Darwin and Modern Science (1909)
Edited by A.C. Seward
V. HEREDITY AND VARIATION IN MODERN LIGHTS.
By W. BATESON, M.A., F.R.S.
Professor of Biology in the University of Cambridge.
arwin'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." ("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 (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 (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." (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.")
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" ("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. (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." ("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 befel
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 (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" (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. (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.)
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. (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. (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. (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.
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. 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 plus 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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