Professor of Biology in the University of Cambridge.
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." ("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.
VI. THE MINUTE STRUCTURE OF CELLS IN RELATION TO HEREDITY. By Eduard
Strasburger.
Professor of Botany in the University of Bonn.
Since 1875 an unexpected insight has been gained into the internal
structure of cells. Those who are familiar with the results of
investigations in this branch of Science are convinced that any modern
theory of heredity must rest on a basis of cytology and cannot be at
variance with cytological facts. Many histological discoveries, both
such as have been proved correct and others which may be accepted as
probably well founded, have acquired a fundamental importance from the
point of view of the problems of heredity.
My aim is to describe the present position of our knowledge of Cytology.
The account must be confined to essentials and cannot deal with
far-reaching and controversial questions. In cases where difference of
opinion exists, I adopt my own view for which I hold myself responsible.
I hope to succeed in making myself intelligible even without the aid of
illustrations: in order to convey to the uninitiated an adequate idea
of the phenomena connected with the life of a cell, a greater number
of figures would be required than could be included within the scope of
this article.
So long as the most eminent investigators (As for example the
illustrious Wilhelm Hofmeister in his "Lehre von der Pflanzenzelle"
(1867).) believed that the nucleus of a cell was destroyed in the course
of each division and that the nuclei of the daughter-cells were produced
de novo, theories of heredity were able to dispense with the nucleus.
If they sought, as did Charles Darwin, who showed a correct grasp of
the problem in the enunciation of his Pangenesis hypothesis, for
histological connecting links, their hypotheses, or at least the best of
them, had reference to the cell as a whole. It was known to Darwin
that the cell multiplied by division and was derived from a similar
pre-existing cell. Towards 1870 it was first demonstrated that
cell-nuclei do not arise de novo, but are invariably the result of
division of pre-existing nuclei. Better methods of investigation
rendered possible a deeper insight into the phenomena accompanying cell
and nuclear divisions and at the same time disclosed the existence of
remarkable structures. The work of O. Butschli, O. Hertwig, W. Flemming
H. Fol and of the author of this article (For further reference to
literature, see my article on "Die Ontogenie der Zelle seit 1875",
in the "Progressus Rei Botanicae", Vol. I. page 1, Jena, 1907.), have
furnished conclusive evidence in favour of these facts. It was found
that when the reticular framework of a nucleus prepares to divide, it
separates into single segments. These then become thicker and denser,
taking up with avidity certain stains, which are used as aids to
investigation, and finally form longer or shorter, variously bent,
rodlets of uniform thickness. In these organs which, on account of their
special property of absorbing certain stains, were styled Chromosomes
(By W. Waldeyer in 1888.), there may usually be recognised a separation
into thicker and thinner discs; the former are often termed Chromomeres.
(Discovered by W. Pfitzner in 1880.) In the course of division of the
nucleus, the single rows of chromomeres in the chromosomes are doubled
and this produces a band-like flattening and leads to the longitudinal
splitting by which each chromosome is divided into two exactly equal
halves. The nuclear membrane then disappears and fibrillar cell-plasma
or cytoplasm invades the nuclear area. In animal cells these fibrillae
in the cytoplasm centre on definite bodies (Their existence and their
multiplication by fission were demonstrated by E. van Beneden and Th.
Boveri in 1887.), which it is customary to speak of as Centrosomes.
Radiating lines in the adjacent cell-plasma suggest that these bodies
constitute centres of force. The cells of the higher plants do not
possess such individualised centres; they have probably disappeared in
the course of phylogenetic development: in spite of this, however, in
the nuclear division-figures the fibrillae of the cell-plasma are seen
to radiate from two opposite poles. In both animal and plant cells a
fibrillar bipolar spindle is formed, the fibrillae of which grasp the
longitudinally divided chromosomes from two opposite sides and arrange
them on the equatorial plane of the spindle as the so-called nuclear
or equatorial plate. Each half-chromosome is connected with one of the
spindle poles only and is then drawn towards that pole. (These important
facts, suspected by W. Flemming in 1882, were demonstrated by E. Heuser,
L. Guignard, E. van Beneden, M. Nussbaum, and C. Rabl.)
The formation of the daughter-nuclei is then effected. The changes
which the daughter-chromosomes undergo in the process of producing the
daughter-nuclei repeat in the reverse order the changes which they went
through in the course of their progressive differentiation from the
mother-nucleus. The division of the cell-body is completed midway
between the two daughter-nuclei. In animal cells, which possess no
chemically differentiated membrane, separation is effected by simple
constriction, while in the case of plant cells provided with a definite
wall, the process begins with the formation of a cytoplasmic separating
layer.
The phenomena observed in the course of the division of the nucleus show
beyond doubt that an exact halving of its substance is of the greatest
importance. (First shown by W. Roux in 1883.) Compared with the method
of division of the nucleus, that of the cytoplasm appears to be very
simple. This led to the conception that the cell-nucleus must be the
chief if not the sole carrier of hereditary characters in the organism.
It is for this reason that the detailed investigation of fertilisation
phenomena immediately followed researches into the nucleus. The
fundamental discovery of the union of two nuclei in the sexual act was
then made (By O. Hertwig in 1875.) and this afforded a new support for
the correct conception of the nuclear functions. The minute study of the
behaviour of the other constituents of sexual cells during fertilisation
led to the result, that the nucleus alone is concerned with handing on
hereditary characters (This was done by O. Hertwig and the author of
this essay simultaneously in 1884.) from one generation to another.
Especially important, from the point of view of this conclusion, is
the study of fertilisation in Angiosperms (Flowering plants); in these
plants the male sexual cells lose their cell-body in the pollen-tube and
the nucleus only--the sperm-nucleus--reaches the egg. The cytoplasm of
the male sexual cell is therefore not necessary to ensure a transference
of hereditary characters from parents to offspring. I lay stress on the
case of the Angiosperms because researches recently repeated with
the help of the latest methods failed to obtain different results.
As regards the descendants of angiospermous plants, the same laws of
heredity hold good as for other sexually differentiated organisms; we
may, therefore, extend to the latter what the Angiosperms so clearly
teach us.
The next advance in the hitherto rapid progress in our knowledge of
nuclear division was delayed, because it was not at once recognised that
there are two absolutely different methods of nuclear division. All
such nuclear divisions were united under the head of indirect or
mitotic divisions; these were also spoken of as karyo-kineses, and
were distinguished from the direct or amitotic divisions which are
characterised by a simple constriction of the nuclear body. So long
as the two kinds of indirect nuclear division were not clearly
distinguished, their correct interpretation was impossible. This was
accomplished after long and laborious research, which has recently
been carried out and with results which should, perhaps, be regarded as
provisional.
Soon after the new study of the nucleus began, investigators were struck
by the fact that the course of nuclear division in the mother-cells, or
more correctly in the grandmother-cells, of spores, pollen-grains, and
embryo-sacs of the more highly organised plants and in the spermatozoids
and eggs of the higher animals, exhibits similar phenomena, distinct
from those which occur in the somatic cells.
In the nuclei of all those cells which we may group together as
gonotokonts (At the suggestion of J.P. Lotsy in 1904.) (i.e. cells
concerned in reproduction) there are fewer chromosomes than in the
adjacent body-cells (somatic cells). It was noticed also that there is a
peculiarity characteristic of the gonotokonts, namely the occurrence of
two nuclear divisions rapidly succeeding one another. It was afterwards
recognised that in the first stage of nuclear division in
the gonotokonts the chromosomes unite in pairs: it is these
chromosome-pairs, and not the two longitudinal halves of single
chromosomes, which form the nuclear plate in the equatorial plane of
the nuclear spindle. It has been proposed to call these pairs gemini.
(J.E.S. Moore and A.L. Embleton, "Proc. Roy. Soc." London, Vol. LXXVII.
page 555, 1906; V. Gregoire, 1907.) In the course of this division
the spindle-fibrillae attach themselves to the gemini, i.e. to entire
chromosomes and direct them to the points where the new daughter-nuclei
are formed, that is to those positions towards which the longitudinal
halves of the chromosomes travel in ordinary nuclear divisions. It
is clear that in this way the number of chromosomes which the
daughter-nuclei contain, as the result of the first stage in division
in the gonotokonts, will be reduced by one half, while in ordinary
divisions the number of chromosomes always remains the same. The first
stage in the division of the nucleus in the gonotokonts has therefore
been termed the reduction division. (In 1887 W. Flemming termed this the
heterotypic form of nuclear division.) This stage in division determines
the conditions for the second division which rapidly ensues. Each of the
paired chromosomes of the mother-nucleus has already, as in an ordinary
nuclear division, completed the longitudinal fission, but in this case
it is not succeeded by the immediate separation of the longitudinal
halves and their allotment to different nuclei. Each chromosome,
therefore, takes its two longitudinal halves into the same
daughter-nucleus. Thus, in each daughter-nucleus the longitudinal halves
of the chromosomes are present ready for the next stage in the
division; they only require to be arranged in the nuclear plate and then
distributed among the granddaughter-nuclei. This method of division,
which takes place with chromosomes already split, and which have only
to provide for the distribution of their longitudinal halves to the next
nuclear generation, has been called homotypic nuclear division. (The
name was proposed by W. Flemming in 1887; the nature of this type of
division was, however, not explained until later.)
Reduction division and homotypic nuclear division are included together
under the term allotypic nuclear division and are distinguished from the
ordinary or typical nuclear division. The name Meiosis (By J. Bretland
Farmer and J.E.S. Moore in 1905.) has also been proposed for these two
allotypic nuclear divisions. The typical divisions are often spoken of
as somatic.
Observers who were actively engaged in this branch of recent
histological research soon noticed that the chromosomes of a given
organism are differentiated in definite numbers from the nuclear
network in the course of division. This is especially striking in the
gonotokonts, but it applies also to the somatic tissues. In the latter,
one usually finds twice as many chromosomes as in the gonotokonts. Thus
the conclusion was gradually reached that the doubling of chromosomes,
which necessarily accompanies fertilisation, is maintained in the
product of fertilisation, to be again reduced to one half in the
gonotokonts at the stage of reduction-division. This enabled us to form
a conception as to the essence of true alternation of generations, in
which generations containing single and double chromosomes alternate
with one another.
The single-chromosome generation, which I will call the HAPLOID, must
have been the primitive generation in all organisms; it might also
persist as the only generation. Every sexual differentiation in
organisms, which occurred in the course of phylogenetic development, was
followed by fertilisation and therefore by the creation of a diploid or
double-chromosome product. So long as the germination of the product
of fertilisation, the zygote, began with a reducing process, a special
DIPLOID generation was not represented. This, however, appeared later
as a product of the further evolution of the zygote, and the reduction
division was correspondingly postponed. In animals, as in plants, the
diploid generation attained the higher development and gradually assumed
the dominant position. The haploid generation suffered a proportional
reduction, until it finally ceased to have an independent existence and
became restricted to the role of producing the sexual products within
the body of the diploid generation. Those who do not possess the
necessary special knowledge are unable to realise what remains of the
first haploid generation in a phanerogamic plant or in a vertebrate
animal. In Angiosperms this is actually represented only by the short
developmental stages which extend from the pollen mother-cells to the
sperm-nucleus of the pollen-tube, and from the embryo-sac mother-cell to
the egg and the endosperm tissue. The embryo-sac remains enclosed in
the diploid ovule, and within this from the fertilised egg is formed
the embryo which introduces the new diploid generation. On the full
development of the diploid embryo of the next generation, the diploid
ovule of the preceding diploid generation is separated from the latter
as a ripe seed. The uninitiated sees in the more highly organised plants
only a succession of diploid generations. Similarly all the higher
animals appear to us as independent organisms with diploid nuclei only.
The haploid generation is confined in them to the cells produced as the
result of the reduction division of the gonotokonts; the development of
these is completed with the homotypic stage of division which succeeds
the reduction division and produces the sexual products.
The constancy of the numbers in which the chromosomes separate
themselves from the nuclear network during division gave rise to the
conception that, in a certain degree, chromosomes possess individuality.
Indeed the most careful investigations (Particularly those of V.
Gregoire and his pupils.) have shown that the segments of the nuclear
network, which separate from one another and condense so as to produce
chromosomes for a new division, correspond to the segments produced from
the chromosomes of the preceding division. The behaviour of such nuclei
as possess chromosomes of unequal size affords confirmatory evidence of
the permanence of individual chromosomes in corresponding sections of an
apparently uniform nuclear network. Moreover at each stage in division
chromosomes with the same differences in size reappear. Other cases are
known in which thicker portions occur in the substance of the resting
nucleus, and these agree in number with the chromosomes. In this
network, therefore, the individual chromosomes must have retained
their original position. But the chromosomes cannot be regarded as the
ultimate hereditary units in the nuclei, as their number is too small.
Moreover, related species not infrequently show a difference in the
number of their chromosomes, whereas the number of hereditary units
must approximately agree. We thus picture to ourselves the carriers of
hereditary characters as enclosed in the chromosomes; the transmitted
fixed number of chromosomes is for us only the visible expression of
the conception that the number of hereditary units which the chromosomes
carry must be also constant. The ultimate hereditary units may, like
the chromosomes themselves, retain a definite position in the resting
nucleus. Further, it may be assumed that during the separation of the
chromosomes from one another and during their assumption of the rod-like
form, the hereditary units become aggregated in the chromomeres and
that these are characterised by a constant order of succession.
The hereditary units then grow, divide into two and are uniformly
distributed by the fission of the chromosomes between their longitudinal
halves.
As the contraction and rod-like separation of the chromosomes serve
to isnure the transmission of all hereditary units in the products of
division of a nucleus, so, on the other hand, the reticular distension
of each chromosome in the so-called resting nucleus may effect a
separation of the carriers of hereditary units from each other and
facilitate the specific activity of each of them.
In the stages preliminary to their division, the chromosomes become
denser and take up a substance which increases their staining capacity;
this is called chromatin. This substance collects in the chromomeres
and may form the nutritive material for the carriers of hereditary units
which we now believe to be enclosed in them. The chromatin cannot itself
be the hereditary substance, as it afterwards leaves the chromosomes,
and the amount of it is subject to considerable variation in the
nucleus, according to its stage of development. Conjointly with the
materials which take part in the formation of the nuclear spindle and
other processes in the cell, the chromatin accumulates in the resting
nucleus to form the nucleoli.
Naturally connected with the conclusion that the nuclei are the carriers
of hereditary characters in the organism, is the question whether
enucleate organisms can also exist. Phylogenetic considerations give an
affirmative answer to this question. The differentiation into nucleus
and cytoplasm represents a division of labour in the protoplast. A
study of organisms which belong to the lowest class of the organic world
teaches us how this was accomplished. Instead of well-defined nuclei,
scattered granules have been described in the protoplasm of several of
these organisms (Bacteria, Cyanophyceae, Protozoa.), characterised by
the same reactions as nuclear material, provided also with a nuclear
network, but without a limiting membrane. (This is the result of the
work of R. Hertwig and of the most recently published investigations.)
Thus the carriers of hereditary characters may originally have been
distributed in the common protoplasm, afterwards coming together and
eventually assuming a definite form as special organs of the cell. It
may be also assumed that in the protoplasm and in the primitive types
of nucleus, the carriers of the same hereditary unit were represented in
considerable quantity; they became gradually differentiated to an extent
commensurate with newly acquired characters. It was also necessary that,
in proportion as this happened, the mechanism of nuclear division must
be refined. At first processes resembling a simple constriction would
suffice to provide for the distribution of all hereditary units to each
of the products of division, but eventually in both organic kingdoms
nuclear division, which alone insured the qualitative identity of the
products of division, became a more marked feature in the course of
cell-multiplication.
Where direct nuclear division occurs by constriction in the higher
organisms, it does not result in the halving of hereditary units. So far
as my observations go, direct nuclear division occurs in the more highly
organised plants only in cells which have lost their specific
functions. Such cells are no longer capable of specific reproduction. An
interesting case in this connection is afforded by the internodal cells
of the Characeae, which possess only vegetative functions. These cells
grow vigorously and their cytoplasm increases, their growth being
accompanied by a correspondingly direct multiplication of the nuclei.
They serve chiefly to nourish the plant, but, unlike the other
cells, they are incapable of producing any offspring. This is a very
instructive case, because it clearly shows that the nuclei are not only
carriers of hereditary characters, but that they also play a definite
part in the metabolism of the protoplasts.
Attention was drawn to the fact that during the reducing division of
nuclei which contain chromosomes of unequal size, gemini are constantly
produced by the pairing of chromosomes of the same size. This led to
the conclusion that the pairing chromosomes are homologous, and that one
comes from the father, the other from the mother. (First stated by T.H.
Montgomery in 1901 and by W.S. Sutton in 1902.) This evidently applies
also to the pairing of chromosomes in those reduction-divisions in
which differences in size do not enable us to distinguish the individual
chromosomes. In this case also each pair would be formed by two
homologous chromosomes, the one of paternal, the other of maternal
origin. When the separation of these chromosomes and their distribution
to both daughter-nuclei occur a chromosome of each kind is provided for
each of these nuclei. It would seem that the components of each pair
might pass to either pole of the nuclear spindle, so that the paternal
and maternal chromosomes would be distributed in varying proportion
between the daughter-nuclei; and it is not impossible that one
daughter-nucleus might occasionally contain paternal chromosomes only
and its sister-nucleus exclusively maternal chromosomes.
The fact that in nuclei containing chromosomes of various sizes, the
chromosomes which pair together in reduction-division are always of
equal size, constitutes a further and more important proof of their
qualitative difference. This is supported also by ingenious experiments
which led to an unequal distribution of chromosomes in the products of
division of a sea-urchin's egg, with the result that a difference was
induced in their further development. (Demonstrated by Th. Boveri in
1902.)
The recently discovered fact that in diploid nuclei the chromosomes are
arranged in pairs affords additional evidence in favour of the unequal
value of the chromosomes. This is still more striking in the case of
chromosomes of different sizes. It has been shown that in the first
division-figure in the nucleus of the fertilised egg the chromosomes of
corresponding size form pairs. They appear with this arrangement in all
subsequent nuclear divisions in the diploid generation. The longitudinal
fissions of the chromosomes provide for the unaltered preservation
of this condition. In the reduction nucleus of the gonotokonts the
homologous chromosomes being near together need not seek out one
another; they are ready to form gemini. The next stage is their
separation to the haploid daughter-nuclei, which have resulted from the
reduction process.
Peculiar phenomena in the reduction nucleus accompany the formation of
gemini in both organic kingdoms. (This has been shown more particularly
by the work of L. Guignard, M. Mottier, J.B. Farmer, C.B. Wilson, V.
Hacker and more recently by V. Gregoire and his pupil C.A. Allen, by the
researches conducted in the Bonn Botanical Institute, and by A. and
K.E. Schreiner.) Probably for the purpose of entering into most
intimate relation, the pairs are stretched to long threads in which the
chromomeres come to lie opposite one another. (C.A. Allen, A. and K.E.
Schreiner, and Strasburger.) It seems probable that these are homologous
chromomeres, and that the pairs afterwards unite for a short time, so
that an exchange of hereditary units is rendered possible. (H. de Vries
and Strasburger.) This cannot be actually seen, but certain facts of
heredity point to the conclusion that this occurs. It follows from
these phenomena that any exchange which may be effected must be one of
homologous carriers of hereditary units only. These units continue to
form exchangeable segments after they have undergone unequal changes;
they then constitute allelotropic pairs. We may thus calculate what sum
of possible combinations the exchange of homologous hereditary units
between the pairing chromosomes provides for before the reduction
division and the subsequent distribution of paternal and maternal
chromosomes in the haploid daughter-nuclei. These nuclei then transmit
their characters to the sexual cells, the conjugation of which in
fertilization again produces the most varied combinations. (A. Weismann
gave the impulse to these ideas in his theory on "Amphimixis".) In this
way all the cooperations which the carriers of hereditary characters are
capable of in a species are produced; this must give it an appreciable
advantage in the struggle for life.
The admirers of Charles Darwin must deeply regret that he did not live
to see the results achieved by the new Cytology. What service would they
have been to him in the presentation of his hypothesis of Pangenesis;
what an outlook into the future would they have given to his active
mind!
The Darwinian hypothesis of Pangenesis rests on the conception that all
inheritable properties are represented in the cells by small invisible
particles or gemmules and that these gemmules increase by division.
Cytology began to develop on new lines some years after the publication
in 1868 of Charles Darwin's "Provisional hypothesis of Pangenesis"
("Animals and Plants under Domestication", London, 1868, Chapter
XXVII.), and when he died in 1882 it was still in its infancy. Darwin
would have soon suggested the substitution of the nuclei for his
gemmules. At least the great majority of present-day investigators in
the domain of cytology have been led to the conclusion that the nucleus
is the carrier of hereditary characters, and they also believe that
hereditary characters are represented in the nucleus as distinct units.
Such would be Darwin's gemmules, which in conformity with the name
of his hypothesis may be called pangens (So called by H. de Vries in
1889.): these pangens multiply by division. All recently adopted
views may be thus linked on to this part of Darwin's hypothesis. It is
otherwise with Darwin's conception to which Pangenesis owes its name,
namely the view that all cells continually give off gemmules, which
migrate to other places in the organism, where they unite to form
reproductive cells. When Darwin foresaw this possibility, the continuity
of the germinal substance was still unknown (Demonstrated by Nussbaum in
1880, by Sachs in 1882, and by Weismann in 1885.), a fact which excludes
a transference of gemmules.
But even Charles Darwin's genius was confined within finite boundaries
by the state of science in his day.
It is not my province to deal with other theories of development which
followed from Darwin's Pangenesis, or to discuss their histological
probabilities. We can, however, affirm that Charles Darwin's idea that
invisible gemmules are the carriers of hereditary characters and that
they multiply by division has been removed from the position of a
provisional hypothesis to that of a well-founded theory. It is supported
by histology, and the results of experimental work in heredity, which
are now assuming extraordinary prominence, are in close agreement with
it.