natural science as a product of heathendom, and therefore felt bound
to look coldly on it, if not even to oppose it. Later, however, even
the Christian Church felt itself forced to give the people some mental
nourishment in the form of natural history, and under its influence,
perhaps actually composed by teachers of the Church, there appeared a
little book, the so-called _Physiologus_, which was meant to instruct
the people in regard to the animal world. This remarkable work, which
has been preserved, must have had a very wide distribution in the
earlier Middle Ages, for it was translated into no fewer than twelve
languages, Greek, Armenian, Syriac, Arabic, Ethiopic, and so on. The
contents are very remarkable, and come from the most diverse sources,
that is, from the most different writers of antiquity, from Herodotus,
from the Bible, and so forth, but never from original observation. The
compilation does not really give descriptions of animals or of their
habits, but, of each of the forty-one animals which the _Physiologus_
recognizes, something remarkable is briefly related in true lapidary
style, sometimes a mere curiosity without further import, or sometimes
a symbolical interpretation. Thus the book says of the panther: 'he
is gaily coloured; after satiating himself he sleeps three days, and
awakes roaring, giving forth such an agreeable odour that all animals
come to him.' Of the pelican the well-known legend is related, that
it tears open its own breast to feed its young with its blood, thus
standing as a symbol of mother-love. Fabulous creatures, too, appear
in these pages. Of the Phœnix, that bird whose plumage glitters with
gold and precious stones, which was known even to Herodotus, and which
has survived through Eastern fairy-tales on to the time of our own
romanticists (Tieck), we read: 'it lives a thousand years, because it
has not eaten of the tree of knowledge'; then 'it sets fire to itself
and arises anew from its own ashes,' a symbol of nature's infinite
power of renewing its youth.
But while among the peoples of Europe all the science of the ancients
was lost, except a few barely recognizable fragments, the old lore was
preserved, both as regards organic nature and other orders of facts,
among the Arabs, through whom so many treasures of antiquity have
eventually been handed down to us, coming in the track of the Arabian
conquests across North Africa and Spain to the nations of Europe.
It was in this way, too, that the writings of Aristotle again found
recognition, after having been translated into Latin at Palermo at
the order of that enthusiast for Science and Art, the Hohenstaufen
Emperor, Frederick the Second. Our Emperor presented one copy of
Aristotle's writings to the University of Bologna, and thus the
wisdom of the ancient Greeks again became the common property of
European culture. From the thirteenth century to the eighteenth, the
study of natural science was limited to repeating and extending the
work of Aristotle. Nothing new, depending upon personal observation,
was added, and it does not even seem to have occurred to any one to
subject the statements of the Stagirite to any test, even when they
concerned the most familiar objects. No one noticed the error which
ascribed to the fly eight legs instead of six; there was in fact as
yet no investigation, and all knowledge of natural history was purely
scholastic, and gave absolute credence to the authority of the ancients.
A revulsion, however, occurred in the century of the Reformation, with
the breaking down of the blind belief in authority which had till
then prevailed in all provinces of human knowledge and thought. After
a long and severe struggle, dry scholasticism was finally overcome,
and natural science, with the rest, turned from a mere reliance on
books to original thinking and personal observation. Thenceforward
interpretations of natural processes were sought for no longer in the
writings of the ancients, but in Nature herself. Of the magnitude
of this emancipation, and of the severity of the struggle against
deep-rooted authority, one could form a faint idea from experience
even in my own youth. Our young minds were so deeply imbued with the
involuntary feeling that the ancients were superior to us moderns
in each and every respect, that not only the hardly re-attainable
plastic art of the Greeks and the immortal songs of Homer, but all the
mental products of antiquity seemed to us models which could never be
equalled; the tragedies of Sophocles were for us the greatest tragedies
that the world had ever seen, the odes of Horace the most beautiful
poems of all time!
In the domain of natural science the new era began with the overthrow
of the Ptolemaic cosmogony, which, for more than a thousand years,
had served as a basis for astronomy. When the German canon, Nicolas
Copernicus (born at Thorn, 1473, died 1543), reversed the old
theory, and showed that the sun did not revolve round the earth, but
the earth round the sun, the ice was broken and the way paved for
further progress. Galilei uttered his famous 'e pur si muove,' Kepler
established his three laws of the movements of the planets, and Newton,
a century later, interpreted their courses in terms of the law of
gravitation.
But we have not here to do with a history of physics or astronomy, and
I only wish to recall these well-known facts, in order that we may
see how increased knowledge in this domain was always accompanied by
advances in that of biology.
Here, however, we cannot yet chronicle any such thoroughgoing
revolution of general conceptions; the basis of detailed empirical
knowledge was not nearly broad enough for that, and it was in the
acquiring of such a foundation that the next three centuries, from the
sixteenth to the end of the eighteenth, were eagerly occupied.
The first step necessary was to collate the items of individual
knowledge in regard to the various forms of life, and to bring the
whole in unified form into general notice. This need was met for the
first time by Conrad Gessner's _Thierbuch_, a handsome folio volume,
printed at Zurich in 1551, and embellished with numerous woodcuts,
some of them very good. This was followed, in 1600, by a great work in
many volumes, written in Latin, by a professor of Bologna, Aldrovandi.
Not native animals alone but foreign ones also were described in these
works, for, after the discovery of America and the opening up of
communication with the East Indies, many new animal and plant forms
came to the knowledge of European nations by way of the sea. Thus
Francesco Hernandez (died 1600), physician in ordinary to Philip II,
described no fewer than forty new Mammals, more than two hundred
Birds, and many other American animals.
Again, in a quite different way, the naturalist's field of vision
was widened, namely, by the invention of the simple microscope, with
which Leeuwenhoek first discovered the new world of Infusorians,
and Swammerdam made his notable observations on the structure and
development of the very varied minute animal inhabitants of fresh
water. In the same century, the seventeenth, anatomists like Tulpius,
Malpighi, and many others extended the knowledge of the internal
structure of the higher animals and of Man, and a foundation was
laid for a deeper insight into the nature of vital functions by the
discovery of the circulation of the blood in Man and the higher
animals. In the following century, the eighteenth, this path of active
research was eagerly followed, and we need only mention such names as
Réaumur, Rösel von Rosenhof, De Geer, Bonnet, J. Chr. Schäfer, and
Ledermüller, to be immediately reminded of the wealth of facts about
the structure, life, and especially the development of our indigenous
animals, which we owe to the labours of these men.
* * * * *
All these advances, great and many-sided as they were, did not at once
lead to a renewal of the attempt of Empedocles to explain the origin
of the organic world. This was as yet not even recognized as a problem
requiring investigation, for men were content to take the world of
life simply as a fact. The idea of getting beyond the naïve, poetic
standpoint of the Mosaic story of Creation was as yet remote from the
minds of naturalists, partly because they were wholly fascinated by
the observation of masses of details, but chiefly because, first by
the English physician, John Ray (died 1678), then by the great Swede,
Carl Linné, the conception of organic 'species' had been formulated and
sharply defined. It is true enough that before the works of these two
men 'species' had been spoken of, but without being connected with any
definite idea; the word was used rather in the same vague sense as the
word 'genus,' to designate one of the smaller groups of organic forms,
but without implying any clear idea of its scope or of its limitations.
Now, however, for the first time, the term 'species' came to be used
strictly to mean the smallest homogeneous group of individual forms
of life upon the earth. John Ray held that the surest indication of a
'species' was that its members had been produced from the same seed;
that is, 'forms which are of different species maintain this specific
nature constantly, and one species does not arise from the seed of
another.' Here we have the germ of the doctrine of the absolute nature
and the immutability of species which Linné briefly characterized in
these words: 'Species tot sunt, quot formæ ab initio creatæ sunt,'
'there are just so many species as there were forms created in the
beginning.' It is here clearly implied, that species as we know them
have been as they are from all time, that, therefore, they exist in
nature as such and unchangeably, and have not been merely read into
nature by man.
This view, though we cannot now regard it as correct, was undoubtedly
reasonable, and thoroughly in accordance with the spirit of the time;
it was congruent with the knowledge, and above all with the scientific
endeavours of the age. In the eighteenth century there was danger that
all outlook on nature as a whole would be lost--smothered under the
enormous mass of isolated facts, and especially under the inundation of
diverse animal and plant forms which were continually being recognized.
It must therefore have been regarded as a real deliverance, when Linné
reduced this chaos of forms to a clearly ordered system, and relegated
each form to its proper place and value in relation to the whole. How,
indeed, could the great systematist have performed his task at all, if
he had not been able to work with definite and sharply circumscribed
groups of forms, if he had not been able to regard at least the lowest
elements of his system, the species, as fixed and definite types?
On the other hand, Linné was much too shrewd an observer not to
entertain, in the course of his long life, and under the influence of
the continually accumulating material, doubts as to the correctness
of his assumption of the fixity and absoluteness of his species. He
discovered from his own experience, what is fully borne out by ours,
that it is easy enough to define a species when there are only a few
specimens of a form to deal with, but that the difficulty increases
in proportion to the number and to the diversity of habitat of those
that are to be brought under one category. In the last edition of
the _Systema Naturæ_ we find very noteworthy passages in which Linné
wonders whether, after all, a species may not change, and in the course
of time diverge into varieties, and so forth. Of these doubts no notice
was taken at the time; the accepted doctrine of the fixity of species
was held to and even raised to the rank of a scientific dogma. Georges
Cuvier, the great disciple of the Stuttgart 'Karlschule,' accentuated
the doctrine still further by his establishment of animal-types, the
largest groups of forms in the animal kingdom within which a definite
and fundamentally distinct plan of architecture prevails. His four
types, Vertebrates, Molluscs, Articulate and Radiate animals, furnished
a further corroboration of the absolute nature of species, since they
seemed to show that even the highest and most comprehensive groups are
sharply defined off from one another.
Let me add that this doctrine of the absolute nature of species was not
fully elaborated till our own day, when the Swiss (afterwards American)
naturalist, Louis Agassiz, went so far as to maintain that not only
the highest and the lowest categories, but all those coming between
them, were categories established and sharply separated by Nature
herself. But in spite of much ingenuity and his wide and comprehensive
outlook he exerted himself in vain to find satisfactory and really
characteristic definitions of what was to be considered a class, an
order, a family, or a genus. He did not succeed in finding a rational
definition of these systematic concepts, and his endeavour may be
regarded as the last important attempt to prop up an interpretation of
nature already doomed to fall. But in referring to Louis Agassiz I have
anticipated the historical course of scientific development, and must
therefore go back to the last quarter of the eighteenth century.
The first unmistakable pioneer of the theory of descent, which now
emerged for the first time as a scientific doctrine, was our great
poet Goethe. He has indeed been often named as the founder of the
theory, but that seems to me saying too much. It is true, however, that
the inquiring mind of the poet certainly recognized in the structure
of 'related' animals the marvellous general resemblances amid all
the differences in detail, and he probed for the reason of these
form-relations. Through the science of 'comparative anatomy,' as it
was taught at the close of the century by Kielmeyer, Cuvier's teacher,
and later by Cuvier himself, Blumenbach, and others, numerous facts
had become known, which paved the way for such questions. It had, for
instance, been recognized that the arm of man, the wing of the bird,
the paddle of the seal, and even the foreleg of the horse, contain
essentially the same chain of bones, and Goethe had already expressed
these relations in his well-known verse,
'Alle Gestalten sind ähnlich, doch keine gleichet der andern,
Und so deutet der Chor auf ein geheimes Gesetz.'
As to what this law was he did not at that time pronounce an opinion,
though he may even then have thought of the transformation of species.
At first he contented himself with seeking for an ideal archetype or
'Urtypus' which was supposed to lie at the foundation of a larger or
smaller group. He discovered the archetypal plant or 'Urpflanze,'
when he rightly recognized that the parts of the flower are nothing
more than modified leaves. He spoke plainly of the 'metamorphosis of
plants,' meaning by that the transformation of his 'archetype' into
the endless diversity of actual plant forms. But at first he certainly
thought of this transformation only in the ideal sense, and not as a
factual evolutionary process.
The first who definitely maintained the latter view was, remarkably
enough, the grandfather of the man who, in our own day, made the theory
of descent finally triumphant, the English physician Erasmus Darwin,
born 1731. This quiet thinker published, in 1794, a book entitled
_Zoonomia_, and in it he takes the important step of substituting for
Goethe's 'secret law' a real relationship of species. He proclaims the
gradual establishment and ennobling of the animal world, and bases
his view mainly on the numerous obvious adaptations of the structure
of an organ to its use. I have not been able to find any passage in
the book in which he has expressly indicated that, because many of
the conditions of life could not have existed from the beginning,
these adaptations are therefore, as such, an argument for the gradual
transformation of species. But he assumed that such exact adaptations
to the functions of an organ could only arise through the exercise of
that function, and in this he saw a proof of transformation. Goethe had
expressed the same idea when he said, 'Thus the eagle has conformed
itself through the air to the air, the mole through the earth to the
earth, and the seal through the water to the water,' and this shows
that he too at one time thought of an actual transformation. But
neither he nor Erasmus Darwin were at all clear as to _how_ the use
of an organ could bring about its variation and transformation. The
latter only says that, for instance, the snout of the pig has become
hard through its constant grubbing in the ground; the trunk of the
elephant has acquired its great mobility through the perpetual use of
it for all sorts of purposes; the tongue of the herbivore owes its
hard, grater-like condition to the rubbing to and fro of the hard grass
in the mouth, and so on. How acute and thoughtful an observer Erasmus
Darwin was, is shown by the fact that he had correctly appreciated the
biological significance of many of the colour-adaptations of animals
to their surroundings, though it was reserved for his grandson to make
this fully clear at a much later date. Thus he regarded the varied
colouring of the python, of the leopard, and of the wild cat as the
best adapted for concealing them from their prey amid the play of light
and shadow in a leafy thicket. The black spot in front of the eye of
the swan he considered an arrangement to prevent the bird from being
dazzled, as would happen if that spot were as snow-white as the rest of
the plumage.
At the end of the book he sums up his views in the following
sentences: 'The world has been evolved, not created; it has arisen
little by little from a small beginning, and has increased through
the activity of the elemental forces embodied in itself, and so has
rather grown than suddenly come into being at an almighty word.' 'What
a sublime idea of the infinite might of the great Architect! the Cause
of all causes, the Father of all fathers, the Ens entium! For if we
could compare the Infinite it would surely require a greater Infinite
to cause the causes of effects than to produce the effects themselves.'
In these words he sets forth his position in regard to religion, and
does so in precisely the same terms as we may use to-day when we say:
'All that happens in the world depends on the forces that prevail in
it, and results according to law; but where these forces and their
substratum, Matter, come from, we know not, and here we have room for
faith.'
I have not been able to discover whether the _Zoonomia_, with its
revolutionary ideas, attracted much attention at the time when it
appeared, but it would seem not. In any case, it was afterwards so
absolutely forgotten, that in an otherwise very complete _History of
Zoology_, published in 1872 by Victor Carus, it was not even mentioned.
About a year after the appearance of _Zoonomia_, Isidore Geoffrey
St.-Hilaire in Paris expounded the view that what are called species
are really only 'degenerations,' deteriorations from one and the same
type, which shows that he too had begun to have doubts as to the fixity
of species. Yet it was not till the third decade of the nineteenth
century that he clearly and definitely took up the position of the
doctrine of transformation, and to this we shall have to return later
on.
But as early as the first decade of the century this position was taken
up by two noteworthy naturalists, a German and a Frenchman, Treviranus
and Lamarck.
Gottfried Reinhold Treviranus, born at Bremen in 1776, an excellent
observer and an ingenious investigator, published, in 1802, a book
entitled _Biologie, oder Philosophie der lebenden Natur_ [_Biology, or
Philosophy of Animate Nature_], in which he expresses and elaborates
the idea of the Evolution theory with perfect clearness. We read
there, for instance: 'In every living being there exists a capacity
for endless diversity of form; each possesses the power of adapting
its organization to the variations of the external world, and it is
this power, called into activity by cosmic changes, which has enabled
the simple zoophytes of the primitive world to climb to higher and
higher stages of organization, and has brought endless variety into
nature.' But where the motive power lies, which brings about these
transformations from the lowliest to ever higher forms of life, was a
question which Treviranus apparently did not venture to discuss. To do
this, and thus to take the first step towards a causal explanation of
the assumed transformations, was left for his successor.
Jean Baptiste de Lamarck, born in 1744 in a village of Picardy, was
first a soldier, then a botanist, and finally a zoologist. He won his
scientific spurs first by his _Flora of France_, and zoology holds him
in honour as the founder of the category of 'vertebrates.' Not that he
occupied himself in particular detail with these, but he recognized the
close alliance of the classes of animals in question--an alliance which
was subsequently expressed by Cuvier by the systematic term 'type' or
'embranchement.'
In his _Philosophie zoologique_, published in 1809, Lamarck set
forth a theory of evolution whose truth he attempted to vindicate by
showing--as Treviranus had done before him--that the conception of
species, on the immutability of which the whole hypothesis of creation
had been based, was an artificial one, read into nature by us; that
sharply circumscribed groups do not exist in nature at all; and that
it is often very difficult, and not infrequently quite impossible, to
define one species precisely from allied forms, because it is connected
with these on all sides by transition stages. Groups of forms which
thus melted into one another indicated that the doctrine of the fixity
of species could not be correct, any more than that of their absolute
nature. Species, he maintained, are not immutable, and are not so old
as nature; they are fixed only for a certain time. The shortness of
our life prevents our directly recognizing this. 'If we lived a much
shorter time, say about a second, the hour-hand of the clock would
appear to us to stand still, and even the combined observations of
thirty generations would afford no decisive evidence as to the hand's
movement, and yet it had been moving.'
The causes on which, according to Lamarck, the transformation of
species, their modification into new species, depends, lie in the
changes in the conditions of life which must have occurred ceaselessly
from the earliest period of the earth's history till our own day, now
here, now there, due in part to changes in climate and in food-supply,
in part to changes in the earth's crust by the rising or sinking of
land-masses, and so forth. These external changes have sometimes been
the _direct_ cause of changes in bodily structure, as in the case
of heat or cold; but they have sometimes and much more effectively
operated _indirectly_. Thus changed conditions may have prompted
an animal of a given species to use certain parts of its body in a
new way, more vigorously, or less actively, or even not at all, and
the more vigorous use, or, conversely, the disuse, has brought about
variations in the organ in question.
Thus the whales lost their teeth when they abandoned their fish diet,
and acquired the habit of feeding on minute and delicate molluscs,
which they swallowed whole without seizure or mastication. Thus, too,
the eyes of the mole degenerated through its life in the dark, and a
still greater degeneration of the eyes has taken place in animals,
like the proteus-salamander, which always inhabit lightless caves.
In mussels both head and eyes degenerated because the animals could
no longer use them after they became enclosed in opaque mantles and
shells. In the same way snakes lost their legs _pari passu_ with the
acquisition of the habit of moving along by wriggling their long
bodies, and of creeping through narrow fissures and holes. On the
other hand, Lamarck interpreted the evolution of the web-feet of
swimming birds by supposing that some land-bird or other had formed the
habit of going into the water to seek for food, and consequently of
spreading out its toes as widely as possible so as to strike the water
more vigorously. In this way the fold of skin between the toes was
stretched, and as the extension of the toes was very frequent and was
continued through many generations, the web expanded and grew larger,
and thus formed the web-foot.