some few animals on distant mountain summits and in the Arctic region is
due to the former extension of ice between these situations during the
last glacial period. He was, before everything else and by necessity for
the examination of his theory, a geologist, and wrote many valuable
geological memoirs. The history of the origin of the species of living
things consists largely in tracing them to extinct creatures, and in
showing what were the possible migrations and what the conditions of
land and water, temperature and vegetation, in past periods, and in
regard to given areas of the globe. The book on the _Fertilisation of
Orchids_ was the first published by Darwin after the _Origin of
Species_. In it he showed how the marvellous shapes and colours and
mechanisms of the flowers of orchids are adapted to ensure
cross-fertilisation by insects, and how they can be explained as
originating by the natural selection of variations—if the value of
cross-fertilisation is once recognised. The explanation of the reason
for the existence of two kinds of primrose flowers—the short-styled and
the long-styled—clearly arrived at by him as being a mechanism to
secure cross-fertilisation, delighted him in 1862, and led him to
discover the same sort of modification in other flowers. Then, in 1864,
he published his researches on _Climbing Plants_, and later a book on
the _Movements of Plants_, in which he discovered the mechanism and the
wonderful variety of movements of plants, and showed their value to the
plant, and consequent origin, by natural selection.
He especially loved to discover evidence that plants can do many things
which had been thought to be only within the powers of the other section
of living things—the animals; and finding during one summer holiday
that the beautiful little sun-dew moves its red-knobbed tentacles so as
to entrap minute insects, he discovered the whole history of
_Insectivorous Plants_, and showed that there are many plants of various
groups which catch insects and digest them in a sort of stomach, as an
animal might do. Thus the water-holding pitchers of the pitcher-plants
of tropical forests were explained as being food-catchers and digesters
of great value to the nutrition of the plant, and their gradual
formation by variation and natural selection rendered comprehensible.
His greatest book next to the _Origin_—containing an immense quantity
of original notes and observations and valuable information from all
kinds of breeders and fanciers—is the _Variation of Animals and Plants
under Domestication_ (1868). The facts recorded are discussed in the
light of the great theory, and honest, fair-minded consideration is
given to those which present difficulties as well as to those which
clearly favour it. In 1871 came the _Descent of Man_, followed in 1872
by the _Expression of the Emotions in Men and Animals_—in which, again,
it was shown that the facts as to the likeness between man and apes can
be explained on the theory that natural selection and survival of
favourable variations have been at work, and that the facts are
hopelessly without meaning or explanation on any other hypothesis. His
last published book was on _The Formation of Vegetable Mould through the
Action of Worms_, in which he not only showed what an important part
earthworms play in burying stones and rocks, and in fitting the ground
for the growth of plants, but recorded some discoveries as to the senses
of worms and as to their treatment of leaves by a digestive fluid exuded
from the mouth so as to soften a leaf before swallowing it.
Every one of Darwin’s books abounds with new facts and new points of
view disclosed by the application to first one thing and then another of
his vivifying discovery-causing theory of natural selection. The
subsidiary theory of the selection of brilliantly coloured males by
females in pairing, as a cause of the brilliant colours and patterns of
many birds and insects, is developed in his _Descent of Man_. It led him
to many important discoveries and observations as to the colouring and
ornamentation of animals, and when considered, together with Wallace’s
and Bates’s theory of mimicry and of the warning and protective
colourings of insects, goes far to explain all the specific colouring of
animals and plants as due to natural selection and survival. A theory
which has produced such prodigious results in the way of “explaining”
all forms, colours, habits, and occurrences of living things—as has
that of Charles Darwin—simply holds the field against all comers. When
Lamarck’s theory has been shown to be consistent with the most
elementary facts as to heredity, and further to afford a rational
explanation of any group of biological facts, it will be time to
consider how far it may be entertained in conjunction with Darwin’s
theory—but not until then.
V
DARWIN’S THEORY UNSHAKEN
It seems ill-mannered, if not ill-natured, that the year of the
centenary of Charles Darwin’s birth should have been chosen by owners of
anonymous pens in order to alarm the public mind with the preposterous
statement that his celebrated and universally accepted theory of the
origin of the species or kinds of plants and animals by natural
selection, or “the survival of favoured races in the struggle for life,”
is undermined and discredited. Such a statement once coolly made in the
public Press is necessarily believed by a large number of uninformed
readers, and, like all calumny, is none the less relished by the
foolish, and, for the moment, none the less harmful, because it is
baseless.
Those who seek to belittle Darwin’s theory show, whenever they venture
to enter into particulars, that they do not know what Darwin’s theory
is. They confuse it with other theories, and even imagine that some
enthusiastic Darwinians who have tried to add a chapter here or there to
Darwin’s doctrine, are opponents of the great theory. Let me briefly
state what that theory is:
It rests on three groups of facts—matters of observation, which are not
theory or guess work at all—but admitted by every one and demonstrated
every day. These are—(1) Living things, each in its kind, produce a
far larger number of young than can possibly grow up to maturity, since
the kind of food and the situation necessary to each kind are limited
and already occupied. Only one oyster embryo out of every five million
produced (the reader may refer to p. 137 on this subject) grows up
through all the successive stages of youth to the adult state. The total
number of a species of animal or plant on the whole area where it is
found does not increase. Even in those which produce a small number of
young, there is great destruction, and taking all the individuals into
consideration, only a single pair of young arrive at maturity to replace
their parents. There is no exception to the rule that every organic
being naturally multiplies at so high a rate that, if not destroyed, the
progeny of a single pair would soon cover the earth. The elephant is
reckoned the slowest breeder of known animals; it commences to breed at
30 years of age, dies at 100, and has six young in the interval. After
750 years, supposing all the offspring of a single pair fulfilled the
rule and were not destroyed in an untimely way, there would be nearly
nineteen million elephants alive descended from the first pair. There is
then no doubt as to the enormous excess in the production of young
living things, nor as to their necessary competition with one another of
the most severe and inexorable kind; nor again as to the necessary
death, in many species, of hundreds and thousands, for every one which
survives to maturity and in its turn breeds.
(2) The second great fact is that among all the young born to a pair of
parents, no two are exactly alike, nor are any exactly like their
parents; nor are any two taken from all produced by all parents of that
species exactly alike. They all resemble their parents at the
corresponding age, in a general way and even very closely; but the
resemblance is far from amounting to identity. This is called
“variation.” It is familiar to us all in the case of the organism which
we know best, and observe most closely, namely, man. It is also a matter
of common observation in the case of dogs, cats, horses, and other
domesticated animals. Many of these “variations” are exhibited in points
of size, proportion, and colour, which are easily noted at once by the
eye. But “variation” is really a deep-seated thing, and depends on
causes which lie below the surface. We know that the offspring of men
and of animals and of plants, give evidence of variations in what we
call constitution, tendency, temperament, aptitude, strength, and that
the colour, and even size of this or that part, are really only
indications of a deep-seated difference in the living chemistry, the
forces of nutrition and growth which reside in the living substance. The
fact that many thousands of a species may be born and only a few
survive, means therefore that many thousand varieties, often varieties
not readily measured by the eye, are produced in each generation, from
which a few individuals are in some way “selected” for survival.
(3) The third great fact is that though there is variation, amongst all
the offspring in each generation, there is also a continual and definite
inheritance by offspring of the qualities and structure of their parents
to a degree which altogether preponderates over the variations. To put
it in another way, we all know that every parental organism transmits to
its young not only the qualities and structure of the species, or of the
race, or of the family, but also transmits its own peculiarities or
variations in which it departed from its parents, and from its brothers
and sisters. This is best illustrated by our daily experience of human
families.
These facts being admitted, and abundantly illustrated and traced in
detail by years of observation and experimental breeding in all kinds
of living things by hundreds of careful observers who have published the
records of their studies, we come to the step where Darwin makes use of
supposition or hypothesis. The question is, “Does the one which, out of
the thousands of slightly different varieties, survives—do so by
haphazard? or is there a necessarily acting state of things which
selects that one special variety for survival?” Gardeners and breeders
of pigeons, dogs, and cattle deliberately select the variations which
they desire, breed from them, and so carry on by inheritance the special
variation—whilst they ruthlessly destroy or restrain from breeding the
numerous other variations in their “stock” which they do not desire.
“If,” said Darwin, “there is any necessarily selective mechanism in
Nature which could act as the breeder does, new varieties might be
‘naturally’ selected, and changes of form and appearance naturally
established, which in the course of long ages would amount to such
marked differences as separate what we call one species from another.”
He showed that there is a natural mechanism of the required kind.
“Since,” he says, “the competition among the members of any one kind or
species for a place in life is so very severe, and the hostile
circumstances so varied, and since all the competing offspring differ by
‘variation’ ever so little from one another, those varieties which are
better suited in even the smallest degree to hold their own not merely
in fighting with the others, but in withstanding injurious influences,
in escaping enemies, and in procuring food, will be the ones which will
survive, when a large number of cases, many thousands, extending over a
large area and many years, are considered. Those which are ‘best fitted’
to get through the exceedingly numerous dangers and difficulties of life
will be the survivors.” Hence we get the survival of the fit—the fit
variations—by natural selection in the struggle for life. This, it
will be observed, is an inference, and not a direct observation.
So long as the conditions remain practically or effectively unchanged,
the animal or plant already “fitted” to them will be succeeded by those
of its offspring which most resemble it in the essential points of
“fitness.” But we know that in the course of ages, more or less rapidly,
climates change, land emerges from the sea, islands join continents,
continents become scattered islands, animals and plants migrate into
regions previously uninhabited by them. As such changes gradually come
on, the natural selection of favoured varieties will necessarily lead to
the survival of others than those previously favoured, other variations
better suited to the new conditions will survive.
The natural selection of favoured variations would not amount to much,
were the variations not perpetuated by transmission to the young which
they produce. This, it is common knowledge [see (3)], does take place.
It is known also that a variation so established is as a result of the
regular process of variation presented in larger volume or emphasised in
character in some individuals of subsequent generations, and by
continued “natural selection” it may become more and more a prominent or
dominant feature of the race.
So far, the only assumption made by Mr. Darwin is that any or some of
the endless variations which occur in all the offspring of wild plants
and animals, in various combinations and degree in each individual, can
be sufficiently important to determine the survival or non-survival of
the organisms possessing them. That is a matter which has been largely
studied and discussed. The verdict of those who have studied on the spot
(as Darwin himself did) the teeming life of the tropics, the insects,
birds, and plants of those regions, is that we are justified in
considering that small variations are sufficiently important to turn
the scale in favour of survival or non-survival. It is not easy for a
man who is not a determined naturalist, constantly observing the ways of
wild living things, to appreciate the evidence as to the efficacy of
small variations, even were I able here to submit it to him. It is to be
found in the published works of an army of investigators. In any case it
is granted that effective variations—whether small or great—occur in
nature, and that natural selection favours and perpetuates the new and
fitter variety to the exclusion of the less fit.
The real difficulty to most people comes in the supposition next made by
Mr. Darwin—namely, that this slow process of change by natural
selection of favoured variations and their transmission and perpetuation
by inheritance is sufficient to effect by its continued operation
through enormous ages of time the conversion of a race of ancestral
three-toed zebras into the one-toed horse of to-day; before that, of
five-toed beasts into three-toed; at an earlier stage of fishlike
creatures into four-footed land animals, and so on. You have to picture
the whole series of animals and of plants which are now or ever have
been, as two gigantic family trees or pedigrees, meeting in common
ancestors of the simplest grade of microscopic life. All the diverging
branches and twigs of these great “family trees” have been determined by
the adaptation of living form to the endlessly varied conditions of life
on this planet, by the natural selection or survival of variations and
the transmission and accumulation of those variations from parent to
offspring. This is a tremendous demand on the imagination. It is,
however, not a difficult one to concede, when one is acquainted with the
facts and conclusions of geology. The history of the crust of the earth
was explained twenty years before the date of Darwin’s theory by Charles
Lyell as due to the continued action through immense periods of time of
the same natural forces which are now at work. And, moreover, the
examination of the successive stratified deposits of the earth’s crust
has yielded the remains of whole series of animals and of plants
(simpler in character the older and deeper the rock in which they
occur), which can be satisfactorily explained and interpreted as the
ancestral forms from which present organisms have been developed.
The theory of the natural selection of variations as the moving spring
in the gradual development of living forms from simplest living matter
is Darwin’s theory. It is not possible to find any naturalist of
consideration who does not accept it. There are various views held and
discussed as to the cause of variation, as to the importance of small
and of big variations, as to the non-transmissibility of some kinds of
variation, and as to various peculiarities in regard to inheritance.
They do not for the most part touch the main features of Mr. Darwin’s
theory. No doubt we are learning and shall learn more about the facts of
variation and the details of the process of hereditary transmission, but
such increase of knowledge has not tended to undermine Mr. Darwin’s
theory, and does not seem at all likely to do so.
On the occasion of the celebration at Cambridge in 1909 of the centenary
of Darwin’s birth, I was invited by the Vice-Chancellor, on behalf of
the University, to deliver in the Senate-house an address, others being
given by representatives of the United States (Prof. Osborne), of
Germany (Prof. Hertwig), and of Russia (Prof. Metchnikoff). The
following is the text of that address:—
“I feel it a great honour to be called upon to speak here to-day, and to
stand, on behalf of the naturalists of the British Empire, by the side
of the distinguished men whose orations you have just heard.
“I think that the one thing about Charles Darwin which the large
majority of British naturalists would wish to be to-day proclaimed, in
the first place—with no doubtful or qualifying phrase—is that, in
their judgment, after these fifty years of examination and testing, his
‘theory of the origin of species by means of natural selection or the
preservation of favoured races in the struggle for life’ remains whole
and sound and convincing, in spite of every attempt to upset it.
“I am not stating more than the simple truth when I say that, in the
judgment of those who are best acquainted with living things in their
actual living surroundings, ‘natural selection’ retains the position
which Mr. Darwin claimed for it of being the main means of the
modification of organic forms.
“Our admiration for the vast series of beautiful observations and
interesting inquiries carried out by Darwin during his long life must
not lead us to forget that they were devised by him in order to test the
truth of his theory and to meet objections to it, and that they were
triumphantly successful. They, together with the work of Alfred Russel
Wallace and many of their followers, have more and more firmly
established Darwin’s theory. On the other hand, no attempt to amend that
theory in any essential particular has been successful.
“The nature of organic variation and of the character of the variations
upon which natural selection can and does act was not, as we are
sometimes asked to believe, neglected or misapprehended by Darwin. The
notion that these variations are large and sudden was considered by him,
and for reasons set forth by him at considerable length rejected. That
notion has in recent years been resuscitated, but its truth has not been
rendered probable by evidence either of such an accurate character or of
such pertinence as would justify the rejection of Darwin’s fundamental
conception of the importance of minute and ubiquitous variations.
“Further, in regard to the important facts of heredity connected with
the cross-breeding of cultivated varieties, especially in regard to the
blending or non-blending of their characters in their offspring and as
to prepotency, it seems to me important that we should now and here call
to mind the full and careful consideration given to this subject by
Darwin. We cannot doubt that he would have been deeply interested in the
numerical and statistical results associated with the name of Mendel.
Those results tend to throw light on the mechanisms concerned in
hereditary transmission, but it cannot be shown that they are opposed in
any way to the truth of Darwin’s great theoretical structure—his
doctrine of the origin of species.
“It has often been urged against Darwin that he did not explain the
origin of variation, and especially that he has not shown how variations
of sufficient moment to be selected for preservation in the struggle for
existence have in the first place originated. The brief reply to the
first objection is that variation is a common attribute of many natural
substances of which living matter is only one. In regard to the second
point, I desire to remind this assembly that Darwin described with
special emphasis instances of what he calls ‘correlated variability.’ In
my opinion he has thus furnished the key to the explanation of what are
called useless specific characters and of incipient organs. That key
consists in the fact that a general physiological property or character
of utility is often selected and perpetuated, which carries with it
distinct, even remote, correlated growths and peculiarities obvious to
our eyes, yet having no functional value. At a later stage in the
history of such a form these correlated growths may acquire value and
become the subject of selection.
“It is thus, as it seems to me, and as, I believe, to the great body of
my brother naturalists, that Darwin’s theory stands after fifty years of
trial and application.
“The greatness of Charles Darwin’s work is, and will be for ever, one of
the glories of the University of Cambridge. It is fitting on the present
occasion that one who speaks on behalf of English men of science should
call to mind the nature of his connection with this great University and
the peculiarly English features of his life-story and of that fine
character which endears his memory to all of us as much as his genius
excites our admiration and reverence. Darwin was not, like so many a
distinguished son of Cambridge, a scholar or a fellow of his college,
nor a professor of the University. His connection with the University
and the influence which it had upon his life belong to a tradition and a
system which have survived longer in our old English universities than
in those of other lands. Darwin entered the University, not seeking a
special course of study with the view of professional training, nor
aiming at success in competitive examinations for honours and emolument.
He came to Cambridge intending to become a clergyman, but blessed with
sufficient means and leisure to enable him to pursue his own devices, to
collect beetles, to explore the fen country, and to cultivate his love
of nature. It was thus that he became acquainted with that rare spirit
Henslow, the Cambridge professor of botany, and it is through Henslow
and the influence of his splendid abilities and high personal character
upon Darwin that Cambridge acquired the right to claim the author of the
‘Origin of Species’ as a product of her beneficence and activity as a
seat of learning.
“As an Oxford man and a member of Exeter College, I may remind this
assembly that in precisely the same way Darwin’s dearest friend and
elder brother in science, Charles Lyell, had a few years earlier
entered at Exeter College, and by happy chance fallen under the
influence of the enthusiastic Buckland, the University reader in geology
and a Canon of Christ Church. The wise freedom of study permitted and
provided for in those long-passed days by Oxford and Cambridge is what
has given the right to claim the discovery, if not the making, of Lyell
to the one and of Darwin to the other.
“Darwin’s love of living nature and of the country life are especially
English characteristics; so, too, I venture to think, are the
unflinching determination and simple courage—I may even say the
audacity—with which he acquired, after he had left the University, the
wide range of detailed knowledge in various branches of science which he
found necessary in order to deal with the problem of the origin of the
species of plants and animals, the investigation of which became his
passion.
“The unselfish generosity and delicacy of feeling which marked Darwin’s
relations with a younger naturalist, Alfred Russel Wallace, are known to
all. I cannot let this occasion pass without citing those words of his
which tell us most clearly what manner of man he was and add to his
splendid achievements as an intellectual force—a light and a beauty of
which every Englishman must be proud. When in old age he surveyed his
life’s work he wrote:—‘I believe that I have acted rightly in steadily
following and devoting my life to science.’