EVOLUTIONARY THEORIES SINCE DARWIN
In considering the value of Charles Darwin's work and its permanent
effect upon the thought of mankind, we must be careful to distinguish
between two phases of his effort. It was his aim to prove two
propositions: first, that there is such a process as evolution;
second, that he had discovered the method by which evolution is
accomplished. Before his time there was no general agreement as to the
fact of evolution. People generally thought the idea absurd, as well
as irreligious. All previous efforts on the part of advanced thinkers
to persuade mankind of the truth of evolution had been nearly without
effect. Among the early philosophers the whole idea was purely
speculative. They made no attempt to prove it, and the conception was
without influence upon the thinking of the ordinary man. This remains
true until the time of Lamarck. This French genius succeeded in
persuading not a few people of the validity of the idea of evolution.
He probably could have convinced many more had it not been for the
hostility of Cuvier. Accordingly, Charles Darwin's "Origin of
Species" fell upon a world entirely hostile to the idea, when it
thought of it at all. Within fifty years of the publication of this
wonderful book, probably the entire scientific world is agreed that
evolution, in some form or other, is the undoubted solution of the
mystery of creation. The materialist may think of it as a mechanical
process relentlessly working itself out without design or purpose. The
theist will accept it as the plan by which Eternal Power steadily
works. The devout Christian or Jew will see in it God's method of
creation. The idea of development has penetrated every science that
has to do with animals or man. It is even beginning to influence such
inorganic sciences as Physics and Chemistry. We now hear of the
evolution of the elements, and the evolution of forces. The world has
been persuaded that evolution is true, and this is primarily the
result of the work of Charles Darwin. It is astonishing that so great
a revolution should have come in so short a time.
The other phase of Darwin's work was his attempt to find the agent
which is bringing about the actual transformation of animals and
plants. As we have seen in the preceding chapters, it was his idea
that natural selection was the efficient agent which constantly
eliminated all unfit variations, leaving only the best to carry on the
work of the world and to reproduce their own fit kind. Many
biologists since his time have doubted whether unaided Natural
Selection will account for the constant advance in organisms. This is
the part of the work which is often seriously questioned.
Weissman and his co-workers have contended that this unaided principle
will serve. Most biologists have asked for some more efficient cause,
and assert that selection does not account for the appearance of
variations, but only for their preservation, and that any valid theory
of evolution must show how variations originate. It is chiefly in this
respect that Darwin's work has failed to satisfy many later
biologists. When we hear a scientist speak of Darwinism as being dead,
this is what he means. He does not think evolution false, but believes
that Natural Selection is not sufficient to account for evolution.
There are three main difficulties involved in Darwin's theory. The
chief defect lies in the fact that selection cannot originate
varieties. In all his earlier works Darwin simply accepted variations
as he found them. He was content to say that all species varied
constantly, and in every direction. He gave no theory to account for
variation. Whenever he took measurements of the dimensions of any
large series of objects of the same kind he found these measurements
to vary, apparently, in all directions. Upon the facts of these
variations, and without accounting for them, he built his own theory
of evolution. He realized his weakness, and acknowledged it in his
book. He probably did not anticipate how insistently later biologists
would demand an explanation that would account for this variation. In
his later work, responding to this criticism, Darwin originated a
theory which he called Pangenesis. He believed that when an adult
animal had responded to his environment and acquired a new character
he could transmit this character to his offspring. At that time no one
doubted this fact. The whole theory of Lamarck was based on the
assumption that this could be done. Darwin suggested that every organ
of the body threw off minute particles, which he called pangenes.
These little bodies, carried by the blood, were taken up by the egg
cells or sperm cells, and the latter cells determined the future
development. Consequently, the character of the new individual was
determined by the parental pangenes. In this way the gain acquired by
one generation could be passed on to the next. This theory was purely
speculative. He never pretended that there was the faintest
corroborating evidence visible to the microscope in the organ, in the
blood, or in the germ cell. It was not an accounting for what is, but
for what it seemed possible to him might be.
This theory of Pangenesis, in the shape in which Darwin promulgated
it, has dropped out of consideration almost entirely. DeVries of
recent years has revised it, but with distinct modifications, and most
biologists pay no attention to it.
There is a school of biologists, headed by Weissman, who have come to
be known as Neo-Darwinians. These men have insisted that Natural
Selection, if properly understood and developed, is quite sufficient
to account for the fact of evolution, including the appearance of
variations. Weissman himself is a microscopist of more than common
skill. He is thoroughly accomplished in the most modern methods of
killing, fixing, staining, and mounting. This worker's acquaintance
with the intimate structure of the cell is probably as great as that
of any other man in the world. Weissman asserts that he has seen
inside the nucleus all the machinery necessary to explain how the
father hands over his qualities to his children. He insists, equally
strongly, that this process is such that no father can hand to his
child any qualities which he himself did not have at least in
potentiality at his birth. Everything the individual acquires during
his lifetime is his own possession, which he may use and develop to
the utmost extent, but it dies with him. His children, born after he
possesses it, can no more inherit it than those born before. Weissman
expressed this in his famous statement that "There is no inheritance
of acquired characters." The biological world has had no shock equal
to this since Darwin's time, and there are few other questions to
which scientists to-day return with such constant vigor.
If what Weissman says is true, that no variation or development which
comes to an animal during his lifetime can be transferred into his own
germ cells and handed on to his children, then it becomes evident that
we must find some cause of variation that acts within the germ cells.
This is the difficulty which Weissman meets. He says that there are
small particles in the nucleus of each cell; that these particles
which he calls determinants decide the form and the course of
development of that cell; that when that cell divides to produce
another cell it gives to this other cell one-half of each determinant.
As a result the second cell grows to be like the first. This tells us
why offspring are like their parents. There is nothing in the theory
thus far to show us why offspring are not exactly like their parents.
In other words, there is no accounting, thus far in the theory, for
variation. When the biologist studies carefully the history of an egg
while it is being formed, he sees that at one stage in its development
it throws away not one-half of each determinant, but one-half of the
determinants. When an egg does this, it deliberately casts aside
one-half of the possibilities of its own development. This throwing
away is quite as effective for all its descendants. Any ancestral
quality now lost is lost from the line forever. In the formation of
the sperm cell set free by the male a similar throwing away of
one-half the characters has taken place. The egg cell and the sperm
cell fuse together. There are as many possibilities now as there were
in either parent, but not all the potentialities of both parents. Half
the possibilities of each have been thrown away, and hence cannot
appear in the offspring. By this constant process we get, in every
generation, new combinations of qualities. This is the main cause,
says Weissman, for variations.
There is, however, another possible cause. Each cell has enough
determinants in it for many individuals, and it seems to be more or
less a matter of accident which qualities shall come out. It has been
suggested that as an egg lies within the gland, a blood vessel may
bring blood to it in such way that a determinant, lying in a certain
position in the egg, may get the richest supply of blood, and hence
develop at the expense of the less nourished determinant. By these two
methods variation comes into an animal's life, if Weissman and his
school are to be believed.
This is a serious blow, if true, to many theories of evolution. The
great mass of evolutionists still feel that somehow there is an
influence by which the environment produces variation. How the
influences of the surrounding world can get down into the body of the
parent and affect the egg is unknown. This is freely confessed by
every biologist. All are agreed that Weissman's work has made us
cautious, and prevented our lightly accepting a belief in the
influence of the environment. Yet it is felt by many that slowly and
gradually, in the long run, the germ is affected in the same manner as
is the body of the parent. In other words, even those who are not
followers of Weissman, have accepted the idea that there is little
inheritance of acquired characters. Yet they return to the belief that
somehow, in some way as yet unexplainable, the main cause for
variation in animals lies in the situation in which they live, and
tends toward better adaptation to that situation.
Whether men with this conviction are merely reactionaries whose
confidence is returning, or bold thinkers whose views will ultimately
prevail, time alone can tell.
A second strong objection was brought against the theory of Natural
Selection. Darwin declared that small variations in favorable
directions are selected and become the starting point of new and
better things. It is soon seen, however, that the effect of unaided
Natural Selection would be but to mix new departures with the old
forms, and soon swamp out any progressive tendency. Whenever a genius
appeared, instead of finding a corresponding genius with which to
pair, it mated with the average of its own species. Hence its
offspring were nearer the average than it was, and their offspring
still nearer. Thus whatever advantage the genius originally possessed
gradually sank into the common level.
It was Moritz Wagner, a German naturalist, who first insisted that if
favorable variations were to amount to anything these possessors must
not only mate with others of their same kind, but must also be
prevented from mating with the old average group. Accordingly, the
belief arose that, under ordinary circumstances, variations returned
to the common level. Wherever a varying group became separated by any
barrier from mating with the rest of its species, and had only its own
kind to pair with, a new species sprang up. This barrier might be a
desert, or an impassable mountain range, an arm of the sea, or
anything else that the animal could not, or would not, cross. Isolated
in this way, the little group that had an advantage in a different
direction could develop its tendencies, and a new species would be
made of what had been previously only a geographical race. In this
matter of geographical isolation Wagner is very strongly supported by
the American zooelogist, David Starr Jordan, who believes that no two
closely related species of animals ever occupied the same geographical
area. Both Wagner and Jordan are ardent admirers of Darwin and his
theory of natural selection, but both believe that it is necessary to
add the idea of isolation in order to make natural selection
effective.
George John Romanes, a British naturalist, has added to Wagner's idea
of isolation, the expanded conception that there may be isolations
that are not geographical. For this phase, Romanes has coined the term
physiological isolation. Something in the structure or habit of the
animals with the new variation prevents them from mating with the
older type. Occasionally it is a difference in the structure of the
reproductive organs themselves. This, however, is not the only
possible divergence. The mating season in one group may come earlier
than that of the other, or may come during the day, while the main
group is in the habit of mating at night. Anything which keeps some
members of a species separate in their mating from the rest, will
result in the course of a longer or shorter time, says Romanes, in the
formation of a new species.
A third great objection was raised against Darwinism. The theory said
that only useful variations were selected by nature. It was asserted
by objectors that the earliest beginnings of any variation must be
too slight to be useful, or as the term went, to have selective value.
It has been noticed by a number of naturalists that certain animals
seem to carry the development of a peculiarity altogether too far. It
is seen for instance that in the Irish Elk, which has for some time
been extinct, the horns were so enormous as to be a source of danger
rather than of assistance to their owner. It was said that the
tendency to produce heavy horns had gained, as it were, a sort of
momentum, and that this impulse had carried the development beyond a
safe limit. The Irish Elk became extinct because his horns were too
heavy. During the Mesozoic period the reptiles grew too large. They
seemed to have carried size to a point at which it became a danger
instead of a help. They completely passed out of existence, leaving
behind them only very much smaller reptiles.
Eimer, of Germany, has based on facts like these his theory of
Orthogenesis. He says that variations in animals are not indefinite
and in every direction, but that they follow along clear and definite
lines. These lines, in the case of the elk and of the Mesozoic
reptiles, developed too far, but ordinarily the effect of such a
tendency is distinctly beneficial to the animal. It particularly
assists in carrying on for a time the variations which have not yet
become useful to the animal. It has always been difficult on Darwinian
principles to understand how the beginnings of the useful variations
could be selected before they were strong enough to be of actual value
to the animal. This tendency to variations in certain directions
instead of at random would account for such early development. This
theory of Orthogenesis has not figured very strongly in the history of
the movement, but it recurs at intervals.
Both in America and France there is a constant tendency on the part of
zooelogists to return to the Lamarckian idea that it is the use of an
organ that develops it, its disuse that makes it fade away. This is
undoubtedly true of the individual, and although Weissman insists that
it is useless to the species as a whole, many zooelogists are slow to
relinquish entirely the idea that somehow these favorable developments
become reproduced in the offspring.
Professor Cope, the American paleontologist, was a strong believer in
the effect of activity, both upon the individual and upon his
descendants. He believed that the insistent beating of the foot of an
animal upon the hard soil of the drying Tertiary plateau, had
influenced the production of a firmer nail, which spread around the
entire end of the toe and made the hoof of the ungulate. He believed
that the use of the teeth in grinding produced a stronger and better
molar tooth, and that the offspring shared in this advantage. Since
Weissmann's time, however, every Lamarckian feels it necessary to
suggest some method by which the altered body of the parent can
produce modifications in the germ plasms from which the young are to
spring. One of our later biologists begins to talk of some effect
comparable with wireless telegraphy or induced electricity. He
believes that organs in the adult, not necessarily by direct action,
but by action from a distance, may alter the germ. Of this, there is
no proof at present. Others have suggested that just as the ductless
glands pour into the blood chemical substances which materially affect
the growth and development of other portions of the body, so similar
enzymes, or other chemical substances, may be sent into the blood,
which subsequently bathes the germ cells of the coming generation and
produces the change. But of this, again, there is no proof. We may
believe that acquired characters are transmitted, but we certainly do
not have a very clear idea as to how it can be done.
One of the strongest objections to Darwin's idea of evolution by
natural selection of small and favorable variations, is that the
process is too inconceivably slow to account for the enormous progress
which has been made. The answer has always been that our observation
ran back so short a time that we really have no clear idea of how
rapid evolution may have been. Again, it has been answered that
transitional geological periods, in which there is much change in the
physical geography of a country, will produce more rapid evolution
than we at present are experiencing.
Hugo DeVries, of Amsterdam, believes he has found the answer to this
difficulty. Outside of his botanical garden an American species of
Evening Primrose had run wild. In looking over a number of these
plants he found, every here and there, certain peculiar members of the
species. They differed noticeably to the practiced eye from the rest
of the group. When they were planted and crossed with each other, and
the resulting seeds were again planted, the peculiarity remained
constant in all the members of the collection. Here then we have a
true variation, not large in amount, but at the same time quite
definite, and which from the first remains true. Here are the
beginnings, says DeVries, of new species. They are true from the
first; they can live among other members of the species and still come
true; they do not need isolation, at least in Wagner's geographical
sense. These forms DeVries calls mutations. It is his thought that a
species may run along uniformly for a long time when, from some cause
which he has not determined as yet, instability comes into the species
and it varies in quite a number of directions. Each of these
variations may be the starting point of a new species. DeVries
believes that he has at least half a dozen mutants of his new Evening
Primrose.
This theory of Mutation has been eagerly seized upon by many
botanists. The zooelogists have not accepted it quite so
enthusiastically. If this is the chief method by which species
transform, it seems strange that we do not find more mutations than we
do. Perhaps we do not look carefully enough; perhaps we shall find
them a little later. Just at present it seems premature to believe
that all evolution is by mutation, although quite possibly some of it
is. The main apparent advantage of mutation is that it hastens the
time in which a new species may arise.
There are certain difficulties which run back into the problem, and
which must first be reasonably solved before a clear understanding of
the idea of evolution is possible. The first of these is as to the
nature of life. What is life? The reply of the biologist will probably
be that so far as its material side is concerned, it must be answered
in terms of physics and chemistry. As to any side not material, if it
have any such side, science says that the chemist can have nothing to
say. The chemist may have an opinion of his own based on some other
ground than his chemistry, but so far as he is a chemist, he has no
opinion. The chemical side of life is being very carefully and very
fully investigated. We are certainly being brought nearer to the
borders of the living substance. We are rapidly gaining fuller
knowledge of the physical and chemical processes which constitute
life, or with which life is always associated. If we gain this
knowledge we shall be in better position to solve many of our other
problems. Even then there is a problem which preceded and which will
possibly always defy solution. How did life originate? Has it
developed out of chemical and physical activities which we know as
heat, light or electricity? If so, what were the conditions under
which it developed? If we understand the nature of life, and the
conditions under which it developed, we may be able to produce it at
will.
A few scientists may hope dimly that this will be attained. I suspect
a great majority believe it to be impossible, and that the question as
to whether life evolved upon this planet, or this planet became
infected with life through meteoric dust from some other center, will
forever remain an unsolved problem.