location in the organism have yielded the most striking and brilliant
results. Darwin himself realized that the doctrine of natural selection
was incomplete, as it accepted at its face value the inheritance of
congenital racial qualities without attempting to describe the way an egg
or any other germ bears them, and he endeavored to round out his doctrine
of selection by adding the theory of pangenesis. According to this, every
cell of every tissue and organ of the body produces minute particles
called gemmules, which partake of the characters of the cells that produce
them. The gemmules were supposed to be transported throughout the entire
body, and to congregate in the germ-cells, which in a sense would be
minute editions of the body which bears them, and would then be capable of
producing the same kind of a body. If true, this view would lead to the
acceptance of Lamarck's or even Buffon's doctrine, for changes induced in
any organ by other than congenital factors could be impressed upon the
germ-cell, and would then be transported together with the original
specific characters to future generations. Darwin was indeed a good
Lamarckian.
But the researches of post-Darwinians, and especially those of the
students of cellular phenomena, have demonstrated that such a view has no
real basis in fact. Many naturalists, like Naegeli and Wiesner, were
convinced that there was a specific substance concerned with hereditary
qualities as in a larger way protoplasm is the physical basis of life. It
remained for Weismann to identify this theoretical substance with a
specific part of the cell, namely, the deeply staining substance, or
chromatin, contained in the nucleus of every cell. Bringing together the
accumulating observations of the numerous cytologists of his time, and
utilizing them for the development of his somewhat speculative theories,
Weismann published in 1882 a volume called "The Germ Plasm," which is an
immortal foundation for all later work on inheritance. The essential
principles of the germ-plasm theory are somewhat as follows. The chromatin
of the nucleus contains the determinants of hereditary qualities. In
reproduction, the male sex-cell, which is scarcely more than a minute mass
of chromatin provided with a thin coat of protoplasm and a motile organ,
fuses with the egg, and the nuclei of the two cells unite to form a double
body, which contains equal contributions of chromatin from the two
parental organisms. This gives the physical basis for paternal inheritance
as well as for maternal inheritance, and it shows why they may be of the
same or equivalent degree. When, now, the egg divides, at the first and
later cleavages, the chromatin masses or chromosomes contained in the
double nucleus are split lengthwise and the twin portions separate to go
into the nuclei of the daughter-cells. As the same process seems to hold
for all the later divisions of the cleavage-cells whose products are
destined to be the various tissue elements of the adult body, it follows
that all tissue-cells would contain chromatin determinants derived equally
from the male and female parents. As of course only the germ-cells of an
adult organism pass on to form later generations, and as their content of
chromatin is derived not from the sister organs of the body, but from the
original fertilized egg, there is a direct stream of the germ plasm which
flows continuously from the germ-cell to germ-cell through succeeding
generations. It would seem, therefore, that the various organic systems
are, so to speak, sister products in embryonic origin. The reproductive
organs are not produced by the other parts of the body, but their cells
are the direct descendants of the common starting-point namely, the egg.
As the cells of the reproductive organs are the only ones that pass over
and into the next and later generations, it will be evident, in the first
place, that the germ plasm of their nuclei is the only essential substance
that connects parent and offspring. This stream of germ plasm passes on in
direct continuity through successive generations--from egg to the complete
adult, including its own germ-cells, through these to the next adult, with
its germ-cells, and so on and on as long as the species exists. It does
not flow circuitously from egg to adult and then to new germ-cells, but it
is direct and continuous, and apparently it cannot pick up any of the
body-changes of an acquired nature. Now we see why individual acquisitions
are not transmitted. The hereditary stream of germ plasm is already
constituted before an animal uses its parts in adult life; we cannot see
how alterations in the structure of mature body parts through use and
adjustment to the environment can be introduced into it to become new
qualities of the species.
It must be clear, I am sure, that this theory supplements natural
selection, for it describes the physical basis of inheritance, it
demonstrates the efficiency of congenital or germ-plasmal factors of
variation in contrast with the Lamarckian factors, and finally in the way
that in the view of Weismann it accounts for the origin of variations as
the result of the commingling of two differing parental streams of germ
plasm.
At first, for many reasons, Weismann's theories did not meet with general
acceptance, but during recent years there has been a marked return to many
of his positions, mainly as the result of further cytological discoveries,
and of the formulation of Mendel's Law and of De Vries's mutation theory.
The first-named law was propounded by Gregor Mendel on the basis of
extensive experiments upon plants conducted during many years, 1860 and
later, in the obscurity of his monastery garden at Altbrünn, in Austria.
It was rescued from oblivion by De Vries, who found it buried in a mass of
literature and brought it to light when he published his renowned Mutation
Theory in 1901. Mendelian phenomena of inheritance, confirmed and extended
by numerous workers with plants and animals, prove that in many cases
portions of the streams of germ plasm that combine to form the hereditary
content of organisms may retain their individuality during embryonic and
later development, and that they may emerge in their original purity when
the germ-cells destined to form a later generation undergo the preparatory
processes of maturation. They demonstrate also the apparent chance nature
of the phenomena of inheritance. To my mind the most striking and
significant result in this field is the demonstration that a particular
chromosome or chromatin mass determines a particular character of an adult
organism, which is quite a different matter from the reference of all the
hereditary characters to the chromatin as a whole. Wilson and others have
brought forward convincing proof that the complex character of sex in
insects actually resides in or is determined by particular and definite
masses of this wonderful physical basis of inheritance.
Mendel's principles also account in the most remarkable way for many
previously obscure phenomena, like reversion, or a case where a child
resembles its grandparent more than it does either of its parents; such
phenomena are due, so to speak, to the rise to the surface of a hidden
stream of germ plasm that had flowed for one or many generations beneath
its accompanying currents. I believe that the law is replacing more and
more the laws of Galton and Pearson, formulated as statistical summaries
of certain phenomena of human inheritance taken _en masse_. According to
Galton's celebrated law of ancestral inheritance, the qualities of any
organism are determined to the extent of a certain fraction by its two
parents taken together as a "mid-parent," that a smaller definite fraction
is contributed by the grandparents taken together as a mid-grandparent,
and so on to earlier generations. But Mendel's Law has far greater
definiteness, it explains more accurately the cases of alternative
inheritance, and it may be shown to hold for blended and mosaic
inheritance as well.
De Vries's new "mutation theory" is clearly not an alternative but a
complementary theory to natural selection, the Weismannian and Mendelian
theories. Like these last, it emphasizes the importance of the congenital
hereditary qualities contained in the germ plasm, though unlike the
Darwinian doctrine it shows that sometimes new forms may arise by sudden
leaps and not necessarily by the slow and gradual accumulation of slight
modifications or fluctuations. The mutants like any other variants must
present themselves before the jury of environmental circumstances, which
passes judgment upon their condition of adaptation, and they, too, must
abide by the verdict that means life or death.
From what has been said of these post-Darwinian discoveries, the
Lamarckian doctrine, which teaches that acquired non-congenital characters
are transmitted, seems to be ruled out. I would not lead you to believe
that the matter is settled. I would say only that the non-transmission of
racial mutilations, negative breeding experiments upon mutilated rats and
mice, the results of further study of supposedly transmitted immunity to
poisons--that all these have led zoölogists to render the verdict of "not
proved." The future may bring to light positive evidence, and cases like
Brown-Séquard's guinea-pigs, and results like those of MacDougal with
plants, and of Tower with beetles, may lead us to alter the opinion
stated. But as it stands now most investigators hold that there are strong
general grounds for disbelief in the principle, and also that it lacks
experimental proof.
* * * * *
The explanation of natural evolution given by Darwinism and the principles
of Weismann, Mendel, and De Vries, still fails to solve the mystery
completely, and appeal has been made to other agencies, even to teleology
and to "unknown" and "unknowable" causes as well as to circumstantial
factors. A combination of Lamarckian and Darwinian factors has been
proposed by Osborn, Baldwin, and Lloyd Morgan, in the theory of organic
selection. The theory of orthogenesis propounded by Naegeli and Eimer, now
gaining much ground, holds that evolution takes place in direct lines of
progressive modification, and is not the result of apparent chance. Of
these and similar theories, all we can say is that if they are true, they
are not so well substantiated as the ones we have reviewed at greater
length.
The task of experimental zoölogy is to work more extensively and deeply
upon inheritance and variation, combining the methods and results of
cellular biology, biometrics, and experimental breeding. We may safely
predict that great advances will be made during the next few years in
analyzing the method of evolution; and that a few decades hence men will
look back to the present time as a period of transition like the era of
reawakened interest and renewed investigation that followed the appearance
of the "Origin of Species." For the present, we can justly say "that
evolution, so far as it is understood, is a real and natural process."
V
THE PHYSICAL EVOLUTION OF THE HUMAN SPECIES AND OF HUMAN RACES
The teachings of science that relate to the origin and history of the
human species constitute for us the most important part of the whole
doctrine of organic evolution and now, having completely outlined this
doctrine as a general one, we are brought to the point where we must deal
frankly and squarely with the insistent questions arising on all sides as
to the way that mankind is involved in the vast mechanism of nature's
order. These questions have been ignored heretofore, in order that the
natural history of animals in general might be discussed without any
interference on the part of purely human interest and concern. It now
becomes our privilege, and our duty as well, to employ and apply the
principles we have learned in order to understand more completely the
origin of the human body as an organic type, the history of human races,
the development of human faculty and of social institutions, and the
evolution finally of even the highest elements of human life. These are
scientific problems, and if we are to solve them we must employ the now
familiar methods of science which only yield sure results.
We must not underestimate the many difficulties to be encountered, for the
field before us is a vast territory of complex human life and of manifold
human relations. Without prolonged exercise in scientific methods, it is
impossible to view our own kind impersonally, as we do the creatures of
lower nature. Furthermore it seems to many that an analysis of human life
and biological history, even if it is possible, must alter or degrade
mankind in some degree; this is no more true than that a knowledge of the
principles of engineering according to which the Brooklyn Bridge has been
constructed renders that structure any different or unsafe for travel. Man
remains man, whether we are in utter ignorance of his mode of origin, or
whether we know all about his ancestry and about the factors that have
made him human. It is because our species appears to occupy a superior and
isolated position above the rest of nature that the mind seems reluctant
to follow the guidance of science when it conducts its investigations into
the history of seemingly privileged human nature. And it is feared also,
that if evolution is proven for man as well as for all other kinds of
animals, our cherished ideas and our outlook upon many departments of
human life must be profoundly affected. This may be so, but science
endeavors only to find out the truth; it cannot alter truth, nor does it
seek to do so. We might well wish that the world were different in many
respects and that we were free from the control of many natural laws
besides that of evolution, but if the real is what it is, then our duty is
plain before us; as we think more widely and deeply on the basis of
ripened experience, it becomes ever clearer that a knowledge of human
history gives the only sure guidance for human life.
To the zoölogist it seems strange that so many are opposed to a scientific
inquiry into the facts of human evolution, and to the conclusions
established by such an inquiry,--though, to be sure, this opposition is
directly proportional to ignorance or misunderstanding of the nature and
purpose of scientific investigation and of human evolution. The naturalist
comes to view our species as a kind of animal, and as a single one of the
hundreds of thousands of known forms of life; thus the question of human
origin is but a small part of organic evolution, which is itself only an
episode in the great sweep of cosmic evolution, endless in past time and
in the future. Were we some other order of beings, and not men, human
evolution would appear to us in its proper scientific proportions, namely,
as a minute fraction of the whole progress of the world.
While the foregoing statements are true, it is nevertheless right that a
close study should be made of the particular case of mankind. No doubt
much of the naturalist's interest in nature at large is due to his
conviction that the laws revealed by the organisms of a lower sphere must
hold true for man, and may explain many things that cannot be so clearly
discerned when only the highest type is the subject of investigation. It
is only too evident that little more than a general outline can be given
of the wide subject or group of subjects included under the head of human
evolution. We must divide the subject logically into parts, so that each
one may be taken up without being complicated by questions relating to
topics of another category, although the findings in any one department
must surely be of importance for comparison with the results established
in another section; for if evolution is universally true, the main
conclusion in any case must assist the investigation of another, just as
comparative anatomy and embryology supplement and corroborate each other
in the larger survey of organic evolution. As before, the illustrations of
each department of the subject must be selected from the stock of everyday
observation and information that we already possess, for we gain much when
we realize that evolution includes all the happenings of everyday life and
thought, as well as the occurrences of the remote past.
For the present, then, the questions relating to the higher aspects of
human life must be put aside, only that they may be taken up at the last.
Social evolution likewise finds its place in a later section, after the
phenomena of mind and mental evolution receive due attention and
description. At the present juncture, the human species presents itself as
a subject for organic analysis and classification, merely as a physical
organism. Just as the study of locomotives must begin with the detailed
structure of machines in the workshop before they can be profitably
understood as working mechanisms, so the physical evolution of mankind
must first be made intelligible before it is possible to prosecute
successfully the studies dealing with the psychology, social relations,
and higher conceptions that seem at first to be the exclusive properties
of our species.
The problems of physical evolution of man and of men fall into two groups.
Those of the first deal with the origin of the human species as a unit,
and its comparative relation to lower organisms, while those of the second
part are concerned with the further evolution of human races that have
come to be different in certain details of structure since the human type
as such arose. In the first part, all men will be assumed to be alike and
the members of a homogeneous species whose fundamental attributes are to
be compared with those of other animals; only afterwards will attention be
directed to the differences, previously ignored, that divide human beings
into well-marked varieties. It must be evident even at this point that the
mode of evolution demonstrated by the first investigation will be likely
to bear some close relation to the methods by which human races have
evolved to their present diverse anatomical situations.
* * * * *
The foregoing classification of the problems concerned with the nature and
origin of the human species renders it possible to restrict the immediate
inquiry to a definite and precise question. It is this: does the evidence
relating to the physical characteristics of our species prove that man is
the product of a supernatural act of creation, or does it show that man's
place in nature has been reached by a gradual process of natural
evolution? In order to obtain an equally precise and definite answer to
this question, referring to the particular case of most concern to us, it
is obvious that the method to be employed is the one which has given us an
understanding of organic evolution as an all-inclusive natural process.
The data must be verified, related, and classified, so that their meaning
may be concisely stated in the form of scientific principles. What are the
facts of human structure, comparatively treated? How does the human body
develop? Does palæontology throw any light on the antiquity of man? Do the
rules of nature's order control the lives of men? Our course is now clear;
we shall take up serially the anatomy, embryology, and fossil history of
the human species, in order to see that there is ample proof of the actual
occurrence of evolution, and then, as before, we may look about for the
causes which have produced this result by natural methods.
While it is necessary to treat the subject directly, namely, by examining
the actual evidences relating to the particular case in question, it is
worthwhile before doing so to point out that, as the whole includes a
part, human evolution has already been proved beyond question. This
conclusion must be accepted, unless reasons can be given for excluding
mankind from the rest of the living world as an absolutely unique type,
supreme and isolated because of some peculiar endowments not shared with
the rest of animate nature. If these reasons are lacking, and the unity of
organic nature be recognized, human evolution cannot be denied unless some
interpretation more reasonable and logical than evolution can be given for
the whole mass of facts exemplified and discussed in the foregoing
chapters. We may accordingly approach the main questions by asking if
there are any reasons for regarding the human species as a unique and
isolated type of organism.
At the outset, we must recognize that in so far as the human body is
material, its movements and mass relations are controlled by physical
principles, like all other masses of matter. It is well, indeed, that this
is so, for if gravitation and the laws of inertia were not consistent and
reliable principles holding true at all times and not intermittently, it
would be difficult to order our lives with confidence. In the next place,
the general principles of biology hold true for the structure and
physiology of the human species as they do for all other living things. A
human body is composed of eight systems of organs, whose functions are
identical with the eight vital tasks of every other animal. All these
organs are made up of cells as ultimate vital units, and the materials of
which human cells are composed belong to the class of substances called
protoplasm. Human protoplasm, like all other living materials, must
replenish itself, and respire and oxidize in obedience to biological laws
that have been found to be uniform everywhere. Thus the human organism is
no more unique in fundamental organic respects than it is apart from the
world of physical processes and laws.
How does the matter stand when the general structural plan of a human
being is examined? Is it entirely different from everything else? It is a
fact of common knowledge that the human body is supported by a bony axis,
the vertebral column, to which the skull is articulated and to which also
the skeletal framework of the limbs is attached. These characteristics
place man inevitably among the so-called vertebrata; he is certainly not
an invertebrate, nor is the basic structure of his body such that a third
group, outside the invertebrata and vertebrata, can be made to include
only the single type--man.
Passing now to the classes that make up the group of vertebrates, we meet
first the lampreys or cyclostomes without jaws, and the others with jaws,
such as the fishes, amphibia, reptiles, birds, and mammals, each class
distinguished by certain definite characters in addition to the vertebral
column. The fishes have gills and scales; amphibia of to-day are
scaleless, and they are provided with gills when they are young and lungs
as adults; reptiles have scales and lungs; birds are warm-blooded and
feathered; while mammals are warm-blooded and haired. Is the human species
a unique kind of vertebrate, or does it find a place in one of these
classes? The occurrence of hair, of a four-chambered heart which propels
warm blood, of mammary glands, and of other systematic characters marks
this species as a kind of mammal and not as a vertebrate in a section by
itself.
The members of the class mammalia differ much among themselves; and now
that we recognize clearly that man is a mammalian vertebrate, the next
question is whether an order exists to which our type must be assigned, or
whether we have at last reached a point where it is justifiable to
establish an isolated division to contain the human species alone. We are
familiar with many representatives of different mammalian orders and with
the kind of structural characteristics that serve as convenient
distinctions in denoting their relationships. Horses and cattle, sheep,
and goats and pigs resemble one another in many respects besides their
hoofs, and they form one natural order; the well-developed gnawing teeth
of rats and rabbits and squirrels place these forms together in the order
rodentia; the structures adapting their possessors for a flesh-eating and
predatory life unite the tribes of the lion, wolf, bear, and seal, in the
order carnivora. Among these and other orders of mammalia is one to which
the lemurs, monkeys, and apes are assigned, because all these forms agree
in certain structural respects that place them apart from the other
mammalia, in the same way, for example, that the races of white men may be
recognized as a group distinct from the black and red races. But
comparative studies, prosecuted not only by those who have been forced to
adopt the evolutionary interpretation, but also by believers in special
creation like Linnæus and Cuvier and other more modern opponents of
evolution, have shown that the peculiar qualities of this order are shared
by the human species. Indeed, the name of primates was given to this
section by Linnæus himself, because the human body found a place in the
array which begins at the lower extreme with the lemurs and the monkeys
and ends with man at the other end. Again it is found that no separate
order of mammals exists to include only the genus _Homo_.