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Henry Edward Crampton - The Doctrine of Evolution

H >> Henry Edward Crampton >> The Doctrine of Evolution




* * * * *

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
formulae 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 pupae.

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.

The competition between and among the individuals of one and the same
species is the third part of the struggle for existence, and it is often
unsurpassed in its ferocity. When two lion cubs of the same litter begin
to shift for themselves, they must naturally compete in the same
territory, and their contest is keener than that which involves either of
them and a young lion born ten or fifteen miles away. The seeds of one
parent plant falling in a restricted area will be engaged in a competitive
struggle for existence that is much more intense than many other parts of
nature's warfare. In brief, the intensity of the competition will be
directly proportional to the similarity of two organisms in constitution
and situation, and to the consequent similarity of vital welfare. The
interests of the white man and the Indian ran counter to each other a few
hundred years ago, and the more powerful colonists won. The assumption of
the white man's burden too often demonstrates the natural effect of
diversity of interest, and the domination of the stronger over the weaker.
In any civilized community the manufacturer, farmer, financier, lawyer,
and doctor must struggle to maintain themselves under the conditions of
their total inorganic and social environments; and in so far as the object
of each is to make a living for himself, they are competitors. But the
contest becomes more absorbing when it involves broker and broker, lawyer
and lawyer, financier and magnate, because in each case the contestants
are striving for an identical need of success.

Although the severity of the conflict imposed by nature is somewhat
modified in the case of social organisms, where community competes with
community and nation with nation, no form of social organization has yet
been developed where the individual contest carried on by the members of
one community has been done away with. It is an inexorable law of nature
that all living things must fight daily and hourly for their very lives,
because so many are brought into the world with each new generation that
there is not sufficient room for all. No organism can escape the struggle
for existence except by an unconditional surrender that results in death.
Everywhere we turn to examine the happenings of organic life we can find
nothing but a wearisome warfare in which it is the ultimate and cruel lot
of every contestant to admit defeat.

* * * * *

What now are the results of variation, over-multiplication, and
competition? Since some must die because nature cannot support all that
she produces, since only a small proportion of those that enter upon life
can find a foothold or successfully meet the hordes of their enemies,
which will be the ones to survive? Surely those that have even the
slightest advantage over their fellows will live when their companions
perish. It is impossible that the result could be otherwise; it must
follow inevitably from what has been described before. The whole process
has its positive and its negative aspects: the survival of the fittest and
the elimination of the unfit. Perhaps it would be more correct to say the
more real element is the negative one, for those which are least capable
of meeting their living foes and the decimating conditions of inorganic
nature are the first to die, while the others will be able to prolong the
struggle for a longer or shorter period before they too succumb. Thus the
destruction of the unfit leaves the field to the better adapted, that is,
to those that vary in such a way as to be completely or at least partially
adapted to carry on an efficient life. In this way Darwinism explains the
universal condition of organic adjustment, showing that it exists because
there is no place in nature for the incompetent.

* * * * *

Finally we come to the process of inheritance as viewed by Darwin, and its
part in the production and perfection of new species. In every case,
Darwin said, the efficiency or inefficiency of an animal depends upon its
characteristics of an inherited or congenital nature. Variations in these
qualities provide the array of more or less different individuals from
which impersonal nature selects the better by throwing out first the
inferior ones. An organism can certainly change in direct response to
environmental influence or by the indirect results of use and disuse, but
not unless it is so constituted by heredity as to be able to change
adaptively. Therefore the final basis of success in life must be sought in
the inherited constitutions of organic forms.

For the reason that the qualities which preserve an animal's existence are
already congenital, they are already transmissible, as Darwin contended.
Since his time much has been learned about the course of inheritance and
its physical basis, and the new discoveries have confirmed the essential
truth of Darwin's statement that the congenital characters only possess a
real power in the evolution of species.

We must devote some time to the subject of inheritance at a later
juncture, but before leaving the matter an additional point must be
established here; the selective process deals immediately with congenital
results, as the heritable characters that make for success or failure in
life, but by doing this it really selects the group of congenital factors
behind and antecedent to their effects. For example, an ape that survives
because of its superior cunning, does so because it varies congenitally in
an improved direction; and the factors that have made it superior are
indirectly but no less certainly preserved through the survival of their
results in the way of efficiency. Hereditary strains are thus the ultimate
things selected through the organic constitutions that they determine and
produce.

Natural selection, as the whole of this intricate process, is simply trial
and error on a gigantic scale. Nature is such that thousands of varying
individuals are produced in order that a mere handful or only one survivor
may be chosen to bear the burden of carrying on the species for another
generation. The effect of nature's process is judicial, as it were. We may
liken the many and varied conditions of life to as many jurymen, before
which every living thing must appear for judgment as to its fitness or
lack of it. A unanimous verdict of complete or partial approval must be
rendered, or an animal dies, for the failure to meet a single vital
condition results in sure destruction. Of course, we cannot regard
selection as involving anything like a primitive conscious choice. It is
because we individualize all of the complex totality of the world as
"Nature" with a capital N that so many people unconsciously come to think
of it as a human-like personality. He who would go further and hold that
all of nature is actually conscious and the dwelling-place of the
supernatural ultimate, must beware of the logical results of such a view.
What must we think of the ethical status of such a conscious power who
causes countless millions of creatures to come into the world and
ruthlessly compels them to battle with one another until a cruel and
tragic death ends their existence?

But that is a metaphysical matter, with which we need not concern
ourselves in this discussion; the important point is that among the
everyday happenings of life are processes that are quite competent to
account for the condition of adaptation exhibited by various animal forms.
These processes are real and natural, not imaginative or artificial, and
so they will remain even though it will become clear that much is still to
be learned about the causes of variation and the course of biological
inheritance. Darwin was the first to contend that natural selection is but
a part of nature's method of accomplishing evolution. As such it is
content to recognize variations and does not concern itself with the
origin of modifications; it accepts the obvious fact that congenital
variations are inherited, although it leaves the question as to how they
are inherited for further examination. Because the doctrine of natural
selection does not profess to answer all the questions propounded by
scientific inquisitiveness, it must not be supposed that it fails in its
immediate purpose of giving a natural explanation of how evolution may be
partly accounted for.

* * * * *

Before proceeding to the post-Darwinian investigations that have done so
much to amplify the account of natural evolution, let us consider the
contrasted explanation given by Lamarck and his followers. As we have
stated earlier, Lamarckianism is the name given to the doctrine that
modifications other than those due to congenital factors may enter into
the heritage of a species, and may add themselves to those already
combined as the peculiar characteristics of a particular species. Let us
take the giraffe and its long neck as a concrete example. The great length
of this part is obviously an adaptive character, enabling the animal to
browse upon the softer leafy shoots of shrubs and trees. The vertebral
column of the neck comprises just the same number of bones that are
present in the short-necked relatives of this form, so that we are
justified in accepting as a fact the evolution of the giraffe's long neck
by the lengthening of each one of originally shorter vertebrae. The
Lamarckian explanation of this fact would be that the earliest forms in
the ancestry of the giraffe as such stretched their necks as they fed, and
that this peculiar function with its correlated structural modification
became habitual. The slight increase brought about by any single
individual would be inherited and transmitted to the giraffes of the next
generation; in other words, an individually acquired character would be
inherited. The young giraffes of this next generation would then begin,
not where their parents did, but from an advanced condition. Thus, by
continued stretching of the neck and by continued transmission of the
elongated condition, the great length of this part of the body in the
modern giraffe would be attained.

The explanation of natural selection would be quite different. The
Darwinian would say that all the young giraffes of any one generation
would vary with respect to the length of the neck. Those with longer necks
would have a slight advantage over their fellows in the extended sphere of
their grazing territory. Being better nourished than the others, they
would be stronger and so they would be more able to escape from their
flesh-eating foes, like the lion. For the reason that their variation
would be congenital and therefore already transmissible, their offspring
would vary about the advanced condition, and further selection of the
longer necked individuals would lead to the modern result.

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