PROBLEMS OF REPRODUCTION (_continued_)
The one outstanding discovery which has placed the science of Embryology
on an absolutely firm basis, and which has made clear so many of the
facts, which were previously puzzling, is this: _that the germ-cells
which give rise to new individuals are themselves produced from
pre-existing germ-cells_. The entire embryo, or young infant, is derived
from one single cell which we have called the fertilised ovum, and that
in its turn was derived from the union of two germ-cells, one from the
male parent, and one from the female. These two cells in their turn were
also derived in a straight line of descent from the fertilised ovum from
which each parent sprang. In other words there has never been any
conjugation between one fertilised ovum and another in spite of the
generations of cells which have been produced between them. Put in
another way the body, or somatic cells, contribute absolutely nothing to
the original material or germ-plasm of which the germ-cells are
composed. They do not produce them in any sense of the word whatsoever,
despite the popular opinion to the contrary. This is the great discovery
of modern Embryology. Until this was known it was assumed that parents
did produce the cells from which their children sprang, and hence--it
was thought--the resemblance between them. The fact is quite otherwise.
No parent ever produces a germ-cell, and the reason why children
resemble parents and ancestors is because the germ-cells which give rise
to individuals in successive generations are produced from the
germ-cells of the previous generation. The line of descent or
inheritance, therefore, is from germ-cell to germ-cell, and not from
parents. Unless the reader makes himself absolutely familiar with the
thought expressed in these facts he will never understand the science of
Embryology.
Dr. Archdall Reid expresses this truth in the following words. "The
somatic cells of the parent, therefore, as far as we know, contribute no
living elements to the child; they merely provide temporary shelter and
nutriment. The child, therefore, does not, as is popularly supposed,
resemble his parent because his several parts are derived from similar
parts of the parent--his head from his parent's head, his hands from his
parent's hands, and so forth; he resembles him only because the
germ-plasm which directed his development was a split-off portion of the
germ-plasm which directed the development of the parent. The egg
produces the fowl, but the fowl as a whole does not produce the
egg--only one cell from the fowl, the fertilised ovum, produces it."
[Illustration: Diagram to show the origin of germ-cells and the embryo.]
Unite in the process of fertilisation to form the fertilised ovum, which
divides a given number of times and forms daughter-cells, which are
germ-cells; one of which, and one only, goes on dividing to form the
body-cells, and so produces the new individual, which as it grows
includes in itself those cells (germ-cells) previously formed.
The rest are germ-cells, which subsequently form the eggs and sperms of
the new individual, _i.e._ they are the germ-cells of the next
generation. They cannot develop independently, but when they unite with
the egg or sperm of another individual, a new fertilised ovum is formed
and the cycle begins again.
This is a startling thought, but it is one which a moment's careful
consideration will show is the only conceivable explanation of all the
facts of physical continuity. Once it is grasped a flood of light is
thrown upon the whole science of Embryology. The individual is seen to
be literally a "chip of the old block," and the "old block" means the
whole sequence of germ-cells which has preceded his formation. In the
light of this fact it is obvious why like produces like; indeed, it is
obvious that it must do so. Further, we now understand at once that
since one generation of germ-cells directly produces those of the next,
there is no reason in the world why an individual should not more nearly
resemble a remote ancestor than his own immediate parents. As a simple
matter of fact this frequently happens. He does so because the germ-cell
from which he sprang is composed of protoplasm handed down in direct
continuity by successive generations of germ-cells from time immemorial.
In fact the problem in the light of this evidence is not so much--as it
always seems to the writer--to understand why children resemble parents
and ancestors, as to understand how it is that they do not resemble them
more.
There is no difficulty now in explaining the fundamental propositions
with which we started, namely, that children resemble their parents.
There is no difficulty in the understanding why a child resembles not
only its immediate parents, but even its ancestors. There is even no
difficulty in understanding why a child should resemble its ancestors,
even though it does not resemble its parents. Given the simple truth
that the germ-plasm is continuous from one generation to another, all
these things become as clear as daylight.
But we also start with another general proposition, namely, that
children differ from their parents, and it is this question of the
variation in offspring which must now claim our attention for a moment.
By this term we mean to convey the fact that although every child has a
real resemblance to its parents or its ancestors, it inevitably and
invariably shows differences even if these be more minute than the
resemblances. In other words the offspring of a human being, though
obviously and necessarily, from the continuity of germ-plasm, it must be
another human being, is never exactly similar to any other. Now these
variations are many of them present from the very beginning, they take
their origin in the germ-plasm of the two germ-cells which form the
fertilised ovum. They are, that is to say, many of them germinal in
origin. These must be carefully distinguished from such characteristics
as are afterwards acquired by the child as the result of its adaptation
to the environment in which it passes its existence.
It would be beyond the scope of the present work to enter into all the
various theories which have been put forward to account for the fact of
variation. It will be sufficient for our purpose here if the reader
remembers that it is a universal tendency in all living protoplasm to
exhibit variations. It is just as universal as is its continuity of
likeness. Moreover, in dealing with the highest animals in which the
fertilised ovum, from which the embryo springs, is produced by the union
of germ-cells from male and female, one may readily understand that the
different lines of descent of the male and female germ-cells may well be
responsible for the differences exhibited in the offspring. Obviously
the fertilised ovum, if it has to give rise to a normal individual,
cannot retain _all the characteristics_ which were possibly existent in
both the male germ-cells and the female germ-cells. Some of them must be
suppressed or got rid of, otherwise there would be too many characters
in the resulting offspring. And, as a matter of fact, such a reduction
does actually take place in the physical tissue comprising the
fertilised cell, and it is probably at this stage that variations take
their origin.
Thus, for example, it is quite impossible that two opposing
characteristics can both be represented in the fertilised ovum. One of
them must be suppressed or thrown out or got rid of in some way or
another. For example, the union of the male element and the female
element will give rise to an embryo which may be eventually either a
male or a female individual, but cannot be both. There were
possibilities of it being either the one or the other at the beginning,
but since the two possibilities are mutually antagonistic, one or the
other must be eliminated. So again, supposing that the colour of the
eyes on the one side were brown, and on the other side blue, the
possibilities are that the fertilised ovum may give rise to an
individual having either blue eyes or brown eyes, but, again, not both.
A variation in offspring then may be regarded as a difference between
that offspring and the parents, which is due to some change in the
germ-plasm, some difference, that is, between the germ-plasm from which
the parent sprang and that from which the next generation arose. Such
differences will, of course, be introduced at the time of fertilisation.
It is important to keep clearly in mind the difference between a true
variation, in the sense that we have just used the term, and a
modification which is caused by the varying effects of influences
affecting parents and offspring. Unless these two things are kept
mentally distinct, much confusion of thought is apt to arise.
The above statement does not necessarily mean that the germ-plasm
carried in the sperms and the ova respectively, cannot be affected in
any way. Indeed one is forced to the conclusion that such germ-cells
must be influenced by the nutrient fluids supplied to them, and by the
existence of toxic or poisonous substances in the body of the parent. It
is quite conceivable, and indeed inevitable, that the individual embryo
resulting from the fusion of such poisoned germ-cells will show
modifications, but these, however, are not to be regarded in any sense
as true variations, for the simple reason that these modifications do
not take their origin in the actual germ-plasm itself, but are simply
the result of abnormal stimuli. Such modifications, no matter in what
direction they may be, are, of course, not transmissible to the next
generation, for the very obvious reason that the germ-cells which are to
be concerned with the next generation have been already produced. The
germ-plasm itself passes on unchanged in so far as its hereditary
possibilities are concerned. We see, therefore, that in order to think
clearly on this matter we must limit the meaning of the word variation
to such differences or changes in germ-plasm which indicates some real
change of an inheritable nature. The term should not be used to apply to
a mere passing environment, in which the germ-plasm happens to be,
caused by the presence of poisons, or similar factors.
Given then the fact that variations do constantly and inevitably occur
in offspring owing to new qualities arising in germ-plasm itself, it is
obvious that these variations are either what are termed "spontaneous,"
or else they must be due to the action of the surroundings on the
germ-plasm. By the term "spontaneous," in this connection, it is not
meant that these variations arise without cause or in a haphazard
manner. It is simply meant to imply that the present state of our
knowledge does not justify us in stating what does actually cause the
variation in germ-plasm, or the laws in accordance with which such
variations occur. That they must be a matter of cause and effect and law
every biologist believes, but until the law can be demonstrated, the
term "spontaneous" may well be retained to distinguish these variations
from those which arise by the obvious action of environment. For
example, the variations which occur as the results of reproduction from
two parents, do so because of the mingling of the respective germ-cells,
and such variations come under the group of "spontaneous"; whereas
changes induced on account of the food supplied, or poisonous substances
in fluids surrounding germ-cells are not spontaneous, but environmental.
A point of great interest to the embryologists is the question whether
the differences of detail which exist between children and their parents
are of the nature of spontaneous variations, taking their origin in the
germ-plasm itself, or mere modifications produced by the action of the
environment of the embryo. Further, should both these factors play a
part in producing these differences, which is of greater importance, and
in what proportion? This question is elaborated in great detail by Dr.
Archdall Reid, in his work, _The Laws of Heredity_, which ought to be
read by every intelligent citizen and parent who is interested in the
welfare of the young. In the main in this subject we follow the ideas so
ably put forward by him. He points out that the offspring of the same
parents always differ not only from the parents, but among themselves,
even if they be twins, and amongst the lower animals every member of a
litter of dogs, or pigs, or kittens, shows differences in size, colour,
activities, temperament, and characteristics. Are those differences due
to the action of environment on the embryo or do they take their origin
in the germ-cells from which the individuals came? Inasmuch as a litter
of puppies is subjected to precisely the same environment during the
whole time of development, it is perfectly obvious that such differences
as they exhibit at the time of birth must have been germinal, an
identical environment could not by any stretch of the imagination be
held responsible for producing variations. They, therefore, must be of
the spontaneous variety. Of course it may be argued that even during
development the environment of each embryo within the mother is not
identical, but it will be a gross abuse of such argument to therefore
conclude that such minute differences of surroundings could account for
one puppy being big and black, and another one small and brown in the
same litter; or that one should resemble one parent, another the other,
and a third a remote ancestor. It is, therefore, clear that _some at
least_ of the variations in offspring are germinal or spontaneous in
origin, and not in any way due to the environment of the embryo.
The question remains whether all variations are due to this cause or
whether some may be traced to environmental factors. One of the best
lines of argument and investigation on this point is that of the
bacteriologist, because microbes with which he is concerned may be
regarded as equivalent in this matter to germ-cells, all microbes being
unicellular. The problems of the germ-cell, and its heredity, therefore,
are very similar in both cases. Tried by this test we may ask whether
the changes produced in these unicellular organisms by the action of
their environment are, or are not, inherited as variations. No one
doubts for a single moment that a microbe as well as a germ-cell may be
changed, or injured, or improved, according to its own special
environment. What is in dispute is whether that change remains fixed in
the succeeding generations to which these unicellular cells give rise.
It is precisely here that the bacteriologist can offer evidence of a
most important character. He will tell us that it is quite easy to
change many of the characteristics of a microbe by altering its
environment, which is undoubtedly true, but the further statement that
they change because their germ-plasm is affected directly by the
environment is not necessarily true. These organisms and germ-cells are
composed of protoplasm whose ultimate constitution permits of their
varying spontaneously. These variations are obviously to enable them to
adapt themselves to the tissues of the animal in which they are living,
and these variations also, or modifications as they really are, are
usually lost when that environment is no longer existent. In other words
they proceed no further than to allow the microbe to exist in a new
environment. This seems to point undoubtedly to the fact that they are
caused by selection of true variations. In other words what is
ultimately produced is a condition of the germ-cell in which it becomes
very highly resistant to any influence immediately exerted upon it by
the environment, and so continues to live in successive generations
without any further modification. The conclusion, therefore, is, in Dr.
Reid's words, "that the germ-plasm is both spontaneously variable and
highly resistant to the direct action of the environment. In other words
we must believe that in any species that is not undergoing extinction
spontaneous variations greatly preponderate over those which are caused
by the direct action of the environment."
This quality of single cells, that is to say of the germ-plasm of all
species which continue to exist, in virtue of which it resists very
strongly any efforts to change it, is a very important matter to grasp.
Without it it is quite obvious that no species could maintain its
characteristic features for any length of time. Were it not for this
resistant power, germ-plasm would be easily destroyed or continually and
readily changed. The descendants from such continually changing
germ-plasm would themselves be of such infinite variety that there would
be no such thing as a definite species, so that there is no doubt
whatever that germ-plasm has become, probably by the action of natural
selection, extremely resistant to all influences of an environmental
character.
That does not mean, of course, that germ-plasm cannot be damaged, or
weakened, or changed in its tendencies. It does mean that when it is so
changed it is principally as the result of injury, which may be indeed
so severe as to destroy the germ-plasm itself. It would seem as if the
inherited tendencies of germ-cells were so intimately bound up in the
constitution of those cells as to be almost a matter of life and death
of the cells. If they be so interfered with as to be destroyed it is
hardly possible for the cell itself to continue to exist. One of the
most interesting examples of this resistance of germ-cells to their
environment is in connection with some human diseases which have existed
from time immemorial, diseases the descriptions of which are to be found
given quite accurately in the most ancient documents, but in spite of
the fact that human germ-cells have been subject to the hostile
surroundings which such human diseases involve they themselves have not
changed to any great extent. That is to say they still produce a type of
embryo and offspring practically identical with that that always was
produced.
The same truth applies to the cells which make up the body of the embryo
and the individual, as well as to the germ-cells. The body-cells, those
which make up bone, and muscle, and gland, and so forth, are constantly
exposed to all sorts of influences which must tend to damage them so far
as it is possible for the cells to be damaged and still live. These
body-cells are sometimes starved, sometimes poisoned with alcohol and
drugs, frozen by extremes of temperature, over-worked by too much
physical strain, and so on, and if it were possible for such external
influences to change the type of cells of their offspring we should
expect to see it here. But it does not occur. The internal hereditary
tendencies of these cells are so strong, and so intimately bound up with
the life of the cells themselves, that when they divide and produce
others these others are precisely similar to the parent cells, in spite
of all the unfavourable environment in which they have been. Slight
variations do, of course, occur, but these are chiefly of a germinal or
spontaneous nature, and not due to the environment.
This thought gives us some vague and imperfect idea of how immensely
complex the constitution of germ-plasm must be. This germ-plasm is very
often subjected to all sorts of unfavourable conditions, especially
those of alcohol and toxins, and such conditions have been acting upon
it more or less for an immense number of generations, and yet the
resistance to modification at the hands of these internal factors is so
great that all the processes which follow upon the fertilisation of the
ovum, all the thousand complications which thereafter ensue in the
building up of the young embryo are hardly ever interfered with. When
they are markedly interfered with such interference generally involves
the death of the embryo.
The conclusion arrived at on this subject by Dr. Archdall Reid, after a
very careful and extensive inquiry into all the evidence from many
points of view, is stated by him as follows: "Though variations may
result from the direct action of the environment, such variations are,
in effect, always injuries, and are of rare occurrence in individuals
who survive and have offspring. Adaptation (_i.e._ evolution) depends
almost exclusively on spontaneous variations. These do not imply damage
to the germ-plasm, but are products of its vital activity. Occurring in
vast abundance all round the specific and parental means, they supply
the sole material for Natural Selection."
"We conceive the germ-plasm, then, as living and active, closely
adjusted to its environment, growing, dividing, varying, capable of
being destroyed and injured, but resisting death and injury, and within
limits capable of repairing damage and returning to its original
state--as behaving exactly as a living individual does."