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
||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.
|Accordingly, he called the tall character 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.
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.
||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.
|Many remarkable things have been learned 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
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
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.||
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.
||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 chapter.
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