essentially similar to the corresponding process in unicellular forms;
for it consists in the continual division of the reproductive cell; the
only difference being that in the former case the reproductive cell
does not form the whole individual, for the latter is composed of the
millions of somatic cells by which the reproductive cell is surrounded.
The question, ‘How can a single reproductive cell contain the germ of a
complete and highly complex individual?’ must therefore be re-stated
more precisely in the following form, ‘How can the substance of the
reproductive cells potentially contain the somatic substance with all
its characteristic properties?’
The problem which this question suggests, becomes clearer when we
employ it for the explanation of a definite instance, such as the
origin of multicellular from unicellular animals. There can be no doubt
that the former have originated from the latter, and that the
physiological principle upon which such an origin depended, is the
principle of division of labour. In the course of the phyletic
development of the organized world, it must have happened that certain
unicellular individuals did not separate from one another immediately
after division, but lived together, at first as equivalent elements,
each of which retained all the animal functions, including that of
reproduction. The _Magosphaera planula_ of Häckel proves that such
perfectly homogeneous cell-colonies exist[37], even at the present day.
Division of labour would produce a differentiation of the single cells
in such a colony: thus certain cells would be set apart for obtaining
food and for locomotion, while certain other cells would be exclusively
reproductive. In this way colonies consisting of somatic and of
reproductive cells must have arisen, and among these for the first time
death appeared. For in each case the somatic cells must have perished
after a certain time, while the reproductive cells alone retained the
immortality inherited from the Protozoa. We must now ask how it becomes
possible that one kind of cell in such a colony, can produce the other
kind by division? Before the differentiation of the colony each cell
always produced others similar to itself. How can the cells, after the
nature of one part of the colony is changed, have undergone such
changes in _their_ nature that they can now produce more than one kind
of cell?
Two theories can be brought forward to solve this problem. We may turn
to the old and long since abandoned _nisus formativus_, or adapting the
name to modern times, to a phyletic force of development which causes
the organism to change from time to time. This _vis a tergo_ or
teleological force compels the organism to undergo new transformations
without any reference to the external conditions of life. This theory
throws no light upon the numerous adaptations which are met with in
every organism; and it possesses no value as a scientific explanation.
Another supposition is that the primary reproductive cells are
influenced by the secondary cells of the colony, which, by their
adaptability to the external conditions of life, have become somatic
cells: that the latter give off minute particles which entering into
the former, cause such changes in their nature that at the next
succeeding cell-division they are compelled to break up into dissimilar
parts.
At first sight this hypothesis seems to be quite reasonable. It is not
only conceivable that particles might proceed from the somatic to the
reproductive cells, but the very nutrition of the latter at the expense
of the former is a demonstration that such a passage actually takes
place. But a closer examination reveals immense difficulties. In the
first place, the molecules of the body devoured are never simply added
to those of the feeding individual without undergoing any change, but
as far as we know, they are really assimilated[38], that is, converted
into the molecules of the latter. We cannot therefore gain much by
assuming that a number of molecules can pass from the growing somatic
cells into the growing reproductive cells, and can be deposited
unchanged in the latter, so that, at their next division, the molecules
are separated to become the somatic cells of the following generation.
How can such a process be conceivable, when the colony becomes more
complex, when the number of somatic cells becomes so large that they
surround the reproductive cells with many layers, and when at the same
time by an increasing division of labour a great number of different
tissues and cells are produced, all of which must originate _de novo_
from a single reproductive cell? Each of these various elements must,
_ex hypothesi_, give up certain molecules to the reproductive cells;
hence those which are in immediate contact with the latter would
obviously possess an advantage over those which are more remote. If
then any somatic cell must send the same number of molecules to each
reproductive cell[39], we are compelled to suspend all known physical
and physiological conceptions, and must make the entirely gratuitous
assumption of an affinity on the part of the molecules for the
reproductive cells. Even if we admit the existence of this affinity,
its origin and means of control remain perfectly unintelligible if we
suppose that it has arisen from differentiation of the complete colony.
An unknown controlling force must be added to this mysterious
arrangement, in order to marshal the molecules which enter the
reproductive cell in such a manner that their arrangement corresponds
with the order in which they must emerge as cells at a later period. In
short, we become lost in unfounded hypotheses.
It is well known that Darwin has attempted to explain the phenomena of
heredity by means of a hypothesis which corresponds to a considerable
extent with that just described. If we substitute gemmules for
molecules we have the fundamental idea of Darwin’s provisional
hypothesis of pangenesis. Particles of an excessively minute size are
continually given off from all the cells of the body; these particles
collect in the reproductive cells, and hence any change arising in the
organism, at any time during its life, is represented in the
reproductive cell[40]. Darwin believed that he had by this means
rendered the transmission of acquired characters intelligible, a
conception which he held to be necessary in order to explain the
development of species. He himself pointed out that the hypothesis was
merely provisional, and that it was only an expression of immediate,
and by no means satisfactory knowledge of these phenomena.
It is always dangerous to invoke some entirely new force in order to
understand phenomena which cannot be readily explained by the forces
which are already known.
I believe that an explanation can in this case be reached by an appeal
to known forces, if we suppose that characters acquired (in the true
sense of the term) by the parent cannot appear in the course of the
development of the offspring, but that all the characters exhibited by
the latter are due to primary changes in the germ.
This supposition can obviously be made with regard to the
above-mentioned colony with its constituent elements differentiated
into somatic and reproductive cells. It is conceivable that the
differentiation of the somatic cells was not primarily caused by a
change in their own structure, but that it was prepared for by changes
in the molecular structure of the reproductive cell from which the
colony arose.
The generally received idea assumes that changes in the external
conditions can, in connection with natural selection, call forth
persistent changes in an organism; and if this view be accepted it must
be as true of all Metazoa as it is of unicellular or of homogeneous
multicellular organisms. Supposing that the hypothetical colonies,
which were at first entirely made up of similar cells, were to gain
some advantages, if in the course of development, the molecules of the
reproductive cells, from which each colony arose became distributed
irregularly in the resulting organism, there would be a tendency
towards the perpetuation of such a change, wherever it appeared as the
result of individual variability. As a result of this change the colony
would no longer remain homogeneous, and its cells would become
dissimilar from the first, because of the altered arrangement of the
molecules in the reproductive cells. Nothing prevents us from assuming
that, at the same time, the nature of a part of the molecule may
undergo still further change, for the molecules are by nature complex,
and may split up or combine together.
If then the reproductive cells have undergone such changes that they
can produce a heterogeneous colony as the result of continual division,
it follows that succeeding generations must behave in exactly the same
manner, for each of them is developed from a portion of the
reproductive cell from which the previous generation arose, and
consists of the same reproductive substance as the latter.
From this point of view the exact manner in which we imagine the
subsequent differentiation of the colony to be potentially present in
the reproductive cell, becomes a matter of comparatively small
importance. It may consist in a different molecular arrangement, or in
some change of chemical constitution, or it may be due to both these
causes combined. The essential point is that the differentiation was
originally due to some change in the reproductive cells, just as this
change itself produces all the differentiations which appear in the
ontogeny of all species at the present day. No one doubts that the
reason why this or that form of segmentation takes place, or why this
or that species finally appears, is to be found in the ultimate
structure of the reproductive cells. And, as a matter of fact,
molecular differentiation and grouping, whether present from the
beginning or first appearing in the course of development, plays a rôle
which can be almost directly observed in certain species. The first
segmentation furrow divides the egg of such species into an opaque and
a clear half, or, as is often the case among Medusae, into a granular
outer layer and a clear central part, corresponding respectively with
the ectoderm and endoderm which are formed at a later period. Such
early differentiations are only the visible proofs of certain highly
complex molecular rearrangements in the cells, and the fact appears to
indicate that we cannot be far wrong in maintaining that
differentiations which appear in the course of ontogeny depend upon the
chemical and physical constitution of the molecules in the reproductive
cell.
At the first appearance of the earliest Metazoa alluded to above, only
two kinds of cells, somatic and reproductive, arose from the
segmentation of the reproductive cell. The reproductive cells thus
formed must have possessed exactly the same molecular structure as the
mother reproductive cell, and would therefore pass through precisely
the same developmental changes. We can easily imagine that all the
succeeding stages in the development of the Metazoa have been due to
the same causes which were efficient at the earliest period. Variations
in the molecular structure of the reproductive cells would continue to
appear, and these would be increased and rendered permanent by means of
natural selection, when their results, in the alteration of certain
cells in the body, were advantageous to the species. The only condition
necessary for the transmission of such changes is that a part of the
reproductive substance (the germ-plasm) should always remain unchanged
during segmentation and the subsequent building up of the body, or in
other words, that such unchanged substance should pass into the
organism, and after the lapse of a variable period, should reappear as
the reproductive cells. Only in this way can we render to some extent
intelligible the transmission of those changes which have arisen in the
phylogeny of the species; only thus can we imagine the manner in which
the first somatic cells gradually developed in numbers and in
complexity.
It is only by supposing that these changes arose from molecular
alterations in the reproductive cell that we can understand how the
reproductive cells of the next generation can originate the same
changes in the cells which are developed from them; and it is
impossible to imagine any way in which the transmission of changes,
produced by the direct action of external forces upon the somatic
cells, can be brought about[41].
The difficulty or the impossibility of rendering the transmission of
acquired characters intelligible by an appeal to any known force has
been often felt, but no one has hitherto attempted to cast doubts upon
the very existence of such a form of heredity.
There are two reasons for this: first, observations have been recorded
which appear to prove the existence of such transmission; and secondly,
it has seemed impossible to do without the supposition of the
transmission of acquired characters, because it has always played such
an important part in the explanation of the transformation of species.
It is perfectly right to defer an explanation, and to hesitate before
we declare a supposed phenomenon to be impossible, because we are
unable to refer it to any of the known forces. No one can believe that
we are acquainted with all the forces of nature. But, on the other
hand, we must use the greatest caution in dealing with unknown forces;
and clear and indubitable facts must be brought forward to prove that
the supposed phenomena have a real existence, and that their acceptance
is unavoidable.
It has never been proved that acquired characters are transmitted, and
it has never been demonstrated that, without the aid of such
transmission, the evolution of the organic world becomes unintelligible.
The inheritance of acquired characters has never been proved, either by
means of direct observation or by experiment[42]. It must be admitted
that there are in existence numerous descriptions of cases which tend
to prove that such mutilations as the loss of fingers, the scars of
wounds, etc., are inherited by the offspring, but in these descriptions
the previous history is invariably obscure, and hence the evidence
loses all scientific value.
As a typical example of the scientific value of such cases I may
mention the frequently quoted instance of the cow, which lost its left
horn from suppuration, induced by some ‘unknown cause,’ and which
afterwards produced two calves with a rudimentary left horn in each
case. But as Hensen[43] has rightly remarked, the loss of the cow’s
horn may have arisen from a congenital malformation, which would
certainly be transmitted, but which was not an acquired character.
The only cases worthy of scientific discussion are the well-known
experiments upon guinea-pigs, conducted by the French physiologist
Brown-Séquard. But the explanation of his results is, in my opinion,
open to discussion. In these cases we have to do with the apparent
transmission of artificially produced malformations. The division of
important nerves, or of the spinal cord, or the removal of parts of the
brain, produced certain symptoms which reappeared in the descendants of
the mutilated animals. Epilepsy was produced by dividing the great
sciatic nerve; the ear became deformed when the sympathetic nerve was
severed in the throat; and prolapsus of the eye-ball followed the
removal of a certain part of the brain—the corpora restiformia. All
these effects were said to be transmitted to the descendants as far as
the fifth or sixth generation.
But we must inquire whether these cases are really due to heredity and
not to simple infection. In the case of epilepsy, at any rate, it is
easy to imagine that the passage of some specific organism through the
reproductive cells may take place, as in the case of syphilis. We are,
however, entirely ignorant of the nature of the former disease. This
suggested explanation may not perhaps apply to the other cases: but we
must remember that animals which have been subjected to such severe
operations upon the nervous system have sustained a great shock, and if
they are capable of breeding, it is only probable that they will
produce weak descendants, and such as are easily affected by disease.
Such a result does not however explain why the offspring should suffer
from the same disease as that which was artificially induced in the
parents. But this does not appear to have been by any means invariably
the case. Brown-Séquard himself says, ‘The changes in the eye of the
offspring were of a very variable nature, and were only occasionally
exactly similar to those observed in the parents.’
There is no doubt, however, that these experiments demand careful
consideration, but before they can claim scientific recognition, they
must be subjected to rigid criticism as to the precautions taken, the
number and nature of the control experiments, etc.
Up to the present time such necessary conditions have not been
sufficiently observed. The recent experiments themselves are only
described in short preliminary notices, which, as regards their
accuracy, the possibility of mistake, the precautions taken, and the
exact succession of individuals affected, afford no data upon which a
scientific opinion can be founded. Until the publication of a complete
series of experiments, we must say with Du Bois Reymond[44], ‘The
hereditary transmission of acquired characters remains an
unintelligible hypothesis, which is only deduced from the facts which
it attempts to explain.’
We therefore naturally ask whether the hypothesis is really necessary
for the explanation of known facts.
At the first sight it certainly seems to be necessary, and it appears
rash to attempt to dispense with its aid. Many phenomena only appear to
be intelligible if we assume the hereditary transmission of such
acquired characters as the changes which we ascribe to the use or
disuse of particular organs, or to the direct influence of climate.
Furthermore, how can we explain instinct as hereditary habit unless it
has gradually arisen by the accumulation, through heredity, of habits
which were practised in succeeding generations?
I will now attempt to prove that even these cases, so far as they
depend upon clear and indubitable facts, do not force us to accept the
supposition of the transmission of acquired characters.
It seems difficult and well nigh impossible to deny the transmission of
acquired characters when we remember the influence which use and disuse
have exercised upon certain special organs. It is well known that
Lamarck attempted to explain the structure of the organism as almost
entirely due to this principle alone. According to his theory the long
neck of the giraffe arose by constant stretching after the leaves of
trees, and the web between the toes of a water-bird’s foot by the
extension of the toes, in an attempt to oppose as large a surface of
water as possible in swimming. There can be no doubt that those muscles
which are frequently used increase in size and strength, and that
glands which often enter into activity become larger and not smaller,
and that their functional powers increase. Indeed, the whole effect
which exercise produces upon the single parts of the body is dependent
upon the fact that frequently used organs increase in strength. This
conclusion also refers to the nervous system, for a pianist who
performs with lightning rapidity certain pre-arranged, highly complex,
and combined movements of the muscles of his hands and fingers has, as
Du Bois Reymond pointed out, not only exercised the muscles, but also
those ganglionic centres of the brain which determine the combination
of muscular movement. Other functions of the brain, such as memory, can
be similarly increased and strengthened by exercise, and the question
to be settled is whether characters acquired in this way by exercise
and practice can be transmitted to the following generations. Lamarck’s
theory assumes that such transmission takes place, for without it no
accumulation or increase of the characters in question would be
possible, as a result of their exercise during any number of successive
generations.
Against this we may urge that whenever, in the course of nature, an
organ becomes stronger by exercise, it must possess a certain degree of
importance for the life of the individual, and when this is the case it
becomes subject to improvement by natural selection, for only those
individuals which possess the organ in its most perfect form will be
able to reproduce them. The perfection of form of an organ does not
however depend upon the amount of exercise undergone by it during the
life of the organism, but primarily and principally upon the fact that
the germ from which the individual arose was predisposed to produce a
perfect organ. The increase to which any organ can attain by exercise
during a single life is bounded by certain limits, which are themselves
fixed by the primary tendencies of the organ in question. We cannot by
excessive feeding make a giant out of the germ destined to form a
dwarf; we cannot, by means of exercise, transform the muscles of an
individual destined to be feeble into those of a Hercules, or the brain
of a predestined fool into that of a Leibnitz or a Kant, by means of
much thinking. With the same amount of exercise the organ which is
destined to be strong, will attain a higher degree of functional
activity than one that is destined to be weak. Hence natural selection,
in destroying the least fitted individuals, destroys those which from
the germ were feebly disposed. Thus the result of exercise during the
individual life does not acquire so much importance, for, as compared
with differences in predisposition, the amount of exercise undergone by
all the individuals of a species becomes relatively uniform. The
increase of an organ in the course of generations does not depend upon
the summation of the exercise taken during single lives, but upon the
summation of more favourable predispositions in the germs.
In criticizing these arguments, it may be questioned whether the single
individuals of a species which is undergoing modification do, as a
matter of fact, exercise themselves in the same manner and to the same
extent. But the consideration of a definite example clearly shows that
this must be the case. When the wild duck became domesticated, and
lived in a farm-yard, all the individuals were compelled to walk and
stand more than they had done previously, and the muscles of the legs
were used to a correspondingly greater degree. The same thing happens
in the wild state, when any change in the conditions of life compels an
organ to be more largely used. No individual will be able to entirely
avoid this extra use, and each will endeavour to accommodate itself to
the new conditions according to its power. The amount of this power
depends upon the predisposition of the germ; and natural selection,
while it apparently decides between individuals of various degrees of
strength, is in truth operating upon the stronger and weaker germs.
But the very conclusions which have been drawn from the increase of
activity which has arisen from exercise, must also be drawn from the
instances of atrophy or degeneration following from the disuse of
organs.