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.
* * * * *
Taking up the first proposition, we return to the subject of variation
that has been discussed previously for the purpose of demonstrating its
reality. The observations of every day are enough to convince us that no
two living things are ever exactly alike in all respects. The reason is
that the many details of organic structure are themselves variable, so
that an entire organism cannot be similar to another either in material or
in functional regards, while furthermore it would be impossible for an
animal to be related to environmental circumstances in the same way as
another member of its species unless it was possible for two things to
occupy the same space at the same time! Individual differences in physical
constitution are displayed by any litter of kittens, with identical
parents; it needs only a careful examination to find the variations in the
shape of the heads, the length of their tails, and in every other
character. Sometimes the differences are less evident in physical
qualities than in disposition and mental make-up, for such variations can
be found among related kittens just as surely as among the children
belonging to a single human family.
Not only do all organisms vary, but they seem to vary in somewhat similar
ways. While modern investigations have thrown much light upon the
relations between variations and their causes, of particular value in the
case of the congenital phenomena, the greatest advance since Darwin's time
consists in the demonstration by the naturalists who have employed the
laborious methods of statistical analysis that the laws according to which
differences occur are the same where-ever the facts have been examined. A
single illustration will suffice to indicate the general nature of this
result. If the men of a large assemblage should group themselves according
to their different heights in inches, we would find that perhaps one half
of them would agree in being between five feet eight inches and five feet
nine inches tall. The next largest groups would be those just below and
above this average class,--namely, the classes of five feet seven to eight
inches and five feet nine to ten inches. Fewer individuals would be in the
groups of five feet five to six inches and five feet ten to eleven inches,
and still smaller numbers would constitute the more extreme groups on
opposite sides of these. If the whole assemblage comprised a sufficient
number of men, it would be found that a class with a given deviation from
the average in one direction would contain about the same number of
individuals as the class at the same distance from the average in the
opposite direction. Taking into account the relative numbers in the
several classes and the various degrees to which they depart from the
average, the mathematician describes the whole phenomenon of variation in
human stature by a concise formula which outlines the so-called "curve of
error." From his study of a thousand men, he can tell how many there would
be in the various classes if he had the measurements of ten thousand
individuals, and how many there would be in the still more extreme classes
of very short and very tall men which might not be represented among one
thousand people.
It is not possible to explain why variation should follow this or any
other mathematical law without entering into an unduly extensive
discussion of the laws of error. The mathematicians themselves tell us in
general terms that the observations they describe so simply by their
formulæ follow as the result of so-called chance, by which they mean that
the combined operation of numerous, diverse, and uncorrelated factors
brings about this result, and not, of course, that there is such a thing
as an uncaused event or phenomenon.
Whenever any extensive series of like organisms has been studied with
reference to the variations of a particular character, the variations
group themselves so as to be described by identical or similar curves of
error. It is certainly significant that this is true for such diverse
characters, cited at random from the lists of the literature, as the
number of ray-flowers of white daisies, the number of ribs of beech
leaves, and of the bands upon the capsules of poppies, for the shades of
color of human eyes, for the number of spines on the backs of shrimps, and
for the number of days that caterpillars feed before they turn into pupæ.
To summarize the foregoing facts, we have learned that variation is
universal throughout the living world, and that the primary factors
causing organic difference--the counterparts of human ingenuity in the
case of dead mechanisms--are the natural influences of the environment, of
organic physiological activity, and of congenital inheritance. These
factors are accorded different values in the evolution of new species, as
we may see more clearly at a later juncture, but the essential point here
is that they are not unreal, although they may not as yet be described by
science in final analytical terms.
* * * * *
We come now to the second element of the whole process of evolution,
namely, what we may call overproduction or excessive multiplication. Like
variation and so many other phenomena of nature, this is so real and
natural that it escapes our attention until science places it before us in
a new light. The normal rate of reproduction _in all species of animals_
is such that if it were unchecked, any kind of organism would cumber the
earth or fill the sea in a relatively short time. That this is universally
true is apparent from any illustration that might be selected. Let us take
the case of a plant that lives for a single year, and that produces two
seeds before it withers and dies; let us suppose that each of these seeds
produces an adult plant which in its turn lives one year and forms two
seeds. If this process should continue without any interference, the
twentieth generation after as many years would consist of more than one
million descendants of the original two-seeded annual plant, provided only
that each individual of the intervening years should live a normal life
and should multiply at the natural rate. But such a result as this is
rendered impossible by the very nature which makes annual plants multiply
in the way they do. Let us take the case of a pair of birds which produce
four young in each of four seasons. Few would be prepared for the figures
enumerating the offspring of a single pair of birds at the end of fifteen
years, if again all individuals lived complete and normal lives: at the
end of the time specified there would be more than two thousand millions
of descendants. The English sparrow has been on this continent little more
than fifty years; it has found the conditions in this country favorable
because few natural enemies like those of its original home have been met,
and as a consequence it has multiplied at an astounding rate so as to
invade nearly all parts of North America, driving out many species of song
birds before it. About twenty years ago David Starr Jordan wrote that if
the English sparrow continued to multiply at the natural rate of that
time, in twenty years more there would be one sparrow to every square inch
of the state of Indiana; but of course nature has seen to it that this
result has not come about. A single conger-eel may produce fifteen million
eggs in a single season, and if this natural rate of increase were
unchecked, the ocean would be filled solid with conger-eels in a few
years. Sometimes a single tapeworm, parasitic in the human body, will
produce three hundred million embryos; the fact that this animal is
relatively rare diverts our attention from the alarming fertility of the
species and the excessive rate of its natural increase. Perhaps the most
amazing figures are those established by the students of bacteria and
other micro-organisms. Many kinds of these primitive creatures are known
where the descendants of a single individual will number sixteen to
seventeen millions after twenty-four hours of development under ordinarily
favorable conditions. Though a single rodlike individual taken as a
starting-point may be less than one five-thousandth of an inch in length,
under natural circumstances it multiplies at a rate which _within five
days_ would cause its descendants _to fill all the oceans to the depth of
one mile_. This is a fact, not a conjecture; the size of one organism is
known, and the rate of its natural increase is known, so that it is merely
a matter of simple arithmetic to find out what the result would be in a
given time.
Even in the case of those animals that reproduce more slowly, an
overcrowding of the earth would follow in a very short time. Darwin wrote
that even the slow-breeding human species had doubled in the preceding
quarter century. An elephant normally lives to the age of one hundred
years; it begins to breed at the age of thirty, and usually produces six
young by the time it is ninety. Beginning with a single pair of elephants
and assuming that each individual born should live a complete life, only
eight hundred years would be requisite to produce nineteen million
elephants; a century or two more and there would be no standing room for
the latest generation of elephants. It is only too obvious that such a
result is not realized in nature, but it is on account of other natural
checks, and not because the natural rate of reproductive increase is
anything but excessive.
The third element of the process of natural selection is the struggle for
existence which is to a large extent the direct consequence of
over-multiplication. Because nature brings more individuals into existence
than it can support, every animal is involved in many-sided battles with
countless foes, and the victory is sometimes with one and sometimes with
another participant in the conflict. A survivor turns from one vanquished
enemy only to find itself engaged in mortal combat with other attacking
forces. Wherever we look, we find evidence of an unceasing struggle for
life, and an apparently peaceful meadow or pond is often the scene of
fierce battles and tragic death that escape our notice only because the
contending armies are dumb.
A community of ants, often comprising more individuals than an entire
European state, depends for its national existence upon its ability to
prevail over other communities with which it may engage in sanguinary wars
where the losses of a single battle may exceed those of Gettysburg. The
developing conger-eels find a host of enemies which greatly deplete their
numbers before they can grow even into infancy. An annual plant does not
produce a million living offspring in twenty years because seeds do not
always fall upon favorable soil, nor do they always receive the proper
amount of sunlight and moisture, or escape the eye of birds and other
seed-eating animals. These three illustrations bring out the fact that
there are three classes of natural conditions which must be met by every
living creature if it is to succeed in life. In detail, the struggle for
existence is _intra-specific_, involving some form of competition or
rivalry among the members of a single species; it is _inter-specific_, as
a conflict is waged by every species with other kinds of living things;
and finally it involves an adjustment of life to _inorganic environmental_
influences. While it may seem unjustifiable to speak of heat and cold and
sunlight as enemies, the direct effects produced by these forces are to be
reckoned with no less certainty than the attacks of living foes.
The three divisions of the struggle for existence are so important not
only in purely scientific respects, but also in connection with the
analysis of human biology, that we may look a little further into their
details, taking them up in the reverse order. Regarding the environmental
influences, the way that unfavorable surroundings decimate the numbers of
the plants of any one generation has already been noted, and it is typical
of the vital situation everywhere. English sparrows are killed by
prolonged cold and snow as surely as by the hawk. The pond in which
bacteria and protozoa are living may dry up, and these organisms may be
killed by the billion. Even the human species cannot be regarded as exempt
from the necessity of carrying on this kind of natural strife, for scores
and hundreds die every year from freezing and sunstroke and the thirsts of
the desert. Unknown thousands perish at sea from storm and shipwreck,
while the recorded casualties from earthquakes and volcanic eruptions and
tidal waves have numbered nearly one hundred and fifty thousand in the
past twenty-eight years. The effects of inorganic influences upon all
forms of organic life must not be underestimated in view of such facts as
these.
In the second place, the vital struggle includes the battles of every
species with other kinds of living things whose interests are in
opposition. The relations of protozoa and bacteria, conger-eels and other
fish, English sparrows and hawks, plants and herbivorous animals, are
typical examples of the universal conflict in which all organisms are
involved in some way. Again it is only too evident that human beings must
participate every day in some form of warfare with other species. In order
that food may be provided for mankind the lives of countless wild
organisms must be sacrificed in addition to the great numbers of
domesticated animals reared by man only that they may be destroyed. The
wolf and the wildcat and the panther have disappeared from many of our
Eastern states where they formerly lived, while no longer do vast herds of
bison and wild horses roam the Western prairies. Because one or another
human interest was incompatible with the welfare of these animals they
have been driven out by the stronger invaders.
That the victory does not always fall to the human contestant is
tragically demonstrated by the effects of the incessant assaults upon man
made by just one kind of living enemy,--the bacillus of tuberculosis.
Every year more than one hundred and twenty-five thousand people of the
United States die because they are unable to withstand its persistent
attacks; five million Americans now living are doomed to death at the
hands of these executioners, and the figures must be more than doubled to
cover the casualties on the human side in the battles with the regiments
of all the species of bacteria causing disease.