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EVOLUTION
BY MEANS OF HYBRIDIZATION

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Biol.
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EVOLUTION

BY MEANS OF HYBRIDIZATION

BY

J. P. LOTSY

THE HAGUE

MARTINUS NIJHOFF
1916

To the Memory of Charles Darwin, this
Sketch is dedicated by the Author as
a Tribute to his Character and to his
Work which lead to the general Recog-
nition of the Principle of Evolution.

PREFACE.

As a Preface is in reality a Postscriptum, the author may be permitted
to open it by mentioning omissions.

The chief sin of omission he committed, is evidently the insufficient
justice he did to the writings of Anton Kerner von Marilaun, who
was — he wants to state this explicitly — the first to recognize fully the
significance of crossing as the underlying cause of the origin of species.

What else should a preface say?

If the work is as condensed as the present one, it may perhaps suffise
to repeat what Linnaeus said to Haller:

Si quos in me vidisti errores, Tu sapientior haec ignoscas.... Quos
plures apud me detegere potes, eo gratior ero, tum possum omnia
corrigere vivus; post mortem non licet emendare propria opuscula.

By which however the author does not consider himself bound to
gratefulness for every kind of criticism.

He is f. i. very little impressed by the kind of criticism which calls it
,inconceivable”’ ,,verging on the absurd”’ etc., to believe that crossing
can ever have been the underlying cause of the origin of new species,
from authors who firmly believe that the origin of new species should be
ascribed to some kind of variability; because it seems to him ,,absurd”’
that those who advocate the origin of new species from a single ances-
tral one, should reproach an author who defends such an origin from
two ancestral species, of stating an ,,inconceivable’”’ opinion.

To save another kind of critics unnecessary trouble, the author is
fully prepared to admit unhesitatingly that his theory explains but part
of the problem of evolution, so that there is ample room for them to ta-
ke part in the exploration of this most interesting field of investigation ;
but he may be permitted to remark that the rest, left unexplained by
theories based on variability, is not smaller than that left unexplained
by his theory.

VIII PREFACE.

The advantage of the theory, here sketched out, seems to the author
to be, that it is based on the experimentally proven fact that crossing
gives rise to new forms, while, in as much as the real existence of here-
ditary variability is not proven, all theories based on variability have
a_ less solid basis.

Unfortunately the final basis of evolution — the constitution of the
germplasma — remains as yet an utter mystery, so that we know next
to nothing of the final cause of evolution; the more reason to keep
facts and mere surmises well separated. This the author has aimed at,
and he flatters himself that, by so doing, he has cut a small peep-hole
in the curtain which hides the stage on which evolution takes its course.
Criticism within ¢his contention, e. g. dealing with the part of evolution
which is discussed in this sketch, will indeed be most welcome to him
and will be considered in a careful manner and a grateful spirit.
Du choc des opinions jaillit la vérité!

HaarceM, February 16th 1916.

PART I.
INTRODUCTION.

WHAT IS EVOLUTION?

From the very moment that mankind began to re-
flect, the desire was born to know how all those diffe-
rent objects which man sees around him, came into
being and many are the opinions, vague and detailed
ones, which he has held and still holds on this momen-
tous question.

To the unexperienced, nature has the aspect of sta-
bility, of being the same to day as yesterday and this
conviction underlies all legends of deities, creating the
earth in a few days, so as to have it ready for the crow-
ning appearance of mankind.

A little experience and a little reflection however,
teaches us that the apparent stability of nature is de-
ceptive, and that this impression is caused exclusively
by the short time of observation, allowed to each single
individual.

A human life lasts but a moment of eternity and
even the lives of many successive individuals are in-
sufficient to observe slow changes in the aspect of
nature.

Even if a man could return to the earth every few
centuries, he would yet be unable to see much change.

A LUTHER, coming back to the Wartburg to day,
could easily revisit all spots once familiar to Junker
Jorg and believe that but a few days had passed since
he last saw them.

4 INTRODUCTION.

The stone which used toserve him as a resting place,
is yet there, it is covered by the same species of mosses
which grew there in the 16th century; the same kinds
of flowers adorn now as then, meadows and rocks, and
Landgrafenschlucht, as well as Hohe-Sonne, would be
familiar landmarks to him, while his feet would carry
him, as in former ages, through Drachenschlucht and
Annathal, back to the Marienthal from where he could
ascend the Wartburg along familiar paths, and once
there, even revisit his former abode.

So it would appear, even to a returning Luther, as
if the landscape of Thuringia — with the exception of
minor alterations made bij man — were a permanent
and unchangeable result of the working of a Creator.

And yet this permanency of Nature is a delusion
and a snare.

We need not go very far back into the history of our
globe, to find on the spot, now occupied by the Thu-
ringian hills, a monotonous flat track of land, and if we
recede a little further into time, we see that this con-
tinent was preceded by an Archipelago of small isles,
while a little further back still, the spot now occupied
by Thuringia, was covered by the waters of an ocean,
on the bottom of which fragments of ancient rocks
were interspersed by a few coral reefs, as a token of the
power of the infinitely small, of the action of myriads
of minute living beings. |

Gradually, slowly, ever so slowly, these fragments
of rocks were kitted together by chalky and other sub-
stances, a kit including the coral reefs also, and for-
ming a new layer around that one, of the numberless

- INTRODUCTION. 5

celestial bodies which we call the globe, a layer des-
tined to persist for countless ages as a horizontal crust.
But below this crust, forces whose action never cea-
ses continued their work of change. Under the influ-
ence of the cold of celestial Space, our globe was forced
to part with much of its heat and consequently had to
contract. This caused the appearance of folds on her
surface, of one of which the Thuringian hills are a
rest. By such folding the fragments of rocks, as well
as the coral reefs enclosed in the, up till then hori-
zontal, ocean bottom underwent a considerable uphea-
val.

And of the fold, so formed, Thuringia is but a small
rest; it has been lowered by the action of wind and
water and the latter especially cut into it those deep
valleys of Drachenschlucht and Mariathal which now
delight our eyes, and isolated the Wartburgtop, enclo-
sing in its conglomerate a coral-reef, destined to form
in a distant future the foundation of the Castle which
should serve Luther as a temporary abode.

So it happened, that the translation of the Bible took
place on the rests of small living beings, who countless
ages ago, peopled an ocean, expanded hundreds of feet
below the point where those corals are now found.
What seemed so permanent, was thus but ashort phase
in the chain of changes which the earth and all it
contains or supports, has to undergo. These changes of

which there is no escape, neither for rocks nor for seas, ©

nor for living beings, we call evolution.
No wonder that man, once convinced of the exis-
tence of so universal a change, wanted to know

6 INTRODUCTION.

how evolution acted, and no wonder that numerous
different opinions have been held on this fascinat-
ing subject. Even yet, we are far from a communis
opinio.

We are best informed as to the non-living part of
the universe. Ingeniously worked out methods have
given us the certainty that all celestial bodies consist
chiefly of the same kind of substances, and we havealso
ascertained that a comparatively small number of
primary substances — the so called elements —suffise
to compose alle those numerous substances which oc-
cur in nature or can be made in our laboratories.

The discovery of the existence of these elements led
at first and had to lead, to the conception that elements
were absolutely independent primary substances, but
Prout, a London physician (obiit 1850), called already
in 1815, attention to the fact that the atomic weights
of all elements are invariably integer multiples of the
atomic weight of the element Hydrogen, and from this
knowledge deducted the hypothesis that all elements
might very well be derivatives, by a process of con-
densation, of Hydrogen.

It has later been shown that the integrity of these
multiples is not perfect, but the ascertained deviations
were not sufficiently great, to force us to discard
Prout’s supposition of the existence of some kind of
connexion between the different elements.

On the contrary, Mendelejeff’s periodical system ga-
ve new support to Prout’s idea.

This system arranges the elements in a series accor-
ding to their atomic weights. Part of the elements so

INTRODUCTION. 7

arranged, forms a series with pretty equal intervals
between the successive atomic weights; at several
points however considerable gaps occur. From this
irregularity MENDELEJEFF concluded that there must
exist other elements — till then unknown — and he
ventured to predict that the gaps in his series would
gradually be filled up by these missing links, of which
he even predicted the atomic weights.

In many cases the gaps have really been thus filled,
and this of course gave strong support tothe idea that
elements are not such independent primary substances,
as one had previously supposed them to be.

Yet, decisive proof for a genetic connexion between
the several elements lacked until the discovery of the
radioactive elements by Monsieur et Madame CuRIE,de-
monstrating the direct transformability of the elements.

We now know that a number of elements, among
which Ionium, Radium, Polonium and very probably
lead also, can be directly derived from Uranium bya
process of splitting off Helium, with the atomic weight
4, and electrons, whose apparent mass is but */s part
of a hydrogen-atom.

Unfortunately, it is as yet impossible to excert even
the slightest influence on the process of radioactivity,
and it is especially unfortunate -that we cannot reverse
the process and thus, by adding Helium and Electrons
to lead f. i., build up Uranium.

But the fact that other elements e. g. Potassium and
Rubidium also, though in a much slighter degree, show
signs of radioactivity, justifies the supposition that
Helium takes part in the building up of all elements,

8 INTRODUCTION.

and this is of so great an importance, because the ne-
bulae which are considered to be the first stages in
the evolution of the celestial bodies consist, if not ex-
clusively, at least chiefly of Helium.

This leads to the conception that during the deve-
lopment of celestial bodies from nebulae, the elements
were formed of Helium or of a similar substance, and
that these elements, later on, united for the greater
part with other elements, thus forming more complica-
ted chemical bodies.

By this process also, water and rocks were formed
which, under the influence of the changes in the form
of the earth’s crust, caused by cooling off, changed
places, so that where mountains once were, oceans now
spread their waters and reversely.

Of course, the time required to bring about all these
changes has- been a long one; its length can be deter-
mined approximately by studying the behaviour of
radioactive minerals.

Radio-activity namely, proceeds with a velocity,
independent of external circumstances such as heat
etc., and this allows us to calculate, if the quantity
of Helium present in a mineral is known, by determi-
nation of the quantity of Helium yearly split off by
this mineral, the minimum age of it. It was thus possi-
ble to state positively, that Titanite or Grothite from
the Archaic Period must, at the very least, be 710 mil-
lion years of age, and this agrees quite well with the
estimate of the earth’s age at 1000 millions of years,
derived from data, concerning the formation of the
great saltbeds. ;

INTRODUCTION. 9

This means, that if the complete history of the earth
couldbecompressed intoacourse of daily one-hour lectu-
res lasting one and a half year, the life of a student —
suppose he could begin attending them immediately
after birth and continue to do so up to the comple-
tion of his 80th year — would be just sufficient to
hear two seconds of these lectures.
Once — or repeatedly — during this long, long pe-
riod, living beings arose from much more complicated
combinations of the elements also.
Keen as this deduction may seem to be, in the face
of the fact that generatio aequivoca has never yet been
demonstrated, we are nevertheless forced to it.
istly. because there has been a time in the history of
the earth, during which life was impossible
upon it.

2dly. because we know that no other elements occur
in the living beings than in non-living nature,
so that both are composed of the same substan-
ces.

3dly. because we know that even at the present time,
the majority of plants is able to transform large
quantities of inorganic matter into living one. So
we know f. i. that the whole body of a poplar is
formed of inorganic matter, thus transformed
by the wee little bit of living substance, once pre-
sent in the seed which sprouted tos form the
poplar.

Where we thus find that all bodies on earth, living
as well as non-living ones, are built up by the same ele-
ments, where we know furthermore that these very

|

|

Io INTRODUCTION.

same primary substances are present on those num-
berless celestial bodies which a clear night reveals to
us, where we know that changed conditions continu-
ously cause new combinations to be formed between
these elements, where we know that infinitely small
quantities of living matter are able to transform large
quantities of non-living matter to living one, and where
finally we know also that some elements at least, can
directly give rise to other ones, it is not too risky to
conclude that whatever wesee —therevelation of every
day — as Prof. Bosscua used to express it, is nothing
but the continuous transformation of that one or of tho-
se few wonderfull primary substance(s), from which
everything, ourselves included, originated.

How?

As yet we know but little about it.

Physicists, chemists, geologists and biologists, all of
us, lift but the tips of the dense veil which hides the
mysterious workshop of nature, and but all too fre-
quently we have to thank those who come after us
for letting the tip, we lifted, fall down again, to hide
the errors we committed. But not-withstanding all
our stooping, we who try to reveal the underlying
secret of nature, feel, even in the midst of our defeats,
that the fundamental idea on which all our efforts are
based, that the conception of continuity in all what
happens, that the Principle of Evolution, which owes so
much to Charles Darwin, is correct that, as he expressed
it, the ,,ordinary succession by generation has never
once been broken”.

PART II.
THE EVOLUTION OF LIVING BEINGS.

CHAPTER I.

DEFINITIONS OF TERMS USED.

The evolution of living beings is, as follows directly
from what has been said in the introduction, but part
of the general problem of Evolution, and again the
Origin of species is but part of the more general pro-
blem of the origin of all those differently constituted
Types which people the globe.

Why, we shall see bye and bye.

For the present we will limit our remarks to the ori-
gin of species, and more especially to the origin of
those species which we now designate as the diploid
ones, e. g. which belong to the class which embraces
all so called higher beings.

He who ventures to write on the origin of species,
ought to define what a species is, so ought he to do
who describes species, no matter whether he considers
his task finished when the description has been made,
or whether he intends to make use of the described
species to build up a more or less elaborate system.
In other words: the systematist, as well as the evoluti-
onist, ought to state clearly what he means by a spe-
cies.

As a matter of fact neither of them usually does.

In 1855 Alphonse DE CANDOLLE said already in his
Géographie botanique raisonnée (T. II. p. 1068):

Enoncer clairement ses opinions sur la nature de l’es-

14 THE EVOLUTION OF LIVING BEINGS.

péce est pour un naturaliste l’épreuve la plus redoutable
de toutes. Il sait que chaque mot sera pesé, que toute
adée nouvelle pourra éive taxée Whérisie et que des
notions fausses sur cette base des sciences naturelles,
gettent ses travaux de description dans un discrédit
meérité.

And Darwin says in his Origin of Species p. 30:

Nor shall I here discuss the various definitions which
have been given of the term species. No one definition has
satisfied all naturalists; yet every naturalist knows vague-
ely what he means when he speaks of a species.

Unfortunately, it is just the vagueness of this kind of
knowledge which has caused endless trouble and there
is a good deal of truth in the ,,badinage’’ of MoriTzI
(Réflexions sur l’espéce. Soleure (Solothurn) 1842) who
tells of a professor of philosophy of his acquaintance,
qui avait admis l’espéce, comme nous tous, a priori, sans
savoir en quot elle consiste. C’est seulement plus tard,
he continues, que l’1dée lui est venue de se rendre compte
de ce sentiment obscur qui l’a guidé depuis son enfance.

All this fits our case; all theories of evolution have,
until quite recently, been guided by a vague knowledge
of what a species is, and consequently have inevitably
been vague themselves.

The remedy is obvious: one has, before one proposes
any theory of evolution, to define the terms which
one is going to use.

This is not so difficult as may appear at first sight,
because we can be guided by the aim which obviously
every systematist, no matter of which period or of
which nationality, had before him when he tried to

THE EVOLUTION OF LIVING BEINGS. 15

establish a species viz, to bring like to one and unlike to
different species.

If we admit thatidentity of the different individuals,
to be included in one species, is the essential quality of
a species, and I doubt whether any naturalist will
object to this, the problem is quite simple: to establish
a species, one ought to bring together individuals of tden-
tical constitution.

C’est simple comme bonjour; the trouble unfor-
tunately lies in the difficulty to determine identity
of constitution.

As a matter of fact, one has long overlooked this
difficulty and thought that one could distinguish dif-
ferent constitutions at sight, even without taking very
much pain.

This led the older botanists to the acceptance of
groups of supposedly identical individuals as species
which later generations have shown to consist of a
mixture of individuals of sometimes very different
constitutions.

One usually expresses this shortly, by saying that
the older botanists considered genera to be species.

It is useless to go into the details of these mistakes
and their causes; the obvious primary cause is a lack
of discriminating power which prevented them from
seeing the differences between the individuals, they
thought to be indentical.

Such a lack of discriminating power can however
not be reproached to LINNAEUS, so that, ifidentity of
constitution can be established at sight, there is every
reason to believe, a priori, that the Linnean species will

16 THE EVOLUTION OF LIVING BEINGS.

constitute a group of individuals of really identical
constitutions. As a matter of fact, one has believed
this to be the case for a considerable time.

Linnaeus himself thought so, or at least, doubted
it only occasionally and then set his doubts aside; yet
he already was convinced himself, that the checking
of his conclusions, based on visible differences, necessi-
tated experiments.

Many before him had felt this: Joun Ray had said
already in the 17th Century, that there was no better
criterium to distinguish species, than the fact that true
species faithfully reproduce their kind by seeds: nulla
certior.... quam distincta propagatio ex semine.

This test was frequently, though not under all the
necessary precautions, resorted to by Linnaeus who,
by applying it, saw that the reproduction was not
faithfull, that, on the contrary, the different indivi-
duals obtained from seeds, supposed to belong to the
same species, showed visible differences. Of some of
these differences, he could demonstrate the dependency
on external conditions and show that they were not
transmitted to the offspring, so that they could be
neglected.

Such differences he called varietates minores, and in
order to call the attention of his students to the rela-
tive unimportance of such differences, he characterised
them as: varietates minores non curat botanicus.
Unfortunately, Linnaeus drew from a few experiments
and observations the conclusion, that all small diffe-
rences between individuals should he considered to be
such uninheritable differences, and that consequently

THE EVOLUTION OF LIVING BEINGS. LZ

the degree of visible difference was sufficient to make
out without experiment, whether it was inheritable
or not, in other words that an experienced systema-
tist could distinguish at sight whether two or more
individuals had the same constitution or not.

This mistaken idea, the famous ,,systematischer
Blick’, unfortunately reigns yet supreme in many her-
baria and musea of the present time. It has been ex-
posed to ridicule already in 1855 by JORDAN, whosaid:
»On s’est efforcé de mettre en vogue une théorie, qui
consiste dans l’admission parmi les végétaux de types
»spécifiques tranchés et dans Vhypothése de Ja varia-
»Oité de ces mémes types.

This hypothesis, as Jordan so justly calls it, the
hypothesis of the variability of the species, has caused,
in my opinion, all the trouble we have experienced in
looking for the causes of the origin of species because
it withdrew from systematics as well as from all theories
of evolution every firm footing.

Very rightly again Jordan has said:

Rejeter le critérium de la permanence héréditaire,
c’est tout réduire 4 de simples hypothéses, 4 l’arbi-
traire, a la fantaisie des appréciations individuelles,
c’est en un mot donner pour fondement a la science, le
scepticisme : ce qui revient a la détruire.

And the cause of these individual appreciations,
cause of this scepticism which refused to accept the
existence of clear-cut species and considered allspecies
to be variable, was in the beginning the total neglect
of all experiments (by the older botanists), subsequent-
ly the insufficient application of the experimental test

2

18 THE EVOLUTION OF LIVING BEINGS.

(by Linnaeus), and finally again the total neglect of
all experimenting.

Here Linnaeus also is at fault, because he knew: that
there existed other than non-transmittable differen-
ces between the individuals, united by him to one
species; he even designated these f. i. the different
forms of cabbage which were cultivated at his time,
by giving them another name than vartetates levissimas
viz varietates tout court.

That he considered these to be of considerable im-
portance, clearly ressorts from his advice to his students:
varietates numerosae plurium specierum attente in-
spiciantur (Am. Acad. I. 1744 p. 55), but later he has
forgotten this distinction which he himself made, fide
his utterance (Philosophia botanica. 100 1751) Vartetas
est planta mutata a causa accidentali, solo, calore,
ventis etc.

It is a great pity that Linnaeus postponed the ex-
perimental study of the gardenvarieties (his _,,varte-
tates’’) until he would find time for them, because that
time never was found; the existence of transmitta-
ble differences within the Linnean species was forgot-
ten and with this the necessity to experiment was for-
gotten also.

HAECKEL says consequently very rightly, in his Gene-
relle Morphologie:

lndessen war man vollkommen zufrieden, wenn man
bei einer untersuchten Anzahl hiochst ahnlicher Indtvi-
duen die Uebereinstimmung in allen wesentlichen Cha-
vacteren festgestellt hatte.”

THE EVOLUTION OF LIVING BEINGS. 19

One neglected all experiments and frequently does
so yet.

Evenaslate as 1901, WETTSTEIN says in his Handbuch
der Systematischen Botanik p. 13:

Man wird daher als Art die Gesamtheit der Indivi-
,duen bezeichnen kénnen, welche in allen, dem Beo-
»bachter wesentlich erscheinenden, Merkmalen unter
,einander und mit ihren Nachkommen uebereinstim-,
men.”

It is true that he adds: ,,und mit ihren Nachkommen
uebereinstimmen,” but the systematical praxis takes
now as little notice of this as it did in 1886 and is per-
fectly content to establish species on morphological
characters only.

Essential (,,wesentliche’’) characters are considered
to be specific characters, minor ones: varietal charac-
ters, but which characters are essential, which minor
ones, nobody says.

Such minor forms are called varieties, formae, ra-
ces, lusus and tutti quanti and if, which happens rare-
ly, one or another investigator takes the trouble to
sow seeds of any of them, and finds to his surprise that
these ,,minor’ forms reproduce their kind faithfully,
this causes not the slightest change in his opinion as to
their varietal or racial rank (these two designations
usually being used indiscriminately.).

Yet, Jordan has already in 1855 ridiculed this casus
positionis in such a scathing manner, thatit is hard to
understand how this view could be held so long.
He said:

Selon les partisans de cette théorie (of the variabili-

20 THE EVOLUTION OF LIVING BEINGS. 4

ty of species) les vrais types spécifiques doivent pou-
voir étre reconnus et distingués entre eux sans aucune
difficulté, méme sans étude ni effort de la part de celui
qui les observe; toutes les formes végétales qui ne se
distinguent pas si facilement.... ne sont que des va-
riétés et ne doivent pas étre élévées au rang d’espéces.
Sil est prouvé qu’elles sont constantes, qu’elles se re-
produisent invariablement par le semis de leurs grains,
c’est indubitablement que le type spécifique a alteré en
elles par les circonstances locales, par \’influence des
stations ou par tout autre cause.

Ne sait-on pas, disent-ils, que les espéces végétales
sont étonnement sujettes 4 varier; et n’en voit-on
pas dans les cultures un grand nombre, qui varient 4
point de devenir presque méconnaissables? Parmi les
variétés des cultures, n’y en a-t-il pas qui sont con-
stantes, que l’on reproduit de leurs graines telles que
celles des Blés par exemple et qui constituent ainsi de
vrates races permanentes? Et une excellente preuve
qu’elles sont effectivement telles qu’on les suppose,
que ce sont bien des races, c’est que c’est la une opinion
generalement admise.

With opinions however, Jordan fortunately was not
satisfied.

He went at the question in the only logical way,
saying: I see differences within the Linnean species;
what are these differences? Speculating about this is
no use, calling them names, in casu calling them varia-
tions, gives no solution, to conclude from multiplicity
to the existence of variability is entirely unwarranted ;
the only right thing to do, is to test their nature by

THE EVOLUTION OF LIVING BEINGS. 21

experiment. So he began to examine the flora of France
carefully and was soon able to show that differences
were observable within a// Linnean species, that whate-
ver Linnean species is carefully observed in nature, itcan
be dismembered into groups of individuals with com-
mon characters, differing from other groups of indivi-
duals with different characters, but also common to
these latter ones.

The only logical conclusion so far was: the Linnean
species is not a unit, but a mixture of individuals of
different constitutions.

But this conclusion had to be proved.

How to do this?

The closest examination, Jordan quite seized this
point, would be insufficient to test the constitution
of an individual, because if such a thing as variability re-
ally did exist, there was no reason to suppose that an
individual, modified in an unheritable way, could be
distinguished from an individual, possessing the same
characters innately, and consequently in a transmit-
table way.

Jordan consequently discarded morphological compari-
son as a criterium for specific purity and, falling back
to Ray, (of whom he may or may not have known) sub-
stituted for it : nulla certior.... quam distincta propaga-
10 ex semine.

This was a great advance, a milestone on the road
to knowledge. Carefully collected seeds, each lot from
a single individual, protected against contamination
by crossing, brought, upon sowing, the important fact
to light that/within the Linnean species there are indi- °

22 THE EVOLUTION OF LIVING BEINGS.

’ viduals of different constitutions and from this, experi-
mentally established, fact Jordan drew the well foun-
ded conclusion:

The Linnean spectes is no spectes.

Yet, up to the present moment, one has continued
to speak of it as such, and this has caused endless
trouble. Most decidedly the time has come to break
with this wrong designation.

I nowpropose to dosoand to replacethename Linnean
species by the neutral name: LINNEON and to define:

A Linneon as the total of individuals which resemble
one another more than they do any other individuals.
_ The different types, faithfully reproducing their

kind which can be distinguished within the Linneon,
Jordan now called, quite reasonably: species. They
have subsequently been called: mikrospecies, Jorda-
nian species, subspecies, small species or elementary
species indiscriminately.

The question remains: are they really species e. g.
have all individuals belonging to a Jordanian species
identical constitutions ?

The answer depends on what answer must be given
to the question:

Is the standing of the test of fatthfull reproduction by
seed, proof of specific purity? The answer unfortunately
is an emphatic: no.

We know that there are f. i. two kinds of white mice,
externally indistinctible, reproducing their kind faith-
fully and yet of different constitutions, as becomes at
onceapparent upon mating a female ofeachkind withthe
same black male. The youngs of the one female will then

THE EVOLUTION OF LIVING BEINGS. 23

be uniformly black, while the young ones of the other
female will be uniformly grey. Breeding true to type is
consequently by itself no reliable test for specific purity
and consequently there is no certainty that a Jorda-
nian species is really a species; forms from which we
know nothing else than that they breed true to type,
may consequently not be designated as species but
must receive another name; as such I propose Jor-
DANONS.

The question remains: what is a species?

The great advance in hybrid analysis in recent years
which allowed us to find out the constitutional diffe-
rences existing between externally indistinctible forms
as the white mice, mentioned above, allows us to give
a definition of the species, viz:

A species consists of the total of individuals of 1dentical
constitution unable to form more than one kind of game-
tes.

All specifically pure individuals are consequently
monogametic. The question remains how to test for
specific purity and the best applicable, although not
always reliable, test is probably the one suggested
recently by Davis:

If two individuals crossed together give a uniform F,,
these two individuals are specifically pure.

The explanation is quite simple: each of them pro-
duces but one kind of gametes; all children therefore
must have the same constitution and consequently
must be alike under like conditions.

Some possible objections however must be conside-
red:

24

THE EVOLUTION OF LIVING BEINGS.

istly. In cases where one and the same individual

2dly

forms one kind of male gametes and one, but a
different kind of female gametes, the crossed
offspring of two such individuals would in F,
be uniform also, because each child is the pro-
duct of the combination of a female gamete
with a male one and all female gametes as well
as all male gametes are alike, al|though the male
ones differ from the female ones.

Such a state of affairs can however easily be
detected by the fact that the reciprocal crosses
will be different, thus indicating that there
must be something amiss with the specific puri-

ty of the tested form. We must therefore make

the Davis-test more severe by extending it as
follows:

Specific purity is indicated by the uniformity
and identity of the F, generations obtained by
crossing the individuals to be tested, RECIPROCALLY.
But in the following case even this more severe
test would remain indecisive viz, if there were
preferential mating between the gametes of two
hybrids, crossed together in sucha way, that only
one kind of the different gametes produced by
the one hybrid were able to combine with one
kind only of the different gametes produced by
the other. In such a case the F, generations,
even the reciprocal ones, would be uniform and
identical and thus simulate specific purity of
the tested individuals.

In dealing with plants however, such cases

THE EVOLUTION OF LIVING BEINGS. 25

would generally be detected by the presence ofa
number of aborted ovules and aborted pollen-
grains.

Forms showing such abortion are however from
the very beginning suspect, so that one does well to
exclude them from experiments necessitating the
use of pure species.

3dly. To make the Davis-test as reliable as possible, we
must furthermore cross the form to be tested with
more than one other form as pure as obtainable.

This will f. i. be absolutely necessary in the
case of albinotic forms, because in these cases
constitutional differences remain invisible in
the F, generations, obtained by crossing two al-
binos of different constitution, f. i. two of the
three kinds of white mice described in Bateson’s
Principles on p. 75.

In such cases crossing with a colored form reveals
the difference.

But from the fact that the F, generation of crossed
albinos does not necessarily reveal the difference in
constitution between those albinos, it is safe to con-
clude that the possibility exists that the F, generation,
obtained by crossing two colored individuals, may also,
in some cases, fail to reveal certain differences in con-
stitution existing between these individuals.

It will therefore be safest to test for specific purity in the
following way.:

Istly. cross the form to be tested with as many sup-
posedly pure species as obtainable, reciprocally.

If the F, generations so obtained, are all uni-

26 THE EVOLUTION OF LIVING BEINGS.

form, and if the reciprocal F, generations of
each cross are identical, there is reason to
suppose the tested form to be specifically
pure.

2dly. submit as many of the F, individuals of each
cross to as extensive hybrid-analysis as possi-
ble, in order to test their constitutional identity.
If they prove to be of identical constitution in
all pointstested,thisfact,combined withtheresult
obtained sub 1, makes it very probable indeed,
that the tested individual is specifically pure.

It must be conceded that even this givesno absolute
certainty that the form tested is specifically pure in all
respects, but it is the best we can do.

But even the simpler Davis-test has the great advan-
tage that we can say, that if two forms crossed recipro-
cally, give a heterogenous progeny, either one or the
other is, or both are, specifically impure, while, if the
progenies are each uniform, but not identical in re-
ciprocal crosses, there is no specific purity either.

Because in these cases there is proof, that at least one
of the individuals crossed, forms more than one kind of
gametes, is polygametic and consequently a hybrid.

One class of differences within the species remains
to be considered: the non-transmittable effect of exter-
nal circumstances on the individuals composing the spe-
cies. That this can be considerable, shows f. i. a com-
parison of specimens of the same species growing at
high altitudes in the mountains, with specimens gro-
wing in the plains. These we will indicate as mods-
fications.

THE EVOLUTION OF LIVING BEINGS. 27

We thus get the following definitions:

LINNEON: fo replace the term species in the Linnean

sense, and to designate a group of individuals which
- resemble one another more than they do any other indt-
viduals.

To establish a Linneon consequently requires
careful morphological comparison only.

JORDANON: fo replace the term species in the Jordanian
sense, viz: mikrospecies, elementary species etc. and
to designate a group of externally alike individuals
which all propagate their kind faithfully, under condi-
tions excluding contamination by crossing with indivi-
duals belonging to other groups, as far as these external
characters are concerned, with the only exception of
noninheritable modifications of these characters, cau-
sed by the influences of the surroundings in the widest
sense, to which these individuals or those composing
the progeny may be exposed.

To establish a Jordanon, morphological compari-
son alone consequently does not suffise; the trans-
mittability of the characters by which the form was
distinguished, must be proved by experimental bree-
ding.

SPECIES: to designate a group of individuals of identical
constitution, unable to form more than one kind of
gametes ; all monogametic individuals of tdentical con-
stitution consequently belong to one species.

To establish a species, neither morphological com-
parison alone, nor experimental breeding by itself is
sufficient, nor are the two combined; hybrid analy-
sis is required in addition.

28 THE EVOLUTION OF LIVING BEINGS.

Hysrips: to designate all individuals able to produce more |
than one kind of gametes, e. g. gametes of different
constitutions, (in some cases all of these or part of these
are non-viable). Hybrids consequently are polygametic.

In many cases the hybrid nature of an individual
can be demonstrated by careful breeding alone, e. g.
in those cases in which the uncontaminated progeny
is visibly heterogeneous; in many other cases f. i.
in all cases of albinism, hybrid analysis is required in
addition.

MopDIFICATION: to designate the non-transmittable effect of
external circumstances.

CHAPTER II.

DO DIPLOID SPECIES VARY?

Armed with the definitions given at the end of the
first chapter, we will consider this momentous question.
All the more generally accepted Theories of Evolution
are based on so called variability.

We have already seen that the existence of variabili-
ty, deduced from the behaviour of individuals belonging
to a Linneon, considered to be a unit, is doubtfull.
Consequently theories like those of Lamarck and Dar-
win, the basis of which is the Linneon considered as a
unit, are untenable, unless it can be shown that pure
species are able to vary.

This species undoubtedly are in an uninheritable
way. Each species is liable to modification, but such
variability is not meant here, the question is whether
a species is variable in a transmittable way or not.

The only sense in which the term ,,variable”’ can pos-
sibly be applied to a homozygous individual, as all indi-
viduals belonging to a pure species necessarily are, is
the demonstration that a homozygous individual can by
itself e. g. without having been crossed, become hetero-
zygous, so that an individual, producing until to day
only one kind of gametes, would proceed. to produce
different kinds of gametes to morrow.

In other words: variability implies the possibility of a
monogametic individual becoming polygametic under

30 THE EVOLUTION OF LIVING BEINGS.

conditions when all poy of a cross is excluded.

This is not inconceivable! it could be brought about
in two ways: certain gametes might loose a property,
, they previously possessed in common with the other
gametes, or some gametes might gain a property which
they, as well as the other gametes, previously lacked.

Such a processs is conceivable, but it would add so-
mething fundamentally new to our stock of knowledge;
something entirely out of the pale of Mendelism f. i.
and to which therefore Mendelian behaviour could give
no direct support, although the contrary is believed so
frequently, because Mendelism deals with the demon-
strated fact that a heterozygous individual produces
more than one, a homoygous individual but one kind
of gametes,and has nothing to do with the way in which
heterozygotism arises, except if it does so by crossing.

But the variability required by evolutionary theories,
based on a process of variation, should be able
to change a homozygous organism,without crossing ,into
a heterozygous one.

We repeat: this is not inconceivable, but very clear
proof for its real existence is wanted, before we are
justified to accept it, as sufficient evidence for such an
unexpected and novel occurrence.

Now how can we possibly get this proof?

Mendelism can ’t give us the clue; zt does not deal
with the internal structure of the gametes, except in so far
as deductions are concerned, derived from the fact that
the hybrid arisen from the product of the fusion of two
gametes of different constitutions, is able to form more
than one kind of gametes.

THE EVOLUTION OF LIVING BEINGS. 31

Mendelism deals with the behaviour of gametes already
formed.

As such however, it can be of service, because, al-
though it is unable to show us how the change necessa-
ry to bring about the heterozygous condition, took
place, it can show us the result viz: the then heterozy-
gous condition of the previously homozygous orga-
nism.

If we now define a mutation as the change of consti-
tution, undergone by a homozygous individual which be-
came heterozygous without having been crossed, we can
say, that PROOF OF MUTATION WOULD BE AT HAND, IF
IT WERE SHOWN THAT A HOMOZYGOUS INDIVIDUAL CAN
BECOME HETEROZYGOUS WHEN ALL POSSIBILITY OF A
CROSS IS EXCLUDED.

Mendelism could show us, that such mutation had
taken place, if we were but sure — it is the old diffi-
culty again — of the homozygous condition, e. g. of the
specific purity of the material from which the supposed
mutants arose. Does the classic subject for mutation:
Oenothera Lamarckiana give us proof for the existence
of such mutations ?

The answer is an unconditional: no.

In the first place, Oenothera Lamarckiana has never
been obtained as yet, in a homozygous condition e. g.
in a condition that it threw no ,,mutants’’; every La-
marckiana-individual, so far examined, ,,mutates’”’, was
consequently impure already, so that the bringing to
light of these mutants was comparable to the bringing
to light of the presence of silver in a lead-ore contai-
ning silver.

32 THE EVOLUTION OF LIVING BEINGS.

As little as the latter fact shows, that silver arose by
mutation from the element lead, as little the former
shows that the aberrant forms, obtained from selfed O.
Lamarckiana, arose from the species O. Lamarckiana.

This is the Lamarckiana-question in a nut-shell.

DE Vries has shown that O. Lamarckiana is a hetero-
zygote and he has shown nothing else; all the rest is
mere hypothesis.

This very obvious conclusion is corroborated on all
sides.

It is easy to prove the heterozygous condition of O.
Lamarckiana by crossing it with its ,mutants”’, and by
examining the F, generation, so obtained, according to
the Davis-test for specific purity, as to its unifor-
mity.

By so doing we cross — according to de Vries’ con-
ception — two pure species (O. Lamarckiana as well as
its mutants being considered to be species by him), and
consequently should obtain a uniform F, generation.

Now the result proves by no means this contention.

In most cases of such crossing we obtain, what de
Vries has called segregation in F,, which means nothing
else than that the F, generation is not uniform, and
from which fact no other conclusion is warranted than
that either O. Lamarckiana or the mutant is able to
form more than one kind of gametes, consequently is a
heterozygote; and as we know that Lamarckiana, as
well as the mutants, is able to throw ,,mutants’”’ it is
most probable that both are heterozygotes.

This makes it highly probable that O. Lamarckiana
only simulates a species, but in reality is nothing but a

THE EVOLUTION OF LIVING BEINGS. 33

Linneon e. g. a group of morphologically similar, but
constitutionally, different individuals.

That this is really the case has been shown admirably
by HERIBERT NILSSON who proved among other things,
that individuals of the Linneon Oenethera Lamarckia-
na produce, after selffertilisation, different ,,mutants’’
and in different proportions, from which he draws the
only warranted conclusion that different individuals of
O. Lamarckiana have different constitutions.

The fact is not challenged by de Vries, but he takes
— indirectly — exception as to the conclusion.

So he says on p. 340 of his Gruppenweise Artbildung:

Die Mutabilitat einer reinen Rasse ist keineswegs
stets dieselbe; die Ernte der einen Mutter ist oft viel
reicher an Mutanten als diejenige einer Nachbarin’’.

Or in €nglish:

» Lhe mutability of a pure Race is by no means al-
,ways the same: the crop of one mother is frequently
»much richer in mutants than the one of a neighbour.”

We naturally expect to find conclusive evidence for
the legitimacy of this startling assertion in the results
of new experiments with a race, the purity of which was
beyond all reasonable doubt.

Unfortunately this is not the case; deVries bases this
assertion solely upon a creed, and on nothing but a
creed: the supposed specific purity of O. Lamarckiana.

It is useless to fight against dogmatic creeds; where
they lead to, thesentences, following immediately upon
the one quoted above, show:

Von Oenothera scintillans, welche gewohnlich aus
»lhren Samen van 15—40% scintillans Pflanzen er-

3

34 THE EVOLUTION OF LIVING BEINGS.

»zeugt, habe ich einmal eine Rasse gehabt, welche
,deren 66—93%, im mittel 84%, hervorbrachtte’’.

And this, to support the assertion that the mutability
of different individuals of a pure race can be different!

On the question: how do you prove that different in-
dividuals of a pure race can be of different mutability ?
one thus obtains the answer: well do’nt you see? I had
once a group of races (O. scintillans) in which indivi-
duals belonging to different races,1) behaved diffe-
rently!

And this is not a slip of the pen on de Vries part!

In a very recent article (Bot. Gazette Nov. 1915) he
says on p. 341:

,1n order to obtain species of O. gigas yielding a high
»percentage of dwarfs from their seeds, I sowed in Ig11
seeds of my pure strain *)’’, cultivated as biennials, and
fertilised them 1912 by their own pollen in bags’’.

The result was an offspring,consisting of plants which
threw no dwarfs, others which threw 0,6% of dwarfs,
others which threw 1.5% of dwarfs, others again which
threw 2.3% of dwarfs, yes even plants which threw
17.8% of dwarfs! Yet de Vries refuses to entertain the
idea that his O. gigas might have been impure!

Nor causes the fact *), that the cross O. Lamarckia-
na x O.nanella gives a heterogenous F, generation, con-

1) Von O. scintillans, welche gewéhnlich von 15—40% scintillans
erzeugt, habe ich friiher einmal eine Rasse gehabt, welche deren....
im mittel 84% hervorbrachtte.

*) The italics are mine.

3) Hugo de Vries: Ueber amphikline Bastarde. Berichte Deutsch
botanische Gesellschaft November 1915.

THE EVOLUTION OF LIVING BEINGS. 35

sisting of Lamarckianas and nanellas in different nu-
merical proportions depending on the,,individuelleKraft
derSamentrager”’ (euphemism for: different genotypical
constitution), de Vries any uneasiness as to the justifia-
bility of considering his Lamarckiana as pure; far from
doubting the purity of his strain, he concludes unblus-
hingly from some rather rough experiments, that ,,die
Kreuzung van O. Lamarckiana mit O. Lam. mut. na-
nella (sic) liefert, je nach den Kulturbedingungen,
o—g0% Zwerge!

The question of the specific purity of O. Lamarckia-
na, the basis of de Vries’ assertions, is not, asit ought to
be to him, an object worthy of the most scrupulous
investigation, but a dogmatic creed.

This involves of course the belief in the existence of
mutations thrown by O. Lamarckiana; we can not de-
ny the possiblity that mutations may exist, but as
scientists, we cannot be satisfied with a mere belief; we
want proof, especially where it concerns such an entire-
ly novel thing, as the real existence of the, so far only
surmised, existence of mutations would be.

The behaviour of O. Lamarckiana gives us not the
slightest cause to suppose that the aberrant forms,
thrown by it, owe their origin to a process of mutation;
they can perfectly well be explained, without ressor-
ting to an ,,explanation’’, lying outside of the pale of
experience, by the simple fact which the experiments
which have been published all tend to show, viz. that O.
Lamarckiana is a mixture of heterozygotes of different
constitutions throwing rogues (the pretended mutants)
by a process of mendelian segregation, in proportions,

36 THE EVOLUTION OF LIVING BEINGS.

deviating from the normal mendelian numbers, by the
failure of certain kinds of gametes to produce viable
offspring with other kinds of gametes, while certain
other kinds, mating together, do produce viable off-
spring; in other words the normal mendelian numbers
are disturbed by preferential mating, a process known
to occur in other cases (gametic coupling and gametic
repulsion).

All this is furthermore complicated by the death of
some kinds of gametes before they are in a position to
mate, as is shown by the sometimes very large proporti-
ons of abortive pollengrains and ovules, and complica-
ted once more by the death, as shown by RENNER, of
certain kinds of embryo’s at a very early stage in their
development, all or part of this, perhaps either caused
by, or complicated through, irregularities in the meiotic
divisions of these heterozygotes.

No unprejudiced investigator will accept the beha-
viour of such an evidently impure form as evidence for
the existence of mutation.

The question remains: is there sufficient evidence to
accept the existence of such a novel process as mutati-
on would be, in other cases?

The difficulty is again to obtain material of unim-
peachable specific purity to experiment with.

We have seen that we possess vo certain means to
prove specific purity in any case, but we know that it
is far easier to obtain the recessives in a pure state than
the dominants.

Now it is a fact, well worthy of our careful conside-
ration, that whenever so called mutants are described,

THE EVOLUTION OF LIVING BEINGS. 37

these are invariably thrown by dominants and not
by recessives, which makes it very probable that
the dominants throwing them were impure, because the
fact that, ,,dominants” throw mutants’ and recessives
_do not, can also be expressed tn this way: that, WHENE-
VER THE PURITY OF A FORM IS VISIBLE (as in recessives)
IN REGARD TO CERTAIN CHARACTERS, IT THROWS NO
MUTANTS’, INVOLVING THESE CHARACTERS, WHILE A
FORM, FROM WHICH IT CAN NOT BE DETERMINED AT
SIGHT, WHETHER IT IS PURE OR NOT (as in the case of a
mixture of pure dominants and impure dominants)
MAY THROW ,,MUTANTS’’.

The only legitimate conclusion which we can draw
from the regrettable fact, that it is so difficult to make
out with certainty that a certain form is specifically
pure in all respects, is that we must require the severest
possible test from him who wants to prove the exis-
tence of mutations. Knowing how difficult it is, toshow
that a given form is free from recessives, we must dis-
qualify,apriori,all claims of having proved the existence
of mutations, based on the demonstration that a cer-
tain form has thrown recessives, no matter in how fee-
ble proportions. In the second place we must refuse to
accept as evidence all cases in which the reciprocal F,
generations, obtained by crossing the supposed mutant
with the form from which it arose, are either not-uni-
form or dissimilar.

In the third place we must refuse to accept as evi-
dence all cases in which the numerical proportions in
the uncontaminated F, generation of such a cross
deviate from the normal mendelian ones, or can only

38 THE EVOLUTION OF LIVING BEINGS.

be explained by the acceptance of numerous so-called

factors, because all this indicates complication and

therefore the possibility of errors of interpretation.

Weare therefore, for the present, forced to require
from him who wants to furnish proof forsuch an unex-
pected novel fact, as mutation would be, at the very
least:

Istly. that he has previously investigated, to the best of
his ability, by hybrid analysis, the purity of the
form from which the supposed mutant arose.

2dly. that the supposed mutant, crossed with the spe-
cies from which it arose, does not behaveas a
recessive, but either dominates in the homo-
geneous and identical, reciprocal F, generati-
ons, or showsin such F,s, characters interme-
diate between those of the original species and
the mutant; furthermore that the mutant and
the form from which it arose reappear in the
uncontaminated F,. generation, either in the
proportion 3 : I or inthe proportion I mutant:
2 intermediates : I original form.

This must be at present the test required because, so
only, it can be proved that a homozygous individual can
become heterozygous by itself, in other words can
mutate.

As far I as am aware, no pretended case of mutu-
tion can stand this test.

There is consequently not the slightest proof for the
existence of mutation and Davis with whom I agree in
the main, goes in my opinion decidedly too far when
he says (Science XLII Nov. 5. 1915):

THE EVOLUTION OF LIVING BEINGS. 39

,One may be a mendelian, firmly believing in the
»principle of segregation following an F, generation,
»which is the principal test of mendelism, and still
,admit the probability of modifications from time to
" ,time of the stereochemistry of germ-plasma even in so
,called ,,pure lines’.

We may admit the ossibility (because to deny a
possibility is unscientific) of such stereochemic modi-
fications, alias mutations, but there is no reason at all
to admit the probability; all we can do, is tosay that
the existence of such an inheritable changability of
the germplasma would be exceedingly interesting, but
that, so long as such proof is not forthcoming, we can
take no account of such a mere possibility in any effort
to explain evolution.

Evolution has suffered quite sufficiently from the
possibilities” with which it is charged, so that I fear
that the addition of another mere possibility might
be like the straw that broke the camel’s back, e. g.
would throw the theory of evolution in universal dis-
credit.

I hear it objected that the appearance of constant
new forms in small numbers from a certain form is good
evidence for mutation, because, if the new form were a
hybrid, it would have to segregate. This is of course not
true, if the original form is a heterozygote; because
then the new form may be the product of the mating of
two identical gametes, produced by this heterozygote,
and consequently be homozygotic itself.

Mutation can therefore, for the present, be discarded
as a factor in evolution; what other kind of transmitta-

40 THE EVOLUTION OF LIVING BEINGS.

blevariation has been claimed to exist, by evolutionists ?

We all know that a great many persons, up to the
present day, express their belief in the inheritance of
so called acquired characters e. g. in the occurence of
changes in the characters of the individual, caused by
external circumstances, and in the transmittal of
such changes to the offspring.

Very few among these, yet believe, in the orthodox
way, that external influences first change the soma
and through this subsequently the constitution of the
gametes.

At present the adherents of theories of a transmis-
sion of acquired characters, usually mean a direct in-
- fluence on the germ-plasma, causing a constitutional
change in the gametes, which as a matter of fact would
be nothing than a mutation, just as mutation is incon-
cievable without some kind of inheritance of acqui-
red characters,

To this supposition of course applies exactly what we
_ have already said about mutations. The possibility can
' not be denied, but proof is lacking.

In as much as nothing happens without cause, all
suppositions of transmittable variability, no matter
whether we call the effect mutation or by some other
name, have to assume that, at the end, such variability
is caused by external circumstances, have consequently
to admit the transmittability of acquired characters.

Darwin clearly perceived this, as results from his let-
ter to Semper, written but little more than half a year
before his death, (Life and Letters III p. 345) in which
he says:

THE EVOLUTION OF LIVING BEINGS. 4I

»1 still must believe that changed conditions give the
impulse to variability but that they act in most cases,
in a very indirect manner. But as I said, it is a most
‘perplexing subject.”

For such an inheritance of acquired characters there
is however no proof whatever, and so we must conclu-
de that the existence of transmittable variability has
never been proved.

The perplexity of the subject, to which Darwin refers,
is caused, in my opinion, by the simple fact that inheri- |
table variability does not exist.

CHAPTER III.

SPECIES, LINNEONS, GENERA AND OTHER ,,HIGHER
GROUPS’ AND EVOLUTION.

The problem of Evolution is not, primarily, as is al-
most generally believed, the problem of the origin of
species, but zs the problem of the origin of all those IN-
DIVIDUALS of different constitution, which people the
earth.

Of some of these individuals we can make clear-cut
definable groups, by uniting those monogametic indi-
viduals which are constitutionally identical, to groups
which we may call species, and explain how such spe-
cies can arise.

But when we find, as we do, that such species are
very rare in nature, we recognize that the problem set
to the evolutionist is not limited to the origin of these
pure species which are but rarely or never met with
in nature, but embraces the problem of that vastly
greater number of impure individuals by which the
earth is peopled. ,

The problem of the species and of its origin 1s conse-
quently comparable to that of the pure chemical substance
and its origin, the problem of the heterozygotes of diffe-
rent constitutions which we find in nature and of thetr
origin is comparable to the problem of the ores found in
nature and their origin. Now to gain a good insight
in the first part which we might call the chemical side

THE EVOLUTION OF LIVING BEINGS. 43

of the problem, it was highly desirable to make groups
of indentical individuals and to name these groups:
‘species, because by so reducing the number of cases
with which we have to deal, we simplified matters
without impairing the exactness of the investigation
in the least.

The question becomes different, when we proceed to
unite the impure forms found in nature, to groups of
SIMILAR, but NOT identical individuals because, by
doing this, we only seem to simplify the problem while,
in fact, we complicate it and impair seriously the exact-
ness of our investigation.

We impair the exactness of our investgation becau-
se we form groups which are undefinable, and in so
doing, open the door to different opinions, as to which
individuals should be received into such groups. That-
as everybody knows causes great trouble in fixing the
limits of such groups as Linneons, Genera, Families,
Classes etc. But what is far worse, forgetting the nature
of these divisions, we are unconsciously and gradually
led to believe — as generally is believed — that such
groups, which we call higher groups +), have necessa-
rily as real an existence as species have, and that con-
sequently, their origin must be explained as an origin
of entities also, while, in fact, they may have no real
existence at all, but exist in our imagination only.

Sixty years ago already, Jordan has, in a discussion
with de Candolle, laid stress on the basic difference
between a species as an entity and a Linneon (which he

1) It were better to speak of larger groups.

44 THE EVOLUTION OF LIVING BEINGS.

considered rightly as a kind of genus, and speaks of it
as such), as a group. About this he said:

»La notion de l’espéce n’est point celle d’un objet
collectif comme l’entend M. de Candolle.... la forme
»spécifique, qui équivaut a l’étre, a lasubstance, est
,identique chez tous les individus d’une méme espéce
,et toujours indépendante du nombre.... ainsi le
»premier homme que Dieu a créé renfermait en lui
»/ humanité entiére’’ ?).

»—En assimilant, comme il le fait, les genres aux es-
»peces M. Alph. de Candolle ne prend pas garde qu'il
,assimile les catégories, qui renferment les étres aux
,etres eux-mémes.

Le genre n’a pas l’étre, il n’est connaissable que
»parceque notre intelligence le constitue étre de raison.
1] existe dans notre intelligence, mais en dehors d’elle
ce n’est pas un étre, c’est un non-étre qui n’a la vérité
que par les conceptions de notre esprit.”’

Now there is a good deal of truth in this, and Jor-
dan’s conclusions are well worth considering, but it is
not the whole truth.

The argumentation looses too much sight of the
fact that primarily nature can make nothing but indi-
viduals.

If but two homozygotic individuals existed — each
of a different constitution — we would yet be able to
say that there lived two species on our globe, so that it
is true that species are realities ,,indépendantes du

1) This example is ill chosen; ,,l">humanitéentiére”’ is not a single spe-
cies, as Jordan would doubtless have been the first to have argued him-
self, if he had not been hampered, in this respect, by religious scruples.

THE EVOLUTION OF LIVING BEINGS. 45

nombre’’. But this does not away with the fact that a spe-
cies, in as much as it consists mostly of a number of in-
dividuals, usually itself is a group. It is true that by
the identity of the constitution of the individuals
which compose it, it remains capable of being considered
as an entity, but this way of looking at a species, con-
sisting of more than one individual, is dangerous be-
cause it leads,almost unconsciously, to the unwarranted.
conclusion that all the individuals of onespecies must
necessarily be the offspring of a single initial pair of
identical constitution, in one word, that species must
necessarily be monophyletic. Now this is unwarranted,
because the constitution of an individual depends ex-
clusively on the constitution of the gametes, from who-
se union the individual sprang, and is independent
of the source of these gametes. Therefore, if iden-
tical gametes can be obtained from different sour-
ces — and we will see in later chapters, that they
can — species need not necessarily have a monophyletic
origin.

Sucha polyphyletic species, though it, by the iden-
ty of the constitution of the individuals composing it,
remains capable of being considered as an entity, yet
is at the same time not only a group, but even a
group of individuals of different origin.

Consequently the real difference between a species
on the one side and a Linneon or a genus on the other
side, is not so much that the one may be, and the other
must be a group, but is that a species is either a special-
ly constituted individual, always homozygotic, or a
group of such individuals of zdentical constitution,

46 THE EVOLUTION OF LIVING BEINGS.

while a Linneon or a genus is a group of individuals of
different constitutions.

And when we have once grasped this fundamental
difference, we can easily demonstrate that Jordan was
perfectly right when he said that l’espéce est indé-
pendante du nombre, while the genus is not. If we
reduce a species to one single individual, by extermi-
nating all others, that species still remains in existence,
but when we reduce a Linneon or a genus to one spe-
cies, that Linneon or that genus disappears, because a
mixture of different types is no longer a mixture after
its reduction to one type.

While thus Jordan ts perfectly right, when he states that
a genus 1s a group made by us, it is not certain yet, that he
is right also when he continues to say that the Linneon or
the genus n'a la verité que par les conceptions de notre
esprit, because it 1s possible that groupings, such as we
make, coincide with similar groupings made by na-
ture.

Because nature primarily can make nothing but
individuals, it does not follow that it cannot se-
condarily group such individuals in various ways; as a
matter of fact we know that it does, by the fact of the
existence of different plant-societies, different flora’s
and fauna’s in different countries etc.

Now if our Linneons, obtained by grouping together
morphologically similar individuals, happened to co-
ver the groupings made by nature, such Linneons would
be something more than mere conceptions of our mind
and therefore be well worth investigating.

IF nature makes groupings more or less coinciding

THE EVOLUTION OF LIVING BEINGS. 47

with our Linneons, it is of course improbable that we
should always have grasped nature’s meaning and soit
may — if natural grouping really does exist — be ex-
pected that some of our Linneons will be mere con-
ceptions (and, in that case, wrong ones) of our mind,
while others may have existence in nature.

How this could come to happen, an example may
show.

Suppose the different races of mankind were pure-
bred, — as our schoolbooks, describing the ancient Nor-
mans as invariably fair-haired, blue eyed, beautiful-
ly proportioned gods and goddesses, rather than men
and women, try to make us believe, they formerly
were — then the races of mankind would be as many
pure species.

Now suppose the world were thus peopled by speci-
fically pure Caucasians, Mongolians, Indians and Ne-
groes, and we were charged to constitute an army of
them, containing the men as well as the women, then
we could solve this problem in different ways.

Suppose we arranged them into 4 bataillons, each
bataillon consisting of one species only, then we would
get a Caucasian bataillon, a Mongolian bataillon, an
Indian bataillon and a Negro one. Now supposea
superior officer came along and said we had united too
many individuals into one bataillon, and charged us to
cut them up into bataillons of the perscribed size, to
unite such bataillons to proper regiments, and such
regiments to an army, we might get four armies of
exactly the same constitution as our bataillons were,
which could very properly be designated as the Cau-

48 THE EVOLUTION OF LIVING BEINGS.

casian army, the Mongolian army, the Indian army
and the Negro-Army.

Suppose now, on the contrary, that the bataillons
we originally made, had been of the proper size, then
uniting them to regiments, would already cause a
change of constitution, because we would get, saya
Caucasian-Mongolian and an Indian-Negro regiment,
in stead of specifically pure regiments, so that the
army, consisting of such regiments, would have to be
designated as the Caucasian-Mongolian-Indian-Negro-
Army. We could of course also constitute a Caucasian-
Mongolian-Indian-Negro army by dividing, from the
beginning, all four species equally over each bataillon.

Now if we compare these different kinds of armies,
we find that each of the armies of the first kind con-
sists of one species only, that in the army of the se-
cond kind the bataillons constitute species, but the
regiments Linneons, and that in the army of the
third kind the bataillons already are Linneons, con-
taining different species, be it, each species consisting
of but a small number of individuals.

Now what does Nature say to such grouping?

Suppose we try to find out. To do this, we discharge.
the men and women, marking each of them by a ta-
too-mark indicating the bataillon to which he or she
belongs, and charge them to mark their progeny to
come, with the same tatoo-marks they got themselves,
each child thus obtaining two tatoo-marks, one from
his father and one from his mother.

Now suppose twenty-five years later, we enlist these
children and arrange them in our armies according to

THE EVOLUTION OF LIVING BEINGS. 49

their tatoo-marks. If the different species have not
intercrossed, we will find that, although the armies we
now compose, consist of different individuals than 25
years ago, they yet have the same constitution as for-
merly, although it may not be possible to place all the
children in the regiments of their parents, because their
tatoo-marks may show that a woman from one regi-
ment has married a man from another regiment,
be it of the same species as she-herself. This howe-
ver does not affect the species, so that nature in this
case has kept our species pure, our species thus coin-
ciding with those of nature.

Suppose on the other hand that crossing has occu-
red, but has been limited to the individuals belonging
to the same regiments of the second army. Then the
tatoo-marks will show this to us, and so we will be
able to put the children — though many of them will
be crossbred, others of course may be pure because
their parents belonged to the same species — into the
same regiments to which their parents belonged.

Nature therefore has made the same groupings as
we did, her regiments coincide with ours, her Linneons
with ours, although each of them contains hybrids
which ours did not.

But suppose crossing had been promiscuous be-
ween all the species in the second army, then we would
be unable to assign to the new recruits places in the
same regiments in which their parents had served, be-
cause many of them would carry tatoo-marks of diffe-
rent regiments.

In that case therefore, nature had not made grou-

4

50 THE EVOLUTION OF LIVING BEINGS.

pings coinciding with our regiments or Linneons; our
regiments or Linneons therefore turned out to be
purely artificial, to be but conceptions of our mind.

Now to this view may be objected, that in the for-
mer case, our Linneons were internally also different
from Nature’s Linneons, because ours contained no
hybrids, while those of nature did. This is true, but we
will perceive this the less, the more we consider — as
we use to do — certain characters as essential to esta-
blish Linneons, certain others as unessential or, as
we usually express it, as to be of varietal value only.

If we admit f. i. that the Caucasian-mongolian regi-
ment or Linneon may ,,vary” in skin-color from
white to yellow, and the Indian-negro one from red to
black, we will accept the regiments or Linneons, nature
sends back to us, to be sufficiently similar to those,
we had made ourselves 25 years ago, to consider them
to be unchanged except by slight variations, too insig-
nificant to change our opinion.

This is no joke; it 7s a fact which we must constantly
keep in mind, that we get the Linnean spectes, ,,das Art-
bild’”’ (our Linneon) only, by choosing certain characters
which we call essential as criteria and by neglecting others;
if we considered ALL characters to be of equal importance
we would never have grouped the different types, each
Linneon contains, together.

Of course there are Linneons, consisting apparently
of one type only; these arise, as we will see later on,
through intra-linneontic crossing and selection of one
type, usually the dominant one; crossing without se-
lection leads, as it does in human Linneons, to Linne-

THE EVOLUTION OF LIVING BEINGS. 51

ons consisting of nothing but hybrids and this may
lead, as in the case of the Linneon Buteo Buteo, toa
most bewildering number of types within the Linne-
on, as splendid material present in the Natural His-
tory Museum of Leiden, shows.

It results from what has been said, that nature prima-
vily makes but individuals, but secondarily groups of in-
dividuals, and that we will have to investigate whether
such groupings, made by nature, are covered by our Lin-
neons or not.

CHAPTER IV.

THE ORIGIN OF DIPLOID SPECIES.

The problem of the species and of its origin is, as was
said in the preceding chapter, comparable to that of the
pure chemical substance and to its origin.

Just as we do’nt study the origin of pure chemical
substances in nature, but investigate this origin
in the laboratory, so the question of the origin of spe-
cies cannot be tackled in the field, but must be studied
in the experiment garden.

It is a curious fact that one has pretty generally
thought, that the problem of heredity lies at bottom of
the question of the origin of species, while heredity
of course can only be concerned in the PERPETUATION
of a species once formed, because heredity implies
the unchanged transmittal of the properties of the
parents to the offspring.

That this confusion has arisen, is due to the fact
that most organisms are not specifically pure, and
consequently get children more or less similar to them,
but not zdentical with them.

This similarity has gradually become looked upon
as a sufficient criterium for heredity, just as the simi-
larity of the individuals composing a Linneon, has been
looked upon as to be sufficiently close to allow one to
consider a Linneon as a species.

In this way, the conception heredity has become as

THE EVOLUTION OF LIVING BEINGS. 53

inexact as the conception species used to be, and such
inexactness is unconsciously carried forward even
when, in itself exact, methods gradually evolve.

So it has even tainted Mendelism, for strictly spea-
king, mendelian behaviour has nothing to do with
heredity and to speak of Mendelian heredity is, au fond
— I beg Mr. Bateson’s pardon — nonsense.

If an organism shows mendelian segregation, it
shows no heredity of the character-complex it posses-
ses, but just the contrary: disintegration of this com-
plex and distribution of the vestiges of it over diffe-
rent individuals e. g. non-transmittability of this
complex.

The problem of the origin of species is not the pro-
blem how a species is perpetuated — how its charac-
ters are inherited by its offspring — butis the problem
howa species (ora pair of them) can give rise to a species
(or to more species) differing from it (or from them).
This problem has been solved by Mendel who showed,
that by mating gametes of different constitution —
brought together by crossing different species — zygo-
tes are formed from which individuals arise, able to
form a number of gametes of different constitution,
from whose matings new species can arise.

By crossing two monogametic individuals of diffe-
rent constitution, one consequently obtains a poly-
gametic hybrid which is the source — and up to the
present the only known source — of the origin of new
forms, some of which are heterozygotes, others of
which are homozygotes e. g. new species.

By isolation of such homozygotes in the experiment

54 THE EVOLUTION OF LIVING BEINGS.

garden, and by selfing them or by mating them with
other individuals of identical constitution, but of dif-
ferent sex, we can multiply them and thus obtain new
species consisting of as many individuals as we choose
to raise. |

Evidently it does not matter from which species the
gametes which form the zygote, that sprouts to the
polygametic hybrid, are derived; it is not even neces-
sary that these gametes should be derived from pure
species; they can just as well be derived from hybrids,
because the result has nothing to do with the origin
of these gametes, but only with their constitution.

Consequently new species can originate, as well in
a monophyletic as in a polyphyletic way.

The spot, where the mating of such gametes takes
place; is of course as indifferent to the effect as the
origin of these gametes, consequently species can arise
polytopically e. g. the same species may be born at
different spots. That this is a fact can easily be shown.

By crossing (Baur 2d edition p 94) a homozygous
yellow Antirrhinum majus with a homozygous red
A. majus we obtain among the segregates in F, ho-
mozygous yellow-red species, which however we can
obtain just as well, by crossing a heterozygous red
form of the Linneon A. majus witha heterozygous pale
yellowred form of the same Linneon.

This as an example, that the same species can arise
in different ways.

That the same species can arise at different spots
and at different times, is ofcourse an experience of
the commonest sort. A rather striking example is

THE EVOLUTION OF LIVING BEINGS. 55

furnished by the Petunias with green-rimmed petals
which were obtained about 1830 in England, and again
in 1914 in my garden at Bennebroek, by crossing the
same Linneons as were crossed 80 years ago in Eng-
land, viz Petunia violacea and Petunia nyctaginiflora.
It is of course equally indifferent to the question of the
origin of species, whether there exist other kinds of
segregations than Mendelian ones or not; all that is
necessary, is that new homozygous combinations arise
finally from a cross. This can evidently even happen
without segregation, if two heterozygotes, containing
among their different gametes some identical ones,
mate, and two of these identical gametes form together
a zygote,

New species consequently are the result of the mating
of identical gametes, derived, usually indirectly, from the
mating of two gametes of different constitution (by cros-
sing heterozygotes or homozygotes of different con-
stitution) or derived directly, from the cross of two hetero-
zygotes, having among their otherwise different gametes
some identical ones in common.

CHAPTER V.

THE PERPETUATION OF THE NEW SPECIES.

As every new individual is the result of the mating
of two gametes, forming a zygote, and as aspecies ori-
ginates either from one, or from a few ofsuch zygotes,
multiplication is necessary to perpetuate the species.

The transmittal of the constitution of the parent to the
offspring we call heredity.

What do we know of it?

Unfortunately: absolutely nothing.

Heredity of course, deals in the last instance with the
constitution of the gametes, and of the constitution
of these gametes we know nothing.

One has tried to get at the constitution of the game-
tes by mating gametes of different constitution, and
by recording the behaviour of the zygote so obtained.

The results show that gametes derived from indivi-
duals, differing from one another in one respect only,
f. i. from plants, differing but slightly in the color of
their flowers, give other results, than follon upon the
mating of gametes derived from individuals which
differ in several respects.

If we mate two gametes, obtained from parents who
differ in one respect only, the hybrid produced is di-
gametic e. g. forms only two kinds of gametes, one
kind of which is identical in constitution with the ga-
metes produced by one of the mondgametic parents

THE EVOLUTION OF LIVING BEINGS. 57

of the hybrid, the other kind being identical in con-
stitution with the gametes produced by the other
parent. ;

This of course teaches us nothing, as to the constitu-
tion of the gametes; it looks asifthe two mated game-
tes remain side by side in the zygote, and form, lin-
ked hand in hand so to speak, the hybrid, to say good
bye to one another, as soon as the vegetative develop-
ment of the hybrid comes at an end and it proceeds to
form gametes itself.

Cytology however teaches us that there is not such
a loose linkage between the gametes in the zygote, as
we would have to accept on the ground of this expla-
nation, but that, on the contrary, a very intimate fu-
sion of the bodies and even of the nuclei of these ga-
metes takes place, in which their individuality, so
far as we can see, gets lost with the exception only of
the individuality of the chromosomes.

As the gametes evidently regain their individuality
as the hybrid proceeds to form gametes, these latter
being identical to those, united in the zygote, from
which the hybrid arose, it looks as if the supposition
of a loose linkage of the gametes in the zygote were
after all correct and that, even during fusion, their in-
dividuality could be upheld unimpaired by their chro-
mosomes, retaining their individuality.

But there is some uncertainty — and this must not
be overlooked, in the assignement of such exclusive
importance to the role of the chromosomes, as this in-
terpretation implies — because it is founded on themere
supposition that the essence of the individuality ofa

58 THE EVOLUTION OF LIVING BEINGS.

gamete lies in its chromosomes, while the only reason
we have for this supposition is, that we can make these
chromosomes visible and observe their behaviour du-
ring nuclear fusion and mitosis.

It must however not be forgotten, that there may
be f. i. in the cytoplasm of the fusing gametes parti-
cles wich retain their individuality, just as well as the
chromosomes do, butwhose behaviour remains intraca-
ble to us, because we lack the means to make them
visible.

If such particles did exist, it would of course be inad-
missable to ascribe to the chromosomes the exclusive
ability to uphold the individuality of the gametes.

Yet — taken, all in all, we know of chromosomes
especially the fact, that where gametes differ greatly
in size, the quantity of cytoplasm introduced by the
male gamete, is insignificantly small compared to that
of the ovum, and yet reciprocal crosses frequently
give the same result — there is certainly good reason
to consider the chromosomes to be very important
constituents of the gametes, so that there is nothing
against the supposition that they are able to maintain
the individuality of the gametes, even during their
fusion in the zygote.

And with this retention of the individuality of the
gametes through their chromosomes, the production
by the hybrid of a number of gametes, half of whichis
identical with the gametes of the one, half of which is
identical with the gametes of the other parent, is in full
accord.

As soon however, as we proceed to mate gametes de-

THE EVOLUTION OF LIVING BEINGS. 59

rived from individuals, differing from one another in
more than one respect, this hypothesis of an individua-
lity of the gametes, maintained unimpaired during the
process of fusion, can no longer be upheld, because then
the hybrid, sprouting from the zygote, no longer forms
but two kinds of gametes, but several kinds, all of
which, except two, consequently must necessarily be
different from the kinds of gametes, produced by the
parents.

How did this change come about?

We know nothing about it.

All so called explanations, all hypotheses of separate
bearers of the several characters of the individual, all
surmises of so called hereditary units which retain
their individuality during nuclear fusion, about in the
same way as the chromosomes do, and which, after
the opinion of some, form parts of the chromosomes,
and, by their interchangability, make new combinati-
ons, and consequently formation of a large number of
different gametes possible, have their footing in Dar-
win’s hypothesis of pangenesis, no matter whether we
call such units pangens, gens, factors or by any other
name.

Their existence is a mere supposition; the so called
fact of the fragmentation or pulverisation of the chro-
mosomes during synapsis, held by some to give consi-
derable support to their real existence, is in itself far
from having been satisfactorily established.

After all, the chief reason for the making of the hypo-
thesis of pangens, is the creed that thesmallest particles
of living matter must necessarily be living themselves,

ha

60 THE EVOLUTION OF LIVING BEINGS.

because life is something so fundamentally different
from non-living, that living substance must necessarily
be fundamentally different from non-living substance,
in its whole composition also.

This however is a mere creed ;/it is perfectly concei-
vable, that life is the resultant of forces in very compli-
cated chemical bodies, mutatis mutandisin the same
way, as heat can be the resultant of forces, in a mixture
of less complicated chemical bodies.

Aslittleasin thelattercaseeach particleofthe chemical
bodies which we mix, need be hot, as little each particle
in the former cases, need be living. The fact demon-
strated by BECQUEREL of Paris, in KAMERLINGH ONNES
Laboratory at Leiden, that seeds, enclosed in tubes de-
void of air, immersed during 3 weeks in fluid air, and
after that during 77 hours in liquid hydrogren, thus ha-
ving been exposed to a cold of 190° till 253° Celsius
below the freezing point of water, sprouted all the
same, after having subsequently been kept for a year
and a half in a vacuum, goes far to throw grave doubt
on the living nature of the protoplasm, present in those
seeds.

Deprived of water and gases, and under a pressure of
almost nihil, exposed to extreme cold, this plasma
must have lost its colloidal phase, and can not have
shown even the slightest traces of physical or chemical
signs of life; we can not designate such a condition,
scientifically speaking, by any other name than death.

Yet this plasma is able to show afterwards all nor-
mal appearancesofgrowth etc.,so that there is every rea-
son to believe that a system of in itself non-living par-

THE EVOLUTION OF LIVING BEINGS. 61

ticles can, under favorable conditions, interact in such
a way that life results.

The conception of entirely independant hereditary
units, retaining their individuality unter all circum-
stances which by temporary linkage, like shuffled stic-
ky, differently colored, lumps of sugar, give rise to new
patterns, comparable to new species, reveals only the
naivité of those minds, who consider this a satisfactory
explanation of the very complicated interactions which
must take place in such a wonderful substance as two
fusing germ-plasma’s

What happens in the zygote is doubtless a very com-
plicated chemical, not a mere physical, process, as the
shuffling of different independant, hereditary units
would be.

Yet, even so rough ascheme can be of service, and has
been of service, to assist us in gaining a first general in-
sight in what may happen, but when we forget the
initial inadequacy of this scheme, and especially when
we proceed to hide the inadequacey of the explanation,
by all kinds of suppositions, notwithstanding the
results of the crosses so ,,explained”’ ought to have war-
ned us against believing in the adequacy of this expla-
nation, we get on very unsafe ground. So f. i. in those
cases, in which we are trying to explain the shortness of
hair, now by the loss of a factor, then again by the
addition of a factor, inhibiting the effect peculiar to a
length factor supposed to be present, we run great risk to
consider our duck a goose, and to gain unwarranted
confidence in a scheme which — at the most — can be,
but even need not be, correct in a very general way.

62 THE EVOLUTION OF LIVING BEINGS.

And this scheme has already obtained an exagerated
value in the opinion of many mendelians who forget,
that it is, at the most, a very rough diagrammation of
the complicated happenings during the interaction of
two mixed germplasmas of different constitution, so
that one has even carried it forward, tointerpret what
happens, when two germplasmas of identical consti-
tution fuse and concluded that every germplasm is a
mixture of independant hereditary units.

Now of this we know absolutely nothing.

The mistaken idea that organisms, showing mende-
lian behaviour, warrant some such kind ofa conclusion,
arose from the fact that nearly all organisms, met with
in nature as well as under cultivation, man included,
are hybrids which were mistakenly considered to be
specifically pure, so that their behaviour was uncon-
sciously held to be that of specifically pure organisms,
while it was that of hybrids; so it happened that segre-
gation was mistaken for heredity.

Yet,if one wanted to make out the way inwhichapure
human species, say one with fair hair and blue eyes,
transmits the fairness of its hair and the blueness of its
eyes to its progeny, one would get no aid whatever
from investigation along mendelian lines; while if
one investigates the transmittal of eye-color or hair-
color in such a society of hybrids,as we are, one finds the
laws by which these hybrids distribute their differently
constituted gametes over their offspring andis apt, ifone
forgets the hybrid nature of one’s material, to mistake
this distribution for heredity, and to speak of Mende-
lian segregation as of mendelian heredity.

THE EVOLUTION OF LIVING BEINGS. 63

_ Whether the charactercomplex of a homozygous in-
dividual is disintegrated, at the moment of the forma-
tion of the gametes, and subsequently rebuilt when two
of these gametes fuse, so as to form azygote, or whe-
ther it is never desintegrated at all,and consequently
really inherited, we know not.

This of course is of importance in connection with
the possibility of the inheritance of engrams in the
sense of SEMON, because such a thing would beat least
conceivable if the charactercomplex were inherited as
an entity, but becomes almost inconceivable if, during
the process of formation of the gametes and during their
subsequent fusion, the engram would first have to be
disintegrated and later have to be rebuilt.

We know, unfortunately, absolutely nothing of the
way in which a homozygous organism transmits its
characters to its offspring, and consequently we know
absolutely nothing of heredity.

If we define heredity as the transmittal of the cha-
ractercomplex of a homozygous individual as an entity
to its offspring, we do not even know whether heredity
exists or not.

We are thus not even in a position to say, whethera
homozygous individual is an entity or a complex of
more or less independant units; it might very well, for
all we know, be the necessary product of a germplas-
ma consisting of a definite chemical substance f. i. of a
definite albuminous body, in a similar way, assomany
cristals are the necessary shape-products of definite
pure chemical substances.

The specific shape might therefore be the expression

64 THE EVOLUTION OF LIVING BEINGS.

of a specific chemical substance ;in suppport of this, one
might bring forward the established fact that different
»species’’ of fishes are characterised by specific prota-
mines in their male gametes.

I wishit to be distinctly understood that Iam neither
pleading for the view that a homozygous individual is
an entity, nor for the view that it is a complex of more
or less independant units; Iam simply stating the fact,
regrettable as it is, that we know nothing about it.

CHAPTER VI.

HOW TO GET AT THE CONSTITUTION OF
THE GAMETES.

It is perfectly correct that the constitution of the
gametes is basic to all questions of heredity, and may
have to do, not only indirectly as it certainly does, but
also directly, with the question of the origin of species.

DE VRIES showed great insight in questions of evolu-
tion by maintaining that the final cause of the origin of
species lies in the constitution of the gametes, but his
mistake (Quis caruit erroribus?) was that he overloo-
ked the fact, that the cause of evolution does not neces-
sarily lie in the behaviour of a single gamete, but might
lie — as it does in my opinion in diploid species at
least — in the interaction of two gametes of different
constitution.

That we complicate matters—probably unnecessarily
— by trying to get at the constitution of the gametes,
through the study of diploid organisms, is plain; we
ought to begin with the study of haploid organisms.

Among these, mosses offer special advantages becau-
se the mossplant — being the haploid generation — is
the result of a single gamete, while the higher plants
— being diploid generations — are the result of the
interaction of two gametes.

If therefore, we ever succeed in changing the consti-
tation of a gamete experimentally, the result will be

5

66 THE EVOLUTION OF LIVING BEINGS.

easier seen in the case of the mossplant, the result of
a single gamete, than in the case of a diploid organism
because here the result is apt to be obscured by the
necessity of such a gamete to mate with another one,
of which we do not know whether it is changed in
the same sense, or not.

The advantage of haploid organisms above diploid
ones is, in general, that we can choose directly the ga-
metes which we want to mate, by pairing morpho-
logically different haploid generations, or better still
—as in the case of Spivogyra— by choosing and ma-
ting morphologically different gametes, while in the
case of diploid organisms we can only pair diploid indi-
viduals, and have no means to find out which of the
gametes produced by these, finally fuse, except by
judging after the result: the character of the diploid
organism, sprouting from the zygote thus obtained.

So the study of haploid organisms may very well be
destined to give us the clue to the final causes of the
origin of species.

For the present we know but very little about them;
of some known facts I will come to speak at the
end of this sketch.

Experiments with mosses, seem to me, tobehighly
recommendable.

For the present we will have tostick to known facts,
and therefore will continue our discussion of the
origin of the differently constituted types of diploid
organisms which people the globe.

CHAPTER VII.

HOW CAN GROUPS OF DIFFERENTLY CONSTITUTED
TYPES FORM A LINNEON, SIMULATING A SPECIES?

As we saw in the fifth Chapter, Mendelian behaviour
and heredity are different things, consequently here-
dity and evolution may very well be quite different
problems.

I would not be at all surprised if definite proof were
some day forthcoming that heredity: and evolution are |
ever antagonistic forces. Evolution, as far as at
present known facts allow us to judge, is caused
by the mating of gametes of different constitu-
tions.

The resulting hybrid gives, after segregation, rise to
a certain number of homozygous individuals whose
constitution is inherited by their offspring, which then
constitute a species, perpetuating its kind faithfully,
up to the moment that a new cross intervenes.

At the moment heredity sets in, evolution therefore
stops, and at the moment evolution sets in heredity
stops.

Now in our experiment garden we have it in hand, to
let evolution begin and stop at our will: by executing
a cross we start evolution, by weeding out the hetero-
zygous combinations, obtained by the mating of the
differently constituted gametes in F,, and by keeping
only the homozygotes and isolating each of these, we

68 THE EVOLUTION OF LIVING BEINGS.

stop evolution and let heredity take its course.

Now this is different from what happens in nature;
there, no homozygous form is ever sure of being al’abri
of a cross, except if strict selffertilisation reigns supre-
me which it but rarely, if ever, does. Consequently we
must look into the question what is likely to happen
in nature after a cross.

Calculation of whatis likely tohappenif
strict selffertilization follows across between
organisms differingin one or morerespects.

If strict selffertilization takes place, the progeny of
each homozygous individual necessarily must be ho-
mozygous also, so that every homozygous individual
starts a species. The heterozygotes on the other hand,
will continue to segregate in all subsequent generations,
and give rise to a mixture of homozygotes and hetero-
zygotes.

The principles which underlie the matter are there-
fore, in Jenning’s words, the following:

(r) In self-fertilized organisms, all characteristics that
become once homozygotic, remain homozygotic
for ever after, since there is no method in self-
fertilization of introducing a gamete that is di-
verse in this respect.

(2) Characteristics, heterozygotically represented be-
come homozygotic in a certain proportion of the
offspring.

The problem becomes essentially this: in what pro-
portion do the heterozygotic characters become homo-
zygotic, and how great a proportion ofall the organisms

THE EVOLUTION OF LIVING BEINGS. 69

will therefore have become homozygotic after a given

number of selffertilizations?

Jennings, American Naturalist 1912 pp. 487 seq.
calculates this as follows:

,Suppose that we begin with an organism in which
all separable characters are heterozygotically repre-
sented.

1) ,,Consider first a single pair of such alternative cha-
,racters, which we may call oe The gametes pro-
duced will be A, a, A and a, and when these com-
bine in all possible ways, they give zygotes AA +
Aa + aA + aa; that is two homozygotes and two
heterozygotes. Thus, the selffertilization of such
,an organism gives 1/, the progeny homozygotic
(with respect to this characteristic) 4/2 hetero-
»zygotic”’

»1f we let x = the proportion of homozygotes, y
= the proportion of heterozygotes (with respect to
one character) then after the first selffertilization

x = 1, of all
y =), of all

Now, after the next selffertilization, of course the
»homozygotes x remain pure, so that half of all
the progeny are still homozygotes on this account.
» Lhe heterozygotes y of course again break up, in the
,way already set forth, one half into x, the other
»halfremaining y. Sincey included half of all,this will
»give YY. of 3/4 (= 1, of all) as x, 2/, of 1/, (= 14 of
,all) as y.

90 the total proportion for the homozygotes x
»becomes after the second fertilization:

70

THE EVOLUTION OF LIVING BEINGS.

Dia a ta
while
Nar es ale

» Lhis process is repeated after each fertilization,
,so that if there are m fertilizations in succession,
»the total number of homozygotes x, becomes

»X = 4+ (4)? + (4)°.... up to (4)”.

»Lhis expression reduces to ¥ = =e where 2

»ls the number of fertilizations.

»For the heterozygotes, y, on the other hand the

»formula is simply

ye

» These then are the formulae in case we deal with

but one pair of characters. They express:

1) the proportion of all the organisms that will be
homozygotic (or heterozygotic as the case may
be), after a given number of n fertilizations.

2) also they of course express the relative proba-
bility for a given case, as to whether it shall be
homozygotic or heterozygotic.

» When we are to deal with two or more pairs of charac-

ters, the problem may be attacked in two ways.

One is by the general principles of probabilities ;

»the other is by analyzing the case of two or more

,characters, in the way exemplified above.

» Lhe two methods give the same results.

» Lhe first method is by far the simpler. It is
,merely an application of the principle that when
,we know the probability for each of two or more
, things separately, the probability that all of them

y
4%

THE EVOLUTION OF LIVING BEINGS. 71

»shall happen is the product of the separate pro-
»babilities for each.

Now we know that the probability x for the
yhomozygotic condition with respect to one cha-
»racter is:

For two characters it is then:

2m_t 2m__1\ 2n__1\2
2" is 7 a Me 2"
,For three characters it is of course.
2n__r\3
2 >
and in general, for any number m of characters, the

»probability x for pure homozygotes (or the pro
»portional number of pure homozygotes) is

Be a
2

»By similar reasoning, the proportion of all the or-
»ganisms that will be heterozygotic with respect to
,all the m characters is

,, With two or more characters, there will be of course
»a considerable number of the organisms that are
,»homozygotic with respect to some characters,
,heterozygotic with respect to others.
»1f we call the proportion of these z, then
25 Bw)

And if we let v be the total proportion that con-
tains any heterozygotic characters (so that v =
»y + 2), then

72

THE EVOLUTION OF LIVING BEINGS.

Qn y\m gmn__(2n__1)m
v=-I— ad
2” Fa

» Lhese formulae may readily be deduced algabrai-
cally, or verified, by a detailed analysis ofa case of
,two or more characters. It may be worth while to
,indicate the method followed, by taking up the
simpler case of two pairs of characters. Call these
»{a and {;. The gametes formed are AB, Ab, aB
,and ab.

When these combine in all possible ways (as
indicated in the diagrams given in Bateson’s Men-
delism), these give the following results:

YTABAB+1AbAbD+1aBaB+ 1abab+ 2ABab+

2 Ab aB+2ABAb+2ABaB-+ 2 Ab ab-+ 2 aB

ab = 16.

itt will be observed that of the entire 16, the
first four are pure homozygotes, the second four
»are pure heterozygotes (heterozygotic with res-
»pect to both characters) ; while the last 8aremixed
,(homozygotic with respect to one character, hete-
,rozygotic with respect to the other).

Letting x = pure homozygotes, y = pure hete-
,rozygotes, z = mixed, we find thus that:

Ha yh ie ae Ona:

Now, by an analysis of the sort already given, it
, will be found that at the next self-fertilization, x
»remains x; y breaks up, 4/4 of these becoming x,
» /g of these becoming z, and!/, remaining y ; z breaks
»up, */2 of these becoming 4%, 1/, remaining z.

» Now, when we recall that before the second ferti-
lization % was 1/4; y 1/4,andz,*/,ofall, we see from

THE EVOLUTION OF LIVING BEINGS. 73

the above that after the second fertilization:
x=4+ (EX8) + EX4) = as = ce |

2”
y = (} X 2) = ae = (4)”
2=(EX4)+ (8X4) =3=()"+ ("7
» These are the formulae for x and y that were ob-
,tained by the other method (since here ” and m
,are each 2). This method however gives in addi-
tion a direct formula for z.

It is easy to verify the formulae for three pairs
,of characters, though of course the conditions be-
,come here somewhat more complex.

», We may now summarize our formulae, and show
the results they give in certain examples.

Let x = the proportional number of organisms
,that are pure homozygotes (with respect to all
the characters considered).

»y = the proportion that are heterozygotic
with respect to all the characters
concerned,

z = the proportion that are mixed,
v = the proportion that have any hetero-
zygotic characters.
» Then, if# = the number of successive self-fertili-
zations and m = the number of pairs
of characters,

e= (SY, (2)

y= (8)™, (2)
2mn — (2%__I)m

eerie Wea (4)

74 THE EVOLUTION OF LIVING BEINGS.

Examples : (1) Suppose that there have been 8 self-
»fertilizations, and that we are dealing with Io pairs
,of characters.

» What proportion x of the organisms will be ho-
»mozygotic with respect to all the 10 characters?
,» What proportion will be homozygotic with respect
»to any given one character? To any given two or
,» three ?

» Laking first the case for ie entire 10 characters, by
»formula (I)

c= eae (2?)"= log. 9.9830020 = 0.961617.

» Lhus, out of 100 individuals, somewhat above 96
,» would be pure homozygotes; or by formula (4), but
,one in 26 would be heterozygotic in any respect
»(V = 0.038383).

» With respect to any one character formula (1) gives:

— (7—1\) _ 255
“= ( = Tee 0.99609375,

so that all but 4 in 1000 would be homozygotes with
»respect to that character”
in the same way we find that with respect to any
»two characters the proportion of homozygotes
, would be 0.9922; with respect to three, 0.9883 ; with
»respect to four, 0.9845, etc.
(2) Suppose that there are 20 pairs of characters,
and that there have been 20 selffertilisations. Then :
20
ne (a) Greet = log 9.999957 = 0.999998.
» Lhat is, ofa million individuals, all but two would be
»pure homozygotes.

THE EVOLUTION OF LIVING BEINGS. 75

,lt thus appears that if the number of separable

»hereditable characters is not very great (say not

» above 100), while the organism has been selffer-

_,tilized for many generations, it is to be expected

»that practically all of the organisms will be homo-

,zygotic with respect to all their characters, they

»will be ,,pure homozygotes”

In other words this calculation leads to a result re-
sembling that of a Linneon, like Triticum vulgare, con-
sisting of a large number of different pure lines.

Now let us see what the calculation of what hap-
pens in an intercrossing population of segregates from
a cross, will teach us.

Calculation of whatis likely to happen, if
free intercrossing follows an initial
crossbetween organisms, diffe-
ring in one or more respects.

This has been calculated repeatedly, always with the
same result f. i. by Baur 2d edition p. 123—129. The
simplest calculation Iam acquainted with is by Reimers
in a forthcoming article on ,, Die Bedeutung des Mende-
lismus fiir die landwirtschaftliche Tierzucht’’ which
will soon be published by Martinus Nihoff in the Ha-
gue. He argues about in the following way:

I. The parents differ in one factor only.

If we call this factor A and its absence a, all F,
individuals must be Aa. In F, we will find the same
proportions as after selffertilisation, because it is
of course indifferent, whether the eggcell A or ais
fertilised by Pollen A or a, derived from the same,

76 THE EVOLUTION OF LIVING BEINGS.

or from any other plant. It is the constitution of the
pollen, not its source which causes the effect.

In the second generation the proportions of the
differently constituted plants will therefore be 1
AA: 2 Aa: I aa.

If each of these plants forms the same number
of gametes, say four, we will get 4 A gametes from
the AA plant, 4 A gametes and 4 a gametes from
the two Aa plants and 4 a gametes from the aa
plant so that we get

8A 8a

8. AN Sra

eee

64 AA 64 Aa
64 Aa 64 aa

which means the same proportion as in Fy, viz
1 AA 32 Aals Fa.

The proportional frequency of the different combina-
tions consequently remains the same, in consecutive Ge-
nerations, if promiscuous mating takes place.

2) The parents differ in several factors.
In this case the proportions of course must remain
the same also in subsequent generations, if the fac-
tors are independent in respect to one another,
because then, the same applies to each set of two
factors, as was calculated above for the set Aa.

We may therefore conclude:

Promiscuous mating brings no change in the proporti-
anal frequency of the different possible combinations in
subsequent generations ; there ts no loss of heterozygotes in

THE EVOLUTION OF LIVING BEINGS. 72

favour to homozygotes, as happens in the case of habitual
selffertilisers.

This agrees fully with Baurs results, calculated on
the basis of promiscuous mating of white and black
mice, about which he says:

» Ueberlassen wir nun eine Population van dieser Zu-
,»sammensetzung sich zur weiteren panmiktischen Ver-
»mehrung, so werden alle folgenden Generationen das
»gleiche Zahlenverhaltniss zwischen den weissen (aa)
,und den beiden sorten (AA und Aa) von schwarzen
» Lieren aufweisen.”’

These calculations consequently lead to the result:

Selffertilisation after a cross leads to a considerable
proportional increase of the homozygotes in subsequent
generations, random mating has no such effect: the pro-
portion of homozygotes to heterozygotes remains the same
im successive generations.

Now all such calculations suffer from the impossible
supposition that all individuals born, reach maturity,
form an equal number of gametes, and that all these
gametes mate and form viable zygotes, by which kind
of calculation — as is well known —a human couple,
in the possession of one cock and six hens, can look for-
wardnot only to a prosperous, but even toa wealthy life.

Unfortunately, the ideal conditions on which such
calculations are based are never realised, so that the
calculated proportion of homozygotes to heterozygo-
tes is valueless, also because strict selffertilisation ne-
ver persists in nature, even not in such ,,typical’’ self-
fertilizers as wheat.

So NILsson-EHLE showed that Pudelweizen can be

78 THE EVOLUTION OF LIVING BEINGS.

inadvertently crossed to the extent of 1 % by other
forms, Rimpavu found similar numbers in the case of
four-rowed barley, and MAYER-GMELIN even higher
ones in beans.

Yet, ina very general way, the difference in behaviour
between selffertilizers and individuals, mating at ran-
dom after a cross, is correct.

Now it is a fact, fully worth our attention, that-—in
a general way also— what we find in nature, agrees with
these calculations, as far as selffertilisers are concerned,
but, as a rule, does not agree when random-mating
occurs.

Selffertilizing Linneons consist really of a great
number of distinct forms — though generally not so
astonishingly pure as the calculation would make us
suppose — while Linneons, within which random-ma-
ting occurs, usually have a rather uniform aspect.

Of course there is no sharp limit between these two
categories of Linneons, and this cannot be expected
either because strict selffertilisation occurs as little as
absolutely promiscuous mating, and what seems, at
first sight to be uniform, proves sometimes to be mul-
tiform on closer examination.

So f. 1. R. E. Ltoyp (The Growth of Groups in the
Animal Kingdom. Longman Green & C®. 1912. 185
pp.) fide American Naturalist 47. 1913, on studying
rats in connexion with the plague problem in India,
found that small groups of rats, differing in some res-
pects from the forms regarded as typical, occur here
and there. But even if we take full account of this lack
of a sharp limit between self-fertilisers and random-

THE EVOLUTION OF LIVING BEINGS. 79

maters, the difference between the Linneons, consis-
ting of self-fertilisers, and those consisting of freely
intercrossing types, is too apparent to be explained
away.

Now what is the reason of this difference?

If it is correct that mutation does not exist — and
a sentence in Nillson-Ehle’s article on: ,,Gibt es erb-
liche Weizenrassen mit mehr oder weniger vollstan-
diger Selbstbefruchtung? Ztschr. f. Pflanzenziichtung
I9QI5 p 5., gives additional support tofthis conten-
tion:

»In der Nachkommenschaft recessiv-merkmaliger F, individuen
,»von Weizenbastardirungen kommen nicht selten vereinzelte Indi-
»viduen mit dem dominanten Merkmal vor und ihre Zuriickfuh-
,»rung auf Vicinismus (crossing with a neighbour of another form)
»kKann, angesichts der bei diesem Versuch gefundendenen oben
»mitgeteilten, Zahlen keineswegs als unbegriindet angesehen wer-
,den”’,

then the only way by which the number of homozy-
gotes can increase in succeeding generations of a freely
intercrossing community is: selection.

So that, if it is correct, that a Linneon is the rest of
what arose from a former cross, the fact that
Linneons inside of which free inter-
crossing occurs, obtain generally
a more or less uniform aspect, is
definite proof for Darwin’s conten-
tion that selection, at least intra-
linneontic one, in nature, is a fact.

It must, of course, be argued that selection will occur
as well within self-fertilizing Linneons as within Lin-
neons inside of which free-intercrossing occurs, but
this must then lead to a different effect.

80 THE EVOLUTION OF LIVING BEINGS.

Now let us look into this question.

Inside of a community of different types, originated
from a cross, which do not further intercross, we will
get, as calculated above, after a number of generations
a large number of species, and but comparatively few
heterozygotes.

The effect of selection can therefore be nothing
else than a reduction of the number of species arisen,
by elimination of the ones less well fitted to survive,
than those which are not eliminated.

Now this is exactly what happens.

About the effect of cold as a selector Nilsson-Ehle
says in the article quoted above:

Besonders schnell degeneriren im Klima Schwedens
wenig winterfeste Sorten (Linneons according to our
nomenclature); die abweichenden Typen nehmen,
wenn sie winterfester als die echten sind, nach stren-
gen Wintern sehr rasch an Zahl zu und kénnen
bald vorherrschend werden. Die Sorte verliert dann
zulezt ganzlich ihren urspriinglichen Charakter.”’

Now what can be expected to happen in a freely
intercrossing community, say if the dominant type is
for some reason or other favored through extermination
of the recessives in proportionally greater numbers
than the dominants?

Such a selection must necessarily also favor the
hybrids with dominant appearance because, if f. i. a
grey rabbit is favored, on account of its protective
color, such a protection is not the outcome of its
genotypical constitution but of its phenotypical as-
pect, and as this does not differ from that of the pure

THE EVOLUTION OF LIVING BEINGS. 81

dominants, pure dominants and dominant-hybrids will
profit equally by their protective coloring.

Suppose we have an F, generation consisting of r AA
plant 2 Aa plants and 1 aa plant and of these aa Plants
50 % perish on account of their conspicuousness, then
we will have 1 AA plant 2 Aa plants and 4 aa plant
left. If we assume now, as in the example on p 76 that
each plant forms 4 gametes, we get 4 A gametes from
the AA plant, 8 gametes from the two Aa plants of
which 4 are A and 4 a, and 2 a gametes from the 3
aa plant, which will combine:

8A 6a
SA. Oa

64 AA 48 Aa
48 Aa 36 aa

64 AA 06 Aa 36 aa

consequently in the proportion 16 AA : 24 Aa: 9 aa
against the proportion 16 AA : 32 Aa: 16 aa in the
preceding generation.

Or, if we count AA and Aa together as phenotypi-
cally indistinctible, we get the proportion 40: 9
against 48 : 16 or 44 : I against 3 : 1. Butnot only has
the number of phenotypically dominant forms increa-
sed considerably, as compared with the recessives,
but also the number of pure dominants, as compared
with the number of dominant-hybrids, this being in the
preceding generation 16 : 32 or 1: 2 and in this ge-
neration 16: 24 or 2: 3 in other words an increase
from 3} to 2.

6

82 THE EVOLUTION OF LIVING BEINGS.

Consequently: favoring of the domi-
nants in a freely intercrossing community tends to
cause an overwhelming majority of the dominant form,
simulating uniformity pretty soon, because the dominant
hybrids are wndistinctible from the pure dominants.

And this is exactly what we see in Linneons like
wild rabbits, where free intercrossing reigns supreme,
thus giving great support to Darwin’s idea of the great
influence of selection in nature. Furthermore, if it is
true, that there are no strict selffertilizers in nature, we
must find it easier to obtain, even in so called selffer-
tilizers, the recessives pure than the dominants, because
then, there must be among the phenotypically domi-
nant forms some dominant-hybrids.

Now this is exactly what does occur. About it Nils-
son Ehle says in his article, quoted already:

»Bei der praktischen Reinhaltung van Weizensor-
,ten mittels Entfernung abweichender Aehrentypen
,aus den Vermehrungen spielen ferner die Merkma-
»le der Weizensorte ob dominant oder recessif eine
»sehr wichtige Rolle. Eine Sorte mit kahlen oder
»weissen Aehren (recessive Merkmale) ist leichter von
,behaarten bezw. braunen Abweichungen rein zu
,halten als eine behaarte oder braune Sorte (domini-
»rende Merkmale) von Kahlen bezw. Weissen. Beider
»Entfernung der Abweichungen werden namlich im
»vorigenFalle auch eventuelle Heterozygoten mitausge-
,zogen. Im letzten Falle bleiben aber die Heterozygo-
,ten zuriick, wenigsten zum Teil, weil sie, ebenso wie
,die typischen Individuefi, braun bezw. behaart sind
»und spalten deshalb im nachsten Jahre wieder weisse

THE EVOLUTION OF LIVING BEINGS. 83

»bezw. kahle Individuen ab; auch nach sorgfaltig-
ster Reinpfliickung erhalt man somit das nachste Jahr
»wieder dieselben Abweichungen, obwohl eventuell in
»geringerer Zahl.”

In cases where crossing is entirely excluded, the
only selection possible is through extermination of cer-
tain species within the Linneon. That, although strict
fertilizers may not exist, at least some forms cross
much less frequently than others, results from Nilsson-
Ehle’s experience in a mixed planting of Pudelweizen
and a wheat designated as n°. 0728, which showed
that the first kind was spontaneously crossed by the
latter, but the latter not by the first.

Yet, so great a selfprotection against crossing is rare
in nature.

So that, if our contention is correct,that a Linneon is
a group ofindividuals of differentconstitutions, which is
but a rest of many moredifferent types which sprang
from across, andif selection causes a freely intercrossing
community in nature to become apparently uniform
in aspect, we may expect to find that, especially
among animals, the Linneons will, by their uniform
aspect, tend to simulate species.

This question is of such considerable importance,
and touches so closely the question how one came to
consider the appearance of individuals, deviating from
the form, considered to be typical for a particular
Linneon, as evidence of variability, that I may be
permitted to enlarge a little upon it.

Let us take as an example the wild rabbits, refer-
red to the Linneon: Lepus cuniculus.

84 THE EVOLUTION OF LIVING BEINGS.

Every child will tell you: wild rabbits are grey, but
people better acquainted with rabbits, gamekeepers
f. i., know that occasionally black and orange-yellow
rabbits occur in nature. As these are evidently much
in the minority compared with the grey ones, they
jump to the conclusion that the grey ones are the orig-
nal form, while the black and the yellow ones are the
varieties derived from this original form.

Systematists use to express this by saying that the
most common form is the ,,species’’, while the rare ones
are ,,varieties’’.

Now the cause of this conception really is nothing
but the overwhelming majority of the greys.

If 1/; of all wild rabbits were grey, 4/3 yellow and 4/;
black, one would probably have made 3 species of them,
and never have entertained the idea of variability or,
if yet some systematist had united them to one species,
one would anyhow, if the idea of a common descent
had cropped up, been at a perfect loss to decide which
form was the original one: the grey, the black or the
yellow one, which had to be considered to be derived.

So that, if it was not exclusively the fact that within
most Linnean species, especially within animal ones,
one form is in the overwhelming majority which cau-
sed the conception of variability, it certainly was this
fact which led to the conception of a single form being
the ancestor of all others.

Nowas we calculated already, there is no reason what-
ever for such a contention.

By crossing the type in the majority with those in
the minority, we find that the type in the majority is

THE EVOLUTION OF LIVING BEINGS. 85

the dominant one, the others prove to be recessives
in regard to this type.

And as we find always that the type in the majority
in nature, 7s the dominant one, this dominant type
has proved — on our contention that its frequency is
due to selection of the most resistant type — to be
stronger than the recessives.

Now although ,,most resistant” and ,,strongest’’ are
rather vague expressions, we all know that when we
are dealing with organisms from the same region in
which our experiment gardens are situated, — which
is essential for comparison, because what is most resis-
tant in our climate may prove to be least resistant say
in the tropics — the recessives usually are by far the
weaker forms.

And this also supports Darwin’s contention of con-
siderable selection taking place in nature.

But.... the effect of changed conditions may also
be the survival of recessives.

We saw that free intercrossing within the Linneon
in nature, must finally cause an impression of unifor-
mity, because the hybrids are indistinctible from
the favored dominant form and thus are mistaken
for the latter. The hybrids among the apparently
pure dominants throw recessives only, if they pair with
another hybrid, while if they pair with a pure domi-
nant, all children have again the aspect of the domi-
nants. Even without selection this causes, as we all
know, already a considerable majority of the type
exhibiting the dominant cahracters, compared to the
frequency in which the recessive type occurs; this

86 THE EVOLUTION OF LIVING BEINGS.

majority being in F, of a cross of two forms differing
in one respect only, 3 : 1. If, as in rabbits, the domi-
nant colour f. 1. is selected for the protective effect
it confers on the animals possessing it, this majority
of course increases rapidly.

We can imagine however easily, that a change of
conditions f. i. a migration of the rabbits from the
dunes to black earth, would divest the dominant color,
grey, from its protective power, and thus cause an
increase of the, now protectively colored, black reces-
cives.

This might even go so far that only one type, reces-
sive in all characters, survives in which case the Lin-
neon has been reduced to a pure species and conse-
quently stops to ,,vary.”’

As ,,variation’”’ by crossing is — by the plasticity it
confers on the Linneon — a means and probably
the chief one, to adapt such a Linneon to changing
conditions the dying out of certain Linneons may
have been caused by such a survival of pure recessi-
ves only, putting a final stop to the possibility of
such a species to adapt itself to changed conditions,
unless a cross with an individual belonging to another
Linneon, creates fresh opportunities to ,,vary.”

If no such cross occurs, if such a Linneon reduced
to a pure species is doomed to remain single, it is —if
conditions change — unable to form a progeny able
to adapt itself to these changed conditions, and conse-
quently must die out, exactly as human families
whose members remain single; though it may remain
in existence for a considerable time, if no untoward

THE EVOLUTION OF LIVING BEINGS. 87

change in the conditions to which it is itself adapted
or, if one prefers, resistant, takes place. While thus
extermination of dominants may lead, by thenarrowing
down of the Linneon to a single recessive species, to
its extinction, elimination of the recessives never leads
to their total extinction, because these recesives
survive cryptomerously in the gametes of those hy-
brids which are indistinctible from the dominants,
and therefore share all protection which the latter
may enjoy.

The idea that the aberrant types — the recessives —
are varieties of the most common type — the do-
minant one — is consequently a mistake; the aberrant
forms never arose from the dominant one, but are se-
gregates from the hybrids indistinctible from the do-
minant type. Hereditable variability
spells segregation.

The Linneon: Lepus cuniculus, consequently is not
a unit but a group of different types and their hybrids.

That this is really the case would be difficult to pro-
ve in the case of the wild rabbit, because we can get
this proof only by mating two hybrid grey forms and, as
these are indistinctible from the pure dominants, a
very large number of matings would be required to
find a pair of such hybrids.

If we disposed of a sufficiently large number of
pure black wild rabbits, it would however be compara-
tively easy to get the impure grey males because a grey
male mated with such a black, giving grey children
only, would be pure; a grey male mated with such a
black (preferably with the same), giving a mixture of

88 THE EVOLUTION OF LIVING BEINGS.

black and grey children, on the other hand, would betray
its hybrid nature.

In the same manner mating grey females witha
black male would reveal to us the purity orimpurity
of the grey female which, if pure, would get grey chil-
dren only, if impure, grey and black ones. Animpure
grey male and an impure grey female, so found out,
paired together, would give a certain percentage of black
children and thus prove our contention that there must
be hybridsamong the apparently pure wild grey rabbits.

The quickest way to obtain such impure greys would
probably be to catch a pregnant black female, as such
a one is very likely to owe its pregnancy toagrey male,
pure or impure, and consequently will either throw a
litter of exclusively impure greys or consisting of a
mixture of impure greys and pure blacks.

The fact that in Java frequently black and spotted
panthers are found in the same litter, is conclusive
proof that among either the black or the spotted
panther in Java, there are hybrid individuals.

Allsuch kinds of impurity are much easier demonstra-
ted in the case of plants which can be selffertilized, be-
cause here isolation of a sufficiently large number of
individuals of the phenotypically dominant type re-
veals to us, by their several progenies, at once the pre-
sence of hybrids.

In this way, I was able to show that among our or-
dinary brown wallflowers (Cheiranthus Cheiri) some
are pure dominants, while others are hybrids, segrega-
ting violets, golden-yellows and whites, thus showing
that the wallflower is not a species but a Linneon, con-

THE EVOLUTION OF LIVING BEINGS. 89

sisting of four different species: brown—violet—gol-
denyellow—and white-flowering ones and their hybrids.

With this demonstration of course the real origin of
these species is not explained.

It probably lies, as experiments with Antirrhinums
show, in a previous cross between two individuals
belonging to different Linneons.

Such a cross gives us — if the forms originally cros-
sed were pure — a uniform F,, each individual of
which, after selfing, gives rise to a large number of differ-
ently constituted individuals, some of which are hetero-
zygotes while others are homozygotes or pure species.

Every cross between two individuals, differing in
many respects, consequently gives rise to the produc-
tion of a mass of differently constituted types, far grea-
ter than the number of separate types usually found
within a wild Linneon.

This is caused by the fact that of the number of
different forms which were born from a cross, only a
small number survives.

ie Lanne ony jis comse quently thie
vestige, consisting of a comparatively small num-
ber of differently constituted types, of that very
large number of types born as the
result of a cross.

The number of types born from crossing in nature,
is usually even much larger than that resulting from
a cross we effect purposely in our experimentgarden,
because in nature crossing is not limited to asingle ini-
tial cross, but usually a large number of crosses occurs
at the same time. This can be esasily demonstrated by

go THE EVOLUTION OF LIVING BEINGS.

planting a single individual of Antirrhinum glutino-
sum in an experimentgarden, in which alargenumber
of different types of Antirrhinum are cultivated.

As Antirrhinum glutinosum is selfsterile, we can
observe the result at once in F,.

Now the result, obtained in such a casein Bennebroek,
was an enormunsly varied F, which could be explained,
either by the fact, that the individual of A. glutino-
sum had been crossfertilized by a number of different
Antirrhinum-types, the pollen of which had been car-
ried by humblebees to the different flowers of the glu-
tinosum-individual, or by a single, but highly hetero-
zygous, type.

A priori, the former was the most probable but this
single experiment was insufficient to settle what had
really happened.

To settle this point, several individuals of A-glutino-
sum were planted out and seeds of one of them collected.

The result was the same: a multiform F,, but
this time there were pure glutinosum-forms among
this F,, which failed to appear in the first one.

It was thus proved that A. glutinosum was fertilised
at least by two of the different types present in the
garden viz by A. glutinosum and by at least one hetero-
zygous type of A. majus, much more probably by
several types, the pollen of which had been brought to
the glutinosum plant by insects.

This experiment is yet in progress with the overwin-
tered F, plants which of course will continue to cross
with one another and with other types present in the
garden, thus giving continuously rise to new types,

THE EVOLUTION OF LIVING BEINGS. gr

just as will happen in nature, and so continue to cre-
ate yearly a number of new types which are able to
fill a good many opportunities for existence, present in
their neighbourhood or within a distance to which
their seeds may be carried, thus offering in other
words, splendid material for adaptation.

What ts born, consequently depends on whatis crossed, \
what survives, on the local circumstances of the birth-place —
of the new forms and on those of its neighbourhood.

If the same kind of cross takes place in Japan andin
Holland, the same forms will be born at those two
distant places. So I found f. i. a white Mus rattus
with a brown head and brown shoulders caught in
Leiden, hardly distinctible from a similar form from
Japan in the Leiden Museum .

As the Linneon Mus rattus ranges from Holland to
Japan, it evidently gives rise to the same recessive
forms, when two hybrids indistinctible from the black
dominants pair, irrespective of the spot where this
pairing takes place.

Now it might very well happen, that the black form
disappeared from Japan and survived in Holland,
through local conditions favoring the survival ofthe
recessive form in Japan, in which case these two forms
would be considered as different ,,species’’, in support
of which distinction, one would not fail to lay stress upon
their occurence in very distant, not overlapping areas.

Such a thing will happen especially frequently with
Linneons of plants, containing forms of a different de-
gree of resistancy against frost which leads to a diffe-
rence in the composition of such Linneons in different

92 THE EVOLUTION OF LIVING BEINGS.

countries, causing them to be considered _,,specifi-
cally” different. That this zs so, is shown by Nilsson-
Ehle’s experience that certain ,,species’” of wheat, in-
troduced into Sweden, and cultivated there for some
time, become of quite another type by the weeding
out of the but little frostresistant strains by the
Swedish climate.

In many cases, as we saw, freely intercrossing Jor-
danons cause the Linneons to get, in nature, a uni-
form aspect.

Yet there are cases in which such Linneons show a
bewildering multiformity either in several or in one
respect, even at first sight.

The most beautiful exemple of this, I ever saw, is fur-
nished by a series of about 200 specimens of Buteo
Buteo in the Leiden Museum, hardly two of which are
alike, and which resemble very much, what one would
expect to obtain, after segregation in F,?).

The reason, that this lot resembles an F, generation
probably is that here no selection has been at work,
because this bird of prey is so strong that it has
practically no ennemies in the regions in which it occurs.

In cases, where the Linneon is more uniform in as-
pect but yet shows great diversity in one respect, the
explanation probably is, that as far as the ,,variable’’
character is concerned, selection did not take place.

So f. i. Symphytum officinale which, at least in the
neighbour hood of Bennebroek is self-sterile and con-
sequently forms an intercrossing community, like that

1) I regret it greatly that it is not possible to publish a colored plate
of this most important series.

THE EVOLUTION OF LIVING BEINGS. 93

of Lepus or Buteo, shows all kinds of colors in its flo-
wers, f.1. dark-violet, light violet, different shades of red,
whites etc.

The same is the case with the blue-eyed, grey-eyed,
brown eyed and black eyed human individuals.

The explanation is probably that none of these co-
lors gives a special advantage in our region and conse-
quently there is no elimination.

That, notwithstanding these differences, we continue
to consider such heterogenous groups as Linneons, is
because we abstract from differences, considered to be
unessential, such as color of flowers etc. and take so cal-
led essential characters as criteria as explained on p. 50.

That other self-sterile forms behave just the same
among plants as among animals, e. g. tend to become
uniform by selection of certain characteristics, is pro-
ved by the behaviour of the strictly-self sterile Car-
damine pratensis inside of which Linneon no such stri-
king differences in the colors of the flowers occur.

In other cases, in which we find great diversity of
form, as f.i. within the Linneon: Taraxacum vulgare, the
cause is very different from that which causes diver-
sity in the case of Symphytum.

Taraxacum vulgare is — with the exception of one
form in Japan —, notwithstanding the fact that its
flowers are perfect and produce quantities of pollen,
never fertilized, but reproduces itself apogamously;
consequently selection can have no other effect than
to decrease the number of different forms, just asin the
case of strict self-fertilizers.

Other polymorphous Linneons, like some in the gene-

94 THE EVOLUTION OF LIVING BEINGS.

ra Hieractum and Antennaria, though occasionally
able to reproduce themselves in the normal sexual way,
and consequently open to an occasional cross, yet
usually, behave like Taraxum which explains their high
degree of polymorphism.

Of course, if apogamy follows directly upon a cross,
it perpetuates not only the homozygous forms but the
heterozygous ones as well — just as cuttings do — so
that the conclusion, drawn by East AND HAyYEs from
the fact (if it be a fact) that such Hieracia occasionally
vary’ and consequently crossing can not be the only
cause of,,variability’’, is unwarranted because of course
heterozygous apogamous forms can give rise to a num-
ber of different forms, can ,,vary’’ as aresult of vegeta-
tive segregation, shown by East himself, to exist f. 1.
among potatoes.

Summa Summarum, I think we may say that a Lin-
neon 1s a vestigial group of a once much larger group of
differently constituted types, born from a cross, which is
apt to simulate a species by the overwhelming majority of
the dominant types it contains, as a result of free-inter-
crossing, combined with a favoring of the dominants by a
process of selection, weeding out the weaker or more con-
spicuous recessives; this uniformity being more apparent
than real, because pure dominants are indistinctible, in
most cases, from dominant-hybrids.

A Linneon consequently is nothing but a group of
morphologically similar individuals. It may consist of
almost nothing but pure species and a few hybrids, as
it does in habitual self-fertilizers; it may consist of a
mixture of homozygotes and heterozygotes, reprodu-

THE EVOLUTION OF LIVING BEINGS. 95

cing themselves apagomously as in the case of Tara-
xum officinale, it may consist chiefly of pure domi-
nants and hybrids, resembling these, with a slight ad-
mixture of pure species (the recessives) as in the case of
the wild rabbits or of the brown-flowered Cheiranthus
Cheiri, or it may consist of almost nothing but hy-
brids of different constitutions, as it does f. i. in the
case of Symphytum officinale and of human beings.

That, even in such cases as these last ones, we keep up
the conception of a Linneon, is caused by the fact that
we are impregnated, from infancy almost, with the con-
viction that some characters are essential for a Linneon
while others are not, so that we refuse to cut up a Lin-
neon into smaller groups, as long as these ,,essential”’
characters e. g. those common to all the different types
within the Linneon, are not brought into play, which
of course they never can, as we ourselves did limit the
Linneon by the criterium of the very presence of these
characters in all its individuals.

CHAPTER VIII.

THE LIMITS OF LINNEONS.

That Linneons are more or less distinctly limited,
is apparent from the fact that in subdivising the
living Kingdom, one made unconsciously halt at the
border of each Linneon, and so was led to consider
them as real species, such as Linnaeus believed them
to be.

In most cases the limits between the smaller units
within the Linneon, are much less conspicuous, so that
it lasted until the middle of the 19th century before
Jordan discovered even the mere existence of such
smaller units within the Linneon.

This difference in the distinctness of the limits between
the Jordanons and the Linneons needs explaining.

We must enquire into this question by asking: are
there cases, in which the Jordanons within a Linneon
are as distinct as the Linneons themselves and are
there cases in which this is not the case?

The answer is that there are, so that the question
remains, what causes this difference?

Investigation shows that poor distinctness of the Jor-
danonswithin the Linneon, occursin all cases where these
Jordanons intercross freely and selection favors acertain
type, and that this distinctness increases in the same ra-
tio as intercrossing decreases, so that it reaches its maxi-
mum when crossing does notoccurat all, asin those cases

THE EVOLUTION OF LIVING BEINGS. 97

in which the different types are perpetuated apoga-
mously.

This gives us the clue as to the difference in distinctness
of the units within the Linneon and between the different
Linneons themselves ; the units within each Linneon form
an intercrossing community, while the Linneons them-
selves usually do not intercross.

Why not?

Because mostly, individuals belonging to different
Linneons, have either an aversion to mating orare pre-
vented from mating, by isolation either in space (occur-
rence in different regions) or in time (different time of
flowering or different time of rutting) or by mechanic
obstacles (differences in size of the male of the one and
the female of the other Linneon, non-fitting copula-
ting organs etc.), all this accentuated by the fact that in
many cases, even if mating occurs, no progeny ora
sterile progeny only, results.

The distinctness of the Linneons ts consequently caused
by the obstacles against mating of the individuals belon-
ging to different Linneons, which obstacles may be rela-
tive obstacles which can be overcome, such as aversion f. 1.
or final obstacles which cannot be overcome: innate stert-
lity as f. 1. between many Linneons of the genus Verbas-
cum .

Consequently it is nature itself which groups the indt-
viduals to Linneons and Linneons are thus something
more than mere conceptions of the human mind; 2t are
natural intercrossing communities of differently constitu-

7

hey

98 THE EVOLUTION OF LIVING BEINGS.

ted types. As we have seen that constant intercrossing,
such as takes place inside of many Linneons between
the different types, by the selection of a certain type,
which is the rule in nature, has finally aswamping effect,
leading to an overwhelming majority of the dominant
type — including both pure dominants and hybrids
indistinctible from these at sight — there is no reason
to suppose that what happens within the Linneon
would not happen between the several Linneons them-
selves, if they also intercrossed freely, so that it is reaso-
nable to suppose that if there were no obstacles of any
kind to a free intercrossing in nature between all the
differently constituted types, which people the earth,
this latter would be chiefly peopled by one type only.

The cause of the possibility of a great diversity of
types, living side by side in the same regions, is the exis-
tence of obstacles to free intercrossing, isolating these
apparently non-isolated types as effectively as if the
different types were put into separate cages.

Within each cage intercrossing freely occurs and
leads, by the aid of selection, to a high degree of pheno-
typical uniformity, while the numerous phenotypically
different types so obtained — the Linneons — remain
distinct because the walls of the cages — in nature the
obstacles to crossing — keep them separate.

Linneons consequently, though being themselves the
vestiges of the result of a cross, are kept distinct in nature
by obstacles against their freelycrossingwith otherLinneons.

If there existed no obstacles to unlimited intercros-
sing in nature, we would be unable to distinguish Lin-
neons; if no crossing took place at all, Linneons—being

THE EVOLUTION OF LIVING BEINGS. 99

themselves the result of a cross — would never have
originated, so that Linneons owe thetr origin to the occa-
sional possibility of a cross and thewr persistence to the
bars which nature, as a rule, keeps closed to prevent inter-
crossing, and but occasionally opens.

The view, that distinctness of the Linneons is caused
by bars to intercrossing with other Linneons, is greatly
supported by the fact, that in cases as those of the Wil-
lows where the Linneons are, as we all know, very badly
limited, this bar against intercrosssing with other
Linneons does not only not exist, but such intercros-
sing is, on the contrary, favored (see Chapter XI).

Whenever nature allows crossing, evolution sets in ;when-
ever crossing 1s made impossible it stops as soon as the
segregation initiated by the cross, 1s at an end.

As the bars, separating the Jordanons within the Lin-
neons, are usually open, evolution within the Linneon,
the formation of so called new ,,varieties’’ is a pheno-
menon occuring daily, and it is stopped permanently
only by the cropping up of apogamy.

As on the contrary, the bars between the different
Linneons are usually closed, the origin of anewLinnean
species is a phenomenon of much rarer occurrence and
very frequently stopped definitely by theinnate sterility
of two Linneons.

The production of new Linneons, usually called ,,spe-
cies’, is generally designated as progressive evolution,
the production of new Jordanons, usually called ,,va-
rieties” is frequently called degressive evolution.

CHAPTER IX.

THE CAUSE OF THE INCREASE OF ,,VARIABILITY”
UNDER DOMESTICATION.

It is almost generally believed that domestication
causes variation by the influence of better food or of
unusual food etc., giving rise to certain transmit-
table tendencies in the domesticated animals or
plants.

So Darwin says:

,»,When we see an animal highly kept, producing off-
»spring with an hereditary tendency to early maturity
,and fatness, when wesee the wild duck, and the austra-
»lian dog, always becoming, when bred for one or a
,few generations in confinement, mottled in their co-
,lours .... we naturally attribute such changes to the
,direct effect of known or unknown agencies acting for
,one or more generations on the parents. It is possible
,that a multitude of peculiarities may thus be caused
»by unknown external agencies’.

Now we have seen that there is no proof for the
existence of a transmittable influence of external agen-
cies, so that we must look for another cause for the
changes we find that frequently follow upon confine-
ment of a wild animal. Darwin himself clearly recogni-
zed that there must be other causes for the greater
, variability’ existing among domesticated animals and
plants, than this direct effect of external agencies, so

THE EVOLUTION OF LIVING BEINGS. IoI

that he ascribes a much greater effect to what he calls
indirect agencies:

»l may add, judging from the vast number of new
varieties of plants which have been produced in the
,same districts and under nearly the same routine of
,culture, that probably the indirect effects of domes-
,tication in making the organisation plastic is a much
»more efficient source of variation than any direct
»effect which external causes may have on the colour,
texture or form of each part’.

Now what may this zmdirect effect of domestication be?

To answer this we must first ask: 1s the fact that we
see a larger number of different forms belonging to the
same Linneon under domestication than in nature, proof
that thereexist more such forms under domestication
than in nature ?

This of course need not be the case, it is also pos-
sible that forms are visible under domestication which,
although existing in nature, remain there hidden
to us.

Suppose this were the case, what then would cause
their appearance under domestication?

The answer is: isolation.

We have seen that every heterozygote isolated, un-
dergoes segregation by which the recessives it con-
tains in a cryptomerous way, become visible.

Now this is exactly what domestication does: it iso-
lates individuals, and this simple fact explains how
savages, of whom no great ,,breeding”’ qualities can be
expected, yet succeed in raising different races of do-
mesticated animals and plants.

I02 THE EVOLUTION OF LIVING BEINGS.

In plants this succeeds, as we saw from the example
of the wallflowers, easily.

Suppose a savage takes home for adornment of his
garden, a wild brown wallflower. Planted, this indivi-
dual, if heterozygous, will, from the seeds it scathers
around it, raise an offspring among which there will be
some with white flowers.

Now suppose the savage prefers this white-flowered
race above the brown-flowered one, and consequently
pulls out all brown-flowered plants which sprang up in
his garden, he succeeds at once in obtaining this white
flowered form — which happens to be the recessive —
pure, and has thus, with very little trouble or insight,
obtained a new constant race.

In the same way, it is easy to obtain new races of rab-
bits. Suppose a heterozygous pregnant wild rabbit has
been caught by a savage and put into a cage, and let us
suppose further — taking the most unfavorable exam-
ple — that this heterozygote was pregnant from a
pure grey male.

The litter thrown, will then consist of pure greys
only, but as some of the males in this litter will be
heterozygotes, such a heterozygous male will, paired
with its mother or with a heterozygous sister, give
some aberrant offspring by segregation, which aber-
rant forms, if bred together, will give easily rise to
new races.

Isolation and subsequent selection of the aberrant
types consequently suffises to obtain the aberrant forms
cryptomerously hidden in wild animals and plants, and
to breed these true to type. This, in all probability, has

THE EVOLUTION OF LIVING BEINGS. I03

almost unconsciously been done in all efforts to tame
wild animals or to cultivate wild plants.

Domestication, even without in-
troducimie anew source for the pro-
duction of new forms, consequently
allows us to gain forms, not or but
weryirarely (met within nature, Dy
the mereisolation of heterozygotes.

But domestication has not acted in this way only; it
certainly has introduced new sources for the produc-
tion of new forms by crossing.

It is a very curious fact, that this evident source of
,variability’”’ under domestication has always been
explained away.

Almost all writers on domestication of animals or
plants had to acknowledge that much pointed towards
a multiple origin of our domesticated races by crossing,
and yet almost all have, notwithstanding this evi-
dence, pleaded for a single origin by variation from one
ancestral form.

We all know, that Darwin ascribed the origin of the
domesticated as well as of the wild new forms to some
sort of heriditary variation. For this conception it is
evidently necessary, to show or at least tomake plau-
sible, that the domestic races, as wellas the wild ,, varie-
ties’, can be considered to belong to one species.

But if we examine the evidence for this contention,
we at once perceive how meagre it is.

In support of this, I will quote from Darwins ,, Varia-
tion of animals and plants under domestication second
edition revised. London John Murray 1893.

104 THE EVOLUTION OF LIVING BEINGS.

Domestic dogs.
Vol. I, p. 26: ,,it is highly probable that the domes-
tic dogs of the world are descended from two well-
defined species of wolf (viz C. lupus and C. latrans)
,and from two or three doubtfull species (namely the
»European, Indian and North African Wolves) ;
»from at least one or two South American canine spe-
cies ; from several races or species of jackal, and per
,haps from one or more extinct species’’.

Domestic Cats.
Vol. I, p. 49. ,,we have seen that distant countries
»possess distinct domestic races of the cat. The diffe-
»rences may in part be due to descent from several
,original species, or at least from crosses with them”.

Domestic Horses.
Vol. I, p. 53. ,,. Whether the whole amount of diffe-
,rence between the various breeds has arisen under
domestication is doubtfull. From the fertility of the
,most distinct breeds, when crossed, naturalists
»have generally looked at all the breeds as having
descended from a single species. Few will agree with
colonel H. Smith, who believes that they have des-
,cended from no less than five primitive and diffe-
»rently coloured stocks. But as several species and
Varieties of the horse existed during the later ter-
,tiary periods and as Riitimeyer found differences in
the size and form of the skull in the earliest known
domesticated horses, we ought not to feel sure
that ,,all our horses are descended from a single
species’.

The ass.

THE EVOLUTION OF LIVING BEINGS. 105

Vol. I, p. 65. ,,There is now little doubt that our
domesticated animal is descended from the Equus
,taeniopus of Abyssinia.

Pigs.
Darwin supposes our Pigs to have arisen from cros-
sing Sus indicus and Sus scrofa and says:
Vol. I, p. 74. ,,Seeing how different the chinese pigs,
»belonging to the Sus indicus type, are in their os-
,teological characters and in external appearance
»from the pigs of the S. scrofa type, so that they
,must be considered specifically distinct,, it isa fact,
»well deserving attention, that Chinese and common
»pigs have repeatedly been crossed in various man-
»ners with unimpaired fertility’’.

Caitle.
Vol. I, p. 82. ,, Domestic cattle are certainly descen-
,dants of more than one wild form, in the same man-
»ner as has been shown to be the case with our dogs
,and pigs.”

Sheep.
Vol. I, p. 97. ,,.Most authors look at our domestic
»sheep as descended from several distinct species.”
p- 98. ,,Another ingenious observer though not a
naturalist, with a bold defiance of everything known
,on geographical distribution, infers that the sheep
,of Great Brittain alone are the descendants of eleven
,endemic British forms! Under such a hopeless state
,of doubt it would be useless for my purpose to give
,»a detailed account of the several breeds.”’

Goats.
Vol. I, p. 105. From the recent researches of M.

106 THE EVOLUTION OF LIVING BEINGS.

, Brandt most naturalists now believe that all our
»goats are descended from the Capra aegagrus of
,the mountains of Asia, possibly mingled with the
,allied species C. falconeri of India”’
Rabbits.
Vol I p. 107 ,,All naturalists, with as far as I know
,,a single exception, believe that the several domestic
breeds of the rabbit are descended from the com-
»mon wild species; I shall therefore describe them
»more carefully than the previous cases.”
Pigeons.
Vol. I, p. 137. _,,I have been lead to study domestic
»pigeons with particular care ,because the evidence
that all the domestic races are descended from one
»kKnown source is far clearer than with any other
,anciently domesticated animal”’.
Fowls.
Vol. I, p. 251. ,,Finally we have not such good evi-
,dence with fowls as with pigeons, of all the breeds
»having descended from a single primitive stock.”
Ducks.
Vol. I, p. 295. ,,From these several facts, more espe-
cially from the drakes of all the breeds having cur-
led tail-feathers and from certain sub-varieties in
,each breed occasionnally resembling in general
»plumage the wild duck, we may conclude with
confidence that all the breeds are descended from
Anas boschas.”’
The Goose.
Vol. I, p. 302. ,,A large majority of capable judges
',are convinced that our geese are descended from

THE EVOLUTION OF LIVING BEINGS. I07

»the wild Grey-leg goose (A. ferus); the young of
»which can easily be tamed’.

The Peacock.

Vol. I, p. 305. ,, This is another bird, which has hard-
»ly varied under domestication, except in someti-
»mes being white or piebald. Mr. Waterhouse care-
»fully compared, as he informs me, skins of the wild
indian and domestic bird and they were identical
»in every respect, except that the plumage of the
latter was perhaps rather thicker.”

The Turkey.

Vol. I, p. 308. ,,It seems fairly well established by
»Mr. Gould, that the turkey, in accordance with the
»history of its first introduction is descended from
»@ Wild Mexican form which had been domesticated
»by the natives before the discovery of America, and
»which is now generally ranked as a local race and
,not as a distinct species.”

The Guinea-fowl.

Vol. I, p. 310. .... ,,is now believed by some natura-
lists to be descended from the Numida ptilorhynca,
»which inhabits very hot, and, in parts, extremely
,arid districts in Eastern Africa. Consequently it has
,been exposed in this country to extremely different
conditions in life. Nevertheless it has hardly varied
,at all except in the plumage being either paler or
darker colored’ +).

The Canary-bird.

Vol. I, p. 311. ,,It has been crossed with nine or ten

1) Italics are mine.

108 THE EVOLUTION OF LIVING BEINGS.

,other species of Fringillidae, and some of the hy-
»brids are almost completely fertile; but we have no
,evidence that any distinct breed has originated
,irom such crosses.

Gold-Fish, Hive-bees and Silk-moths. No definite state-
ments.

Plants.

Cereal plants.
Vol. I, p. 338. ,,Finally, every one must judge for
himself whether it is more probable that theseveral
»forms of wheat, barley, rye and oats are descended
,4rom between ten and fifteen species, most of which
,are now either unknown or extinct, or whether they
,are descended from between four and eight species
»which may have either closely resembled our pre-
,sent cultivated forms, or have been so widely diffe-
»rent as to escape identification.”

Zea Mays.
Vol. I, p. 338. ,, Botanists are nearly unanimous that
all the cultivated kinds belong to the same species’.

Cabbage.
Vol. I, p. 343. ,,. Most authors believe that all the ra-
ces are descended from the wild cabbage found on
»the Western shores of Europe, but Alph. de Can-
,dolle forcibly argues, on historical and other grounds,
that itis more probable that two or three closely
allied forms, generally ranked as distinct species, still
living in the mediterranean region, are the parents,
,now all mingled together, of the various cultivated
forms’.
Pp. 344. ,,The other cultivated forms of the genus

THE EVOLUTION OF LIVING BEINGS. I09

» Brassica are descended, according to the view a-
,dopted by Godron and Mezger, from twospecies, B.
,»napus and B. rapa; but according to other bota-
»nists from three species, whilst others again strong-
ly suspect that all these forms, both wild and cul-
, tivated ought to be ranked as a single species.
Brassica napus has given rise to two large groups,
,»namely swedish turnips (believed to be of hybrid
,origin); and Colzas the seeds of which yield oil.
Brassica rapa (of Koch) has also given rise to two
»races, namely common turnips and the oil-giving
»rape.”’

Peas.

Vol. I, p. 349. ,, Whether many of the new varieties
»Which incessantly appear are due to such occasio-
,nal and accidental crosses I do not know.”

Beans.

Vol. I, p. 349. ,,With respect to beans (Faba vulgu-
»ris), I will say but little.... As in the case of the
»pea, our existing varieties were preceded during
»the Bronze age in Switzerland by a peculiar and
»now extinct variety producing very small beans.”
Potato (Solanum tuberosum).

Vol. I, p. 350. ,, There is little doubt about the paren-
»tage of this plant, for the cultivated varieties differ
,extremely little in general appearance from the wild
species which can be recognized in its native land
,at the first glance.

The Vine (Vitis vinifera).

Vol. I, p. 352. ,, The best authorities consider all our
»grapes as the descendants of one species which

IIo THE EVOLUTION OF LIVING BEINGS.

»now grows wild in Western Asia.... some authors
,however, entertain much doubt about the single
»parentage of our cultivated varieties”.

White Mulberry (Morus albus).
Vol. I, p. 354. ,,Jn India the mulberry has also
»given rise to many varieties. The Indian form is
»thought bij many botanists to be a distinct spe-
,cies, but as Royle remarks, so many varieties
,have been produced by cultivation that it is dif-
ficult to ascertain whether they all belong to one
»species.

The orange group.
Vol. I, p. 355. ,,We here meet with great confusion
»in the specific distinction and parentage of the
,several kinds.”

Peach and Nectarine (Amygdalus persica).
Vol. I, p. 360. ,, Whether or not the peach has procee-
,ded from the almond, it has certainly given rise to
, nectarines.”

Apricot (Prunus armeniaca).
Vol. I, p. 365. ,,It is commonly admitted that the
tree is descended from a single species, now found
»wild in the Caucasion region.”’

Plums (Prunus insititia).
Vol. I, 366. ,,Formerly the sloe, P. spinosa, was
»thought to be the parent of all our plums but now
,this honour is very commonly accorded to P. in-
,sititia or the bullace, which is found wild in the
Caucasus and N. Western India.... another sup-
»posed parent-form, the P. domestica is said to be
»found wild in the region of the Caucasus.”’

THE EVOLUTION OF LIVING BEINGS. LEE

Cherries (Prunus cerasus, avium etc.).
Vol. I, p. 368. ,, Botanists believe that our cultivated
»cherries are descended from one, two, four or even
,»more wild stocks’.

Apple (Pyrus malus).
Vol. I, p. 369. ,, The one source of doubt felt by bo-
,tanists with respect to the parentage of the apple,
»ls whether, besides P. malus, two or three other
closely allied wild forms, namely P. acerba and
»praecox or paradisiaca, do not deserve to be ranked
»as distinct species’.

Pears (Pyrus communis).
Vol. I, p. 372. ,,1 need say little on this fruit, which
»Vvaries much in the wild state, and to an extraor-
,dinary degree, when cultivated, in its fruit, flowers
,and foliage. One of the most celebrated botanists
,im Europe, M. Decaisne has carefully studied the
»Many varieties; although he formerly believed that
»they were derived from more than one species, he
,now: thinks that they all belong to one.”

Strawberries (Fragaria).
Vol I, p. 374. ,, The blending together of two or more
aboriginal forms, which there 1s every reason to be-
,,lleve has occurred with some of our anciently cultiva-
, ted productions, we see now actually occurring with
,,0ur strawberries,” 1)

Gooseberry (Ribes grossularia).
Vol. I, p. 376. ,,No one, I believe has hitherto doub-
,ted that all the cultivated kinds are sprung from
»the wild plant bearing the name, which is common

1) Italics are mine.

[1I2 THE EVOLUTION OF LIVING BEINGS.

»in central and Northern Europe.”’
Walnut (Juglans regia).
Vol. I, p. 379. Description of different ,,varieties”’ only.
Nuts (Corylus avellana).
Vol. I, p. 379. Most botaniss rank all the varieties
under the same species, the common wild nut.
Cucurbitaceous plants.
Vol. I, p. 384. ,, Finally M. Naudin remarks that the
,extraordinary production of races and varieties
by a single species and their permanence, when not
interfered with by crossing, are phenomena well
calculated to cause reflection.
Trees.
Vol. I, p. 384. ,,Deserve a passing notice on account
_,of the numerous varieties which they present.”
Flowers.
Vol. I, p. 388. ,,. Many of our favourite kinds in their
present state are the descendants of two or more
species crossed and commingled together and this
circumstance alone would render it difficult to de-
tect the difference due to variation.”

As we see, a single origin is proved in no case, while an
| origin from different sources by crossing, is made pro-
\ bable on the other hand in many cases.

Darwin himself resumes the question in the follo-

wung sentence on p. 12 of the Origin:

»In the case of most of our anciently domesticated
,animals and plants it is not possible to come to any
,definite conclusion, whether they are descended from
,»one or several wild species”

THE EVOLUTION OF LIVING BEINGS. £3

In the face of this fact and of the knowledge, we have
obtained, that a transmittable influence of external
circumstances has never been proved to exist, as little
as any other form of hereditable variability, it is safe
to say that:

Our domesticated plants and animals are the results of
isolation of heterozygotes, caught in nature, followed by
selection and isolation of the recessives, or the results of
crossing, followed also by segregation and selection of the
desirable segregates.

Now is it possible to get definite proof for this con-
tention?

It seems to me that it is.

If crossing is at the bottom of the origin of our do-
mestic races, such races must have originated as he-
terozygotes ; andas such heterozygotes frequently must
have had desirable qualities already, it stands to rea-
son, that in cases, in which such heterozygotes could be
multiplied asexually, e. g. by budding, by grafting, by
cuttings, by bulbs etc. one will have resorted to one of
thesemeans. Iftherefore, crossing is the final cause of the
origin of new domestic races,it is reasonable to expect,
that a great majority of the different kinds of trees
and flowers, multiplied habitually in an asexual way,
as f. i. fruittrees and flower bulbs are, must be hete-
rozygotes.

Now this is doubtless the case.

We know that nearly all kinds of apples, hyacinths,
tulips etc. when sown, segregate into a great number
of different types — frequently in a most astonishing
number of them — thus proving their hybrid origin.

8

a
N

II4 THE EVOLUTION OF LIVING BEINGS.

We are thus justified to conclude:

Domestication spells segregation, followed by selec-
tion and isolation of the desirable segregates.

Crossing is always at the bottom of it, this may have
taken place already before the animal or plant was
domesticated in nature; in most cases however will
have taken place after domesticion.

All breeders of animals and plants know this and
continuously obtain novelties by crossing.

The introduction of new forms from distant coun-
tries is therefore diligently resorted to, always with
the view of crossing them with stock already in hand,
in the hope to obtain novelties.

On the other hand, there is not the slightest proof,
that the mere change of conditions, following upon do-
mestication, itself causes ,,variability’’; in all known
cases such ,,variability” was the result of a cross.

That it seems occasionaily, as if spontaneous varia-
bility occurs in breeds, is caused by the fact that hard-
ly any breed is homozygotic in all its individuals, so
that aberrant types are born, whenever two hetero-
zygotes happen to mate. This f. i. is the reason that
from time to time red ,,variants’ occur in the dutch
white and black cattle.

CHAPTER X.

PROGRESSION IN EVOLUTION.

We saw in the eighth chapter, that the production of
new Linneons, usually called ,,species’”’, is generally
designated as progressive evolution, and that this
kind of evolution is considered by many the evolution
par excellence.

We must therefore devote a chapter to the question
of Progression in Evolution.

This question of progression is a vexed one; there
is in the human mind a craving towards improve-
ment in everything, which makes mankind believe,
but all too readily, that things are better to day than
they were yesterday, and wil be better still to morrow.

Perhaps the only good, that will come of the dam-
nable war which is reigning over the greater part of
the ,,civilised”’ world, will be the recognition that we
did not progress as much as we thought we did, that
man is little better or rather worse than he was f. 1.
in Koman times.

But man is such a curious animal, that he will forget
in a few years how he behaved in these years as a
beast, and pride himself again on his ,,progression’”’!
There is no more conceited being in all nature than
man, and this conceit is a snare, in which he is caught
every time he looks at the universe from the stand-
point, so tickling to his vanity, that he almost un-

I16 THE EVOLUTION OF LIVING BEINGS.

consciously dwells upon it, that all was made for his
pleasure. This anthropocentric standpoint causes him
to believe that the world could not possibly have been
so good before he was pleased to make his appearance
into it, than after this never-sufficiently-to-be-appre-
ciated condescension of his, and so we have had, in all
seriousness, a discussion on the supposed inadequacy
of the adaptation of the plants of coal measure times.

It is this same anthropocentric pedantry which
leads man to consider himself the crown of the universe,
and to arrange the different kinds of animals accor-
ding to their degree of lesser or greater similarity
to himself, claiming that those most differing from
his Nibbs, are the lower ones, those resembling him
most, the higher ones, thus creating the conception of
progression.

So a little chit of a monkey becomes ,,higher’’ than
an elephant, a mouse higher than a Condor, (is not
sucking the young, like a human mother, much ,,higher’”’
than feeding them on carrion) a slow-worm higher
than a shark, poor little Amphioxus higher than a
giant cuttle-fish, a clam higher than the most beauti-
ful of yelly-fishes and all such nonsense.

Very rightly Victor FRANz has maintained in the
Biologisches Centralblatt 1911 p. 1,/that we possess no
reliable measure whatever for the determination of
, which organisms are higher, which lower on thescale.

The result depends entirely on which characters we
choose for comparison. If we choose the brain and the
urogenital system as our measure, one can indeed ar-
gue that man is the highest animal, but if we choose

THE EVOLUTION OF LIVING BEINGS. EI7

as criteria the protective measures against wet feet,
a cow is an infinitely higher animal than man. /

The much met with criterium: higher differentiation
indicates a higher, less differentiation a lower degree of
development, is no good either; by this measure am-
phibia, generally considered to be ,,higher’’ than fis-
hes, would have to be placed lower than these on the
progressive scale, because they lack a good deal of dif-
ferentation in the skeleton, in the sense-organs of
the skin, in the covering of it, in the brains, with the
exception of the Pallium, and in the ovarium, which the
Teleosts possess.

Comparison of the characters consequently gives
no criterium for progression, so that it is very possible
that progression has as litte real existence in nature
as genera have, but that progression is an abstraction
of the human mind just as the genus is.

But.... we are not yet justified to draw this con-
slusion; there is something, called the geological re-
cord, the record — even if it be very incomplete — of
the development in time which according to the
opinion of many, proves that progression does exist!

It proves nothing of the kind; it proves, at the most,
that the succession was: Coelenterata, Molluscs, Fis-
hes, Reptils, Birds, Mammals, to mention only the
groups referred to above, but this of course does not
prove, that the forms which made their appearance
the latest, are the highest ones (did not much ,,higher’”’
reptils appear much earlier than the present, ,,lower”’
ones?). One might, with equal justice, claim mental
superiority for the child which is last born!

118 THE EVOLUTION OF LIVING BEINGS.

Consequently the geological record gives no support
to progression either, and we are perfectly justified to
say that progression is a human conception and that

| progressive evolution does not exist.

But the geological record does show — even if we
take full account of its incompleteness — that man
appeared very late on the globe, that fishes and amphi-
bia preceded reptilia and mammalia etc.

This has to be accounted for.

But this ts no accounting for progressive evolu-
tion, but for successive evolution, as opposed to
simultaneous creation.

Now this succession shows that the later types have
arisen from gametes, produced by the immediately
preceding types, and consequently that under the
conditions existing and having existed on the earth —
in the widest sense — no other way was possible.

But this shows, by no means, that there is something
innate to living matter which necessitated this parti-
cular sequence; we might very well have had an ear-
lier appearance, say of man, than has taken place, if
conditions — in the widest sense — had been different.

This is clear, when we keep in mind that in our ex-
periment gardens we have it in hand, to postpone to
any desirable moment, the production of a new species,
by not executing the cross, as a result of which, that
species would arise, or to hasten its production by exe-
cuting that particular cross at once.

So it may be, that if the cross from which man has
arisen, had been executed by nature earlier than it has
been executed, man would have appeared earlier in

THE EVOLUTION OF LIVING BEINGS. IIg

the history of the earth, than he has. His production
may have been, what is usually, called the result of
chance e. g. of the meeting and mating of two forms,
at that particular moment and not at an earlier one,
just as the moment of the birth of children depends,
in the last instance, on the chance meeting of the man
and woman who, after this meeting, decide to marry.

In how far chance, in this sense, has played a role
in successive evolution is an unanswerable question;
could man f. i. have originated from gametes, pro-
duced by two particular reptils, if these had happened
to mate, or is a shortening of the sequence: reptils—
lower’? mammals — man, impossible?

We know not; and the so-called Biogenetisches
Grundgesetz gives no solution either, although it ra-
ther points towards the possibility of shortening.

All this is speculation; we have anyhow not to deal
with the question in which other way evolution might
have taken place, but how it has taken place and this
we can deduce from the geological record only,
which shows clearly that it took place successively.

With such a successive evolution, our theory that
new Linneons arise from a cross of preexisting ones, is
in full accord, so that the moment has come to consider
the evidence for the occurrence of crossing of different
Linneons in nature.

CHAPTER XI.

THE EVIDENCE FOR THE OCCURENCE OF CROSSES
BETWEEN INDIVIDUALS BELONGING TO DIFFERENT
LINNEONS IN NATURE.

Crossing of individuals belonging to different Linne-
ons will be brought about by opportunity or by ne-
cessity.

Opportunity is offered f. i. whenever pollen gets
on a foreign stigma and nothing prevents its subse-
quent fertilizing action; necessity is born by the sexu-
al desire which no animals and few men can resist in
the long run.

So, while most white men in Europe would not think
of mating with a negress, white men not infrequently
do so in Africa, where no white women live, and while
bantamcocks don’t pair with pheasants in nature,
they do so in captivity, if locked up with pheasant-
hens, in the absence of hens of their own kind.

While hare and rabbit don’t pair in nature, a male
hare doubtless would do so if sufficiently long isola-
ted with female rabbits, in the absence of male rabbits
on an island, as ressorts from the experiments of Mr.
Houwink, showing that the hare looses its inborn aver-
sion of a tame rabbit, if it is taken soon after birth
from its mother, and sucked by a tame rabbit foster-
mother.

This secret for obtaining Leporids, Mr. Houwink ob-

THE EVOLUTION OF LIVING BEINGS. I2r

tained from a poacher and it worked so splendidly
that he is now in the possession of a fine fullgrown F,
generation of Leporids, a male of which has already
paired with one of its sisters. 3)

I would therefore not be a bit surprised if it were
subsequently proved that our tame rabbits were evol-
ved from an original cross between the wild rabbit
and the hare, which cross Mr. Houwink is trying to
obtain in a similar way.

Perhaps this trick of having the individuals belon-
ging to a foreign Linneon nurtured by an individual
belonging to the Linneon, with which one desires to
cross them, is very widely applicable and may lead
to the obtention of many interesting hybrids.

Dr. van Oort tells me that it is well known, that a
bitch of our domestic dogs refuses nearly always to
mate with a male wolf, but shows no aversion if she
herself was raised by a female wolf.

The aversion to mating between individuals belon-
ging to different Linneons can consequently be overco-
me, and that this happens in nature also, is proved by
the fact that Linneons which intercross in nature are
known from almost all groups of animals and plants.
Of animals we will mention of the class of the Echino-
dermata: Echinus esculentus and E. acutus in the
neighbourhood of Plymouth (cf. Shearer, de Morgan
and Fuchs Phil. Frans act. Royal Society B 204 pp.-
255—362), of the Class of the Vermes: hybrids between
Ascaris univalens and A. bivalens with three chromoso-

1) This copulation has since proved to be fertile, but the young
ones died soon after birth.

I22 THE EVOLUTION OF LIVING BEINGS.

mes by Herla and Zoja (cf. Biol. CBl. 1912 p. 718), of
Arthropoda it is well known, that in the group of the
insects such hybrids are common. Among Mollusca
Helix hortensis and H. nemoralis frequently cross,
while among Pisces it is well known that f. i. in the ri-
ver IJssel in Holland every hoal of the big nets called
»blessings’’ (not from the fish’s standpoint!) contains
hybrids between the genera Leuciscus and Blicca,
while hybrids between Blicca and Abramis and be-
tween different Linneons of Leuciscus are common in
many rivers. It appears that generally hybrids be-
tween different Linneons of fishes are by no means
rare (cf. Claus). Among Amphibia natural hybrids be-
tween different Linneons of Triton are known (cf. Poll.
Biol. CBl. 1909 p. 30) and also between different Lin-
neons of frogs (cf. Boulenger). Among Aves hybrids are
especially common between different Linneons, yes
even between different genera f.i. of ducks, even such
between individuals belonging to different families
viz between Penelope (fam. Cracidae) and Phasianus
colchicus (cf. Haecker p. 211) but this one not in
nature. In Sweden Birkhahn and Auerhenne cross
frequently, probably because the Auerhahne are
decimated by shooting (cf. Naumann Naturgesch.
der Végel Mitteleuropas. Vol. 6 p. 106 Anm. 13.108.
Among Mammalia, Darwin says in the Origin p. 224,
that he has reason to believe that the hybrids from
Cervulus vaginalis and Reevisii are perfectly fertile.
So are the Indian humped and common cattle when
crossed.

Among plants, hybrids between different Linneons

THE EVOLUTION OF LIVING BEINGS. I23

are quite common, and certain peculiarities in flower
structure or behaviour f. i. dichogamy even favor such
crossfertilisation among Linneons. An exemple of this
is furnished by the willows.

I will here quote what Kerner’s Pflanzenleben says
about this (Vol. II p. 311)

»Auch die zweihausigen Pflanzen sind in der Mehr-
zahl noch proterogyn. In den ausgedehnten Weiden-
bestanden an den Ufern unserer Fliisse sieht man bis-
weilen einzelne Arten durch Tausende von Strauchern
vertreten. Ein Teil derselben tragt Pollenbliiten, der
andere Fruchtbliiten. Sie wachsen auf demselben Bo-
den, sind in gleicher Weise der Besonnung ausgesetzt
and werden van denselben Luftstr6mungen bestrichen,
und trotz dieser gleichen aiissern Einfliissen eilen die
Stécke mit Fruchtbliiten ihren Nachbarn mit Pollen-
bliiten deutlich voraus. Die Narben der Salix amyg-
dalina sind schon 2—3 Tage hindurch belegungsfahig
und dennoch hat sich weit und breit noch keine einzige
Anthere dieser Weidenart gedffnet. Dasselbe gilt van
der Purpurweide, der Korbweide, der Bruchweide
etc. Da das ungleichzeitige Eintreffen der Reife der
zweierlei Geschlechter einer Art eine Einrichtung ist,
welche bei den meisten, ja vielleicht bei allen Pflanzen
vorkommt, so kann auch nicht angenommen werden,
dassdieser Einrichtunggar keine Bedeutungzukomt. Ich
will es nun versuchen die Bedeutung der Dichogamie
zu erklaren, und lade den Leser ein mit mir zunachst
eines der Weidengebiete zu betreten welches in vorher-
gehendem kurz geschildert wurde. Die Purpurweide be-
ginnt grade zu bliihen. Die Fruchtbliiten derselben

I24 THE EVOLUTION OF LIVING BEINGS.

zeigen bereits belegungsfahige Narben, aber die Pollen-
bliiten sind noch in der Entwicklung zurtick und es ist
noch keine einzige Anthere derselben gedffnet. Dage-
gen stehen die Pollenbliiten an der Korbweide (Salix
viminalis) welche untermischt mit der Purperweide im
selben Bestande wachst, auf dem Hohepunkt der
Entwicklung. Pollen der Korbweide ist in Hiille und
Fiille zu haben. Durch den Duft und die Farbe der
Bliitenkatzchen angelockt, haben sich zahlreiche Bie-
nen eingestellt, schwirren von Strauch zu Strauch,
saugen Honig und sammeln Pollen. Sie sind bei dieser
Arbeit nicht wahlerisch und beschranken sich nicht auf
eine einzige Art, sondern fliegen ebenso gerne zur Pur-
purweide wie zur Korbweide.... Indem aber die Nar-
ben der Purpurweide mit dem Pollen der Korbweide
belegt wurden, hat eine zweiartige Kreuzung stattge-
funden. Erst zwei oder drei Tage spater kann eine ein-
artige Kreuzung stattfinden, denn nun haben sich
auch aus den Pollenbliiten der Purpurweide die Anthe-
ren hervorgeschoben ....BetBeginn des Bliihens ist also
bei der genannten Weide, infolge der Dichogamie nur
eine zweiartige, spdter erst eime einartige Kreuzung
moglich.

At the commencement of flowering, the Dichogamy of
the willow mentioned above, allows only crossing with
individuals of another Linneon, while later only ferith-
sation by members of the same Linneon becomes possible.

Of course it is indifferent in principle, whether the
pollen is carried by the wind or by insects, but as the
pollen, in order to be transferable by wind, must pos-
sess certain properties and the flowers which produce

THE EVOLUTION OF LIVING BEINGS. I25

such pollen also (in the first place to cover the great
loss of such pollen during transportation, they must pro-
duce it in very large quantities) crossfertilisation
between many Linneons of plants must have been al-
most impossible before insects came into existence.

The birth of insects consequently offered new posst- |
bilities of crossing and consequently of the birth of new |
species. |

Darwin clearly perceived this, as results from a letter
he wrote to HOOKER on Aug. 6th 1881 (Life and Letters
III p. 248).

Nothing is more extraordinary in the history of the
»vegetable kingdom as it seems to me, than the appa-
»renily very sudden or abrupt development of the
»higher plants. ... Hence I was greatly interested by a
»view which Saporta propounded to me a few years
,ago.... viz, that as soon as flower-frequenting in-
,sects mere developed, during the latter part of the
,secondary period, an enormous impulse was given to
»the development of the higher plants by crossfertili-
,sation being thus suddenly formed.”

Of course the influence of dichogamy remains the
same in favoring crossfertilisation of Linneons whe-
ther the wind or insects are the transporters of the
pollen, and so it is quite correct that Kerner makes the
general statement, that dichogamy favours bi-specific
crossing especially at the beginning and at the end of
the flowering period ofall plants possessing this peculi-
arity (cf. l. c. II p. 315).

That this hybridization of Linneons is by no means
of rare occurrence, KERNER, who was the first to recog-

126 THE EVOLUTION OF LIVING BEINGS.

nize the great importance of hybrids for evolution
shows on p. 570, where he states that the number of
such hybrids growing wild in Europe, which has beco-
me known in the last 40 years preceding the writing of
his book, can safely be estimated at 1000 and that the
41 Linneons of Coniferae growing in Europe have pro-
duced no less than 7 hybrids.

When we consider further, that not only different
Linneons but also different genera intercross f. i.
Secale and Triticum, Triticum and Aegilops etc. and
that up to the present, we know with certainty of no
other way of the formation of new species and new Lin-
neons than as a result of crossing, we may, I think, ac-
cept safely that the underlying cause of the diversity of
the different types which people the earth, the under-
lying cause of evolution, at least as far as diploid orga-
nisms is concerned, is hybridization.

It draws a more complete paralell between the ori-
gin of individuals and that ofspecies, on which Darwin
insisted already, than his theory did, because according
to his conception, species had but one parent, the vary-
ing ancestral species, while individuals had two pa-
rents; while according to our view, species as well as in-
dividuals have two parents, the first the two parental
species from whose cross the new species arose, the lat-
ter their father and mother.

CHAPTER XII.

THE EFFECT OF CROSSING LINNEONS.

On talking with different scientific friends about the
views which have gradually developped in my mind
about crossing being the origin of species, I have fre-
quently heard the objection — which is pretty gene-
rally offered against all theories of evolution — that
the effect is inconsiderable, compared with the changes
wanted to explain the origin of such different groups,
say as fishes, reptils and mammals.

I will return to this question in the next chapter, li-
miting myself in the present one, to show what the ef-
fect of an actually executed cross between individuals
belonging to different Linneons has been.

It is impossible to treat here of all the numerous
forms which result from such a cross; for this I must
refer the reader to a book by the author, on the
hybrids between different Linneons of Antirrhinum
which would have appeared long ago, if the war had not
inferfered with the making of the plates.

But as all that is necessary here, is toshow the great
diversity which can be the result of across, I can suffice
by restating, what I said at the IVe Conférence de
Génétique in Paris in Ig11 and by reproducing some
photographs as illustration.

In 1910 Professor Baur of Berlin succeeded in cros-
sing certain species of Antirrhinum, the hybrids of

128 THE EVOLUTION OF LIVING BEINGS.

which proved to be fully fertile. The seeds of two of
these hybrid-combinations he very kindly gave tome
for further investigation.

One of these combinations was Antirrhinum glutino-
sum crossfertilised witha peloric form of Antirrhinum
majus.

As Antirrhinum glutinosum contains several types,
all of which are completely selfsterile, the F, genera-
tion obtained was somewhat polymorphous, though not
to a considerable extent. On the whole, it was fairly
intermediate between the two parent species.

The F, obtained from the F, plants was exceedingly
polymorphous; one of the self-fertilised F, plants gave
255 children, not two of which were alike. They dif-
fered in a large number of characters, such as colour,
form of flowers, habit of growth, leaf-characters, hoa-
riness, self-fertility, resistance to draught and frost etc.
Zygomorphous and peloric flowers were always present,
and it appears that the segregation was a complicated
mendelian one.

The most interesting results perhaps were obtained
from the cross A. glutinosum by a red zygomorphous
form of A. majus.

In the F, of this cross, several remarkable forms oc-
curred, in one f. i. the sepals were coloured and peta-
loid, another showed several spur-like excressences at
the lower lip of the flower and some had flowers asto-
nishingly different from those of the parent-species,
resembling more a Rhinanthus thanan Antirrhinumand
of a type entirely unknown hitherto within this latter
genus, as the photographs here reproduced show.

THE EVOLUTION OF LIVING BEINGS. I29

The first photograph shows at the top, indicated as P#
and P?, the two parents crossed.

P? is the flower of Antirrhinum glutinosum, which is
white, except that it has pink striae on the upperlip.

The flower indicated by P?, is the dark red flower of An-

tirrhinum majus with which the former was crossed. Of

the two other flowers on this photograph, the one which

appeared in the fourth selfed generation (F4), was pure

ivory and characterized by the possession of spur-like
9

130 THE EVOLUTION OF LIVING BEINGS.

organs at the lower lip, while the one which appeared
in F, was light pink, and showed beautiful petaloid
sepals of the same color but in a lighter shade.

The second photograph shows one flower of a plant
of the 2d generation, indicated by F,, which was flesh-
colored and resembled the flower of a Rhinanthus, the
other flowers, all belonging to the 4th generation (F,),
were magenta and showed shapes very different indeed
also, from the parent-species crossed.

THE EVOLUTION OF LIVING BEINGS. I31

It seems to me, that this little suffises to show that
very unexpected results can arise from a cross, forms
so different from the Linneons crossed, that no syste-
matist could possibly say to which of these Linneons
they belonged, yes so different, that if their origin were |
not known, they might easily be referred to different |
genera.

CHAPTER XIII.
THE ORIGIN OF THE GREAT CLASSES.

The origin of the great classes of the vegetable and
animal kingdom is a historical problem in which geo-
logy has the first word.
The most important facts geology teaches us are:
Istly. that the origin of the different classes lies very
far back.

2dly. that a new class appears suddenly with a great
many, mostly highly differentiated, different
forms.

3dly. that the further we get away from the birth of
the class, the more the number of different
forms diminishes, and the more ,,reduced”’ these
become.

We will illustrate these important facts by some
examples.

SCOTT says (p. 6. of the Introduction to his study in
fossil Botany 1908) about the very ancient origin of the
great divisions:

»Lhere are probably no biologists left now who
Oppose in toto the doctrine of evolution, but if there
,»were, they might draw a telling, though fallacious
,argument from the high organization of the Devonian
ilonait

The sudden appearance in a large number of diffe-
rent forms, ofsuch groups as the Reptils, Cycadophytes

THE EVOLUTION OF LIVING BEINGS. 133

and Angiosperms is generally known; also their gradu-
al diminution in types (except in the latter group,
which is yet flourishing) until but a few remain, or total
extinction follows.
If we compare the Cycadophytes of Mesozoic times,
with their highly developed Bennettitales, with the few
Cycadaceae of to day, or the Lycopodiales of the Coal-
measures, with their giant Lepidodendrons, with the
miserable Lycopods of to day, or the Equisetales of the
Coal-measurés, with their numerous giant Calamites,
with the few horse-tails of the present time, or the fossil
Reptils and Ammonites of the Jura with the recent
ones, the exactness of the facts mentioned above, sub 2
and 3, is apparent.
Now all this coincides splendidly with what we see
happening after across between two differentLinneons,viz.
Istly. appearance, among the progeny of the hybrids,
of a very large number of different forms.

adly. gradual decrease of that large number of diffe-
rent forms until but a few remain within each
Linneon.

Soon after the formation of a large number of diffe-
rent forms from a cross, these continue to intercross,
leading thus to a constant increase of new types, but
gradually this intercrossing ceases, either by isolation,
sterility or by any other obstacles among which aversion
may play a considerable réle, and so smaller groups, Lin-
neons, findthemselves together as communities to which
intercrossing, if such continues, is limited. If it does not
continue, mere segregation will rapidly cause an increa-
se of the homozygotes in proportion to the heterozy-

i

134 THE EVOLUTION OF LIVING BEINGS.

gotes, and these homozygotes will be decimated in the
struggle for life; while if intercrossing continues, selec-
tion of the dominant type which seems to be the rule,
causes apparent uniformity within such Linneons! asin
the case of the grey rabbits.

The result is, that from the large number of types
arisen from the cross, finally but a few survive.

_ Exactly what we see en grand after the appearance
of a new Class of organisms.

- Within each Linneon the number of smaller units,

of Jordanons, decreases also by selection, — which

spells: extermination (of the less fitted types) —

until finally but a few or even but onesurvives.

If one survives only within aself-fertilising Linneon,
its ,, variability” is at an end, and therefore it must soo-
ner or later die out, and the same must happen if, wi-
thin a Linneon, while intercrossing of the different
types it contains continues, selection leads to the sur-
vival of the recessive type, in stead of to that of the do-
minant one, because then its ,,variability”’ is at an end
also, and extinction must follow.

That exclusive survival of recessives is one of the
chief-reasons of extinction, gains support from the fact
that the recent forms of ancient groups are all weak
ones, compared to the older ones, and experience goes
far to prove that the recessive types are usually the
weaker ones.

Crossing therefore is the cause of the origin of new
types, heredity perpetuates them, selection 1s the cause —
not of their origin as was formerly supposed — but of
their extinction.

THE EVOLUTION OF LIVING BEINGS. 135

Selection,if notinterfered with by crossing, inevitably
ends with extinction because selection can act through
extermination only, and by thus diminishing the num-
ber of differently constituted types, decreases the possi-
bility of crossing and thus the formation of new types,
which formation not only means the origin of new
species, but also the possibility of adaptation, as the
only way in which a Linneon can adapt itself to new
circumstances, isa putting into the world of new types.

All this agrees perfectly with what we saw that hap-
pens, in the appearance and gradual extinction of new
classes of plants or animals, so that we are justified to
conclude:

Crossing was the origin of the new classes ; selection, the
result of extermination by the struggle for life, the cause
of their gradual extinction.

Such extinction of classes must proceed continuously,
until a happy meeting between two sufficiently different-
ly constituted gametes, causes the origin of a new class,

We will therefore have periods of new-formations of
classes and periods of gradual extinction of such classes. |

In the present time we live in a period, in which the
extinction of many classes is almost completed, andin
which no new classes are formed.

This is easily demonstrated for the vascular plants
by a glance at Scott’s system:

Equisetales.
Pseudoborniales.
Sphenophyllales.
Psilotales.

Sphenopsida

136 THE EVOLUTION OF LIVING BEINGS.

Lycopsida Lycopodiales.
Filicales.

Pteropsida Pteridospermeae.
Gymnospermeae.
Angiospermeae.

Of these three great divisions the two first are ex-
tinct, with the exception of a few Equisetaceae, Psilo-
taceae and Lycopadiaceae; of the Pteropsida the Pte-
ridospermeae are extinct, Gymnospermeae on the way
to extinction, so that only the Filicales and the An-
giosperms remain, of which the latter alone, are flou-
rishing.

A formation of new classes is not in action at the pre-
sent moment, so that it is illegitimate to claim, that one
who wants to explain evolution, must demonstrate how
such a formation of new classes goes on.

One can ’t demonstrate something which does not
happen, at the time one is living; we must conclude to
the way of the origin of new classes by analogy, and we
have done so, by comparing it with what happens after
crossing two Linneons. What happens there, is demon-
strable and we have demonstrated it.

Whether new classes will in future be formed again
on our globe, or whether we assist at the last trial of
life to maintain itself on our globe, in other words
whether weare living at the beginning of the period of
extinction of the latest born classes, we of course know
not.

CHAPTER XIV.
THE QUESTION OF RELATIONSHIP.

Relationship among different groups of animals
and plants has generally been considered to be of the
nature of bloodrelationship.

So Scott says: ,,In these days most of us, when we
speak ofrelationship among organisms, mean toimply
»a real affinity, that is to say a bloodrelationship.”’

And a little further on he specifies:

,/xisting organisms are related to each other more
,or less as brothers or cousins are related.”

Now the question is: ave brothers related ?

As all men are hybrids, the bloodrelationship be-
ween members of the same family has been very
much exagerated; as a matter of fact the constitu-
tional similarity — which, though wrongly, is meant
when one speaks of bloodrelationship — can be very
slight between brothers and sisters, yes can be much
less even, than between individuals of much more
remote parentage.

As similarity of the individuals depends — abstrac-
tion made from external influences — on the constitu-
tion of the gametes which formed these individuals,
this, similarity will reach its maximum, e. g. become
identity, in case gametes of identical constitution mate,
irrespective of the source of these gametes, while
it is perfectly indifferent, whether such gametes of

138 THE EVOLUTION OF LIVING BEINGS.

identical constitution derive from hybrids or from spe-
cifically pure individuals.

Consequently, we may confidently expect to meet oc-
casionally very similar human individuals, who are
not af all related in the usual sense, and we all know
that this happens so frequently, that it is even said
that each man has his duplicate somewhere.

What value then have we to ascribe to the blood-
test of UHLENHUTH and NUTTALL?

No other, than revealing, perhaps, similarity of con-
stitution, which is quite another thing than relati-
onship.

As little as the result of testing for Cainchemistry,

revealing the presence of this element in the stalactites
of a grotto and in the substance of our bones, reveals
the derivation of the calcium in the stalactites‘and in our
bones from the same source — much less the deriva-
tion of the stalactites and of ourselves from the same
source — as little the results of the bloodtest reveal
relationship. Yet,in a sense, it is correct, that species are
velated to one another in a similar way as human bro-
_ thers or cousins are, viz im as much, as this expresses the
_ fact that both are segregates from a former cross.
. In the case of brothers and cousins, we can trace
their origin if there is a family register, if no such re-
gister of births exists, we can not; so that, if species are
really ,,related”’ in a similar way, it is, in the absence
of species-birth registers, a priory improbable that we
shall ever be able to reconstruct their phylogeny.

CHAPTER XV.
THE QUESTION OF PHYLOGENY.

As long as one accepted an uninterrupted progression
say from amoeba to man, it was justifiable to deduce
from the degree of similarity of a given form to the
amoeba, or to man, the place which this form ought to
occupy on the ladder leading from the amoeba to man.

Ofcourse, the stronger the belief in orthogenetic
evolution, the greater the degree of certainty with
which the place of a given form in phylogeny could be
determined.

Some such degree of certainty could be attained in
Lamarck’s evolutionary scheme, because this supposed
all progress to come about by response to necessity,
so that, if it was for-ordained that the amoeba should
better its constitution so as to reach that of man, there
was reason to suppose a rectilinear succession of types
also.

NAGELI’s Vervollkommnungstrieb allows a similar
degree ofcertainty. This idea of a rectilinear progression
was given up by Darwin, who saw the cause of progres-
sion in the selection of the best of a number of allsi-
ded variations, comparing the road along which phylo-
geny proceeded, to a branched tree, in which the posi-
tion, to be assigned to a given individual, became very
doubtfull.

Yet, as long as one believed similarity to bea measu-

140 THE EVOLUTION OF LIVING BEINGS.

re for the degree of relationship, optimistic natures
could continue to believe that arrangement of the
different types according to their degree of similarity,
would give a picture — be it a blurred one — of phy-
logeny.

Now we know that similarity, neither phenotypical
nor genotypical similarity, is a criterium for relation-
ship.

Sisters can be more dissimilar than two girls of quite
different extraction, and consequently similarity gives
us no measure whatever of relationship.

And as we know furthermore, that crossing has oc-
curred and does occur daily, that it produces many dif-
ferent types, from which but a few survive, after having
recrossed in alle directions, we know that evolution
’ proceeds forwards, sidewards and backwards, along
the meshes of a net, so that it is absolutely hopeless to
choose out of the many ways, in which one can draw
a broken line on such a netting, the one along which
evolution has proceeded.

Phylogeny e. g. reconstruction of what has happened in
the past, is no science but a product of phantastic specu-
lations which can be held but little in check by the geo-
logical record, on account of the incompleteness of the lat-
ter.

Those, who know that I have spent a considerable
part of my life in efforts to trace the phylogeny of the
vegetable kingdom, will know, that this is not writ-
ten down lightly; nobody cares to destroy his own
efforts.

Every author, who cares to be taken as a man of

THE EVOLUTION OF LIVING BEINGS. I4I

science, has however to expose his own errors in the
first place.

Adhering to ideas venerable by age, irrespective
of the possibility of proof, is the prerogative of reli-
gious creeds; science ruthlessly destroys what it no lon-
ger recognizes as well established.

There is something very dear to the human heart
in sticking to venerable traditions; it is even questio-
nable whether the upholding of illusions is not bet-
ter worth our while than the restless pursuit of the
truth, which forces us to destroy to day, what we built
up yesterday, but this is a philosophical question
which does not concern us here, science demands the
total eradication of convictions as soon as these are
recognised to be ill-founded.

Happily we but rarely err completely, and though
Phylogeny is untraceable and consequently, as said

above, a product of phantastic speculations, yet the ~

basic truth of it remains unimpaired, which is so well
expressed by Darwin in the conslusions to his origin:

»As all the living forms of life are the lineal descen-
,aants of those which lived long before the Cambrian
»epoch, we may feel certain that the ordinary succession
»oy generation has never once been broken.”

In this sense Phylogeny is a fact; it are our recon-
structions of the way in which it proceeds (which we
use to call Phylogeny) only, which are phantastic.

The recognition of this is, to my mind, a fact added
to our stock of knowledge, be it, in a sense, a negative
one.

CHAPTER XVI.
HOMOLOGOUS PARTS AND RUDIMENTARY ORGANS.

What is meant by homology?

Darwin expresses it as follows:

,, Lhe members of thesame class,independently oftheir
»habit of life resemble each other in the general plan
,of their organisation. This resemblance is often
expressed by the term ,,unity of type” or by saying
»that the several parts and organs in the different
»species of the class are homologous”

Hence homologous paris are parts occupying corres-
ponding positions in the general groundplan.

Therefore one is tempted to consider the corollae of
two flowers to be homologous, however different,
while bracts, however corolla-like, are certainly not
homologous with the petals which form the corolla,
but only analoga of these.

Now the determination of what is homologous and
what is analogous, consequently depends on the cer-
tainty with which we can determine the position
of a part in the general groundplan, and as the name
»general’” groundplan already indicates some un-
certainty in this respect, it is no wonder that one
meets in the literature with endless discussions as to
which parts are homologous and which analogous. It
was to call attention to this fact, that Isaid above, that
we are tempted to consider all corollae to be homolo-

THE EVOLUTION OF LIVING BEINGS. 143

gous, because we all know that it is exceedingly diffi-
cult to say, ina particular case, whether the place, occu-
pied by the corolla in the general groundplan, is the
rightfull place of the stamens, that of the calyx or e-
ven that between the outer whorl of stamens and the
calyx.

In the first case, such a corolla would be the homo-
logon of the stamens, in the second that of the calyx,
and in the third place, it would have no homologon at
all, but be a new formation, an intercalation in the
general groundplan.

The trouble in questions, like those of a general
groundplan, and of the character, the type, the essence
(wesentliche charactere) of a Linnean species, is that
we are unable to grasp all the characters of a group or
even of an individual, and so are led, unconsciously,
to consider the most conspicuous ones, as the essen-
tial ones. Gradually we find out, that among these
most conspicuous characters some are common to
more members of the group than others and as men are
great believers in the principle of majority (does not
a superior minister when he finds himself unable to
solve a difficult problem, appoint a commission of
M. P’s, probably each less capable than himself, and
accept their majority-report as decisive?) we are led to
take these characters in the majority to be the best, to
be the most essential, to constitute in one word the
groundplan. It ought to be plain to us, that this is
nonsense, that we have thus succeeded in construc-
ting a common groundplan only, by divesting the
plans of the different individuals from all what made

144 THE EVOLUTION OF LIVING BEINGS.

them different, and that we can do this just as well
with members of very different classes as with members
of the same class, by trimming their differences a little
more yet.

Itiseasy,in this way, to construct a general ground-
plan even between a sea-urchin and a hedgehog, by
abstraction of all the differences they show, until we
have nothing left than the one point they have in
common: the covering with spines and then claim
that the groundplan of Echinus and Erinaceus al-
lows us, to unite them to the group of the Spiniferae,
which is not very much worse than the group of the
Chordata as defined in the Handworterbuch of the
Naturwissenschaften II p. 623 as: ,,die durch den Be-
sitz einer Riickensaite, Chorda dorsalis, ausgezeichne-
ten Tiere: Tunicaten, Amphioxus und Vertebraten.”’

Man’s mind is a curious one, because, after having
thus divested the different individuals he sees in na-
ture, of all their differences, he assigns such immense
importance to the shabby rest thus obtained, that
he proceeds to explain the differences (he has just
succeeded in explaining away) by speculations about
the causes which could have changed this common
rest to all the different structures which are actually
presented by the different organisms, forgetting ent-
vely, that there is not the slightest reason to consider this
vest as ever having been transformed.

It is always the same mistake: looking for the (non
existing) single origin of different types.

So one reaches all kinds of attractive, but quite un-
founded, conclusions as f. i. that the flapper of a seal

THE EVOLUTION OF LIVING BEINGS. 145

is a metamorphosed hindleg of a landanimal, which
conclusion is about as well founded as that the door of
my house is a metamorphosis of the door of my neigh-
bour’s.

This may have happened, but need not have hap-
pened; my door may have been made from that of my
neighbour’s, by taking off something here and adding
perhaps something there, so as to fit my house, but it
may just as well, and much more probably has been,
constructed anew.

So why should we accept the flapper of a seal to
have been formed by the changing of the hindleg of
a land-animal? We know, that from a cross not one
type, but several types arise, so that there is no reason
whatever to consider all these different types to be
derived from the one with the simplest constitution;
we know on the contrary that it are just the compara-
tively simplest types, the recessive ones, which are en-
tirely unable to give rise directly to other ones.

A type giving rise to a large number of forms must
on the contrary be a complicated one, allowing segrega-
tion.

I hear it objected, that I am proceeding to do the
same I reproach to those I criticise, viz that Iam on
the way to explain away the differences which do
exist between different classes.

This is not my purpose; I only claim that the
»groundplan”’ is but a very general one, that we are
not justified to consider the simplified ,,groundplan’’,
obtained by cutting off all differences in structure
existing between the different organisms, as primitive,

10

146 THE EVOLUTION OF LIVING BEINGS.

as the one possessed by the common ancestor of those
organisms, and that consequently we have no reason
to suppose that each of the parts of the different orga-
nisms had already a position in that ancestralorganism,
through which we can spot the homologies of a given
part of an organism, now existing.

It remains perfectly true however, that the members °
of not too large a group resemble each other in their
general plan of organization.

Now this is stating a fact of the same kind as when
we say that the different forms resulting from a cross
between two Linneons resemble each other in the ge-
neral groundplan of their organisation. The cause
must lie — in the latter case at least, in the former we
do not even know whether the members of the group
belong genetically together — in the constitution of the
gametes which united to form the hybrid, which initi-
ated the group of new species, and as long as we know
nothing of the constitution of these gametes, we can
explain, as little how this comes about,as we can explain
why one chemical substance cristallizes in the one,
another in another form.

Now one hears it often said, that there is a basic dif-
ference between the structure of new Linneons, arising
from the cross of two preexisting ones, and between the
structure of a new class of organisms, supposedly also
arisen from the cross of two individuals — be it from
two more different ones — in as much as the general
plan of the new Linneons is the same as that of the
Linneons crossed, while the general plan of the new
class must, on the contrary, be different from that of

THE EVOLUTIONS OF LIVING BEINGS. I47

the class to which the crossed individuals belonged.

But this statement is not correct. The difference is
but one of degree. A cross between different Linneons
also gives rise to differently constituted types and to
consider these to be of the same general plan as the
Linneons crossed, is again only possible by abstracting
from the differences.

If we cross f. i. two white Sweetpeas — even such
belonging to the same Linneon — we may obtain
colored types, some of which have exclusively colored
descendants although they may segregate into many
differently colored types.

Now a group of such colored types has a different
»groundplan” from the uncolored types from whose
cross they arose; we are even able to say in what these
groundplans differ. They differ in so far as the color-
less types crossed, possess only one substance
of a certain class, either a chromogen or an
oxydase, while the colored groups, resulting from
their union possess both substances in their ,,ground-
plan.”

Now, mutatis mutandis, the skelettal groundplan of
the vertebrates may have arisen by the crossing of
two invertebrates, each of which possessed some of
the substances necessary to form askeleton, but lacked
some of the others, which combined by crossing these
two types.

And just as in the colored Sweetpeas mentioned
above, the groundplan ,,color’ remains, but is different
in the different segregates from some of these colored
types, so the groundplan of the skeleton may remain in

148 THE EVOLUTION OF LIVING BEINGS.

some of the descendants of the crossed invertebrates
— which descendants we then unite to the class of the
vertebrates — but be different in the segregates arisen
from them.

So the different forms of extremities we meet with, in
different vertebrates, can be explained in a similar
way as the different forms and colors of flowers, viz by
segregation, and those existing now, are the rests of pro-
bably many others which have existed formerly, as
becomes evident upon comparison of the extremities
of the now existing reptils with those of former geo-
logical periods.

The principle of selection holds good in as far as it
shows the result of the extermination by the struggle
for life; it shows us which forms could resist this, but
it is no principle which explains the origin of certain
types; selection also spells extermination, the types
last to be exterminated, obtaining the epitheton ornans:
selected types.

Consequently, we have to drop the idea of homology
in the sense of parts, occupying corresponding posi-
tions in the ancestral groundplan, but may continue
to use it, for convenience sake, cum grano salis, as in-
indicating corresponding positions in the general
groundplan, if we only keep in mind that such map-
ping outhasno other significancethan asa pons asinorum
for our memory and never allows us, with certainty, to
distinguish between analogous and homologous parts
or between these and new formations, as is shown by the
example of the corollae, mentioned at the beginning of
this chapter.

THE EVOLUTION OF LIVING BEINGS. I49

The question of rudimentary organs is of no special
significance to us; it only states the fact of the exis-
tence of organs which evidently lack something to be-
come completely developed, just as the white flowe-
red sweetpeas lack a substance, the presence of which
would allow them to develop color, and can therefore
be described as to be rudimentary colored.

When we look back on what has been said, it results
that it is very difficult to say in a given case whether
a form is primitive in its class or reduced, in other
words whether it is near to the original hybrid which
initiated the group, or further removed from it, because
as we have seen, a F, generation already can contain
types of very different constitution, so that the desig-
nation primitive or derived, looses much of its mea-
ning.

In a very general way however, we can say that

there is evidence of the non-primitiveness of most
of the simpler types, in this sense that the sim-
pler types usually are recessive segregates and
consequently unable to give rise to new forms, unless
crossed.
"It certainly speaks volumes for the genius of Char-
les Darwin, that although differing completely from
him in my opinion as to the way in which evolution
takes place, I can nevertheless conclude this chapter
with the very words with which he concluded his
corresponding chapter:

»Finally the several classes of facts which have been
,considered in this chapter, seem to me to proclaim so
»plainly, that the innumerable species, genera and

I50 THE EVOLUTION OF LIVING BEINGS

families with which this world is peopled are all des-
,cended, each within its own class or group from com-
»mon parents, and have all been modified in the course
,of descent, that I should without hesitation adopt
»this view, even if it were unsupported by other facts
,or arguments.”

CHAPTER XVII.

MIGRATION.

That the types put into the world by a cross, do not
always remain at the spot of their birth, but migrate, is
a generally known fact.

That they can do so in an astonishingly extensive
way is known also, and that it is done sometimes — fre-
quently perhaps even — by the intermediancy of man,
such as by travelling man himself, by his beasts of bur-
den or by his trains or vessels, of course, abstracts not a
iota from the fact of such migration existing, but only
justifies the conclusion that this kind of transport can-
not have existed in times before man appeared on our
globe.

It does not prove of course, that migration was less
extensive in former periods than now, because other
ways of migration then existed than now, f. i. land-
connections allowing intercourse between continents
now separated, more rainfall preventing the erection of
such formidable bars against migration as the Sahara-
desert, higher temperature of the sea, allowing ani-
mals and plants of the aequator to migrate further to-
wards the north and south, than is now possible, less
interference of man, by non-extermination of types
considered a nuisance by him etc. etc. So that we have
no measure for the relative degree of migration going

152 THE EVOLUTION OF LIVING BEINGS.

on in different geological periods; all we can say is that
migration formerly existed as it does now.

Yet migration has been exagerated in one respect, in
as much, as here also, as in former theories of evolution,
the dogma of the single origin reigned supreme.

So, if Primula acaulis is found in England and in the
Tirolese Alps, it is concluded that it must necessarily
have originated either in England or in the Tirolese
Alps or on a single third spot from where it has migra-
ted to the different countries it new occupies.

Now thiscreed was the consequence of the supposi-
tion that species arose by selection of favorable varia-
tions, and that, what was favorableat the onespot, could
not be expected to be favorable — at least not to the
same degree — at another spot, so that the result at
different spots could not be the same e. g. that the pro-
duction of identical species at different spots, was well
nigh impossible. This objection — in itself not well
tenable because during this selective process the ances-
tral species itself does not remain stationary, but also
migrates — of course falls away entirely if species are
not the result of a selective accumulation of favorable
variations, but are born, ready made,asa result ofacross
and left to try to find a place fit to support them, or...
perish.

If we dwell a moment on the fact, that identical
gametes, irrespective of their source, must give the sa-
me kind of zygote, irrespective of the spot where the
mating occured, it follows from the theory of the origin
ofnewtypesbycrossing ,thatidenticaltypescan originate
at different spots, that species can arise polytopically.

THE EVOLUTION OF LIVING BEINGS. a Oe 3

This has long been conceded for ,,varieties”’ ; it was
BrIQvuET who first claimed its validity for species and he
is in my opinion perfectly right.

This way ofa polytopic origin of identical types, ma-
kes it possible to divest migration of many very un-
certain attempts to explain the present occurrence of
identical types in very far distant places, but does not
tend to make the present geographical distribution of
animals and plants more easy of explanation in detail
than it formerly was.

Geographical distribution is, just like evolution in
the past, a historical problem which can never be
reconstructed completely, but from which we can de-
rive no more than the very general underlying prin-
ciples.

CHAPTER XVIII.

GEOLOGY AND THE CONSTANCY OF SPECIES.

The selection-theory, as well as the theories based on
inheritance of acquired characters, must suppose that
variability, will it have any effect, must occur more or
less continuously, that in other words organisms can ’t
survive very long without showing variation. It is even
inconceivable, on the ground of these theories, that the
groups considered by them to be primitive, say like the
Flagellates, should yet remain in existence, notwith-
standing all the changes which the earth has under-
gone must have been inductive to their varying. This
applies even, though in a lesser degree, to the mutation
theory, the only conception able to explain the unchan-
ged continuation of a homozygotic type through long
ages, is the conception that a species is constant, unless
it happens to cross with another one.

Now what does the geoloigcal record teach us to
this effect ? Let us quote here what GRAND’ EvEy than
whom no one, has greater experience in this matter,
says:

»Un fait notoire domine tous les autres, la permanence
des espéces durant la majorité ou la presque totalité de
leur ,,existence’’.

Venu il y a 25 ans a St. Etienne avec l’idée contraire
que les espéces ont varié d’une maniére continue, D.

THE EVOLUTION OF LIVING BEINGS. ES5

StuR me conseilla de m’en assurer sur le terrain. Au
lieu de cela, dans des dépéts interrompus, qui auraient
conservé les formes gradiées d’espéces variables je n’ai
rien rencontré que les débris d’espéces constantes; a
l’appui de ce dire il me serait facile de citer plus de dix
espéces communes aux deux grands bassins houilliers
francais, plus de dix espéces immuables de la base au
sommet du bassin de la Loire et plus de dix autres trans-
versant sans changer la moitié supérieure de ce bassin”’
If we consider further that M. Grand’ Eury’s ex-
perience shows that ,,les genres les plus naturelles com-
mencent par d’espéces peu distinctes et mélangées et
une fois fixées se séparent et ne changent plus’’, we can
only say that this agrees astonishingly well with what
happens after a cross, and that thus geology gives
strong support to the conception that crossing is the
underlying cause of the origin of new types.

CHAPTER XIX.

CONCLUSIONS FROM THE BEHAVIOUR OF DIPLOID
ORGANISMS.

New forms arise as the result of a cross; they can
gradually become specifically pure by segregation if
self-fertilization prevails.

Once pure, they perpetuate themselves by heredity
and constitute a species.

Several of such species are united by systematists to
Linneons. A Linneon of this kind e. g. a Linneon consist-
ing of habitual selffertilizers, gradually looses species by
extermination through the struggle for life, which pro-
cess may result in thesurvival of onespecies within such
a Linneon only, which species is then called selected.

Selection therefore spells: extermination.

Such a Linneon, reduced to one species, becomes sy-
nonymous with species and can withstand changes of
conditions only, by non-transmittable plasticity, which
is frequently called adaptability.

The usual idea that a species can survive by adapting
itself to changed conditions by transmittable variabili-
ty, isnot only wrong, because such transmittable varia-
bility does not exist, but also because the result of such
a process would be the creation of new species and
consequently not assure the survival of the old one,
but its replacement by others.

THE EVOLUTION OF LIVING BEINGS. 157

A Linneon of the class here treated, once having be-
come monospecific, must sooner or later be extermina-
ted without leaving any descendant, unless it crosses
with a form belonging to another Linneon. Such a pro-
geny of course is not identical with the parent-spe-
cies, but gives rise to new species.

The conclusion therefore is: sfecies arise by crossing,
perpetuate themselves by heredity and are gradually ex-
terminated by the struggle for life, those last exterminated
obtaining the epitheton ornans: selected ones.

The result of a cross consequently can lead to the for-
mation ofnew species but need not do so;italways
must lead to the production of new forms.

If however such new forms continue to intercross,
no new species will ever be formed; usually however
crossing is not so promiscuous as all that, and new spe-
cies do arise by segregation; to such applies what has
been said above.

Among the other new forms, promiscuous crossing
sooner or later is limited also to intercrossing within
certain groups, between which bars against intercros-
sing are erected by isolation, aversion or sterility.

Such smaller intercrossing communities are also uni-
ted by systematists to Linneons.

They can resist untoward circumstances much bet-
ter than Linneons, consisting of strict selffertilizers,
because they have a far greater degree of plasticity
through the fact, that the intercrossing of the forms wi-
thin them, gives rise continually to the birth of new
forms, offering new material, resistant to the extermi-
nating effect of the struggle for life.

158 THE EVOLUTION OF LIVING BEINGS.

As long as the new forms, resulting from such inter-
crossing, do not transgress the border of the Linneon,
we can say, with justice, that such a Linneon maintains
itself by adaptation, although of course the adapted
Linneon is then internally changed e. g. is a group of
types of other constitutions, than the ones it previously
contained.

In such a Linneon also, the struggle for life gradually
plays havoc because, by extermination of certain types,
it gradually reduces the scope of crossing and conse-
quently of adaptability. The struggle for life usually
results here, first in the selection of the dominant type
which can never proceed so far that pure dominants
only survive, because the hybrids with the dominant
characters enjoy the same advantages as the constitu-
tionally pure dominants in the struggle for life

Iftherefore, the dominant form is selected, this is of
great advantage to such a Linneon, because then it
continues to contain the recessive forms — be it cry-
ptomerously — and consequently can call on them in
case of emergency. As however such a call is hampered
by the fact, that such recessives can come into being
only, if two impure dominants happen to mate, even
the selection of the dominants must in the long run
lead to extinction. Such extinction will however come
about much quicker, if the struggle for life resultsin the
selection of the recessives, because then the Linneon
finally becomes really mono-specific and so must
perish sooner or later, exactly like the self-fertilising
Linneons treated off above.

THE EVOLUTION OF LIVING BEINGS. 159

The conclusion therefore is: crossing of allogamous
forms leads to the production of new forms, most of which
sooner or later fall into separate non-intercrossing groups,
each of which however consists of different intercrossing
forms. Such groups are called Linneons.

Such Linneons can adapt themselves to changed cir-
cumstances by giving birth to new forms by crossing
the different types they contain. These new forms do
not transgress the limits of the Linneon and are gradually
also exterminated by the struggle for life, which ends in
the selection, either of a dominant hybrid type tll fitted to
give rise to new-forms, or of a recessive type entirely
unable to form new forms.

Such Linneons consequently also, are gradually exter-
minated by the struggle for life, and here also the forms
within tt, last exterminated, obtain the epitheton ornans:
selected ones.

We can express this shortly by saying:

Linneons arise by crossing and are gradually extermi-
nated by the struggle for life, the last surviving ones ob-
taining the epitheton ornans: selected ones.

All this is probably parallelled, en grand,inthe appea-
vance and extinction of the great classes. A cross between
two greatly differing gametes gives rise to a great diversity
of new forms which we group into families, genera and
Linneons, to all of which applies what has been said
above of the Linneons: gradual dimunttion of the oppor-
tunities of crossing and consequently of the birth of new
types, able to withstand changed conditions. This must

160 THE EVOLUTION OF LIVING BEINGS. '

finally lead to extinction of the classes also. There is
consequently periodicity in the production of new
types, nomatter whether these are so different that we
put them into different classes, or solittle different that
we put them into different Linneons only.

Such a period of production of new types is always
immediately followed by the beginning of extinction
through the struggle for life, which is withstood as long
as crossing remains possible, and becomes complete
sooner or later, after the last possibility of a cross has

disappeared.
Selection is only ,,une belle phrase”’ for extinction, the
forms last exterminated, being called the selected ones.

Adaptation has a double meaning: adaptation of Lin-
neons to changed conditions by changing their internal
composition through the production of new types as the
result of crossing, which do not transgress the limits the
Linneon and adaptation of individuals by the plasticity
which every individual enjoys, within limits, to respond
to the call of new necessities.

The vera causa of the production of new types conse-
quently is: crossing; the vera causa of their extinction:
the struggle for life; the selection resulting from the latters
is by no means a revival, but is the sign of struggle of the
doomed.

The production of new classes can evidently only be
studied and demonstrated experimentally in a period of
production of such classes; in a period, as the present one,
we must be content with the demonstration of the origin of

THE EVOLUTION OF LIVING BEINGS. 161

newLinneons, and must conclude by analogy to the way by
which new classes originated.

I should like to finish this chapter by calling once
more attention to one of the chief modern mistakes,
which, according to my view, has been made in the in-
terpretation of evolutionary facts. This is: that one has
looked for the cause of the origin of the different types
within a Linneon exclusively within the limits of that
Linneon, and so has been led to conclude, in most cases,
that the most common wild form within that Linneon,
was the ancestral one.

So BATESON says, after crossing differently constitu-
ted white flowered types of the Linneon, Lathyrus
odoratus:

,», When F, was grown however, it was clear that here
was ,,a remarkable opportunity of studyng areversion
in colour, due to crossing, for these plants instead of
being white were purple, like the wild Sicilian plant
{from which our cultivated sweet peas are descended.”

And in his presidential Australian address he says
about this same point:

In spite of repeated trials no one has yet succeeded
yin crossing the sweet Pea with other leguminous
»species. We know that early in its cultivated history
it produced at least two marked varieties which I can
,»only conceive as spontaneously arising, though no
doubt, the profusion of forms we now have, was made
»by the crossing of those original varieties’.

Now why accept another origin e. g.spontaneously
arising for these two ,,original”’ varieties than for those
arisen later?

reat

*

162 THE EVOLUTION OF LIVING BEINGS.

My contention is that they were segregates from im-
pure purples, indistinctible from pure purples, which
grew intermixed with the pure purples in Sicily, and
which revealed their impure nature after isolation in
domestication, just as my impure brown-yellow wall-
flowers segregate, ifisolated and selfed: violets, golden-
yellows and whites.

The real origin of the different types which we unite to a
Linneon lies not inside that Linneon — although part of
them arose from secondary intra-linneontic segregation
— but lies further back in the cross of two individuals,
belonging to different other pre-existing Linneons.

CHAPLER XX.

HAPLOID ORGANISMS AND MUTATION.

For the investigation of the existence or non-exis-
tence of mutation, no group of plants offers better op-
portunities than mosses.

Bacteria are all too uncertain for such investiga-
tions on account of the possibility of contamination,
especially if great care is not taken to start with abso-
lute certaintly from a single cell. The delution-method,
so frequently ressorted to, never gives absolute cer-
tainty in this respect.

Organisms with multinucleate cells are never safe
objects because BURGEFF has shown, in his most interes-
ting investigations on the results of crossing different
types of Phycomyces that such cells may be hetero-
caryotic e. g. may contain nuclei of different constitu-
tions, so that with such polyenergid organisms one can
at best, obtain the same relative certainty for their
specific purity, as is obtainable in the case of diploid
organisms, but no greater certainty.

_ But mosses offer very much better opportunities for
the final settlement of the vexed mutation-question.

As each mossplant arises from a single gamete, it can
not bean inter-gametic hybrid as diploid organisms so
frequently are. Leaving for the present the question
of the possibility of intragametic hybrids aside, we

|

164 THE EVOLUTION OF LIVING BEINGS.

can therefore say that, if it could be proved that a moss
plant were able to produce more than one kind of ga-
metes, mutation would have been proved in so far at
least, as we would be justified to conclude, fromsucha
result, that a monogametic organism can become poly-
gametic without the direct interference of a cross.

Experiments with mosses, from which of course
hybrid diploid generations can be obtained, viz hybrid
capsules, are therefore highly advisable, also because
they can throw light on the question of gametic purity,
the nature of every separate gamete here becoming vi-
sible in the mossplant, which arises from it, whilein
diploid organisms we can see but the effect of the in-
teraction of two gametes.

CHAPTER XXI.

DIPLOID ORGANISMS AND MUTATION.

We have given our reasons for being sceptical as to
the existence of mutations.

This scepsis is partly based on fact, partly on cir-
cumstantial evidence. The fact is, that we possess no
means to establish complete specific purity experimen-
tally, and consequently are never sure of the purity of
our material.

Certainty of purity however 1s a condttio sine qua non
to obtain proof of the existence of mutation in living
beings, just as chemical purity is a conditio sine qua non
to obtain proof of the existence of mutability of the ele-
ments.

The circumstantial evidence is, that allso called mu-
tants are recessives e. g. arise in material in which
impure individuals are indistinctible at sight from
pure ones, which makes it very probable that the aber-
rant individuals were no mutants at all, butsegregates
from heterozygotes, indistinctible from the pure do-
minants.

Circumstantial evidence always contains an element
of uncertainty and the evidence from the recessives is
no exception to the rule.

The fact that the ,.mutants” are always recessives is
namely no proof that they are segregates.

166 THE EVOLUTION OF LIVING BEINGS.

If the presence- and absence-hypothesis holds good,
they may really be mutants, arisen by a loss of a gen
or factor, as a result of faulty inheritance e. g.,
after our present conceptions, as a result of accidental
irregularities in karyokinesis, but even if this were the
case, such an occurence would not materially assist the
mutation theory because evolution by a process of re-
peated losses is inconceivable.

The mutation theory requires proof of the existence

/ of progressive mutants e. g. proof that dominants can

arise from recessives.

As we have seen, there is not a particle of proof for
such an occurence. It need hardly be added, that it is
not sufficient to prove the occurence of a dominant in a
bed of recessives, but that it is necessary to prove that
the dominant arose from a recessive.

The mere presence of a dominant in a bed of reces-
sives, proves as little that it arose from a recessiveas
the presence of a cuckoo’s egg in the nest of a sparrow
proves that this egg arose from a sparrow’s egg.

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