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Darwin's work has the property of greatness
in that it may be admired from more aspects
than one. For some the perception of the
principle of Natural Selection stands out
as his most wonderful achievement to which
all the rest is subordinate. Others, among
whom I would range myself, look up to him
rather as the first who plainly distinguished,
collected, and comprehensively studied that
new class of evidence from which hereafter
a true understanding of the process of Evolution
may be developed. We each prefer our own
standpoint of admiration; but I think that
it will be in their wider aspect that his
labours will most command the veneration
of posterity.
A treatise written to advance knowledge may
be read in two moods. The reader may keep
his mind passive, willing merely to receive
the impress of the writer's thought; or he
may read with his attention strained and
alert, asking at every instant how the new
knowledge can be used in a further advance,
watching continually for fresh footholds
by which to climb higher still. Of Shelley
it has been said that he was a poet for poets:
so Darwin was a naturalist for naturalists.
It is when his writings are used in the critical
and more exacting spirit with which we test
the outfit for our own enterprise that we
learn their full value and strength.
Whether we glance back and compare his performance
with the efforts of his predecessors, or
look forward along the course which modern
research is disclosing, we shall honour most
in him not the rounded merit of finite accomplishment,
but the creative power by which he inaugurated
a line of discovery endless in variety and
extension. Let us attempt thus to see his
work in true perspective between the past
from which it grew, and the present which
is its consequence.
Darwin attacked the problem of Evolution
by reference to facts of three classes:
Variation;
Heredity;
Natural Selection.
His work was not as the laity suppose, a
sudden and unheralded revelation, but the
first fruit of a long and hitherto barren
controversy. The occurrence of variation
from type, and the hereditary transmission
of such variation had of course been long
familiar to practical men, and inferences
as to the possible bearing of those phenomena
on the nature of specific difference had
been from time to time drawn by naturalists.
Maupertuis, for example, wrote "Ce qui
nous reste a examiner, c'est comment d'un
seul individu, il a pu naitre tant d'especes
si differentes." And again "La
Nature contient le fonds de toutes ces varietes:
mais le hasard ou l'art les mettent en oeuvre.
C'est ainsi que ceux dont l'industrie s'applique
a satisfaire le gout des curieux, sont, pour
ainsi dire, creatures d'especes nouvelles."
{Footnote: "Venus Physique, contenant
deux Dissertations, l'une sur l'origine des
Hommes et des Animaux: Et l'autre sur l'origine
des Noirs" La Haye, 1746, pages 124
and 129. For an introduction to the writings
of Maupertuis I am indebted to an article
by Professor Lovejoy in "Popular Sci.
Monthly", 1902.}
Such passages, of which many (though few
so emphatic) can be found in eighteenth century
writers, indicate a true perception of the
mode of Evolution. The speculations hinted
at by Buffon, {Footnote: For the fullest
account of the views of these pioneers of
Evolution, see the works of Samuel Butler,
especially "Evolution, Old and New"
(2nd edition) 1882. Butler's claims on behalf
of Buffon have met with some acceptance;
but after reading what Butler has said, and
a considerable part of Buffon's own works,
the word "hinted" seems to me a
sufficiently correct description of the part
he played. It is interesting to note that
in the chapter on the Ass, which contains
some of his evolutionary passages, there
is a reference to "plusieurs idees tres-elevees
sur la generation" contained in the
Letters of Maupertuis.} developed by Erasmus
Darwin, and independently proclaimed above
all by Lamarck, gave to the doctrine of descent
a wide renown. The uniformitarian teaching
which Lyell deduced from geological observation
had gained acceptance. The facts of geographical
distribution {Footnote: See especially W.
Lawrence, "Lectures on Physiology",
London, 1823, pages 213 f.} had been shown
to be obviously inconsistent with the Mosaic
legend. Prichard, and Lawrence, following
the example of Blumenbach, had successfully
demonstrated that the races of Man could
be regarded as different forms of one species,
contrary to the opinion up till then received.
These treatises all begin, it is true, with
a profound obeisance to the sons of Noah,
but that performed, they continue on strictly
modern lines. The question of the mutability
of species was thus prominently raised.
Those who rate Lamarck no higher than did
Huxley in his contemptuous phrase "buccinator
tantum," will scarcely deny that the
sound of the trumpet had carried far, or
that its note was clear. If then there were
few who had already turned to evolution with
positive conviction, all scientific men must
at least have known that such views had been
promulgated; and many must, as Huxley says,
have taken up his own position of "critical
expectancy." {Long Footnote: See the
chapter contributed to the "Life and
Letters of Charles Darwin" II. page
195. I do not clearly understand the sense
in which Darwin wrote
(Autobiography, ibid. I. page 87):
"It has sometimes been said that the
success of the "Origin" proved
'that the subject was in the air,' or 'that
men's minds were prepared for it.' I do not
think that this is strictly true, for I occasionally
sounded not a few naturalists, and never
happened to come across a single one who
seemed to doubt about the permanence of species."
This experience may perhaps have been an
accident due to Darwin's isolation. The literature
of the period abounds with indications of
"critical expectancy." A most interesting
expression of that feeling is given in the
charming account of the "Early Days
of Darwinism" by Alfred Newton, "Macmillan's
Magazine", LVII. 1888, page 241. He
tells how in 1858 when spending a dreary
summer in Iceland, he and his friend, the
ornithologist John Wolley, in default of
active occupation, spent their days in discussion.
"Both of us taking a keen interest in
Natural History, it was but reasonable that
a question, which in those days was always
coming up wherever two or more naturalists
were gathered together, should be continually
recurring. That question was, 'What is a
species?' and connected therewith was the
other question, 'How did a species begin?'...
Now we were of course fairly well acquainted
with what had been published on these subjects."
He then enumerates some of these publications,
mentioning among others T. Vernon Wollaston's
"Variation of Species"--a work
which has in my opinion never been adequately
appreciated. He proceeds:
"Of course we never arrived at anything
like a solution of these problems, general
or special, but we felt very strongly that
a solution ought to be found, and that quickly,
if the study of Botany and Zoology was to
make any great advance." He then describes
how on his return home he received the famous
number of the "Linnean Journal"
on a certain evening.
"I sat up late that night to read it;
and never shall I forget the impression it
made upon me. Herein was contained a perfectly
simple solution of all the difficulties which
had been troubling me for months past...
I went to bed satisfied that a solution had
been found." But he knew Herbert. Trans.
Hort. Soc. IV, 1819, pp. 16-
17. Also see Samuel Butler, A Memoir, I.
p. 165.} Why, then, was it, that Darwin succeeded
where the rest had failed? The cause of that
success was two-fold. First, and obviously,
in the principle of Natural Selection he
had a suggestion which would work. It might
not go the whole way, but it was true as
far as it went. Evolution could thus in great
measure be fairly represented as a consequence
of demonstrable processes. Darwin seldom
endangers the mechanism he devised by putting
on it strains much greater than it can bear.
He at least was under no illusion as to the
omnipotence of Selection; and he introduces
none of the forced pleading which in recent
years has threatened to discredit that principle.
For example, in the latest text of the "Origin"
{Footnote: "Origin", (6th edition
(1882), page 421.} we find him saying:
"But as my conclusions have lately been
much misrepresented, and it has been stated
that I attribute the modification of species
exclusively to natural selection, I may be
permitted to remark that in the first edition
of this work, and subsequently, I placed
in a most conspicuous position-- namely,
at the close of the Introduction--the following
words: 'I am convinced that natural selection
has been the main but not the exclusive means
of modification.'"
But apart from the invention of this reasonable
hypothesis, which may well, as Huxley estimated,
"be the guide of biological and psychological
speculation for the next three or four generations,"
Darwin made a more significant and imperishable
contribution. Not for a few generations,
but through all ages he should be remembered
as the first who showed clearly that the
problems of Heredity and Variation are soluble
by observation, and laid down the course
by which we must proceed to their solution.
{Footnote: Whatever be our estimate of the
importance of Natural Selection, in this
we all agree. Samuel Butler, the most brilliant,
and by far the most interesting of Darwin's
opponents -- whose works are at length emerging
from oblivion--in his Preface (1882) to the
2nd edition of "Evolution, Old and New",
repeats his earlier expression of homage
to one whom he had come to regard as an enemy:
"To the end of time, if the question
be asked, 'Who taught people to believe in
Evolution?' the answer must be that it was
Mr. Darwin. This is true, and it is hard
to see what palm of higher praise can be
awarded to any philosopher."} The moment
of inspiration did not come with the reading
of Malthus, but with the opening of the "first
note-book on Transmutation of Species."
{Footnote: "Life and Letters",
I. pages 276 and 83.} Evolution is a process
of Variation and Heredity. The older writers,
though they had some vague idea that it must
be so, did not study Variation and Heredity.
Darwin did, and so begat not a theory, but
a science.
The extent to which this is true, the scientific
world is only beginning to realise. So little
was the fact appreciated in Darwin's own
time that the success of his writings was
followed by an almost total cessation of
work in that special field. Of the causes
which led to this remarkable consequence
I have spoken elsewhere. They proceeded from
circumstances peculiar to the time; but whatever
the causes there is no doubt that this statement
of the result is historically exact, and
those who make it their business to collect
facts elucidating the physiology of Heredity
and Variation are well aware that they will
find little to reward their quest in the
leading scientific Journals of the Darwinian
epoch.
In those thirty years the original stock
of evidence current and in circulation even
underwent a process of attrition. As in the
story of the Eastern sage who first wrote
the collected learning of the universe for
his sons in a thousand volumes, and by successive
compression and burning reduced them to one,
and from this by further burning distilled
the single ejaculation of the Faith, "There
is no god but God and Mohamed is the Prophet
of God," which was all his maturer wisdom
deemed essential:--so in the books of that
period do we find the corpus of genetic knowledge
dwindle to a few prerogative instances, and
these at last to the brief formula of an
unquestioned creed.
And yet in all else that concerns biological
science this period was, in very truth, our
Golden Age, when the natural history of the
earth was explored as never before; morphology
and embryology were exhaustively ransacked;
the physiology of plants and animals began
to rival chemistry and physics in precision
of method and in the rapidity of its advances;
and the foundations of pathology were laid.
In contrast with this immense activity elsewhere
the neglect which befell the special physiology
of Descent, or Genetics as we now call it,
is astonishing. This may of course be interpreted
as meaning that the favoured studies seemed
to promise a quicker return for effort, but
it would be more true to say that those who
chose these other pursuits did so without
making any such comparison; for the idea
that the physiology of Heredity and Variation
was a coherent science, offering possibilities
of extraordinary discovery, was not present
to their minds at all. In a word, the existence
of such a science was well nigh forgotten.
It is true that in ancillary periodicals,
as for example those that treat of entomology
or horticulture, or in the writings of the
already isolated systematists, {Footnote:
This isolation of the systematists is the
one most melancholy sequela of Darwinism.
It seems an irony that we should read in
the peroration to the "Origin"
that when the Darwinian view is accepted
"Systematists will be able to pursue
their labours as at present; but they will
not be incessantly haunted by the shadowy
doubt whether this or that form be a true
species. This, I feel sure, and I speak after
experience, will be no slight relief. The
endless disputes whether or not some fifty
species of British brambles are good species
will cease." "Origin", 6th
edition
(1882), page 425. True they have ceased to
attract the attention of those who lead opinion,
but anyone who will turn to the literature
of systematics will find that they have not
ceased in any other sense. Should there not
be something disquieting in the fact that
among the workers who come most into contact
with specific differences, are to be found
the only men who have failed to be persuaded
of the unreality of those differences?} observations
with this special bearing were from time
to time related, but the class of fact on
which Darwin built his conceptions of Heredity
and Variation was not seen in the highways
of biology. It formed no part of the official
curriculum of biological students, and found
no place among the subjects which their teachers
were investigating.
During this period nevertheless one distinct
advance was made, that with which Weismann's
name is prominently connected. In Darwin's
genetic scheme the hereditary transmission
of parental experience and its consequences
played a considerable role. Exactly how great
that role was supposed to be, he with his
habitual caution refrained from specifying,
for the sufficient reason that he did not
know. Nevertheless much of the process of
Evolution, especially that by which organs
have become degenerate and rudimentary, was
certainly attributed by Darwin to such inheritance,
though since belief in the inheritance of
acquired characters fell into disrepute,
the fact has been a good deal overlooked.
The "Origin" without "use
and disuse" would be a materially different
book. A certain vacillation is discernible
in Darwin's utterances on this question,
and the fact gave to the astute Butler an
opportunity for his most telling attack.
The discussion which best illustrates the
genetic views of the period arose in regard
to the production of the rudimentary condition
of the wings of many beetles in the Madeira
group of islands, and by comparing passages
from the "Origin" {Footnote:
6th edition pages 109 and 401. See Butler,
"Essays on Life, Art, and Science",
page 265, reprinted 1908, and "Evolution,
Old and New", chapter XXII. (2nd edition),
1882.} Butler convicts Darwin of saying first
that this condition was in the main the result
of Selection, with disuse aiding, and in
another place that the main cause of degeneration
was disuse, but that Selection had aided.
To Darwin however I think the point would
have seemed one of dialectics merely. To
him the one paramount purpose was to show
that somehow an Evolution by means of Variation
and Heredity might have brought about the
facts observed, and whether they had come
to pass in the one way or the other was a
matter of subordinate concern.
To us moderns the question at issue has a
diminished significance. For over all such
debates a change has been brought by Weismann's
challenge for evidence that use and disuse
have any transmitted effects at all. Hitherto
the transmission of many acquired characteristics
had seemed to most naturalists so obvious
as not to call for demonstration. {Footnote:
W. Lawrence was one of the few who consistently
maintained the contrary opinion. Prichard,
who previously had expressed himself in the
same sense, does not, I believe repeat these
views in his later writings, and there are
signs that he came to believe in the transmission
of acquired habits. See Lawrence, "Lect.
Physiol." 1823, pages 436-437, 447 Prichard,
Edin. Inaug. Disp. 1808 (not seen by me),
quoted ibid. and "Nat. Hist. Man",
1843, pages 34 f. See also Godron passim}
Weismann's demand for facts in support of
the main proposition revealed at once that
none having real cogency could be produced.
The time-honoured examples were easily shown
to be capable of different explanations.
A few certainly remain which cannot be so
summarily dismissed, but -- though it is
manifestly impossible here to do justice
to such a subject -- I think no one will
dispute that these residual and doubtful
phenomena, whatever be their true nature,
are not of a kind to help us much in the
interpretation of any of those complex cases
of adaptation which on the hypothesis of
unguided Natural Selection are especially
difficult to understand. Use and disuse were
invoked expressly to help us over these hard
places; but whatever changes can be induced
in offspring by direct treatment of the parents,
they are not of a kind to encourage hope
of real assistance from that quarter. It
is not to be denied that through the collapse
of this second line of argument the Selection
hypothesis has had to take an increased and
perilous burden. Various ways of meeting
the difficulty have been proposed, but these
mostly resolve themselves into improbable
attempts to expand or magnify the powers
of Natural Selection.
Weismann's interpellation, though negative
in purpose, has had a lasting and beneficial
effect, for through his thorough demolition
of the old loose and distracting notions
of inherited experience, the ground has been
cleared for the construction of a true knowledge
of heredity based on experimental fact.
In another way he made a contribution of
a more positive character, for his elaborate
speculations as to the genetic meaning of
cytological appearances have led to a minute
investigation of the visible phenomena occurring
in those divisions by which germ-cells arise.
Though the particular views he advocated
have very largely proved incompatible with
the observed facts of heredity, yet we must
acknowledge that it was chiefly through the
stimulus of Weismann's ideas that those advances
in cytology were made; and though the doctrine
of the continuity of germ-plasm cannot be
maintained in the form originally propounded,
it is in the main true and illuminating.
{Footnote: It is interesting to see how nearly
Butler was led by natural penetration, and
from absolutely opposite conclusions, back
to this underlying truth: "So that each
ovum when impregnate should be considered
not as descended from its ancestors, but
as being a continuation of the personality
of every ovum in the chain of its ancestry,
which every ovum it actually is quite as
truly as the octogenarian is the same identity
with the ovum from which he has been developed.
This process cannot stop short of the primordial
cell, which again will probably turn out
to be but a brief resting-place. We therefore
prove each one of us to be actually the primordial
cell which never died nor dies, but has differentiated
itself into the life of the world, all living
beings whatever, being one with it and members
one of another," "Life and Habit",
1878, page 86.}
Nevertheless in the present state of knowledge
we are still as a rule quite unable to connect
cytological appearances with any genetic
consequence and save in one respect (obviously
of extreme importance -- to be spoken of
later) the two sets of phenomena might, for
all we can see, be entirely distinct.
I cannot avoid attaching importance to this
want of connection between the nuclear phenomena
and the features of bodily organisation.
All attempts to investigate Heredity by cytological
means lie under the disadvantage that it
is the nuclear changes which can alone be
effectively observed. Important as they must
surely be, I have never been persuaded that
the rest of the cell counts for nothing.
What we know of the behaviour and variability
of chromosomes seems in my opinion quite
incompatible with the belief that they alone
govern form, and are the sole agents responsible
in heredity. {Footnote: This view is no doubt
contrary to the received opinion. I am however
interested to see it lately maintained by
Driesch ("Science and Philosophy of
the Organism", London, 1907, page 233),
and from the recent observations of Godlewski
it has received distinct experimental support.}
If, then, progress was to be made in Genetics,
work of a different kind was required. To
learn the laws of Heredity and Variation
there is no other way than that which Darwin
himself followed, the direct examination
of the phenomena. A beginning could be made
by collecting fortuitous observations of
this class, which have often thrown a suggestive
light, but such evidence can be at best but
superficial and some more penetrating instrument
of research is required. This can only be
provided by actual experiments in breeding.
The truth of these general considerations
was becoming gradually clear to many of us
when in 1900 Mendel's work was rediscovered.
Segregation, a phenomenon of the utmost novelty,
was thus revealed. From that moment not only
in the problem of the origin of species,
but in all the great problems of biology
a new era began. So unexpected was the discovery
that many naturalists were convinced it was
untrue, and at once proclaimed Mendel's conclusions
as either altogether mistaken, or if true,
of very limited application. Many fantastic
notions about the workings of Heredity had
been asserted as general principles before:
this was probably only another fancy of the
same class.
Nevertheless those who had a preliminary
acquaintance with the facts of Variation
were not wholly unprepared for some such
revelation. The essential deduction from
the discovery of segregation was that the
characters of living things are dependent
on the presence of definite elements or factors,
which are treated as units in the processes
of Heredity. These factors can thus be recombined
in various ways. They act sometimes separately,
and sometimes they interact in conjunction
with each other, producing their various
effects. All this indicates a definiteness
and specific order in heredity, and therefore
in variation. This order cannot by the nature
of the case be dependent on Natural Selection
for its existence, but must be a consequence
of the fundamental chemical and physical
nature of living things. The study of Variation
had from the first shown that an orderliness
of this kind was present. The bodies and
the properties of living things are cosmic,
not chaotic. No matter how low in the scale
we go, never do we find the slightest hint
of a diminution in that all-pervading orderliness,
nor can we conceive an organism existing
for a moment in any other state. Moreover
not only does this order prevail in normal
forms, but again and again it is to be seen
in newly-sprung varieties, which by general
consent cannot have been subjected to a prolonged
Selection. The discovery of Mendelian elements
admirably coincided with and at once gave
a rationale of these facts. Genetic Variation
is then primarily the consequence of additions
to, or omissions from, the stock of elements
which the species contains. The further investigation
of the species-problem must thus proceed
by the analytical method which breeding experiments
provide.
In the nine years which have elapsed since
Mendel's clue became generally known, progress
has been rapid. We now understand the process
by which a polymorphic race maintains its
polymorphism. When a family consists of dissimilar
members, given the numerical proportions
in which these members are occurring, we
can represent their composition symbolically
and state what types can be transmitted by
the various members. The difficulty of the
"swamping effects of intercrossing"
is practically at an end. Even the famous
puzzle of sex-limited inheritance is solved,
at all events in its more regular manifestations,
and we know now how it is brought about that
the normal sisters of a colour-blind man
can transmit the colour-blindness while his
normal brothers cannot transmit it.
We are still only on the fringe of the inquiry.
It can be seen extending and ramifying in
many directions. To enumerate these here
would be impossible. A whole new range of
possibilities is being brought into view
by study of the interrelations between the
simple factors. By following up the evidence
as to segregation, indications have been
obtained which can only be interpreted as
meaning that when many factors are being
simultaneously redistributed among the germ-cells,
certain of them exert what must be described
as a repulsion upon other factors. We cannot
surmise whither this discovery may lead.
In the new light all the old problems wear
a fresh aspect. Upon the question of the
nature of Sex, for example, the bearing of
Mendelian evidence is close. Elsewhere I
have shown that from several sets of parallel
experiments the conclusion is almost forced
upon us that, in the types investigated,
of the two sexes the female is to be regarded
as heterozygous in sex, containing one unpaired
dominant element, while the male is similarly
homozygous in the absence of that element.
{Footnote: In other words, the ova are each
either female, or male (i. e. non-female),
but the sperms are all non-female.} It is
not a little remarkable that on this point--which
is the only one where observations of the
nuclear processes of gameto-genesis have
yet been brought into relation with the visible
characteristics of the organisms themselves--there
should be diametrical opposition between
the results of breeding experiments and those
derived from cytology.
Those who have followed the researches of
the American school will be aware that, after
it had been found in certain insects that
the spermatozoa were of two kinds according
as they contained or did not contain the
accessory chromosome, E. B. Wilson succeeded
in proving that the sperms possessing this
accessory body were destined to form females
on fertilisation, while sperms without it
form males, the eggs being apparently indifferent.
Perhaps the most striking of all this series
of observations is that lately made by T.
H. Morgan {Morgan, "Proc. Soc. Exp.
Biol. Med." V. 1908, and von Baehr,
"Zool. Anz." XXXII. page 507, 1908.},
since confirmed by von Baehr, that in a Phylloxeran
two kinds of spermatids are formed, respectively
with and without an accessory (in this case,
double) chromosome. Of these, only those
possessing the accessory body become functional
spermatozoa, the others degenerating. We
have thus an elucidation of the puzzling
fact that in these forms fertilisation results
in the formation of females only. How the
males are formed -- for of course males are
eventually produced by the parthenogenetic
females -- we do not know.
If the accessory body is really to be regarded
as bearing the factor for femaleness, then
in Mendelian terms female is DD and male
is DR. The eggs are indifferent and the spermatozoa
are each male, or female. But according to
the evidence derived from a study of the
sex-limited descent of certain features in
other animals the conclusion seems equally
clear that in them female must be regarded
as DR and male as RR. The eggs are thus each
either male or female and the spermatozoa
are indifferent. How this contradictory evidence
is to be reconciled we do not yet know. The
breeding work concerns fowls, canaries, and
the Currant moth (Abraxas grossulariata).
The accessory chromosome has been now observed
in most of the great divisions of insects,
{As Wilson has proved, the unpaired body
is not a universal feature even in those
orders in which it has been observed. Nearly
allied types may differ. In some it is altogether
unpaired. In others it is paired with a body
of much smaller size, and by selection of
various types all gradations can be demonstrated
ranging to the condition in which the members
of the pair are indistinguishable from each
other.} except, as it happens, Lepidoptera.
At first sight it seems difficult to suppose
that a feature apparently so fundamental
as sex should be differently constituted
in different animals, but that seems at present
the least improbable inference. I mention
these two groups of facts as illustrating
the nature and methods of modern genetic
work. We must proceed by minute and specific
analytical investigation. Wherever we look
we find traces of the operation of precise
and specific rules.
In the light of present knowledge it is evident
that before we can attack the Species-problem
with any hope of success there are vast arrears
to be made up. He would be a bold man who
would now assert that there was no sense
in which the term Species might not have
a strict and concrete meaning in contradistinction
to the term Variety. We have been taught
to regard the difference between species
and variety as one of degree. I think it
unlikely that this conclusion will bear the
test of further research. To Darwin the question,
What is a variation? presented no difficulties.
Any difference between parent and offspring
was a variation. Now we have to be more precise.
First we must, as de Vries has shown, distinguish
real, genetic, variation from fluctuational
variations, due to environmental and other
accidents, which cannot be transmitted. Having
excluded these sources of error the variations
observed must be expressed in terms of the
factors to which they are due before their
significance can be understood. For example,
numbers of the variations seen under domestication,
and not a few witnessed in nature, are simply
the consequence of some ingredient being
in an unknown way omitted from the composition
of the varying individual. The variation
may on the contrary be due to the addition
of some new element, but to prove that it
is so is by no means an easy matter. Casual
observation is useless, for though these
latter variations will always be dominants,
yet many dominant characteristics may arise
from another cause, namely the meeting of
complementary factors, and special study
of each case in two generations at least
is needed before these two phenomena can
be distinguished.
When such considerations are fully appreciated
it will be realised that medleys of most
dissimilar occurrences are all confused together
under the term Variation. One of the first
objects of genetic analysis is to disentangle
this mass of confusion.
To those who have made no study of heredity
it sometimes appears that the question of
the effect of conditions in causing variation
is one which we should immediately investigate,
but a little thought will show that before
any critical inquiry into such possibilities
can be attempted, a knowledge of the working
of heredity under conditions as far as possible
uniform must be obtained. At the time when
Darwin was writing, if a plant brought into
cultivation gave off an albino variety, such
an event was without hesitation ascribed
to the change of life. Now we see that albino
gametes, germs, that is to say, which are
destitute of the pigment-forming factor,
may have been originally produced by individuals
standing an indefinite number of generations
back in the ancestry of the actual albino,
and it is indeed almost certain that the
variation to which the appearance of the
albino is due cannot have taken place in
a generation later than that of the grandparents.
It is true that when a new dominant appears
we should feel greater confidence that we
were witnessing the original variation, but
such events are of extreme rarity, and no
such case has come under the notice of an
experimenter in modern times, as far as I
am aware. That they must have appeared is
clear enough. Nothing corresponding to the
Brown-breasted Game fowl is known wild, yet
that colour is a most definite dominant,
and at some moment since Gallus bankiva was
domesticated, the element on which that special
colour depends must have at least once been
formed in the germ-cell of a fowl; but we
need harder evidence than any which has yet
been produced before we can declare that
this novelty came through over-feeding, or
change of climate, or any other disturbance
consequent on domestication. When we reflect
on the intricacies of genetic problems as
we must now conceive them there come moments
when we feel almost thankful that the Mendelian
principles were unknown to Darwin. The time
called for a bold pronouncement, and he made
it, to our lasting profit and delight. With
fuller knowledge we pass once more into a
period of cautious expectation and reserve.
In every arduous enterprise it is pleasanter
to look back at difficulties overcome than
forward to those which still seem insurmountable,
but in the next stage there is nothing to
be gained by disguising the fact that the
attributes of living things are not what
we used to suppose. If they are more complex
in the sense that the properties they display
are throughout so regular {I have in view,
for example, the marvellous and specific
phenomena of regeneration, and those discovered
by the students of "Entwicklungsmechanik".
The circumstances of its occurrence here
preclude any suggestion that this regularity
has been brought about by the workings of
Selection. The attempts thus to represent
the phenomena have resulted in mere parodies
of scientific reasoning.} that the Selection
of minute random variations is an unacceptable
account of the origin of their diversity,
yet by virtue of that very regularity the
problem is limited in scope and thus simplified.
To begin with, we must relegate Selection
to its proper place. Selection permits the
viable to continue and decides that the non-viable
shall perish; just as the temperature of
our atmosphere decides that no liquid carbon
shall be found on the face of the earth:
but we do not suppose that the form of the
diamond has been gradually achieved by a
process of Selection. So again, as the course
of descent branches in the successive generations,
Selection determines along which branch Evolution
shall proceed, but it does not decide what
novelties that branch shall bring forth.
"La Nature contient le fonds de toutes
ces varietes, mais le hazard ou l'art les
mettent en oeuvre," as Maupertuis most
truly said.
Not till knowledge of the genetic properties
of organisms has attained to far greater
completeness can evolutionary speculations
have more than a suggestive value. By genetic
experiment, cytology and physiological chemistry
aiding, we may hope to acquire such knowledge.
In 1872 Nathusius wrote {"Vortrage uber
Viehzucht und Rassenerkenntniss", page
120, Berlin, 1872.}: "Das Gesetz der
Vererbung ist noch nicht erkannt; der Apfel
ist noch nicht vom Baum der Erkenntniss gefallen,
welcher, der Sage nach, Newton auf den rechten
Weg zur Ergrundung der Gravitationsgesetze
fuhrte." We cannot pretend that the
words are not still true, but in Mendelian
analysis the seeds of that apple-tree at
last are sown.
If we were asked what discovery would do
most to forward our inquiry, what one bit
of knowledge would more than any other illuminate
the problem, I think we may give the answer
without hesitation. The greatest advance
that we can foresee will be made when it
is found possible to connect the geometrical
phenomena of development with the chemical.
The geometrical symmetry of living things
is the key to a knowledge of their regularity,
and the forces which cause it. In the symmetry
of the dividing cell the basis of that resemblance
we call Heredity is contained. To imitate
the morphological phenomena of life we have
to devise a system which can divide. It must
be able to divide, and to segment as -- grossly
-- a vibrating plate or rod does, or as an
icicle can do as it becomes ribbed in a continuous
stream of water; but with this distinction,
that the distribution of chemical differences
and properties must simultaneously be decided
and disposed in orderly relation to the pattern
of the segmentation. Even if a model which
would do this could be constructed it might
prove to be a useful beginning.
This may be looking too far ahead. If we
had to choose some one piece of more proximate
knowledge which we would more especially
like to acquire, I suppose we should ask
for the secret of interracial sterility.
Nothing has yet been discovered to remove
the grave difficulty, by which Huxley in
particular was so much oppressed, that among
the many varieties produced under domestication
-- which we all regard as analogous to the
species seen in nature -- no clear case of
interracial sterility has been demonstrated.
The phenomenon is probably the only one to
which the domesticated products seem to afford
no parallel. No solution of the difficulty
can be offered which has positive value,
but it is perhaps worth considering the facts
in the light of modern ideas. It should be
observed that we are not discussing incompatibility
of two species to produce offspring (a totally
distinct phenomenon), but the sterility of
the offspring which many of them do produce.
When two species, both perfectly fertile
severally, produce on crossing a sterile
progeny, there is a presumption that the
sterility is due to the development in the
hybrid of some substance which can only be
formed by the meeting of two complementary
factors. That some such account is correct
in essence may be inferred from the well-known
observation that if the hybrid is not totally
sterile but only partially so, and thus is
able to form some good germ-cells which develop
into new individuals, the sterility of these
daughter-individuals is sensibly reduced
or may be entirely absent. The fertility
once re-established, the sterility does not
return in the later progeny, a fact strongly
suggestive of segregation.
Now if the sterility of the cross-bred be
really the consequence of the meeting of
two complementary factors, we see that the
phenomenon could only be produced among the
divergent offspring of one species by the
acquisition of at least two new factors;
for if the acquisition of a single factor
caused sterility the line would then end.
Moreover each factor must be separately acquired
by distinct individuals, for if both were
present together, the possessors would by
hypothesis be sterile. And in order to imitate
the case of species each of these factors
must be acquired by distinct breeds. The
factors need not, and probably would not,
produce any other perceptible effects; they
might, like the colour-factors present in
white flowers, make no difference in the
form or other characters. Not till the cross
was actually made between the two complementary
individuals would either factor come into
play, and the effects even then might be
unobserved until an attempt was made to breed
from the cross-bred.
Next, if the factors responsible for sterility
were acquired, they would in all probability
be peculiar to certain individuals and would
not readily be distributed to the whole breed.
Any member of the breed also into which both
the factors were introduced would drop out
of the pedigree by virtue of its sterility.
Hence the evidence that the various domesticated
breeds say of dogs or fowls can when mated
together produce fertile offspring, is beside
the mark. The real question is, Do they ever
produce sterile offspring? I think the evidence
is clearly that sometimes they do, oftener
perhaps than is commonly supposed.
These suggestions are quite amenable to experimental
tests. The most obvious way to begin is to
get a pair of parents which are known to
have had any sterile offspring, and to find
the proportions in which these steriles were
produced. If, as I anticipate, these proportions
are found to be definite, the rest is simple.
[Comment: Note that Bateson here assumes
the reader knows the expected outcome of
the experiment. It is difficult to know what
outcome he was actually predicting at that
time. It is argued in one quarter that this
reveals that Bateson belonged to the "genic"
school of Dobzhansky and Muller. (Click Here)]
In passing, certain other considerations
may be referred to. First, that there are
observations favouring the view that the
production of totally sterile cross- breds
is seldom a universal property of two species,
and that it may be a matter of individuals,
which is just what on the view here proposed
would be expected. Moreover, as we all know
now, though incompatibility may be dependent
to some extent on the degree to which the
species are dissimilar, no such principle
can be demonstrated to determine sterility
or fertility in general. For example, though
all our Finches can breed together, the hybrids
are all sterile. Of Ducks some species can
breed together without producing the slightest
sterility; others have totally sterile offspring,
and so on. The hybrids between several genera
of Orchids are perfectly fertile on the female
side, and some on the male side also, but
the hybrids produced between the Turnip
(Brassica napus) and the Swede (Brassica
campestris), which, according to our estimates
of affinity should be nearly allied forms,
are totally sterile. {See Sutton, A. W.,
"Journ. Linn. Soc." XXXVIII. page
341, 1908.}
Lastly, it may be recalled that in sterility
we are almost certainly considering a meristic
phenomenon. Failure to divide is, we may
feel fairly sure, the immediate "cause"
of the sterility. Now, though we know very
little about the heredity of meristic differences,
all that we do know points to the conclusion
that the less-divided is dominant to the
more-divided, and we are thus justified in
supposing that there are factors which can
arrest or prevent cell-division. My conjecture
therefore is that in the case of sterility
of cross-breds we see the effect produced
by a complementary pair of such factors.
This and many similar problems are now open
to our analysis.
The question is sometimes asked, Do the new
lights on Variation and Heredity make the
process of Evolution easier to understand?
On the whole the answer may be given that
they do. There is some appearance of loss
of simplicity, but the gain is real. As was
said above, the time is not ripe for the
discussion of the origin of species. With
faith in Evolution unshaken -- if indeed
the word faith can be used in application
to that which is certain -- we look on the
manner and causation of adapted differentiation
as still wholly mysterious.
As Samuel Butler so truly said: "To
me it seems that the 'Origin of Variation,'
whatever it is, is the only true 'Origin
of Species'" {"Life and Habit",
London, page
263, 1878.}, and of that Origin not one of
us knows anything. But given Variation --
and it is given: assuming further that the
variations are not guided into paths of adaptation
-- and both to the Darwinian and to the modern
school this hypothesis appears to be sound
if unproven -- an evolution of species proceeding
by definite steps is more, rather than less,
easy to imagine than an evolution proceeding
by the accumulation of indefinite and insensible
steps.
Those who have lost themselves in contemplating
the miracles of Adaptation (whether real
or spurious) have not unnaturally fixed their
hopes rather on the indefinite than on the
definite changes. The reasons are obvious.
By suggesting that the steps through which
an adaptative mechanism arose were indefinite
and insensible, all further trouble is spared.
While it could be said that species arise
by an insensible and imperceptible process
of variation, there was clearly no use in
tiring ourselves by trying to perceive that
process. This labour-saving counsel found
great favour. All that had to be done to
develop evolution- theory was to discover
the good in everything, a task which, in
the complete absence of any control or test
whereby to check the truth of the discovery,
is not very onerous. The doctrine "que
tout est au mieux" was therefore preached
with fresh vigour, and examples of that illuminating
principle were discovered with a facility
that Pangloss himself might have envied,
till at last even the spectators wearied
of such dazzling performances.
But in all seriousness, why should indefinite
and unlimited variation have been regarded
as a more probable account of the origin
of Adaptation? Only, I think, because the
obstacle was shifted one plane back, and
so looked rather less prominent. The abundance
of Adaptation, we all grant, is an immense,
almost an unsurpassable difficulty in all
non-Lamarckian views of Evolution; but if
the steps by which that adaptation arose
were fortuitous, to imagine them insensible
is assuredly no help. In one most important
respect indeed, as has often been observed,
it is a multiplication of troubles. For the
smaller the steps, the less could Natural
Selection act upon them. Definite variations
-- and of the occurrence of definite variations
in abundance we have now the most convincing
proof -- have at least the obvious merit
that they can make and often do make a real
difference in the chances of life.
There is another aspect of the Adaptation
problem to which I can only allude very briefly.
May not our present ideas of the universality
and precision of Adaptation be greatly exaggerated?
The fit of organism to its environment is
not after all so very close -- a proposition
unwelcome perhaps, but one which could be
illustrated by very copious evidence. Natural
Selection is stern, but she has her tolerant
moods.
We have now most certain and irrefragable
proof that much definiteness exists in living
things apart from Selection, and also much
that may very well have been preserved and
so in a sense constituted by Selection. Here
the matter is likely to rest. There is a
passage in the sixth edition of the "Origin"
which has I think been overlooked. On page
70 Darwin says "The tuft of hair on
the breast of the wild turkey-cock cannot
be of any use, and it is doubtful whether
it can be ornamental in the eyes of the female
bird." This tuft of hair is a most definite
and unusual structure, and I am afraid that
the remark that it "cannot be of any
use" may have been made inadvertently;
but it may have been intended, for in the
first edition the usual qualification was
given and must therefore have been deliberately
excised.{Origin, 1st edition, p. 90, "which
can hardly be either useful or ornamental
to this bird".} Anyhow I should like
to think that Darwin did throw over that
tuft of hair, and that he felt relief when
he had done so. Whether however we have his
great authority for such a course or not,
I feel quite sure that we shall be rightly
interpreting the facts of nature if we cease
to expect to find purposefulness wherever
we meet with definite structures or patterns.
Such things are, as often as not, I suspect
rather of the nature of tool-marks, mere
incidents of manufacture, benefiting their
possessor not more than the wire-marks in
a sheet of paper, or the ribbing on the bottom
of an oriental plate renders those objects
more attractive in our eyes.
If Variation may be in any way definite,
the question once more arises, may it not
be definite in direction? The belief that
it is has had many supporters, from Lamarck
onwards, who held that it was guided by need,
and others who, like Nageli, while laying
no emphasis on need, yet were convinced that
there was guidance of some kind. The latter
view under the name of "Orthogenesis,"
devised I believe by Eimer, at the present
day commends itself to some naturalists.
The objection to such a suggestion is of
course that no fragment of real evidence
can be produced in its support. On the other
hand, with the experimental proof that variation
consists largely in the unpacking and repacking
of an original complexity, it is not so certain
as we might like to think that the order
of these events is not pre-determined. For
instance the original "pack" may
have been made in such a way that at the
nth division of the germ-cells of a sweet
pea a colour-factor might be dropped, and
that at the n + n' prime division the hooded
variety be given off, and so on. I see no
ground whatever for holding such a view,
but in fairness the possibility should not
be forgotten, and in the light of modern
research it scarcely looks so absurdly improbable
as before.
No one can survey the work of recent years
without perceiving that evolutionary orthodoxy
developed too fast, and that a great deal
has got to come down; but this satisfaction
at least remains, that in the experimental
methods which Mendel inaugurated, we have
means of reaching certainty in regard to
the physiology of Heredity and Variation
upon which a more lasting structure may be
built.
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