SELECTION AND EVOLUTION
Darwin's Theory of Natural Selection still holds today first place in every
discussion of evolution, and for this very reason the theory calls for
careful scrutiny; for it is not difficult to show that the expression
"natural selection" is to many men a metaphor that carries many meanings,
and sometimes different meanings to different men. While I heartily agree
with my fellow biologists in ascribing to Darwin himself, and to his work,
the first place in biological philosophy, yet recognition of this claim
should not deter us from a careful analysis of the situation in the light
of work that has been done since Darwin's time.
THE THEORY OF NATURAL SELECTION
In his great book on the _Origin of Species_, Darwin tried to do two
things: first, to show that the evidence bearing on evolution makes that
explanation probable. No such great body of evidence had ever been brought
together before, and it wrought, as we all know, a revolution in our modes
of thinking.
Darwin also set himself the task of showing _how_ evolution might have
taken place. He pointed to the influence of the environment, to the effects
of use and disuse, and to natural selection. It is to the last theory that
his name is especially attached. He appealed to a fact familiar to
everyone, that no two individuals are identical and that some of the
differences that they show are inherited. He argued that those individuals
that are best suited to their environment are the most probable ones to
survive and to leave most offspring. In consequence their descendants
should in time replace through competition the less well-adapted
individuals of the species. This is the process Darwin called natural
selection, and Spencer the survival of the fittest.
Stated in these general terms there is nothing in the theory to which
anyone is likely to take exception. But let us examine the argument more
critically.
[Illustration: FIG. 71. Series of leaves of a tree arranged according to
size. (After de Vries.)]
If we measure, or weigh, or classify any character shown by the individuals
of a population, we find differences. We recognize that some of the
differences are due to the varied experiences that the individuals have
encountered in the course of their lives, i.e. to their environment, but we
also recognize that some of the differences may be due to individuals
having different inheritances--different germ plasms. Some familiar
examples will help to bring home this relation.
If the leaves of a tree are arranged according to size (fig. 71), we find a
continuous series, but there are more leaves of medium size than extremes.
If a lot of beans be sorted out according to their weights, and those
between certain weights put into cylinders, the cylinders, when arranged
according to the size of the beans, will appear as shown in figure 72. An
imaginary line running over the tops of the piles will give a curve (fig.
73) that corresponds to the curve of probability (fig. 74).
[Illustration: FIG. 72. Beans put into cylindrical jars according to the
sizes of the beans. The jars arranged according to size of contained beans.
(After de Vries.)]
[Illustration: FIG. 73. A curve resulting from arrangement of beans
according to size. (After de Vries.)]
If we stand men in lines according to their height (fig. 75) we get a
similar arrangement.
[Illustration: FIG. 74. Curve of probability.]
[Illustration: FIG. 75. Students arranged according to size. (After
Blakeslee.)]
The differences in size shown by the individual beans or by the individual
men are due in part to heredity, in part to the environment in which they
have developed. This is a familiar fact of almost every-day observation. It
is well shown in the following example. In figure 76 the two boys and the
two varieties of corn, which they are holding, differ in height. The
pedigrees of the boys (fig. 77) make it probable that their height is
largely inherited and the two races of corn are known to belong to a tall
and a short race respectively. Here, then, the chief effect or difference
is due to heredity. On the other hand, if individuals of the same race
develop in a favorable environment the result is different from the
development in an unfavorable environment, as shown in figure 78. Here to
the right the corn is crowded and in consequence dwarfed, while to the left
the same kind of corn has had more room to develop and is taller.
[Illustration: FIG. 76. A short and a tall boy each holding a stalk of
corn--one stalk of a race of short corn, the other of tall corn. (After
Blakeslee.)]
[Illustration: FIG. 77. Pedigree of boys shown in Fig. 76. (After
Blakeslee.)]
Darwin knew that if selection of particular kinds of individuals of a
population takes place the next generation is affected. If the taller men
of a community are selected _the average_ of their offspring will be taller
than the average of the former population. If selection for tallness again
takes place, still taller men will _on the average_ arise. If, amongst
these, selection again makes a choice the process would, he thought,
continue (fig. 79).
[Illustration: FIG. 78. A race of corn reared under different conditions.]
We now recognize that this statement contains an important truth, but we
have found that it contains only a part of the truth. Any one who repeats
for himself this kind of selection experiment will find that while his
average class will often change in the direction of his selection, the
process slows down as a rule rather suddenly (fig. 80). He finds, moreover,
that the limits of variability are not necessarily transcended as the
process continues even although the average may for a while be increased.
More tall men may be produced by selection of this kind, but the tallest
men are not necessarily any taller than the tallest in the original
population.
[Illustration: FIG. 79. Curves showing how (hypothetically) selection might
be supposed to bring about progress in direction of selection. (After
Goldschmidt.)]
Selection, then, has not produced anything new, but only more of certain
kinds of individuals. Evolution, however, means producing more new things,
not more of what already exists.
Darwin seems to have thought that the range of variation shown by the
offspring of a given individual about that type of individual would be as
wide as the range shown by the original population (fig. 79), but Galton's
work has made it clear that this is not the case in a general or mixed
population. If the offspring of individuals continued to show, as Darwin
seems to have thought, as wide a range on each side of their parents' size,
so to speak, as did the original population, then it would follow that
selection could slide successive generations along in the direction of
selection.
[Illustration: FIG. 80. Diagram illustrating the results of selection for
extra bristles in D. ampelophila. Selection at first produces decided
effects which soon slow down and then cease. (MacDowell.)]
Darwin himself was extraordinarily careful, however, in the statements he
made in this connection and it is rather by implication than by actual
reference that one can ascribe this meaning to his views. His
contemporaries and many of his followers, however, appear to have accepted
this _sliding scale_ interpretation as the cardinal doctrine of evolution.
If this is doubted or my statement is challenged then one must explain why
de Vries' mutation theory met with so little enthusiasm amongst the older
group of zoologists and botanists; and one must explain why Johannsen's
splendid work met with such bitter opposition from the English school--the
biometricians--who amongst the post-Darwinian school are assumed to be the
lineal descendants of Darwin.
And in this connection we should not forget that just this sort of process
was supposed to take place in the inheritance of use and disuse. What is
gained in one generation forms the basis for further gains in the next
generation. Now, Darwin not only believed that acquired characters are
inherited but turned more and more to this explanation in his later
writings. Let us, however, not make too much of the matter; for it is much
less important to find out whether Darwin's ideas were vague, than it is to
make sure that our own ideas are clear.
If I have made several statements here that appear dogmatic let me now
attempt to justify them, or at least give the evidence which seems to me to
make them probable.
The work of the Danish botanist, Johannsen, has given us the most carefully
analyzed case of selection that has ever been obtained. There are,
moreover, special reasons why the material that he used is better suited to
give definite information than any other so far studied. Johannsen worked
with the common bean, weighing the seeds or else measuring them. These
beans if taken from many plants at random give the typical curve of
probability (fig. 74). The plant multiplies by self-fertilization. Taking
advantage of this fact Johannsen kept the seeds of each plant separate from
the others, and raised from them a new generation. When curves were made
from these new groups it was found that some of them had different modes
from that of the original general population (fig. 81 A-E, bottom group).
They are shown in the upper groups (A, B, C, D, E). But do not understand
me to say that the offspring of each bean gave a different mode.
[Illustration: FIG. 81. Pure lines of beans. The lower figure gives the
general population, the other figures give the pure lines within the
population. (After Johannsen.)]
On the contrary, some of the lines would be the same.
The result means that the general population is made up of definite kinds
of individuals that may have been sorted out.
That his conclusion is correct is shown by rearing a new generation from
any plant or indeed from several plants of any one of these lines. Each
line repeats the same modal class. There is no further breaking up into
groups. Within the line it does not matter at all whether one chooses a big
bean or a little one--they will give the same result. In a word, the germ
plasm in each of these lines is pure, or homozygous, as we say. The
differences that we find between the weights (or sizes) of the individual
beans are due to external conditions to which they have been subjected.
In a word, Johannsen's work shows that the frequency distribution of a pure
line is due to factors that are extrinsic to the germ plasm. It does not
matter then which individuals in a pure line are used to breed from, for
they all carry the same germ plasm.
We can now understand more clearly how selection acting on a general
population brings about results in the direction of selection.
An individual is picked out from the population in order to get a
particular kind of germ plasm. Although the different classes of
individuals may overlap, so that one can not always judge an individual
from its appearance, nevertheless on the whole chance favors the picking
out of the kind of germ plasm sought.
In species with separate sexes there is the further difficulty that two
individuals must be chosen for each mating, and superficial examination of
them does not insure that they belong to the same group--their germ plasm
cannot be inspected. Hence selection of biparental forms is a precarious
process, now going forward, now backwards, now standing still. In time,
however, the process forward is almost certain to take place if the
selection is from a heterogeneous population. Johannsen's work was
simplified because he started with pure lines. In fact, had he not done so
his work would not have been essentially different from that of any
selection experiment of a pure race of animals or plants. Whether Johannsen
realized the importance of the condition or not is uncertain--curiously he
laid no emphasis on it in the first edition of his "Elemente der exakten
Erblichkeitslehre".
It has since been pointed out by Jennings and by Pearl that a race that
reproduces by self-fertilization as does this bean, automatically becomes
pure in all of the factors that make up its germ plasm. Since
self-fertilization is the normal process in this bean the purity of the
germ plasm already existed when Johannsen began to experiment.
HOW HAS SELECTION IN DOMESTICATED ANIMALS AND PLANTS BROUGHT ABOUT ITS
RESULTS?
If then selection does not bring about transgressive variation in a general
population, how can selection produce anything new? If it can not produce
anything new, is there any other way in which selection becomes an agent in
evolution?
We can get some light on this question if we turn to what man has done with
his domesticated animals and plants. Through selection, i.e., artificial
selection, man has undoubtedly brought about changes as remarkable as any
shown by wild animals and plants. We know, moreover, a good deal about how
these changes have been wrought.
(1) By crossing different wild species or by crossing wild with races
already domesticated new combinations have been made. Parts of one
individual have been combined with parts of others, creating new
combinations. It is possible even that characters that are entirely new may
be produced by the interaction of factors brought into recombination.
(2) New characters appear from time to time in domesticated and in wild
species. These, like the mutants in Drosophila, are fully equipped at the
start. Since they breed true and follow Mendel's laws it is possible to
combine them with characters of the wild type or with those of other mutant
races.
Amongst the new mutant factors there may be some whose chief effect is on
the character that the breeder is already selecting. Such a modification
will be likely to attract attention. Superficially it may appear that the
factor for the original character has varied, while the truth may be that
another factor has appeared that has modified a character already present.
In fact, many or all Mendelian factors that affect the same organ may be
said to be modifiers of each other's effects. Thus the factor for vermilion
causes the eye to be one color, and the factor for eosin another color,
while eosin vermilion is different from both. Eosin may be said to be a
modifier of vermilion or vermilion of eosin. In general, however, it is
convenient to use the term "modifier" for cases in which the factor causes
a detectable change in a character already present or conspicuous.
[Illustration: FIG. 82. Scheme to indicate influence of the modifying
factors, cream and whiting. Neither produces any effect alone but they
modify other eye colors such as eosin.]
One of the most interesting, and at the same time most treacherous, kinds
of modifying factors is that which produces an effect _only_ when some
other factor is present. Thus Bridges has shown that there is a factor
called "cream" that does not affect the red color of the eye of the wild
fly, yet makes "eosin" much paler (fig. 82). Another factor "whiting" which
produces no effect on red makes eosin entirely white. Since cream or
whiting may be carried by red eyed flies without their presence being seen
until eosin is used, the experimenter must be continually on the lookout
for such factors which may lead to erroneous conclusions unless detected.
As yet breeders have not realized the important role that modifiers have
played in their results, but there are indications at least that the
heaping up of modifying factors has been one of the ways in which highly
specialized domesticated animals have been produced. Selection has
accomplished this result not by changing factors, but by picking up
modifying factors. The demonstration of the presence of these factors has
already been made in some cases. Their study promises to be one of the most
instructive fields for further work bearing on the selection hypothesis.
In addition to these well recognized methods by which artificial selection
has produced new things we come now to a question that is the very crux of
the selection theory today. Our whole conception of selection turns on the
answer that we give to this matter and if I appear insistent and go into
some detail it is because I think that the matter is worth very careful
consideration.
ARE FACTORS CHANGED THROUGH SELECTION?
As we have seen, the variation that we find from individual to individual
is due in part to the environment; this can generally be demonstrated.
Other differences in an ordinary population are recognized as due to
different genetic (hereditary) combinations. No one will dispute this
statement. But is all the variability accounted for in these two ways? May
not a factor itself fluctuate? Is it not _a priori_ probable that factors
do fluctuate? Why, in a word, should we regard factors as inviolate when we
see that everything else in organisms is more or less in amount? I do not
know of any _a priori_ reason why a factor may not fluctuate, unless it is,
as I like to think, a chemical molecule. We are, however, dealing here not
with generalities but with evidence, and there are three known methods by
means of which it has been shown that variability, other than environmental
or recombinational, is not due to variability in a factor, nor to various
"potencies" possessed by the same factors.
(1) By making the stock uniform for all of its factors--chief factors and
modifiers alike. Any change in such a stock produced by selection would
then be due to a change in one or more of the factors themselves.
Johannsen's experiment is an example of this sort.
[Illustration: FIG. 83 a. Drosophila ampelophila with truncate wings.]
(2) The second method is one that is capable of _demonstrating_ that the
effects of selection are actually due to modifiers. It has been worked out
in our laboratory, chiefly by Muller, and used in a particular case to
demonstrate that selection produced its effect by isolating modifying
factors. For example, a mutant type called truncate appeared, characterized
by shorter wings, usually square at the end, (fig. 83a). The wings varied
from those of normal length to wings much shorter (fig. 83b). For three
years the mutant stock was bred from individuals having the shorter wings
until at last a stock was obtained in which some of the individuals had
wings much shorter than the body. By means of linkage experiments it was
shown that at least three factors were present that modified the wings.
These were isolated by means of their linkage relations, and their mutual
influence on the production of truncate wings was shown.
[Illustration: FIG. 83 b. Series of wings of different length shown by
truncate stock of D. ampelophila.]
An experiment of this kind can only be carried out in a case where the
groups of linked gens are known. At present Drosophila is the only animal
(or plant) sufficiently well known to make this test possible, but this
does not prove that the method is of no value. On the contrary it shows
that any claim that factors can themselves be changed can have no finality
until the claim can be tested out by means of the linkage test. For
instance, bar eye (fig. 31) arose as a mutation. All our stock has
descended from a single original mutant. But Zeleny has shown that
selection within our stock will make the bar eye narrower or broader
according to the direction of selection. It remains to be shown in this
case how selection has produced its effects, and this can be done by
utilizing the same process that was used in the case of truncate.
Another mutant stock called beaded (fig. 84), has been bred for five years
and selected for wings showing more beading. In extreme cases the wings
have been reduced to mere stumps (see stumpy, fig. 5), but the stock shows
great variability. It is probable here as Dexter has shown, that a number
of mutant factors that act as modifiers have been picked up in the course
of the selection, and when it is recalled that during those five years over
125 new characters have appeared elsewhere it does not seem improbable that
factors also have appeared that modify the wings of this stock.
[Illustration: FIG. 84. Two flies showing beaded wings.]
(3) The third method is one that has been developed principally by East for
plants; also by MacDowell for rabbits and flies. The method does not claim
to prove that modifiers are present, but it shows why certain results are
in harmony with that expectation and can not be accounted for on the basis
that a factor has changed. Let me give an example. When a Belgian hare with
large body was crossed to a common rabbit with a small body the hybrid was
intermediate in size. When the hybrid was crossed back to the smaller type
it produced rabbits of various sizes in apparently a continuous series.
MacDowell made measurements of the range of variability in the first and in
the second generations.
_Classification in relation to parents based on skull lengths and ulna
lengths, to show the relative variability of two measurements and of
the first generation (F_1) and the back cross (B. C.)_
CHARACTER|GENERATION|-13|-12|-11|-10| -9| -8| -7| -6| -5| -4| -3| -2| -1|
---------+----------+---+---+---+---+---+---+---+---+---+---+---+---+---+
Length of{ F_1 | | | | | | | | | | | | | |
skull { B.C. | | | | | | | | | | | | | 3|
Length of{ F_1 | | | | | | | | | | | | | |
ulna { B.C. | 1| | | | | 1| | 2| 3| 1| 2| 4| 4|
_same table continued_
CHARACTER|GENERATION| 0| 1| 2| 3| 4| 5| 6| 7| 8| 9| 10| 11| 12|
---------+----------+---+---+---+---+---+---+---+---+---+---+---+---+---+
Length of{ F_1 | | | | | | | | 2| 2| 8| 5| 10| 7|
skull { B.C. | 6| 4| 13| 18| 42| 32| 38| 34| 16| 16| 8| 4| 3|
Length of{ F_1 | | 1| | | 2| | 1| 1| 1| 2| 2| 5| 3|
ulna { B.C. | 12| 11| 20| 26| 17| 19| 18| 15| 12| 13| 15| 11| 5|
_same table continued_
CHARACTER|GENERATION| 13| 14| 15| 16| 17| 18| 19| 20| 21| 22| 23| 24| 25|
---------+----------+---+---+---+---+---+---+---+---+---+---+---+---+---+
Length of{ F_1 | 3| 2| 2| | | | | | | | | | |
skull { B.C. | 1| | | | | | | | | | | | |
Length of{ F_1 | 1| 7| 3| 2| 1| | | | 2| | 1| | 1|
ulna { B.C. | 2| 4| 2| 2| | | 1| 1| | | | | |
He found that the variability was smaller in the first generation than in
the second generation (back cross). This is what is expected if several
factor-differences were involved, because the hybrids of the first
generation are expected to be more uniform in factorial composition than
are those in the second generation which are produced by recombination of
the factors introduced through their grandparents. Excellent illustrations
of the same kinds of results have been found in Indian corn. As shown in
figure 85 the length of the cob in F_1 is intermediate between the parent
types while in F_2 the range is wider and both of the original types are
recovered. East states that similar relations have been found for 18
characters in corn. Emerson has recently furnished further illustrations of
the same relations in the length of stalks in beans.
[Illustration: FIG. 85. Cross between two races of Indian corn, one with
short cobs and one with long cobs. The range of variability in F_1 is less
than that in F_2. (After East.)]
A similar case is shown by a cross between fantail and common pigeons (fig.
86). The latter have twelve feathers in the tail, while the selected race
from which the fantails came had between 28 and 38 feathers in the tail.
The F_1 offspring (forty-one individuals) showed (fig. 87) between 12 and
20 tail feathers, while in F_2 the numbers varied between 12 and 25. Here
one of the grand-parental types reappears in large numbers, while the
extreme of the other grand-parental type did not reappear (in the counts
obtained), although the F_2 number would probably overlap the lower limits
of the race of fantail grandparents had not a selected (surviving) lot been
taken for the figures given in the table.
[Illustration: FIG. 86. Cross of pigeon with normal tail P_1 and fantail
P_1; F_1, bird below.]
[Illustration: FIG. 87. Cross of normal and fantail pigeons. (See Fig. 86.)
The F_2 range is wider than that of F_1. The normal grand-parental type of
12 feathers was recovered in F_2 but the higher numbers characteristic of
fantails were not recovered.]
The preceding account attempts to point out how I should prefer to
interpret the problem of selection in the light of the most recent work on
breeding. But I would give a very incomplete account of the whole situation
if I neglected to include some important work which has led some of my
fellow-workers to a very different conclusion.
[Illustration: FIG. 88. Scheme to show classes of hooded rats used by
Castle. (After Castle.)]
Castle in particular is the champion of a view based on his results with
hooded rats. Starting with individuals which have a narrow black stripe
down the back he selected for a narrower stripe in one direction and for a
broader stripe in the other. As the diagram shows (fig. 88) Castle has
succeeded in producing in one direction a race in which the dorsal stripe
has disappeared and in the other direction a race in which the black has
extended over the back and sides, leaving only a white mark on the belly.
Neither of these extremes occurs, he believes, in the ordinary hooded race
of domesticated rats. In other words no matter how many of them came under
observation the extreme types of his experiment would not be found.
Castle claims that the factor for hoodedness must be a single Mendelian
unit, because if hooded rats are crossed to wild gray rats with uniform
coat and their offspring are inbred there are produced in F_2 three uniform
rats to one hooded rat. Castle advances the hypothesis that factors--by
which he means Mendelian factors--may themselves vary in much the same way
as do the characters that they stand for. He argues, in so many words, that
since we judge a factor by the kind of character it produces, when the
character varies the factor that stands for it may have changed.
As early as 1903 Cuenot had carried out experiments with spotted mice
similar to those of Castle with rats. Cuenot found that spotted crossed to
uniform coat color gave in F_2 a ratio of three uniform to one spotted, yet
selection of those spotted mice with more white in their coat produced mice
in successive generations that had more and more white. Conversely Cuenot
showed that selection of those spotted mice that had more color in their
coat produced mice with more and more color and less white. Cuenot does not
however bring up in this connection the question as to how selection in
these spotted mice brings about its results.
Without attempting to discuss these results at the length that they deserve
let me briefly state why I think Castle's evidence fails to establish his
conclusion.
In the first place one of the premises may be wrong. The three to one ratio
in F_2 by no means proves that all conditions of hoodedness are due to one
factor. The result shows at most that one factor that gives the hooded
types is a simple Mendelian factor. The changes in this type may be caused
by modifying factors that can show an effect only when hoodedness is itself
present. That this is not an imaginary objection but a real one is shown by
an experiment that Castle himself made which furnishes the ground for the
second objection.
Second. If the factor has really changed its potency, then if a very dark
individual from one end of the series is crossed to a wild rat and the
second generation raised we should expect that the hooded F_2 rats would
all be dark like their dark grandparent. When Castle made this test he
found that there were many grades of hooded rats in the F_2 progeny. They
were darker, it is true, as a group than were the original hooded group at
the beginning of the selection experiment, but they gave many intermediate
grades. Castle attempts to explain this by the assumption that the factor
made pure by selection became contaminated by its normal allelomorph in the
F_1 parent, but not only does this assumption appear to beg the whole
question, but it is in flat contradiction with what we have observed in
hundreds of Mendelian cases where no evidence for such a contamination
exists.
Later Castle crossed some of the extracted rats of average grade (3.01)
from the plus series to the same wild race and got F_2 hooded rats from
this cross. These F_2 hooded rats did not further approach the ordinary
range but were nearer the extreme selected plus hooded rats (3.33) than
were the F_2's extracted from the first cross (2.59). Castle concludes from
this that multiple factors can not account for the result. As a matter of
fact, Castle's evidence _as published_ does not establish his conclusion
because the wild rats used in the second experiment may have carried plus
modifiers. This could only be determined by suitable tests which Castle
does not furnish. This is the crucial point, without which the evidence
carries no conviction.
Furthermore, from Castle's point of view, these latest results would seem
to increase the difficulty of interpretation of his first F_2 extracted
cross, and it is now the first result that calls for explanation if one
accepts his later conclusion.
These and other objections that might be taken up show, I think, that
Castle's experiment with hooded rats fails entirely to establish his
contention of change in potency of the germ or of contamination of factors,
while on the contrary they are in entire accord with the view that he is
dealing with a case of modifying factors.
[Illustration: FIG. 89. Races of Paramecium. (After Jennings.)]
Equally important are the results that Jennings has obtained with certain
protozoa. Paramecium multiplies by dividing across in the middle, each half
replacing its lacking part. Both the small nucleus (micronucleus) and the
large nucleus (macronucleus) divide at each division of the body. Jennings
found that while individuals descended from a single paramecium vary in
size (fig. 89), yet the population from a large individual is the same as
the population derived from a small individual. In other words, selection
produces no result and the probable explanation is, of course, that the
different sizes of individuals are due to the environment, while the
constancy of the type is genetic. Jennings found a number of races of
paramecium of different sizes living under natural conditions. The largest
individual of a small race might overlap the smallest individual of other
larger races (fig. 89); nevertheless each kind reproduced its particular
race. The results are like those of Johannsen in a general way, but differ
in that reproduction takes place in paramecium by direct division instead
of through self-fertilization as in beans, and also in that the paramecia
were probably not homozygous. Since, however, so far as known no
"reduction" takes place in paramecium at each division, the genetic
composition of parent and offspring should be the same. Whether
pseudo-parthenogenesis that Woodruff and Erdmann have found occurring in
paramecium at intervals involves a redistribution of the hereditary factors
is not clear. Jennings's evidence seems incompatible with such a view.
[Illustration: FIG. 90. Stylonychia showing division into two. (After
Stein.)]
More recently one of Jennings's students, Middleton, has made a careful
series of selection experiments with Stylonychia (fig. 90) in which he
selected for lines showing more rapid or slower rates of division. His
observations seem to show that his selection separated two such lines that
came from the same original stock. The rapidity of the effects of selection
seems to preclude the explanation that pseudo-parthenogenesis has
complicated the results. Nevertheless, the results are of such a kind as to
suggest that they were due to selection of vegetative (somatic) differences
and that no genetic change of factors was involved, for his conclusion that
the rapidity with which the effects gained by long selection might be
suddenly reversed when selection was reversed is hardly consistent with an
interpretation of the results based on changes in the "potencies" of the
factors present.
Equally striking are the interesting experiments that Jennings has recently
carried out with Difflugia (fig. 91). This protozoon secretes a shell about
itself which has a characteristic shape, and often carries spines. The
opening at one end of the shell through which the protoplasm protrudes to
make the pseudopodia is surrounded by a rim having a characteristic
pattern. The protoplasm contains several nuclei and in addition there is
scattered material or particles called chromidia that are supposed to be
chromatic in nature and related to the material of the nuclei, possibly by
direct interchange.
[Illustration: FIG. 91. Difflugia Corona. (After Cash.)]
When Difflugia divides, part of the protoplasm protrudes from the opening
and a new shell is secreted about this mass which becomes a daughter
individual. The behavior of the nucleus and of the chromidia at this time
is obscure, but there is some evidence that their materials may be
irregularly distributed between parent and offspring. If this is correct,
and if in the protozoa the chromatin has the same influence that it seems
to have in higher animals, the mode of reproduction in Difflugia would be
expected to give little more than random sampling of the germ plasm.
[Illustration: FIG. 92. Races of Difflugia. (After Leidy.)]
Jennings was able by means of selection to get from the descendants of one
original individual a number of different types that themselves bred true,
except in so far as selection could affect another change in them. In this
connection it is interesting to note that Leidy has published figures of
Difflugia (fig. 92) that show that a great many "types" exist. If through
sexual union (a process that occurs in Difflugia) the germ plasm
(chromatin) of these wild types has in times past been recombined, then
selection would be expected to separate certain types again, if, at
division, irregular sampling of the germ plasm takes place. Until these
points are settled the bearing of these important experiments of Jennings
on the general problem of selection is uncertain.
HOW DOES NATURAL SELECTION INFLUENCE THE COURSE OF EVOLUTION?
The question still remains: Does selection play any role in evolution, and,
if so, in what sense? Does the elimination of the unfit influence the
course of evolution, except in the negative sense of leaving more room for
the fit? There is something further to be said in this connection, although
opinions may differ as to whether the following interpretation of the term
"natural selection" is the only possible one.
[Illustration: FIG. 93. Evolution of elephant's skulls. (After Dendy.)]
If through a mutation a character appears that is neither advantageous nor
disadvantageous, but indifferent, the chance that it may become established
in the race is extremely small, although by good luck such a thing may
occur rarely. It makes no difference whether the character in question is a
dominant or a recessive one, the chance of its becoming established is
exactly the same. If through a mutation a character appears that has an
_injurious_ effect, however slight this may be, it has practically no
chance of becoming established.
[Illustration: FIG. 94. Evolution of elephant's trunk. (After Lull.)]
If through a mutation a character appears that has a _beneficial_ influence
on the individual, the chance that the individual will survive is
increased, not only for itself, but for all of its descendants that come to
inherit this character. It is this increase in the number of individuals
possessing a particular character, that might have an influence on the
course of evolution. This gives a better chance for improvement by several
successive steps; but not because the species is more likely to mutate
again in the same direction. An imaginary example will illustrate how this
happens: When elephants had trunks less than a foot long, the chance of
getting trunks more than one foot long was in proportion to the length of
trunks already present and to the number of individuals; but increment in
trunk length is no more likely to occur from an animal having a trunk more
than one foot long than from an animal with a shorter trunk.
The case is analogous to tossing pennies. At any stage in the game the
chance of accumulating a hundred heads is in proportion to the number of
heads already obtained, and to the number of throws still to be made. But
the number of heads obtained has no influence on the number of heads that
will appear in the next throw.
[Illustration: FIG. 95. Evolution of elephant's trunk: above Maeritherium,
in the middle Tetrabelodon (After Lancaster); below African elephants
(After Gambier Bolton).]
Owing then to this property of the germ plasm to duplicate itself in a
large number of samples not only is an opportunity furnished to an
advantageous variation to become extensively multiplied, but the presence
of a large number of individuals of a given sort prejudices the probable
future result.
The question may be raised as to whether it is desirable to call selection
a _creative_ process. There are so many supernatural and mystical
implications that hang around the term creative that one can not be too
careful in stating in what sense the term is to be used. If by creative is
meant that something is made out of nothing, then of course there is no
need for the scientist to try to answer such a question. But if by a
creative process is meant that something is made out of something else,
then there are two alternatives to be reckoned with.
First, if it were true that selection of an individual of a certain kind
determines that new variations in the same direction occur as a consequence
of the selection, then selection would certainly be creative. How this
could occur might be quite unintelligible, but of course it might be
claimed that the point is not whether we can explain how creation takes
place, but whether we can get verifiable evidence that such a kind of thing
happens. This possibility is disposed of by the fact that there is no
evidence that selection determines the direction in which variation occurs.
Second, if you mean by a creative process that by picking out a certain
kind of individual and multiplying its numbers a better chance is furnished
that a certain end result will be obtained, such a process may be said to
be creative. This is, I think, the proper use of the term creative in a
mechanistic sense.
CONCLUSIONS
In reviewing the evidence relating to selection I have tried to handle the
problem as objectively as I could.
The evidence shows clearly that the characters of wild animals and plants,
as well as those of domesticated races, are inherited both in the wild and
in the domesticated forms according to Mendel's Law.
The causes of the mutations that give rise to new characters we do not
know, although we have no reason for supposing that they are due to other
than natural processes.
Evolution has taken place by the incorporation into the race of those
mutations that are beneficial to the life and reproduction of the organism.
Natural selection as here defined means both the increase in the number of
individuals that results after a beneficial mutation has occurred (owing to
the ability of living matter to propagate) and also that this preponderance
of certain kinds of individuals in a population makes some further results
more probable than others. More than this, natural selection can not mean,
if factors are fixed and are not changed by selection.
* * * * *
INDEX
Abnormal abdomen 109
Abraxas 78-81
Allantois 17
Allelomorphs 83-84
Altenburg 112
Amnion 16-17
Andalusian fowl 45, 46
Annelids 22
Antlered wing 111
Apterous wing 11
Arc wing 111
Aristae 104
Bar eye 67, 108, 169
Bateson 18, 34, 36
Beaded wing 11, 115
Beans 147-149, 157
Belgian hare 171
Bent wing 116
Bergson 30, 31
Bildungstrieb 34
Biogenetic law 15, 18, 19, 21
Biometricians 156
Bird 21, 23
Bithorax 65, 112, 113
Black body color 111, 133
Blakeslee 152
Bridges 114, 143, 163
British Association 36
Bruenn 40
Buff eye color 109
Bufon 27
Castle 176-180
Cat 33
Cell 90, 91
Chance variations 37
Chick 16, 17, 20
Chromatin 184
Chromosome group of Drosophila 102
Chromosomes 91, 95, 96, 98, 130, 131, 132
Cleavage 21, 22, 94
Clover butterfly 62
Club wing 69, 70, 108
Colias philodice 62
Color blindness 77, 125
Comb of Drosophila 103
Combs of fowls 33, 54
Comparative anatomy 7, 8, 9, 14
Corn 150, 153, 172
Correns 41
Cosmogonies 27
Cream eye color 163, 164
Crepidula 22
Criss-cross inheritance 78
Crossing over 131-133
Cuenot 178
Curled wing 115
Curved wing 111
Curve of probability 149
Cut wing 11, 104
Dachs legs 112
Dahlgren 62
Darwin 15, 24, 28, 32, 35-37, 64, 145, 146, 152, 154-156
Dendy 188
De Vries 18, 147, 156
Dexter 170
Dichaete 114
Difflugia 184-187
Discontinuous variation 13
Disuse 31
Drosophila ampelophila 10, 12, 13, 48-50, 60, 75, 84, 85, 93, 100, 103,
119, 155, 162, 169
Drosophila repleta 76
Duplication of legs 109
Dwarf 114
East 170, 172
Ebony 50, 55, 56, 115
Egg 91, 94
Elephant 191
Elephants' skulls 188
Elephants' trunks 190
Embryology 13-23
Emerson 172
Environment 27
Eosin eye color 61, 107, 163
Erdmann 183
Evolution Creatrice 30
Evolution--three kinds of 1, 2, 4
Eye color 13
Eyeless 66, 115
Factorial theory 89
Factors of Drosophila 143
Fantails 172, 175
Fertilization 91
Fish 16, 20, 21
Flatworms 22
Fluctuations 12
Forked bristles 106
Fowl 77
Fused veins 107, 108
Galton 154
Geneticist 26
Germ-plasm 142
Geoffroy St. Hilaire 27
Giant 114
Gill-slits 20, 21, 23
Groups I, II, III, IV 100-118
Haeckel 15
Haemophilia 77
Heliotropism 106, 107
Himalyan rabbits 83
History 1, 6
Hoge 66
Horse, evolution of 6
Indian corn 172, 173
Interference 137, 138
Janssens 132
Jaunty wing 111
Jennings 161, 181-184, 186
Johannsen 156, 157, 159-161, 166, 182
Lamarck 31-34
Langshan 77
Leaves 147
Leidy 186
Lethal 105
Linkage groups 103
Lizard 23
Localization of factors 118
MacDowell 155, 170, 171
Macritherium 191
Mammal 16, 21, 23
Man 20, 77, 125, 126
Map of Chromosomes 136
Maroon eye color 114
Mendel 40, 41, 52, 89
Mendelian heredity 39
Mendel's law 41-59, 64, 124
Mendel's second law 52
Mesenchyme cells 22
Mesoderm cells 22
Metaphysician 30
Mice 33, 178
Middleton 183
Miniature wing 108
Mirabilis 42
Modifiers 163, 164, 170, 171
Molluscs 22
Mouse 83
Muller 112, 167
Mutations 35, 39, 84
Naegeli 34, 35
Natural Selection 36, 145, 146, 187-194
Nisus formativus 34
Non-disjunction 139-142
Notch wing 104-106
Nucleus 91
Origin of Species 35, 145
Orthogenesis 34
Paleontology 24-27
Papilio polytes 63
Papilio turnus 63
Paramecium 181, 182
Paratettix 81
Peach eye color 114
Pea comb 54
Pearl 161
Peas 47
Pigeons 172, 174, 175
Pink eye color 114, 115
Planarian 22
Plymouth Rock 77
Podarke 22
Polar bodies 126
Pole arms 5
Protozoa 181
Pseudo-parthenogenesis 183
Purple eye color 109
Purpose 4
Rabbits 83, 170
Rats 176-180
Reduction division 182
Reproductive cells 96
Ruby eye color 106
Rudimentary organ 116
Rudimentary wing 70, 71, 107
Sable body color 107
Science definition of 6
Segregation 41
Selenka 94
Sepia eye color 13, 114
Sex chromosomes 118
Sex linked inheritance 75, 118-130
Sexual dimorphism 62
Sheep 33
Single comb 54
Sooty body color 50, 114, 115
Speck 68, 69, 111
Spencer 145
Spermatozoon 91, 98
Stars, evolution of 6
St. Hilaire 27-30
Strap wing 110, 111
Stumpy wing 11
Sturtevant 76, 143
Stylonychia 183
Survival of the fittest 146
Systematist 85
Tails 33
Tan flies 106, 107
Tetrabelodon 191
Trefoil 111
Truncate wing 111, 112, 167, 168
Unfolding principle 34
Unio 22
Unit character 74, 75
Use 31
Variation discontinuous 13
Vermilion eye color 108, 163
Vestigial wing 11, 55, 56, 109, 133
Vital force 34
Wallace 36
Walnut comb 54
Weismann 17, 31-33
Wilson, E. B. 125
Wingless 67
Winiwarter 126
White eye color 13, 75, 119-130
Whiting eye color 163, 164
Woodruff 183
Yellow body color 108, 133
Yolk sac 16, 17
Zeleny 169
* * * * *
Corrections made to printed original.
page 104, "shown in figures 53, 54, 55, 56": '52, 53, 54, 55' in original.