Henry Edward Crampton - The Doctrine of Evolution

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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
Achatinellidae 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 Achatinellidae,
namely, the Partulae, which are thus in relative proximity to the
Achatinellidae and not on the other side of the world. Furthermore, the
Partulae 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 Achatinellidae 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 zooelogical 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 zooelogical 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.

But by far the greater number of variations seem to be due to the
so-called congenital causes, which are sharply contrasted with the
influences of the first and second classes. It is quite true that the
influences of the third class cannot be surely and directly demonstrated
like the others, but however remote and vague they themselves may appear to
be, their effects are obvious and real, while at the same time their
effects are to be clearly distinguished from the products of the other two
kinds. Congenital factors reside in the physical heritage of an organism,
and their results are often evident before an individual is subjected to
environmental influences and before it begins to use its various organs.
For example, it is a matter of common observation that a child with light
hair and blue eyes may have dark-eyed and brown-haired parents. The fact
of difference is a phenomenon of variation; the causes for this fact
cannot be found in any other category than that comprising the hereditary
and congenital influences of parent upon offspring. _How_ the effect is
produced by such causes is less important in the present connection than
the natural _fact_ of congenital variation. Science, however, has learned
much about the causes in question, as we shall see at a later point.

Thus the first step which is necessary for an evolution and transformation
of organic mechanisms proves to be entirely natural when we give only
passing attention to certain obvious phenomena of life. The fact of
"becoming different" cannot be questioned without indicting our powers of
observation, and we must believe in it on account of its reality, even
though the ultimate analysis of the way variations of different kinds are
produced remains for the future.

Having learned that animals are able to change in various ways, the next
question is whether variations can be transmitted to future generations
through the operation of secondary factors. Long ago Buffon held that the
direct effects of the environment are immediately heritable, although the
mode of this inheritance was not described; it was simply assumed and
taken for granted. Thus the darker color of the skin of tropical human
races would be viewed by Buffon as the cumulative result of the sun's
direct effects. Lamarck laid greater stress upon the indirect or
functional variations due to the factors of use and disuse, and he also
assumed as self-evident that such effects were transmissible as "acquired
characters." This expression has a technical significance, for it refers
to variations that are added during individual life to the whole group of
hereditary qualities that make any animal a particular kind of organism.
If evolution takes place at all, any new kind of organism originating from
a different parental type must truly acquire its new characteristics, but
few indeed of the variations appearing during the lifetime of an animal
owe their origin to the functional and environmental influences, whose
effects only deserve the name of "acquired characters" in the special
biological sense.

In sharp contrast to Lamarckianism, so called,--although it did not
originate in the mind of the noted man of science whose name it bears,--is
the doctrine of natural selection, first proposed in its full form by
Charles Darwin. This doctrine presents a wholly natural description of the
method by which organisms evolve, putting all of the emphasis upon the
congenital causes of variation, although the reality of other kinds of
change is not questioned. But the contrast between Darwinism and the other
descriptions of secondary factors can best be made after a somewhat
detailed discussion of the former, which has gained the adherence of the
majority of the naturalists of to-day. However, we must not pass on
without pointing out that however much the explanations given by various
men of science may differ, they all agree in expressly recognizing the
complete naturalness of the secondary as well as of the primary factors of
evolution.

* * * * *

The doctrine of natural selection forms the best basis for the detailed
discussion of the way evolution has come about in the past and how it is
going on to-day. This is true because it was the first description of
nature's program to carry conviction to the scientific world, and because
its major elements have stood the test of time as no other doctrine has
done. Much has been added to our knowledge of natural processes during
post-Darwinian times, and new discoveries have supplemented and
strengthened the original doctrine in numerous ways, although they have
corrected certain of the minor details on the basis of fuller
investigation.

At the outset it must be clearly understood that Darwin's doctrine is
concerned primarily with the _method_ and not with the evidences as to the
actual _fact_ of evolution. Most of those who are not familiar with the
principles of science believe that Darwin discovered this process; but
their opinion is not correct. The reality of natural change as a universal
attribute of living things had been clearly demonstrated long before
Darwin wrote the remarkable series of books whose influence has been felt
outside the domains of biology and to the very confines of organized
knowledge everywhere. The "Origin of Species" was published in 1859, and
only the last of its fourteen chapters is devoted to a statement of the
evidence that evolution is true. In this volume Darwin presented the
results of more than twenty-five years of patient study of the phenomena
of nature, utilizing the observations of wild life in many regions visited
by him when he was the naturalist of the "Beagle" during its famous voyage
around the world. He also considered at length the results of the
breeder's work with domesticated animals, and he showed for the first time
that the latter have an evolutionary significance. Because his logical
assembly of wide series of facts in this and later volumes did so much to
convince the intellectual world of the reasonableness of evolution, Darwin
is usually and wrongly hailed as the founder of the doctrine. It is
interesting to note in passing that Alfred Russel Wallace presented a
precisely similar outline of nature's workings at about the same time as
the statement by Darwin of his theory of natural selection. But Wallace
himself has said that the greater credit belongs to the latter
investigator who had worked out a more complete analysis on the basis of
far more extensive observation and research.

The fundamental point from which the doctrine of natural selection
proceeds is the fact that all creatures are more or less perfectly adapted
to the circumstances which they must meet in carrying on their lives; this
is the reason why so much has been said in earlier connections regarding
the universal occurrence of organic adaptation. An animal is not an
independent thing; its life is intertwined with the lives of countless
other creatures, and its very living substance has been built up out of
materials which with their endowments of energy have been wrested from the
environment. Every animal, therefore is engaged in an unceasing struggle
to gain fresh food and new energy, while at the same time it is involved
in a many-sided conflict with hordes of lesser and greater foes. It must
prevail over all of them, or it must surrender unconditionally and die.
There is no compromise, for the vast totality we individualize as the
environment is stern and unyielding, and it never relents for even a
moment's truce.

To live, then, is to be adapted for successful warfare; and the question
as to the mode of origin of species may be restated as an inquiry into the
origin of the manifold adaptations by which species are enabled to meet
the conditions of life. Why is adaptation a universal phenomenon of
organic nature?

The answer to this query given by Darwinism may be stated so simply as to
seem almost an absurdity. It is, that if there ever were any unadapted
organisms, they have disappeared, leaving the world to their more
efficient kin. Natural selection proves to be a continuous process of
trial and error on a gigantic scale, for all of living nature is involved.
Its elements are clear and real; indeed, they are so obvious when our
attention is called to them that we wonder why their effects were not
understood ages ago. These elements are (1) the universal occurrence of
variation, (2) an excessive natural rate of multiplication, (3) the
struggle for existence entailed by the foregoing, (4) the consequent
elimination of the unfit and the survival of only those that are
satisfactorily adapted, and (5) the inheritance of the congenital
variations that make for success in the struggle for existence. It is true
that these elements are by no means the ultimate causes of evolution, but
their complexity does not lessen their validity and efficiency as the
immediate factors of the process.

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