of the extinct types of animals that occupy the position of "links"
between groups now widely separated by their divergence in evolution from
the same ancestors. Perhaps the most famous example is _Archæopteryx_
found in a series of slates in Germany. This animal is at once a
feathered, flying reptile, and a primitive bird with countless reptilian
structures. Its short head possesses lizard-like jaws, all of which bear
teeth; its wings comprise five clawed digits; its tail is composed of a
long series of joints or vertebræ, bearing large feathers in pairs; its
breastbone is flat and like a plate, thus resembling that of reptiles and
differing markedly from the great keeled breastbone of modern flying
birds, whose large muscles have necessitated the development of the keel
for purposes of firm attachment. In brief, this animal was close to the
point where reptiles and birds parted company in evolution, and although
it was a primitive bird, it is in a true sense a "missing link" between
reptiles and the group of modern birds. Other fossil forms like
_Hesperornis_ and _Ichthyornis_, whose remains occur in the strata of a
later date, fill in the gap between _Archæopteryx_ and the birds at the
present time, for among other things they possess teeth which indicate
their origin from forms like _Archæopteryx_, while in other respects they
are far nearer the birds of later epochs. That these links are not unique
is proved by numerous other examples known to science, such as those which
connect amphibia and reptiles, ancient reptiles and primitive mammals, as
well as those which come between the different orders of certain
vertebrate classes.
In summarizing the foregoing facts, and the larger bodies of evidence that
they exemplify, we learn how surely the testimony of the rocks establishes
evolution in its own way, how it confirms the law of recapitulation
demonstrated by comparative embryology, and how it proves that the greater
and smaller divisions of animals have followed the identical order in
their evolution that the comparative study of the present day animals has
independently described.
* * * * *
The facts of geographical distribution constitute the fifth division of
zoölogy, and an independent class of evidences proving the occurrence of
evolution. This department of zoölogy assumed its rightful status only
after the other divisions had attained considerable growth. Many
naturalists before Darwin and Wallace and Wagner had noticed that animals
and plants were by no means evenly distributed over the surface of the
globe, but until the doctrine of evolution cleared their vision they did
not see the meaning of these facts. As in the case of all the other
departments of zoölogy the immediate data themselves are familiar, but
because they are so obvious the mind does not look for their
interpretation but accepts the facts at their face value. While the
phenomena of distribution are no less fascinating to the naturalist, and
no less effective in their demonstration of evolution, their comprehensive
treatment would demand more space than the whole purpose of the present
description of organic evolution would justify. Thus a brief outline only
can be given of the salient principles of this subject in order that their
bearing upon the problem of species may be indicated.
Even as children we learn many facts of animal distribution; every one
knows that lions occur in Africa and not in America, that tigers live in
Asia and Malaysia, that the jaguar is an inhabitant of the Brazilian
forests, and that the American puma or mountain lion spreads from north to
south and from east to west throughout the American continents. The
occurrence of differing human races in widely separated localities is no
less familiar and striking, for the red man in America, the Zulu in
Africa, the Mongol and Malay in their own territories, display the same
discontinuity in distribution that is characteristic of all other groups
of animals and of plants as well. As our sphere of knowledge increases, we
are impressed more and more forcibly by the diversity and unequal extent
of the ranges occupied by the members of every one of the varied divisions
of the organic world. Another fact which becomes significant only when
science calls our attention to it is the absence from a land like
Australia of higher mammals such as the rabbit of Europe. The hypothesis
of special creation cannot explain this absence on the assumption that the
rabbit is unsuited to the conditions obtaining in the country named, for
when the species was introduced into Australia by man, it developed and
spread with marvelous rapidity and destructive effect. It may seem
impossible that facts like these could possess an evolutionary
significance, but they are actual examples of the great mass of data
brought together by the naturalists who have seen in them something to be
interpreted, and who have sought and found an explanation in the
formularies of science.
The general principles of distribution appear with greatest clearness when
an examination is made of the animals and plants of isolated regions like
islands. The Galapagos Islands constitute a group that has figured largely
in the literature of the subject, partly because Darwin himself was so
impressed by what he found there in the course of his famous voyage around
the world in the "Beagle." They form a cluster on the Equator about six
hundred miles west of the nearest point of the neighboring coast of South
America. Although the lizards and birds that live in the group differ
somewhat among themselves as one passes from island to island, on the
whole they are most like the species of the corresponding classes
inhabiting South America. Why should this be so? On the hypothesis of
special creation there is no reason why they should not be more like the
species of Africa or Australia than like those of the nearest body of the
mainland. The explanation given by evolution is clear, simple, and
reasonable. It is that the characteristic island forms are the descendants
of immigrants which in greatest probability would be wanderers from the
neighboring continent and not from far distant lands. Reaching the
isolated area in question the natural factors of evolution would lead
their offspring of later generations to vary from the original parental
types, and so the peculiar Galapagos species would come into being. The
fact that the organisms living on the various islands of this group differ
somewhat in lesser details adds further justification for the evolutionary
interpretation, because it is not probable that all the islands would be
populated at the same time by similar stragglers from the mainland. The
first settlers in one place would send out colonies to others, where
independent evolution would result in the appearance of minor differences
peculiar to the single island. In this manner science interprets the
general agreement between the animals of the Azores Islands and the fauna
of the northwestern part of Africa, the nearest body of land, from which
it would be most natural for the ancestors of the island fauna to come.
The land-snails inhabiting the various groups of islands scattered
throughout the vast extent of the Pacific Ocean provide the richest and
most ideal material for the demonstration of the principles of
geographical distribution. In the Hawaiian Islands snails of the family of
Achatinellidæ occur in great abundance, and like the lizards of the
Galapagos Islands different species occur on the different members of the
group. Within the confines of one and the same island, they vary from
valley to valley, and the correlation between their isolation in
geographical respects and specific differences on the other hand, first
pointed out by Gulick, makes this tribe of animals classical material. In
Polynesia and Melanesia are found close relatives of the Achatinellidæ,
namely, the Partulæ, which are thus in relative proximity to the
Achatinellidæ and not on the other side of the world. Furthermore, the
Partulæ are not alike in all of the groups of Polynesia where they occur;
the species of the Society Islands are absolutely distinct from those of
the Marquesas, Tonga, Samoan, and Solomon Islands, although they agree
closely in the basic characters that justify their reference to a single
genus. The geological evidence tells us that these islands were once the
peaks of mountain ranges rising from a Pacific continent which has since
subsided to such an extent that the mountain tops have become separate
islands. Thus the resemblances between Hawaiian and Polynesian snails, and
the closer similarities exhibited by the species of the various groups of
Polynesia, are intelligible as the marks of a common ancestry in a
widespread continental stock, while the observed differences show the
extent of subsequent evolution along independent lines followed out after
the isolation of the now separated islands. The principle may be worked
out in even greater detail, for it appears that within the limits of one
group diverse forms occupy different islands, evolved in different ways in
their own neighborhoods; while in one and the same island, the populations
of the different valleys show marked effects of divergence in later
evolution, precisely as in the case of the classic Achatinellidæ of the
Hawaiian Islands.
The broad and consistent principle underlying these and related facts is
this: _there is a general correspondence between the differences displayed
by the organisms of two regions and the degree of isolation or proximity
of these two areas_. Thus the disconnected but neighboring areas of the
Galapagos Islands and South America support species that resemble each
other closely, for the reasons given before; long isolated areas like
Australia and its surroundings possess peculiar creatures like the
egg-laying mammals, and all of the pouched animals or marsupials with only
one or two exceptions like our own American opossum,--a correlation
between a geological and geographical discontinuity on the one hand and a
peculiarity on the other that reinforces our confidence in the
faunal evolutionary interpretation of the facts of distribution.
It is true that the various classes of animals do not always appear with
coextensive ranges. The barriers between two groups of related species
will not be the same in all cases. A range like the Rocky Mountains will
keep fresh-water fish apart, while birds and mammals can get across
somewhere at some time. All these things must be taken into account in
analyzing the phenomena of distribution, and many factors must be given
due attention; but in all cases the reasons for the particular state of
affairs in geographical and biological respects possess an evolutionary
significance.
Having then all the facts of animal natural history at his disposal, and
the uniform principles in each body of fact that demonstrate evolution, it
is small wonder that the evolutionist seems to dogmatize when he asserts
that descent with adaptive and divergent modification is true for all
species of living things. The case is complete as it stands to-day, while
it is even more significant that every new discovery falls into line with
what is already known, and takes its natural place in the all-inclusive
doctrine of organic evolution. Because this explanation of the
characteristics of the living world is more reasonable than any other,
science teaches that it is true.
IV
EVOLUTION AS A NATURAL PROCESS
The purpose of the discussions up to this point has been to present the
reasons drawn from the principal classes of zoölogical facts for believing
that living things have transformed naturally to become what they now are.
Even if it were possible to make an exhaustive analysis of all of the
known phenomena of animal structure, development, and fossil succession,
the complete bodies of knowledge could not make the evolutionary
explanation more real and evident than it is shown to be by the simple
facts and principles selected to constitute the foregoing outline. We have
dealt solely with the evidences as to the fact of evolution; and now,
having assured ourselves that it is worth while to so do, we may turn to
the intelligible and reasonable evidence found by science which proves
that the familiar and everyday "forces" of nature are competent to bring
about evolution if they have operated in the past as they do to-day.
Investigation has brought to light many of the subsidiary elements of the
whole process, and these are so real and obvious that they are simply
taken for granted without a suspicion on our part of their power until
science directs our attention to them.
For one reason or another, those who take up this subject for the first
time find it difficult to banish from their minds the idea that evolution,
even if it ever took place, has been ended. They think it futile to expect
that a scrutiny of to-day's order can possibly find influences powerful
enough to have any share in the marvelous process of past evolution
demonstrated by science. The naturalists of a century ago held a similar
opinion regarding the earth, viewing it as an immutable and unchanged
product of supernatural creation, until Lyell led them to see that the
world is a plastic mass slowly altering in countless ways. It is no more
true that living things have ceased to evolve than that mountains and
rivers and glaciers are fixed in their final forms; they may seem
everlasting and permanent only because a human life is so brief in
comparison with their full histories. Like the development of a continent
as science describes it, the origin of a new species by evolution, its
rise, culmination, and final extinction may demand thousands of years; so
that an onlooker who is himself only a conscious atom of the turbulent
stream of evolving organic life does not live long enough to observe more
than a small fraction of the whole process. Therefore living species seem
unchanged and unchangeable until a conviction that evolution is true, and
a knowledge of the method of science by which this conviction is borne
upon one, guide the student onwards in the further search for the
efficient causes of the process.
The biologist employs the identical methods used by the geologist in
working out the past history of the earth's crust. The latter observes the
forces at work to-day, and compares the new layers of rock now being
formed with the strata of deeper levels; these are so much alike that he
is led to regard the constructive influences of the past as identical with
those he can now watch at work. Similarly the biologist must first learn,
as we have done, the principles of animal construction and development,
and of other classes of zoölogical facts, and then he must turn his
attention from the dead object of laboratory analysis to the workings of
organic machines. The way an organism lives its life in dynamic relations
to the varied conditions of existence, as well as the mutual physiological
relations of the manifold parts of a single organism, reveal certain
definite natural forces at work. Therefore his next task is to compare the
results accomplished by these factors in the brief time they may be seen
in operation with the products of the whole process of organic evolution,
to learn, like the geologist in his sphere, that the present-day natural
forces are able to do what reason says they have done in the past.
When the subject of inquiry was the reality of evolution, it was perhaps
surprising to find that even the most familiar animals like cats and frogs
provided adequate data for science to use in formulating its principles.
So it is with the matter of method; it is unnecessary to go beyond the
observations of a day or a week of human life to find forces at work, as
real and vital as animal existence and organic life themselves. This is
true, because evolution is true, and because the lives of all creatures
follow one consistent law. Our task is therefore much more simple than
most people suppose it to be; let us look about us and classify what we
may observe, increasing our knowledge from the wide array of equally
natural facts supplied by the biologist.
The analogies of the steamship and the locomotive proved useful at many
times during the discussion of the fact of evolution, and even in the
present connection they will still be of service. The evolution of these
dead machines has been brought about by man, who, as an element of their
environment, has been their creator as well as the director of their
historical transformations. The result of their changes has been greater
efficiency and better adjustment or adaptation to certain requirements
fixed by man himself. The whole process of improvement has been one, in
brief, of trial and error; new inventions have often been worthless, and
they have been relegated to the scrap-heap, while the better part has been
finally incorporated in the type machine. In brief, then, the important
elements in the evolution of these examples have been three; first,
_adaptation_, second, the _origination of new parts_, and third, the
_retention of the better invention_.
Are the creatures of the living world so constituted that biological
equivalents of these three essential elements of mechanical evolution can
be found? Are organisms adapted to the circumstances controlling their
lives, and are they capable of changing naturally from generation to
generation, and of transmitting their qualities to their offspring? These
are definite questions that bring us face to face with the fundamental
problems relating to the dynamics or workings of evolution. We need not
ask for or expect to find complete answers, for we know that it is
impossible to obtain them. But we may expect to accomplish our immediate
object, which is to see that evolution is natural. Our attention must be
concentrated upon the three biological subjects of _adaptation_,
_variation,_ and _inheritance_, and we must learn why science describes
them as real organic phenomena and the results of natural causes.
* * * * *
At the very outset, when the general characteristics of living things were
considered, much was said on the subject of adaptation as a universal
phenomenon of nature. It was not contended that perfection is attained by
any living mechanism, but it was held that no place exists in nature for
an organism that is incapable of adjusting itself to the manifold
conditions of life. A _modus vivendi_ must be established and some
satisfactory degree of adaptation must be attained, or else an animal or a
species must perish. With this fundamental point as a basis, we look to
nature for two kinds of natural processes or factors, first, those which
may originate variations as _primary factors_,--the counterparts of human
ingenuity and invention in the case of locomotive evolution,--and the
_secondary factors_ of a preservative nature which will perpetuate the
more adaptive organic changes produced by the first influences; it is
clear that the latter are no less essential for evolution than the first
causes for the appearance of variations.
The term "variation" is employed for the natural phenomenon of being or
becoming different. It is an obvious fact that no child is ever exactly
like either of its parents or like any one of its earlier ancestors; while
furthermore in no case does an individual resemble perfectly another of
its own generation or family. This departure from the parental condition,
and the lack of agreement with others even of its closest blood-relatives,
are two familiar forms of variation. As a rule, the degree to which a
given organism is said to vary in a given character is most conveniently
measured by the difference between its actual condition and the general
average of its species, even though there is no such thing as a specimen
of average nature in all of its qualities. In brief, then, variation means
the existence of some differences between an individual and its parents,
its fraternity, and, in a wider sense, all others of its species.
Passing now to the causes of variation, all of the countless deviations of
living things can be referred to three kinds of primary factors; namely,
the _environmental_, _functional_, and _congenital_ influences that work
upon the organism in different ways and at different times during its
life. We shall learn that the evolutionary values of these three classes
are by no means equal, but we take a long step forward when we realize
that among the things we see every day are facts demonstrating the reality
of three kinds of natural powers quite able to change the characters of
organic mechanisms.
The "environment" of an organism is everything outside the creature
itself. In the case of an animal it therefore includes other members of
its own kind, and other organisms which prey upon its species or which
serve it as food, as well as the whole series of inorganic influences
which first come to mind when the term is used. For example, the
environment of a lion includes other lions which are either members of its
own family, or else, if they live in the same region, they are its more or
less active rivals and competitors. In the next place, other kinds of
animals exist whose lives are intimately related to the lion's life, such
as the antelopes or zebras that are preyed upon, and the human hunter to
whom the lion itself may fall a victim. In addition, there are the
contrasted influences of inorganic nature which demand certain adjustments
of the lion's activities. Light and darkness, heat and cold, and other
factors have their direct and larger or smaller effects upon the life of a
lion, although these effects are less obvious in this instance than in the
case of lower organisms.
The reality of variations due to the inorganic elements of the environment
is everywhere evident. Those who have spent much time in the sun are aware
that sunburn may result as a product of a factor of this class. The amount
of sunlight falling upon a forest will filter through the tree-tops so as
to cause some of the plants beneath to grow better than others, thus
bringing about variations among individuals that may have sprung from the
myriad seeds of a single parent plant. In times of prolonged drought,
plants cannot grow at the rate which is usual and normal for their
species, and so many variations in the way of inhibited development may
arise.
Then there are the variations of a second class, more complex in nature
than the direct effects of environment,--namely, the functional results of
use and disuse. A blacksmith uses his arm muscles more constantly than do
most other men, and his prolonged exercise leads to an increase of his
muscular capacity. All of the several organic systems are capable of
considerable development by judicious exercise, as every one knows. If the
functional modifications through use were unreal, then the routine of the
gymnasium and the schoolroom would leave the body and the mind as they
were before. Furthermore, we are all familiar with the opposite effects of
disuse. Paralysis of an arm results in the cessation of its growth. When a
fall has injured the muscles and nerves of a child's limb, that structure
may fail to keep pace with the growth of the other parts of the body as a
result of its disuse. These are simple examples of a wide range of
phenomena exhibited everywhere by animals and even by the human organism,
demonstrating the plasticity of the organic mechanism and its modification
by functional primary factors of variation.