to Lamarck, gradually evolved by the continual stretching of the limbs
by wading in deeper and deeper water, and similarly for the long necks
and bills of the waders, the herons and the storks. Finally we may
mention the case of the giraffe, whose enormously long neck and tall
forelegs are interpreted as due to the fact that the animal feeds on
the foliage of trees, and was always stretching as far as possible, in
order to reach the higher leaves.
We shall see later in what a different way Charles Darwin explained
this case of the giraffe. Lamarck's idea is at once clear; it is true
that exercising an organ strengthens it, that disuse makes it weaker.
Through much gymnastic exercise the muscles of the arm become thicker
and more capable, and memory too may be improved, that is to say,
even a definite part of the brain may be considerably strengthened
by use. Indeed, we may now go so far as to admit that every organ is
strengthened by use and weakened by disuse, and so far the foundations
of Lamarck's interpretations are sound. But he presupposes something
that cannot be admitted so readily, namely, that such 'functional'
improvement or diminution in the strength of an organ can be
transmitted by inheritance to the succeeding generation. We shall have
to discuss this question in detail at a later stage, and I shall only
say now that opinions as to whether this is possible or not are very
much divided. I myself doubt this possibility, and therefore cannot
admit the validity of the Lamarckian evolutionary principle in so far
as it implies the directly transforming effect of the functioning of
an organ. But even if we recognize the Lamarckian factor as a _vera
causa_, it is easy to show that there are a great many characters
which it is not in a position to interpret. Many insects which live
upon green leaves are green, and not a few of them possess exactly the
shade of green which marks the plant on which they feed; they are thus
protected in a certain measure from injuries. But how could this green
colour of the skin have been brought about by the activity of the skin,
since the colour of the surroundings does not usually stimulate the
skin to activity at all? Or how should a grasshopper, which is in the
habit of sitting on dry branches of herbs, have thereby been incited
to an activity which imparts to it the colour and shape of a dry twig?
Just as little, or perhaps still less, can the protective green colour
of a bird's or insect's eggs be explained through the direct influence
of their usually green surroundings, even if we disregard the fact that
the eggs are green when they are laid--that is, before the environment
can have had any influence on them.
The Lamarckian principle of modification through use does not, in
any case, nearly suffice as an interpretation of the transformations
of the organic world. It must be allowed that Lamarck's theory of
transformation was well founded at the time when it was advanced; it
not only attacked the doctrine of the immutability of species, but
sought for the first time to indicate the forces and influences which
must be operative in the transformations of species; it was therefore
well worth careful testing. Nevertheless it did not divert science from
its chosen path; very little notice was taken of it, and in the great
Cuvier's chronicle of scientific publications for 1809, not a syllable
is devoted to Lamarck's book, so strong was the power of prejudice.
But, although the new doctrine was thus ignored, it did not altogether
fall to the ground; it glimmered for a while in Germany, where it found
its champions in the 'Naturphilosophie' of the time, and especially in
Lorenz Oken, a peasant's son, born at Ortenau, near Offenburg, in 1783.
Oken professed views similar to those of Erasmus Darwin, Treviranus,
and Lamarck, though they were not clothed in such purely scientific
garb, being, in fact, bound up with the general philosophical
speculations which came increasingly into favour at that time, chiefly
through the writings of Schelling. In the same year, 1809, in which
Lamarck published his _Philosophie zoologique_, Oken's _Lehrbuch der
Naturphilosophie_ appeared.
This book is by no means simply a theory of descent; its scope is much
wider, including the phenomena of the whole cosmos; on the other hand,
it goes too little into details and is too indefinite to deserve its
title. Its way of playing with ideas, its conjectures and inferences
from a fanciful basis, make it difficult for us now to think ourselves
into its mode of speculation, but I should like to give some indication
of it, for it was just these speculative encroachments of the
'categories' of the so-called 'Naturphilosophie' which played a fatal
part in causing the temporary disappearance of the Evolution-theory
from science, so that, later on, it had to be established anew.
Oken defines natural science as 'the science of the everlasting
transmutations of God (the Spirit) in the world': Every thing,
considered in the light of the genetic process of the whole, includes,
besides the idea of being, that of not-being, in that it is involved
in a higher form. 'In these antitheses the category of polarity is
included. The simpler elementary bodies unite into higher forms,
which are thus merely repetitions at a potential higher than that of
their causes. Thus the different genera of bodies form parallel and
corresponding series, the reasonable arrangement of which results as an
intrinsic necessity from their genetic connexion. In individuals these
lowlier series make their appearance again during development. The
contrasts in the solar system between planets and sun are repeated in
plants and animals, and, as light is the principle of movement, animals
have the power of independent movement in advance of the plants which
belong to the earth.'
Obviously enough, this is no longer the study of nature; it is
nature-construction from a basis of guesses and analogies rather than
of knowledge and facts. Light is the principle of motion, and as
animals move, they correspond to the sun, and plants to the planets!
Here there is not even a hint of a deepening of knowledge, and all
these deductions now seem to us quite worthless.
On the other hand, it must be allowed that good ideas are by no means
absent from this 'philosophy,' nor can we deny to this restlessly
industrious man a great mind always bent on discovering what was
general and essential. Much of what we now _know_ he even then guessed
at and taught, as, for instance, that the basis of all forms of life
in this infinitely diverse world of organisms was one and the same
substance--'primitive slime,' 'Urschleim' as he called it, or, as we
should now say, 'protoplasm.' We can therefore, _mutatis mutandis_,
agree with Oken when he says,'Everything organic has come from slime,
and is nothing but diversely organized slime.' Many naturalists of the
present day would go further, and agree with Oken when he suggests
that 'this primitive slime has arisen in the sea, in the course of the
planet's (the earth's) evolution out of inorganic material.'
Thus Oken postulated, as the specific vehicle of life, a primitive
substance, in essence at least homogeneous. But he went further, and
maintained that his 'Urschleim' assumed _the form of vesicles_, of
which the various organisms were composed. 'The organic world has as
its basis an infinitude of such vesicles.' Who is not at once reminded
of the now dominant _Cell-theory_? And, in fact, thirty years later,
when the cell was discovered, Oken did claim priority for himself. In
so doing, he obviously confused the formulating of a problem with the
solving of it; he had, quite rightly, divined that organisms must be
built up of very minute concentrations of the primitive substance, but
he had never seen a cell, or proved the necessity for its existence, or
even attempted to prove it. His vesicle-theory was a pure divination, a
prevision of genius, but one which could not directly deepen knowledge;
it did not prompt, or even hasten, the discovery of the cell. Here,
as throughout in his natural philosophy, Oken built, not from beneath
upwards, by first establishing facts and then drawing conclusions from
them, but, inversely, he invented ideas and principles, and out of
them reconstructed the world. In this he differs essentially from his
predecessors Erasmus Darwin, Treviranus, and Lamarck, who all reasoned
inductively, that is, from observed data.
Thus the whole evolutionary movement was lost in indefiniteness;
because men wanted to find a reason for everything, they missed
even what might then have been explained. Moreover, the theory of
evolution still lacked a sufficiently broad basis of facts; the
'Naturphilosophie,' by its want of moderation, robbed it of all
credit; and it is not to be wondered at that men soon ceased to occupy
themselves with the problem of the evolution of the living world. A few
indeed held fast to the doctrine of evolution during the first third
of the century, but then it disappeared completely from the realm of
science.
Its last flicker of life was seen in France, in 1830, at the time of
the July revolution, when the legitimate sovereignty of Charles X was
overthrown. It is interesting to note the lively interest that Goethe,
the first forerunner of the theory, and then aged eighty-one, had in
the intellectual combat that took place in the French Academy between
Cuvier and Isidore Geoffroy St.-Hilaire. A friend of Goethe's, Soret,
relates that on August 2, 1830, he went into the poet's room, and was
greeted with the words: 'Well, what do you think of this great event?
The volcano is in eruption, and all is in flames. There can no longer
be discussion with closed doors.' Soret replied: 'It is a terrible
business! But what else was to be expected with things as they are,
and with such a ministry, than that it should end in the expulsion
of the reigning family?' To which Goethe answered: 'We don't seem to
understand each other, my dear friend. I am not talking of these people
at all; I am thinking of quite different affairs. I refer to the open
rupture in the Academy between Cuvier and Geoffroy St.-Hilaire; it is
of the utmost importance to science.'
In this conflict of opinions, Cuvier opposed Geoffroy's conception of
the unity of the plan of structure in all animals, confronting him with
the four Cuvierian types, in each of which the plan of structure was
altogether different, and strongly insisting on the doctrine of the
fixity of species, which he maintained to be the necessary postulate of
a scientific natural history.
The victory fell to Cuvier, and it cannot be denied that there was much
justification for his opinions at the time, for the knowledge of facts
at that stage was not nearly comprehensive enough to give security to
the Evolution theory, and moreover the quiet progress of science might
have been hindered rather than furthered by premature generalization
and theorizing. It had now been seen how far the interpretation of
general biological problems could be carried with the available
material; the 'Naturphilosophie' had not merely exploited it as far as
possible, but had burdened it much beyond its carrying power, and the
world was weary of insecure speculations. The 'Naturphilosophie' was
for the time quite worked out, and a long period set in, during which
all energies were devoted to detailed research.
LECTURE II
THE DARWINIAN THEORY
Period of detailed research--Appearance of Darwin's _Origin
of Species_--Darwin's life--Voyage round the world--His
teaching--Domesticated animals, dog, horse--Pigeons--Artificial
selection--Unconscious selection--Correlated variations.
THE period of wholly unphilosophical, purely detailed research may be
reckoned as from about 1830 to 1860, though, of course, many of the
labours of the earlier part of the century must be counted among the
investigations which were carried out without any reference to general
questions, and even after 1860 numerous such works appeared. Nor could
it be otherwise, for the basis of all science must be found in facts,
and the thorough working up of the fact-material will always remain
the first and most indispensable condition of our scientific progress.
During the period referred to, however, it had become the sole end to
be striven for; and all energies were concentrated exclusively on the
accumulation of facts.
The previous century had added much to the knowledge of the inner
structure of animals, the so-called 'comparative anatomy,' and in
the nineteenth century this line of investigation was pursued even
more extensively and energetically, so that the knowledge increased
enormously. Up till this time it was chiefly the structure of the
backboned animals and of a few 'backboneless' animals, so called, that
had been studied, but now all the lower groups of the animal kingdom
were also investigated, and became known better and in more detail as
the methods of research improved.
Not content, however, with a knowledge of the adult animal, naturalists
began to investigate its development. In the year 1814 the first
great work on development appeared, on the development of the chick,
by Pander and Von Baer. It was there shown for the first time, how
the chick begins as a little disk-shaped membrane on the surface of
the yolk of the egg, at first simply as a pale streak, the 'primitive
streak,' then as a groove, the 'primitive groove,' by the side of which
arise two folds, the 'medullary folds,' and further how a system of
blood-vessels is developed around this primitive rudiment on the upper
surface of the yolk, how a heart arises before the rest of the body is
complete, and how the blood begins to circulate; in short, there was
disclosed all the marvel of development to which we are now so much
accustomed, that we can hardly understand the sensation it made at that
time.
Later on, attention was turned to the development of Fishes and
Amphibians (Agassiz and Vogt, later Remak), then to that of the Worms
(Bagge), of Insects (Kölliker), and gradually the development of
all the groups of the animal-kingdom--from Sponges to Man--was so
thoroughly investigated that it almost seems to-day as if there could
not be much that is new to discover in this department. This impression
may indeed be true as far as the less complex processes and the more
obvious questions are concerned, but it is impossible to predict what
new problems may confront us, whose solution will depend on a still
more detailed study of development.
As embryology is a science of the nineteenth century, so also is
histology, the science of tissues. Its pioneer was Bichat, but its
real foundations were not laid till Schwann and Schleiden formulated
the conception of the 'cell,' and proved that all animals and plants
were composed of cells. What Oken had only guessed at they now proved,
that there are very minute form-elements of life which build up all the
parts of animals and plants or produce them by processes of secretion.
New light was thus shed on embryonic development, and this gradually
led to the recognition of the fact that the egg, too, is a cell, and
that development depends on a cell-division process in this egg-cell.
This led further to the conception of many-celled and single-celled
organisms, and so on to many items of knowledge to speak of which here
would carry us too far.
For it is not my intention to attempt a complete review of the
development of biology in the nineteenth century, or even in the
period which we have mentioned as devoted to detailed research; it
is rather my desire to convey a general impression of the enormous
extent and many-sidedness of the progress that was made in this time.
Let us therefore briefly recall the entirely new facts which were
brought to light in this period with regard to the reproduction of
animals. Asexual reproduction by budding and division was already
known, but parthenogenesis is a discovery of this period, and so also
is alternation of generations, so far-reaching in its bearing on
general problems. It was first observed (1819) by Chamisso in Salpa,
then by Steenstrup in Medusæ and trematodes, and was later made fully
clear in its most diverse forms and relations by the researches of
Leuckart, Vogt, Kölliker, Gegenbaur, Agassiz, and other illustrious
investigators. Reproduction by heterogony, too, which occurs in
many crustaceans, and in aphides and certain worms, was recognized
at that time, and in the sixties Carl Ernst von Baer added to the
list precocious reproduction, or pædogenesis, which is illustrated in
certain insects which reproduce in the larval state.
This may suffice to convey some idea of the great mass of new, and in
some cases startling facts previously unguessed at, which were then
brought to light in the department of animal biology alone. To this
must be added the vast increase in the number of known species and
varieties, their distribution on the earth, and all this, _mutatis
mutandis_, for plants also. Nor can we omit to mention the rapidly
growing number of fossil species of animals and plants.
Thus there gradually accumulated a new mass of material; investigation
became more and more specialized, and the danger became imminent that
workers in the various departments would be unable to understand
each other, so completely were they independent of one another in
their specialist researches. There was lack of any unifying bond, for
workers had lost sight of the general problem in which all branches
of the science meet, and through which alone they can be united into
a general science of biology. The time had come for again combining
and correlating the details, lest they should grow into an unconnected
chaos, through which it would be impossible to find one's way, because
no one could overlook it and grasp it as a whole. In a word, it was
high time to return to general questions.
* * * * *
Though I have called the period from 1830 to 1860 that of purely
detailed research, I do not mean to ignore the fact that, during that
time, there were a few feeble attempts to return to the great questions
which had been raised at the beginning of the century. But the point
is, that all such attempts remained unnoticed. Thus there appeared, in
1844, a book entitled _Vestiges of the Natural History of Creation_,
the anonymous author of which revealed himself much later as Robert
Chambers, an Edinburgh publisher. In this book the evolution of species
was ascribed to two powers, a power of transformation and a power of
adaptation. Two Frenchmen, Naudin and Lecoq, also published a work in
which the theory of evolution was set forth, and from 1852 to 1854 the
well-known German anthropologist Schaafhausen was writing on similar
lines. But all these calls sounded unheard, so deeply were naturalists
plunged in detailed investigations, and it required a much mightier
voice to command the ear of the scientific world.
It is impossible to estimate the effect of Darwin's book on _The
Origin of Species_, published in English in 1858, in German in 1859
unless we fully realize how completely the biologists of that time
had turned away from general problems. I can only say that we, who
were then the younger men, studying in the fifties, had no idea that
a theory of evolution had ever been put forward, for no one spoke of
it to us, and it was never mentioned in a lecture. It seemed as if all
the teachers in our universities had drunk of the waters of Lethe,
and had utterly forgotten that such a theory had ever been discussed,
or as if they were ashamed of these philosophical flights on the part
of natural science, and wished to guard their students from similar
deviations. The over-speculation of the 'Naturphilosophie' had left in
their minds a deep antipathy to all far-reaching deductions, and, in
their legitimate striving after purely inductive investigation, they
forgot that the mere gathering of facts is not enough, that the drawing
of conclusions is an essential part of the induction, and that a mass
of bare facts, however enormous, does not constitute a science.
One of my most stimulating teachers at that time, the gifted anatomist,
Jacob Henle, had written as a motto under his picture, 'There is a
virtue of renunciation, not in the province of morality alone, but
in that of intellect as well,' a sentence which expressly indicated
the desirability of refraining from all attempts to probe into the
more general problems of life. Thus the young students of that time
were nourished only on the results of detailed research, in part
indeed interesting enough, but in part dry and, because uncorrelated,
unintelligible in the higher sense, and only here and there awakening a
deeper interest, when, as in physiology and in embryology, they formed
a connected system in themselves. Without being fully clear as to what
was lacking, we certainly missed the deeper correlation of the many
separate disciplines.
It is therefore not to be wondered that Darwin's book fell like a
bolt from the blue; it was eagerly devoured, and while it excited in
the minds of the younger students delight and enthusiasm, it aroused
among the older naturalists anything from cool aversion to violent
opposition. The world was as though thunderstruck, as we can readily
see from the preface with which the excellent zoologist of Heidelberg,
Bronn, introduced his translation of Darwin's book, where he asks this
question among others, 'How will it be with you, dear reader, after you
have read this book?' and so forth.
But before I enter on a detailed examination of the contents of this
epoch-making book, I should like to say a few words about the man
himself, who thus revolutionized our thinking.
Charles Darwin was born in 1809, the year of the publication of
Lamarck's _Philosophie zoologique_, and of Oken's _Lehrbuch der
Naturphilosophie_. There was thus a whole generation between the first
emergence of the Evolution theory and its later revival. Darwin's
father was a physician, and his education was not a regular one. In his
youth he seems to have devoted much time and enthusiasm to hunting, and
only very slowly to have taken up regular studies towards a definite
end. In accordance with his father's wishes, he studied medicine for a
time, but soon abandoned it to devote himself to botany and zoology.
Before he had had time to distinguish himself in any special way in
these subjects, he was offered, in his twenty-first year, the post of
naturalist on an English war-ship which was to make a voyage round the
world, and that at a leisurely rate.
This was decisive not only for Darwin's immediate studies, but for
the work of his life, for, as he tells us himself, it was during this
voyage on the _Beagle_ that the idea of the Evolution theory first came
to him. While the vessel made a stay at the Galapagos Islands, west
of South America, he noticed that quite a number of little land-birds
occurred there which closely resembled those of the neighbouring
mainland, but yet were different from them. Almost every little island
had its own species, and so he concluded that all these might be
descended from representatives of a few species which had long before
drifted over from the mainland to these volcanic islands, become
established there, and in the course of time taken on the character of
new species. The problem of the transformation of species opened up
before him, and he made up his mind to follow up the idea after his
return, in the hope that by a patient collecting of facts, he would by
and by arrive at some security with regard to this great question.