THE FACTORS CONCERNED IN THE BUILDING OF THE LIVING MACHINE.
Having now outlined the results of our study into the mechanism of the
living machine, we turn our attention next to the more difficult problem
of the method by which this machine was built. From the facts which we
have been considering in the last two chapters it is evident that the
problem we have before us is a mechanical rather than a chemical one. Of
course, chemical forces lie at the bottom of vital activity, and we must
look upon the force of chemical affinity as the fundamental power to
which the problems must be referred. But a chemical explanation will
evidently not suffice for our purpose; for we have absolutely no reason
for believing that the phenomena of life can occur as the results of the
chemical properties of any compound, however complex. The simplest known
form of matter which manifests life is a machine, and the problem of the
origin of life must be of the origin of that machine. Are there any
forces in nature which are of a sort as to enable us to use them to
explain the building of machines? Plants and animals are the only
machines which nature has produced. They are the only instances in
nature of a structure built with its parts harmoniously adjusted to each
other to the performance of certain ends. All other machines with which
we are acquainted were made by man, and in making them intelligence came
in to adapt the parts to each other. But in the living organism is a
similarly adapted machine made by natural means rather than artificial.
How were they built? Does nature, apart from human intelligence, possess
forces which can achieve such results?
Here again we must attack the problem from what seems to be the wrong
end. Apparently it would be simpler to discover the method of the
manufacture of the simplest machine rather than the more complex ones.
But this has proved contrary to the fact. Perhaps the chief reason is
that the simplest living machine is the cell whose study must always
involve the use of the microscope, and for this reason is more
difficult. Perhaps it is because the problem is really a more difficult
one than to explain the building of the more complex machines out of the
simpler ones. At all events, the last fifty years have told us much of
the method of the building of the complex machines out of the simpler
ones, while we have as yet not even a hint as to the solution of the
building of the simplest machine from the inanimate world. Our attention
must, therefore, be first directed to the method by which nature has
constructed the complex machines which we find filling the world to-day
in the form of animals and plants.
==History of the Living Machine.==--In the first place, we must notice
that these machines have not been fashioned suddenly or rapidly, but
have been the result of a very slow growth. They have had a history
extending very far back into the past for a period of years which we can
only indefinitely estimate, but certainly reaching into the millions. As
we look over this past history in the light of our present knowledge we
see that whatever have been the forces which have been concerned in the
construction of these machines they have acted very slowly. It has taken
centuries, and, indeed, thousands of years, to take the successive steps
which have been necessary in this construction. Secondly, we notice that
the machines have been built up step by step, one feature being added to
another with the slowly progressing ages. Thirdly, we notice that in one
respect this construction of the living machine by nature's processes
has been different from our ordinary method of building machines. Our
method of building puts the parts gradually into place in such a way
that until the machine is finished it is incapable of performing its
functions. The half-built engine is as useless and as powerless as so
much crude iron. Its power of action only appears after the last part is
fitted into place and the machine finished. But nature's process in
machine building is different. Every step in the process, so far as we
can trace it at least, has produced a complete machine. So far back as
we can follow this history we find that at every point the machine was
so complete as to be always endowed with motion and life activity.
Nature's method has been to take simpler types of machines and slowly
change them into more complicated ones without at any moment impairing
their vigour. It is something as if the steam engine of Watt should be
slowly changed by adding piece after piece until there was finally
produced the modern quadruple expansion engine, but all this change
being made upon the original engine without once stopping its motion.
[Illustration: FIG. 45. A group of cells resulting from division,
representing the first step in machine making.]
This gradual construction of the living machines has been called
_Organic Evolution_, or the _Theory of Descent_. It will be necessary
for us, in order to comprehend the problem which we have before us, to
briefly outline the course of this evolution. Our starting point in this
history must be the cell, for such is the earliest and simplest form of
living thing of which we have any trace. This cell is, of course,
already a machine, and we must presently return to the problem of its
origin. At present we will assume this cell as a starting point endowed
with its fundamental vital powers. It was sensitive, it could feel,
grow, and reproduce itself. From such a simple machine, thus endowed,
the history has been something as follows: In reproducing itself this
machine, as we have already seen, simply divided itself into two halves,
each like the other. At first all the parts thus arising separated from
each other and remained independent. But so long as this habit continued
there could be little advance. After a time some of the cells failed to
separate after division, but remained clinging together (Fig. 45). The
cells of such a mass must have been at first all alike; but, after a
little, differences began to appear among them. Those on the outside of
the mass were differently affected by their surroundings from those in
the interior, and soon the cells began to share among themselves the
different duties of life. The cells on the outside were better situated
for protection and capturing food, while those on the inside could not
readily seize food for themselves, and took upon themselves the duty of
digesting the food which was handed to them by the outer cells. Each of
these sets of cells could now carry on its own special duties to better
advantage, since it was freed from other duties, and thus the whole mass
of cells was better served than when each cell tried to do everything
for itself. This was the first step in the building of the machine out
of the active cells (Fig. 46). From such a starting point the subsequent
history has been ever based upon the same principle. There has been a
constant separation of the different functions of life among groups of
cells, and as the history went on this division of labor among the
different parts became greater and greater. Group after group of cells
were set apart for one special duty after another, and the result was a
larger and ever more complicated mass of cells, with a greater and
greater differentiation among them. In this building of the machine
there was no time when the machine was not active. At all points the
machine was alive and functional, but each step made the total function
of the machine a little more accurately performed, and hence raised
somewhat the totality of life powers. This parcelling out of the
different duties of life to groups of cells continued age after age,
each step being a little advance over the last, until the result has
been the living machine as we know it in its highest form, with its
numerous organs, all interrelated in such a way as to form a
harmoniously acting whole.
[Illustration: FIG. 46. A later step in machine building in which the
outer cells have acquired different form and function from the inner
cells: _ec_, the outer cells, whose duties are protective; _en_, the
inner cells engaged in digesting food.]
But a second principle in this growth of the machine was needed to
produce the variety which is found in nature. As the different cells in
the multicellular mass became associated into groups for different
duties, the method of such division of labor was not alike in all
machines. A city in China and one in America are alike made up of
individuals, and the fundamental needs of the Chinaman and the American
are alike. But differences in industrial and political conditions have
produced different combinations and associations, so that Pekin is
wonderfully unlike New York. So in these early developing machines,
quite a variety of method of organization was adopted by the different
groups. Now as soon as any special type of organization was adopted by
any animal or plant, the principle of heredity transmitted the same kind
of organization to its descendants, and there thus arose lines of
descent differing from each other, each line having its own method of
organization. As we follow the history of each line the same thing is
repeated. We find that the representatives of each line again separate
into groups, each of which has acquired some new type of organization,
and there has thus been a constant divergence of these lines of descent
in an indefinite number of directions. The members of the different
lines of descent all show a fundamental likeness with each other since
they retain the fundamental characters of their common ancestor, but
they show also the differences which they have themselves acquired. And
thus the process is repeated over and over again. This history of the
growth of these different machines has thus been one of divergence from
common centres, and is to be diagrammatically expressed after the
fashion of a branching tree. The end of each branch represents the
highest state of perfection to which each line has been carried.
One other point in this history must be noted. As the development of the
complication of the machine progressed the possibility of further
progress has been constantly narrowed. When the history of these
machines began as a simple mass of cells, there was a possibility of an
almost endless variety of methods of organization. But as a distinct
type of organization was adopted by one and another line of descendants
all subsequent productions were limited through the law of heredity to
the general line of organization adopted by their ancestors. With each
age the further growth of such machines must consist in the further
development in the perfection of its parts, and not in the adoption of
any new system of organization. Hence it is that the history of the
living machine has shown a tendency toward development along a few
well-marked lines, and although this complication becomes greater, we
still see the same fundamental scheme of organization running through
the whole. As the ages have progressed the machines have become more
perfect in the adjustment of their parts, i.e., they have become more
perfect machines, but the history has been simply that of perfecting
the early machines rather than the production of new types.
==Evidence for this History.==--As just outlined, we see that the living
machines have been gradually brought into their present condition by a
process which has been called organic evolution. But we must pause for a
moment to ask what is our evidence that such has been the history of the
living machine. The whole possibility of understanding living nature
depends upon our accepting this history and finding an explanation of
it. At the outset we have the question of fact, and we must notice the
grounds upon which we stand in assuming this history to be as outlined.
This problem is the one which has occupied such a prominent place in the
scientific world during the last forty years, and which has contributed
so largely toward making modern biology such a different subject from
the earlier studies of natural history. It is simply the evidence for
organic evolution, or the theory of descent. The subject has for forty
years been thoroughly sifted and tested by every conceivable sort of
test. As a result of the interest in the question there has been
disclosed an immense mass of evidence, relevant and irrelevant. As the
evidence has accumulated it has become more and more evident that the
evolution theory must be recognized as the only one which is in accord
with the facts, and the outcome has been a practical unanimity among
thinkers that the theory of descent must be the foundation of our
further study. The evidence which has forced this conclusion upon
scientists we must stop for a moment to consider, since it bears very
directly upon the subject we are studying.
==Historical.==--The first source of evidence is naturally a historical
one. This long history of the construction of the living machine has
left its record in the rocks which form the earth's surface. During this
long period the rocks of the earth's crust have been deposited, and in
these rocks have been left samples of many of the steps in this history
of machine building. The history can be traced by the study of these
samples just as the history of any machine might be traced from a study
of the models in a patent office. One might very easily trace, with most
strict accuracy and minute detail, the history of the printing machine
from the models which are preserved in the patent offices and elsewhere.
So is it with the history of the living machine. To be sure, the history
is rather incomplete and at times difficult to read. Many a period in
the development has left no samples for our inspection and must be
interpreted in our history between what went before and what comes
after. Many of the machines, especially the early ones, were made of
such fragile material that they could not be preserved in the rocks. In
many a case, too, the rocks in which the specimens were deposited have
been subjected to such a variety of heatings and pressures, that they
have been twisted out of shape and even crushed out of recognizable
form. But in spite of this the record is showing itself more complete
each year. Our paleontologists are opening layer after layer of these
rocks, and thus examining each year new pages in nature's history. The
more recent epochs in the history have been already read with almost
historic accuracy. From them we have learned in great detail how the
finishing touches were given to these machines, and are able to trace
with accuracy how the somewhat more generalized forms of earlier days
were changed to produce our modern animals.
This fossil record has given us our best knowledge of the course by
which the present living world has been brought into its existing
condition. But its accuracy is largely confined to the recent periods.
Of the very early history fossils tell us little or nothing. All the
early rocks, which we may believe were formed during the period when the
first steps in this machine building were taken, have been so changed by
heat and pressure that whatever specimens they may have originally
contained have been crushed out of shape. Furthermore, the earliest
organisms had no hard skeletons, and it was not until living beings had
developed far enough to have hard parts that it was possible for them to
leave traces of themselves in the rocks. Hence, so far as concerns this
earliest history, we can get no record of it in the rocks.
==Embryological.==--But here comes in another source of evidence which
helps to fill up the gap. In its development every animal to-day begins
as an egg. This is a simple cell, and the animal goes through a series
of changes which eventually lead to the adult. Now these changes appear
for the most part to be parallel to the changes through which the
earlier forms of life passed in their development from the simple to the
more complicated forms. Where it is possible to follow the history of
the groups of animals from their fossil remains and compare it with the
history of the individual animal as it progresses from the egg to the
adult, there is found a very decided parallelism. This parallelism
between embryology and past history has been of great service in
helping us toward the history of the past. At one time it was believed
that it was the key which would unlock all doors, and for a decade
biologists eagerly pursued embryology with the expectation that it would
solve all problems in connection with the history of animals. The result
has been somewhat disappointing. Embryology has, it is true, been of the
utmost service in showing relationships of forms to each other, and in
thus revealing past history. But while this record is a valuable one, it
is a record which has unfortunately been subject to such modifying
conditions that in many cases its original meaning has been entirely
obliterated and it has become worthless as a historical record. These
imperfections in regard to the record were early seen after the
attention of biologists was seriously turned to the study of embryology,
but it was expected that it would be possible to correct them and
discover the true meaning underlying the more apparent one. Indeed, in
many cases this has been found possible. But many of the modifications
are so profound as to render it impossible to untangle them and discover
the true meaning. As a result the biologist to-day is showing less
confidence in embryology, and is turning his attention in different
directions as more promising of results in the line desired.
But although the teachings of embryology have failed to realize the
great hopes that were placed upon them, their assistance in the
formulation of this history of the machine has been of extreme value.
Many a bit of obscurity has been cleared up when the embryology of
puzzling animals has been studied. Many a relationship has been made
clear, and this is simply another way of saying that a portion of this
history of life has been read. This aid of embryology has been
particularly valuable in just that part of the history where the
evidence from the study of fossils is wanting. The study of fossils, as
we have seen, gives little or no data concerning the early history of
living machines; and it is just here that embryology has proved to be of
the most value. It is a source of evidence that has told us of most of
the steps in the progress from the single-celled animal to the
multicellular organisms, and gives us the clearest idea of the
fundamental principles which have been concerned in the evolution of
life and the construction of the complicated machine out of the simple
bit of protoplasm. In spite of its limits, therefore, embryology has
contributed a large quota of the evidence which we have of the evolution
of life.
==Anatomical.==--A third source of this history is obtained from the facts
of comparative anatomy. The essential feature of this subject is the
fact that animals and plants show relationships. This fact is one of the
most patent and yet one of the most suggestive facts of biology. It has
been recognized from the very beginning of the study of animals and
plants. One cannot be even the most superficial observer without seeing
that certain forms show great likeness to each other while others are
much more unlike. The grouping of animals and plants into orders,
genera, and species is dependent upon this relationship. If two forms
are alike in everything except some slight detail, they are commonly
placed in the same genus but in different species, while if they show a
greater unlikeness they may be placed in separate genera. By thus
grouping together forms according to their resemblance the animal and
vegetable kingdoms are classified into groups subordinate to groups. The
principle of relationship, i.e., fundamental similarity of structure,
runs through the whole animal and vegetable kingdom. Even the animals
most unlike each other show certain points of similarity which indicates
a relationship, although of course a distant one.
The fact of such a relationship is too patent to demand more words, but
its significance needs to be pointed out. When we speak of relationship
among men we always mean historical connection. Two brothers are closely
related because they have sprung from common parents, while two cousins
are less closely related because their common point of origin was
farther back in time. More widely we speak of the relationship of the
Indo-European races, meaning thereby that back in the history of man
these races had a common point of origin. We never speak of any real
relation of objects unless thereby we mean to imply historical
connection. We are therefore justified in interpreting the manifest
relationships of organisms as pointing to history. Particularly are we
justified in this conclusion when we find that the relationships which
we draw between the types of life now in existence run parallel to the
history of these types as revealed to us by fossils and at the same time
disclosed by the study of embryology.
This subject of comparative anatomy includes a consideration of what is
called homology, and perhaps a concrete example may be instructive both
in illustration and as suggesting the course which nature adopts in
constructing her machines. We speak of a monkey's arm and a bird's wing
as homologous, although they are wonderfully different in appearance and
adapted to different duties. They are called homologous because they
have similar parts in similar relations. This can be seen in Figs. 47
and 48, where it will be seen that each has the same bones, although in
the bird's wing some of the bones have been fused together and others
lost. Their similarity points to a relationship, but their dissimilarity
tells us that the relationship is a distant one, and that their common
point of origin must have been quite far back in history. Now if we
follow back the history of these two kinds of appendages, as shown to us
by fossils, we find them approaching a common point. The arm can readily
be traced to a walking appendage, while the bird's wing, by means of
some interesting connecting links, can in a similar way be traced to an
appendage with its five fingers all free and used for walking. Fig. 49
shows one of these connecting links representing the earliest type of
bird, where the fingers and bones of the arm were still distinct, and
yet the whole formed a true wing. Thus we see that the common point of
origin which is suggested by the likenesses between an arm and a wing is
no mere imaginary one, for the fossil record has shown us the path
leading to that point of origin. The whole tells us further that
nature's method of producing a grasping or flying organ was here, not to
build a new organ, but to take one that had hitherto been used for other
purposes, and by slow changes modify its form and function until it was
adapted to new duties.
[Illustration: FIG. 47.--The arm of a monkey, a prehensile appendage.]
[Illustration: FIG. 48.--The arm of a bird, a flying appendage. In life
covered with feathers.]
[Illustration: FIG. 49.--The arm of an ancient half-bird half-reptile
animal. In life covered with feathers and serving as a wing.]
==Significance of these Sources of History.==--The real force of these
sources of evidence comes to us only when we compare them with each
other. They agree in a most remarkable fashion. The history as disclosed
by fossils and that told by embryology agree with each other, and these
are in close harmony with the history as it can be read from comparative
anatomy. If archæologists were to find, in different countries and
entirely unconnected with each other two or more different records of a
lost nation, the belief in the actual existence of that nation would be
irresistible. When researches at Nineveh, for example, unearth tablets
which give the history of ancient nations, and when it proves that among
the nations thus mentioned are some with the same names and having the
same facts of history as those mentioned in the Bible, it is absolutely
impossible to avoid the conclusion that such a nation with such a
history did actually exist. Two independent sources of record could not
be false in regard to such a matter as this.
Now, our sources of evidence for this history of the living machine
prove to be of exactly this kind. We have three independent sources of
evidence which are so entirely different from each other that there is
almost no likeness between them. One is written in the rocks, one in
bone and muscle, while the third is recorded in the evanescent and
changing pages of embryology and metamorphosis. Yet each tells the same
story. Each tells of a history of this machine from simple forms to more
complex. Each tells of its greater and greater differentiation of labour
and structure as the periods of time passed. Each tells of a growing
complexity and an increasing perfection of the organisms as successive
periods pass. Each tells us of common points of origin and divergence
from these points. Each tells us how the more complicated forms have
arisen as the results of changes in and modifications of the simpler
forms. Each shows us how the individual parts of the organisms have been
enlarged or diminished or changed in shape to adapt them to new duties.
Each, in short, tells the same story of the gradual construction of the
living machine by slow steps and through long ages of time. When these
three sources of history so accurately agree with each other, it is as
impossible to disbelieve in the existence of such history as it is to
disbelieve in the existence of the ancient Hittite nation, after its
history has been told to us by two different sources of record.
Now all this is very germane to our subject. We are trying to learn how
this living machine, with its wonderful capabilities, was built. The
history which we have outlined is undoubtedly the history of the
building of this machine, and the knowledge that these complicated
machines have been produced as the result of slow growth is of the
utmost importance to us. This knowledge gives us at the very start some
idea of the nature of the forces which have been at work. It tells us
that in searching for these forces we must look for those which have
been acting constantly. We must look for forces which produce their
effects not by sudden additions to the complication of the machine. They
must be constant forces whose effect at any one time is comparatively
slight, but whose total effect is to increase the complexity of the
machine. They must be forces which produce new types through the
modification of the old ones. We must look for forces which do not adapt
the machine for its future, but only for its present need. Each step in
the history has been a complete animal with its own fully developed
powers. We are not to expect to find forces which planned the perfect
machine from the start, nor forces which were engaged in constructing
parts for future use. Each step in the building of the machine was taken
for the good of the machine at the particular moment, and the forces
which we are to look for must therefore be only such as can adapt the
organisms for its present needs. In other words, nothing has been
produced in this machine for the purpose of being developed later into
something of value, but all parts that have been produced are of value
at the time of their appearance. We must, in short, look for forces
constantly in action and always tending in the same direction of
greater complexity of structure.
Is it possible to discover these forces and comprehend their action?
Before the modern development of evolution this question would
unhesitatingly have been answered in the negative. To-day, under the
influence of the descent theory, stimulated, in the first place, by
Darwin, the question will be answered by many with equal promptness in
the affirmative. At all events, we have learned in the last forty years
to recognize some of the factors which have been at work in the
construction of this machine. We must turn, therefore, to the
consideration of these factors.
==Forces at Work in the Building of the Living Machine.==--There are three
primary factors which lie at the bottom of the whole process. They are--
1. _Reproduction_, which preserves type from generation to generation.
2. _Variation_, which modifies type from generation to generation.
3. _Heredity_, which transmits characters from generation to generation.
Each must be considered by itself.
==Reproduction.==--Reproduction is the primary factor in this process of
machine building, heredity and variation being simply phases of
reproduction. The living machine has developed by natural processes, all
other machines by artificial methods. Reproduction is the one essential
point of difference between the living machine and the others which has
made their construction by natural processes a possibility. What, then,
is reproduction? Reproduction is in all cases at the bottom simple
division. Whether we consider the plant that multiplies by buds or the
unicellular animal that simply divides into two equal parts, or the
larger animal that multiplies by eggs, we find that in all cases the
fundamental feature of the process is division. In all cases the
organism divides into two or more parts, each of which becomes in time
like the original. Moreover, when we trace this division further we find
that in all cases it is to be referred back to the division of the cell,
such as we have described in a previous chapter. The egg is a single
cell which has come from the parent by the division of one of the cells
in the body of the parent. A bud is simply a mass of cells which have
all arisen from the parent cells by division. The foundation of
reproduction is thus in all cases cell division. Now, this process of
division is dependent upon the properties of the cell. Firstly, it is a
result of the assimilative powers of the cell, for only through
assimilation can the cell increase in size, and only as it increases in
size can it gain sustenance for cell division. Secondly, it is
dependent, as we have seen, upon the mechanism of the cell body, and
especially the nucleus and centrosome. These structures regulate the
cell division, and hence the reproduction of all animals and plants. We
can not, therefore, find any explanation of reproduction until we have
explained the mechanism of the cell. The fundamental feature, of
nature's machine building is thus based upon the machinery of the
nucleus and centrosome of the organic cell.
Aside from the simple fact that it preserves the race, the most
important feature connected with this reproduction is its wonderful
fruitfulness. Since it results from division, it always tends to
increase the offspring in geometrical ratio. In the simplest case, that
of the unicellular animals, the cell divides, giving rise to two
animals, each of which divides again, producing four, and these again,
giving eight, etc. The rapidity of this multiplication is sometimes
inconceivable. It depends, of course, upon the interval of time between
the successive divisions, but among the lower organisms this interval is
sometimes not more than half an hour, the result of which is that a
single individual could give rise in the course of twenty-four hours to
sixteen million offspring. This is doubtless an extreme case, but among
all the lower animals the rate is very great. Among larger animals the
process is more complicated; but here, too, there is the same tendency
to geometrical progression, although the intervals between the
successive reproductions may be quite long and irregular. But it is
always so great that if allowed to progress unhindered at its normal
rate the offspring would, in a few years, become so numerous as to crowd
other life out of existence. Even the slow-breeding elephant would, if
allowed to breed unhindered for seven hundred and fifty years, produce
nineteen million offspring--a rate of increase plainly incompatible with
the continued existence of other animals.
Here, then, we have the foundation of nature's method of building
animals and plants of the higher classes. In the machinery of the cell
she has a power of reproduction which produces an increase in
geometrical ratio far beyond the possibility for the surface of the
earth to maintain.
==Heredity.==--The offspring which arise by these processes of division
are like each other, and like the parent from which they sprung. This
is the essence of what is called heredity. Its significance in the
process of machine building is evident at once. It is the conserving
force which preserves the forms already produced and makes it possible
for each generation to build upon the structures of the earlier ones.
Without it each generation would have to begin anew at the beginning,
and nothing could be accomplished. But since this principle brings each
individual to the same place where its parents stand, and thus always
builds the offspring into a machine like the parent, it makes it
possible for the successive generations to advance. Heredity is thus
like the power of memory, or better still, like the invention of
printing in the development of civilization. It is a record of past
achievements. By means of printing each age is enabled to benefit by the
discoveries of the previous age, and without it the development of
civilization would be impossible. In the same way heredity enables each
generation to benefit by the achievements of its ancestors in the
process of machine building, and thus to devote its own energies to
advancement.
The fact of heredity is patent enough. It has been always clearly
recognized that the child has the characters of its parents, and this
belief is so well attested as to need no proof. It is still a question
as to just what characters may be inherited, and what influences may
affect the inheritance. There are plenty of puzzling problems connected
with heredity, but the fact of heredity is one of the foundation stones
of biological science. Upon it must be built all theories which look
toward the explanation of the origin of the living machine.
This factor of heredity again we must trace back to the machinery of
the cell. We have seen in the previous pages evidence for the wonderful
nature of the chromosomes of the cells. We can not pretend to understand
them, but they must be extraordinarily complex. We have seen proof that
these chromosomes are probably the physical basis of heredity, since
they are the only parts of each parent which are handed down to
subsequent generations. With these various facts of cell division and
cell fertilization in mind, we can reach a very simple explanation of
fundamental features of heredity. The following is an outline of the
most widely accepted view of the hereditary process.
Recognizing that the chromosomes are the physical basis of hereditary
transmission, we can picture to ourselves the transmission of hereditary
characters something as follows: As we have seen, the fertilized egg
contains an equal number of chromosomes from each parent (Fig. 42). Now
when this fertilized cell divides, each of the rods splits lengthwise,
half of each entering each of the two cells arising from the cell
division. From this method of division of the chromosomes it follows
that the daughter cells would be equivalent to each other and equivalent
also to the undivided egg. If the original chromosomes contained
potentially all the hereditary traits handed down from parent to child,
the chromosomes of each daughter cell will contain similar hereditary
traits. If, therefore, the original fertilized egg possessed the power
of developing into an adult like the parent, each of the daughter cells
should likewise possess the power of developing into a similar adult.
And thus each cell which arises as the result of such division should
possess similar characters so long as this method of division continues.
But after a little in the development of the egg a differentiation among
the daughter cells arises. They begin to acquire different shapes and
different functions. This we can only believe to be the result of a
differentiation in their chromatin material. In the cell division the
chromosomes no longer split into equivalent halves, but some characters
are portioned off to some cells and others to other cells. Those cells
which are to carry on digestive functions when they are formed receive
chromatin material which especially controls them in the performance of
this digestive function, while those which are to produce sensory organs
receive a different portion of the chromatin material. Thus the adult
individual is built up as the cells receive different portions of this
hereditary substance contained in the original chromosomes. The original
chromosomes contained _all_ hereditary characters, but as development
proceeds these are gradually portioned out among the daughter cells
until the adult is formed.
From this method of division it will be seen that each cell of the adult
does not contain all the characters concealed in the original
chromosomes of the egg, although each contains a part which may have
been derived from each parent. It is thought, however, that a part of
the original chromatin material does not thus become differentiated, but
remains entirely unchanged as the individual is developing. This
chromatin material may increase in amount by assimilation, but it
remains unchanged during the entire growth of the individual. It thus
follows that the adult will contain, along with its differentiated
material, a certain amount of the original physical basis of heredity
which still retains its original powers. This undifferentiated chromatin
material originally possessed powers of producing a new individual, and
of course it still possesses these powers, since it has remained dormant
without alteration. Further, it will follow that if this dormant
undifferentiated chromatin should start into activity and produce a new
individual, the new individual thus produced would be identical in all
characters with the one which actually did develop from the egg, since
both individuals would have come from a bit of the same chromatin. The
child would be like the parent. This would be true no matter how much
this undifferentiated material should increase in amount by
assimilation, _so long as it remained unaltered in character_, and it
hence follows that every individual carries around a certain amount of
undifferentiated chromatin material in all respects identical with that
from which he developed.
Now whether this undifferentiated _germ plasm_, as we will now call it,
is distributed all over the body, or is collected at certain points, is
immaterial to our purpose. It is certain that portions of it find their
way into the reproductive organs of the animal or plant. Thus we see
that part of the chromatin material in the egg of the first generation
develops into the second generation, while another part of it remains
dormant in that second generation, eventually becoming the chromatin of
its eggs and spermatozoa. Thus each egg of the second generation
receives chromosomes which have come directly from the first generation,
and thus it will follow that each of these eggs will have identical
properties with the egg of the first generation. Hence if one of these
new eggs develops into an adult it will produce an adult exactly like
the second generation, since it contains chromosomes which are
absolutely identical with those from which the second generation sprung.
There is thus no difficulty in understanding why the second generation
will be like the first, and since the process is simply repeated again
in the next reproduction, the third generation will be like the second,
and so on, generation after generation. A study of the accompanying
diagram will make this clear.
In other words, we have here a simple understanding of at least some of
the features of heredity. This explanation is that some of the chromatin
material or germ plasm is handed down from one generation to another,
and is stored temporarily in the nucleii of the reproductive cells.
During the life of the individual this germ plasm is capable of
increasing in amount without changing its nature, and it thus continues
to grow and is handed down from generation to generation, always endowed
with the power of developing into a new individual under proper
conditions, and of course when it does thus give rise to new individuals
they will all be alike. We can thus easily understand why a child is
like its parent. It is not because the child can inherit directly from
its parent, but rather because both child and parent have come from the
unfolding of two bits of the same germ plasm. This fact of the
transmission of the hereditary substance from generation to generation
is known as the theory of the _continuity of germ plasm_.
Such appears to be, at least in part, the machinery of heredity. This
understanding makes the germ substance perpetual and continuous, and
explains why successive generations are alike. It does not explain,
indeed, why an individual inherits from its parents, but why it is like
its parents. While biologists are still in dispute over many problems
connected with heredity, all are agreed to-day that this principle of
the continuity of the heredity substance must be the basis of all
attempts to understand the machinery of heredity. But plainly this whole
process is a function of the cell machinery. While, therefore, the idea
of the continuity of germ substance greatly simplifies our problem, we
must acknowledge that once more we are thrown back upon the mysteries of
the cell. Until we can more fully explain the cell machine we must
recognize our inability to solve the fundamental question of why an
individual is like its parents.
[Illustration: FIG. 50.--Diagram illustrating the principle of
heredity.
_A_ represents an egg of a starfish. From one half, the unshaded
portion, develops the starfish of the next generation, _B_. The other is
distributed without change in the ovaries, _ov_, of the individual, _B_.
From these ovaries arises the next egg, _A'_, with its germ plasm. This
germ plasm is evidently identical with that in _A_, since it is merely a
bit of the same handed down through the individual, _B_. In the
development of the next generation the process is repeated, and hence
_B'_ will be like _B_, and the third generation of eggs identical with
the first and second. The undifferentiated part of the germ plasm is
thus simply handed on from one generation to the next.]
But plainly reproduction and heredity, as we have thus far considered
them, will be unable to account for the slow modification of the
machine; for in accordance with the facts thus far outlined, each
generation would be _precisely like the last_, and there would be no
chance for development and change from generation to generation. If the
individual is simply the unfolding of the powers possessed by a bit of
germ plasm, and if this germ plasm is simply handed on from generation
to generation, the successive generations must of necessity be
identical. But the living machine has been built by changes in the
successive generation, and hence plainly some other factor is needed.
This factor is _variation_.
==Variation.==--Variation is the principle that produces _modification of
type_. Heredity, as just explained, would make all generations alike.
But nothing is more certain than that they are not alike. The fact of
variation is patent on every side, for no two individuals are alike.
Successive generations differ from each other in one respect or
another. Birds vary in the length of their bills or toes; butterflies,
in their colours; dogs, in their size and shape and markings; and so on
through an endless category. Plants and animals alike throughout nature
show variations in the greatest profusion. It is these variations which
must furnish us with the foundation of the changes which have gradually
built up the living machine.
Of the fact of these variations there is no question, and the matter
need not detain us. Every one has had too many experiences to ask for
proof. Of the nature of the variations, however, there are some points
to be considered which are very germane to our subject. In the first
place, we must notice that these variations are of two kinds. There is
one class which is born with the individual, so that they are present
from the time of birth. In saying that these variations are born with
the individual we do not necessarily mean that they are externally
apparent at birth. A child may inherit from its parents characters which
do not appear till adult life. For example, a child may inherit the
colour of its father's hair, but this colour is not apparent at birth.
It appears only in later life, but it is none the less an inborn
character. In the same way, we may have many inborn variations among
individuals which do not make themselves seen until adult life, but
which are none the less innate. The offspring of the same parents may
show decided differences, although they are put under similar
conditions, and such differences are of course inherent in the nature of
the individual. Such variations are called _congenital variations_.
There is, however, a second class of variations which are not born in
the individual, but which arise as the result of some conditions
affecting its after-life. The most extreme instances of this kind are
mutilations. Some men have only one leg because the other has been lost
by accident. Here is a variation acquired as the result of
circumstances. A blacksmith differs from other members of his race in
having exceptionally large arm muscles; but here, again, the large
muscles have been produced by use. A European who has lived under a
tropical sun has a darkened skin, but this skin has evidently been
darkened by the action of the sun, and is quite a different thing from
the dark skin of the dark races of men. In such instances we have
variations produced in individuals as the result of outside influences
acting upon them. They are not inborn, but are secondarily acquired by
each individual. We call them _acquired variations_.
It is not always possible to distinguish between these two types of
variation. Frequently a character will be found in regard to which it is
impossible to determine whether it is congenital or acquired. If a child
is born under the tropical sun, how can we tell whether its dark skin
was the result of direct action of the sun on its own skin, or was an
inheritance from its dark-skinned parents? We might suppose that this
could be answered by taking a similar child, bringing it up away from
the tropical sun, and seeing whether his skin remained dark. This would
not suffice, however; for if such a child did then develop a white skin,
we could not tell but that this lighter-coloured skin had been produced
by the direct bleaching effect of the northern climate upon a skin
which otherwise would have been dark. In other words, a conclusive
answer can not here be given. It is not our purpose, however, to attempt
to distinguish between these two kinds of variations, but simply to
recognize that they occur.
Our next problem must be to search for an explanation of these
variations. With the acquired variations we have no particular trouble,
for they are easily explained as due to the direct action of the
environment upon animals. One of the fundamental characters of the
living protoplasm (using the word now in its widest sense) is its
extreme instability. So unstable is it that any disturbing influence
will affect it. If two similar unicellular organisms are placed under
different conditions they become unlike, since their unstable protoplasm
is directly affected by the surrounding conditions. With higher animals
the process is naturally a little more complicated; but here, too, they
are easily understood as part of the function of the machine. One of the
adjustments of the machine is such that when any organ is used more than
usual the whole machine reacts in such a way as to send more blood to
this special organ. The result is a change in the nutrition of the organ
and a corresponding variation in the individual. Thus acquired
variations are simply functions of the action of the machine.
Congenital variations, however, can not receive such an explanation.
Being born with the individual, they can not be produced by conditions
affecting him, but rather to something affecting the germ plasm from
which he sprung. The nature of the germ plasm controls the nature of the
individual, and congenital variations must consequently be due to its
variations. But it is not so easy to see how this germ plasm can
undergo variation. The conditions which surround the individual would
affect its body, but it is not easy to believe that they would affect
the germinal substance. Indeed, it is not easy to see how any external
conditions can have influence upon this germinal material if it is not
an active part of the body, but is simply stored within it for future
use in reproduction. How could any changes in the environment of the
individual have any effect upon this dormant material stored within it?
But if we are correct in regarding this germ material in the
reproductive bodies as the basis of heredity and the guiding force in
development, then it follows that the only way in which congenital
variations can occur is by some variations in the germ plasm. If a child
developed from germ plasm _identical_ with that from which its parents
developed, it would inherit identical characters; and if there are any
congenital variations from its parents, they must be due to some
variations in the germ plasm. In other words, in order to explain
congenital variations we must account for variations in the germ plasm.
Now, there are two methods by which we may suppose that these variations
in the germ may arise. The first is by the direct influence upon the
germ plasm of certain unknown external conditions. The life substance of
organisms is always very unstable, and, as we have seen, acquired
variations are caused by external influences directly affecting it. Now,
the hereditary material is also life substance, and it is plainly a
possibility for us to imagine that this germ material is also subject to
influences from the conditions surrounding it. That such variations do
occur appears to be hardly doubtful, although we do not know what sort
of influences can produce them. If the germ plasm is wholly stored
within the reproductive gland, it is certainly in a position to be only
slightly affected by surrounding conditions which affect the animal. We
can readily understand that the use of an organ like the arm will affect
it in such a way as to produce changes in its protoplasm, but we can
hardly imagine that such use of the _arm_ would produce any change in
the hereditary substance which is stored in the reproductive organs.
External conditions may thus readily affect the body, but not so readily
the germ material. Even if such material is distributed more or less
over the body instead of being confined to the reproductive glands, as
some believe, the difficulty is hardly lessened. This difficulty of
understanding how the germ plasm can be affected by external conditions
has led one school of biologists to deny that it is subject to any
variation by external conditions, and hence that all modification of the
germ plasm must come from some other source. Probably no one, however,
holds this position to-day, and it is the general belief that the germ
plasm may be to some slight extent modified by external conditions. Of
course, if such variations do occur in the germ plasm they will become
congenital variations of the next generation, since the next generation
is the unfolding of the germ plasm.
The second method by which the variations of germ plasm may arise is
apparently of more importance. It is based upon the fact that, with all
higher animals and plants at least, each individual has two parents
instead of one. In our study of cells we have seen that the machinery
of the cell is such that it requires in the ordinary process of
reproduction the union of germinal material from two different
individuals to produce a cell which can develop into a new individual.
As we have seen, the egg gets rid of half its chromosomes in order to
receive an equal number from a male parent; and thus the fertilized egg
contains chromosomes, and hence hereditary material, from two different
individuals. Now, this sexual reproduction occurs very widely in the
organic world. Among some of the lowest forms of unicellular organisms
it is not known, but in most others some form of such union is
universal. Now, here is plainly an abundant opportunity for congenital
variations; for it is seen that each individual does not come from germ
material _identical with that from which either parent came, but from
some of this material mixed with a similar amount from a different
parent_. Now, the two parents are never exactly alike, and hence the
germ plasm which each contributes to the offspring will not be exactly
alike. The offspring will thus be the result of the unfolding of a bit
of germ plasm which will be different from that from which either of its
parents developed, and these differences will result in _congenital
variations_. Sexual reproduction thus results in congenital variations;
and if congenital variations are necessary for the evolution of the
living machine--and we shall soon see reason for believing that they
are--we find that sexual reproduction is a device adopted for bringing
out such congenital variations.
==Inheritance of Variations.==--The reason why congenital variations are
needed for the evolution of the living machine is clear enough.
Evanescent variations can have no effect upon this machine, for they
would disappear with the individual in which they appeared. In order
that they should have any influence in the process of machine building
they must be permanent ones; or, in other words, they must be inherited
from generation to generation. Only as such variations are transmitted
by heredity can they be added to the structure of the developing
machine. Therefore we must ask whether the variations are inherited.
In regard to the congenital variations there can be no difficulty. The
very fact that they are congenital shows us that they have been produced
by variations in the germ plasm, and as such they must be transmitted,
not only to the next generation, but to all following generations, until
the germ plasm becomes again modified. This germ plasm is handed on from
generation to generation with all its variations, and hence the
variations will be added permanently to the machine. Congenital
variations are thus a means for permanently modifying the organism, and
by their agency must we in large measure believe that evolution through
the ages has taken place.
With the acquired variations the matter stands quite differently. We can
readily understand how influences surrounding an animal may affect its
organs. The increase in the size of the muscles of the blacksmith's arm
by use we understand readily enough. But with our understanding of the
machinery of heredity we can not see how such an effect can extend to
the next generation. It is only the organ directly affected that is
modified by external conditions. Acquired variations will appear in the
part of the body influenced by the changed conditions. But the germ
plasm within the reproductive glands is not, so far as we can see,
subject to the influence of an increased use, for example, in the arm
muscles. The germ material is derived from the parents, and, if it is
simply stored in the individual, how could an acquired variation affect
it? If an individual lose a limb his offspring will not be without a
corresponding limb, for the hereditary material is in the reproductive
organs, and it is impossible to believe that the loss of the limb can
remove from the hereditary material in the reproductive glands just that
part of the germ plasm which was designed for the production of the
limb. So, too, if the germ plasm is simply stored in the individual, it
is impossible to conceive any way that it can be affected by the
conditions around the individual in such a way as to explain the
inheritance of acquired variations. If acquired variations do not affect
the germ plasm they cannot be inherited, and if the germ plasm is only a
bit of protoplasmic substance handed down from generation to generation,
we can not believe that acquired variations can influence it.
From such considerations as these have arisen two quite different views
among biologists; and, while it is not our purpose to deal with disputed
points, these views are so essential to our subject that they must be
briefly referred to. One class of biologists adhere closely to the view
already outlined, and insist for this reason that acquired variations
_can not_ under any conditions be inherited. They insist that all
inherited variations are congenital, and due therefore to direct
variations in the germ plasm, and that all instances of seeming
inheritance of acquired variations are capable of other explanation. The
other school is equally insistent that there are abundant instances of
the inheritance of acquired characters, claiming that these proofs are
so strong as to demand their acceptance. Hence this class of biologists
insist that the explanation of heredity given as a simple handing down
from generation to generation of a germ plasm is not complete, and that
while it is doubtless the foundation of heredity, it must be modified in
some way so as to admit of the inheritance of acquired characters. There
is no question that has excited such a wide interest in the biological
world during the last fifteen years as this one of the inheritance of
acquired characters. Until about 1884 the question was not seriously
raised. Heredity was known to be a fact, and it was believed that while
congenital characters are more commonly inherited, acquired characters
may also frequently be handed down from generation to generation. The
facts which we have noted of the continuity of germ plasm have during
the last fifteen years led many biologists to deny the possibility of
the latter. The debate which arose has continued vigorously, and can not
be regarded as settled at the present time. One result of this debate is
clear. It has been shown beyond question that while the inheritance of
congenital characters is the rule, the inheritance of acquired
characters is at all events unusual. At the present time many
naturalists would be inclined to think that the balance of evidence
indicates that under certain conditions certain kinds of acquired
characters may be inherited, although this is still disputed by others.
Into this discussion we cannot enter here. The reason for referring to
it at all is, however, evident. We are searching for nature's method of
building machines. It is perfectly clear that variations among animals
and plants are the foundations of the successive steps in advance made
in this machine building, but of course only such variations as can be
transmitted to posterity can serve any purpose in this development. If
therefore it should prove that acquired characters can not be inherited,
then we should no longer be able to look upon the direct influence of
the surroundings as a factor in the machine building. We should then
have nothing left except the congenital variations produced by sexual
union, or the direct variation of the germ plasm as a factor for
advance. If, however, it shall prove that acquired characters may even
occasionally be inherited, then the direct effect of the environment
upon the individual will serve as a decided assistance in our problem.
Here, then, we have before us the factors which have been concerned in
the building of the living machine under nature's hands. Reproduction
keeps in existence a constantly active, unstable, readily modified
organism as a basis upon which to build. Variation offers constantly new
modifications of the type, while heredity insures that the modifications
produced in the machine by the influences which give rise to the
variations shall be permanently fixed.
==Method of Machine Building.==--_Natural Selection._ The method by which
these factors have worked together to build up the living machines is
easily understood in its general aspects, although there are many
details as yet unsolved. The general facts connected with the evolution
of animals are matters of common knowledge. We need do no more than
outline the subject, since it is well understood by all. The basis of
the method is _natural selection_, which acts in this machine building
something as follows:
The law of reproduction, as we have seen, produces new individuals with
extraordinary rapidity, and as a result more individuals are born than
can possibly find sustenance in the world. Hence only a few of the
offspring of any animal or plant can live long enough to produce
offspring in turn. The many must die that the few may live; and there
is, therefore, a constant struggle among the individuals that are born
for food or for room in the world. In this _struggle for existence_ of
course the weakest will go to the wall, while those that are best
adapted for their place in life will be the ones to get food, live, and
reproduce their kind. This is at all events true among the lower
animals, although with mankind the law hardly applies. Now, among the
individuals that are born there will be no two exactly alike, since
variations are universal, many of which are congenital and thus born
with the individual and transmitted by inheritance. Clearly enough those
animals that have a variation which makes them a little better adapted
for the struggle will be the ones to live and hence to produce
offspring, while those without such advantage will be the ones to die.
We may suppose, for example, that some of the individuals had longer
necks than the average. In time of scarcity of food these individuals
would be able to get food that the short-necked individuals could not
reach. Hence in times of famine the long-necked individuals would be the
ones to survive. Now if this peculiarity were a congenital variation it
would be already represented in the germ plasm, and consequently it
would be inherited by the next generation. The short-necked individuals
being largely destroyed in this struggle for food, it would follow that
the next generation would be a little better off than the last, since
all would inherit this tendency toward a long neck. A few generations
would then see the disappearance of all individuals which did not show
either this or some other corresponding advantage, and in this way the
lengthened neck would be added permanently as a _part of the machine_.
When this time came this peculiarity would no longer give its possessors
any advantage over its rivals, since all would possess it. Now,
therefore, some new variation would in the same way determine which
animals should live and which should die in the struggle, and in time a
new modification would be added to the machine. And thus this process
continues, one variation after another being added, until the machine is
slowly built into a more and more complicated structure, always active
but with a constantly increasing efficiency. The construction is a
natural one. A mixing of germ plasm in sexual reproduction or some other
agencies produce congenital variations; natural selection acting upon
the numerous progeny selects the best of the new variations, and
heredity preserves and hands them down to posterity.
All students of whatever school recognize the force of this principle
and look upon natural selection as an efficient agency in machine
building. It is probably the most fundamental of the external laws that
have guided the process. There are, however, certain other laws which
have played a more or less subordinate part. The chief of these are the
influence of migration and isolation, and the direct influence of the
environment. Each of these laws has its own school of advocates, and
each has been given by its advocates the chief role in the process of
machine building.
==Migration and Isolation.==--The production of the various types of
machines has been undoubtedly facilitated by the migrations of animals
and the isolation of different groups of descendants from each other by
various natural barriers. The variations which occur in organisms are so
great that they would sometimes run into abnormal structures were it not
for the fact that sexual reproduction constantly tends to reduce them.
In an open country where animals and plants interbreed freely, it will
commonly happen that individuals with certain peculiarities will mate
with others without such peculiarities, and the offspring will therefore
inherit the peculiarity not in increased degree but in decreased degree.
This constant interbreeding of individuals will tend to prevent the
formation of many modifications in the machine which become started by
variations. Now plainly if some such individuals, with a peculiar
variation, should migrate into a new territory or become isolated from
their relatives which do not have similar variations, these individuals
will be obliged to breed with each other. The result will be that the
next generation, arising thus from two parents each of which shows the
same variation, will show it also in equal or increased degree.
Migrations and isolations will thus tend to _fix_ in the machine
variations which sexual union or other influences inaugurate. Now in the
history of the earth's surface there have been many changes which tend
to bring about such migration and isolations, and this factor has
doubtless played a more or less important part in the building of the
machines. How great a part we cannot say, nor is it necessary for our
purpose to decide; for in all these cases the machine building has only
been the result of the hereditary transmission of congenital variation
under certain peculiar conditions. The fundamental process is the same
as already considered, only the details of its working being in
question.
==Direct Influence of the Environment.==--Under this head we have a
subject of great importance. It is an undoubted fact that the
environment has a very decided effect upon the machine. These direct
effects of the environment are very positive and in great variety. The
tropical sun darkens the human skin; cold climate stunts the growth of
plants; lack of food dwarfs all animals and plants, and hundreds of
other similar examples could be selected. Another class of similar
influences are those produced by _use_ and _disuse_. Beyond question the
use of an organ tends to increase its size, and disuse to decrease it.
Combats of animals with each other tend to increase their strength,
flight from enemies their running powers, etc.
Now all these effects are direct modifications of the machine, and if
they are only transmitted to following generations so as to become
_permanent_ modifications, they will be most important agencies in the
machine building. If, on the other hand, they are not transmitted by
heredity, they can have no permanent effect. We have here thus again the
problem of the inheritance of acquired characters. We have already
noticed the uncertainty surrounding this subject, but the almost
universal belief in the inheritance of such characters requires us to
refer to it again. It is uncertain whether such direct effects have any
influence upon the offspring, and therefore whether they have anything
to do with this machine building. Still, there are many facts which
point strongly in this direction. For example, as we study the history
of the horse family we find that an originally five-toed animal began to
walk more and more on its middle toe, in such a way that this toe
received more and more use, while the outer toes were used less and
less. Now that such a habit would produce an effect upon the toes in any
generation is evident; but apparently this influence extended from
generation to generation, for, as the history of the animals is
followed, it is found that the outer toes became smaller and smaller
with the lapse of ages, while the middle one became correspondingly
larger, until there was finally produced the horse with its one toe only
on each foot. Now here is a line of descent or machine building in the
direct line of the effects of use and disuse, and it seems very natural
to suppose that the modification has been produced by the direct effect
of the use of the organs. There are many other similar instances where
the line of machine building has been quite parallel to the effects of
use and disuse. If, therefore, acquired characters can be inherited to
_any_ extent, we have, in the direct influences of the environment an
important agency in machine building. This direct effect of the
conditions is apparently so manifest that one school of biologists finds
in it the chief cause of the variations which occur, telling us that the
conditions surrounding the organism produce changes in it, and that
these variations, being handed down to subsequent generations,
constitute the basis of the development of the machine. If this factor
is entirely excluded, we are driven back upon the natural selection of
congenital variations as the only kind of variations which can
permanently effect the modification of the machine.
==Consciousness.==--It may be well here to refer to one other factor in
the problem, because it has somewhat recently been brought into
prominence. This factor is consciousness on the part of the animal.
Among plants and the lower animals this factor can have no significance,
but consciousness certainly occurs among the higher animals. Just when
or how it appeared are questions which are not answered, and perhaps
never will be. But consciousness, after it had once made its appearance,
became a controlling factor in the development of the machine. It must
not be understood by this that animals have had any consciousness of the
development of their body, or that they have made any conscious
endeavours to modify its development. This has not always been
understood. It has been frequently supposed that the claim that
consciousness has an influence upon the development of an animal means
that the animal has made conscious efforts to develop in certain
directions. For example, it has been suggested that the tiger, conscious
of the advantage of being striped, had a desire to possess stripes, and
the desire caused their appearance. This is absurd. Consciousness has
been a factor in the development of the machine, but an _indirect_ one.
Consciousness leads to effort, and effort has a direct influence in
development. For example, an animal is conscious of hunger, and this
leads to efforts on his part to obtain food. His efforts to obtain food
may lead to migration or to the adoption of new kinds of food or to
conflicts with various kinds of rivals, and all of these efforts are
potent factors in determining the direction of development.
Consciousness, again, may lead certain animals to take pleasure in each
other's society, or to recognize that in mutual association they have
protection against common enemies. Such a consciousness will give rise
to social habits, and social habits are a very potent factor in
determining the direction in which the inherited variations will tend;
not, perhaps, because it effects the variations themselves, but rather
because it determines which variations among the many shall be preserved
and which rejected by natural selection. Consciousness may lead the
antelope to recognize that he has no chance in a combat with a lion, and
this will induce him to flee. The _habit_ of flight would then develop
the _power_ of flight, not because the antelope desired such power, but
because the animals with variations which gave increased power of flight
would be the ones to escape the lion, while the slower ones would die
without offspring. Thus consciousness would indirectly, though not
directly, result in the lengthening of the legs of the animal and in the
strengthening of his running muscles. Beyond a doubt this factor of
consciousness has been a factor of no little moment in the development
of the higher types of organic machines. We can as yet only dimly
understand its action, but it must hereafter be counted as one of the
influences in the evolution of the living machine.
But, after all, these are only questions of the method of the action of
certain well demonstrated, fundamental factors. Whether by natural
selection, or by the inheritance of acquired characters produced by the
environment, or whether by the effect of isolation of groups of
individuals, the machine building has always been produced in the same
way. A machine, either through the direct influence of the environment,
or as a result of sexual combination of germ plasm, shows a variation
from its parents. This variation proves of value to its possessor, who
lives and transmits it permanently to posterity. Thus step by step, one
part is added to another, until the machine has grown into the
intricately adapted structure which we call the animal or plant. This
has been nature's method of building machines, all based upon the three
properties possessed by the living cell--reproduction, variation, and
heredity.
==Summary of Nature's Power of Building Machines.==--Let us now notice the
position we have reached. Our problem in the present chapter has been to
find out whether nature possesses forces adequate to explain the
building of machines with their parts accurately adapted to each other
so as to act harmoniously for certain ends. Astronomy has shown that she
has forces for the building of worlds; geology, that she has forces for
making mountain and valley; and chemistry, that she has forces for
building chemical compounds. But the organism is neither a world, nor a
mass of matter, nor a chemical compound. It is a machine. Has nature any
forces for machine building? We have found that by the use of the three
factors, reproduction, variation, and heredity, nature is able to
produce a machine of ever greater and greater complexity, with the parts
all adapted to each other. Now the difference between a machine and a
mass of matter is simply in the adaptation of parts to act harmoniously
for definite ends. Hence if we are allowed these three factors, we can
say that nature _does possess forces adequate to the manufacture of
machines_. These forces are not chemical forces, and the construction of
the machine has thus been brought about by forces entirely different
from those which produced the chemical molecule.
But we have plainly not reached the bottom of the matter in our attempt
to explain the machinery of living things. We have based the whole
process upon three factors. Reproduction, variation, and heredity are
the properties of all living matter; but they are not, like gravity and
chemism, universal forces of nature. They occur in living organisms
only. Why should they occur in living organisms, and here alone? These
three properties are perhaps the most marvellous properties of nature;
and surely we have not finished our task if we have based the whole
process of machine building upon these mysterious phenomena, leaving
them unintelligible. We must therefore now ask whether we can proceed
any farther and find any explanation of these fundamental powers of the
living machine.
It must be confessed that here we are at present forced to stop. We can
proceed no further with any certainty, or even probability. We may say
that variation and heredity are only phases of reproduction, and
reproduction is a property of the living cell. We may say that this
power of reproduction is dependent upon the power of assimilation and
growth, for cell division is a result of cell growth. We may further say
that growth and assimilation are chemical processes resulting from the
oxidation of food, and that thus all of these processes are to be
reduced to chemical forces. In this way we may seem to have a chemical
foundation for life phenomena. But clearly this is far from
satisfactory. In the first place, it utterly fails to explain why the
living cell has these properties, while no other body possesses them,
nor why they are possessed by living protoplasms _alone_, ceasing
instantly with death. Indeed it does not tell us what death can be.
Secondly, it utterly fails to explain the marvels of cell division with
resulting hereditary transmission. For all this we must fall back upon
the structure of protoplasm, and say that the cell machinery is so
adjusted that the machine, when acting as a whole, is capable of
transforming the energy of chemical composition in certain directions.
These fundamental properties are then the properties of the cell
_machine_ just as surely as printing is the property of the printing
press. We can no more account for the life phenomena by chemical powers
than we can for printing by chemical forces manifested in the burning of
the coal in the engine room. To be sure, it is the chemical forces in
the engine room that furnishes the energy, but it is the machinery of
the press that explains the printing. So, while chemical forces supply
life energy, it is the cell machinery that must explain the fundamental
living factors. So long as this machine is intact it can continue to run
and perform its duties. But it is a very delicate machine and is easily
broken. When it is broken its activities cease. A broken machine can not
run. It is dead. In short, we come back once more to the idea of the
machinery of protoplasm, and must base our understanding of its
properties upon its structure.
It is proper to state that there are still some biologists who insist
that the ultimate explanation of protoplasm is purely chemical and that
life phenomena may be manifested in mixtures of compounds that are
purely physical mixtures and not machines. It is claimed that much of
this cell structure described above is due to imperfection in
microscopic methods and does not really exist in living protoplasm,
while the marvellous activities described are found only in the highly
organized cell, but do not belong to simple protoplasm. It is claimed
that simple protoplasm consists of a physical mixture of two different
compounds which form a foam when thus mixed, and that much of the
described structure of protoplasm is only the appearance of this foam.
This conception is certainly not the prevalent one to-day; and even if
it should be the proper one, it would still leave the cell as an
extremely complicated machine. Under any view the cell is a mechanism
and must be resolved into subordinate parts. It may be uncertain whether
these subordinate parts are to be regarded simply as chemical compounds
physically mixed, or as smaller units each of which is a smaller
mechanism. At all events, at the present time we know of no such simple
protoplasm capable of living activities apart from machinery, and the
problem of explaining life, even in the simplest form known, remains the
problem of explaining a mechanism.
==The Origin of the Cell Machine.==--We have thus set before us another
problem, which is after all the fundamental one, namely, to ask whether
we can tell anything of nature's method of building the protoplasmic
machine. The building of the higher animal and plant, as we have seen,
is the result of the powers of protoplasm; but protoplasm itself is a
machine. What has been its history?
We must first notice that no notion of _chemical evolution_ helps us
out. It has been a favourite thought with some that the origin of the
first living thing was the result of chemical evolution. As the result
of physical forces there was produced, from the original nebulous mass,
a more and more complicated system until the world was formed. Then
chemical phenomena became more and more complicated until, with the
production of more and more complicated compounds, protoplasm was
finally produced. A few years ago, under the impulse of the idea that
protoplasm was a compound, or at least a simple mixture of compounds,
this thought of protoplasm as the result of chemical evolution was quite
significant. _Physical forces_, _chemical forces_, and _vital forces_,
explain successively the origin of _worlds_, _protoplasm_, and
_organisms_. This conception has, however, no longer much significance.
We know of no such living chemical compound apart from cell machinery. A
new conception of protoplasm has arisen which demands a different
explanation of its origin. Since it is a machine rather than a compound,
mechanical rather than chemical forces are required for its explanation.
Have we then any suggestion as to the method of the origin of this
protoplasmic machine? Our answer must, at the present, be certainly in
the negative. The complexity of the cell tells us plainly that it can
not be the ultimate living substance which may have arisen from chemical
evolution. It is made up of parts delicately adapted to act in harmony
with each other, and its activity depends upon the relation of these
parts. Whatever chemical forces may have accomplished, they never could
have combined different bodies into linin, centrosomes, chromosomes,
etc., which, as we have seen, are the basis of cell life. To account for
this machine, therefore, we are driven to assume either that it was
produced by some unknown intelligent power in its present condition of
complex adjustment, or to assume that it has had a long history of
building by successive steps, just as we have seen to be the case with
the higher organisms. The latter assumption is, of course, in harmony
with the general trend of thought. To-day protoplasm is produced only
from other protoplasm; but, plainly, the first protoplasm on the earth
must have had a different origin. We must therefore next look for facts
which will enable us to understand its origin. We have seen that the
animal and plant machines have been built up from the simple cell as the
result of its powers acting under the ordinary conditions of nature.
Now, in accordance with this general line of thought, we shall be
compelled to assume that previous to the period of building machinery
which we have been considering, there was another period of machine
building during which this cell machine was built by certain natural
forces.
But here we are forced to stop, for nothing which we yet know gives even
a hint as to the method by which this machine was produced. We have,
however, seen that there are forces in nature efficient in building
machines, as well as those for producing chemical compounds; and this,
doubtless, suggests to us that there may be similar forces at work in
building protoplasm. If we can find natural forces by which the simplest
bit of living matter can be built up into a complicated machine like the
ox, with its many delicately adjusted parts, it is certainly natural to
imagine that the same forces may have built this simpler machine with
which we started. But such a conclusion is for a simple reason
impossible. We have seen that the essential factor in this machine
building is reproduction, with the correlated powers of variation and
heredity. Without these forces we could not have advanced in this
machine building at all. But these properties are themselves the result
of the machinery of protoplasm. We have no reason for thinking that this
property of reproduction can occur in any other object in nature except
this protoplasmic machine. Of course, then, if reproduction is the
result of the structure of protoplasm we can not use this factor in
explaining the origin of this protoplasm. The powers of the completed
machine can not be brought forward to account for its origin. Thus the
one fundamental factor for machine building is lacking, and if we are to
explain nature's method of producing protoplasm from simpler structures,
we must either suppose that the _parts_ of the cell are capable of
reproduction and subject to heredity, or we must look for some other
method. Such a road has however not yet been found, nor have we any idea
in what direction to look. But the fact that nature has methods of
machine building, as we have seen, may hold out the possibility that
some day we may discover her method of building this primitive living
machine, the cell.
It is useless to try to go further at present. The origin of living
matter is shrouded in as great obscurity as ever. We must admit that the
disclosures of the modern microscope have complicated rather than
simplified this problem. While a few years ago chemists and biologists
were eagerly expecting to discover a method of manufacturing a bit of
living matter by artificial means, that hope has now been practically
abandoned. The task is apparently hopeless. We can manipulate chemical
forces and produce an endless series of chemical compounds. But we can
not manipulate the minute bits of matter which make up the living
machine. Since living matter is made of the adjustment of these
microscopic parts of matter, we can not hope to make a bit of living
matter until we find some way of making these little parts and adjusting
them together. Most students of protoplasm have therefore abandoned all
expectation of making even the simplest living thing. We are apparently
as far from the real goal of a natural explanation of life as we were
before the discovery of protoplasm.
==General Summary.==--It is now desirable to close this discussion of
seemingly somewhat unconnected topics by bringing them together in a
brief summary. This will enable us to see more clearly the position in
which science stands to-day upon this matter of the natural explanation
of living phenomena, and to picture to ourselves more concisely our
knowledge of the living machine.
The problem we have set before us is to find out to what extent it is
possible to account for vital phenomena by the application of ordinary
natural laws and forces, and therefore to find out whether it is
necessary to assume that there are forces needed to explain life which
are different from those found in other realms of nature, or whether
vital forces are all correlated with physical forces. It has been
evident at a glance that the living body is a machine. Like other
machines it consists of parts adjusted to each other for the
accomplishment of definite ends, and its action depends upon the
adjustment of its parts. Like other machines, it neither creates nor
destroys energy, but simply converts the potential energy of its foods
into some form of active energy, and, like other machines, its power
ceases when the machine is broken.
With this understanding the problem clearly resolved itself into two
separate ones. The first was to determine to what extent known physical
and chemical laws and forces are adequate to an explanation of the
various phenomena of life. The second was to determine whether there are
any known forces which can furnish a natural explanation of the origin
of the living machine. Manifestly, if the first of these problems is
insolvable, the second is insolvable also.
In the study of the first problem we have reached the general conclusion
that the secondary phenomena of life are readily explained by the
application of physical and chemical forces acting in the living
machine. These secondary phenomena include such processes as the
digestion and absorption of food, circulation, respiration, excretion,
bodily motion, etc. Nervous phenomena also doubtless come under this
head, at least so far as concerns nervous force. We have been obliged,
however, to exclude from this correlation the mental phenomena. Mental
phenomena can not as yet be measured, and have not yet been shown to be
correlated with physical energy. In other words, it has not yet been
proved that mental force is energy at all; and if it is not energy, then
of course it can not be included in the laws which govern the physical
energy of the universe. Although a close relation exists between
physical changes in the brain cells and mental phenomena, no further
connection has yet been drawn between mental power and physical force.
All other secondary phenomena, however, are intelligently explained by
the action of natural forces in the machinery of the living organism.
While we have thus found that the secondary phenomena of life are
intelligible as the result of the structure of the machine, certain
other fundamental phenomena have been constantly forcing themselves upon
our attention as a _foundation_ of these secondary activities. The power
of contraction, the power of causing certain kinds of chemical change to
occur which result in metabolism, the property of sensibility, the
property of reproduction--these are fundamental to all living activity,
and are, after all, the real phenomena which we wish to explain. But
these are not peculiar to the complicated machines. We can discard all
the apparent machinery of the animal or plant and find these properties
still developed in the simplest bit of living matter. To learn their
significance, therefore, we have turned to the study of the simplest
form of matter in which these fundamental properties are manifested.
This led us at once to the study of the so-called protoplasm, for
protoplasm is the simplest known form of matter that is alive.
Protoplasm itself at first seemed to be a homogeneous body, and was
looked upon as a chemical compound of high complexity. If this were true
its properties would depend upon its composition and would be explained
by the action of chemical forces. Such a conception would have quickly
solved the problem, for it would reduce living properties to chemical
powers. But the conception proved to be delusive. Protoplasm, at least
the simplest form known to possess the fundamental life properties, soon
showed itself to be no chemical compound, but a machine of wonderful
intricacy.
The fundamental phenomena of life and of protoplasm have proved to be
both chemical and mechanical. Metabolism is the result of the oxidation
of food, and motion is an instance of transference of force. Our problem
then resolved itself into finding the power that guides the action of
these natural forces. Food will not undergo such an oxidation except in
the presence of protoplasm, nor will the phenomena of metabolism occur
except in the presence of _living_ protoplasm. Clearly, then, the living
protoplasm contains within itself the power of guiding this play of
chemical force in such a way as to give rise to vital phenomena, and our
search must be not for chemical force but for this guiding principle.
Our study of protoplasm has told us clearly enough that we must find
this guiding principle in the interaction of the machinery within the
protoplasm. The microscope has told us plainly that these fundamental
principles are based upon machinery. The cell division (reproduction) is
apparently controlled by the centrosomes; the heredity by the
chromosomes; the constructive metabolism by the nucleus in general,
while the destructive metabolism is also seated in the cell substance
outside the nucleus. Whether these statements are strictly accurate in
detail does not particularly affect the general conclusion. It is
clearly enough demonstrated that the activities of the protoplasmic body
are dependent upon the relation of its different parts. Although we have
got rid of the complicated machinery of the organism in general, we are
still confronted with the machinery of the cell.
But our analysis can not, at present, go further. Our knowledge of this
machine has not as yet enabled us to gain any insight as to its method
of action. We can not yet conceive how this machine controls the
chemical and physical forces at its disposal in such a way as to produce
the orderly result of life. The strict correlation between the forces of
the physical universe and those manifested by this protoplasm tells us
that a transformation of energy occurs within it, but of the method of
that transformation we as yet know nothing. Irritability, movement,
metabolism, and reproduction appear to be not chemical properties of a
compound, but mechanical properties of a machine. Our mechanical
analysis of the living machine stops short before it reaches any
foundation in the chemical forces of nature.
It is thus clearly apparent that the phenomena of life are dependent
upon the machinery of living things, and we have therefore the second
question of the _origin_ of this machinery to answer. Chemical forces
and mechanical forces have been laboriously investigated, but neither
appear adequate to the manufacture of machines. They produce only
chemical compounds and worlds with their mountains and seas. The
construction of artificial machines has demanded intelligence. But here
is a natural machine--the organism. It is the only machine produced by
natural methods, so far as we know; and we have therefore next asked
whether there are, in nature, simple forces competent to build machines
such as living animals and plants?
In pursuance of this question we have found that the complicated
machines have been built out of the simpler ones by the action of known
forces and laws. The factors in this machine building are simply those
of the fundamental vital properties of the simplest protoplasmic
machine. Reproduction, heredity, and variation, acting under the
ever-changing conditions of the earth's surface, are apparently all that
are needed to explain the building of the complex machines out of the
simpler ones. Nature _has_ forces adequate to the building of machines
as well as forces adequate to the formation of chemical compounds and
worlds.
But here again we are unable to base our explanation upon chemical and
physical forces. Reproduction, heredity, and variation are properties of
the cell machine, and we are therefore thrown back upon the necessity of
explaining the origin of this machine. Can we find a mechanical or
chemical explanation of the origin of protoplasm? A chemical explanation
of the cell is impossible, since it is not a chemical compound, but a
piece of mechanism. The explanation given for the origin of animals and
plants is also here apparently impossible. The factors upon which that
explanation depended are factors of this completed machine itself, and
can not be used to explain its origin. We are left at present therefore
without any foundation for further advance. The cells must have had a
history of construction, but we do not as yet conceive any forces which
may be looked upon as contributing to that history. Whether life
phenomena can be manifested by any mixture of compounds simpler than the
cell we do not yet know.
The great problems still remaining for solution, which have hardly been
touched by modern biology in all its endeavours to find a mechanical
explanation of the living machine, are, therefore, three. First, the
relation of mentality to the general phenomena of the correlation of
force; second, the intelligible understanding of the mechanism of
protoplasm which enables it to guide the blind chemical and physical
forces of nature so as to produce definite results; third, the kind of
forces which may have contributed to the origin of that simplest living
machine upon whose activities all vital phenomena rest--the living cell.
INDEX.
A.
Absorption of food, 20.
Acquired characters, inheritance of, 164, 165, 166, 167, 171.
--variations, 159, 160.
Amoeba, 73.
Anatomical evidence for evolution, 142.
Aquacity, 80.
Arm compared with wing, 144.
Aristotle, 1.
Assimilation, 80, 124, 149, 176.
Asters of dividing cells, 98.
B.
Barry, 63, 64.
Bathybias, 84.
Biology a new science, 1, 5, 15.
Blood, 35, 36, 38, 69, 73.
Blood-vessels, 35, 36.
Body as a machine, 22, 25, 49.
Bone cells, 69.
Building of the living machine, 131, 134, 136, 137, 167, 175, 180.
C.
Cartilage cells, 68.
Cell as a machine, 126, 128.
--description of, 69.
--division, 95, 96, 101.
--discovery of, 58.
--doctrine, 60.
--substance, 65, 125.
Cells, 56, 84, 86, 118, 119.
Cellular structure of organisms, 65.
Cell wall, 64, 72.
Centrosome, 94, 96, 97, 101, 103, 105, 110.
Challenger expedition, 83.
Chemical evolution, 179.
Chemical theory of vitality, 14;
of life, 78, 116.
Chemism or mechanism, 57, 176.
Chemistry of digestion, 27, 28;
of protoplasm, 76;
of respiration, 38.
Chromatin, 92, 94, 96, 102, 149, 153.
Chromosomes, 97, 98, 101, 105, 108, 110, 113, 152.
Circulation, 34.
Colonies of cells, 85.
Comparison of the body and a machine, 22.
Congenital variations, 158, 160, 163;
inheritance of, 164.
Connective-tissue cells, 70.
Conservation of energy, 7, 17.
Consciousness as a factor in machine building, 173.
Constructive chemical processes, 50, 51, 52, 124.
Continuity of germ plasm, 155.
Correlation of vital and physical forces, 13, 16, 22, 23, 24, 25.
Cytoblastema. 62.
Cytology, 10.
D.
Darwin, 81.
Death of the cell, 127.
Decline of the reign of protoplasm, 85.
Destructive chemical processes, 50, 51, 52, 125.
Dialysis, 29, 30, 31.
Digestion, 27.
E.
Egg, 103, 120, 152.
division of, 63.
Egg, fertilization of, 102.
Embryological evidence for evolution, 140.
Energy of nervous impulse, 43, 54.
Environment, 171.
Evidence for evolution as a method of machine building, 139, 145.
Evolution, 9, 16, 81, 134.
Experiments with developing eggs, 121.
F.
Fat, absorption of, 32.
Female pronucleus, 110.
Fern cells, section of, 67.
Fertilization of the egg, 95, 102;
significance of, 112.
Fibres in protoplasm, 87;
--in spindle, 98, 101.
Forces at work in machine building, 148, 176, 181.
Formed material, 64.
Free cell formation, 64.
G.
Geological evidence for evolution, 139.
Germ plasm, 154.
H.
Heart as a pump, 35.
Heat, 24, 44, 45.
Heredity, 148, 150, 176;
--explanation of, 152.
Hereditary traits, 113, 153.
Historical geology, 6.
History of the living machine, 133, 147.
Horses' toes, loss of, 172.
Huxley, 11, 75, 83, 84.
I.
Irritability, 54.
Isolation, theory of, 170.
K.
Karyokinesis, 96, 101.
Kidneys, 41.
L.
Leaf, section of, 66.
Life the result of a mechanism, 115, 177.
Linin, 92, 103.
Linnæus, 1.
Lyell, 6.
Lymph, 36, 37.
M.
Machine defined, 20.
Machines the result of mechanical forces, 116.
Male cell, 104, 107.
---- pronucleus, 109.
Maturation of the egg, 104.
Mechanical nature of living organisms, 12.
Mechanical theory of life, 81, 144.
Membrane of the nucleus, 92, 101.
Mental phenomena, 47, 48.
Metabolism, 54.
Microsomes, 87.
Migration, theory of, 170.
Monera, 88.
Movement, 54.
Muscle, 36, 71.
N.
Natural selection, 167.
Nerve-fibre cell, 70.
Nervous energy, 42, 44.
---- system, 41.
New biological problems, 15.
Nucleolus, 65, 92, 94.
Nucleus, 65, 84, 87, 93, 101, 103, 113, 124, 149;
formation of new, 101.
---- function of, 89, 90, 95.
---- presence of, 87, 88, 89.
---- structure of, 91.
O.
Organic chemistry, 78.
Organic compounds, artificial manufacture of, 78, 82.
Origin of cell machine, 178, 179, 180.
Origin of life, 81, 182.
Osmosis, 29.
Oxidation, 80, 176.
---- as a vital process, 39, 56.
P.
Philosophical biology, 4.
Physical basis of life, 75.
Polar cells, 107.
Potato, section of cells, 67.
Properties of chemical compounds, 79.
Protoplasm, 14, 74, 82, 83, 84, 114, 115, 179.
---- artificial manufacture of, 82.
---- as a machine, 86, 178.
---- discovery of, 74.
---- nature of, 76.
---- structure of, 86, 87.
Purpose _vs._ cause, 11, 12.
R.
Reaction against the cell doctrine, 117.
Reign of law, 4.
---- of the nucleus, 91.
---- of protoplasm, 81, 85.
Relationship, significance of, 143.
Removal of waste, 39, 40.
Reproduction, 54, 80, 124, 148, 176;
--rapidity of, 149.
Respiration, 37.
Reticulum of cell, 87;
--of nucleus, 92.
Root tip, section of, 66.
S.
Schultze, 74, 75.
Schwann, 61, 62, 72.
Secretion, 39, 40.
Segmentation nucleus, 110.
Sensations, 46.
Separation of chromosomes, 100.
Sexual reproduction, 102.
Spermatozoan, 107, 109, 154.
Splitting of chromosomes, 99.
Spindle fibres, 101.
Struggle for existence, 168.
Summary of Part I, 128.
---- general, 182.
U.
Undifferentiated protoplasm, 83.
Unicellular animals, 71.
Units of vital activity, 53.
Use and disuse, 171, 172.
V.
Variation, 148, 157, 160, 176.
Variation from sexual union, 162.
Variation in germ plasm, 161.
Vegetative functions, 41.
Villi, 31.
Vital force, vitality, 13, 15, 34, 37, 52, 80, 85.
Vital properties, 54;
--located in cells, 123.
W.
Wing compared with arm, 144.
Wood cells, 68.
THE END.
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_PIONEERS OF EVOLUTION, from Thales to Huxley_ By EDWARD
CLODD, President of the Folk-Lore Society; Author of "The Story of
Creation," "The Story of 'Primitive' Man," etc. With Portraits, 12mo.
Cloth, $1.50.
"The mass of interesting material which Mr. Clodd has got together and
woven into a symmetrical story of the progress from ignorance and theory
to knowledge and the intelligent recording of fact is prodigious.... The
'goal' to which Mr. Clodd leads us in so masterly a fashion is but the
starting point of fresh achievements, and, in due course, fresh
theories. His book furnishes an important contribution to a liberal
education."--_London Daily Chronicle._
"We are always glad to meet Mr. Clodd. He is never dull; he is always
well informed, and he says what he has to say with clearness and
precision.... The interest intensifies as Mr. Clodd attempts to show the
part really played in the growth of the doctrine of evolution by men
like Wallace, Darwin, Huxley, and Spencer.... We commend the book to
those who want to know what evolution really means."--_London Times._
"This is a book which was needed.... Altogether, the book could hardly
be better done. It is luminous, lucid, orderly, and temperate. Above
all, it is entirely free from personal partisanship. Each chief actor is
sympathetically treated, and friendship is seldom or never allowed to
overweight sound judgment"--_London Academy._
"We can assure the reader that he will find in this work a very useful
guide to the lives and labors of leading evolutionists of the past and
present. Especially serviceable is the account of Mr. Herbert Spencer
and his share in rediscovering evolution, and illustrating its relations
to the whole field of human knowledge. His forcible style and wealth of
metaphor make all that Mr. Clodd writes arrestive and
interesting."--_London Literary World._
"Can not but prove welcome to fair-minded men.... To read it is to have
an object-lesson in the meaning of evolution.... There is no better book
on the subject for the general reader.... No one could go through the
book without being both refreshed and newly instructed by its masterly
survey of the growth of the most powerful idea of modern times."--_The
Scotsman._
D. APPLETON AND COMPANY, NEW YORK.
==BOOKS ON SOCIAL SCIENCE.==
==Socialism New and Old.==
By Prof. WILLIAM GRAHAM, 12mo. Cloth, $1.75.
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interested in the study of socialism, and not so intoxicated with its
promises of a new heaven and a new earth as to be impatient of temperate
and reasoned criticism."_--London Times._
"Professor Graham presents an outline of the successive schemes of three
writers who have chiefly influenced the development of socialism, and
dwells at length upon the system of Rousseau, that of St. Simon, and on
that of Karl Marx, the founder of the new socialism, 'which has gained
favor with the working classes in all civilized countries,' which agrees
with Rousseau's plan in being democratic, and with St. Simon's in aiming
at collective ownership.... The professor is an independent thinker,
whose endeavor to be clear has resulted in the statement of definite
conclusions. The book is a remarkably fair digest of the subject under
consideration."_--Philadelphia Ledger._
==Dynamic Sociology:==
_Or, Applied Social Science, as based upon Statical Sociology and the
less Complex Sciences._ By LESTER F. WARD, A.M. In 2 vols.
12mo. Cloth, $4.00.
"A book that will amply repay perusal.... Recognizing the danger in
which sociology is, of falling into the class of dead sciences or polite
amusements, Mr. Ward has undertaken to 'point out a method by which the
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Post-Express._
"Mr. Ward has evidently put great labor and thought into his two
volumes, and has produced a work of interest and importance. He does not
limit his effort to a contribution to the science of sociology.... He
believes that sociology has already reached the point at which it can be
and ought to be applied, treated as an art, and he urges that 'the
State' or Government now has a new, legitimate, and peculiar field for
the exercise of intelligence to promote the welfare of men."--_New York
Times._
==Criminal Sociology.==
By Prof. E. FERRI. A new volume in the Criminology Series,
edited by W. Douglas Morrison, 12mo. Cloth, $1.50.
In this volume Professor Ferri, a distinguished member of the Italian
Parliament, deals with the conditions which produce the criminal
population, and with the methods by which this anti-social section of
the community may be diminished. He divides the causes of crime into two
great classes, individual and social. The individual causes consist of
physical and mental defects; the social causes consist of social
disadvantages of every description. His view is that the true remedy
against crime is to remove individual defects and social disadvantages
where it is possible to remove them. He shows that punishment has
comparatively little effect in this direction, and is apt to divert
attention from the true remedy--the individual and social amelioration
of the population as a whole.
D. APPLETON AND COMPANY, NEW YORK.
BOOKS FOR NATURE LOVERS.
==Insect Life. (New Edition in Colors.)==
By JOHN HENRY COMSTOCK, Professor of Entomology in Cornell
University. With 12 full-page plates reproducing butterflies and various
insects in their natural colors, and with many wood engravings by Anna
Botsford Comstock, Member of the Society of American Wood Engravers,
12mo. Cloth, $1.75 net; postage, 20 cents additional.
"The volume is admirably written, and the simple and lucid style is
a constant delight.... It is sure to serve an excellent purpose in
the direction of popular culture, and the love of natural science
which it will develop in youthful minds can hardly fail to bear
rich fruit."--_Boston Beacon._
==Familiar Fish: Their Habits and Capture.==
A Practical Book on Fresh-Water Game Fish. By EUGENE MCCARTHY.
With an Introduction by Dr. David Starr Jordan, President of Leland
Stanford Junior University, and numerous Illustrations, 12mo. Cloth,
$1.50.
"One of the handsomest, most practical, most informing books that
we know. The author treats his subject with scientific
thoroughness, but with a light touch that makes the book easy
reading.... The book should be the companion of all who go
a-fishing."--_New York Mail and Express._
==The Art of Taxidermy.==
By JOHN ROWLEY, Chief of the Department of Taxidermy in the
American Museum of Natural History. Illustrated, 12mo. Cloth, $2.00.
"Mr. Rowley will long be gratefully remembered by taxidermists,
amateurs, and others, for the care he has used in thus meeting a
long-felt want."--_Bangor, Me., Sportsman._
"The book is not an elaborate treatise upon the abstract principles
which lie at the foundation of artistic taxidermy, but is rather a
compendium full of practical hints and suggestions, recipes, and
formulas for the working taxidermist."--_The Dial._
==Plants. (Plant Relations and Plant Structures in one volume.)==
By JOHN M. COULTER, A.M., Ph.D., Head of Department of Botany,
University of Chicago, 12mo. Cloth, $1.80 net. (One of the Twentieth
Century Text-Books.)
D. APPLETON AND COMPANY, NEW YORK.
_EVOLUTION OF MAN AND CHRISTIANITY._
New edition. By the Rev. HOWARD MACQUEARY. With a new Preface,
in which the Author answers his Critics, and with some important
Additions, 12mo. Cloth, $1.75.
"This is a revised and enlarged edition of a book published last
year. The author reviews criticisms upon the first edition, denies
that he rejects the doctrine of the incarnation, admits his doubts
of the physical resurrection of Christ, and his belief in
evolution. The volume is to be marked as one of the most profound
expressions of the modern movement toward broader theological
positions."--_Brooklyn Times._
_HISTORY OF THE CONFLICT BETWEEN RELIGION AND SCIENCE._ By Dr. JOHN
WILLIAM DRAPER. 12mo. Cloth, $1.75.
"The keynote to this volume is found in the antagonism between the
progressive tendencies of the human mind and the pretensions of
ecclesiastical authority, as developed in the history of modern
science. No previous writer has treated the subject from this point
of view, and the present monograph will be found to possess no less
originality of conception than vigor of reasoning and wealth of
erudition."--_New York Tribune._
_A CRITICAL HISTORY OF FREE THOUGHT IN REFERENCE TO THE CHRISTIAN
RELIGION._ By Rev. Canon ADAM STOREY FARRAR, D.D., F.R.S., etc.
12mo. Cloth, $1.50.
"A conflict might naturally be anticipated between the reasoning
faculties of man and a religion which claims the right, on
superhuman authority, to impose limits on the field or manner of
their exercise. It is the chief of the movements of free thought
which it is my purpose to describe, in their historic succession
and their connection with intellectual causes. We must ascertain
the facts, discover the causes, and read the moral."--_The Author._
_CREATION OR EVOLUTION? A Philosophical Inquiry._ By GEORGE TICNOR
CURTIS. 12mo. Cloth, $2.00.
"A treatise on the great question of Creation or Evolution by one
who is neither a naturalist nor theologian, and who does not
profess to bring to the discussion a special equipment in either of
the sciences which the controversy arrays against each other, may
seem strange at first sight; but Mr. Curtis will satisfy the
reader, before many pages have been turned, that he has a
substantial contribution to make to the debate, and that his book
is one to be treated with respect. His part is to apply to the
reasonings of the men of science the rigid scrutiny with which the
lawyer is accustomed to test the value and pertinency of testimony,
and the legitimacy of inferences from established facts."--_New
York Tribune._
D. APPLETON AND COMPANY, NEW YORK.
End of Project Gutenberg's The Story of the Living Machine, by H. W. Conn