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The Predicament of Evolution
by George McCready Price (1870-1963)
This was ©1925 by Southern Publishing Assoc.
Chapter Two -
Heredity and Variation
TWO ideas that are very intimately connected
with any theory of organic development, are heredity and variation. Heredity is
shown in all the various ways in which an animal or a plant is like its parent.
Variation is illustrated in the ways in which it is unlike its parents or its
ancestors. The two ideas are antagonistic; if variation had full sway there
would be no stability of type; if heredity only prevailed there could be no
evolution. In Darwin's day very little was known about either of these
principles; but this ignorance of the real facts permitted Darwin to assume
almost anything he wished regarding variation. Within modern times Mendelism has
taught us many exact facts regarding heredity, with the result that, as Edwin
Grant Conklin says, "At present it is practically certain that there is no other
kind of inheritance than Mendelian."—"Heredity and Environment," p. 99.
This leaves a very slim chance for variation in the Darwinian sense to affect
the offspring, so that, as D. H. Scott says, "it is clear that we know
astonishingly little about variation."
Mendelism is the term which embraces pretty
much all we know about heredity and variation. Gregor Mendel (1822-1884) did his
work during the third quarter of the nineteenth century, working chiefly with
the common garden pea (Pisum sativum). Charles Darwin was then living,
but neither he nor any one else seemed to give much attention to the queer
experiments in breeding which were being so patiently and accurately carried on
by the obscure monk of Brunn, Austria. Mendel used to say, "Meine Zeit wird
schon kommen," ("My time will yet come"); but he had been dead some sixteen
years before the wonderful facts that he had discovered were brought to the
attention of the scientific world. Since then these facts and principles have
worked a complete revolution in biology.
The Discoveries of Mendel
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Bateson has told us that: "Had Mendel's
work come into the hands of Darwin, it is not too much to say that the
history of the development of evolutionary philosophy would have been very
different from that which we have witnessed." What the difference would have
been, I shall leave the reader to decide after reading the remainder of this
chapter.
Mendel differed in his methods from all
previous students of heredity in that he concentrated his attention each
time upon some one pair of contrasted characters, giving no attention
to the other characters which were present. In this way he arrived at the
great truth that all the various characters of the organism are
separately transmitted in heredity. For example, when he crossed a tall
pea with a dwarf, he found that all the first hybrid generation were always
talls, with no dwarfs and no intermediates.
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dominant, and the dwarf character recessive; and a pair of
contrasted characters that act in this way are now called unit
characters. The hereditary principle that is back of this behavior, as
the cause of the dominance or the recessiveness, is termed a factor;
and these factors are now thought to be carried along from one generation to
another by the chromosomes of the cell nucleus. But this matter will
come up again later.
But when Mendel allowed these hybrid talls
to pollinate and produce seeds in the usual way, he found that in the next
hybrid generation he always got three talls to one dwarf out of every
four. By carrying the experiment further, it was proved that these
dwarfs of the second hybrid generation always bred true ever afterwards,
proving to be just as purely dwarfs as if they had been bred from a thousand
generations of pure dwarf stock.
One out of the three talls also was always
found to be pure bred for tallness, always coming true, thus making another
quarter of the total. The remaining fifty per cent, which were talls, proved
to be mixed, always acting like the first hybrids, splitting up in the next
generation with the same mathematical regularity.
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The Thunder of Facts
These experiments have been verified
repeatedly in all parts of the world. Thousands of such unit characters of size,
form, color, etc., have been separated out as pure dominants or pure recessives,
until it is now generally recognized that there is no other kind of inheritance
than the Mendelian.
The diagram at the bottom of the page
(below) illustrates these principles in the case of the tall and the dwarf
peas.
Among the most extensive and careful
experiments along this line are those by Thomas Hunt Morgan and his associates
at Columbia University, Their work has been chiefly with the fruit fly (Drosophila)
and related types; and it has been carried on now for over ten years.
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During this time over two hundred new
types of this fly have been produced, each with a definite pedigree, and
each capable of being again produced at will by the same combination of
parents. Every portion of the fly has been affected by one or another of
these changes. The wings have been shortened or greatly changed in shape, or
eliminated entirely. A number of different colors of the eye have been
produced, even totally blind types having been developed. And each of these
changes or mutations has been produced, not gradually, as the
Darwinians would have supposed, but at a single step.
Darwin's Armchair Theories
One cannot fail to appreciate the
sarcastic references that Morgan makes to the armchair theories of the
Darwinians, which have so long and so harmfully dominated all biological
studies.
"Formerly," says Morgan, "we were told
that eyeless animals arose in caves. This case shows that they may also
arise suddenly in glass milk bottles, by a change in a single factor. . .
. We used to be told that wingless insects occurred on desert islands
because those insects that had the best developed wings had been blown out
to sea. Whether this is true or not, I will not pretend to say; but at any
rate wingless insects may also arise, not through a slow process of
elimination, but at a single step."—"A Critique of the Theory of
Evolution" (1916), p. 67.
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regarding those parts of the ovum and the sperm that have now been proved to
be the carriers of the hereditary characters. These carriers of heredity are
the chromosomes, small threadlike portions of the nucleus of the cell
that can be watched under the microscope during the various processes
through which the cell passes.
All the higher forms of life invariably
arise from a single fertilized ovum, this ovum being thus a blending of two
cells, the male and the female. Before fertilization, both the sperm and the
ovum undergo some complicated changes which need not be described here, but
which result in the original number of the chromosomes being reduced in
number to exactly half the original number for the particular species
represented. This half number of the chromosomes is given as 7 in the garden
pea; in corn 10; in the mouse 20; in the tomato 12 ; in wheat 8; and in man
"probably 24" (Morgan). Every cell in one of these species always carries
the same number of chromosomes.
Nothing New Evolved |
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Reduction is thus a preparation for the
union of the two cells; and by this union, or fertilization, the original
number of chromosomes is restored, the sperm and the ovum each having the half
or reduced number.
In the examples of hybridization mentioned
above, only one pair of contrasted characters was dealt with. What would happen
if two pairs of such unit characters are combined?
It has been found that when a kind with two
dominants is crossed with one possessing two recessives, the results become
more complicated. For out of every sixteen hybrids thus produced, nine will show
both dominant characters, one will show both recessives, while the remaining six
specimens will show two distinctly new types, three of one and three of
another.
| For example, if we cross a tall yellow
pea with a dwarf green pea, the first hybrid generation will be all tall
yellows; for both tallness and yellowness are dominant. But in the second
hybrid generation, out of every sixteen plants, we get nine tall yellows,
one dwarf green, with three dwarf yellows, and three tall greens.
These last two kinds are wholly new forms, which are thus called mutants.
Many other and even more extraordinary mutants have been produced among both
plants and animals. |
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When such mutants were first produced they
were hailed as "elementary species," on the supposition that in some such way
strictly new species might be produced. But further study of the matter has
shown that all these new types can by back-crossing be bred back to the original
kinds. Hence in Mendelian breeding we are evidently only marking time, only
working around in a circle, much the same as the chemist does in his laboratory
by mixing compounds. The latter certainly never hopes to get new elements
that he did not have in his original mixtures.
Accordingly, where is there any organic
evolution in all this?
Acquired Characters Not Transmitted
Obviously there is no room for absolutely new
characters to be shown in the offspring, unless we may suppose that some
external effect could become registered in one or more of the chromosomes of
either the sperm or the ovum. Unfortunately, there is no known means by which
this could be imagined to take place.
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One of the chief difficulties in this
connection is that the reproductive cells apparently are not in any way
affected by what may happen to the body cells, or to the body as a whole. In
all the sexually reproduced animals, the reproductive cells constitute a
class apart, a sort of cellular aristocracy, which take no part in the
metabolism or other functions of the body, and hence are not in any way
affected by what may happen to the body cells in the way of use or disuse,
or in the way of effects brought about by the environment. It is on this
account that acquired characters are not transmitted in heredity,
because no experiences that the soma, or the body, passes through can
become registered in the germ cells.
We now know that the variations
wherein one of the offspring differs from its parents always come under the
one or the other of two very distinct classes.
1. Fluctuations. These are
sometimes called continuous variations, and are produced by whatever affects
the body organism, such as variations in the food or the surroundings. But
these fluctuations are not capable of being transmitted to the offspring.
2. Mutations. These may be
large or small in degree; but they are not produced by the surroundings.
They have been inherited from the one or the other of the parents; and in
turn they will always be passed along to the succeeding generation, either
as dominants or recessives. |
But where are we now, in the light of all these modern
discoveries in genetics, or the science of breeding?
This is a large question, and can best be considered in
another c
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