THE BEARING OF MENDEL'S DISCOVERY ON THE ORIGIN OF HEREDITARY CHARACTERS
Between the years 1857 and 1868 Gregor Mendel, Augustinian monk, studied
the heredity of certain characters of the common edible pea, in the garden
of the monastery at Bruenn.
In his account of his work written in 1868, he said:
"It requires indeed some courage to undertake a labor of such a
far-reaching extent; it appears, however, to be the only right way by
which we can finally reach the solution of a question the importance of
which cannot be over-estimated in connection with the history of the
evolution of organic forms."
He tells us also why he selected peas for his work:
"The selection of the plant group which shall serve for experiments of
this kind must be made with all possible care if it be desired to avoid
from the outset every risk of questionable results."
"The experimental plants must necessarily
1. Possess constant differentiating characters.
2. The hybrids of such plants must, during the flowering period, be
protected from the influence of all foreign pollen, or be easily
capable of such protection."
Why do biologists throughout the world to-day agree that Mendel's discovery
is one of first rank?
A great deal might be said in this connection. What is essential may be
said in a few words. Biology had been, and is still, largely a descriptive
and speculative science. _Mendel showed by experimental proof that heredity
could be explained by a simple mechanism. His discovery has been
exceedingly fruitful._
Science begins with naive, often mystic conceptions of its problems. It
reaches its goal whenever it can replace its early guessing by verifiable
hypotheses and predictable results. This is what Mendel's law did for
heredity.
MENDEL'S FIRST DISCOVERY--SEGREGATION
[Illustration: FIG. 13. Diagram illustrating a cross between a red (dark)
and a white variety of four o'clock (Mirabilis jalapa).]
Let us turn to the demonstration of his first law--the law of segregation.
The first case I choose is not the one given by Mendel but one worked out
later by Correns. If the common garden plant called four o'clock (Mirabilis
jalapa) with red flowers is crossed to one having white flowers, the
offspring are pink (fig. 13). The hybrid, then, is intermediate in the
color of its flowers between the two parents. If these hybrids are inbred
the offspring are white, pink and red, in the proportion of 1:2:1. All of
these had the same ancestry, yet they are of three different kinds. If we
did not know their history it would be quite impossible to state what the
ancestry of the white or of the red had been, for they might just as well
have come from pure white and pure red ancestors respectively as to have
emerged from the pink hybrids. Moreover, when we test them we find that
they are as pure as are white or red flowering plants that have had all
white or all red flowering ancestors.
Mendel's Law explains the results of this cross as shown in figure 14.
The egg cell from the white parent carries the factor for white, the pollen
cell from the red parent carries the factor for red. The hybrid formed by
their union carries both factors. The result of their combined action is to
produce flowers intermediate in color.
When the hybrids mature and their germ cells (eggs or pollen) ripen, each
carries only one of these factors, either the red or the white, but not
both. In other words, the two factors that have been brought together in
the hybrid separate in its germ cells. Half of the egg cells are white
bearing, half red bearing. Half of the pollen cells are white bearing, half
red bearing. Chance combinations at fertilization give the three classes of
individuals of the second generation.
[Illustration: FIG. 14. Diagram illustrating the history of the factors in
the germ cells of the cross shown in Fig. 13.]
The white flowering plants should forever breed true, as in fact they do.
The red flowering plants also breed true. The pink flowering plants, having
the same composition as the hybrids of the first generation, should give
the same kind of result. They do, indeed, give this result i.e. one white
to two pink to one red flowered offspring.
[Illustration: FIG. 15. Diagram illustrating a cross between special races
of white and black fowls, producing the blue (here gray) Andalusian.]
Another case of the same kind is known to breeders of poultry. One of the
most beautiful of the domesticated breeds is known as the Andalusian. It is
a slate blue bird shading into blue-black on the neck and back. Breeders
know that these blue birds do not breed true but produce white, black, and
blue offspring.
[Illustration: FIG. 16. Diagram showing history of germ cells of cross of
Fig. 15. The larger circles indicate the color of the birds; their enclosed
small circles the nature of the factors in the germ cells of such birds.]
The explanation of the failure to produce a pure race of Andalusians is
that they are like the pink flowers of the four o'clock, i.e., they are a
hybrid type formed by the meeting of the white and the black germ cells. If
the whites produced by the Andalusians are bred to the blacks (both being
pure strains), all the offspring will be blue (fig. 15); if these blues are
inbred they will give 1 white, to 2 blues, to 1 black. In other words, the
factor for white and the factor for black separate in the germ cells of the
hybrid Andalusian birds (fig. 16).
[Illustration: FIG. 17. Diagram of Mendel's cross between yellow (dominant)
and green (recessive) peas.]
The third case is Mendel's classical case of yellow and green peas (fig.
17). He crossed a plant belonging to a race having yellow peas with one
having green peas. The hybrid plants had yellow seeds. These hybrids inbred
gave three yellows to one green. The explanation (fig. 18) is the same in
principle as in the preceding cases. The only difference between them is
that the hybrid which contains both the yellow and the green factors is in
appearance not intermediate, but like the yellow parent stock. Yellow is
said therefore to be dominant and green to be recessive.
[Illustration: FIG. 18. Diagram illustrating the history of the factors in
the cross shown in Fig. 17.]
Another example where one of the contrasted characters is dominant is shown
by the cross of Drosophila with vestigial wings to the wild type with long
wings (fig. 19). The F_1 flies have long wings not differing from those of
the wild fly, so far as can be observed. When two such flies are inbred
there result three long to one vestigial.
[Illustration: FIG. 19. Diagram illustrating a cross between a fly
(Drosophila ampelophila) with long wings and a mutant fly with vestigial
wings.]
The question as to whether a given character is dominant or recessive is a
matter of no theoretical importance for the principle of segregation,
although from the notoriety given to it one might easily be misled into the
erroneous supposition that it was the discovery of this relation that is
Mendel's crowning achievement.
Let me illustrate by an example in which the hybrid standing between two
types overlaps them both. There are two mutant races in our cultures of the
fruit fly Drosophila that have dark body color, one called sooty, another
which is even blacker, called ebony (fig. 20). Sooty crossed to ebony gives
offspring that are intermediate in color. Some of them are so much like
sooty that they cannot be distinguished from sooty. At the other extreme
some of the hybrids are as dark as the lightest of the ebony flies. If
these hybrids are inbred there is a continuous series of individuals,
sooties, intermediates and ebonies. Which color here shall we call the
dominant? If the ebony, then in the second generation we count three
ebonies to one sooty, putting the hybrids with the ebonies. If the dominant
is the sooty then we count three sooties to one ebony, putting the hybrids
with the sooties. The important fact to find out is whether there actually
exist three classes in the second generation. This can be ascertained even
when, as in this case, there is a perfectly graded series from one end to
the other, by testing out individually enough of the flies to show that
one-fourth of them never produce any descendants but ebonies, one-fourth
never any but sooties, and one-half of them give rise to both ebony and
sooty.
[Illustration: FIG. 20. Cross between two allelomorphic races of
Drosophila, sooty and ebony, that give a completely graded series in F_2.]
MENDEL'S SECOND DISCOVERY--INDEPENDENT ASSORTMENT
Besides his discovery that there are pairs of characters that disjoin, as
it were, in the germ cells of the hybrid (law of segregation) Mendel made a
second discovery which also has far-reaching consequences. The following
case illustrates Mendel's second law.
If a pea that is yellow and round is crossed to one that is green and
wrinkled (fig. 21), all of the offspring are yellow and round. Inbred,
these give 9 yellow round, 3 green round, 3 yellow wrinkled, 1 green
wrinkled. All the yellows taken together are to the green as 3:1. All the
round taken together are to the wrinkled as three to one; but some of the
yellows are now wrinkled and some of the green are now round. There has
been a recombination of characters, while at the same time the results, for
each pair of characters taken separately, are in accord with Mendel's Law
of Segregation, (fig. 22). The second law of Mendel may be called the law
of independent assortment of different character pairs.
[Illustration: FIG. 21. Cross between yellow-round and green-wrinkled peas,
giving the 9: 3: 3: 1 ratio in F_2.]
We can, as it were, take the characters of one organism and recombine them
with those of a different organism. We can explain this result as due to
the assortment of factors for these characters in the germ cells according
to a definite law.
[Illustration: FIG. 22. Diagram to show the history of the factor pairs
yellow-green and round-wrinkled of the cross in Fig. 21.]
As a second illustration let me take the classic case of the combs of
fowls. If a bird with a rose comb is bred to one with a pea comb (fig. 23),
the offspring have a comb different from either. It is called a walnut
comb. If two such individuals are bred they give 9 walnut, 3 rose, 3 pea, 1
single. This proportion shows that the grandparental types differed in
respect to two pairs of characters.
[Illustration: FIG. 23. Cross between pea and rose combed fowls. (Charts of
Baur and Goldschmidt.)]
A fourth case is shown in the fruit fly, where an ebony fly with long wings
is mated to a grey fly with vestigial wings (fig. 24). The offspring are
gray with long wings. If these are inbred they give 9 gray long, 3 gray
vestigial, 3 ebony long, 1 ebony vestigial (figs. 24 and 25).
[Illustration: FIG. 24. Cross between long ebony and gray vestigial flies.]
The possibility of interchanging characters might be illustrated over and
over again. It is true not only when two pairs of characters are involved,
but when three, four, or more enter the cross.
[Illustration: FIG. 25. Diagram to show the history of the factors in the
cross shown in Fig. 24.]
It is as though we took individuals apart and put together parts of two,
three or more individuals by substituting one part for another.
Not only has this power to make whatever combinations we choose great
practical importance, it has even greater theoretical significance; for, it
follows that the individual is not in itself the unit in heredity, but that
within the germ-cells there exist smaller units concerned with the
transmission of characters.
The older mystical statement of the individual as a unit in heredity has no
longer any interest in the light of these discoveries, except as a past
phase of biological history. We see, too, more clearly that the sorting out
of factors in the germ plasm is a very different process from the influence
of these factors on the development of the organism. There is today no
excuse for confusing these two problems.
If mechanistic principles apply also to embryonic development then the
course of development is capable of being stated as a series of
chemico-physical reactions and the "_individual_" is merely a term to
express the sum total of such reactions and should not be interpreted as
something different from or more than these reactions. So long as so little
is known of the actual processes involved in development the use of the
term "individuality", while giving the appearance of profundity, in reality
often serves merely to cover ignorance and to make a mystery out of a
mechanism.
THE CHARACTERS OF WILD ANIMALS AND PLANTS FOLLOW THE SAME LAWS OF
INHERITANCE AS DO THE CHARACTERS OF DOMESTICATED ANIMALS AND PLANTS.
Darwin based many of his conclusions concerning variation and heredity on
the evidence derived from the garden and from the stock farm. Here he was
handicapped to some extent, for he had at times to rely on information much
of which was uncritical, and some of which was worthless.
Today we are at least better informed on _two_ important points; one
concerning the _kinds_ of variations that furnish to the cultivator the
materials for his selection; the other concerning the modes of inheritance
of these variations. We know now that new characters are continually
appearing in domesticated as well as in wild animals and plants, that these
characters are often sharply marked off from the original characters, and
whether the differences are great or whether they are small they are
transmitted alike according to Mendel's law.
Many of the characteristics of our domesticated animals and cultivated
plants originated long ago, and only here and there have the records of
their first appearance been preserved. In only a few instances are these
records clear and definite, while the complete history of any large group
of our domesticated products is unknown to us.
Within the last five or six years, however, from a common wild species of
fly, the fruit fly, Drosophila ampelophila, which we have brought into the
laboratory, have arisen over a hundred and twenty-five new types whose
origin is completely known. Let me call attention to a few of the more
interesting of these types and their modes of inheritance, comparing them
with wild types in order to show that the kinds of inheritance found in
domesticated races occur also in wild types. The results will show beyond
dispute that the characters of wild types are inherited in precisely the
same way as are the characters of the mutant types--a fact that is not
generally appreciated except by students of genetics, although it is of the
most far-reaching significance for the theory of evolution.
A mutant appeared in which the eye color of the female was different from
that of the male. The eye color of the mutant female is a dark eosin color,
that of the male yellowish eosin. From the beginning this difference was as
marked as it is to-day. Breeding experiments show that eosin eye color
differs from the red color of the eye of the wild fly by a single mutant
factor. Here then at a single step a type appeared that was sexually
dimorphic.
Zoologists know that sexual dimorphism is not uncommon in wild species of
animals, and Darwin proposed the theory of sexual selection to account for
the difference between the sexes. He assumed that the male preferred
certain kinds of females differing from himself in a particular character,
and thus in time through sexual selection, the sexes came to differ from
each other.
[Illustration: FIG. 26. Clover butterfly (Colias philodice) with two types
of females, above; and one type of male, below.]
In the case of eosin eye color no such process as that postulated by Darwin
to account for the differences between the sexes was involved; for the
single mutation that brought about the change also brought in the
dimorphism with it.
In recent years zoologists have carefully studied several cases in which
two types of female are found in the same species. In the common clover
butterfly, there is a yellow and a white type of female, while the male is
yellow (fig. 26). It has been shown that a single factor difference
determines whether the female is yellow or white. The inheritance is,
according to Gerould, strictly Mendelian.
[Illustration: FIG. 27. Papilio turnus with two types of females above and
one type of male below.]
In Papilio turnus there exist, in the southern states, two kinds of
females, one yellow like the male, one black (fig. 27). The evidence here
is not so certain, but it seems probable that a single factor difference
determines whether the female shall be yellow or black.
Finally in Papilio polytes of Ceylon and India three different types of
females appear, (fig. 28 to right) only one of which is like the male. Here
the analysis of the breeding data shows the possibility of explaining this
case as due to two pairs Mendelian factors which give in combination the
three types of female.
[Illustration: FIG. 28. Papilio polytes, with three types of female to
right and one type of male above to left.]
Taking these cases together, they furnish a much simpler explanation than
the one proposed by Darwin. They show also that characters like these shown
by wild species may follow Mendel's law.
[Illustration: FIG. 29. Mutant race of fruit fly with intercalated
duplicate mesothorax on dorsal side.]
There has appeared in our cultures a fly in which the third division of the
thorax with its appendages has changed into a segment like the second (fig.
29). It is smaller than the normal mesothorax and its wings are imperfectly
developed, but the bristles on the upper surface may have the typical
arrangement of the normal mesothorax. The mutant shows how great a change
may result from a single factor difference.
A factor that causes duplication in the legs has also been found. Here the
interesting fact was discovered (Hoge) that duplication takes place only in
the cold. At ordinary temperatures the legs are normal.
[Illustration: FIG. 30. Mutant race of fruit fly, called eyeless; a, a'
normal eye.]
In contrast to the last case, where a character is doubled, is the next one
in which the eyes are lost (fig. 30). This change also took place at a
single step. All the flies of this stock however, cannot be said to be
eyeless, since many of them show pieces of the eye--indeed the variation is
so wide that the eye may even appear like a normal eye unless carefully
examined. Formerly we were taught that eyeless animals arose in caves. This
case shows that they may also arise suddenly in glass milk bottles, by a
change in a single factor.
I may recall in this connection that wingless flies (fig. 5 f) also arose
in our cultures by a single mutation. We used to be told that wingless
insects occurred on desert islands because those insects that had the best
developed wings had been blown out to sea. Whether this is true or not, I
will not pretend to say, but at any rate wingless insects may also arise,
not through a slow process of elimination, but at a single step.
The preceding examples have all related to recessive characters. The next
one is dominant.
[Illustration: FIG. 31. Mutant race of fruit fly called bar to the right
(normal to the left). The eye is a narrow vertical bar, the outline of the
original eye is indicated.]
A single male appeared with a narrow vertical red bar (fig. 31) instead of
the broad red oval eye. Bred to wild females the new character was found to
dominate, at least to the extent that the eyes of all its offspring were
narrower than the normal eye, although not so narrow as the eye of the pure
stock. Around the bar there is a wide border that corresponds to the region
occupied by the rest of the eye of the wild fly. It lacks however the
elements of the eye. It is therefore to be looked upon as a rudimentary
organ, which is, so to speak, a by-product of the dominant mutation.
The preceding cases have all involved rather great changes in some one
organ of the body. The following three cases involve slight changes, and
yet follow the same laws of inheritance as do the larger changes.
[Illustration: FIG. 32. Mutant race of fruit fly, called speck. There is a
minute black speck at base of wing.]
At the base of the wings a minute black speck appeared (fig. 32). It was
found to be a Mendelian character. In another case the spines on the thorax
became forked or kinky (fig. 52b). This stock breeds true, and the
character is inherited in strictly Mendelian fashion.
[Illustration: FIG. 33. Mutant race of fruit fly called club. The wings
often remain unexpanded and two bristles present in wild fly (b) are absent
on side of thorax (c).]
In a certain stock a number of flies appeared in which the wing pads did
not expand (fig. 33). It was found that this peculiarity is shown in only
about twenty per cent of the individuals supposed to inherit it. Later it
was found that this stock lacked two bristles on the sides of the thorax.
By means of this knowledge the heredity of the character was easily
determined. It appears that while the expansion of the wing pads fails to
occur once in five times--probably because it is an environmental effect
peculiar to this stock,--yet the minute difference of the presence or
absence of the two lateral bristles is a constant feature of the flies that
carry this particular factor.
In the preceding cases I have spoken as though a factor influenced only one
part of the body. It would have been more accurate to have stated that the
_chief_ effect of the factor was observed in a particular part of the body.
Most students of genetics realize that a factor difference usually affects
more than a single character. For example, a mutant stock called
rudimentary wings has as its principle characteristic very short wings
(fig. 34). But the factor for rudimentary wings also produces other effects
as well. The females are almost completely sterile, while the males are
fertile. The viability of the stock is poor. When flies with rudimentary
wings are put into competition with wild flies relatively few of the
rudimentary flies come through, especially if the culture is crowded. The
hind legs are also shortened. All of these effects are the results of a
single factor-difference.
[Illustration: FIG. 34. Mutant race of fruit fly, called rudimentary.]
One may venture the guess that some of the specific and varietal
differences that are characteristic of wild types and which at the same
time appear to have no survival value, are only by-products of factors
whose most important effect is on another part of the organism where their
influence is of vital importance.
It is well known that systematists make use of characters that are constant
for groups of species, but which do not appear in themselves to have an
adaptive significance. If we may suppose that the constancy of such
characters may be only an index of the presence of a factor whose _chief_
influence is in some other direction or directions, some physiological
influence, for example, we can give at least a reasonable explanation of
the constancy of such characters.
I am inclined to think that an overstatement to the effect that each factor
may affect the entire body, is less likely to do harm than to state that
each factor affects only a particular character. The reckless use of the
phrase "unit character" has done much to mislead the uninitiated as to the
effects that a single change in the germ plasm may produce on the organism.
Fortunately, the expression "unit character" is being less used by those
students of genetics who are more careful in regard to the implications of
their terminology.
There is a class of cases of inheritance, due to the XY chromosomes, that
is called sex linked inheritance. It is shown both by mutant characters and
characters of wild species.
For instance, white eye color in Drosophila shows sex linked inheritance.
If a white eyed male is mated to a wild red eyed female (fig. 35) all the
offspring have red eyes. If these are inbred, there are three red to one
white eyed offspring, but white eyes occur only in the males. The
grandfather has transmitted his peculiarity to half of his grandsons, but
to none of his granddaughters.
[Illustration: FIG. 35. Diagram showing a cross between a white eyed male
and a red eyed female of the fruit fly. Sex linked inheritance.]
The reciprocal cross (fig. 36) is also interesting. If a white eyed female
is bred to a red eyed male, all of the daughters have red eyes and all of
the sons have white eyes. We call this criss-cross inheritance. If these
offspring are inbred, they produce equal numbers of red eyed and white eyed
females and equal numbers of red eyed and white eyed males. The ratio is 1:
1: 1: 1, or ignoring sex, 2 reds to 2 whites, and not the usual 3:1
Mendelian ratio. Yet, as will be shown later, the result is in entire
accord with Mendel's principle of segregation.
[Illustration: FIG. 36. Diagram illustrating a cross between a red eyed
male and white eyed female of the fruit fly (reciprocal cross of that shown
in Fig. 35).]
It has been shown by Sturtevant that in a wild species of Drosophila, viz.,
D. repleta, two varieties of individuals exist, in one of which the thorax
has large splotches and in the other type smaller splotches (fig. 37). The
factors that differentiate these varieties are sex linked.
[Illustration: FIG. 37. Two types of markings on thorax of Drosophila
repleta, both found "wild". They show sex linked inheritance.]
Certain types of color blindness (fig. 38) and certain other abnormal
conditions in man such as haemophilia, are transmitted as sex linked
characters.
[Illustration: FIG. 38, A. Diagram illustrating inheritance of color
blindness in man; the iris of the color-blind eye is here black.]
[Illustration: FIG. 38, B. Reciprocal of cross in Fig. 38 a.]
In domestic fowls sex linked inheritance has been found as the
characteristic method of transmission for at least as many as six
characters, but here the relation of the sexes is in a sense reversed. For
instance, if a black Langshan hen is crossed to a barred Plymouth Rock cock
(fig. 39), the offspring are all barred. If these are inbred half of the
daughters are black and half are barred; all of the sons are barred. The
grandmother has transmitted her color to half of her granddaughters but to
none of her grandsons.
[Illustration: FIG. 39. Sex-linked inheritance in domesticated birds shown
here in a cross between barred Plymouth Rock male and black Langshan
female.]
[Illustration: FIG. 40. Reciprocal of Fig. 39.]
In the reciprocal cross (fig. 40) black cock by barred hen, the daughters
are black and the sons barred--criss-cross inheritance. These inbred give
black hens and black cocks, barred hens and barred cocks.
There is a case comparable to this found in a wild species of moth, Abraxas
grossulariata. A wild variation of this type is lighter in color and is
known as A. lacticolor. When these two types are crossed they exhibit
exactly the same type of heredity as does the black-barred combination in
the domestic fowl. As shown in figure 41, lacticolor female bred to
grossulariata male gives grossulariata sons and daughters. These inbred
give grossulariata males and females and lacticolor females. Reciprocally
lacticolor male by grossulariata female, (fig. 42) gives lacticolor
daughters and grossulariata sons and these inbred give grossulariata males
and females and lacticolor males and females.
[Illustration: FIG. 41. Sex-linked inheritance in the wild moth, Abraxas
grossulariata (darker) and A. lacticolor.]
[Illustration: FIG. 42. Reciprocal of Fig. 41.]
[Illustration: FIG. 43. Four wild types of Paratettix in upper line with
three hybrids below.]
It has been found that there may be even more than two factors that show
Mendelian segregation when brought together in pairs. For example, in the
southern States there are several races of the grouse locust (Paratettix)
that differ from each other markedly in color patterns (fig. 43). When any
two individuals of these races are crossed they give, as Nabours has shown,
in F_2 a Mendelian ratio of 1: 2: 1. It is obvious, therefore, that there
are here at least nine characters, any two of which behave as a Mendelian
pair. These races have arisen in nature and differ definitely and
strikingly from each other, yet any two differ by only one factor
difference.
[Illustration: FIG. 44. Diagram illustrating four allelomorphs in mice,
viz. gray bellied gray (wild type) (above, to left); white bellied gray
(above, to right); yellow (below, to right); and black (below, to left).]
Similar relations have been found in a number of domesticated races. In
mice there is a quadruple system represented by the gray house mouse, the
white bellied, the yellow and the black mouse (fig. 44). In rabbits there
is probably a triple system, that includes the albino, the Himalayan, and
the black races. In the silkworm moth there have been described four types
of larvae, distinguished by different color markings, that form a system of
quadruple allelomorphs. In Drosophila there is a quintuple system of
factors in the sex chromosome represented by eye colors, a triple system of
body colors, and a triple system of factors for eye colors in the third
chromosome.
MUTATION AND EVOLUTION
What bearing has the appearance of these new types of Drosophila on the
theory of evolution may be asked. The objection has been raised in fact
that in the breeding work with Drosophila we are dealing with artificial
and unnatural conditions. It has been more than implied that results
obtained from the breeding pen, the seed pan, the flower pot and the milk
bottle do not apply to evolution in the "open", nature "at large" or to
"wild" types. To be consistent, this same objection should be extended to
the use of the spectroscope in the study of the evolution of the stars, to
the use of the test tube and the balance by the chemist, of the
galvanometer by the physicist. All these are unnatural instruments used to
torture Nature's secrets from her. I venture to think that the real
antithesis is not between unnatural and natural treatment of Nature, but
rather between controlled or verifiable data on the one hand, and
unrestrained generalization on the other.
If a systematist were asked whether these new races of Drosophila are
comparable to wild species, he would not hesitate for a moment. He would
call them all one species. If he were asked why, he would say, I think,
"These races differ only in one or two striking points, while in a hundred
other respects they are identical even to the minutest details." He would
add, that as large a group of wild species of flies would show on the whole
the reverse relations, _viz._, they would differ in nearly every detail and
be identical in only a few points. In all this I entirely agree with the
systematist, for I do not think such a group of types differing by one
character each, is comparable to most wild groups of species because the
difference between wild species is due to a large number of such single
differences. The characters that have been accumulated in wild species are
of significance in the maintenance of the species, or at least we are led
to infer that even though the visible character that we attend to may not
itself be important, one at least of the other effects of the factors that
represent these characters is significant. It is, of course, hardly to be
expected that _any_ random change in as complex a mechanism as an insect
would improve the mechanism, and as a matter of fact it is doubtful whether
any of the mutant types so far discovered are better adapted to those
conditions to which a fly of this structure and habits is already adjusted.
But this is beside the mark, for modern genetics shows very positively that
adaptive characters are inherited in exactly the same way as are those that
are not adaptive; and I have already pointed out that we cannot study a
single mutant factor without at the same time studying one of the factors
responsible for normal characters, for the two together constitute the
Mendelian pair.
And, finally, I want to urge on your attention a question that we are to
consider in more detail in the last lecture. Evolution of wild species
appears to have taken place by modifying and improving bit by bit the
structures and habits that the animal or plant already possessed. We have
seen that there are thirty mutant factors at least that have an influence
on eye color, and it is probable that there are at least as many normal
factors that are involved in the production of the red eye of the wild fly.
Evolution from this point of view has consisted largely in introducing new
factors that influence characters already present in the animal or plant.
Such a view gives us a somewhat different picture of the process of
evolution from the old idea of a ferocious struggle between the individuals
of a species with the survival of the fittest and the annihilation of the
less fit. Evolution assumes a more peaceful aspect. New and advantageous
characters survive by incorporating themselves into the race, improving it
and opening to it new opportunities. In other words, the emphasis may be
placed less on the competition between the individuals of a species
(because the destruction of the less fit does not _in itself_ lead to
anything that is new) than on the appearance of new characters and
modifications of old characters that become incorporated in the species,
for on these depends the evolution of the race.
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