleus already considerably larger than the migratory
nucleus.

J, the migratory nucleus passing through the membrane that sep-
arates the two mated animals, into the other individual.

K, union of the migratory and the stationary nuclei. The latter
(above) is much larger than the former. L, the two nuclei almost
completely united.

M, the first division of the nucleus formed by the union of the migra-
tory and stationary nuclei. The union is not quite complete, so that
at the right end two spindles can be seen. Each of the 16 chromosomes
has divided into two, so that two groups of 16 are now present.

180 Life and Death, Heredity and Evolution

have 16 chromosomes, — until another reduction occurs
preparatory to another mating.

In the infusorian Anoplophrya branchiarum, which is a
parasite in the blood of the fresh water crustacean Gam-
marus, the number of chromosomes is 6. These are reduced
to 3 before the mating (Figure 48) ; by the union of two
at mating the number 6 is restored (Collin, 1909).

In Opercularia coarctata, a relative of Vorticella, accord-
ing to Enriques (1907), the unreduced number is 16; the

Figure 48. Conjugation and reduction in the number of chromosomes
in the infusorian Anoplophrya branchiarum, after Collin, 1909. A,
the two micronuclei (after the first maturation division) have each 6
small chromosomes. B, each micronucleus dividing anew, showing the
separation of the group of 6 into two groups of 3, one at each end
of the spindle.

reduced number 8. The reduction occurs at the second of
the three maturation divisions; and the number 16 is re-
stored at mating. In the infusorian Chilodon uncinatus,
according to the same author (Enriques, 1908), the number
of chromosomes before reduction is 4. At the second
maturation division, these are reduced to 2, in the usual way.
Mating restores the original number 4.

In Carchesium, according to Popoff (1908), the micro-
nuclei have, before the time of mating, 16 chromosomes.
At the first maturation division 8 of these go into one of the
resulting half nuclei, 8 into the other. At the second and
third divisions each of these 8 chromosomes divides into two,

Reduction m the Protozoa 181

so that finally the two half nuclei that mate have each 8
chromosomes. The original number, 16, is restored by the
mating.

In Opalina intestinalis, a large infusorian parasitic in the
alimentary canal of amphibians, the number of chromosomes
during ordinary reproduction by fission is 8. In the spring
there appear small animals, which divide several times, then
encyst; when they come out of the cysts they mate, two
individuals completely uniting. These small individuals
before mating have but 4 chromosomes in place of 8. How
the reduction is brought about is not known (Metcalf, 1909).

In the two common species of Paramecium, aurelia and
caudatum, the nuclear processes at mating appear to differ
considerably. They are best known in Paramecium cau-
datum, through the work of Calkins and Cull (1907). In
this animal the matter is greatly complicated by the fact
that a very large number of chromosomes is present (Figure
49). The number is so great that they cannot be counted,
but Calkins and Cull estimate them at about 165.

In individuals beginning mating, the chromosomes appear
as double rods (Figure 49, A). Calkins and Cull suspect
that this is due to the pairing of two chromosomes, such as
we saw in Monocystis (Figure 46). At both the first and
second maturation divisions these double chromosomes split
lengthwise. One of these divisions therefore apparently
separates the paired chromosomes (reducing the number to
half in each resulting half nucleus ) ; the other divides each
chromosome lengthwise. The reduced number is apparently
therefore present in the micronuclei before the third division.
This third division takes place in a very different way from
the other two. The chromosomes, instead of being long
threads, fall into strings of granules ; and each of these
strings is broken transversely, at about its middle (G).

182 Life and Death, Heredity and Evolution

Figure 49. The chromosomes and the divisions preparatory to mating
in Paramecium caudatum, after Calkins and Cull, 1907. A, B, C,
first of the three maturation divisions. A, the numerous chromosomes
united in pairs lengthwise (or split?). B, the two chromosomes (or
halves?) separating lengthwise. C, the two chromosomes (or halves)
almost separated; two groups forming. D, one of the two nuclei
resulting from the first maturation division. E, F, second maturation
division. E, the two halves of the chromosomes pulling apart. F, the
two groups separated; the two new micronuclei united by a narrowed
connecting zone. G, third division (that producing the migratory and
stationary half nuclei). The chromosomes formed of rows of particles;
the rows have broken in the middle and the halves are separating. H,
union of migratory and stationary half nuclei, in the two individuals.
The oblique fine is the surface of separation of the two mates. In each
individual the migratory half nucleus is the smaller one.

Thus at this division each of the two half nuclei formed
receives a half of each of the chromosomes (then present in
the reduced number). The original number would of course
be restored by the mating of the migratory .and the sta-
tionary nuclei (H).

Reduction is not so well known in the other groups of

Reduction in the Protozoa

183

Protozoa as in the ciliate infusoria. In the gregarine
Monocystis however we find a particularly beautiful example
(Figure 46). In the flagellates, Schaudinn (1904) and
Prowazek (1904) described a reduction from 8 chromosomes
to 4, in a number of species (Trypanosoma noctuae, T.
lewisi, T. brucei, and Herpetomonas). The accuracy of
these accounts has been called in question.

Bott (1907) has described the reduction of the number of
chromosomes in the rhizopod Pelomyxa, an animal re-

Figure 50. Reduction of the number of chromosomes before mating
in the rhizopod Pelomyxa, after Bott, 1907. A, the eight oval chromo-
somes of the parent. B, first of the two maturation divisions; the 8
chromosomes separating into two groups of 4. C, the second matura-
tion division; in the spindle to the left each of the 4> chromosomes (of
which but 3 are in view) is dividing into two.

sembling a large amoeba. The single animal contains many
nuclei. At times these go through two divisions in succes-
sion, which may be called the maturation divisions (see
Figure 50). At first there are eight oval chromosomes
(A). At the first division (B), four of these go into one
of the resulting cells, four into the other; the number is
thus reduced. At the second division each of the four
chromosomes is divided (C), so that all the nuclei produced
have four chromosomes. Later each of these nuclei, along
with a little cytoplasm, separates from the body of the

184 Ltfe and Death, Heredity and Evolution

mother, forming a free cell or gamete. When two of these
gametes meet, they unite; thus the original number of
chromosomes is restored.

Only a few of the Protozoa have been examined with suf-
ficient thoroughness to reveal this process of reduction and
recombination in the chromosomes, and in some the chromo-
somes are so numerous, minute and crowded that just what
occurs cannot be directly determined. But the cases al-
ready worked out, scattered as they are through the dif-
ferent classes of the group, show that the process is one
of general occurrence, here as in the higher organisms ; they
make it possible to recognize the occurrence of reduction
even when the chromosomes cannot be counted. Just be-
fore mating, the nuclei, both in the Protozoa and in higher
organisms, go through the two successive divisions (in the
infusoria, owing to special conditions, three), known as the
maturation divisions. It is in one of these as a rule that
the reduction in number occurs, through the distribution of
half the chromosomes to one nucleus, half to the other. In
most cases these two (or three) divisions are distinguishable
from all others by marked peculiarities connected with the
reducing process ; and these make it possible to recognize
the reducing divisions even when the number of chromosomes
cannot be counted. Whenever two (or three) peculiar divi-
sions occur in rapid succession just before the nuclei are
ready to mate, we may be practically certain that in these
the reduction in the number of chromosomes has occurred.
Such divisions we saw in the cases of autogamy (page 134
and Figure 41 ); in our examination of the preparations
for mating in the infusoria (Figures 46 to 50) ; they occur
indeed almost universally in preparation for mating. All
such divisions indicate a process of reduction in number

Recombinations of the Primary Hereditary Characters 185

of chromosomes, with resulting formation of a new combina-
tion through mating.

For the Protozoa it is clear therefore that so far as the
primary hereditary characters — the chromosomes — are con-
cerned, mating is a process of producing new combinations
of hereditary characters. In higher organisms we know
that these primary hereditary characters are what determine
also the later or secondary hereditary characters, — those
that appear in the developed body. There can be no doubt
that the same is true of the Protozoa. Everything indicates
that in these respects mating in the Protozoa is the same
sort of thing that it is in higher organisms, and that when
the matter is fully studied it will be found to produce the
same kind of results. Mating may be defined as the process
of producing new groups of hereditary characters, primary
and secondary, by combining diverse half groups from dif-
ferent nuclei.

But what shall be said from this point of view of the cases
in which the two half nuclei are produced from a single one,
and these two later unite in mating, as in the numerous cases
of autogamy (Figure 41, etc.)? These cases form a diffi-
culty for almost any other way of looking at the matter, but
not for this one. For new combinations of the primary
hereditary characters are formed likewise when the mating is
between two nuclei that are recent products of a single one.

In all cases of such autogamy, a fact is observed which
becomes of the greatest significance. After a single nucleus
has divided into two, these two do not reunite at once, but
there are always one or two intervening divisions. And it
is these intervening divisions that bring about the reduction
in number of the chromosomal packets, with the consequent
formation of new combinations of the primary hereditary

186 Life and Death, Heredity and Evolution

characters in mating. Thus, in Figure 41, we see that after
the separation of the original nucleus into two, each of these
two divides twice, giving off two very small nuclei, which
are absorbed and disappear.

It is worth while to notice just how new combinations are
formed in these cases in which the two nuclei that mate
originally came from the same single nucleus. Let us sup-
pose that the original single nucleus had four pairs of the
chromosomal packets ; these we may designate as follows :

A B C D
abed

Now when this nucleus divides into two, the division is of
the usual sort, in which each single packet divides inio two
like itself, so that each of the two nuclei produced has the
same set of chromosomal packets that its parent had.

But now each of these two goes through the "reducing
division," in which the set of eight divides into two sets of
four each, — one member of each pair going to each resulting
set of four. Then evidently many different combinations
may be formed, depending on how the members are dis-
tributed. In one nucleus the group of four will be A B C D,
in another A B C d, in another A b c D, in another a b C d,
and so on (16 different combinations are possible). Now
two of these combinations of four unite. It is practically
certain that they will have different combinations ; let us
suppose that one contained the combination A b c D, the
other the combination a b C d; then the nucleus formed by
their union will show the combination

A b c D
a b C d

That is, from a nucleus showing the combination

A B C D
abed

Recombinations of the Primary Hereditary Characters 187

we have by division, reduction, and reunion obtained a
totally different combination.

In other cases, nuclei with the same original combination
will give still different results ; there are 81 diverse resultant
combinations that may be produced in this way from a sin-
gle nucleus having the combination of packets supposed
above.

Since the facts as to the recombination and distribution of
these primary hereditary characters are the same in Pro-
tozoa and in higher organisms, we may expect them to pro-
duce the same results. That is, we may expect to find Men-
delian inheritance in Protozoa, when the facts are fully stud-
ied. Mendelian inheritance is nothing more nor less than a
recombination and distribution of the secondary hereditary
characters in the manner that the primary hereditary char-
acters are recombined and distributed, — without doubt in
consequence of this recombination and distribution of the
primary characters.

There is, of course, no reason to expect such a Mendelian
distribution of inherited characters among the progeny of a
single individual that divides after conjugation. All such
progeny will probably be alike, save for any accidental vari-
ations in the splitting of the chromosomal packets ; this, as
we have seen, observation shows to be the case. But if the
progeny are examined from a large number of pairs coming
from two diverse races that have been induced to conjugate,
we may expect to find among these the typical Mendelian
distribution of characters.

This may be illustrated most directly by the inheritance
of the primary characters (the chromosomes) ; we know that
the secondary hereditary characters follow the primary ones.

Let us suppose, for example, that in the two races a pair
of chromosomes differ ; we will say that they are black in one

188 Life and Death, Heredity and Evolution

race, white in the other (see Figure 51, P). Now we know
that in each individual before mating, one chromosome of
each pair is gotten rid of, the half nucleus that remains con-
taining but a single chromosome of this pair; and that this
half nucleus divides to form the migratory and stationary
half nuclei of that individual; so that migratory and sta-
tionary half nuclei contain the same set of chromosomes. In
the one race they will, of course, therefore both contain a
black chromosome, in the other a white one. When the ex-
change and union of half nuclei occur, the resulting nucleus
contains one white chromosome, one black (Figure 51, F 1).
This will happen in every pair of the two races that mate
together; every family will have a white-black pair of
chromosomes.

In ordinary fission, however, the black and white do not
separate, but each merely splits, so that all the individuals
produced have this white-black chromosome pair (F 1).
This is also just the situation of affairs in the first genera-
tion of offspring (the "F 1 generation") from a cross in
higher organisms; this F 1 generation is composed of indi-
viduals that are alike with respect to their characters.

But suppose that after a long series of generations, the
white-black individuals mate among themselves (as at P 2,
Figure 51). What will happen?

In all the individuals, one of the two chromosomes of this
pair will be gotten rid of at the second maturation division,
leaving the other. In half the cases it will be the black
chromosome that is left ; in half the white one. That is, half
the individuals will have black chromosomes in their migra-
tory and stationary nuclei, while half will have white ones.

Of those that contain black chromosomes, half will mate
with other individuals that contain black ones, half with
those that contain white ones. Similarly, of course, of those

Mendelian Inheritance

n

189

Figure 51. Diagram to illustrate how Mendelian inheritance would
occur in an infusorian (Paramecium). The circles represent a pair of
diverse chromosomes, the diversity being indicated by making one black,
the other white. The diagram shows that in successive conjugations
these would be distributed according to Mendelian rules. After the
first conjugation (P), the ex-con j ugants and their descendants by
fission (F\) would all have one black, one white, chromosome of this
pair. At the next conjugation (P2), by the varied reductions and
matings the ex-con j ugants and their descendants by fission (Fa) would
exist in the proportions: — 1 white-white: 2 white-black: 1 black-black.

190 Life and Death, Heredity and Evolution

that bear white chromosomes, half will have mates with black
chromosomes, half with white. Then all the different com-
binations so producible are shown at P 2, Figure 51 ; each
of these combinations occurs as frequently as any other. By
two of the four combinations we get offspring ( ex-con ju-
gants, F 2) with one chromosome white, one black ; by one
we get offspring with both chromosomes white; by one, off-
spring with both chromosomes black. Summing up the
eight offspring, we get the following proportions for the
offspring of the generation F 2:

2 white-white -j- 4 white-black -f- 2 black-black.

Now this is exactly what is called Mendelian inheritance.
If we call white A and black a, the proportions give the fa-
miliar Mendelian formula

AA + 2 Aa + aa

Any other chromosome pair, or any character that de-
pends on a chromosome pair, will give the same result.

We have found then, so far as knowledge has gone in this
direction, that mating produces the same kinds of results
in the Protozoa that it does in higher animals; it gives bi-
parental inheritance, and also gives many diverse hereditary
stocks, and these results are produced in the same way in
the Protozoa that they are in higher organisms.

The Effect of Mating on the Stock

What effect has this on the entire species in which mat-
ing occurs? To answer this question, another fact as to
conjugation is of importance.

Assortative Mating: — When we place together in the same
vessel members of two different races of Paramecium, one
having large individuals, the other small ones, and then in-
duce conjugation, we observe a surprising fact. Members

Assortative Mating

191

of each race mate only with members of their own race ; the
large individuals only with other large ones; the small in-
dividuals only with other small ones (Figure 52). There is
no crossing between the two races when they thus differ
considerably in size. This is a highly inconvenient fact
when one wishes to study heredity in such crosses !

r\

\J

Figure 52. Conjugants and non-con jugants from a culture composed
of a mixture of two races (k and i) of different size, of Paramecium
aurelia. The members of the race k are larger than those of the race
i; only members of the same race mate together. After Jennings, 1911.

The same thing is observed when a culture of Paramecium
contains individuals of many different sizes (whether of the
same race, or of different races). There is a marked as-
sortative mating; that is, individuals of the same size tend
to mate, while individuals of diverse size do not readily mate
(Figure 53). There are some exceptions; we find a few in-
stances in which a small individual has mated with a larger
one, but such cases are rare. In general, we find that all
the pairs can be arranged in a rather regular series such as
Figure 53 shows, — the two members being of about the same
size.

This assortative mating takes place with respect to other

192 Life and Death, Heredity and Evolution

characters also. It has been demonstrated by careful study
that it occurs in Paramecium with reference to rate of fis-
sion; the two animals which mate have on the whole similar
rates of fission. It appears clear that the mating is between
animals of similar physiological characteristics.

Such assortative mating has been shown to occur with
respect to size in certain other infusoria, — noticeably,
Blepharisma (Watters, 1912) and Anoplophrya (Collin,
1909). Assortative mating is common, too, in higher ani-
mals and man. It is well known how strong a reluctance

Figure 53. Pairs from a single race of Paramecium aurelia, illus-
trating assortative mating; individuals alike in size mate together. The
lines A-C and B-B1 are parallel. After Jennings, 1911.

there is in man for strikingly different races to mate; a re-
luctance that is reenforced by all sorts of social and legal
regulations (which regulations, of course, are manifestations
of the biological characteristics of the organisms). In the
case of blacks and whites among human beings, for example,
an observer from Mars, examining in the United States the
two stocks objectively, would find that in the overwhelming
majority of cases white is mated with white, black with
black, — although some exceptions occur.

In higher animals this assortative mating manifests itself
in details also, as it does in Paramecium; study shows, for
example, that on the whole tall persons tend to mate with
tall, short with short. Although detailed studies have been

General Results of Mating 193

made for but few cases, this tendency for like to mate with
like, and to refuse to mate with unlike, probably exists in
considerable degree throughout the world of organisms.
This is one of the important facts to be reckoned with in
attempting to get any general picture of the results of
mating.

General Results of Mating: — To a picture of the general
results of mating we now turn. In what respect does the
world of organisms, or any particular group of organisms,
differ from the condition which we would find if no mating
occurred? We leave out of account here the results of the
replacement of the old active nucleus by the reserve nucleus,
since this is not a distinctive feature of mating, occurring as
it does in the infusoria without mating; and in most organ-
isms not occurring even at mating.

We hear it maintained on the one hand that mating pro-
duces variation; some assert, indeed, that it is the great
source of variation. On the other hand, some maintain that
the result of mating is to prevent or destroy variation ; to
keep the species of organisms uniform. Facts can be ad-
duced that support both these propositions.

The difficulty here is that the expressions "increase varia-
tion" or "decrease variation" are ambiguous, and that
neither of them precisely touches the essential point. The
increase and decrease of variation are mere diverse aspects
of what really occurs ; sometimes one of these may result
from mating, sometimes the other. The really fundamental
thing that mating does is to produce new combinations of
hereditary characters. And in so doing it quite changes the
face of the world of organisms.

We may illustrate this most simply by noticing again what
happens in the case of the primary hereditary characters, —
the chromosomal packets ; we know that the secondary char-

194> Life and Death, Heredity and Evolution

acters follow the primary ones. If there are two races of
organisms, in each of which both members of each pair of
chromosomes are alike (so-called pure lines, or pure homozy-
gotes), we may call their two sets of chromosomes

ABCD abed

ABCD and abed

Now, when the two sets are reduced in number by division
into two groups, with one member of each pair in each group,
evidently the only possible groups are ABCD from one
race, and abed from the other ; all the half nuclei of each
race will be alike. When two half nuclei from the different
races mate, all the resulting nuclei will show the combination

ABCD
abed

That is, by the crossing of these two diverse races, progeny
are produced that are all alike. A new combination has
been produced, — but only one combination from the original
two. So the progeny of the cross will be uniform, while the
parents were diverse. Variation has been greatly decreased.

But now suppose that the progeny, showing this new and
uniform combination, mate among themselves. We have al-
ready for another purpose examined the results of this
(page 186); we found that a great number of different
hereditary combinations would be produced, such as

AbcD , abCd , etc., etc.
AbCd aBCD

Progeny of 81 different kinds of hereditary combinations
will result. By this mating of two parents that were just
alike variation has been greatly increased.

Suppose that we compare this group of 81 diverse com-
binations with the two sets of grandparents. Has variation

Effect of Mating on Variability 195

been increased or decreased? In the original stocks the two
sets of parents were diverse in all their characters, while in
their grandchildren, although there are 81 sets instead of
two, they show all possible intermixtures and gradations.
It may be maintained therefore that the result of mating has
been to reduce the degree of diversity. And if we find the
"coefficient of variation" for the two sets, which is a measure
of how much on the whole the individuals differ from the
average intermediate condition, we shall doubtless find this
to be much greater for the two original stocks, where none
of the individuals are like the average of the two sets, than
for their grandchildren. By this measure, therefore, varia-
tion will be found reduced by mating. Walton (1915) has
shown that precisely this is what occurs when diverse stocks
of certain higher organisms are mated ; from which he argues
that mating decreases variation. The whole is an excellent
illustration of the way in which average measures (like the
coefficient of variation) may conceal important biological
facts. What has happened is the production of many di-
verse hereditary combinations bridging the gap between the
two that first existed. This is an increase of variation, if
one means thereby an increase in the number of hereditarily
diverse stocks ; it is a decrease of variation, if one means by
.ariation the average diversity from the intermediate con-
dition. But this superficial increase or decrease of variation
is merely a consequence of the underlying fact, — the pro-
ductions of new combinations through mating.4

4 Mast (1917) studied this phenomenon of increase or decrease in
variability of the fission rate as a result of mating in the infusorian
Didinium. He was not able to detect with certainty any consistent
increase or decrease in the coefficient of variation as a result of mating.
This need not surprise us, in view of the points brought out above; but
Mast's data were in any case hardly such as to give dependable results
on the matter. The study of variation was made on the results of
experiments designed for entirely different purposes, so that the num-
bers of cases were too small to give significant data. Most of his coeffi-

196 Life and Death, Heredity and Evolution

The facts of assortative mating, of course, limit the ex-
tent to which new combinations of the characters are pro-
ducible through mating. After stocks have reached a cer-
tain degree of diversity, assortative mating prevents their
union, and so prevents the formation of stocks combining
their characteristics. But this still leaves a wide field for
the formation through mating of varied combinations of the
hereditary characteristics of differing stocks.

We now come to one of the most striking biological re-
sults of mating. In this formation of new combinations,
characters which were previously in diverse stocks become
united in one individual. Sometimes the characters so com-
bined are more or less incompatible; they do not develop
well together ; or they do not function harmoniously ; or they
produce secondary characters which do not function well
under the particular conditions in which the organisms are
found. Individuals resulting from such combinations may
be weak or pathological; they may not develop at all; or
if they do, they lack vigor, and do not multiply. Many ex-
amples of such consequences we saw in our study of the re-
sults of mating in Paramecium; many individuals or
families produced died out, or were weak, multiplying
slowly; or showed hereditary deformities. Other individu-
als on the other hand received combinations of characters

cients of variation were worked out on the basis of but 2, 3 or 4 diverse
lines of descent! In only one single case were there as many as eight
lines in both the sets compared (conjugants and non-con j u gants ); in
this case the variation was much greater in the lines that had conju-
gated. But it is obvious that such small numbers cannot give clear and
consistent results, particularly when we find that the coefficients of
variation brought to light range from 0 to 55.3 per cent. If probable
errors had been worked out, they would doubtless have shown the com-
parisons of coefficients to be without significance. A study of the ques-
tion whether mating actually produces lines with diverse hereditary
characters would be of great interest in Didinium, as in any other Pro-
tozoa, but the determination of coefficients of variation is a most iu^
feet and uncertain index of this matter.

Mating and Selection 197

that worked well together and under the conditions of the
environment; the families produced were vigorous, some of
them flourishing even better than the original stocks through
whose mating they were produced. In the same manner a
poor family of human beings may produce a Lincoln among
its children; a combination of characteristics is formed by
mating that never before existed and that meets the condi-
tions of existence in a more vigorous and successful manner
than the individuals from which it was derived.

Experimentally, or under natural conditions, of course
these newly formed stocks in which life and reproduction are
vigorous, gradually replace the weakened stocks, in which
mating has resulted, not in reinvigoration but in degenera-
tion ; in the formation of combinations of primary hereditary
characters that cannot develop vigorously under the condi-
tions. The formation of such "degenerated" stocks is as
much a characteristic of mating as the formation of more
vigorous ones.

By this formation of new combinations with gradual re-
placement of the unenduring ones by those that are vigor-
ous, the stocks in existence come to be very diverse from
those that existed at an earlier time. Mating is a continued
process of forming new combinations of the primary heredi-
tary characters ; of the chemicals on which the vigor and
the nature of development depend ; with suppression of the
combinations that are weak or imperfect, leaving the more
harmonious and vigorous combinations in existence, to carry
the process further.

VIII

Comparison of the Genetic Phenomena in the Protozoa
with Those in Higher Organisms. General View of Develop-
ment, Mating and Evolution.

HOW far does the condition of affairs which we have set
forth for the lower organisms hold for the higher
ones ?

As we have remarked at various points in our earlier
lectures, many, perhaps most, of the general relations are
similar in the lower and higher organisms. But there are
certain points in regard to which questions may be raised;
particularly as to the origin of new hereditary characters.
The points needing examination are mainly (1) as to the
effect of the environment in causing inherited changes, and
(2) as to the nature and extent of the changes in hereditary
characters that arise in nature; with the relation of these
to the process of evolution.

We have dealt in our fourth lecture with alteration of the
hereditary constitution of lower organisms by external con-
ditions. In the higher, more complex organisms the under-
lying conditions as to this are somewhat different from those
found in organisms composed of but a single cell; though
perhaps not so completely diverse as is sometimes conceived.
But in the higher organisms there is a great mass of cells,
the body, which finally disintegrates completely, without
propagating itself by division ; the body of the next genera-
tion is formed, not from the body of the preceding genera-

198

Inheritance of Acquired Characters 199

tion, but by the divisions of a single cell, which is formed by
the union of two half cells, one from each parent. This was
illustrated in Figure 5, on page 21.

It is clear that if any of the later (secondary) hereditary
characters are to become modified, this must be accomplished
by some modification of the primary hereditary characters,
— those passed on bodily from parent to offspring. For
if two germ cells are exactly alike, the characters inherited
through them are bound to be alike; two individuals that
are to differ in their later hereditary characters must be
diverse in their primary hereditary characters. If environ-
mental agencies are to produce diversities that are to be
hereditary, they must change the germ cells, in the particular
way required to bring about the observed later changes in
the body ; for the body is derived from these germ cells.

This has always been the theoretical difficulty with the
"inheritance of acquired characters," — if we mean by that
abused expression the inheritance of modifications produced
directly on the body by the outer world. If the form of the
hand is changed by certain outward conditions, how is that
change to modify the primary hereditary characters in the
germ cells, which are not directly touched by the given
outer conditions, — in just such a way as to cause them to
produce the same new form of hand in the next generation?

Theoretical difficulties of this sort of course must not
be allowed to stand in the way of our recognizing how nature
actually does operate, if she does indeed operate without
regard to these difficulties. There is nothing so little worthy
of confidence in science as assertions that particular events,
not yet observed, are impossible ; such propositions have been
falsified a thousand times, and the careful man of science,
will not permit his researches to be guided by them. But in
this case the great weight of evidence thus far is that this

200 Life and Death, Heredity and Evolution

particular theoretical difficulty corresponds to an actual
one; that the direct effect of the environment on the body
cells is not inherited through the germ cells in the next
generation. It is only fair to say however that there is
certain evidence produced by the Austrian investigator
Kammerer (1913) in long-continued experimental studies on
amphibians which seems to imply such inheritance through
the germ cells of changes primarily produced in the body
cells of the animal. But practically all students of biology
will agree that this evidence is far from establishing heredity
of this sort, and that the overwhelming mass of evidence is
against it.

But it is another question whether external agents may
not act directly on the germ cells, in such a way as to induce
them to produce a body with new characteristics, and to
transmit the same changes by cell division to the germ cells
that are to produce the later generations, so that these too
show the altered hereditary characters. This sort of action
would correspond to the hereditary changes produced by
external agents in the Protozoa and bacteria, such as we
described in Lecture 4.

There is no theoretical difficulty whatever as to this; the
difficulties are purely observational ones; it turns out that
such changes do not occur so readily or frequently as one
would expect. There is no a priori reason why the sub-
stances of the germ cells should not be as readily altered
as any other mass of chemicals. But in practice it turns
out that most agents which produce chemical alterations
of the germ cells at the same time kill the organism. Further,
the germ cells, like other living systems, have a great
tendency to compensate for any disturbances induced them ;
their condition is one of stable equilibrium, in which an alter-
ation is followed by a return to the original condition. Add

Production of Inherited Variations 201

to this the fact that in higher organisms the germinal mate-
rial is commonly hidden deeply within a great mass of body
cells, by which its surroundings are kept uniform and it is
protected from marked changes of all sorts, — and it becomes
more intelligible why in higher organisms inherited changes
due to the action of the environment are much less commonly
observed than general theory might lead us to expect. In
the bacteria and Protozoa the germinal material is not pro-
tected by a great mass of body cells, but is more directly
exposed to the action of environmental agents, so that in
these, heritable results of environmental action are better
known.

When we examine the experimental evidence on this matter
in higher organisms, we find that scientific opinion looks
upon it as being in a somewhat less satisfactory condition
than appeared to be the case a few years ago. Cases had
been described in which the inheritance of the action of the
environment on the germ cells appeared clear; and the evi-
dence on these particular cases has not altered. But the
lack of further confirmation, of other instances; the failure
of other tests under similar conditions, has shaken the
opinion of most students of biology as to the conclusiveness
of the evidence that had been given, and has made them
inclined to wait for further evidence before accepting the
principles of action involved. No subject in biology is more
in need of further work than this one.

The principal evidence for actual modification of the
germ cells by the environment in such a way as to cause the
appearance in the body of new hereditary characters has
come perhaps from the work of Standfuss (1906) and
Fischer (1907); of Tower (1906); of Kammerer (1913),
and of Stockard (1913); these at least are typical. It is
not our present task to give an account of these researches,

202 Life and Death, Heredity and Evolution

but a few words of comment will aid in obtaining an outline
of the present situation in science.

Standfuss and Fischer, by the application of low tem-
peratures to the pupae of certain butterflies, induced in a
small proportion of the adults the appearance of darker
tinges of color than usual. In a small proportion of these
aberrant adults, the modified color reappeared in later gen-
erations; it was hereditary.

Tower tried the effects of hot, moist conditions on the
potato beetle at the time that the germ cells were under-
going their growth and transformation. He found that
the germ cells so treated produced a considerable proportion
of individuals differing from the typical ones; lighter in-
dividuals and darker ones. And in later generations these
aberrant colors showed themselves to be hereditary.

Kammerer experimented for many years in breeding vari-
ous sorts of amphibians, attempting through climatic
changes, alterations of temperature and moisture, changes in
color of the background on which they live, and by other
means, including operations, to modify their colors, habits
and other characters. According to his detailed reports,
published in the technical scientific journals, he has been re-
markably successful in this ; many sorts of diversities in en-
vironmental conditions have produced inherited alterations;
including even operative procedures. These positive results
have come to him so easily and regularly, in experiments of a
sort in which others have practically universally reached neg-
ative results, as to arouse in most investigators a feeling that
they must be confirmed by others before they can be accepted.
It must be said however that Kammerer's work appears to
have been done with great thoroughness and care, and he
has given full account of his methods and results, in such a
way as to leave little room for criticism of details. On the

Production of Inherited Variations 203

other hand the phenomena with which he deals are excessively
complicated and variable, making errors of interpretation
easy to one who is strongly convinced of a particular
doctrine, as Kammerer evidently is of the inheritance of en-
vironmental effects. Some results and interpretations re-
ported by him have put a strong strain on the powers of
acceptance of other investigators; notably his report of
the inherited effect of cutting off a certain organ in the
ascidian ; * and his report that when the ovary of a given
sort of salamander is transplanted to the body of another
kind, the germ cells of this ovary transmit the characters of
the body to which they were transplanted. Kammerer's
work is distinctly in need of confirmation.

Stockard dealt with the effects of alcohol on the germ
cells, and through these on the later generations of off-
spring in the Guinea pig. He found that continued ad-
ministration of alcohol to the parents so injures their germ
cells that their progeny and their descendants of later gen-
erations are weak, imperfect, diseased, deformed, in many
ways.

Somewhat similar results had been reached years before by
Brown-Sequard (1869), in studying the results of mutilation
of the parent in Guinea pigs. Repetition of his work by
later investigators has not convinced students of the subject
that it was correctly interpreted ; it is believed that he was
dealing with diseased stocks. The study made by Pearl
(1917 o) of the effects of alcohol in the fowl did not bring
to light any such results as Stockard's on the Guinea pig,
so that there is on the whole a tendency to suspend judg-
ment as to the interpretation of the results until Stockard's
work has been repeated on a new stock.

Thus all along the line there is a feeling of uncertainty
1 See the comment of Castl6, 1916o, page 29, on this point.

204 Life and Death, Heredity and Evolution

and a desire for further tests before judgment is passed as to
the inherited effects of environmental action on the germ
cells, in higher organisms; more work along these lines is
greatly needed.

But the results set forth are, in all cases save perhaps
certain of those described by Kammerer, of the sort that
agree in principle with the inherited effects of environmental
action in the lower organisms, as described in Lecture 4.
The action of the external agents was directly on the germ
cells, modifying the primary (directly transmitted) heredi-
tary characters. In consequence the later or secondary
characters were altered. Such cases are perhaps better
established in the Protozoa and bacteria than in higher
organisms.

As to the nature and extent of the changes in hereditary
characters arising in higher organisms, aside from the direct
effects of environmental action, an enormous volume of work
has been done. In higher organisms it is a prevalent con-
viction that changes in the hereditary characters occur by
mutation. What relation has this and the facts on which
it is based to the hereditary changes seen in the Protozoa, —
in such a case as Difflugia, for example?

The concept of mutation has been based, in different
minds, on a number of different points, — sometimes held,
separately, sometimes together. One basis for the distinc-
tion of mutations from other variations is this: Many
variations in the characteristics of organisms are not in-
herited; such are the common superficial effects of environ-
mental diversities. We have given illustrations of this in
our account of the Protozoa. It is therefore convenient to
have a distinctive name for those that are inherited, and
some call these mutations. In this usage, of course it is a
mere matter of definition to state that any new heritable

Mutation 805

variation is a mutation. The word so used implies nothing
as to extent or nature of the variations that are herit-
able; it is a name for all that occur.

But in many minds the term mutation means more than
this. Many new heritable characters in higher organisms
are found to be, when the character has reached its com-
plete development in the adult, changes of considerable ex-
tent. They differ from the original condition by a large
step ; they affect many parts of the organism ; or profoundly
change particular organs. Such were most of the hereditary
changes found by de Vries (1901) in the evening primroses,
CEnothera ; from this work arises the general use of the word
mutation for hereditary changes. Such too are many of the
hereditary changes observed in the fruit-fly, Drosophila, by
Morgan and his associates. Thus, in the typical individuals
the eyes are red ; these sometimes produce offspring in which
the eyes are white, and this mutation is inherited. The
hereditary change has come, not by minute changes of shade,
gradually altering from generation to generation; but by a
complete change in one generation from red to white.

From such cases, the word mutation has come to mean in
the minds of many persons an extensive change; a sudden
jump from one condition to another; a "saltation." And
the statement that evolutionary changes occur by mutation
has come to mean that they do not take place in gradations ;
in minute, almost imperceptible alterations from generation
to generation, but always by large leaps. Possibly this is
the usual idea of what is meant by the mutation theory of
evolution.

Such slight changes as we have described in the preceding
lectures as occurring in Difflugia and other Protozoa do not
agree with this idea. Is there a contrast in this respect
between what occurs in the higher and the lower organisms?

206 Life and Death, Heredity and Evolution

It is natural that an alteration of a primary hereditary
character in the germ cell of a higher organism should, when
the long development from that germ cell is completed, pro-
duce a much more extensive and more marked effect than in
a Protozoan. For in the latter it is the same cell that is
altered which forms the adult, with relatively little develop-
ment, and with no intervening multiplication of cells. But
in the higher organism the altered germ cell goes through a
great number of cell divisions, accompanied by continuous
interactions of the different substances in the nuclei, result-
ing in an enormous increase in differentiation, in numbers of
cells, and in bulk. All these cells, and this entire bulk, may
therefore show the results of the slight original change. If
some substance necessary for the production of the red eye
color of the fruit-fly were omitted from the germ cell, it is
probable that the change in the germ cell would itself be
so slight that it could not be detected by any physical or
chemical tests at present available. It is little more than
changes corresponding to this that we may expect to find
in the organisms made up of but one cell.

But must a hereditary change in the adult characters of
a higher organism necessarily be such a saltation; a change
of large extent? On this point the state of knowledge has
greatly changed with the thorough studies made in recent
years, although the change has as yet been little appreciated
outside the field of specialists working on these matters.
We shall attempt to give a brief sketch of the position of
this question in higher organisms, for comparison with what
we have seen in the lower ones.

It has been found that in many higher organisms it is
possible through long continued breeding with careful selec-
tion of the parents, to gradually cause a change in the hered-
itary characteristics shown by the stock. Often this change

Nature of Inherited Variations 207

is a mere quantitative alteration, in the extent or intensity of
pigmentation or other characteristics. In many cases such
alterations have occurred in characters that in other re-
spects behave like "single unit characters." Such work was
done on the rat by Castle and his associates ; on Drosophila
by MacDowell, Zeleny and Mattoon, Reeves, Morgan, Stur-
tevant, and others.

Two views have been held by investigators as to the nature
of the change in such cases. Castle and a number of others
have long held that there was occurring a gradual change,
perhaps merely quantitative in nature, in the single unit
factor on which the adult character depends. On the other
side, many have maintained that these gradual alterations
are due to the fact that the adult character depends on
many distinct genes or unit factors, each affecting the adult
character but little. By selective breeding many of these
factors are gradually accumulated in one set of progeny,
few in the other; so that the adult features become slowly
very diverse. That is, it is maintained that the apparent
changes in the hereditary characters are really due, like all
Mendelian inheritance, to recombinations of the existing fac-
tors.

This explanation, commonly called the hypothesis of mul-
tiple modifying factors, has recently been accepted, on the
basis of crucial experiments, by Castle himself.2 There can
hardly be doubt that it is correct for at least most cases of
this kind.

Let us, therefore, accept this explanation, and proceed to
an examination of its relation to the questions in which we
are interested. What bearing have the facts, so interpreted,
on the nature of hereditary variations and on the method of
evolution ?
'Castle, W. E., Proc. Nat. Acad., April, 1919.

208 Life and Death, Heredity and Evolution

In no other organism have heritable variations been stud-
ied so thoroughly as in Drosophila, and no other body of
men have been more thoroughgoing upholders of mutation-
ism and of the multiple factor explanation of the effects of
selection, than the students of Drosophila — Morgan, Sturte-
vant, Bridges, Dexter, Muller, MacDowell, and the others.
We may therefore turn to the evidence from Drosophila
with confidence that it will be presented with fairness to the
mutationist point of view. We shall first ask (1) what we
learn from the work on Drosophila as to the possibility of
finding finely graded variations in a single unit character.
Next we shall inquire (2) as to the relation of the assumed
modifying factors to changes in hereditary constitution ; to
the nature of the effects of selection.

1. First, then, what are the facts as to numerous finely
graded variations in a single unit factor? Here we have
certain remarkable data as to the eye-color of Drosophila;
data that are of great interest with relation to the nature
of evolutionary change. This fruit-fly has normally a red
eye. Some years ago a variation occurred by which the
eye lost its color, becoming white, a typical mutation.
Somewhat later another variation came, by which the eye
color became eosin. By these wonderfully ingenious meth-
ods which the advanced state of knowledge of the genetics
of Drosophila has made possible, it was determined that the
mutations white and eosin are due to changes in a particular
part of a particular chromosome, namely, of the so-called
X-chromosome, or chromosome I. And further, it was dis-
covered that the two colors are due to different conditions
of the same locus of the chromosome; in other words, they
represent two different variations of the same unit. More-
over, the normal red color represents a third condition of
that same unit.

Somewhat later a fourth condition of this same unit was

Inheritance of Small Variations 209

found, giving a color which lies nearer the red, between the
red and eosin ; this new color was called cherry. So we have
four grades or conditions of this single unit character.

And now, with the minute attention paid to the distinction
of these grades of eye color, new grades began to come fast.
In the number of Genetics for November, 1916, Hyde adds
two new grades, one called "blood," near the extreme red end
of the series, the other called "tinged," near the extreme
white end ; in fact, from the descriptions it requires careful
examination to distinguish these two from red and white, re-
spectively. Thus we have now six grades of this unit. And in
the same number of the same journal, Safir (1916) adds an-
other intermediate grade, lying between "tinged" and eosin ;
this he calls "buff." All these seven grades are diverse con-
ditions of the single unit factor, having its locus in a certain
definite spot in the X-chromosome. Such diverse conditions
of a single factor are known as multiple allelomorphs.

So, up to date we know from the mutationists' own stud-
ies of Drosophila that a single unit factor presents seven
gradations of color between white and red, each gradation
heritable in the usual Mendelian manner. These grades are
the following: (1) red; (2) blood; (3) cherry; (4) eosin;
(5) buff; (6) tinged; (7) white.

It would not require a bold prophet to predict that as the
years pass we shall come to know more of these gradations,
till all detectible differences of shade have been distinguished,
and each shown to be inherited as a Mendelian unit. Con-
sidering that the work on Drosophila has been going on only
about seven or eight years, this is remarkable progress
toward a demonstration that a single unit factor can pre-
sent as many grades as can be distinguished, that the grades
may give a pragmatically continuous series.

Besides showing that a unit factor may thus exist in
numerous minutely differing grades, this case shows that a

210 Life and Death, Heredity and Evolution

heritable variation may occur so small as to be barely de-
tectible. Although the variations do not usually occur in
this way, the case present the conditions which would allow
of a gradual transition from one extreme to the other, by
means of numerous intermediate conditions.

2. But, as we have seen, the gradual changes in heredi-
tary characters seen in selective breeding usually do not oc-
cur in this way, but rather by the slow accumulation of
many factors each having a slight effect, — the multiple modi-
fying factors. But what sort of things are these factors and
what is their relation to actual changes in the heritable con-
stitution of the organism?

Our direct experimental knowledge of these "modifying
factors" is scanty; it comes mainly from the studies of
Drosophila. We find data as to certain known modifying
factors by Bridges (1916) in his important paper on non-
disjunction of the chromosomes. And here we are taken
back again to the series of eye colors, and indeed to one par-
ticular member of the series, the middle member, called eosin.
Bridges tells us that he found a factor whose only effect was
to lighten the eosin color in a fly with eosin eyes ; this factor
indeed nearly or quite turns the eosin eye white. This factor
Bridges calls "whiting." Another factor has the effect of
lightening the eosin color a little less, giving a sort of cream
color; this is called "cream b." A third factor dilutes the
eosin color not so much ; it is called "cream a." In addition
to these, Bridges tells us that he has discovered three other
diluters of the eosin color ; we will call them the fourth, fifth,
and sixth diluters. And finally Bridges tells us of another
factor whose only effect is to modify eosin in the direction of
a darker color ; this factor he calls "dark." None of these
factors has any effect save on eosin-eyed flies.

As you see, these things add tremendously to our grada-

Inheritance of Small Variations

211

tions in eye color. We had already been furnished seven
grades, from white to red; now we have seven secondary
grades within a single one of these seven primary grades.
Our list of gradations of eye color in Drosophila therefore
takes now the following form:

Heritable grades of eye color,
due to diverse variations of a
single unit located in Chromo-
some I.

1. White

2. Tinged

3. Buff

4. Eosin

5. Cherry

6. Blood

7. Red

Variations that give modifica-
tions of the intensity of eosin, but
are located in other chromosomes.

1. Whiting

2. Cream b

3. Cream a

4. Fourth diluter

5. Fifth diluter

6. Sixth diluter

7. Dark

Here again then we have minutely differing conditions of
a single shade of color, brought about by seven modifying
factors.

But what are these modifying factors? And here we
come to the essential point. These modifying factors are
themselves alterations in the hereditary constitution. Bridges
leaves no doubt upon this point. He lists and describes them
specifically as mutations ; as actual changes in the hereditary
material.

What then is the difference in principle between such cases
and the theory of gradual alterations in a single unit fac-
tor? The difference is that in the case of the multiple modi-
fying factors the minute changes occur, not all in one factor
— in one locus of the chromosome — but in a number of di-
verse parts of the germinal material; this appears to have
been clearly demonstrated. But this is a matter of detail;
it does not touch the fundamental question.

This fundamental question is as to the occurrence of these
minute changes in the hereditary constitution, and as to the

Life and Death, Heredity and Evolution

possibility of getting therefrom by selection various grades
of a given external characteristic. In this, so far as I can
see, there is complete agreement.

It appears then that under the recent careful studies
made, it can no longer be maintained that hereditary changes,
even in higher organisms, must be large leaps or saltations.
They may be of this character, but they may equally well be
graded changes so slight as to be hardly detectible when
taken singly.

This appears to be recognized by those who have proposed
and defended the mutation theory. De Vries (1916) in a
recent summary of the theory emphasizes throughout exten-
sive mutations, and speaks repeatedly of their origin as
"sudden and without transitional conditions," but admits
also that "not only very small, but also much greater" dif-
ferences between species arise all of a sudden ("mit einem
Sprunge") ; and that most mutations affect only a single
character. He sets forth further that in a single stock one
such mutation after another may arise, at intervals, until in
the course of time the stock has become very diverse from
the original one; and has become differentiated into a num-
ber of different types on which selection may act. Now, so
far as the mutations are "very small," the condition after
but one or a few mutations had appeared would be prac-
tically indistinguishable from a "transitional condition" to
the state after many mutations had occurred. Morgan
(1917) recently insists that it must be recognized and has
always been urged by de Vries that "mutations may be very
small so far as the character change is concerned."

The true and important points insisted on by the muta-
tion theory appear to be these: —

(1) There are many differences ("variations") between
individuals that are not heritable. Hence by selection of
such diversities no evolutionary change is produced.

Nature of Inherited Variations 213

(2) Actual hereditary changes in characters occur rather
rarely. This is apparently what is meant by the statement
that they appear suddenly ("sprungweise") ; for a time they
do not exist; then they do. (But the changes thus suddenly
occurring may be so minute as to be hardly detectible until
later changes in the same direction have accentuated them.)

(3) Heritable changes may and often do occur in large
steps, so far as their effect on the developed characters of
adults is concerned. (This is an important fact; equally
important is the fact that heritable changes may be, and
often are, very minute.)

(4) Appearances indicate that the changes are analogous
to (or actually are) chemical changes. When one chemical
compound changes into another, there is, it is held, no transi-
tional condition between the two, and the same is believed
to be true for hereditary variations. This conception of
the nature of hereditary variations accounts for the fact that
they often show in the adult as changes of large extent ; and
at the same time it fits equally well the minutely graded
hereditary changes that likewise occur. For there is no
change so minute that it may not be chemical in its nature.
In the immense organic molecule, with its thousands of
atoms, a shift of a single radical or single atom from one
position to another is a chemical change, though it may make
a difference so slight as to be almost beyond detection by
the most refined means.

Whether a doctrine embodying these ideas differs from
that set forth by Darwin to such an extent as to deserve the
name of a new theory may be doubted. This will be a mat-
ter of individual opinion.

The doctrine that hereditary changes must occur by large
steps evidently cannot be held. But what bearing on the
method of progressive evolution has the fact that they often
do occur by such steps, — not by a series of gradual altera-

Life and Death, Heredity and Evolution

tions? In the eye of Drosophila variation may occur from
red to white directly, without transitional stages; or from
any grade to any other; the continuous scale of colors we
have mentioned is obtained only by arranging the steps in
order. Some maintain therefore that evolution has occurred
by such large steps, not by gradations. This conception
has evidently lost the compelling force it seemed to have be-
fore hereditary variations in minute grades were detected.
The very facts in such an organism as Drosophila show that
there is nothing to prevent a passage from one extreme to
another by minute changes, such as are held to occur by
palaeontologists and selectionists. Further, in such cases
as the eye-color of Drosophila we are dealing with charac-
ters that are already highly developed, and the changes we
observe are mainly retrogressive. We know, for example,
that the red eye color of Drosophila is formed by the co-
operation of many separate parts of diverse chromosomes ;
it is a highly complex product of evolution. Now, we find
that one or another of these parts may suddenly cease to
perform its function, so that the red color is not completely
formed ; there is a sudden change in it ; or it may disappear
completely. But it may be doubted whether this implies
that in the original production of this complex character,
with its numerous underlying functional parts, there was
the same change by sudden large steps. Is there any rea-
son to suppose for example that at one time there was a
complete eye save for the absence of the red color ; and that
this suddenly appeared? Our knowledge that this red color
is made by the cooperation of many diverse parts makes
such a notion almost inconceivable. Destructive changes in
a fully formed character, such as we see in the large major-
ity of cases of mutation, could hardly be expected to throw
light on how that character was built up. The observed

Nature of Inherited Variations 215

facts leave readily open the possibility of the building up of
a character by minute graded changes.

In essentials, therefore, the study of mutations, when
carried so far as in Drosophila, is not in disagreement with
our observations of gradual variation in the Protozoa nor
with the conclusions of palaeontologists as to the gradual de-
velopment of the characteristics of organisms in past ages.

These conclusions of the palasonologists are well stated
in the recent work of Osborn (1917). He sets forth that in
following given stocks from earlier to later ages, characters
arise from minutest beginnings and pass by continuous
gradations to a highly developed condition. This seems
in agreement with the experimental results on both higher
and lower organisms, as I have tried to set them forth. The
palaeontogolical evidence, he holds further, indicates that the
hereditary changes as one passes from age to age do not oc-
cur in random directions, but follow a definite course, which
might seem to have been predetermined in the constitution
of the organisms, or otherwise. In the experimental work
on the lower organisms little that indicates this has thus far
been observed. By selection we can move in more than one
direction; though it is also true, of course, that the varia-
tions possible are limited by the constitution of the organ-
ism. The experimental work has hardly gone far enough
to offer important evidence on this problem.

There is one other point in the work on higher organisms
that we may briefly consider. This is the point made by
Bateson (1914) in his Presidential Address before the Brit-
ish Association, and further developed in a recent paper by
Davenport (1916). It is the paradoxical proposition that
since practically all observed variations are cases of loss
and disintegration, we are driven to suppose that evolution
has occurred by loss and disintegration. Davenport com-

816 Life and Death, Heredity and Evolution

bines this idea with the theory that these disintegrating
variations follow a definite course, predetermined in large
measure by the constitution of the disintegrating material.

There are two points that need consideration in dealing
with this theory. The first is one of observational fact ; al-
though it is true that many mutations appear to be cases of
loss and disintegration, yet there is no indication that this
is the case in such results of selection as have been described
in the Protozoa; heritable variations are not limited to any
particular direction.

But secondly, it appears to me that this conclusion that
evolution is by disintegration and loss is based on an error
in logic, which, being detected, puts it out of consideration.
As we examine the series of organisms, from amoaba to man ;
or as we examine the palaeontological series, we find a grada-
tion from those showing little visible differentiation to those
showing great visible diff erentiation ; the problem of evolu-
tion is as to how the passage was made from those visibly
little differentiated to those visibly highly differentiated.

But now, according to the doctrine we are considering,
when we come to examine the actual changes in hereditary
characters, we detect on the whole only the disintegration
of organisms already visibly differentiated ; only a change
from greater visible differentiation to less.

The doctrine then proceeds to draw from this fact the
absurd conclusion that the visibly more differentiated must
have arisen from the visibly less differentiated by decrease
in the differentiation of the latter !

So preposterous a conclusion can be drawn only from the
fact that while in our premises we are talking of visible in-
crease and decrease of differentiation, in the conclusion as
ordinarily drawn the ground is shifted to mean something
entirely different, — an inner, invisible, purely theoretical

Process of Evolution £17

kind of disintegration and differentiation. If we recall that
we were dealing in the premises with visible increase and de-
crease of differentiation, and that therefore in the conclusion
we must deal with the same, the absurdity of the conclusion
becomes manifest. All that we can legitimately conclude,
if we accept the premise that the observed changes in hered-
itary characters are cases of loss and disintegration, is that
we have not seen the process of evolution occurring. But
we are not compelled to accept that premise. In the lower
organisms at least it cannot be asserted that all changes of
hereditary characters are cases of loss and disintegration.

General View of the Processes and Problems of Development,

Mating and Evolution, in the Light of What We

Find in Lower Organisms

Let us now attempt an outline of what our examination of
the processes of mating and development have shown us in
the lower organisms, in so far as it agrees with what we find
also in the higher organisms.

We saw at the beginning that each species of organism,
so far as studied, is differentiated into many slightly diverse
stocks, each diversity hereditary. This we found to be true
both in Protozoa and in higher organisms. We saw, too,
that in simple reproduction from a single parent, by division
of a cell or of an individual, there is a high degree of con-
stancy in the hereditary characters of these slightly differ-
ing stocks. The constancy is so great that for a long time
the search for hereditary changes was at a standstill; the
stocks seemed permanent.

But with intensified study, it was found that in the
Protozoa changes in the hereditary characters do occur
even in reproduction by simple division. Such an organism
as Difflugia gradually differentiates, even without mating,

218 Life and Death, Heredity and Evolution

into slightly differing stocks, similar to those found in na-
ture. The changes are either continuous, or by steps so
slight that single ones are hardly detectible.

In the higher organisms, where the matter is greatly com-
plicated by the fact that most reproduction is from two
parents, it first appeared that gradual or minute hereditary
changes did not occur. But as we have tried to bring out
above, with more thorough study it has come to light that
such changes do occur, as well as do hereditary alterations,
which, when they reach the adult condition, form a sudden
marked change. In this way all contrast in principle be-
tween what we find in the lower organisms and what we find
in the higher ones disappears. In both the process of evo-
lution by minute gradations is visible.

Then we proceeded to examine how far in lower organ-
isms diversity of external conditions brings about hereditary
changes. Here again the first examination, even though
long continued, seemed to show constancy; diversity of ex-
ternal conditions appeared to have no permanent effect on
the stocks. But again, intensified study reveals that the
hereditary characters gradually do become changed by di-
versities of external conditions. Through such diversities,
continuing for great numbers of generations, single stocks,
uniform in their hereditary characters, gradually differen-
tiate into many with faintly differing hereditary features.
Again the process is gradual, or by steps so small that sin-
gle ones are imperceptible. In higher organisms the state
of knowledge on this point appears less satisfactory. But
the evidence so far as it goes indicates that the processes
here are in agreement with those in lower organisms. Ap-
parently diversities in external agents may, under condi-
tions which seem rather rarely met, so modify the germ cells
that they produce progeny with changed hereditary char-

Process of Evolution 219

acters. On the other hand there is little indication that
when an agent produces a direct effect on a part of the body,
this so changes the germ cells that in later generations they
produce bodies with the same alterations.

All together we find then, that even independently of any
mating processes, diversity of stocks is being produced, but
most slowly and gradually.

Next we turned to a study of reproduction from two par-
ents, and its relation to these general questions. We found
that while in many organisms (particularly the higher
ones), the two individuals or cells that mate are unlike, be-
longing to separate sexes, this seems not to be universal.
Mating is apparently often between like parts. This ap-
pears clearest in the final act of mating, the conjugation of
the chromosomes, for in these there is no indication of a
sex difference. But it seems to be true also in many cases
for the germ cells and for the individuals that mate. It is
certainly not clear that sex diversity is a general and fun-
damental requisite for mating ; rather does the contrary ap.-
pear true.

Yet where sex diversity does occur, as in the higher or-
ganisms, it is manifest in the most fundamental features of
the organism. Every cell of the male, in many organisms,
differs from every cell of the female, and precisely in the
most fundamental features of the cell ; in the nuclei ; and in
the essential chemical operations in which the nuclei are in-
volved. But this seems to be a condition derived from the
simpler state where no such diversity exists, but in which
mating nevertheless occurs.

In search for what is fundamental in mating and its re-
sults, we came upon theories that mating produces re-
juvenescence; that mating is a necessity for continued ex-
istence and multiplication; that without it vitality is lost;

220 Life and Death, Heredity and Evolution

and that it must take place in order that the lost vitality
shall be restored. But when we examine the evidence on
this in lower organisms, we find a whole series of facts that
will not range themselves under this doctrine, along with some
that will. After mating, some organisms are less vigorous
than before; some little altered; some more vigorous. The
latter may be held to show rejuvenescence. But the gen-
eral result is to produce many diverse stocks with new sets
of hereditary characters. Some of these new combinations
show greater vigor, others less ; and they differ in many
other ways. Such diverse stocks resulting from mating
show similarities among themselves, resulting from their de-
pendence on the union of two parents ; that is, they show
biparental inheritance. But the stocks produced by a given
mating differ, too, in their hereditary characters.

These relations, as yet little known in the lower organisms,
receive illustration on a vast and conspicuous scale in the
higher organisms, where they are known as Mendelian in-
heritance. The offspring of a given pair are more alike
than are the offspring of diverse pairs. Nevertheless, the
offspring of a single pair show combinations of hereditary
characters diverse from each other and from their parents.

Examining the minute processes that occur in reproduc-
tion from two parents, we find that there is a visible forma-
tion of new combinations of the chemicals on which develop-
ment and function depend; new combinations of the pri-
mary hereditary characters. These chemicals are in vis-
ible packets, and the method of forming new combinations
of them can be seen under the microscope. Even when, as
often happens, the mating is between parts of the same
nucleus of the same cell, the processes are such as to bring
about new combinations of the primary hereditary sub-
stances ; organisms with new combinations of hereditary

Process of Evolution

characters are produced. These recombinations occur in
the same general way in the lower and the higher organisms.

This formation of new combinations of the primary hered-
itary substances is then the general feature of mating. It
is this of which we were in search when we asked: Is there
any general result of mating, comparable with the produc-
tion of energy as the general result of the taking of food?
Mating is a process of forming new combinations of the
primary hereditary materials.

As a result of these new combinations, the organisms pro-
duced are very diverse in their hereditary characters. In
the infusorian some are weak and unenduring; the things
combined do not work together harmoniously; they die out.
Others are strong and vigorous; they persist and multiply.
In other organisms similar differences appear, along with di-
versities in respect to all possible hereditary characteris-
tics. Thus mating steadily changes the face of organic na-
ture, continuously producing new combinations, some of
which are extinguished, while others flourish.

This process is greatly assisted in the lower organisms by
the fact that after a set of new combinations is produced by
mating, each combination is multiplied greatly by vegeta-
tive reproduction, which does not make a change in the
grouping. Thus each combination is given an opportunity
to meet many diverse external conditions, with some of which
it may work in harmony; further, the number of possible
diverse combinations which may result from the next period
of mating is greatly increased.

This continued formation of new combinations is the
great corrective of the uniformity which would result from
more rigid laws of heredity. In general, no one can predict
what combinations will actually result from a given mating,
for the number possible is much greater than the number

222 Life and Death, Heredity and Evolution

that can be realized. In other words, no one can predict
with certainty the characteristics of the offspring to be
produced by a given pair. Parents in which certain char-
acteristics are developed in but a mediocre degree may pro-
duce by their union offspring in which these characteristics
are developed in a high degree, as we saw in Paramecium that
parents of low vigor may produce a few offspring of high
vigor. This is as true for the qualities which in human in-
dividuals we tend to class as good or bad, as for vigor in
Paramecium. From mediocre parents may arise, by the
formation of new combinations of the hereditary material,
offspring that are distinguished for good or for ill.

This is the fact that undermines all exclusively aristo-
cratic theories of breeding and inheritance in such an organ-
ism as man; this is the possibility that must underlie any
democratic theory of society and of progress. Possibly
from the great mass of mediocre humanity there may arise
by new combinations in every generation so great a number
of distinguished men as to make the contribution of offspring
from the relatively few distinguished individuals unimpor-
tant. It is not certain that the relative infertility of the
intellectual classes decreases the existing proportion of in-
tellectual men. It may be that there continually arises from
the great average mass of mankind a proportion of dis-
tinguished men that remains relatively constant, even though
these distinguished men may not reproduce at all.

To return to our general relations, this process of re-
combination does not evidently, of itself, result in the pro-
duction of any new characteristics. We cannot say posi-
tively that it does not, but if the chemicals on which devel-
opment depends — the primary hereditary characters — were
permanent, unchangeable things, then recombination could
produce merely kaleidoscopic regrouping of these; and the

Process of Evolution

number of possible diverse combinations that could appear
would be definitely limited. All change would be a regroup-
ing of what already exists.

Therefore it becomes most important to examine what
happens when there is no such regrouping, in heredity from
a single parent. Here we discover, as we have already set
forth, that actual changes in the hereditary materials are
occurring, independently of recombinations. New charac-
teristics appear that are heritable.

Are such new characters bound up with the other primary
hereditary characters, and subject to the same processes of
recombination at mating? In Drosophila this question is
answered clearly in the affirmative; the new characters that
appear recombine at mating as do the old ones. There is lit-
tle reason to doubt that they do so in all organisms.

Thus a new character arising in a particular individual
having a given combination of characters, is transferred by
mating to other individuals with other combinations. With
some it may work harmoniously ; with others, not. Further,
several new characters arising in diverse individuals may by
mating become transferred to a single one, where they may
form a combination working more harmoniously together
and with the environment than any that has before existed.
Thus all sorts of combinations arise, of new and old charac-
ters, such as could not possibly occur without mating.
Some are more vigorous and harmonious; some less; some
fit one set of outward conditions; some another. Mating
thus contributes enormously to the differentiation of organ-
isms into hereditarily diverse stocks. Its general result is to
give all sorts of combinations of characters, new or old;
some persisting, others not. Added to the slow production
of new characters, it greatly hastens the changes in organic
nature that we call evolution.

WORKS REFERRED TO IN THE TEXT.

(This is not a general bibliography of the subject, but only a list of
the works to which reference is made. Where there is general reference
to a large series of investigations by a particular author, as a rule only
one reference is given, to some important contribution; from which the
reader can follow up the subject if he desires. This is done particularly
when the subject lies to one side of our main field.)

Ackert, J. E.f 1916. On the effect of selection in Paramecium. Ge-
netics, 1:387-405. — Agar, W. E., 1914. Experiments on inheritance in
parthenogenesis. Phil. Trans. Roy. Soc., London, 205 B: 421-489.

Baitsell, G. A., 1912. Experiments on the reproduction of the Hypo-
trichous Infusoria. I. Conjugation between closely related individuals
of Stylonychia pustulata. Jour. Exper. Zool., 13:47-77. — Id., 1914. Ex-
periments on the reproduction of the Hypotrichous Infusoria. II. A
study of the so-called life cycle in Oxytricha fallax and Pleurotricha
lanceolata. Jour. Exper. Zool., 16:211-235.— Balbiani, E. G., 1881-1882.
Les Protozoaires. Journ. de Microg., T. 5 et 6. (Cited from Biitschli,
1880.) — Barber, M. A., 1907. On heredity in certain micro-organisms.
Kansas Univ. Sci. Bui., 4 (1); 1-47— Bateson, W., 1914. The Address
of the President of the British Association for the Advancement of
Science. Science, 40:287-302, 319-333.— Blakeslee, A. P., 1904. Sexual
reproduction in the Mucorineae. Proc. Amer. Acad., 40:205-319. — Id.,
1913. Conjugation in the heterogamic genus Zygorhynchus. Mycolog.
Centralbl., 2: 241-244.— Bott, K., 1907. Ueber die Fortpflanzung von
Pelomyxa palustris, nebst Mitteilungen tiber ihren Bau. Arch. f. Protis-
tenkunde, 8: 120-158. — Bridges, C. B., 1916. Non-disjunction as proof of
the chromosome theory of heredity. Genetics, 1:1-52, 107-163. — Brown-
Stquard, C. E., 1869. Nouvelles recherches sur 1'^pilepsie due a certaines
Idsions de la moelle 6piniere et des nerfs rachidiens. Arch. Physiol.
Norm. Path., 2:211-220, 422-437, 496-503.— Butschli, O., 1875. Vor-
laufige Mittheilung einiger Resultate von Studien tiber die Conjuga-
tion der Infusorien und die Zelltheilung. Zeitschr. f. wiss. Zool.,
25:426-441.— Id., 1880-82. Protozoa. Bronn's Klassen und Ordnungen
des Thierreichs, Bd. 1. 2035 pp.

Calkins, G. N., 1902. Studies on the life-history of Protozoa. I. The
life-cycle of Paramoecium caudatum. Arch. f. Entw.-Mech., 15:139-
186. — Id., 1902a. Studies on the life-history of Protozoa. III. The six
hundred and twentieth generation of Paramoecium caudatum. Biol. Bui.,
S: 192-205.— Id., 1904. Studies on -the life-history of Protozoa. IV. Death
of the A series. Conclusions. Jour. Exp. Zool., 1 : 423-461.— Id., 1906.
The Protozoan life cycle. Biol. Bui., 11:229-244.— Id., 1911. Regenera-
tion and cell division in Uronychia. Jour. Exper. Zool., 10:95-116. — Id.,
1915. Cycles and rhythms and the problem of "immortality" in Para-
mecium. Amer. Nat., 49:65-76. — Id., 1915a. Didinium nasutum. I. The
life history. Jour. Exper. Zool., 19: 225-241.— Id., 1916. General biology

224

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Cull., 8. W., 1907. The conjugation of Paramecium aurelia (caudatum).
Arch. f. Protistenkunde, 10:375-415.— Calkins, G. N., and Gregory,
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INDEX

Abnormalities, 150, 159
Acclimatization, 97
Ackert, 63

Acquired characters, 44
Agar, 64
Alcohol, 203
Amoeba, 15, 39
Amphibia, 200
Anabolism, 115
Anoplophrya, 180, 192
Arcella, 56, 77, 82
Arsenic, 101
Assortative mating, 191
Aster, 117
Autogamy, 134, 185

Bacillus coli, 88

Bacillus prodigiosus, 63, 88

Bacteria, 15, 50, 58, 63, 86-96

Balbiani, 24

Barber, 63, 87

Bateson, 68, 215

Beans, 64

Biparental inheritance, 151, 152,

168, 170-190

Biparental reproduction, 21, 219
Blakeslee, 123
Blepharisma, 192
Bott, 183
Bridges, 208, 210
Brown-S6quard, 203
ButschK, 24
Butterflies, 202

Caenomorpha, 16

Calkins, 24, 29, 32, 120, 141, 151

Calkins and Cull, 181

Calkins and Gregory, 142

Carchesium, 180

Castle, 203, 207

Catabolism, 116

Centropyxis, 56, 77

Centrosome, 116

Chemical agents, 88-96, 102, 203

Chilodon, 128, 180
Chlamydomonas, 155-157
Chromidium, 83
Chromosomes, 108, 115, 116, 138,

171-190

Climacostomum, 150
Collin, 126, 180, 192
Colored bacteria, 88
Colpoda, 165

Constancy of stocks, 62, 65, 67, 86
Corrosive sublimate, 89
Coulter, 126

Conditions of conjugation, 164
Conjugation, 22, 25, 36, 102, 106-

169

Continuity of life, 19, 21, 30, 35
Corpse, 35
Correlation, 73
Crustacea, 64
Cryptochilum, 165
Cull, 120

Dallinger, 98

Dallingeria, 98

Davenport, 68, 215

Death, 19, 28, 39

Democracy, 222

Depression, 148

Dexter, 208

Didinium, 16, 32, 133, 142, 150,

177, 195

Difflugia, 40, 45, 50-55, 68, 70-76
Diplodinium, 17
Dobell, 93, 95, 96, 106, 153
Doflein, 119
Draba, 59
Drosophilia, 205, 207, 208, 223

Endomixis, 31

Enriques, 29, 33, 128, 165, 180

Environmental modifications, 85-

103, 198, 218
Epistylis, 121
Erdmann, 166

231

232

Index

Euplotes, 146

Evolution, 38, 65-73, 83, 213, 223

Ewing, 64

Exchange of nuclei, 25, 36, 110

Exconjugant, 112

Eye color, 208-211

Female, 115, 116, 133

Fertnor, 32

Fertilization, 36, 106

Fischer, 202

Fission rate, 24, 81, 141-147, 154,

158, 161
Flagellate, 134
Folliculina, 16
Fowl, 203

Geddes and Thompson, 115
Genotype, 65, 86
Glaucoma, 29, 33
Gradations, 76, 209, 213-218
Gregarinidae, 175
Guinea pig, 203

Hance, 58

Hartmann, 137

Hegner, 56, 77, 82

Heredity, 37-49, 67-84, 152-157,

170-190
Heritable variations, 76, 81, 84,

212

Herpetomonas, 183
Hertwig, R., 24, 142, 144-148
Hutchison, 57
Hydra, 64
Hypotricha, 42

Immortality of unicellular organ-
isms, 20, 30, 35

Infusoria, 16, 17, 24

Inheritance, 37-49, 67-84, 152-157,
170-190

Inheritance of acquired characters,
44, 48, 85-105, 199, 218

Isolation of bacterium, 87

Johannsen, 64, 65

Jollos, 33, 34, 57, 63, 95, 101

Jordan, 59, 66

Kammerer, 200, 202
Kinetic, 116

Kinetonucleus, 96, 97, 116
Kinetoplasm, 116

Lacrymaria, 16
Lashley, 64, 124
Leucophrys, 28, 146, 150
Lotsy, 59

MacDowell, 207, 208
Macronucleus, 25, 31, 35, 44, 103,

110

Male, 115, 116, 133
Massini, 92
Mast, 142, 195
Maturation, 172-186
Maupas, 24, 28, 30, 64, 114, 121, 122,

131, 132, 142, 145, 146, 147, 150
Mendelian inheritance, 65, 187-190
Metcalf, 181
Micronucleus, 25, 31, 44, 58, 103,

110

Middleton, 80, 100
Migratory nucleus, 110, 132
Minchin, 126
Modification, 103
Modification of inherited characters

by external agents, 85-103, 198,

218

Modifying factors, 207, 210
Monas, 98
Monocystis, 116
Monstrosities, 150, 159
Morgan, 205, 207, 208
Mould, 123, 136
Mucor, 123
Muller, 208
Mulsow, 175

Multiple allelomorphs, 209
Mutation, 67, 68, 102, 204-211
Mutilations, inheritance of, 45
Myxobacteria, 93
Myxococcus, 93

Newman, 60
Non-disjunction, 210
Nucleo-plasmic relation, 148
Nucleus, 24, 25, 82

CEnothera, 205
Onychodromus, 28, 146
Opalina, 181
Opercularia, 128, 180
Osborn, 215
Oxytricha, 28

Index

233

Palaeontology, 69, 215
Paramecium, 22, 25, 29, 31, 34, 46,

50, 56, 58, 63, 68, 97, 110, 119,

130, 133, 142, 143, 154, 158, 165,

181, 191

Parthenogenesis, 64
Pearl, 68, 203
Pelomyxa, 183
Plant lice, 64
Pneumonia bacillus, 90
Popoff, 180
Potassium bichromate, effects of,

89

Potato beetle, 202
Powers and Mitchell, 58
Prandtl, 133, 177
Primary hereditary characters, 170,

185-187, 222
Pringsheim, 95
Protozoa, 15
Prowazek, 183
Pure line, 65
Purpose, 106

Quehl, 93

Radl, 13

Rate of reproduction, 24, 81, 141-

147, 154, 158, 161
Reduction, 172, 175-186
Reeves, 207
Rejuvenescence, 23, 26, 36, 114,

141-147, 150, 162, 220
Reproduction, rate of5 24, 81, 141-

147, 154, 158, 161
Reserve nucleus, 25
Resistance, 57, 101
Root, 56, 77

Saltation, 205

Schaudinn, 183

Secondary hereditary characters,

185

Selection, 60-73, 81
Senility, 30

Sex, 109, 114-140, 168, 219
Size of nuclei, 82
Spirogyra, 135
Spirostomum, 150

Stand fuss, 202
Stentor, 16
Stephanosphsera, 126
Stockard, 203
Structure of Protozoa, 15
Sturtevant, 207, 208
Stylonychia, 16, 28, 32, 43, 81, 100,
145, 150

Temperature, acclimatization to,

98-100

Tetramitus, 98
Toenniessen, 90
Tower, 202
Trichomastix, 134
Trophic, 116
Trophonucleus, 116
Trophoplasm, 116
Trypanosoma, 96, 183
Twins, 60

Unicellular organisms, 15, 16
Uniparental inheritance, 38-84
Uniparental reproduction, 21
Unit factor, 208

Uroleptus, 32, 142, 144, 149, 151,
163

Vacuoles, contractile, 58
Variation, 50, 61, 66, 67-72, 76, 193
Vitality, 24, 141-147, 161, 196
Vries, de, 205, 212
Vorticella 16, 120, 127

Wallengren, 121
Walton, 195
Watters, 192
Wolf, 63, 88, 93
Woodruff, 29, 30, 33, 149, 167
Woodruff and Erdmann, 30, 32, 34,
103, 145

Yeasts, 63
Young, 33

Zeleny and Mattoon, 207
Zweibaum, 165
Zygorhynchus, 136
Zygospore, 123

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