The opportunity of addressing fellow-students pursuing lines of inquiry
other than his own falls seldom to a scientific man. One of these rare
opportunities is offered by the constitution of the Professorship to
which I have had the honour to be called. That Professorship, though
bearing the comprehensive title "of Biology," is founded with the
understanding that the holder shall apply himself to a particular class
of physiological problems, the study of which is denoted by the term
Genetics. The term is new; and though the problems are among the oldest
which have vexed the human mind, the modes by which they may be
successfully attacked are also of modern invention. There is therefore
a certain fitness in the employment of this occasion for the deliverance
of a discourse explaining something of the aims of Genetics and of the
methods by which we trust they may be reached.
You will be aware that the claims put forward in the name of Genetics
are high, but I trust to be able to show you that they are not high
without reason. It is the ambition of every one who in youth devotes
himself to the search for natural truth, that his work may be found
somewhere in the main stream of progress. So long only as he keeps
something of the limitless hope with which his voyage of discovery
began, will his courage and his spirit last. The moment we most dread is
one in which it may appear that, after all, our effort has been spent in
exploring some petty tributary, or worse, a backwater of the great
current. It is because Genetic research is still pushing forward in the
central undifferentiated trunk of biological science that we confess no
guilt of presumption in declaring boldly that whatever difficulty may be
in store for those who cast in their lot with us, they need fear no
disillusionment or misgiving that their labour has been wasted on a
paltry quest.
In research, as in all business of exploration, the stirring times come
when a fresh region is suddenly unlocked by the discovery of a new key.
Then conquest is easy and there are prizes for all. We are happy in that
during our own time not a few such territories have been revealed to the
vision of mankind. I do not dare to suggest that in magnitude or
splendour the field of Genetics may be compared with that now being
disclosed to the physicist or the astronomer; for the glory of the
celestial is one and the glory of the terrestrial is another. But I
will say that for once to the man of ordinary power who cannot venture
into those heights beyond, Mendel's clue has shown the way into a realm
of nature which for surprising novelty and adventure is hardly to be
excelled.
It is no hyperbolical figure that I use when I speak of Mendelian
discovery leading us into a new world, the very existence of which was
unsuspected before.
The road thither is simple and easy to follow. We start from a common
fact, familiar to everyone, that all the ordinary animals and plants
began their individual life by the union of two cells, the one male, the
other female. Those cells are known as germ-cells or _gametes_, that is
to say, "marrying" cells.
Now obviously the diversity of form which is characteristic of the
animal and plant world must be somehow represented in the gametes, since
it is they which bring into each organism all that it contains. I am
aware that there is interplay between the organism and the circumstances
in which it grows up, and that opportunity given may bring out a
potentiality which without that opportunity must have lain dormant. But
while noting parenthetically that this question of opportunity has an
importance, which some day it may be convenient to estimate, the one
certain fact is that all the powers, physical and mental that a living
creature possesses were contributed by one or by both of the two
germ-cells which united in fertilisation to give it existence. The fact
that _two_ cells are concerned in the production of all the ordinary
forms of life was discovered a long while ago, and has been part of the
common stock of elementary knowledge of all educated persons for about
half a century. The full consequences of this double nature seem
nevertheless to have struck nobody before Mendel. Simple though the
fact is, I have noticed that to many it is difficult to assimilate as a
working idea. We are accustomed to think of a man, a butterfly, or an
apple tree as each _one_ thing. In order to understand the significance
of Mendelism we must get thoroughly familiar with the fact that they are
each _two_ things, double throughout every part of their composition.
There is perhaps no better exercise as a preparation for genetic
research than to examine the people one meets in daily life and to try
in a rough way to analyse them into the two assemblages of characters
which are united in them. That we are assemblages or medleys of our
parental characteristics is obvious. We all know that a man may have his
father's hair, his mother's colour, his father's voice, his mother's
insensibility to music, and so on, but that is not enough.
Such an analysis is true, inasmuch as the various characters _are_
transmitted independently, but it misses the essential point. For in
each of these respects the individual is double; and so to get a true
picture of the composition of the individual we have to think how _each_
of the two original gametes was provided in the matter of height, hair,
colour, mathematical ability, nail-shape, and the other features that go
to make the man we know. The contribution of each gamete in each respect
has thus to be separately brought to account. If we could make a list of
all the ingredients that go to form a man and could set out how he is
constituted in respect of each of them, it would not suffice to give one
column of values for these ingredients, but we must rule two columns,
one for the ovum and one for the spermatozoon, which united in
fertilisation to form that man, and in each column we must represent how
that gamete was supplied in respect of each of the ingredients in our
list. When the problem of heredity is thus represented we can hardly
avoid discovering, by mere inspection, one of the chief conclusions to
which genetic research has led. For it is obvious that the contributions
of the male and female gametes may in respect of any of the ingredients
be either the same, or different. In any case in which the contribution
made by the two cells is the same, the resulting organism--in our
example the man--is, as we call it, _pure-bred_ for that ingredient, and
in all respects in which the contribution from the two sides of the
parentage is dissimilar the resulting organism is _cross-bred_.
To give an intelligible account of the next step in the analysis without
having recourse to precise and technical language is not very easy.
We have got to the point of view from which we see the individual made
up of a large number of distinct ingredients, contributed from two
sources, and in respect of any of them he may have received two similar
portions or two dissimilar portions. We shall not go far wrong if we
extend and elaborate our illustration thus. Let us imagine the contents
of a gamete as a fluid made by taking a drop from each of a definite
number of bottles in a chest, containing tinctures of the several
ingredients. There is one such chest from which the male gamete is to be
made up, and a similar chest containing a corresponding set of bottles
out of which the components of the female gamete are to be taken. But in
either chest one or more of the bottles may be empty; then nothing goes
in to represent that ingredient from that chest, and if corresponding
bottles are empty in both chests, then the individual made on
fertilisation by mixing the two collections of drops together does not
contain the missing ingredient at all. It follows therefore that an
individual may thus be "pure-bred," namely alike on both sides of his
composition as regards each ingredient in one of two ways, either by
having received the ingredient from the male chest and from the female,
or in having received it from neither. Conversely in respect of any
ingredient he may be "cross-bred," receiving the presence of it from one
gamete and the absence of it from the other.
The second conception with which we have now to become thoroughly
familiar is that of the individual as composed of what we call presences
and absences of all the possible ingredients. It is the basis of all
progress in genetic analysis. Let me give you two illustrations. A blue
eye is due to the absence of a factor which forms pigment on the front
of the iris. Two blue-eyed parents therefore, as Hurst has proved, do
not have dark-eyed children. The dark eye is due to either a single or
double dose of the factor missing from the blue eye. So dark-eyed
persons may have families all dark-eyed, or families composed of a
mixture of dark and light-eyed children in certain proportions which on
the average are definite.
Two plants of _Oenothera_ which I exhibit illustrate the same thing. One
of them is the ordinary _Lamarckiana_. I bend its stem. It will not
break, or only breaks with difficulty on account of the tough fibres it
contains. The stem of the other, one of de Vries' famous mutations,
snaps at once like short pastry, because it does not contain the factor
for the formation of the fibres. Such plants may be sister-plants
produced by the self-fertilisation of one parent, but they are distinct
in their composition and properties--and this distinction turns on the
presence or absence of elements which are treated as definite entities
when the germ-cells are formed. When we speak of such qualities as the
formation of pigment in an eye, or the development of fibres in a stem,
as due to transmitted elements or factors, you will perhaps ask if we
have formed any notion as to the actual nature of those factors. For my
own part as regards that ulterior question I confess to a disposition to
hold my fancy on a tight rein. It cannot be very long before we shall
_know_ what some of the factors are, and we may leave guessing till
then. Meanwhile however there is no harm in admitting that several of
them behave much as if they were ferments, and others as if they
constructed the substances on which the ferments act. But we must not
suppose for a moment that it is the ferment, or the objective substance,
which is transmitted. The thing transmitted can only be the power or
faculty to produce the ferment or the objective substance.
So far we have been considering the synthesis of the individual from
ingredients brought into him by the two gametes. In the next step of our
consideration we reverse the process, and examine how the ingredients of
which he was originally compounded are distributed among the gametes
that are eventually budded off from him.
Take first the case of the components in respect of which he is
pure-bred. Expectation would naturally suggest that all the germ-cells
formed from him would be alike in respect of those ingredients, and
observation shows, except in the rare cases of originating variations,
the causation of which is still obscure, that this expectation is
correct.
Hitherto though without experimental evidence no one could have been
certain that the facts were as I have described them, yet there is
nothing altogether contrary to common expectation. But when we proceed
to ask how the germ-cells will be constituted in the case of an
individual who is cross-bred in some respect, containing that is to say,
an ingredient from the one side of his parentage and not from the other,
the answer is entirely contrary to all the preconceptions which either
science or common sense had formed about heredity. For we find definite
experimental proof in nearly all the cases which have been examined,
that the germ-cells formed by such individuals do either contain or not
contain a representation of the ingredient, just as the original gametes
did or did not contain it.
If _both_ parent-gametes brought a certain quality in, then all the
daughter gametes have it; if neither brought it in, then none of the
daughter gametes have it. If it came in from one side and not from the
other, then on an average in half the resulting gametes it will be
present and from half it will be absent. This last phenomenon, which is
called segregation, constitutes the essence of Mendel's discovery.
So recurring to the simile of the man as made by the mixing of
tinctures, the process of redistribution of his characters among the
germ-cells may be represented as a sorting back of the tinctures again
into a double row of bottles, a pair corresponding to each ingredient;
and each of the germ-cells as then made of a drop from one or other
bottle of each pair: and in our model we may represent the phenomenon of
segregation in a crude way by supposing that the bottles having no
tincture in them, instead of being empty contained an inoperative fluid,
say water, with which the tincture would not mix. When the new
germ-cells are formed, the two fluids instead of diluting each other
simply separate again. It is this fact which entitles us to speak of the
purity of germ-cells. They are pure in the possession of an ingredient,
or in not possessing it; and the ingredients, or factors, as we
generally call them, are units because they are so treated in the
process of formation of the new gametes and because they come out of the
process of segregation in the same condition as they went in at
fertilisation.
As a consequence of these facts it follows that however complex may be
the origin of two given parents the composition of the offspring they
can produce is limited. There is only a limited number of types to be
made by the possible recombinations of the parental ingredients, and the
relative numbers in which each type will be represented are often
predicable by very simple arithmetical rules.
For example, if neither parent possesses a certain factor at all, then
none of the offspring will have it. If either parent has two doses of
the factor then all the children will have it; and if either parent has
one dose of the factor and the other has none, then on an average half
the family will have it, and half be without it.
To know whether the parent possesses the factor or not may be difficult
for reasons which will presently appear, but often it is quite easy and
can be told at once, for there are many factors which cannot be present
in the individual without manifesting their presence. I may illustrate
the descent of such a factor by the case of a family possessing a
peculiar form of night-blindness. The affected individuals marrying with
those unaffected have a mixture of affected and unaffected children, but
their unaffected children not having the responsible ingredient cannot
pass it on[1].
In such an observation two things are strikingly exemplified, (1) the
fact of the permanence of the unit, and (2) the fact that a _mixture_ of
types in the family means that one or other parent is cross-bred in some
respect, and is giving off gametes of more than one type.
The problem of heredity is thus a problem primarily analytical. We have
to detect and enumerate the factors out of which the bodies of animals
and plants are built up, and the laws of their distribution among the
germ-cells. All the processes of which I have spoken are accomplished by
means of cell-divisions, and in the one cell-union which occurs in
fertilisation. If we could watch the factors segregating from each
other in cell-division, or even if by microscopic examination we could
recognize this multitudinous diversity of composition that must
certainly exist among the germ-cells of all ordinary individuals, the
work of genetics would be much simpler than it is.
But so far no such direct method of observation has been discovered. In
default we are obliged to examine the constitution of the germ-cells by
experimental breeding, so contrived that each mating shall test the
composition of an individual in one or more chosen respects, and, so to
speak, sample its germ-cells by counting the number of each kind of
offspring which it can produce. But cumbersome as this method must
necessarily be, it enables us to put questions to Nature which never
have been put before. She, it has been said, is an unwilling witness.
Our questions must be shaped in such a way that the only possible
answer is a direct "Yes" or a direct "No." By putting such questions we
have received some astonishing answers which go far below the surface.
Amazing though they be, they are nevertheless true; for though our
witness may prevaricate, she cannot lie. Piecing these answers together,
getting one hint from this experiment, and another from that, we begin
little by little to reconstruct what is going on in that hidden world of
gametes. As we proceed, like our brethren in other sciences, we
sometimes receive answers which seem inconsistent or even contradictory.
But by degrees a sufficient body of evidence can be attained to show
what is the rule and what the exception. My purpose today must be to
speak rather of the regular than of the irregular.
One clear exception I may mention. Castle finds that in a cross between
the long-eared lop-rabbit and a short-eared breed, ears of intermediate
length are produced: and that these intermediates breed approximately
true.
Exceptions in general must be discussed elsewhere. Nevertheless if I may
throw out a word of counsel to beginners, it is: Treasure your
exceptions! When there are none, the work gets so dull that no one cares
to carry it further. Keep them always uncovered and in sight. Exceptions
are like the rough brickwork of a growing building which tells that
there is more to come and shows where the next construction is to be.
You will readily understand that the presentation here given of the
phenomena is only the barest possible outline. Some of the details we
may now fill in. For example, I have spoken of the characters of the
organism, its colour, shape, and the like, as if they were due each to
one ingredient or factor. Some of them are no doubt correctly so
represented; but already we know numerous bodily features which need the
concurrence of several factors to produce them. Nevertheless though the
character only appears when all the complementary ingredients are
together present, each of these severally and independently follows, as
regards its transmission, the simple rules I have described.
This complementary action may be illustrated by some curious results
that Mr Punnett and I have encountered when experimenting with the
height of Sweet Peas. There are two dwarf varieties, one the prostrate
"Cupid," the other the half-dwarf or "Bush" Sweet Peas. Crossed together
they give a cross-bred of full height. There is thus some element in the
Cupid which when it meets the complementary element from the Bush,
produces the characteristic length of the ordinary Sweet Pea. We may
note in passing that such a fact demonstrates at once the nature of
Variation and Reversion. The Reversion occurs because the two factors
that made the _height_ of the old Sweet Pea again come together after
being parted: and the Variations by which each of the dwarfs came into
existence must have taken place by the dropping out of one of these
elements or of the other.
Conversely there are factors which by their presence can prevent or
inhibit the development and appearance of others present and
unperceived.
For example, all the factors for pigmentation may be present in a plant
or an animal; but in addition there may be another factor present which
keeps the individual white, or nearly so.
There are cases in which the action of the factors is superposed one on
top of the other, and not until each factor is removed in turn can the
effects of the underlying factors be perceived. So in the mouse if no
other colour-factor is present, the fur is chocolate. If the next factor
in the series be there, it is black. If still another factor be added,
it has the brownish grey of the common wild mouse. Conversely, by the
variation which dropped out the top factor, a black mouse came into
existence. By the loss of the black factor, the chocolate mouse was
created, and for aught we can tell there may be still more possibilities
hidden beneath.
In the disentanglement of the properties and interactions of these
elementary factors, the science we must call to our aid is Physiological
Chemistry. The relations of Genetics with the other branches of biology
are close. Such work can only be conducted by those who have the good
fortune to be able to count upon continual help and advice from
specialists in the various branches of Zoology, Physiology, and Botany.
Often we have questions with which only a cytologist can deal, and
often it is the experience of a systematist we must invoke. The school
of Genetics in Cambridge starts under happy auspices in that we are
surrounded by colleagues qualified, and as we have often found, willing
to give us such aid unstinted. But with chemical physiology, we stand in
an even closer relation; and from the little I have dared to say
respecting the action and interaction of factors, it is evident that for
their disentanglement there must one day be an intimate and enduring
partnership arranged with the physiological chemists.
Now, as the whole of the elaborate process by which the various elements
are apportioned among the gametes must be got through in a few
cell-divisions at most, and perhaps in one division only, it is not
surprising that there is sometimes an interaction between factors that
have quite distinct rĂ´les to perform. These interactions are probably
of several kinds. One, which I shall illustrate presently, is probably
to be represented as a repulsion between two factors. As a consequence
of its operations when the various factors are sorted out into the
gametes, if the individual be cross-bred in respect of the _two_
repelling factors, having received so to speak only a single dose of
each, then the gametes are made up in such a way that each takes one or
other of the two repelling factors, not both.
Mutual repulsions of this kind probably play a significant part in the
phenomena of heredity. A single concrete case which Mr Punnett and I
have been investigating for some years will illustrate several of these
principles. We crossed together a pure white Sweet Pea having an erect
standard, with another pure white Sweet Pea having a hooded standard.
The result is, as you see, a purple flower with an erect standard. The
colour comes from the concurrence of complementary elements. A dose of a
certain ingredient from one parent meets a dose of another ingredient
from the other parent and the two make pigment in the flower. From other
experiments we know that the _purple_ colour of the pigment is due to a
dose of a third ingredient brought in from the hooded parent; and that
in the absence of that blue factor, as we may call it, the flower would
be red. The standard is erect because it contains a dose of the
erectness-factor from the erect parent, and the hooded parent can
readily be proved to owe its peculiar shape to the absence of that
element.
Our purple plant is thus cross-bred for four factors, containing only
one dose of each.
We let it fertilise itself, and its offspring show all the possible
combinations of the four different factors and their absences which the
genetic constitution of the plant can make.
Note that one of the combinations we expect to find is missing. There
are white erect and white hooded--white because they are lacking one or
other of the complementary ingredients necessary to the production of
pigment. There are purple erect and purple hooded, of which the purple
erect must perforce contain all the four factors, and the purple hooded
must similarly contain all of them except that for erectness. But when
we turn to the red class we are surprised to find that they are all
erect, none hooded. One of the possible combinations is missing. If you
examine this series of facts you will find there is only one possible
interpretation: namely that the ingredient which turns the flower
purple--alkalinity, perhaps we may call it--never goes into the same
germ-cell as the ingredient which makes the standard erect. There are
plenty of ways of testing the truth of this interpretation. For example,
it follows that the purple erects from such a family will in perpetuity
have offspring 1 purple hooded: 2 purple erect: 1 red erect; also that
all the white hooded crossed with pure reds will give purples, and so
on. These experiments have been made and the result has in each case
been conformable to expectation.
Between these two factors, the purpleness and the erectness of standard,
some antagonism or repulsion must exist. In some way therefore the
chemical and the geometrical phenomena of heredity must be
inter-related.
Some one will say perhaps this is all very well as a scientific
curiosity, but it has nothing to do with real life. The right answer to
such criticism is of course the lofty one that science and its
applications are distinct: that the investigator fixes his gaze solely
on the search for truth and that his attention must not be distracted by
trivialities of application. But while we make this answer and at least
try to work in the spirit it proclaims, we know in our hearts that it is
a counsel of perfection. I suspect that even the astronomer who at his
spectroscope is analysing the composition of Vega or Capella has still
an eye sometimes free for the affairs of this planet, and at least the
fact that his discoveries may throw light on our destinies does not
diminish his zeal in their pursuit. And surely to the study of Heredity,
preeminently among all the sciences, we are looking for light on human
destiny. To pretend otherwise would be mere hypocrisy. So while
reserving the higher line of defence I will reply that again and again
in our experimental work we come very near indeed to human affairs.
Sometimes this is obvious enough. No practical dog-breeder or seeds-man
can see the results of Mendelian recombination without perceiving that
here is a bit of knowledge he can immediately apply. No sociologist can
examine the pedigrees illustrating the simple descent of a deformity or
a congenital disease, and not see that the new knowledge gives a solid
basis for practical action by which the composition of a race could be
modified if society so chose. More than this: we know for certain in one
case, from the work of Professor Biffen, that the power to resist a
disease caused by the invasion of a pathogenic organism, wheat-rust, is
due to the absence of one of the simple factors or ingredients of which
I have spoken, and what we know to be true in that one case we are
beginning to suspect to be true of resistance to certain other diseases.
No pathologist can see such an experiment as this of Professor Biffen's
without realizing that here is a contribution of the first importance
to the physiology of disease.
There is no lack of utility and direct application in the study of
Genetics. I have alluded to some strictly practical results. If we want
to raise mangels that will not run to seed, or to breed a cow that will
give more milk in less time, or milk with more butter and less water, we
can turn to Genetics with every hope that something can be done in these
laudable directions. But here I would plead what I cannot but regard as
a higher usefulness in our work. Genetic inquiry aims at providing
knowledge that may bring, and I think will bring, certainty into a
region of human affairs and concepts which might have been supposed
reserved for ages to be the domain of the visionary. We have long known
that it was believed by some that our powers and conduct were dependent
on our physical composition, and that other schools have maintained
that nurture not nature, to use Galton's antithesis, has a
preponderating influence on our careers; but so soon as it becomes
common knowledge--not a philosophical speculation, but a certainty--that
liability to a disease, or the power of resisting its attack, addiction
to a particular vice, or to superstition, is due to the presence or
absence of a specific ingredient; and finally that these characteristics
are transmitted to the offspring according to definite, predicable
rules, then man's views of his own nature, his conceptions of justice,
in short his whole outlook on the world, must be profoundly changed. Yet
as regards the more tangible of these physical and mental
characteristics there can be little doubt that before many years have
passed the laws of their transmission will be expressible in simple
formulae.
The blundering cruelty we call criminal justice will stand forth
divested of natural sanction, a relic of the ferocious inventions of
the savage. Well may such justice be portrayed as blind. Who shall say
whether it is crime or punishment which has wrought the greater
suffering in the world? We may live to know that to the keen satirical
vision of Sam Butler on the pleasant mountains of Erewhon there was
revealed a dispensation, not kinder only, but wiser than the terrific
code which Moses delivered from the flames of Sinai.
If there are societies which refuse to apply the new knowledge, the
fault will not lie with Genetics. I think it needs but little
observation of the newer civilisations to foresee that _they_ will apply
every scrap of scientific knowledge which can help them, or seems to
help them in the struggle, and I am good enough Selectionist to know
that in that day the fate of the recalcitrant communities is sealed.
The thrill of discovery is not dulled by a suspicion that the discovery
can be applied. No harm is done to the investigator if he can resist the
temptation to deviate from his aim. With rarest exceptions the
discoveries which have formed the basis of physical progress have been
made without any thought but for the gratification of curiosity. Of this
there can be few examples more conspicuous than that which Mendel's work
presents. Untroubled by any itch to make potatoes larger or bread
cheaper, he set himself in the quiet of a cloister garden to find out
the laws of hybridity, and so struck a mine of truth, inexhaustible in
brilliancy and profit.
I will now suggest to you that it is by no means unlikely that even in
an inquiry so remote as that which I just described in the case of the
Sweet Pea, we may have the clue to a mystery which concerns us all in
the closest possible way. I mean the problem of the physiological nature
of Sex. In speaking of the interpretation of sexual difference
suggested by our experimental work as of some practical moment, I do not
imply that as in the other instances I have given, the knowledge is
likely to be of immediate use to our species; but only that if true it
makes a contribution to the stock of human ideas which no one can regard
as insignificant.
In the light of Mendelian knowledge, when a family consists of more than
one type the fact means that the germ-cells of one or other parent must
certainly be of more than one kind. In the case of sex the members of
the family are thus of two kinds, and the presumption is overwhelming
that this distinction is due to a difference among the germ-cells. Next,
since for all practical purposes the numbers of the two sexes produced
are approximately equal, sex exhibits the special case in which a family
consists of two types represented in equal numbers, half being male,
half female. But I called your attention to the fact that equality of
types results when _one_ parent was cross-bred in the character
concerned, having received one dose only of the factor on which it
depends. So we may feel fairly sure that the distinction between the
sexes depends on the presence in one or other of them of an unpaired
factor. This conclusion appears to me to follow so immediately on all
that we have learnt of genetic physiology that with every confidence we
may accept it as representing the actual fact.
The question which of the two sexes contains the unpaired factor is less
easy to answer, but there are several converging lines of evidence which
point to the deduction that in Vertebrates at least, and in some other
types, it is the female, and I feel little doubt that we shall succeed
in proving that in them femaleness is a definite Mendelian factor
absent from the male and following the ordinary Mendelian rules.
Before showing you how the Sweet Pea phenomenon aids in this inquiry I
must tell you of some other experimental results. The first concerns the
common currant moth, _Abraxas grossulariata_. It has a definite pale
variety called _lacticolor_. With these two forms Doncaster has made a
remarkable series of experiments. When he began, _lacticolor_ was only
known as a female form. This was crossed with the _grossulariata_ male
and gave _grossulariata_ only, showing that the male was pure to type.
The hybrids bred together gave _grossulariata_ males and females and
_lacticolor_ females only. But the hybrid males bred to _lacticolor_
females produced all four combinations, _grossulariata_ males and
females, and _lacticolor_ males and females. When the _lacticolor_ males
were bred to _grossulariata_ females, whether hybrid, or wild from a
district where _lacticolor_ does not exist, the result was that all the
males were _grossulariata_ and all the females _lacticolor_! It is
difficult to follow the course of such an experiment on once hearing and
all I ask you to remember is first that there is a series of matings
giving very curious distributions of the characters of type and variety
among the two sexes. And then, what is perhaps the most singular fact of
all, that the wild typical _grossulariata_ female can when crossed with
the _lacticolor_ male produce all females _lacticolor_. This last fact
can, we know, mean only one thing, namely that these wild females are in
reality hybrids of _lacticolor_; though since the males are pure
_grossulariata_, that fact would in the natural course of things never
be revealed.
When we encounter such a series of phenomena as this, our business is to
find a means of symbolical expression which will represent all the
factors involved, and show how each behaves in descent. Such a system or
scheme we have at length discovered, and I incline to think that it must
be the true one. If you study this case you will find that there are
nine distinct kinds of matings that can be made between the variety, the
type and the hybrid, and the scheme fits the whole group of results. It
is based on two suppositions:
1. That the female is cross-bred, or as we call it heterozygous for
femaleness-factor, the male being without that factor. The eggs are thus
each destined from the first to become either males or females, but as
regards sex the spermatozoa are alike in being non-female.
2. That there is a repulsion between the femaleness-factor and the
_grossulariata_ factor.
Such a repulsion between two factors we are justified in regarding as
possible because we have had proof of the occurrence of a similar
repulsion in the case of the two factors in the Sweet Pea.
If the case of this moth stood alone it would be interesting, but its
importance is greatly increased by the fact that we know two cases in
birds which are closely comparable. The simpler case to which alone I
shall refer has been observed in the Canary. Like the Currant moth it
has a kind of albino, called Cinnamon, and males of this variety when
mated with ordinary dark green hen canaries produce dark males and
Cinnamons which are always hens; while the green male and the Cinnamon
hen produce nothing but greens of both sexes. This case, which has been
experimentally studied by Miss Durham, offers a certain complication,
but in its main outlines it is exactly like that of the moth, and the
same interpretation is applicable to both.
The particular interpretation may be imperfect and even partially
wrong; but that we are at last able to form a working idea of the course
of such phenomena at all is a most encouraging fact. If we are right, as
I am strongly inclined to believe, we get a glimpse of the significance
of the popular idea that in certain respects daughters are apt to
resemble their fathers and sons their mothers; a phenomenon which is
certainly sometimes to be observed.
There are several collateral indications that we are on the right track
in our theory of the nature of sex. One of these, derived from the
peculiar inheritance of colour-blindness, is especially interesting.
That affection is common in men, rare in women. Men who are colour-blind
can transmit the affection but men who have normal vision cannot. Women
however who are ostensibly normal may have colour-blind sons; and women
who are colour-blind have, so far as we know, no sons who are not
colour-blind[2].
Mendelian analysis of these facts shows that colour-blindness is due,
not, as might have been supposed, to the absence of something from the
composition of the body, but to the presence of something which affects
the sight. Just as nicotine-poisoning can paralyse the colour sense, so
may we conceive the development of a secretion in the body which has a
similar action. The comparative exemption of the woman must therefore
mean that there is in her a positive factor which counteracts the
colour-blindness factor, and it is not improbable that the counteracting
element is no other than the femaleness-factor itself[3].
I think I have said enough to prove that after all, those curiosities
collected from observation of Sweet Peas and Canaries have no remote
bearing on some very fascinating problems of human life.
Lastly I suppose it is self-evident that they have a bearing on the
problem of Evolution. The facts of heredity and variation are the
materials out of which all theories of Evolution are constructed. At
last by genetic methods we are beginning to obtain such facts of
unimpeachable quality, and free from the flaws that were inevitable in
older collections. From a survey of these materials we see something of
the changes which will have to be made in the orthodox edifice to admit
of their incorporation, but he must be rash indeed who would now attempt
a comprehensive reconstruction. The results of genetic research are so
bewilderingly novel that we need time and an exhaustive study of their
inter-relations before we can hope to see them in proper value and
perspective. In all the discussions of the stability and fitness of
species who ever contemplated the possibility of a wild species having
one of its sexes permanently hybrid? When I spoke of adventures to be
encountered in genetic research I was thinking of such astonishing
discoveries as that.
There are others no less disconcerting. Who would have supposed it
possible that the pollen-cells of a plant could be all of one type, and
its egg-cells of two types? Yet Miss Saunders' experiments have provided
definite proof that this is the condition of certain Stocks, of which
the pollen grains all bear doubleness, while the egg-cells are some
singles and some doubles. We cannot think yet of interpreting these
complex phenomena in terms of a common plan. All that we know is that
there is now open for our scrutiny a world of varied, orderly and
specific physiological wonders into which we have as yet only peeped. To
lay down positive propositions as to the origin and inter-relation of
species in general, now, would be a task as fruitless as that of a
chemist must have been who had tried to state the relationship of the
elements before their properties had been investigated.
For the first time _Variation_ and _Reversion_ have a concrete, palpable
meaning. Hitherto they have stood by in all evolutionary debates,
convenient genii, ready to perform as little or as much as might be
desired by the conjuror. That vaporous stage of their existence is over;
and we see Variation shaping itself as a definite, physiological event,
the addition or omission of one or more definite elements; and Reversion
as that particular addition or subtraction which brings the total of the
elements back to something it had been before in the history of the
race.
The time for discussion of Evolution as a problem at large is closed. We
face that problem now as one soluble by minute, critical analysis. Lord
Acton in his inaugural lecture said that in the study of history we are
at the beginning of the documentary age. No one will charge me with
disrespect to the great name we commemorate this year, if I apply those
words to the history of Evolution: Darwin, it was, who first showed us
that the species have a history that can be read at all. If in the new
reading of that history, there be found departures from the text laid
down in his first recension, it is not to his fearless spirit that they
will bring dismay.