THE THEORY OF EVOLUTION
One of the most important considerations in connection with the problem
of adaptation is that in all animals and plants the individuals sooner
or later perish and new generations take their places. Each new
individual is formed, in most cases, by the union of two germ-cells
derived one from each parent. As a result of this process of
intermixing, carried on from generation to generation, all the
individuals would tend to become alike, unless something else should
come in to affect the result.
So far as our actual experience reaches, we find that the succeeding
generations of individuals resemble each other. It is true that no two
individuals are absolutely alike, but if a sufficiently large number are
examined at a given time, they will show about the same variations in
about the same proportionate numbers. Such a group of similar forms,
repeating itself in each generation, is the unit of the systematists,
and is called a species.
It has been said that within each species the individuals differ more or
less from each other, but our experience teaches that in each generation
the same kinds of variations occur, and, moreover, that from any one
individual there may arise in the next generation any one of the
characteristic variations. Certain limitations will have to be made in
regard to this statement, but for the present it will suffice. The Law
of Biogenesis states that each living thing arises from another living
thing; that there is no life without antecedent life, _i.e._ spontaneous
generation does not occur. The law is not concerned with the likeness or
unlikeness of the different individuals that descend from each other.
The theory of evolution includes the same idea, but in addition it has
come to mean nowadays, that there have been changes, as the succeeding
generations have arisen. The transmutation theory, and even the descent
theory, have come to mean nearly the same thing as the theory of
evolution. It is unfortunate that one of these terms cannot be used to
signify simply the repetition, generation after generation, of groups of
similar individuals. The theory of descent might be used to convey only
this idea, but unfortunately it too has come to include also the idea of
change. I shall attempt nevertheless to discriminate between the descent
and the transmutation theory, and use the term _descent theory_ when I
do not wish to convey the idea of change, and _transmutation theory_
when I do wish to emphasize this idea.
On the transmutation theory it is assumed that a group (species) may
give rise to one or more groups of forms differing from their ancestors;
the original group being now replaced by its new kinds of offspring, or
the old and the new may remain in existence at the same time. This
process repeating itself, each or some of the new groups giving rise in
turn to one or more new species, there will be produced a larger group
of species having certain similar characters which are due to their
common descent. Such a group of species is called a genus. The
resemblances of these species is accounted for by their common descent;
but their differences must be due to those factors that have caused them
to depart from the original type. We may now proceed to consider the
evidence on which this idea of transmutation rests.
Evidence in Favor of the Transmutation Theory
EVIDENCE FROM CLASSIFICATION AND FROM COMPARATIVE ANATOMY
It does not require any special study to see that there are certain
groups of animals and of plants that are more like each other than they
are like the members of any other group. It is obvious to every one that
the group known as mammals has a combination of characters not found in
any other group; such, for instance, as a covering of hair, mammary
glands that furnish milk to the young, and a number of other less
distinctive features. These and other common characteristics lead us to
put the mammals into a single class. The birds, again, have certain
common characters such as feathers, a beak without teeth, the
development of a shell around the egg, etc., and on account of these
resemblances we put them into another class. Everywhere in the animal
and plant kingdoms we find large groups of similar forms, such as the
butterflies, the beetles, the annelidan worms, the corals, the snails,
the starfishes, etc.
Within each of these groups we find smaller groups, in each of which
there are again forms more like each other than like those of other
groups. We may call these smaller groups families. Within the families
we find smaller groups, that are more like each other than like any
other groups in the same family, and these we put into genera. Within
the genus we find smaller groups following the same rule, and these are
the species. Here we seem to have reached a limit in many cases, for we
do not always find within the species groups of individuals more like
each other than like other groups. Although we find certain differences
between the individuals of a species, yet the differences are often
inconstant in the sense that amongst the descendants of any individual
there may appear any one of the other variations. If this were the whole
truth, it would seem that we had here reached the limits of
classification, the species being the unit. This, however, is far from
being the case, for, in many species we find smaller groups, often
confined to special localities. These groups are called varieties.
In some cases it appears, especially in plants, these smaller groups of
varieties resemble in many ways the groups of species in other forms,
since they breed true to their kind, even under changed conditions. They
have been recognized as “smaller species” by a number of botanists.
In this connection a point must be brought up that has played an
important rôle in all discussion as to what limits can be set to a
species. As a rule it is found that two distinct species cannot be made
to cross with each other, _i.e._ the eggs of an individual of one
species cannot be fertilized by spermatozoa derived from individuals of
another species; or, at least, if fertilization takes place the embryo
does not develop. In some cases, however, it has been found possible to
cross-fertilize two distinct species, although the offspring is itself
more or less infertile. Even this distinction, however, does not hold
absolutely, for, in a few cases, the offspring of the cross is fertile.
It cannot be maintained, therefore, that this test of infertility
between species invariably holds, although in a negative sense the test
may apply, for if two different forms are infertile, _inter se_, the
result shows that they are distinct species. If they cross they may or
may not be good species, and some other test must be used to decide
their relation.
We should always keep in mind the fact that the individual is the only
reality with which we have to deal, and that the arrangement of these
into species, genera, families, etc., is only a scheme invented by man
for purposes of classification. Thus there is no such thing in nature as
a species, except as a concept of a group of forms more or less alike.
In nature there are no genera, families, orders, etc. These are
inventions of man for purposes of classification.
Having discovered that it is possible to arrange animals and plants in
groups within groups, the question arises as to the meaning of this
relation. Have these facts any other significance than that of a
classification of geometric figures, or of crystals according to the
relations of their axes, or of bodies as to whether they are solids,
liquids, or gases, or even whether they are red, white, or blue?
If we accept the transmutation view, we can offer an explanation of the
grouping of living things. According to the transmutation theory, the
grouping of living things is due to their common descent, and the
greater or less extent to which the different forms have diverged from
each other. It is the belief in this principle that makes the
classification of the biologist appear to be of a different order from
that in any other science; and it is this principle that appears to give
us an insight into a large number of phenomena.
For example, if, as assumed in the theory, a group of individuals
(species) breaks up into two groups, each of these may be supposed to
inherit a large number of common characteristics from their ancestors.
These characters are, of course, the resemblances, and from them we
conclude that the species are related and, therefore, we put them into
the same genus. The differences, as has been said, between the species
must be explained in some other way; but the principle of classification
with which we are here concerned is based simply on the resemblances,
and takes no account of the differences between species.
In this argument it has been tacitly assumed that the transformation of
one species into another, or into more than one, takes place by adding
one or more new characters to those already present, or by changing over
a few characters without altering others. But when we come to examine
any two species whatsoever, we find that they differ, not only in one or
in a few characters, but in a large number of points; perhaps in every
single character. It is true that sometimes the differences are so small
that it is difficult to distinguish between two forms, but even in such
cases the differences, although small, may be as numerous as when they
are more conspicuous. If, then, this is what we really find when we
carefully examine species of animals or of plants, what is meant when we
claim that our classification is based on the characters common to all
of the forms that have descended from the same ancestor? We shall find,
if we press this point that, in one sense, there is no absolute basis of
this sort for our classification, and that we have an unreal system.
If this is admitted, does our boasted system of classification, based as
it is on the principle of descent, give us anything fundamentally
different from an artificial classification? A few illustrations may
make clearer the discussion that follows. If, for example, we take a
definition of the group of vertebrates we read: “The group of craniate
vertebrates includes those animals known as Fishes, Amphibians,
Reptiles, Birds, and Mammals; or in other words, Vertebrates with a
skull, a highly complex brain, a heart of three or four chambers, and
red blood corpuscles.” If we attempt to analyze this definition, we find
it stated that the skull is a characteristic of all vertebrates, but if
we ask what this thing is that is called skull, we find not only that it
is something different in different groups, being cartilaginous in
sharks, and composed of bones in mammals, but that it is not even
identical in any two species of vertebrates. If we try to define it as a
case of harder material around the brain, then it is not something
peculiar to the vertebrates, since the brain of the squid is also
encased in a cartilaginous skull. What has been said of the skull may be
said in substance of the brain, of the heart, and even of the red blood
corpuscles.
If we select another group, we find that the birds present a sharply
defined class with very definite characters. The definition of the group
runs as follows: “Birds are characterized by the presence of feathers,
their fore-limbs are used for flight, the breast-bone is large and
serves for the attachment of the muscles that move the wings; outgrowths
from the lungs extend throughout the body and even into the bones and
serve as air sacs which make the body more buoyant. Only one aortic arch
is present, the right, and the right ovary and oviduct are not
developed. The eyes are large and well developed. Teeth are absent. We
have here a series of strongly marked characteristics such as
distinguish hardly any other class. Moreover, the organization of
existing birds is, in its essential features, singularly uniform; the
entire class presenting less diversity of structure than many orders of
Fishes, Amphibians, and Reptiles.”[1] The feathers are the most unique
features of birds, and are not found in any other group of the animal
kingdom; moreover the plan on which they are formed is essentially the
same throughout the group, yet in no two species are the feathers
identical, but differ not only in form and proportions, but even in the
character of the barbs and hooks for holding the vane together. The
modification of the fore-limbs for flight is another characteristic
feature; yet in some birds, as the ostrich and kiwi, although the wing
has the same general plan as in other birds, it is not used for flight.
In the latter it is so small that it does not project beyond the
feathers, and in some birds, as in the penguins, the wings are used only
as organs for swimming.
Footnote 1:
Parker and Haswell: “Text Book of Zoology.”
In spite of these differences we have no difficulty in recognizing
throughout the group of birds a similarity of plan or structure,
modified though it be in a thousand different ways.
Enough has been said to illustrate what is meant by the similarities of
organisms on which we base our system of classification. When we
conclude from the statement that all vertebrates have a skull that they
owe this to a common descent, we do not mean that a particular structure
has been handed down as a sort of entailed heirloom, but that the
descendants have followed the same plan of structure as that of their
ancestors, and have the brain enclosed in a covering of harder material,
although this material may not have exactly the same form, or be made of
the same substance in all cases. Furthermore while we may recognize that
the cartilaginous skull of the shark is simpler in structure than that
of the cartilaginous-bony skull of the frog, and that the skull of the
frog is simpler than that of the rabbit, yet we should not be justified
in stating, except in a metaphorical sense, that something has been
added to the skull of the shark to make that of the frog, and something
to the latter to make that of the rabbit. On the contrary, while
something may have been added, and the plan made more complicated, the
skull has also been changed throughout in every single part.
There is another point of some importance to be taken into account in
this connection; namely, that each new generation begins life as a
single cell or egg. The egg does not contain any preformed adult
structures that it hands down unaltered, but it is so constructed that,
under constant conditions, the same, or nearly the same, kind of
structure is produced. Should something affect the egg, we can imagine
that it might form a new combination on the same general plan as that of
the old, yet one that differed from the original in every detail of its
structure. It is this idea, I believe, that lies at the base of the
transmutation theory. On some such assumption as this, and on this
alone, can we bring the theory of transmutation into harmony with the
facts of observation.
What has been said in regard to individuals as a whole may be repeated
also in respect to the study of the single organs. Selecting any one
group of the animal or plant kingdom, we find the same organ, or the
same combination of organs present in whole groups of forms. We can
often arrange these organs in definite series passing from the simple to
the complex, or, in case of degeneration, in the reverse order. However
convenient it may be to study the structure of organisms from this point
of view, the artificiality of the procedure will be obvious, since here
also the organs of any two species do not differ from each other in only
one point, but in many, perhaps in all. Therefore to arrange or to
compare them according to any one scheme gives only an incomplete idea
of their structure. We should apply here the same point of view that we
used above in forming a conception of the meaning of the zoological and
botanical systems. We must admit that our scheme is only an ideal, which
corresponds to nothing real in nature, but is an abstraction based on
the results of our experience. It might be a pleasing fancy to imagine
that this ideal scheme corresponds to the plan of structure or of
organization that is in every egg, and furnishes the basis for all the
variations that have come or may come into existence; but we should find
no justification whatsoever for believing that our fiction corresponds
to any such real thing.
To sum up the discussion: we find that the resemblances of animals and
plants can be accounted for on the transmutation theory, not in the way
commonly implied, but in a somewhat different sense. We have found that
the resemblances between the different members of a group are only of a
very general sort, and the structures are not identically the same in
any two species—in fact, perhaps in no two individuals. This conclusion,
however, does not stand in contradiction to the transmutation
hypothesis, because, since each individual begins as an egg which is not
a replica of the original adult from which it is derived, there can be
no identity, but at most a very close similarity. Admitting, then, that
our scheme is an ideal one, we can claim, nevertheless, that on this
basis the facts of classification find a legitimate explanation in the
transmutation theory.
THE GEOLOGICAL EVIDENCE
On the theory of descent, as well as on the theory of transmutation, the
ancestors of all present forms are supposed to have lived at some time
in the past on the surface of the earth. If, therefore, their remains
should have been preserved, we should expect on the descent theory to
find some, at least, of these remains to be like present forms, while on
the transmutation theory we should expect to find most, if not all, of
the ancestral forms to be different from the present ones.
The evidence shows that fossil forms are practically all different from
living forms, and the older they are the greater the difference from
present forms. In general, therefore, it may be said that the evidence
is in favor of the transmutation theory. It can scarcely be claimed that
the evidence is absolutely conclusive, however probable it may appear,
for the problem is complicated in a number of ways.
In the first place, there is convincing evidence that some forms have
been entirely exterminated. Other groups have very few living
representatives, as is the case in the group containing nautilus, and in
that of the crinoids. It is therefore always possible that a given
fossil form may represent an extinct line, and may be only indirectly
connected with forms alive at the present time. Again the historical
record is so broken and incomplete in all but a few cases that its
interpretation is largely a question of probability. We can easily
conceive that it would be only in very exceptional cases that successive
generations of the same form would be buried one above the other, so
that we should find the series unbroken. This is evident not only
because the conditions that were at one time favorable for the
preservation of organic remains might not be favorable at another time,
but also because if the conditions remained the same the organisms
themselves might also remain unchanged. A new form, in fact, would be,
_ex hypothese_, better suited to live in a different environment, and
consequently we should not expect always to find its remains in the same
place as that occupied by the parent species. This possibility of
migration of new forms into a new locality makes the interpretation of
the geological record extremely hazardous.
Nevertheless, if the evolution of the entire animal and plant kingdoms
had taken place within the period between the first deposits of
stratified rocks and the present time, we might still have expected to
find, despite the imperfections of the record, sufficient evidence to
show how the present groups have arisen, and how they are related to one
another. But, unfortunately, at the period when the history of the rocks
begins, nearly all the large groups of animals were in existence, and
some of them, indeed, as the trilobites and the brachiopods, appear to
have reached the zenith of their development.
On the other hand, the subdivisions of the group of vertebrates have
evolved during the period known to us. It is true that the group was
already formed when our knowledge of it begins, but, from the fishes
onwards, the history of the vertebrates is recorded in the rocks. The
highest group of all, the mammals, has arisen within relatively modern
times. The correctness of the transmutation theory could be as well
established by a single group of geological remains as by the entire
animal kingdom. Let us, therefore, examine how far the theory is
substantiated by the paleontological record of the vertebrates. We find
that the earliest vertebrates were fishes, and these were followed
successively by the amphibians, reptiles, birds, and mammals, one of the
last species of all to appear being man himself. There can be little
doubt that this series, with certain limitations to be spoken of in a
moment, represents a progressive series beginning with the simpler forms
and ending with the more complicated. Even did we not know this
geological sequence we would conclude, from the anatomical evidence
alone, that the progression had been in some such order as the
geological record shows. The limitation referred to above is this: that
while the mammals arose later than the birds, we need not suppose that
the mammals arose from the birds, and not even perhaps from the
reptiles, or at least not from reptiles like those living at the present
day. The mammals may in fact, as some anatomists believe, have come
direct from amphibian-like forms. If this is the case, we find the
amphibians giving rise on one hand to reptiles and these to birds, and
on the other hand to mammals.
This case illustrates how careful we should be in interpreting the
record, since two or more separate branches or orders may arise
independently from the same lower group. If the mammals arose from the
amphibians later than did the reptiles, it would be easy to make the
mistake, if the record was incomplete at this stage, of supposing that
the mammals had come directly from the reptiles.
That the birds arose as an offshoot from reptile-like forms is not only
probable on anatomical grounds, but the geological record has furnished
us with forms like archæopteryx, which in many ways appears to stand
midway between the reptiles and birds. This fossil, archæopteryx, has a
bird-like form with feathered wings, and at the same time has a beak
with reptilian teeth, and a long, feathered tail with a core of
vertebræ.
From another point of view we see how difficult may be the
interpretation of the geological record, when we recall that throughout
the entire period of evolution of the vertebrates the fishes,
amphibians, reptiles, and birds remained still in existence, although
they, or some of them, may have at one time given origin to new forms.
In fact, all these groups are alive and in a flourishing condition at
the present time. The fact illustrates another point of importance,
namely, that we must not infer that because a group gives rise to a
higher one, that it itself goes out of existence, being exterminated by
the new form. There may be in fact no relation whatsoever between the
birth of a new group and the extermination of an old one.
On the transmutation theory we should expect to find not only a sequence
of forms, beginning with the simplest and culminating with the more
complex, but also, in the beginning of each new group, forms more or
less intermediate in structure. It is claimed by all paleontologists
that such forms are really found. For example, transitional forms
between the fishes and the amphibia are found in the group of dipnoans,
or lung-fishes, a few of which have survived to the present day. There
are many fossil forms that have characters between those of amphibians
and reptiles, which if not the immediate ancestors of the reptiles, yet
show that at the time when this group is supposed to have arisen
intermediate forms were in existence. The famous archæopteryx remains
have been already referred to above, and it appears in this case that we
have not only an intermediate form, but possibly a transitional one. In
the group of mammals we find that the first forms to appear were the
marsupials, which are undoubtedly primitive members of the group.
The most convincing evidence of transmutation is found in certain series
of forms that appear quite complete. The evolution of the horse series
is the most often cited. As this case will be discussed a little later,
we need not go into it fully here. It will suffice to point out that a
continuous series of forms has been found, that connect the living
horses having a single toe through three-toed, with the five-toed
horses. Moreover, and this is important, this series shows a
transformation not only in one set of structures, but in all other
structures. The fossil horses with three toes are found in the higher
geological layers, and those with more toes in the deeper layers
progressively. In some cases, at least, the fossils have been found in
the same part of the world, so that there is less risk of arranging them
arbitrarily in a series to fit in with the theory.
EVIDENCE FROM DIRECT OBSERVATION AND EXPERIMENT
Within the period of human history we do not know of a single instance
of the transformation of one species into another one, if we apply the
most rigid and extreme tests used to distinguish wild species from each
other.[2] It may be claimed that the theory of descent is lacking,
therefore, in the most essential feature that it needs to place the
theory on a scientific basis. This must be admitted. On the other hand,
the absence of direct observation is not fatal to the hypothesis, for
several reasons. In the first place, it is only within the last few
hundred years that an accurate record of wild animals and plants has
been kept, so that we do not know except for this period whether any new
species have appeared. Again, the chance of observing the change might
not be very great, especially if the change were sudden. We would simply
find a new species, and could not state where it had come from. If, on
the other hand, the change were very slow, it might extend over so many
years that the period would be beyond the life of an individual man. In
only a few cases has it been possible to compare ancient pictures of
animals and plants with their prototypes living at the present time, and
it has turned out in all cases that they are the same. But these have
been almost entirely domesticated forms, where, even if a change had
been found, it might have been ascribed to other factors. In other
cases, as in the mummified remains of a few Egyptian wild animals (which
have also been found to be exactly like the same animals living at the
present day), it was pointed out by Geoffroy Saint-Hilaire that, since
the conditions of the Egyptian climate are the same to-day as they were
two thousand years ago, there is no reason to expect any change would
have taken place. But waiving this assumption, we should not forget that
the theory of evolution does not postulate that a change must take place
in the course of time, but only that it may take place sometimes.
Footnote 2:
The transformation of “smaller species,” described by De Vries, will
be described in a later chapter.
The position that we have here taken in regard to the lack of evidence
as to the transformation of species is, perhaps, extreme, for, as will
be shown in some detail in later chapters, there is abundant evidence
proving that species have been seen to change greatly when the
conditions surrounding them have been changed; but never, as has been
stated, so far, or rather in such a way, that an actual new species that
is infertile with the original form has been produced. Whether, after
all, these changes due to a change in the environment are of the kind
that makes new species, is also a question to be discussed later.
The experimental evidence, in favor of the transformation of species,
relates almost entirely to domesticated forms, and in this case the
conscious agency of man seems, in some cases, to have played an
important part; but here, even with the aid of the factor of isolation,
it cannot be claimed that a single new species has been produced,
although great changes in form have been effected. It is clear,
therefore, that we must, at present, rely on other data, less
satisfactory in all respects, to establish the probability of the theory
of transformation.
MODERN CRITICISM OF THE THEORY OF EVOLUTION
Throughout the whole of the nineteenth century a steady fire of
criticism was directed against the theory of evolution; the names of
Cuvier and of Louis Agassiz stand out preëminent in this connection, yet
the theory has claimed an ever increasing number of adherents, until at
the present time it is rare to find a biologist who does not accept in
one form or another the general principle involved in the theory. The
storm of criticism aroused by the publication of Darwin’s “Origin of
Species,” was directed more against the doctrine of evolution than
against Darwin’s argument for natural selection. The ground has been
gone over so often that there would be little interest in going over it
again. It will be more profitable to turn our attention to the latest
attack on the theory from the ranks of the zoologists themselves.
Fleischmann, in his recent book, “Die Descendenztheorie,” has made a new
assault on the theory of evolution from the three standpoints of
paleontology, comparative anatomy, and embryology. His general method is
to try to show that the recognized leaders in these different branches
of biology have been led to express essentially different views on the
same questions, or rather have compromised the doctrine by the examples
they have given to illustrate it. Fleischmann is fond of bringing
together the antiquated and generally exaggerated views of writers like
Haeckel, and contrasting them with more recent views on the same
subject, without making sufficient allowances for the advances in
knowledge that have taken place. He selects from each field a few
specific examples, by means of which he illustrates the weakness, and
even, as he believes, the falsity of the deductions drawn for the
particular case. For example, the plan of structure of the vertebrates
is dealt with in the following way: In this group the limbs, consisting
typically of a pair of fore-legs and a pair of hind-legs, appear under
the form of cylindrical outgrowths of the body. In the salamander, in
the turtle, in the dog, the cylindrical legs, supporting the body and
serving to support it above the ground, are used also for progression.
The general purpose to which the limbs are put as organs of locomotion
has not interfered with an astonishing number of varieties of structure,
adapted to different conditions of existence, such as the short legs
used for creeping in salamanders, lizards, turtles, crocodiles; the long
and thin legs of good runners, as the hoofed animals; the mobile legs of
the apes used for climbing; and the parachute legs of some squirrels
used for soaring. Even more striking is the great variety of hands and
feet, as seen in the flat, hairy foot of the bear; the fore-foot of the
armadillos, carrying long, sickle-shaped claws; the digging foot of the
mole; the plump foot of the elephant, ending in a broad, flat pad with
nails around the border, and without division into fingers; the hand of
man and of the apes ending with fine and delicate fingers for grasping.
To have discovered a general plan of structure running through such a
great variety of forms was proclaimed a triumph of anatomical study.[3]
Footnote 3:
This paragraph is a free translation of Fleischmann’s text.
A study of the bony structure of the limb shows that typically it
consists of a single proximal bone (the humerus in the upper arm, the
femur in the thigh), followed by two bones running parallel to each
other (the radius and ulna in the arm and the tibia and fibula in the
shank); these are succeeded in the arm by the two series of carpal
bones, and in the leg by the two series of tarsal bones, and these are
followed in each by five longer bones (the metacarpals and metatarsals),
and these again by the series of long bones that lie in the fingers and
toes. Despite the manifold variety of forms, Fleischmann admits that
both the hind- and the fore-limbs are constructed on the same plan
throughout the vertebrates. Even forms like the camel, in which there
are fewer terminal bones, may be brought into the same category by
supposing a reduction of the bones to have taken place, so that three of
the digits have been lost. In the leg of the pig and of the reindeer,
even a greater reduction may be supposed to have taken place.
Fleischmann points out that these facts were supposed to be in full
harmony with the theory of descent.
The analysis of the origin of the foot of the horse gave even better
evidence, it was claimed, in favor of the theory. The foot consists of a
single series of bones corresponding to the middle finger and toe. When,
as sometimes happens, individual horses are found in which in addition
to the single middle finger two smaller lateral fingers with small hoofs
appear, the followers of the descent theory rejoiced to be able to bring
this forward as a confirmation of their doctrine. The occurrence was
explained as a sporadic return to an ancestral form. The naïve
exposition of the laws of inheritance that were supposed to control such
phenomena was accepted without question. And when finally a large number
of fossil remains were found by paleontologists,—remains showing a
gradual increase in the middle finger, and a decrease in size of the
lateral fingers,—it was supposed that the proof was complete; and
anatomists even went so far as to hold that the original ancestor of the
horse was a five-fingered animal.
This same law of type of structure was found to extend to the entire
vertebrate series, and the only plausible explanation appeared to be
that adopted by Darwin and his followers, namely, that the resemblance
is the result of the blood-relationship of the different forms. But a
simple comparison of the skeleton of the limbs if carried out without
theoretical prejudice would show, Fleischmann thinks, that there is only
a common style, or plan of structure, for the vertebrates. This
anatomical result has about the same value as the knowledge of the
different styles of historical architecture—that, for instance, all
large churches of the Gothic period have certain general principles in
common. The believers in the theory of descent have, however, he thinks,
gone beyond the facts, and have concluded that the common plan in
animals is the consequence of a common descent. “I cannot see the
necessity for such a conclusion, and I certainly should unhesitatingly
deny that the common plan of the Gothic churches depended on a common
architect. The illustration is, however, not perfect, because the
influence of the mediæval school of stone-cutters on its wandering
apprentices is well known.”
Fleischmann adds that if the descent theory is true we should expect to
find that if a common plan of structure is present in one set of organs,
as the limbs, it should be present in all other organs as well, but he
does not add that this is generally the case.
The weakness of Fleischmann’s argument is so apparent that we need not
attempt an elaborate refutation. When he says there is no absolute proof
that the common plan of structure must be the result of
blood-relationship, he is not bringing a fatal argument against the
theory of descent, for no one but an enthusiast sees anything more in
the explanation than a very probable theory that appears to account for
the facts. To demand an absolute proof for the theory is to ask for more
than any reasonable advocate of the descent theory claims for it. As I
have tried to show in the preceding pages, the evidence in favor of the
theory of descent is not absolutely demonstrative, but the theory is the
most satisfactory one that has as yet been advanced to account for the
facts. Fleischmann’s reference to the common plan of structure of the
Gothic churches is not very fortunate for his purpose, since he admits
himself that this may be the result of a common tradition handed down
from man to man, a sort of continuity that is not very dissimilar in
principle from that implied in the descent theory; in the latter the
continuity of substance taking the place of the tradition in the other.
Had the plan for each, or even for many of the churches, originated
independently in the mind of each architect, then the similarity in
style would have to be accounted for by a different sort of principle
from that involved in the theory of descent; but as a matter of fact the
historical evidence makes it probable that similar types of architecture
are largely the result of imitation and tradition. Certain variations
may have been added by each architect, but it is just the similarity of
type or plan that is generally supposed to be the outcome of a common
tradition.
Fleischmann’s attempt in the following chapter to belittle Gegenbaur’s
theory of the origin of the five-fingered type of hand from a fin, like
that of a fish, need not detain us, since this theory is obviously only
a special application which like any other may be wrong, without in the
least injuring the general principle of descent. That all phylogenetic
questions are hazardous and difficult is only too obvious to any one
familiar with the literature of the last thirty years.
Fleischmann devotes a long chapter to the geological evidences in
connection with the evolution of the horse, and attempts to throw
ridicule on the conclusions of the paleontologists by emphasizing the
differences of opinion that have been advanced in regard to the descent
of this form. After pointing out that the horse, and its few living
relatives, the ass and the zebra, are unique in the mammalian series in
possessing a single digit, he shows that by the discovery of the fossil
horses the group has been simply enlarged, and now includes horses with
one, three, and five toes. The discovery of the fossil forms was
interpreted by the advocates of the descent theory as a demonstration of
the theory. The series was arranged by paleontologists so that the
five-toed form came first, then those with three and one toe, the last
represented by the living horses. But the matter was not so simple,
Fleischmann points out, as it appeared to be to the earlier writers, for
example to Haeckel, Huxley, Leidy, Cope, Marsh. Different authors came
to express different opinions in regard to the genealogical connection
between the fossil forms. Several writers have tried to show that the
present genus, Equus, has not had a single line of descent, but have
supposed that the European horses and the original American horses had
different lines of ancestry, which may have united only far back in the
genus Epihippus. Fleischmann points out that the arrangement of the
series is open to the criticism that it is arbitrary, and that we could
equally well make up an analogous series beginning with the
five-fingered hand of man, then that of the dog with the thumb
incompletely developed, then the four-fingered hind-foot of the pig
without a big toe and with a weak second and fifth digit, then the foot
of the camel with only two toes, and lastly the foot of the horse with
only one toe. It sounds strange that Fleischmann should make such a
trivial reply as this, and deliberately ignore the all-important
evidence with which he is, of course, as is every zoologist, perfectly
conversant. Not only are there a hundred other points of agreement in
the horse series, but also the geological sequence of the strata, in
which some at least of the series have been found, shows that the
arrangement is not arbitrary, as he implies.
Fleischmann then proceeds to point out that when the evidence from other
parts of the anatomy is taken into account, it becomes evident that all
the known fossil remains of horses cannot be arranged in a single line,
but that there are at least three families or groups recognizable. Many
of these forms are known only from fragments of their skeletons—a few
teeth, for instance, in the case of Merohippus, which on this evidence
alone has been placed at the uniting point of two series. At present
about eight different species of living horses are recognized by
zoologists, and paleontological evidence shows only that many other
species have been in existence, and that even three- and one-toed forms
lived together at the same time.
Fleischmann also enters a protest against the ordinary arrangement of
the fossil genera Eo-, Oro-, Meso-, Merohippus in a series, for these
names stand not for single species, but for groups containing no less
than six species under Protohippus, fourteen under Equus, twelve under
Mesohippus, and twenty under Hipparion. Fleischmann concludes: “The
descent of the horses has not been made out with the precision of an
accurate proof, and it will require a great deal of work before we get
an exact and thorough knowledge of the fossil forms. What a striking
contrast is found on examination between the actual facts and the crude
hopes of the apostles of the descent theory!...”
In so far as this criticism of Fleischmann’s applies to the difficulties
of determining the past history of the horse, it may be granted that he
has scored a point against those who have pretended that the evidence is
simple and conclusive; but we should not fail to remember that this
difficulty has been felt by paleontologists themselves, who have been
the first to call attention to the complexity of the problem, and to the
difficulties of finding out the actual ancestors of the living
representative of the series. And while we may admit that the early
enthusiasts exaggerated, unintentionally, the importance of the few
forms known to them, and went too far in supposing that they had found
the actual series of ancestors of living horses, yet we need not let
this blind us to the importance of the facts themselves. Despite the
fact that it may be difficult and, perhaps, in most cases, impossible,
to arrange the fossil forms in their relations to one another and to
living forms, yet on an unprejudiced view it will be clear, I think,
that so far as the evidence goes it is in full harmony with the theory
of descent. This is especially evident if we turn our attention to a
part of the subject that is almost entirely ignored by Fleischmann, and
yet is of fundamental importance in judging of the result. The series of
forms beginning with the five-toed horses and ending with those having a
single toe has not been brought together haphazard, as Fleischmann’s
comparison might lead one to suppose, but the five-fingered forms are
those from the older rocks, and the three-toed forms from more recent
layers. The value of this kind of evidence might have been open to
greater doubt had the series been made up of forms found scattered over
the whole world, for it is well known how difficult it is to compare in
point of time the rocks of different continents. But in certain parts of
the world, especially in North America, series of fossil horses have
been found in sedimentary deposits that appear to be perfectly
continuous. This series, by itself, and without regard to the point as
to whether in other parts of the world other series may exist, shows
exactly those results which the theory of descent postulates, and we
find here, in all probability, a direct line of descent. While it may be
freely admitted that no such series can demonstrate the theory of
descent with absolute certainty, yet it would be folly to disregard
evidence as clear as this.
In regard to the other point raised by Fleischmann concerning the large
number of species of fossil horses that have existed in past times, it
is obvious that while this greatly increases the difficulty of the
paleontologist it is not an objection to the descent theory. In fact,
our experience with living species would lead us to expect that many
types have been represented at each geological period by a number of
related species that may have inhabited the same country. On the descent
theory, one species only in each geological period could have been in
the line of descent of the present species of horse. The difficulty of
determining which species (if there were several living in a given
epoch) is the ancestor of the horse is increased, but this is not in
itself an objection to the theory.
The descent of birds from flying reptiles is used by Fleischmann as
another point of attack on the transmutation theory. The theory
postulates that the birds have come from ancestors whose fore-legs have
been changed into highly specialized wings. The long vertebrated tail of
the ancestral form is supposed to have become very short, and long
feathers to have grown out from its stump which act as a rudder during
flight. Flying reptiles with winged fore-legs and a long vertebrated
tail have been actually found as fossil remains, as seen in the
pterodactyls and in the famous archæopteryx. The latter, which is
generally regarded either as the immediate ancestor of living birds, or
at least as a closely similar form, possessed a fore-leg having three
fingers ending in claws, and feathers on the forearm similar to those of
modern birds. It had a long tail, like that of a lizard, but with
well-developed feathers along its sides. It had pointed teeth in the
horn-covered jaws. Fleischmann proceeds to point out that the
resemblance of the hand of archæopteryx to that of the reptiles is not
very close, for two fingers are absent as in modern birds. The typical
form of the foot is that of the bird, and is not the simple reptilian
type of structure. Feathers and not scales cover the body, and give no
clew as to how the feathers of birds have arisen. He concludes,
therefore, that archæopteryx, having many true bird-like characters,
such as feathers, union of bones in the foot, etc., has other characters
not possessed by living birds, namely, a long, vertebrated tail, a flat
breastbone, biconcave vertebræ, etc. Therefore, it cannot be regarded as
an intermediate form. Fleischmann does not point out that it is just
these characters that would be postulated on the descent theory for the
ancestor of the birds, if the latter arose from reptiles. Even if it
should turn out that archæopteryx is not the immediate forefather of
living birds, yet the discovery that a form really existed intermediate
in many characters between the reptiles and the birds is a gain for the
transmutation theory. It is from a group having such characters that the
theory postulates that the birds have been evolved, and to have
discovered a member of such a group speaks directly and unmistakably in
favor of the probability of the transmutation theory.
Fleischmann again fails to point out that the geological period in which
the remains of archæopteryx were found, is the one just before that in
which the modern group of birds appeared, and, therefore, exactly the
one in which the theory demands the presence of intermediate forms. This
fact adds important evidence to the view that looks upon archæopteryx as
a form belonging to a group from which living birds have arisen. That a
number of recent paleontologists believe archæopteryx to belong to the
group of birds, rather than to the reptiles, or to an intermediate
group, does not in the least lessen its importance, as Fleischmann
pretends it does, as a form possessing a number of reptilian characters,
such as the transmutation theory postulates for the early ancestors of
the birds.
The origin of the mammalian phylum serves as the text for another attack
on the transmutation theory. Fleischmann points out that the discovery
of the monotremes, including the forms ornithorhynchus and echidna, was
hailed at first as a demonstration of the supposed descent of the
mammals from a reptilian ancestor. The special points of resemblance
between ornithorhynchus and reptiles and birds are the complete fusion
of the skull bones, the great development of the vertebræ of the neck
region, certain similarities in the shoulder girdle, the paired oviducts
opening independently into the last part of the digestive tract
(cloaca), and the presence of a parchment-like shell around the large,
yolk-bearing egg. These are all points of resemblance to reptiles and
birds, and were interpreted as intermediate stages between the latter
groups and the group of mammals. In addition to these intermediate
characters, ornithorhynchus possesses some distinctive, mammalian
features—mammary glands and hair, for instance. Fleischmann takes the
ground, in this case, that there are so many points of difference
between the monotremes and the higher mammals, that it is impossible to
see how from forms like these the higher groups could have arisen, and
that ornithorhynchus cannot be placed as an intermediate form, a link
between saurians and mammals, as the followers of the transmutation
theory maintain. He shows, giving citations, that anatomists themselves
are by no means in accord as to the exact position of ornithorhynchus in
relation to the higher forms.
In reply to this criticism, the same answer made above for archæopteryx
may be repeated here, namely, that because certain optimists have
declared the monotremes to be connecting forms, it does not follow that
the descent theory is untrue, and not even that these forms do not give
support to the theory, if in a less direct way. I doubt if any living
zoologist regards either ornithorhynchus or echidna as the ancestral
form from which the mammals have arisen. But on the other hand it may be
well not to forget that these two forms possess many characters
intermediate between those of mammals and reptiles, and it is from a
group having such intermediate characters that we should expect the
mammals to have arisen. These forms show, if they show nothing else,
that it is possible for a species to combine some of the characters of
the reptiles with those of the mammals; and the transmutation theory
does no more than postulate the existence at one time of such a group,
the different species of which may have differed in a number of points
from the two existing genera of monotremes.
The origin of lung-bearing vertebrates from fishlike ancestors, in which
the swim-bladder has been changed into lungs, has been pointed to by the
advocates of the transmutation theory as receiving confirmation in the
existence of animals like those in the group of dipnoan fishes. In these
animals both gills and a swim-bladder, that can be used as a lung, are
present; and through some such intermediate forms it is generally
supposed that the lung-bearing animals have arisen. Fleischmann argues,
however, that, on account of certain trivial differences in the position
of the duct of the swim-bladder in living species, the supposed
comparison is not to the point; but the issue thus raised is too
unimportant to merit further discussion. Leaving aside also some even
more doubtful criticisms which are made by Fleischmann, and which might
be added to indefinitely without doing more than showing the credulity
of some of the more ardent followers of the transmutation theory, or
else the uncertainty of some of the special applications of the theory,
let us pass to Fleischmann’s criticism of the problem of development.[4]
Footnote 4:
The long argument of Fleischmann in regard to the origin of the
fresh-water snails, as illustrated by the planorbis series, and also
the origin of the nautiloid group, has been recently dealt with fully
by Plate, and, therefore, need not be considered here.
With fine scorn Fleischmann points to the crudity of the ideas of Oken
and of Haeckel in regard to the embryology (or the ontogeny) repeating
the ancestral history (or the phylogeny). We may consider briefly (since
we devote the next chapter almost entirely to the same topic) the
exceptions to this supposed recapitulation, which Fleischmann has
brought together. The young of beetles, flies, and butterflies creep out
of the egg as small worm-like forms of apparently simple organization.
They have a long body, composed of a series of rings; the head is small
and lacks the feelers, and often the faceted eyes. The wings are absent,
and the legs are short. At first sight the larva appears to resemble a
worm, and this led Oken to conclude that the insects appear first in the
form of their ancestors, the segmented worms. If we examine the
structure of the larva more carefully, we shall find that there are a
great many differences between it and the segmented worms; and that even
the youngest larva is indeed a typical insect. The tracheæ, so
characteristic of the group of insects, are present, the structure of
the digestive tract with its Malpighian tubes, the form of the heart,
the structure of the head, as well as the blastema of the reproductive
organs, show in the youngest larva the type of the insects. In other
words the body of the caterpillar is formed on exactly the same
fundamental plan as that of the butterfly.
In regard to the larval forms of other groups we find the same
relations, as, for example, in the amphibians. The young of salamanders,
toads, and frogs leave the egg not in the completed form, but as small
tadpoles adapted to life in the water. A certain resemblance to fish
cannot be denied. They possess a broad tail, gills (rich in blood
vessels) on each side of the neck, and limbs are absent for a long time.
These are characters similar to those of fish, but a more careful
anatomical examination destroys the apparent resemblance. The
superficial resemblances are due to adaptation to the same external
conditions.
Fleischmann ridicules the idea that the young chick resembles at any
stage an adult, ancestral animal; the presence of an open digestive
tract shows how absurd such an idea is. The obvious contradiction is
explained away by embryologists, by supposing that the ancestral adult
stages have been crowded together in order to shorten the period of
development; and that, in addition, larval characters and provisional
organs have appeared in the embryo itself, which confuse and crowd out
the ancestral stages.
In regard to the presence of gill-slits in the embryo of the higher
vertebrates, in the chick, and in man, for example, Fleischmann says: “I
cannot see how it can be shown by exact proof that the gill-slits of the
embryos of the higher vertebrates that remain small and finally
disappear could once have had the power of growing into functional
slits.” With this trite comment the subject is dismissed.
On the whole, Fleischmann’s attack cannot be regarded as having
seriously weakened the theory of evolution. He has done, nevertheless,
good service in recalling the fact that, however probable the theory may
appear, the evidence is indirect and exact proof is still wanting.
Moreover, as I shall attempt to point out in the next chapter, we are
far from having arrived at a satisfactory idea of how the process has
really taken place.
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