readily understand how a voyage round the world, lasting for five
years, would offer to the inquiring mind of a Darwin rich opportunities
for the most varied observations. That he did not fail to make use of
these is evidenced not only by his book on _The Origin of Species_, but
by several more special works, published shortly after his return--his
natural history of those remarkable sessile crustaceans, the barnacles
or Cirripedia, and his studies on the origin of coral reefs. The
first-named book still holds its own as a classic monograph on this
animal group, with its wealth of forms; and the theory of the origin of
coral reefs which Darwin elaborated has still many adherents, in spite
of various rival interpretations.
But Darwin would hardly have achieved what he did if he had been
compelled to secure for himself a professional position in order to
obtain bread and butter. Such great problems demand not only the whole
of a man's mental energy, they monopolize his time. Studies of detail
may well be taken up in leisure hours, but big problems absorb all the
thoughts and must always be present to the mind, lest the connexion
between the many individual inquiries, which make up the whole task, be
lost sight of. Darwin had the good fortune to be a free investigator,
and to be able to retire, on his return from his travels, to a small
property at Down in Kent, there to live for his family and his work.
Here he followed up the idea of evolution which he had already
formulated, and it has always seemed to me the most remarkable thing
about him, that he was able to keep in mind and work up the hundreds
of isolated inquiries that were eventually to be brought together to
form the main fabric of his theory. When one studies his many later
writings, one cannot but be surprised afresh by the number of different
sets of facts he collected at the same time, partly from others,
partly from personal observation, and continually also from his own
experiments. He made experiments on plants and on animals, and the
number of people with whom he carried on a scientific correspondence
is simply astounding. In this way he brought together, in the course
of twenty years, an extraordinarily rich material of facts, from the
fullness of which he was able later to write his book on _The Origin
of Species_. Never before had a theory of evolution been so thoroughly
prepared for, and it is undoubtedly to this that it owed a great part
of its success; not to this alone, however, but still more, if not
mainly, to the fact that it presented a principle of interpretation
that had never before been thought of, but whose importance was
apparent as soon as attention was called to it--the principle of
selection.
Charles Darwin championed, in the main, the same fundamental ideas as
had been promulgated by his grandfather, Erasmus Darwin, by Treviranus,
and by Lamarck: species only seem to us immutable; in reality they can
vary, and become transformed into other species, and the living world
of our day has arisen through such transformations, through a sublime
process of evolution which began with the lowest forms of life, but
by degrees, in the course of unthinkably long ages, progressed to
organisms more and more complex in structure, more and more effective
in function.
It is interesting to note at what point Darwin first put in his lever
to attempt the solution of the problem of evolution. He started from
quite a different point from the investigators of the early part of
the century, for he began with forms of life which had previously
been markedly neglected by science, the varieties of our domesticated
animals and cultivated plants.
Previously these had been in a sense mere step-children of biology,
inconvenient existences which would not fit properly into the system,
which were therefore as far as possible ignored or dismissed as outside
the scope of 'the natural,' because it was difficult to know what else
to do with them. I can quite well remember that, even as a boy, I was
struck by the fact that one could find nothing in the systematic books
about the many well-established garden forms of plants, or about our
domestic animals, which seemed to be regarded as in a sense artificial
products, and as such not worthy of scientific consideration. But it
was in these that Darwin particularly interested himself, making them
virtually the basis of his theory, for he led up from them to the very
principle of transformation, which was his most important addition to
the earlier presentations of the Evolution theory.
He started from the existence of varieties which may be observed in
so many wild species. His line of thought was somewhat as follows: If
species have really arisen through a gradual process of transformation,
then varieties must be regarded as possible first steps towards new
species; if, therefore, we can only succeed in finding out the causes
which underlie the formation of any varieties whatever, we shall have
discovered the causes of the transformation of species. Now we find by
far the greatest number of varieties, and the most marked ones, among
our domesticated animals and plants, and unless we are to assume that
each of these is descended from a special wild species, the reason why
there has been such a wealth of variety-formation among them must lie
in the conditions which influence the relevant species in the course of
domestication; and it remains for us to analyse these conditions till
we come upon the track of the operative factors. With this conviction,
Darwin devoted himself to the study of domesticated animals and plants.
The first essential was to prove that every variety had not a separate
wild species as ancestor, but that the whole wealth of our domesticated
breeds originated, in each case, from one, or at least from a few wild
species. Of course I cannot here recapitulate the multitudinous facts
which were marshalled by Darwin, especially in his later works, notably
his _Animals and Plants under Domestication_, but this is not necessary
to an understanding of his conclusions, and I shall therefore restrict
myself to a few examples.
Let us take first the domestic dog, _Canis familiaris_, Linné. We
have at the present day no fewer than seven main breeds, each of
which has its sub-breeds, often numerous. Thus there are forty-eight
sub-breeds which are used as guardians of our houses, 'house-dogs'
in the restricted sense, thirty sub-breeds of dogs with silk-like
hair (King Charles dogs, Newfoundland dogs, &c.), twelve of terriers,
and thirty-five of sporting dogs, among them such different forms as
the deerhound and the pointer. We have further nineteen sub-breeds
of bulldogs, thirty-five of greyhounds, and six of naked or hairless
dogs. Not only the main breeds, but even the sub-breeds often differ
as markedly from one another as wild species do, and the question must
first be decided whether each of the very distinct breeds has not a
special wild species as ancestor.
Obviously, however, this cannot be maintained, for so many species of
wild dog have never existed on the earth at any time. We know, too,
that 4,000 or 5,000 years ago a large number of breeds of dogs were
in existence in India and Egypt. There were Pariah dogs, coursers,
greyhounds, mastiffs, house-dogs, lapdogs and terriers. It is not
possible that the products of all lands could, at that time, have been
gathered into one, and it is inconceivable that so many wild species
could have existed in the one country of India.
On the other hand, however, it cannot be maintained that all our
present breeds have descended from _a single_ wild species; it is much
more probable that several wild species were domesticated in different
countries.
It has often been supposed that the manifold diversity of our present
breeds has been brought about by crossing the various tamed species.
That cannot be the case, however, because crossing gives rise only to
hybrid mongrel forms, not to distinct breeds with quite new characters.
It is true that all breeds of dogs can be very readily crossed with
each other, but the result is not new breeds, but those numberless
and transient intermediate forms which the dog-breeder despises as
worthless for his purpose. It must therefore have been through the
influence of domestication, combined with crossing, that a few wild
species gave rise to the various breeds of dogs.
The pedigree of the horse is rather more clear than that of the dog.
Even in this case, indeed, one cannot definitely name the ancestral
wild form, but it is very probable that it was of a grey-brown colour,
and similar to the wild horses of our own day. Darwin supposes that
it must also have had the black stripe on the back which is exhibited
by the domestic ass, and by several wild species of ass, basing his
opinion on the fact that the spinal stripe often occurs in foals,
especially in those of a grey-brown colour.
But though there can be no doubt that this is to be interpreted as a
reversion to a character of a remote ancestor, it by no means follows
that the _direct_ ancestral form must have had this stripe. I am
more inclined to believe that the ancestor which bore this mark was
considerably more remote, and lived before the differentiation of the
horse from the ass. Darwin himself noted the remarkable fact that in
rare cases, especially in foals, not only may the stripe on the back be
present, but there may be more or less distinct zebra-striping on the
legs and withers: this, however, must be interpreted as a reversion to
the character of a very much more remote ancestor, to a common ancestor
of all our present-day horses and asses, which must have been striped
over its whole body, like the zebra living in Africa now.
It cannot be proved of any of the wild horses of to-day that they are
not descended from domesticated ancestors; indeed, we can say with
certainty that the thousands of wild horses which roam the plains of
North and South America are descended from domestic horses, for there
was no horse in America at the time it was discovered by the Europeans.
In all probability our horse originated in Middle Asia, was there
first domesticated, and has thence been gradually introduced into
other countries. In Egypt it appears first on the monuments in the
seventeenth century B.C., and it seems to have been introduced by the
conquering Hyksos. On the ancient Assyrian monuments the chase after
wild horses is depicted, and they were not caught, but killed with
arrow and lance, like the lion and the gazelle.
But even if two wild species of horse had been tamed in different parts
of the great continent of Asia, these two domesticated animals would
have varied much and in the most diverse manner, as we may infer from
our different breeds of horses at the present day. There are a great
many of these, and many of them differ very considerably from each
other. If we think of the lightly built Arab horse, and place beside
it the little pony, or the enormous Percheron, the powerful cart-horse
from the old French province of La Perche, which easily draws a load of
fifty kilograms, we are face to face with differences as great as those
between natural species. And we may realize how many breeds of horses
there are now upon the earth if we remember that nearly every oceanic
island has its special breed of ponies. Not only in the cold Shetland
Islands, England, Sardinia and Corsica, but in almost every one of the
larger islands of the extensive Indian Archipelago there is one, and
Borneo and Sumatra have several.
But the most conclusive proof of descent from a single wild species
is afforded by the pigeons, and as the production of new breeds among
them has been, and will continue to be, carried on with particular
enthusiasm and deliberateness, I propose to deal with them somewhat
more in detail.
Darwin's work proves beyond a doubt that all our present-day breeds
of pigeons are descended from a single wild species, the rock-dove,
_Columba livia_. In appearance, this form, which still lives in a wild
state, differs little from our half-wild blue-grey field-pigeon. It has
the same metallic shimmer on the feathers of the neck, the same two
black cross-bars on the wings as well as the band over the tail, and it
has also the same slate-blue general colour. Now, all breeds of pigeons
are without restriction fertile _inter se_, so that any breed can be
crossed with any other, and it often happens that, in the products
of such crossing, characters appear which the parents, that is, the
two or more crossed breeds, did not possess, but which are among the
characters of the rock-dove. Thus Darwin obtained, by crossing a pure
white fantail with a black barb, hybrids which were partly blackish
brown, partly mixed with white, but when he crossed these hybrids with
others from two breeds which were likewise not blue, and had no bars,
he obtained a slate-blue rock-pigeon, with bars on the wings and tail.
We shall inquire later on how far it is correct to regard such cases
as reversions to remote ancestors, but if we take it for granted in
the meantime, we have here a proof of the descent of our breeds from a
single wild species. This is corroborated, too, by everything that we
know about the distribution of the rock-pigeon and the place and time
of its domestication. It still lives on the cliff-guarded shores of
England, Brittany, Portugal, and Spain, and both in India and in Egypt
there were tame pigeons at a very early period. Pigeons appear on the
menu of a Pharaoh of the fourth dynasty (3000 B.C.), and of India we
know at least that in 1600 A.D. there were 20,000 pigeons belonging to
the court of one of the princes.
The beauty of this bird, and the ease with which it can be tamed,
obviously called man's attention to it at a very early date, and it has
been one of man's domestic companions for several thousands of years.
Now we can distinguish at least twenty main races (Fig. 1), which
differ from each other as markedly as, if not more markedly than, the
most nearly allied of the 288 wild species of pigeons which inhabit the
earth. We have carriers and tumblers, runts and barbs, pouters, turbits
and Jacobins, trumpeters and laughers, fantails, swallows, Indian
pigeons, &c.
[Illustration: FIG. 1. Group of various races of domestic pigeons
(after Prütz). 1. Pouter. 2. Indian barb. 3. Bucharest trumpeter with
a whorl of feathers (_Nelke_) on its forehead. 4. Nürnberger swallow.
5. Nürnberger bagadotte. 6. English carrier. 7. Fantail. 8. Eastern
turbit. 9. Schmalkaldener Jacobin. 10. Chinese owl. 11. German turbit.]
Each of these races falls into sub-races; thus there is a German, an
English, and a Dutch pouter-pigeon. The books on pigeons mention over
150 kinds which are quite distinct from one another, and breed true,
that is, always produce young similar to themselves.
Without entering upon a detailed description of any of these, I should
like to call attention to the way in which certain characters have
varied among them. Colour is a subordinate race-character, in so far
that colour alone does not constitute a race, yet the colouring within
a particular sub-race is usually very sharply defined, and in every
breed there are sub-races of different colours. Thus there are white,
black, and blue fantails, there are white turbits with red-brown wings,
but also red ones with white heads, and white tumblers with black
heads, &c. Very unusual colours and colour-markings sometimes occur.
Thus one sub-race of tumblers exhibits a peculiar clayey-yellow colour
splashed with black markings, otherwise rare among pigeons, and almost
suggestive of a prairie-hen; there is also a copper-red spot-pigeon, a
cherry-red 'Gimpel'-pigeon, lark-coloured pigeons, &c. Then we find all
possible juxtapositions of colours, limited to quite definite regions
of the body; thus we have white tumblers with a red head, red tail,
and red wing-tips, or white tumblers with a black head, red turbits
with white head, Indian pigeons quite black except for white wing-tips,
and so on. The distribution of colour is often very complicated, but
nevertheless, all the individuals of the breed show it in exactly the
same manner. Thus there are the so-called blondinettes in which almost
the whole body is copper-red, but the wings white, save that each quill
bears at the rounded end of its vane a black and red fringe. I should
never come to an end, if I were to try to give anything like a complete
idea of the diversity of colouring among the various breeds of pigeons.
Even such an important and, among wild species, unusually constant
organ as the bill has varied among pigeons to an astonishing degree.
Carrier-pigeons (Fig. 1, No. 6) have an enormously long and strong
bill, which is moreover covered with a thick red growth of the cere,
while in the turbits and owls (Fig. 1, Nos. 8 and 10) the bill is
shorter than any we find among wild birds. In many breeds even the
_form_ of the bill deviates far from the normal, as in the bagadottes
(No. 5) with crooked bill.
Like the bill, the legs vary in regard to their length. The pouters
(No. 1) stand on their long legs as on stilts, while the legs of the
'Nürnberger swallow' are strikingly small. Remarkable, too, and very
different from the wild species, is the thick growth of feathers on the
feet and toes of the pouters and trumpeters (Fig. 1, No. 1), as well
as of some other breeds, which suggests the arrangement of feathers on
a wing.
Furthermore, the number and size of wing and tail-feathers in the
different breeds often deviate considerably from the normal. The
fantail (No. 7) in its most perfect form possesses forty tail-feathers,
instead of the twelve usual in the wild rock-pigeon, and they are
carried upright like a fan, while the head and neck of the bird are
bent sharply backwards. In the hen-like pigeons the tail-feathers are
few and short, so that they show an upright tail like that of a hen. I
have already referred to the extraordinary carunculated skin-growth on
the bill of many breeds; such folds also often surround the eye, and,
as in the Indian barb (No. 3), are developed into well-formed thick
circular ridges, while in the English carrier (No. 6) they lie about
the bill as a formless mass of flesh.
Even the skull has undergone many variations, as can be observed
even in the living bird in many of the breeds with short forehead.
Differences are to be found, too, in the number and breadth of the
ribs, the length of the breast-bone, the number and size of the
tail-vertebræ in different breeds. Of the internal organs, the crop in
many breeds, but particularly in the pouters (No. 1), has attained an
enormous size, and with this size is usually associated the habit of
blowing it out with air, and assuming the characteristically upright
position.
That variations have taken place, too, in the most delicate structure
of the brain, is shown by certain new instincts, such as the trumpeting
of the trumpeters, the cooing of others, and the silence of yet other
breeds, as well as by the curious habit of the tumblers of ascending
quickly and vertically to a considerable height, and then turning
over once, or even several times, in the course of their descent. In
contrast to this, other breeds like the fantails have altogether given
up the habit of flying high, and usually remain close to the dove-cot.
Lastly, let me mention that the unusual development of individual
feathers, or of groups of feathers, has become a race-character, upon
which depend such remarkable structures as the feather-mantle turned
over the head in the Jacobins (No. 9), the cap or plume on the head of
various breeds, the white beard in the bearded tumbler, the collars
which lie like a shirt-collar on the breast, or run down the sides of
the neck (Nos. 8 and 10), and the circle of feathers which marks the
root of the bill in the Bucharest trumpeter (No. 3).
After what has been said, it is hardly necessary to add that the size
of the whole body differs in different races. But the differences
are very considerable, for, according to Darwin, one of the largest
runt-pigeons weighed exactly five times as much as one of the smallest
tumblers with short forehead, and in the illustration (Fig. 1) the
pouter looks a giant beside the little barb to its left.
Thus we see that nearly every part of the body of the pigeon has varied
under domestication in the most diverse ways, and to a high degree;
and the same is true of several other domesticated animals, poultry,
horses, sheep, cattle, pigs, and so on, though the matter is not
altogether so clear in their case, since descent from a single wild
species cannot be proved, and is in many cases improbable. But in the
case of pigeons this common descent is certain, and we have now to
inquire in what manner all these variations from the parent form have
been brought about.
The answering of this question is rendered easier by the fact that new
breeds arise even now, and that, to some extent at least, they can be
caused to arise, consciously and intentionally. In England, as well
as in Germany and France, there are associations for the breeding of
birds, and in England especially pigeon and poultry clubs are numerous
and highly developed. These by no means confine themselves to simply
preserving the purity of existing breeds, they are continually striving
to improve them, by increasing and accentuating their characters, or
even by introducing quite new qualities, and in many cases they succeed
even in this last. Prizes are offered for particular new variations,
and thus a spirit of rivalry is fostered among the breeders, and each
strives to produce the desired character as quickly as possible. Darwin
says: 'The English judges decided that the comb of the Spanish cock,
which had previously hung limply down, should stand erect, and in
five years this end was achieved; they ordained that hens should have
beards, and six years later fifty-seven of the groups of hens exhibited
at the Crystal Palace in London were bearded.' The transformation
does not always come about so quickly, however; thus, for instance,
it required thirteen years before a certain breed of tumblers was
furnished with a white head. But the breeders cause every visible part
of the body to vary as seems good to them, and within the last fifty
years they have really brought about very considerable changes in many
breeds. Their method of procedure is carefully to select for breeding
those birds which already possess a faint beginning of the desired
character. Domesticated animals have on the whole a higher degree of
variability than wild species, and the breeder takes advantage of
this. Suppose it is a question of adding a crown of feathers to a
smooth-headed breed, a bird is chosen which has the feathers on the
back of the head a little longer than usual, and mated for breeding.
Among its descendants there will probably be some which also exhibit
these slightly prominent feathers, and possibly there may be one or
other of them which has these feathers considerably lengthened. This
one is then used for breeding, and by continually proceeding thus,
and selecting for breeding, from generation to generation, only
the individuals which approach most nearly to the desired end, the
wished-for character is at last secured.
Thus it is not by crossing of different breeds, but by a patient
accumulating of insignificant little variations through many
generations, that the desired transformations are brought about. That
is the magic wand by means of which the expert breeder produces his
different breeds, we might almost say, as the sculptor moulds and
remoulds his clay model according to his fancy. Quite according to his
fancy the breeder has brought about all the fantastic forms we are
familiar with among pigeons, mere variations which are of no use either
to the bird itself or to man, which simply gratify man's whim without
in many cases even satisfying his sense of beauty. For many of the
existing breeds of pigeons, hens, and other domesticated animals, are
anything but beautiful, the body being often unharmonious in structure
and sometimes actually monstrous.