ORGANIC EVOLUTION, VARIATION, AND HEREDITY
Science developed when primitive man began pondering over the problems
of the creation. He sought the causes of life, of the development of
life forms, and the authorship or origin of the uniformity and apparent
design in nature. It is, therefore, probable that what we now study as
the science of organic evolution is one of the oldest of the sciences.
As the ages have rolled on, the origin of life has been explained in
turn by theories of: (1) eternity of present conditions; (2) miraculous
creation; (3) catastrophism with (a) increases by immigration (b)
increases by successive creations; and, finally, by (4) organic
evolution.
The term organic evolution means the forming of new combinations of the
elements of organisms. It does not mean the arising of an animal or
plant out of nothing--a new creation. That idea was exploded long ago.
The science which Darwin started surveys the whole course of natural
history in terms of four dimensions--length, breadth, depth, and
duration. This was the plan which led Darwin to his great discoveries.
While studying the minor changes taking place in common animals and
plants, and looking over the broad vistas of nature back to the
remotest times, he saw how each year countless weak and ill-adapted
plants, insects, and animals were killed off. When he reflected that
this process has been going on throughout all time, the idea flashed
into his mind that it is through this testing ordeal that adaptability
of surviving organisms is derived.
One of the grandest conceptions of the human mind is that the
apparently complex, inharmonic system of nature has developed from a
simple beginning on a cooled globe from a jellylike cell.
The theory of the permanence of species was generally held by
biologists before publication of Darwin's first great book. Darwin said
that no naturalist of his time doubted the accuracy of the theory of
the eternity of existing conditions and they refused to listen to his
views regarding the mutability of species.
Darwin put forth the theory of organic evolution by natural selection
and the survival of the fittest. The great beauty of this theory
lies in its simplicity and its appeal to agencies which we can see
in full operation every day and night. The skillful manner in which
Darwin marshaled data to substantiate his theory quickly converted
the scientific world, and led to revolutionary changes in the general
tendencies of knowledge, and in practically all fields of human
activity.
Darwin's terse statement of his conception was: "As many more
individuals of each species are born than can possibly survive, and as
consequently there is a frequently recurring struggle for existence, it
follows that any being, if it vary in any manner profitable to itself,
under the complex and sometimes varying conditions of life, will have
a better chance of surviving, and thus be naturally selected. From the
strong principle of inheritance any selected variety will tend to
propagate its new and modified form." ("Origin of Species," Intro.)
This statement of the doctrine of the survival of the fittest,
suggesting a glimpse at the great pageant of nature from the remotest
times, shows how the organisms existing at this moment are the
descendants of the victors in the world's greatest battles. The
struggles for life, always keen and persistent, shared in by every
individual organism, both animal and vegetable, are the instigators
of all progress in the natural world. They are nature's means for the
attainment of beauty, usefulness, and perfection.
The Darwinian theory was based upon the observed facts that members
of any given species are not alike, while their offspring may differ
in numerous ways from their parents. The data furnished by zoölogy,
botany, physiology, and other sciences supply overwhelming evidence
that the present species of animals and plants have arisen through the
modification from various causes of many pre-existing species. The
organisms with which we are familiar owe their characteristics to the
accumulation of a long series of changes similar to those that we may
see that they are still undergoing.
The methods pursued in studying variation in species, and its
important accompaniment, heredity, consists in comparison, statistical
examinations, cultural experiments, and crossbreeding.
Evolution is the process of differentiation accompanying the operations
of nature. All the great naturalists before Darwin's time noted
facts indicating this universal differentiation, but it required the
particularly wide sweep of Darwin's mind to phrase and demonstrate it.
The law of origin by evolution, as Herbert Spencer showed, is not
confined to the method of bringing into existence new species of
animals and plants. The stars, planets, the geological strata and earth
contours and forms, human institutions, social customs, and practically
everything in nature are obedient to it.
Much research work in evolution has been done since Darwin stated his
theory, but the basic principle of the survival of the fittest remains
untouched by criticism. Some of his views respecting minor details of
selection and the effects of various factors have been modified or
enlarged, and many new evolutionary forces have been discovered. It has
also been found that a single cause is usually followed by more than
one effect.
Weissmann has drawn attention to the importance of adaptations. Most
organic beings are usually closely fitted for the conditions under
which their lives are spent.
The principal parts of every animal and plant, and all the points in
which one species differs from a nearly related species, have been
shown to have arisen on account of their usefulness to the creatures
possessing them. As natural selection is always progressive, it follows
that no adaptation is ever perfect. There is always progress from the
useful to the more useful--a continual striving for greater beauty of
form and color and higher efficiency.
Works on evolution furnish an abundance of interesting evidence showing
how adaptation works. A single instance may be cited here.
One of the Mexican yucca plants common in our Southern States is
pollinated by a moth of the Pronuba family. This moth is adapted for
its work by several special organs including a special ovipositor
and peculiar maxillary tentacles which are not found in other moths.
The female moth collects pollen with these tentacles from several
yucca flowers, rolls it into a ball and kneads it into a pellet.
When the pellet is ready the moth seeks an unvisited flower and,
after depositing a few of her own eggs in the ovary, she climbs the
style and forces the pollen pellet into the stigma. This is the way
the yucca is pollinated and fertilized. Two important purposes are
served by this arrangement: a species of plant and a species of moth,
together with those dependent upon them, are enabled to survive by this
moth's activities. There are many known cases of similar cooperative
adaptation to living conditions.
Quetelet, in 1845, followed by Francis Goltin and Karl Pearson, have
applied statistical methods in dealing with evolutionary problems,
and a new science called biometry has been developed. This science
has yielded much important data regarding the effects of inherited
characteristics.
The studies of variations in plants by mutations, made by the Dutch
botanist De Vries, have opened up wide fields of study regarding
the causes of variation. He has shown that increased bulk or better
coloration may result from improved nutrition and more light, and that
such improved characteristics may be inherited.
A law of ancestral heredity has been worked out for men by
biometricians, and this has been confirmed by the experiments of
Professor Johannsen, of Copenhagen, on self-fertilizing beans, and by
Jennings on protozoa. This hypothesis suggests that every ancestor
of a particular man or woman contributes its quota to the heritable
qualities displayed by that individual. The average amount of
resemblance between an individual and any of his particular ancestors
is capable of being numerically expressed.
The experiments and conclusions of Gregor Mendel (1822-1882) tend
to oppose the law of ancestral heredity, but it is believed that
any exceptional cases may be explained by the operation of special
conditions.
Karl Pearson has shown by the analyses of numerous statistical records
of Englishmen that by artificial selection any selected characteristic,
such as facial contour or stature, can be changed within a few
generations. But when the character has been changed about 90 per
cent within a short time another method must be employed, because the
original one then becomes less efficient.
Individuals in any given population who differ in size from the mean
of the population give rise to offspring which differs from that mean
value in the same direction but to a smaller extent. The same law
applies to the color of the hair or to intelligence or constitution.
Selection will always produce a change in the average character of a
population taken as a whole. But selection within a pure line, or one
which shows only normal variability about a mean or type value, does
not produce marked changes.
The usual selection within any particular population consists in the
partial separation of extreme types.
The personal characteristics of any ancestor do not influence his
descendants. Only the typical characteristics are handed down.
These and many other facts developed by investigations in biometry
should be of value in regulating immigration, so as to guard against
degenerative influences, and they have greatly increased the efficiency
of farming by showing how to improve farm stocks and crops so as to
yield larger returns. Farmers have been more ready than politicians to
avail of their advantages. We note how the speed of racing and trotting
horses, and the milking capacity of cows, have been improved by the
past century, but we are doing little to reform national health and
efficiency.
Mutation is the name given to the process of origination of a new
species or character accomplished by a single step or by a series of
steps.
Bateson, in 1894, showed that symmetry is a characteristic common
to all organisms. This may affect the whole or parts of an organ.
Major symmetry involves the whole organism and minor symmetry only
an organ or part. There are meristic variations, involving the
symmetrical pattern, and substantive variations involving changes in
the constitution or substance of the organism. Red-flowering plants,
for example, may yield offspring bearing white flowers. Substantive
variations are often discontinuous, or accidental, and are infrequent.
Organic bodies are built up of a number of cells. The living material
of cells is protoplasm formed out of many elements, of which carbon,
oxygen, hydrogen, nitrogen, and sulphur are the more important. New
cells arise from bipartition of existing cells. Therefore by following
back the history of any animal or plant we will arrive at a stage
when its ancestors had only one cell. Every animal or plant which is
propagated sexually actually starts as a cell and develops through its
main evolutionary changes in the embryonic state. Cells are liable to
all the evolutionary changes that the organism as a whole is subject to.
Studies of embryology have shown that the fusion of biparental
reproductive cells results in the formation of a complete new
individual which, at the time of the fusing of the two conjugating
cells, called gametes, or germ cells, inherits the characteristics of
each parent and its ancestors.
The determination of the sex of the cell, plant, or animal, depends
upon the presence of extra male or female sex-chromosomes, or
sex-determinant fibers of the cell nucleus. Certain animals and plants
transmit male characteristics to the female descendants, while the
female transmits her characteristics to the male descendants. There are
many variations of this kind. These strange movements in heredity are
explained by the laws governing chromosomes and idio-chromosomes and
elementary cells.
According to the germ plasm theory of inheritance, the separate parts
of living organisms are assumed to be represented by separate material
particles in the germ cells. In the Mendelian theory each cell is
assumed to contain a large number of _ids_, or complete sets of sex
determinants, half the total being derived from each parent. This
permits the germ cells to contain a certain number of ids from each
parent.
Studies of these subjects show that the great harmonies of the natural
world are manifested in form, number, pattern, and color, which we find
to be basically simple and, when studied systematically, they appear
quite clearly, so as to be capable of being described and expressed as
laws.
The study of the agencies under social control which may improve, or
impair, the racial qualities of future generations, either physically,
socially, or mentally, is called the science of eugenics. This new
science is another outgrowth of the revolution in intellectual
development originating with the publication of Darwin's theory. Sir
Francis Galton was the pioneer worker, and he has been followed by
Pearson, Yule, Lombroso (1836-1909), and others.
Eugenic studies, confirmed by those of genetics and biometry, show
that the human race, which is the masterpiece of Nature's evolutionary
processes, is capable of much further development through the careful
guiding of the very forces used in evolving man to his present state.
Man can be improved by selection and education to greater beauty,
clearer intellect, larger stature, sounder character, and better
physique. The measure of what man has done is a good criterion of what
he is capable of doing under the guidance and encouragement of science.
Genetics, the study of the hereditary phenomena of organisms, is
based upon the law of inheritance discovered by Mendel in 1865. This
law relates to the inheritance of certain definite characters called
allelomorphs. These characters are found to group themselves in pairs
which exhibit more or less antagonistic qualities. A knowledge of
these characteristics is necessary to conduct selective breeding
experiments scientifically. It is found that when two similar germ
cells, each bearing the same new combination of allelomorphs, meet
in fertilization, they result in the development of a new zygotic
combination of a pure type which breeds true. This accounts for the
establishment of new species. When, on the other hand, the coupling
is unequal, or only partial, there will be irregularities in the
characters of the offspring and no new species is likely to develop.
Immense value is attached to this law by naturalists working in all
fields. The three new sciences of eugenics, genetics, and biometry have
prepared the way for a regeneration of humanity through breeding in the
desirable and breeding out the undesirable.