CONTINUOUS AND DISCONTINUOUS VARIATION AND HEREDITY
The two terms _continuous_ and _discontinuous variation_ refer to the
succession or inheritance of the variations rather than to the actual
conditions amongst a group of individuals living at the same time; but
this distinction has only a subordinate value. The term _fluctuating_,
or _individual variation_, expresses more nearly the conditions of the
individuals of a species at any one time, and the continuation of this
sort of difference is the continuous variation spoken of above. The
discontinuous variations are probably of the same nature as those that
have been called mutations, and what Darwin sometimes called sports, or
single variations, or definite variations.
Continuous Variation
If we examine a number of individuals of the same species, we find that
no two of them are exactly alike in all particulars. If, however, we
arrange them according to some one character, for example, according to
the height, we find that there is a gradation more or less perfect from
one end of the series to the other. Thus, if we were to take at random a
hundred men, and stand them in line arranged according to their height,
the tops of their heads, if joined, would form a nearly continuous line;
the line will, of course, incline downward from the tallest to the
shortest man. This illustrates individual variation. An arrangement of
this kind fails to bring out one of the most important facts connected
with individual differences. If the line is more carefully examined, it
will be found that somewhere near the middle the men are much more
nearly of the same height, or rather there are more men having about the
same height than there are near the ends of the line. Another
arrangement will bring this out better. If we stand in a line all the
men from 60 to 61.9 inches, and in another parallel line all those
between 62 and 63.9, then those between 64 and 65.9, then between 66 and
67.9 inches in__ height, etc., it will be found that there are more men
in some of these lines than in others. The longest line will be that
containing the men of about 65 inches; the two lines formed out of men
on each side of this one will contain somewhat fewer men, and the next
ones fewer still, and so on. If we looked at our new group of men from
above, we should have a figure triangular in outline, the so-called
frequency polygon, Figure 3 B. With a larger amount of data of this sort
it is possible to construct a curve, the curve of frequency, Figure 3 A.
In order to obtain this curve of frequency, it is of course not
necessary to actually put the individuals in line, but the curve can be
drawn on paper from the measurements. We sort out the measurements into
classes as in the case given above. The classes are laid off at regular
intervals along a base-line by placing points at definite intervals.
Perpendiculars are then erected at each point, the height of each being
proportional to the frequency with which each class occurs. If now we
join the tops of these perpendiculars, the curve of frequency is the
result.
[Illustration:
Fig. 3.—Curves of frequency, etc.
A, normal curve.
B, showing the method of arranging individuals in lines containing
similar kinds of individuals.
C, curve that is skew to the right.
D, polygon of frequencies of horns of rhinoceros beetles.
(After Davenport.)]
“In arranging the individuals it will be found, as has been said, that
certain groups contain more individuals. They will form the longest
line. This value that occurs with the greatest frequency is called the
mode. The position of this modal class in the polygon is one of the
points of importance, and the spread of the polygon at its base is
another. A polygon with a low mode and a broad range means great
variability. The range may, however, be much affected by a single
individual standing far removed from the rest, so that a polygon
containing such an individual might appear to show greater variation
than really exists. Therefore we need a measure of variability that
shall take into account the departures of all the individuals from the
mode. One such measure is the arithmetical average of all the departures
from the mean in both directions; and this measure has been widely
employed. At present another method is preferred, namely, the square
root of the squared departures. This measure is called the standard
deviation. The standard deviation is of great importance, because it is
the index of variability.”[21]
Footnote 21:
Davenport, C. B., “The Statistical Study of Biological Problems,”
_Popular Science Monthly_, September, 1900.
Of the different kinds of polygons there are two main sorts, the simple
and the complex. The former have only a single mode, the latter have
more than one mode. Some simple polygons lie symmetrically on each side
of the mode, Figure 3 A; others are unsymmetrical or skew, Figure 3 B.
The skew polygon generally extends out on one side farther than on the
other. It has been suggested that when a polygon is symmetrical the
species is not changing, and when skew that the species is evolving in
the direction of the longer base. This assumes that the sort of
variation measured by these curves is of the kind of which evolution is
made up, but this is a question that we must further consider. How far
the change indicated by the skew curve may be carried is also another
point for further examination.
A complex polygon of variation, Figure 3 D, has been sometimes
interpreted to mean that two subgroups exist in a species, as is well
shown in the case of the rhinoceros beetle described by Bateson. Two
kinds of male individuals exist, some with long horns, others with short
horns; each with a mode of its own, the two polygons overlapping. Other
complex polygons may be due to changes occurring at different times in
the life of the individual, as old age, for example.
If, instead of examining the variations of the individuals of the race,
we study the variations in the different organs of the same individual,
we find in many cases that certain organs vary together. Thus the right
and the left leg nearly always vary in the same direction, also the
first joints of the index and middle fingers, and the stature and the
forearm. On the other hand, the length of the clavicle and that of the
humerus do not vary together to the same extent; and the breadth and
height of the skull even less so.
════════════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════
No. of Veins│ 10│ 11│ 12│ 13│ 14│ 15│ 16│ 17│ 18│ 19│ 20│ 21│ 22
────────────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────
First Tree │ —│ —│ —│ —│ —│ 1│ 4│ 7│ 9│ 4│ 1│ —│ —
Second Tree │ —│ —│ —│ 3│ 4│ 9│ 8│ 2│ —│ —│ —│ —│ —
════════════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════
We may also study those cases in which a particular organ is repeated a
number of times in the same individual, as are the leaves of a tree. If
the leaves of the same tree are examined in respect, for example, to the
number of veins that each contains, we find that the number varies, and
that the results give a variation polygon exactly like that when
different individuals are compared with one another. Let us take the
illustration given by Pearson. He counted the veins on each side of the
midrib of the leaves of the beech. If a number of leaves be collected
from one tree, and the same number from another, and if all those having
fifteen veins are put in one vertical column, and all those with sixteen
in another, as shown in the following table, it will be found that each
tree has a mode of its own. Thus in the first tree the mode is
represented by nine individuals having eighteen veins, and in the second
by nine individuals having fifteen veins. So far as this character is
concerned we might have interchanged certain of the individual leaves,
but we could not have interchanged the two series. They are _individual_
to the two trees. Now in what does this individuality consist? Clearly
there are most leaves in one tree with eighteen ribs, and most in the
other with fifteen ribs.
If we contrast these results with those obtained by picking at random a
large number of leaves from different beech trees, we have no longer
types of individuals, but racial characters. Pearson has given the
following table to illustrate these points:
Frequency of Different Types of Beech Leaves
════════════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════
No. of Veins│ 10│ 11│ 12│ 13│ 14│ 15│ 16│ 17│ 18│ 19│ 20│ 21│ 22
────────────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────
Frequency │ 1│ 7│ 34│ 110│ 318│ 479│ 595│ 516│ 307│ 181│ 36│ 15│ 1
════════════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════
Thus the mode for beech trees in general is sixteen; but, as shown in
the other table, this mode does not correspond with either of the two
individual modes here ascertained. The illustration shows that the
racial mode may differ from the individual mode. There are also cases
known in which the mode of a group of individuals living in one locality
is different from that of another group living in another locality. This
difference may be a constant one from year to year, although so slight,
that unless actual measurements are made, the difference cannot be
detected, because of the overlapping of the individuals from different
localities. If evolution took place by slow changes of this sort, it
might be possible to detect its action, even when very slow, by means of
measurements made on a large number of individuals. At least this has
been suggested by those who believe new species may result from changes
of this sort.
There is some evidence showing that by selecting particular individuals
of a series, and breeding from them, the mode may be changed in the
direction of selection. Thus it has been stated by Davenport that the
descendants of twelve- and thirteen-rayed daisies give a polygon with a
skewness of +1.92; while the descendants of twenty-one-rayed plants give
a polygon with a skewness of -.13.
Pearson has described very concisely the possibilities involved in the
selective action of the environment. He states that if we examine the
frequency distribution of a set of organisms that have just become
mature, and later make a similar examination on the same number of
individuals (but not the same individuals) during the period of
reproduction, we shall probably find that a change has taken place which
may have been due to selection of some sort. The same thing might be
found in the next generation, and, if it did, this would indicate that
“selection does not necessarily mean a permanent or a progressive
change.” The selection in this imaginary case would be purely periodic
and suffice only to maintain a given race under given conditions. “Each
new adolescent generation is not the product of the entire preceding
generation, but only of selected individuals. This is certainly the case
for civilized man, in which case twenty-six per cent of the married
population produce fifty per cent of the next generation.”
Pearson believes that “if a race has been long under the same
environment it is probable that only periodic selection is at work,
maintaining its stability. Change the environment and a secular change
takes place, the deviations from the mode previously destroyed giving
the requisite material.” “Clearly periods of rapidly changing
environment, of great climatological and geological change, are likely
to be associated with most marked secular selection. To show that there
is little or no change year by year in the types of rabbit and wild
poppy in our English fields, or of daphnia in our English ponds, is to
put forward no great argument for the inefficiency of natural selection.
Take the rabbit to Australia, the wild poppy to the Cape, the daphnia
into the laboratory, and change their temperature, their food supply,
and the chemical constituents of water and air, and then the existence
of no secular selection would indeed be a valid argument against the
Darwinian theory of evolution.” In regard to the last point, it should
be noted that, even if under the changed conditions a change in the mode
took place, as Pearson assumes, it does not follow necessarily that
selection has had anything to do with it, but the environment may have
directly changed the forms. Furthermore, and this is the essential
point, even if selection does act to the extent of changing the mode, we
should not be justified in concluding that this sort of change could go
on increasing as long as the selection lasts. All that might happen
would be to keep the species up to the highest point to which
fluctuating variation can be held. This need not lead to the formation
of new species, or direct the course of evolution.
Pearson points out further that, even if we suppose that a secular
change is produced in a new environment, we cannot explain how species
may break up into two or more races that are relatively infertile.
Suppose two groups of individuals, subjected to different environments,
become isolated geographically. Two local races will be produced.
“Isolation may account for the origin of local races, but never for the
origin of species unless it is accompanied by a differential fertility.”
In other words, Pearson thinks that, unless the reproductive organs are
correlated with other organs, in such a way that as these organs change
the interracial fertility of the germ-cells is altered, so that in the
two changed groups the individuals are no longer interfertile, new
species cannot be accounted for, since their mutual infertility is one
of their most characteristic features. “Without a barrier to
intercrossing during differentiation the origin of species seems
inexplicable.”
We need not discuss the various suggestions that have been made to
explain this difficulty, none of which, as Pearson points out, have been
satisfactory. He himself believes that a process of segregation of like
individuals must occur, during the incipient stages at least, in the
formation of species. Afterwards a correlation may exist between the new
organs and the germ-cells, of such a sort that a relative or an absolute
sterility between the incipient species is attained. After this
condition has been reached the two new species may freely intermix
without a return to the primitive type, since they are no longer fertile
_inter se_. It seems to me, also, that this would be an essential
requisite if we assume that species are slowly formed out of races from
individual differences, as Pearson supposes to be the case. There are,
however, other possibilities that Pearson does not take into account,
namely, that from the very beginning the change may be so great that the
new form is not fertile with the original one; and there is also another
possibility as well, that, although the new and the old forms are
fertile, the hybrids may be like one or the other parent, as in several
cases to be given later. Not that I mean to say that in either of these
two ways can we really offer a solution of the question of infertility,
for, from the evidence that we possess, it appears improbable that the
infertility of species _inter se_ has been the outcome of either of
these causes.
In support of his main thesis Pearson gives certain data in respect to
preferential mating in the human race. By this is meant that selection
of certain types of individuals is more likely to take place, and also
that the fertility of certain types of individuals is greater than that
of other types. The calculations are based on stature, color of hair,
and of eyes. The results appear to show in all cases examined that there
is a slight tendency to form new races as the result of the more
frequent selection of certain kinds of individuals. But even if this is
the case, what more do the results show than that local races may be
formed,—races having a certain mode for height, for color of eyes or of
hair? That changes of this kind can be brought about we knew already
without any elaborate measurements, yet we should not conclude from this
that new species will be formed by a continuation of the process.
Pearson writes: “As to the problem of evolution itself we are learning
to see it under a new light. Natural selection, combined with sexual
selection [by which Pearson means segregation of certain types through
individual selection] and heredity, is actually at work changing types.
We have quantitative evidence of its effects in many directions.” Yes!
but no evidence that selection of this sort can do anything more than
keep up the type to the upper limit attained in each generation by
fluctuating variations. Pearson adds, “Variations do not occur
accidentally, or in isolated instances; autogamic and assortive mating
are realities, and the problem of the near future is not whether
Darwinism is a reality, but what is quantitively the rate at which it is
working and has worked.” This statement expresses no more than Pearson’s
conviction that the process of evolution has taken place by means of
selection. He ignores other possibilities, which if established may put
the whole question in a very different light.
Heredity and Continuous Variation
It has been to a certain extent assumed in the preceding pages that both
parents are alike, or, if different, that they have an equal influence
on the offspring. This may be true in many cases for certain
characteristics. Thus a son from a tall father and a short mother may be
intermediate in height, or if the father is white and the mother black,
the children are mulattoes. But other characters rarely or never blend.
In such cases the offspring is more like one or the other parent, in
which case the inheritance is said to be exclusive. Thus if one parent
has blue eyes and the other black, some of the children may have black
eyes and others blue. There are also cases of particular inheritance
where there may be patches of color, some like the color of one parent,
some like that of the other parent. The latter two kinds of inheritance
will be more especially considered in the subsequent part of this
chapter; for the present we are here chiefly concerned with blended
characters.
How much in such cases does each parent contribute to the offspring?
This has been expressed by Galton in his law of ancestral heredity. This
law takes into account not only the two parents, but also the four
grandparents, and the eight great-grandparents, etc. There will be 1024
in the tenth generation. These 1024 individuals may be taken as a fair
sample of the general population, provided there has not been much
interbreeding. Are we then to look upon the individual as the fused or
blended product of the population a few generations back? If this were
true, should we not expect to find all the individuals of a community
very much alike, except for the fluctuating variations close around the
mode?
As a result of his studies on the stature of man, and on the coat color
of the Basset hounds, Galton has shown that the inheritance from the
parents can be represented by the fraction 1/2; that is one-half of the
peculiarities of the individual comes from the two parents. The four
grandparents together count for 1/4 of the total inheritance, the
great-grandparents 1/8, and so on, giving the series 1/2, 1/4, 1/8.
Pearson, taking certain other points into consideration, believes the
following series more fully represents the inheritance from the
ancestors, .3, .15, .075, .0375, etc. He concludes that, “if Darwinism
be the true view of evolution, _i.e._ if we are to describe evolution by
natural selection combined with heredity, then the law which gives us
definitely and concisely the type of the offspring in terms of the
ancestral peculiarities is at once the foundation stone of biology and
the basis upon which heredity becomes an exact branch of science.”
The preceding statements give some idea of what would occur in a
community in which no selection was taking place. The results will be
quite different, although the same general law of inheritance will hold,
if selection takes place in each generation. If, for instance, selection
takes place, the offspring after four generations will have .93 of the
selected character, and without further selection will not regress, but
breed true to this type.[22] “After six generations of selection the
offspring will, selection being suspended, breed true to under two per
cent divergence from the previously selected type.”
Footnote 22:
In this statement the earlier ancestors are assumed to be identical
with the general type of the population.
If, however, we do not assume that the ancestors were mediocre, it is
found that after six generations of selection the offspring will breed
true to the selected type within one per cent of its value. Thus, if
selection were to act on a race of men having a mode of 5 feet 9 inches,
and the 6-foot men were selected in each generation, then in six
generations this type would be permanently established, and this change
could be effected in two hundred years.[23]
Footnote 23:
Quoted from Pearson’s “Grammar of Science.”
Thus we have exact data as to what will happen on the average when
blended, fluctuating variations are selected. Important as such data
must always be to give us accurate information as to what will occur if
things are left to “chance” variations, yet if it should prove true that
evolution has not been the outcome of chance, then the method is
entirely useless to determine how evolution has occurred.
More important than a knowledge of what, according to the theory of
chances, fluctuating variations will do, will be information that would
tell us what changes will take place in each individual. In this field
we may hope to obtain data no less quantitative than those of chance
variations, but of a different kind. A study of some of the results of
discontinuous variation will show my meaning more clearly.
Discontinuous Variation
Galton, in his book on “Natural Inheritance,” points out that “the
theory of natural selection might dispense with a restriction for which
it is difficult to see either the need or the justification, namely,
that the course of evolution always proceeds by steps that are severally
minute and that become effective only through accumulation.” An apparent
reason, it is suggested, for this common belief “is founded on the fact
that whenever search is made for intermediate forms between widely
divergent varieties, whether they are of plants or of animals, of
weapons or utensils, of customs, religion, or language, or of any other
product of evolution, a long and orderly series can usually be made out,
each member of which differs in an almost imperceptible degree from the
adjacent specimens. But it does not at all follow because these
intermediate forms have been found to exist, that they were the very
stages that were passed through in the course of evolution. Counter
evidence exists in abundance, not only of the appearance of considerable
sports, but of their remarkable stability in hereditary transmission.”
Comparing such an apparently continuous series with machines, Galton
concludes, “If, however, all the variations of any machine that had ever
been invented were selected and arranged in a museum, each would differ
so little from its neighbors as to suggest the fallacious inference that
the successive inventions of that machine had progressed by means of a
very large number of hardly discernible steps.”
Bateson, also, in his “Materials for the Study of Variation,” speaks of
the two possible ways in which variations may arise. He points out that
it has been tacitly assumed that the transitions have been continuous,
and that this assumption has introduced many gratuitous difficulties.
Chief of these is the difficulty that in their initial and imperfect
stages many variations would be useless. “Of the objections that have
been brought against the Theory of Natural Selection, this is by far the
most serious.” He continues: “The same objection may be expressed in a
form which is more correct and comprehensive. We have seen that the
differences between species on the whole are Specific, and are
differences of kind forming a discontinuous Series, while the
diversities of environment to which they are subject are, on the whole,
differences of degree, and form a continuous Series; it is, therefore,
hard to see how the environmental differences can thus be made in any
sense the directing cause of Specific differences, which by the Theory
of Natural Selection they should be. This objection of course includes
that of the utility of minimal Variations.”
“Now the strength of this objection lies wholly in the supposed
continuity of the process of Variation. We see all organized nature
arranged in a discontinuous series of groups differing from each other
by differences which are Specific; on the other hand, we see the diverse
environments to which these forms are subject passing insensibly into
each other. We must admit, then, that if the steps by which the diverse
forms of life have varied from each other have been insensible,—if, in
fact, the forms ever made up a continuous series,—these forms cannot
have been broken into a discontinuous series of groups by a continuous
environment, whether acting directly as Lamarck would have, or as
selective agent as Darwin would have. This supposition has been
generally made and admitted, but in the absence of evidence as to
Variation it is nevertheless a gratuitous assumption, and, as a matter
of fact, when the evidence as to Variation is studied, it will be found
to be in a great measure unfounded.”
There is a fair number of cases on record in which discontinuous
variations have been seen to take place. Darwin himself has given a
number of excellent examples, and Bateson, in the volume referred to
above, has brought together a large and valuable collection of facts of
this kind.
Some of the most remarkable of these instances have been already
referred to and need only be mentioned here. The black-shouldered
peacock, the ancon ram, the turnspit dog, the merino sheep, tailless and
hornless animals, are all cases in point. In several of these it has
been discovered that the young inherit the peculiarities of their
parents if the new variations are bred together; and what is more
striking, if the new variation is crossed with the parent form, the
young are like one or the other parent, and not intermediate in
character. This latter point raises a question of fundamental importance
in connection with the origin of species.
Darwin states that he knows of _no cases in which, when different
species or even strongly marked varieties are crossed, the hybrids are
like one form or the other_. They show, he believes, always a blending
of the peculiarities of the two parents. He then makes the following
significant statement: “All the characters above enumerated which are
transmitted in a perfect state to some of the offspring and not to
others—such as distinct colors, nakedness of skin, smoothness of leaves,
absence of horns or tail, additional toes, pelorism, dwarfed structure,
etc., have all been known to appear suddenly in individual animals or
plants. From this fact, and from the several slight, aggregated
differences which distinguish domestic races and species from each
other, not being liable to this peculiar form of transmission, we may
conclude that it is in some way connected with the sudden appearance of
the characters in question.”
Darwin has, incidentally, raised here a question of the most
far-reaching import. If it should prove true, as he believes, that
inheritance of this kind of discontinuous variation is also
discontinuous, and that we do not get the same result when distinct
species are intercrossed, or even when well-marked domestic races are
interbred, then he has, indeed, placed a great obstacle in the path of
those who have tried to show that new species have arisen through
discontinuous variation of this sort.
If wild species, when crossed, give almost invariably intermediate
forms, then it may appear that we are going against the only evidence
that we can hope to obtain if we claim that discontinuous variation, of
the kind that sports are made of, has supplied the material for
evolution. If, furthermore, when distinct races of domesticated animals
are crossed, we do not get discontinuous inheritance, it might, perhaps,
with justness be claimed that this instance is paralleled by what takes
place when wild species are crossed. And if domesticated forms have been
largely the result of the selection of fluctuating variations, as Darwin
believes, then a strong case is apparently made out in favor of Darwin’s
view that continuous variation has given the material for the process of
evolution in nature. Whether selection or some other factor has directed
the formation of the new species would not, of course, be shown, nor
would it make any difference in the present connection.
Before we attempt to reach a conclusion on this point let us analyze the
facts somewhat more closely.
In the first place, a number of these cases of discontinuous variation
are of the nature of abnormalities. The appearance of extra fingers or
toes in man and other mammals is an example of this sort. This
abnormality is, if inherited at all, inherited completely; that is, if
present the extra digit is perfect, and never appears in an intermediate
condition, even when one of the parents was without it. The most obvious
interpretation of this fact is that when the material out of which the
fingers are to develop is divided up, or separated into its component
parts, one more part than usual is laid down. Similarly, when a flower
belonging to the triradiate type gives rise to a quadriradiate form,—as
sometimes occurs,—the new variation seems to depend simply on the
material being subdivided once more than usual; perhaps because a little
more of it is present, or because it has a somewhat different shape. My
reasons for making a surmise of this sort are based on certain
experimental facts in connection with the regeneration of animals. It
has been shown in several cases that it is possible to produce more than
the normal number of parts by simply dividing the material so that each
part becomes more or less a new whole, and the total number of parts
into which the material becomes subdivided is increased. It seems not
improbable that phenomena of this sort have occurred in the course of
evolution, although it is, of course, possible that those characters
that define species do not belong to this class of variation. To take an
example. There are nine neck-vertebræ in some birds, but in the swan the
number is twenty-five. We cannot suppose that the ancestor of the swan
gradually added enough materially to make up one new vertebra and then
another, but at least one new whole vertebra was added at a time; and we
know several cases in which the number of vertebræ in the neck has
suddenly been increased by the addition of one more than normal, and the
new vertebra is perfectly formed from the first.
In cases of this sort we can easily understand that the inheritance must
be either of one kind or the other, since intermediate conditions are
impossible, when it comes to the question of one or not one; but if one
individual had one and another six vertebræ, then it would be
theoretically possible for the hybrid to have three.
This brings us to a question that should have been spoken of before in
regard to the inheritance of discontinuous variation. It sometimes
occurs that a variation, which appears in other respects to be
discontinuous, is inherited in a blended form. Thus the two kinds of
variation may not always be so sharply separated as one might be led to
believe. There may be two different kinds of discontinuous variation in
respect to inheritance, or there may be variations that are only to a
greater or a less extent inherited discontinuously; and it seems not
improbable that both kinds occur.
This diversion may not appear to have brought us any nearer to the
solution of the difficulty that Darwin’s statement has emphasized,
except in so far as it may show that the lines are not so sharply drawn
as may have seemed to be the case. The solution of the difficulty is, I
believe, as follows:—
_The discontinuity referred to by Darwin relates to cases in which only
a single step (or mutation) has been taken, and it is a question of
inheritance of one or not one. If, however, six successive steps should
be taken in the same direction, then when such a form is crossed with
the original form, the hybrid may inherit only three of the steps and
stand exactly midway between the parent forms; or it may inherit four,
or five, or three, or two steps and stand correspondingly nearer to the
one_ _or to the other parent. Thus while it may not be possible to halve
a single step (hence one-sided inheritance), yet when more than one step
has been taken the inheritance may be divided. There is every evidence
that most of the Linnæan (wild) species that Darwin refers to have
diverged from the parent form, and from each other, by a number of
successive steps; hence on crossing, the hybrid often stands somewhere
between the two parent forms. On this basis not only can we meet
Darwin’s objection, but the point of view gives an interesting insight
into the problem of inheritance and the formation of species._
The whole question of inheritance has assumed a new aspect; first on
account of the work of De Vries in regard to the appearance of
discontinuous variation in plants; and secondly, on account of the
remarkable discoveries of Gregor Mendel as to the laws of inheritance of
discontinuous variations. Mendel’s work, although done in 1865, was long
neglected, and its importance has only been appreciated in the last few
years. We shall take up Mendel’s work first, and then that of De Vries.
Mendel’s Law[24]
Footnote 24:
Bateson, in his book on “Mendel’s Principles of Heredity,” has given
an admirable presentation of Mendel’s results. I have relied largely
on this in my account.
The importance of Mendel’s results and their wide application is
apparent from the results in recent years of De Vries, Correns,
Tschermak, Bateson, Castle, and others. Mendel carried out his
experiments on the pea, _Pisum sativum_. Twenty-two varieties were used,
which had been proven by experiment to be pure breeds. When crossed they
gave perfectly fertile offspring. Whether they all have the value of
varieties of a single species, or are different subspecies, or even
independent species, is of little consequence so far as Mendel’s
experiments are concerned. The flower of the pea is especially suitable
for experiments of this kind. It cannot be accidentally fertilized by
foreign pollen, because the reproductive organs are inclosed in the keel
of the flower, and, as a rule, the anthers burst and cover the stigma of
the same flower with its own pollen before the flower opens. In order to
cross-fertilize the plants it is necessary to open the young buds before
the anthers are mature and carefully remove all the anthers. Foreign
pollen may be then, or later, introduced.
The principle involved in Mendel’s law may be first stated in a
theoretical case, from which a certain complication that appears in the
actual results may be removed.
If _A_ represent a variety having a certain character, and _B_ another
variety in which the same character is different, let us say in color,
and if these two individuals, one of each kind, are crossed, the hybrid
may be represented by _H_. If a number of these hybrids are bred
together, their descendants will be of three kinds; some will be like
the grandparent, _A_, in regard to the special character that we are
following, some will be like the other grandparent, _B_, and others will
be like the hybrid parent, _H_. Moreover, there will be twice as many
with the character _H_, as with _A_, or with _B_.
A B
↘ ↙
H
↙|↘
A | B
↙ | ↘
A | B
H
↙|↘
A | B
↙ | ↘
A | B
H
↙|↘
A | B
H
If now we proceed to let these _A_’s breed together, it will be found
that their descendants are all _A_, forever. If the _B_’s are bred
together they produce only _B_’s. But when the _H_’s are bred together
they give rise to _H_’s, _A_’s, and _B_’s, as shown in the accompanying
diagram. In each generation, the _A_’s will also breed true, the _B_’s
true, but the _H_’s will give rise to the three kinds again, and always
in the same proportion.
Thus it is seen that the hybrid individuals continue to give off the
pure original forms, in regard to the special character under
consideration. The numerical relation between the numbers is also a
striking fact. Its explanation is, however, quite simple, and will be
given later.
In the actual experiment the results appear somewhat more complicated
because the hybrid cannot be distinguished from one of the original
parents, but the results really conform exactly to the imaginary case
given above. The accompanying diagram will make clearer the account that
follows.
A B
↘ ↙
↘ ↙
A(B)
↙ | ↘
↙ | ↘
A | B
|
A(B)
↙ | ↘
↙ | ↘
A | B
|
A(B)
↙ | ↘
↙ | ↘
A | B
|
A(B)
The hybrid, _A_(_B_), produced by crossing _A_ and _B_ is like _A_ so
far as the special character that we will consider is concerned. In
reality the character that _A_ stands for is only dominant, that is, it
has been inherited discontinuously, while the other character,
represented by _B_, is latent, or recessive as Mendel calls it.
Therefore, in the table, it is included in parentheses. If the hybrids,
represented by this form _A_(_B_), are bred together, there are produced
two kinds of individuals, _A_’s and _B_’s, of which there are three
times as many _A_’s as _B_’s. It has been found, however, that some of
these _A_’s are pure forms, as indicated by the _A_ on the left in our
table, while the others, as shown by their subsequent history, are
hybrids, _A_(_B_). There are also twice as many of these _A_(_B_)’s as
of the pure _A_’s (or of the _B_’s). Thus the results are really the
same as in our imaginary case, only obscured by the fact that the _A_’s
and the _A_(_B_)’s are exactly alike to us in respect to the character
chosen. We see also why there appear to be three times as many _A_’s as
_B_’s. In reality the results are 1 _A_, 2 _A_(_B_), 1 _B_.
In subsequent generations the results are the same as in this one, the
_A_’s giving rise only to _A_, the _B_’s to _B_, and the _A_(_B_)’s
continuing to split up into the three forms, as shown in our diagram.
Mendel found the same law to hold for all the characters he examined,
including such different ones as the form of the seed, color of
seed-albumen, coloring of seed-coat, form of the ripe pods, position of
flowers, and length of stem.
Mendel also carried out a series of experiments in which several
differentiating characters are associated. In the first experiment the
parental plants (varieties) differed in the form of the seed and in the
color of the albumen. The two characters of the seed plant are
designated by the capital letters _A_ and _B_; and of the pollen plant
by small _a_ and _b_. The hybrids will be, of course, combinations of
these, although only certain characters may dominate. Thus in the
experiments, the parents are _AB_ (seed plant) and _ab_ (pollen plant),
with the following seed characters:—
Seed parent {A form round Pollen parent {a form angular
_AB_ {B albumen yellow _ab_ {b albumen green
When these two forms were crossed the seeds appeared round and yellow
like those of the parent, _AB_, _i.e._ these two characters dominated in
the hybrid.
The seeds were sown, and in turn yielded plants which when
self-fertilized gave four kinds of seeds (which frequently all appeared
in the same pod). Thus 556 seeds were produced by 15 plants, having the
following characters:—
_AB_ 315 round and yellow
_Ab_ 101 angular and yellow
_aB_ 108 round and green
_ab_ 32 angular and green
These figures stand almost in the relation of 9 : 3 : 3 : 1.
These seeds were sown again in the following year and gave:—
From the round yellow seeds:—
_AB_ 38 round and yellow seeds
_ABb_ 65 round yellow and green seeds
_AaB_ 60 round yellow and angular yellow seeds
_AaBb_ 138 round yellow and green, angular yellow and green
seeds
From the angular yellow seeds:—
_aB_ 28 angular yellow seeds
_aBb_ 68 angular yellow and green seeds
From the round green seeds:—
_Ab_ 35 round green seeds
_Aab_ 67 round angular seeds
From the angular green seeds:—
_ab_ 30 angular green seeds
Thus there were 9 different kinds of seeds produced. There had been
separated out at this time 38 individuals like the parent seed plant,
_AB_, and 30 like the parent pollen plant, _ab_. Since these had come
from similar seeds of the preceding generation they may be looked upon
as pure at this time. The forms _Ab_ and _aB_ are also constant forms
which do not subsequently vary. The remainder are still mixed or hybrid
in character. By successive self-fertilizations it is possible gradually
to separate out from these the pure types of which they are compounded.
Without going into further detail it may be stated that the offspring of
the parent hybrids, having two pairs of differentiating characters, are
represented by the series:—
_AB_ _Ab_ _aB_ _ab_ 2_ABb_ 2_aBb_ 2_Aab_ 2_ABa_ 2_AaBb_
This series is really a combination of the two series:—
_A_ + 2_Aa_ + _a_
_B_ + 2_Bb_ + _b_
Mendel even went farther, and used two parent varieties having three
differentiating characters, as follows:—
_ABC seed parent_ _abc pollen plant_
{ A form round { a form angular
{ B albumen yellow { b albumen green
{ C seed-coat grey { c seed-coat white
brown
The results, as may be imagined, were quite complex, but can be
expressed by combining these series:—
_A_ + 2_Aa_ + _a_
_B_ + 2_Bb_ + _b_
_C_ + 2_Cc_ + _c_
In regard to the two latter experiments, in which two and three
characters respectively were used, it is interesting to point out that
the form of the hybrid more nearly approaches “to that one of the
parental plants which possesses the greatest number of dominant
characters.” If, for instance, the seed plant has short stem, terminal
white flowers, and simply inflated pods; the pollen plant, on the other
hand, a long stem, violet-red flowers distributed along the stem, and
constricted pods,—then the hybrid resembles the seed parent only in the
form of the pod; in its other characters it agrees with the pollen
plant. From this we may conclude that, if two varieties differing in a
large number of characters are crossed, the hybrid might get some of its
dominant characters from one parent, and other dominant characters from
the other parent, so that, unless the individual characters themselves
were studied, it might appear that the hybrids are intermediate between
the two parents, while in reality they are only combinations of the
dominant characters of the two forms. But even this is not the whole
question.
Mendel points out that, from knowing the characters of the two parent
forms (or varieties), one could not prophesy what the hybrid would be
like without making the actual trial. Which of the characters of the two
parent forms will be the dominant ones, and which recessive, can only be
determined by experiment. Moreover, the hybrid characters are something
peculiar to the hybrid itself, and to itself alone, and not simply the
combination of the characters of the two forms. Thus in one case a
hybrid from a tall and a short variety of pea was even taller than the
taller parent variety. Bateson lays much emphasis on this point,
believing it to be an important consideration in all questions relating
to hybridization and inheritance.
The theoretical interpretation that Mendel has put upon his results is
so extremely simple that there can be little doubt that he has hit on
the real explanation. The results can be accounted for if we suppose
that the hybrid produces egg-cells and pollen-cells, each of which is
the bearer of only one of the alternative characters, dominant or
recessive as the case may be. If this is the case, and if on an average
there are the same number of egg-cells and pollen-cells, having one or
the other of these kinds of characters, then on a random assortment
meeting of egg-cells and pollen-cells, Mendel’s law would follow. For,
25 per cent of dominant pollen grains would meet with 25 per cent
dominant egg-cells; 25 per cent recessive pollen grains would meet with
25 per cent recessive egg-cells; while the remaining 50 per cent of each
kind would meet each other. Or, as Mendel showed by the following
scheme:—
A A a a
| / |
| × |
| / |
A A a a
Or more simply by this scheme:—
A a
| /|
| × |
|/
A a
Mendel’s results have received confirmation by a number of more recent
workers, and while in some cases the results appear to be complicated by
other factors, yet there can remain little doubt that Mendel has
discovered one of the fundamental laws of heredity.
It has been found that there are some cases in which the sort of
inheritance postulated by Mendel’s law does not seem to hold, and, in
fact, Mendel himself spoke of such cases. He found that some kinds of
hybrids do not break up in later generations into the parent forms. He
also points out that in cases of discontinuity the variations in each
character must be separately regarded. In most experiments in crossing,
forms are chosen which differ from each other in a multitude of
characters, some of which are continuous and others discontinuous, some
capable of blending with their contraries while others are not. The
observer in attempting to discover any regularity is confused by the
complications thus introduced. Mendel’s law could only appear in such
cases by the use of an overwhelming number of examples which are beyond
the possibilities of experiment.[25]
Footnote 25:
This statement is largely taken from Bateson’s book.
Let us now examine the bearing of these discoveries on the questions of
variation which were raised in the preceding pages. It should be pointed
out, however, that it would be premature to do more than indicate, in
the most general way, the application of these conclusions. The chief
value of Mendel’s results lies in their relation to the theory of
inheritance rather than to that of evolution.
In the first place, Mendel’s results indicate that we cannot make any
such sharp distinction as Darwin does between the results of inheritance
of discontinuous and of continuous variations. As Mendel’s results show,
it is the separate characters that must be considered in each case, and
not simply the sum total of characters.
The more general objection that Darwin has made may appear to hold,
nevertheless. He thinks that the evolution of animals and plants cannot
rest primarily on the appearance of discontinuous variations, because
they occur rarely and would be swamped by intercrossing. If Mendel’s law
applies to such cases, that is, if a cross were made between such a
sport and the original form, the hybrid in this case, if
self-fertilized, would begin to split up into the two original forms.
But, on the other hand, it could very rarely happen that the hybrid did
fertilize its own eggs, and, unless this occurred, the hybrid, by
crossing with the parent forms in each generation, would soon lose all
its characters inherited from its “sport” ancestor. Unless, therefore,
other individuals gave rise to sports at the same time, there would be
little chance of producing new species in this way. We see then that
discontinuity in itself, unless it involved infertility with the parent
species, of which there is no evidence, cannot be made the basis for a
theory of evolution, any more than can individual differences, for the
swamping effect of intercrossing would in both cases soon obliterate the
new form. If, however, a species begins to give rise to a large number
of individuals of the same kind through a process of discontinuous
variation, then it may happen that a new form may establish itself,
either because it is adapted to live under conditions somewhat different
from the parent form, so that the dangers of intercrossing are lessened,
or because the new form may absorb the old one. It is also clear, from
what has gone before, that the new form can only cease to be fertile
with the parent form, or with its sister forms, after it has undergone
such a number of changes that it is no longer able to combine the
differences in a new individual. This result will depend both on the
kinds of the new characters, as well as the amounts of their difference.
This brings us to a consideration of the results of De Vries, who has
studied the first steps in the formation of new species in the
“mutations” of the evening primrose.
The Mutation Theory of De Vries
De Vries defines the mutation theory as the conception that “the
characters of the organism are made up of elements (‘Einheiten’) that
are sharply separated from each other. These elements can be combined in
groups, and in related species the same combinations of elements recur.
Transitional forms like those that are so common in the external
features of animals and plants do not exist between the elements
themselves, any more than they do between the elements of the chemist.”
This principle leads, De Vries says, in the domain of the descent theory
to the conception that species have arisen from each other, not
continuously, but by steps. Each new step results from a new combination
as compared with the old one, and the new forms are thereby completely
and sharply separated from the species from which they have come. The
new species is all at once there; it has arisen from the parent form
without visible preparation and without transitional steps.
The mutation theory stands in sharp contrast to the selection theory.
The latter uses as its starting-point the common form of variability
known as individual or fluctuating variation; but according to the
mutation theory there are two kinds of variation that are entirely
different from each other. “The fluctuating variation can, as I hope to
show, not overstep the bounds of the species, even after the most
prolonged selection,—much less can this kind of variation lead to the
production of new, constant characters.” Each peculiarity of the
organism has arisen from a preceding one, not through the common form of
variation, but through a sudden change that may be quite small but is
perfectly definite. This kind of variability that produces new species,
De Vries calls mutability; the change itself he calls a mutation. The
best-known examples of mutations are those which Darwin called “single
variations” or “sports.”
De Vries recognizes the following kinds of variation:—
First, the polymorphic forms of the systematists. The ordinary groups
which, following Linnæus, we call species, are according to De Vries
collective groups, which are the outcome of mutations. Many such Linnæan
species include small series of related forms, and sometimes even large
numbers of such forms. These are as distinctly and completely separated
from each other as are the best species. Generally these small groups
are called varieties, or subspecies,—varieties when they are separated
by a single striking character, subspecies when they differ in the
totality of their characters, in the so-called habitus.
These groups have already been recognized by some investigators as
elementary species, and have been given corresponding binary names. Thus
there are recognized two hundred elementary species of the form formerly
called _Draba verna_.
When brought under cultivation these elementary species are constant in
character and transmit their peculiarities truly. They are not local
races in the sense that they are the outcome in each generation of
special external conditions. Many other Linnæan species are in this
respect like _Draba verna_, and most varieties, De Vries thinks, are
really elementary species.
Second, the polymorphism due to intercrossing is the outcome of
different combinations of hereditary qualities. There are here, De Vries
says, two important classes of facts to be kept strictly
apart,—scientific experiment, and the results of the gardener and of the
cultivator. The experimenter chooses for crossing, species as little
variable as possible; the gardener and cultivator on the other hand
prefer to cross forms of which one at least is variable, because the
variations may be transmitted to the hybrid, and in this way a new form
be produced.
New elementary characters arise in experiments in crossing only through
variability, not through crossing itself.
Third, variability in the ordinary sense, that is, individual
variability, includes those differences between the individual organs
that follow Quetelet’s theory of chance. This kind of variability is
characterized by its presence at all times, in all groups of
individuals.
De Vries recalls Galton’s apt comparison between variability and a
polyhedron which can roll from one face to another. When it comes to
rest on any particular face, it is in stable equilibrium. Small
vibrations or disturbances may make it oscillate, but it returns always
to the same face. These oscillations are like the fluctuating
variations. A greater disturbance may cause the polyhedron to roll over
on to a new face, where it comes to rest again, only showing the ever
present fluctuations around its new centre. The new position corresponds
to a mutation. It may appear from our familiarity with the great changes
that we associate with the idea of discontinuous variability, that a
mutation must also involve a considerable change. Such, however, De
Vries says, is not the case. In fact, numerous mutations are smaller
than the extremes of fluctuating variation. For example, the different
elementary species of _Draba verna_ are less different from each other
than the forms of leaves on a tree. The essential differences between
the two kinds of variation is that the mutation is constant, while the
continuous variation fluctuates back and forth.
The following example is given by De Vries to illustrate the general
point of view in regard to varieties and species. The species _Oxalis
corniculata_ is a “collective” species that lives in New Zealand. It has
been described as having seven well-characterized varieties which do not
live together or have intermediate forms. If we knew only this group,
there would be no question that there are seven good species. But in
other countries intermediate forms exist, which exactly bridge over the
differences between the seven New Zealand forms. For this reason all the
forms have been united in a single species.
Another example is that of the fern, _Lomaria procera_, from New
Zealand, Australia, South Africa, and South America. If the forms from
only one country be considered, they appear to be different species; but
if all the forms from the different parts of the world be taken into
account, they constitute a connected group, and are united into one
large species.
It will be seen, therefore, that the limits of a collective species are
determined solely by the deficiencies in the genealogical tree of the
elementary species. If all the elementary species in one country were
destroyed, then the forms living in other countries that had been
previously held together because of those which have now been destroyed,
would, after the destruction, become true species. In other words: “The
Linnæan species are formed by the disappearance of other elementary
species, which at first connected all forms. This mode of origin is a
purely historical process, and can never become the subject of
experimental investigation.” Spencer’s famous expression, the “survival
of the fittest,” is incomplete, and should read the “survival of the
fittest species.” It is, therefore, not the study of Linnæan species
that has a physiological interest, but it is the study of the elementary
species of which the Linnæan species are made up, that furnishes the
all-important problem for experimental study.
De Vries gives a critical analysis of a number of cases in which new
races have been formed under domestication. He shows very convincingly
that, whenever the result has been the outcome of the selection of
fluctuating variations, the product that is formed can only be kept to
its highest point of development by the most rigid and ever watchful
care. If selection ceases for only a few generations, the new form sinks
back at once to its original level. Many of our cultivated plants have
really arisen, not by selection of this sort, but by mutations; and
there are a number of recorded cases where the first and sudden
appearance of a new form has been observed. In such cases as these there
is no need for selection, for if left to themselves there is no return
to the original form. If, however, after a new mutation has appeared in
this way, we subject its fluctuating variations to selection, we can
keep the new form up to its most extreme limit, but can do nothing more.
Another means, frequently employed, by which new varieties have been
formed is by bringing together different elementary species under
cultivation. For instance, there are a large number of wild elementary
species of apples, and De Vries believes that our different races of
apples owe their origin in part to these different wild forms. Crossing,
cultivation, and selection have done the rest.
De Vries points out some of the inconsistencies of those who have
attempted to discriminate between varieties and species. The only rule
that can be adhered to is that a variety differs from a species to which
it belongs in only one or in a few characters. Most so-called varieties
in nature are really elementary species, which differ from their nearest
relatives, not in one character only, but in nearly all their
characters. There is no ground, De Vries states, for believing them to
be varieties. If it is found inconvenient to rank them under the names
of the old Linnæan species, it will be better, perhaps, to treat them as
subspecies, but De Vries prefers to call them elementary species.
In regard to the distribution of species in nature, it may be generally
stated that the larger the geographical domain so much the larger is the
number of elementary species. They are found to be heaped up in the
centre of their area of distribution, but are more scattered at the
periphery.
In any one locality each Linnæan species has as a rule only one or a few
elementary species. The larger the area the more numerous the forms.
From France alone Jordan had brought together in his garden 50
elementary species of _Draba verna_. From England, Italy, and Austria
there could be added 150 more. This polymorphism is, De Vries thinks, a
general phenomenon, although the number of forms is seldom so great as
in this case.
Amongst animals this great variety of forms is not often met with, yet
amongst the mammalia and birds of North America there are many cases of
local forms or races, some of which at least are probably mutations.
This can only be proven, however, by actually transferring the forms to
new localities in order to find out if they retain their original
characters, or become changed into another form. It seems not improbable
that many of the forms are not the outcome of the external conditions
under which the animal now lives, but would perpetuate themselves in a
new environment.
From the evidence that his results have given, De Vries believes it is
probable that mutation has occurred in all directions. In the same way
that Darwin supposed that individual or fluctuating variations are
scattering, so also De Vries believes that the new forms that arise
through mutation are scattering. On this point it seems to me that De
Vries may be too much prejudiced by his results with the evening
primrose. If, as he supposes, many forms, generally ranked as varieties,
are really elementary species, it seems more probable that the mutation
of a form may often be limited to the production of one or of only a
very few new forms. The single variations, or sports, point even more
strongly in favor of this interpretation. Moreover, the general problem
of evolution from a purely theoretical point of view is very much
simplified, if we assume that the kinds of mutating forms may often be
very limited, and that mutations may often continue to occur in a direct
line. On this last point, De Vries argues that the evidence from
paleontology cannot be trusted, for all that we can conclude from fossil
remains is that certain mutations have dominated, and have been
sufficiently abundant to leave a record. In other words, the conditions
may have been such that only certain forms could find a foothold.
De Vries asks whether there are for each species periods of mutation
when many and great changes take place, and periods when relatively
little change occurs. The evidence upon which to form an opinion is
scanty, but De Vries is inclined to think that such periods do occur. It
is at least certain from our experience that there are long periods when
we do not see new forms arising, while at other times, although we know
very few of them, epidemics of change may take place. The mutative
period which De Vries found in the evening primrose is the best-known
example of such a period of active mutation. Equally important for the
descent theory is the idea that the same mutation may appear time after
time. There is good evidence to show that this really occurs, and in
consequence the chances for the perpetuation of such a form are greatly
increased. Delbœuf, who advocated this idea of the repeated reappearance
of a new form, has also attempted to show that if this occurs the new
form may become established without selection of any kind taking
place,—the time required depending upon the frequency with which the new
form appears. This law of Delbœuf, De Vries believes, is correct from
the point of view of the mutation theory. It explains, in a very simple
way, the existence of numerous species-characters that are entirely
useless, such, for instance, as exist between the different elementary
species of _Draba verna_. “According to the selection theory only useful
characters can survive; according to the mutation theory, useless
characters also may survive, and even those that may be hurtful to a
small degree.”
We may now proceed to examine the evidence from which De Vries has been
led to the general conclusions given in the preceding pages. De Vries
found at Hilversam, near Amsterdam, a locality where a number of plants
of the evening primrose, _Œnothera lamarckiana_, grow in large numbers.
This plant is an American form that has been imported into Europe. It
often escapes from cultivation, as is the case at Hilversam, where for
ten years it had been growing wild. Its rapid increase in numbers in the
course of a few years may be one of the causes that has led to the
appearance of a mutation period. The escaped plants showed fluctuating
variations in nearly all of their organs. They also had produced a
number of abnormal forms. Some of the plants came to maturity in one
year, others in two, or in rare cases in three, years.
A year after the first finding of these plants De Vries observed two
well-characterized forms, which he at once recognized as new elementary
species. One of these was _O. brevistylis_, which occurred only as
female plants. The other new species was a smooth-leafed form with a
more beautiful foliage than _O. lamarckiana_. This is _O. lævifola_. It
was found that both of these new forms bred true from self-fertilized
seeds. At first only a few specimens were found, each form in a
particular part of the field, which looks as though each might have come
from the seeds of a single plant.
ŒNOTHERA LAMARCKIANA
Elementary Species
===============+======+======+======+======+========+========+====+========
GENERATION |GIGAS |ALBIDA|OBLON-|RUBRI-|LAMARCK-|NANNELLA|LATA|SCINTIL-
| | |GATA |NERVIS| IANA | | | LANS
===============+======+======+======+======+========+========+====+========
|8 Gener. |
VIII | 1899 | 5 1 0 1700 21 1
|annual | ‘———————————v—————————’
| |
|7 Gener. |
VII | 1898 | 9 0 3000 11
|annual | ‘———————v——————’
| |
|6 Gener. |
VI | 1897 | 11 29 3 1800 9 5 1
|annual | ‘———————————v—————————————’
| |
|5 Gener. |
V | 1896 | 25 135 20 8000 49 142 6
|annual | ‘———————————v—————————————’
| |
|4 Gener. |
IV | 1895 | 1 15 176 8 14000 60 73 1
|annual | ‘—————————————v—————————————’
| |
|3 Gener. |
III | 1890-91 | 1 10000 3 3
|biennial | ‘———v——————————’
| |
|2 Gener. |
II | 1888-89 | 15000 5 5
|biennial | ‘—v——————————’
| |
|1 Gener. |
I | 1886-87 | 9
|biennial |
=====+=========+===========================================================
These two new forms, as well as the common _O. lamarckiana_, were
collected, and from these plants there have arisen the three groups or
families of elementary species that De Vries has studied. In his garden
other new forms also arose from those that had been brought under
cultivation. The largest group and the most important one is that from
the original _O. lamarckiana_ form. The accompanying table shows the
mutations that arose between 1887 and 1899 from these plants. The seeds
were selected in each case from self-fertilized plants of the
_lamarckiana_ form, so that the new plants appearing in each horizontal
line are the descendants in each generation of _lamarckiana_ parents. It
will be observed that the species, _O. oblongata_, appeared again and
again in considerable numbers, and the same is true for several of the
other forms also. Only the two species, _O. gigas_ and _O. scintillans_,
appeared very rarely.
Thus De Vries had, in his seven generations, about fifty thousand
plants, and about eight hundred of these were mutations. When the
flowers of the new forms were artificially fertilized with pollen from
the flowers on the same plant, or of the same kind of plant, they gave
rise to forms like themselves, thus showing that they are true
elementary species.[26] It is also a point of some interest to observe
that all these forms differed from each other in a large number of
particulars.
Footnote 26:
_O. lata_ is always female, and cannot, therefore, be self-fertilized.
When crossed with _O. lamarckiana_ there is produced fifteen to twenty
per cent of pure _lata_ individuals.
Only one form, _O. scintillans_, that appeared eight times, is not
constant as are the other species. When self-fertilized its seeds
produce always three other forms, _O. scintillans_, _O. oblongata_, and
_O. lamarckiana_. It differs in this respect from all the other
elementary species, which mutate not more than once in ten thousand
individuals.
From the seeds of one of the new forms, _O. lævifolia_, collected in the
field, plants were reared, some of which were _O. lamarckiana_ and
others _O. lævifolia_. They were allowed to grow together, and their
descendants gave rise to the same forms found in the _lamarckiana_
family, described above, namely, _O. lata_, _elliptica_, _nannella_,
_rubrinervis_, and also two new species, _O. spatulata_ and
_leptocarpa_.
In the _lata_ family, only female flowers are produced, and, therefore,
in order to obtain seeds they were fertilized with pollen from other
species. Here also appeared some of the new species, already mentioned,
namely, _albida_, _nannella_, _lata_, _oblongata_, _rubrinervis_, and
also two new species, _elliptica_ and _subovata_.
De Vries also watched the field from which the original forms were
obtained, and found there many of the new species that appeared under
cultivation. These were found, however, only as weak young plants that
rarely flowered. Five of the new forms were seen either in the Hilversam
field, or else raised from seeds that had been collected there. These
facts show that the new species are not due to cultivation, and that
they arise year after year from the seeds of the parent form, _O.
lamarckiana_.
Conclusions
From the evidence given in the preceding pages it appears that the line
between fluctuating variations and mutations may be sharply drawn. If we
assume that mutations have furnished the material for the process of
evolution, the whole problem appears in a different light from that in
which it was placed by Darwin when he assumed that the fluctuating
variations are the kind which give the material for evolution.
From the point of view of the mutation theory, species are no longer
looked upon as having been slowly built up through the selection of
individual variations, but the elementary species, at least, appear at a
single advance, and fully formed. This need not necessarily mean that
great changes have suddenly taken place, and in this respect the
mutation theory is in accord with Darwin’s view that _extreme_ forms
that rarely appear, “sports,” have not furnished the material for the
process of evolution.
As De Vries has pointed out, each mutation may be different from the
parent form in only a slight degree for each point, although all the
points may be different. The most unique feature of these mutations is
the constancy with which the new form is inherited. It is this fact, not
previously fully appreciated, that De Vries’s work has brought
prominently into the foreground. There is another point of great
interest in this connection. Many of the groups that Darwin recognized
as varieties correspond to the elementary species of De Vries. These
varieties, Darwin thought, are the first stages in the formations of
species, and, in fact, cannot be separated from species in most cases.
The main difference between the selection theory and the mutation theory
is that the one supposes these varieties to arise through selection of
individual variations, the other supposes that they have arisen
spontaneously and at once from the original form. The development of
these varieties into new species is again supposed, on the Darwinian
theory, to be the result of further selection, on the mutation theory,
the result of the appearance of new mutations.
In consequence of this difference in the two theories, it will not be
difficult to show that the mutation theory escapes some of the gravest
difficulties that the Darwinian theory has encountered. Some of the
advantages of the mutation theory may be briefly mentioned here.
1. Since the mutations appear fully formed from the beginning, there is
no difficulty in accounting for the incipient stages in the development
of an organ, and since the organ may persist, even when it has no value
to the race, it may become further developed by later mutations and may
come to have finally an important relation to the life of the
individual.
2. The new mutations may appear in large numbers, and of the different
kinds those will persist that can get a foothold. On account of the
large number of times that the same mutations appear, the danger of
becoming swamped through crossing with the original form will be
lessened in proportion to the number of new individuals that arise.
3. If the time of reaching maturity in the new form is different from
that in the parent forms, then the new species will be kept from
crossing with the parent form, and since this new character will be
present from the beginning, the new form will have much better chances
of surviving than if a difference in time of reaching maturity had to be
gradually acquired.
4. The new species that appear may be in some cases already adapted to
live, in a different environment from that occupied by the parent form;
and if so, it will be isolated from the beginning, which will be an
advantage in avoiding the bad effects of intercrossing.
5. It is well known that the differences between related species
consists largely in differences of unimportant organs, and this is in
harmony with the mutation theory, but one of the real difficulties of
the selection theory.
6. Useless or even slightly injurious characters may appear as
mutations, and if they do not seriously affect the perpetuation of the
race, they may persist.
In Chapters X and XI, an attempt will be made to point out in detail the
advantages which the mutation theory has over the Darwinian theory.
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