GENERAL DISCUSSION.
A. RELATION OF HEREDITY AND ONTOGENY.
In studying heredity our attention must often be focused on the
ontogenesis of the different characters, and we are sometimes
inclined to regard the adult character as the product of the course
of ontogenesis. But this is a superficial way of looking at things;
the determiners of all characters are in the germ-plasm and together
they direct the development of one part after another in orderly
succession; a modernized form of the pre-formation doctrine seems
logically necessary.
What do we know of the processes that take place in bringing the
fertilized egg, freighted with its specific heredity, to its
destination--the adult form? Modern embryological and cytological
studies give us an insight into many of them. First of all, the egg
has a certain organization that foreshadows something of its fate.
Then cell-divisions begin, at first synchronous, but later becoming
accelerated here and retarded there. Eventually (especially among
animals) these cells become arranged into a membrane whose unequal
growth in limited areas produces foldings. The folding of membranes,
their stretching, local thickenings, or thinnings are largely
the result of local inhibitions of water. Sometimes movements of
individual cells occur out of the membranes into and through cavities
or solid yolk-masses, and by the aggregation of such cells massive
organs are sometimes formed. Local absorption of tissues already
established may be effected in later life by such migratory cells.
Membranes once established may form pockets or linear folds, as in
gastrulation and gland formation; they may become perforated; two
membranes may fuse along areas or lines and a perforation may even
occur at the region of fusion. Linear strands or tubules may grow
out, making connections, as nerves do, with distant organs; tubes
may unite to form a network, or split lengthwise. Finally, membranes
and masses undergo vacuolization, or masses may split apart or fuse
together. Thus in the ontogeny that is proceeding under the control
of heredity all is motion and change.
What are the factors that control all these movements--for these are
the true factors of heredity? We do not know much about them, but we
know some things. We know that cell-divisions occur at particular
times and places under the influence of preceding division planes;
but their normal occurrence may be interfered with by an abnormal
chemical condition of the environment.
We have reason for concluding that each developmental process is
a "response"--a reaction of the living, streaming protoplasm to
changing environment. The nature of the response to any stimulus
probably depends on the chemical constitution of the protoplasm--and
this is hereditary. In an important sense heredity is the control of
ontogeny.
The _specific_ characteristics are mostly those that appear late in
ontogeny. The integumentary folds over the nasal bones of the chick
appear on or about the tenth day. At that time it can be ascertained
whether the comb is median, or multiple, or Y-shaped, or
cup-shaped, or consists of 2 papillæ. In the case of the single-comb
the fold is linear and single; in the case of the pea-comb, linear
and triple; in the case of the rose-comb, quintuple or irregularly
wrinkled over the whole area; in the case of the Polish-comb, there
is a pair of "pocket folds." In the single-combed fowl the single
linear fold grows quickly to a great height and very thin, while
in the pea-comb, with its additional pair of wrinkles, the median
element is not so high as in typical single-combed races; in the
pea-comb there is an additional folding stimulus and a reduced growth
stimulus. In the heterozygote both stimuli are weakened; the lateral
folds are usually much reduced--"are hard to make out," as I stated
in 1906 (p. 35); and the factor that determines the continued growth
(elevation) of the fold is weakened, so that the pea-comb--although
"abnormally high" (1906, p. 35, figs. 20 and 21)--is not nearly as
high as the single-comb of the Minorca (1906, fig. 4).
Two results are evident: first, each character in the heterozygous
condition is reduced, and, second, each is much more variable than
in the homozygous condition. Why is the character reduced? If the
reaction to continued growth of the fold is strong in one race and
weak in the other, then in the heterozygote that reaction, whatever
its nature, is reduced. Why is the reduction in the response so
variable? There is a variation in the irritability or other growing
factor of the embryonic material that is destined to form the fold.
Even Minorcas vary in the growth of the comb, and so do the Dark
Brahmas. Let _G_ be a constant element of the growth factor of the
Minorca's comb; then _G + a_ or _G - a_ will indicate its variants.
Let _g_ be the growth factor of the Brahma's comb, and _g + a_ and
_g - a_ its variants. Then the hybrids of these two races may be of
the following types: _Gg_, _Gg + a_, _Gg - a_, _Gg + 2a_, _Gg - 2a_.
This gives 5 varying conditions instead of 3 and greater extremes of
variation.
In the foregoing case I have assumed that the positive character is
that of increased growth in the Minorca; but the positive character
may be an _inhibition_ to indefinite growth of the pea-comb.
Heredity may be conceived of as exerting at all points a control on
developmental processes--sometimes initiating and continuing this;
but often, on the other hand, slowing down or wholly inhibiting
that. The inhibition of a process is quite as _positive_ a function
of heredity as its initiation. The hair of a young rabbit grows
until it attains a certain length and then the growth ceases. The
growing character is a youthful, embryonic one; the new character is
the stoppage of growth. Similarly the young feathers of birds grow
continuously until something intervenes that stops the growth and
dries up the sheath. Now, in Angora rabbits and long-tailed fowl
the epidermal organ continues its embryonic growth indefinitely;
the something that intervenes to stop growth is absent. There is no
reason for regarding the long hair or long feather as a positive
condition and short hair or feather as due to its absence.
Again, Mediterranean fowl have non-feathered shanks; but in Asiatics
the feet are feathered like the rest of the body (except the soles
and face). It has been assumed that _boot_ is an additional character
and should be dominant over absence of boot. But, on the other hand,
we may well think of the capacity of producing feathers as general to
the skin. From this point of view the real question is, what prevents
feather production on the eyelids, comb, wattles, and shank? It
seems equally probable that there is an inhibitor of feather-growth
for these few areas as that every conceivable area of the body has
its special stimulus factor for feather development; or even as
that there is such a factor to each separate feather-tract. In the
Minorca, then, the inhibitor of boot is present; in the Silkie a weak
heterozygous inhibition appears; but in the Dark Brahma there is no
inhibitor and feathers extend down from the heel over the whole of
front and sides of the foot and even on the upper surface of the
toes--just as they do over the anterior appendages.
The case of the rumpless fowl is important in relation to the
hypothesis of inhibitors. Either tail-production depends on a special
factor _TT_, which is diluted, as _Tt_, in the heterozygote; or else
there is a tail inhibitor, _II_, which is diluted, as _Ii_, in the
heterozygote. In F2 we expect, on the one hypothesis, 25 per cent
_tt_, giving no tail, and 25 per cent _TT_, giving tail; on the other
hypothesis 25 per cent _ii_, giving tail, and 25 per cent _II_,
giving no tail. Actually we get all tailed in some cases; in others
25 per cent with no tail. Which hypothesis best fits the facts?
Which is the more probable--that the 25 per cent recessive no-tail
should produce a tail (as it were, out of nothing) or that the 25
per cent dominant tail inhibitor should be ineffective, permitting
the development of a tail? It is clear that the ontogenetic failure
of an inhibitor is easier to understand than the development of a
character that is not represented at all in the germ-plasm. This
matter is treated in another connection in the next section. But
the present point is that it is equally in accord with the facts to
regard heredity as initiating and inhibiting processes. If, indeed,
processes were not regularly inhibited, they must, when once started,
go on indefinitely, as do the hairs of Angora goats and wonder-horses.
As we have seen, ontogeny is not completed at hatching or birth. Many
characters are at that time undeveloped. Hence, not infrequently
the recessive condition is at first seen and is only later
replaced by the dominant condition. The reverse sequence will
rarely be followed, because development rarely, except in cases of
degeneration, moves backward. One of the familiar cases of this sort
is human hair-color. In youth this is frequently flaxen, later it
becomes light brown, and eventually it may become dark brown. Darwin
gives a number of examples in his Chapter XII of Animals and Plants
under Domestication. To these I may add some from my own experience.
The hybrids between white and gray Java sparrows are at first light
and later become of a slaty gray like the dark parent. Many black
fowl gain white feathers as they grow older, and every fancier knows
that birds with complex white-and-black patterns can usually be
"exhibited" only once, on account of loss of "standard" coloration
late in life. In these cases the advanced condition in the series
of melanic colors appears only late in ontogeny.[13] Similarly Lang
(1908, p. 54) finds that in snail hybrids often the young shells
have the recessive yellow color, only later in life showing the
dominant red color. This is, of course, no reversal of dominance in
ontogeny, but mere ontogenesis of pigmentation. So in general, since
the recessive condition is absence of the character or its low stage
of development and the dominant condition is presence of the full
character, the individual in ontogenesis may exhibit in succession
the recessive and then the dominant character, but not in the reverse
order.
[13]: Does the graying of human hair represent an
ontogenetically advanced condition of the melanic pigment as
yellow represents the embryonic condition?
B. DOMINANCE AND RECESSIVENESS.
If segregation is the cornerstone of modern studies in heredity,
dominance forms an important part, at least, of the foundation. In
any case, a critical examination of dominance is now required; the
more so since its significance and value have often been doubted.
First, how is a dominant character to be defined? It has been defined
both on the basis of visible results in mating and on the basis of
its essential nature. On the basis of visible results in hybridizing
dominant characters may be defined as Mendel (1866, p. 11) defined
them: "jene Merkmale, welche ganz oder fast unverändert in die
Hybride-Verbindung übergehen." Bateson's translation (1902, p. 49)
renders this passage: "those characters which are transmitted entire,
or almost unchanged in the hybridization."
On the basis of the essential nature of the dominant character there
has obtained a great diversity of definitions. Thus de Vries (1900,
p. 85) suggested that the "systematically higher" character is the
dominating one, and, again (1902, pp. 33, 145), that the dominant
character is the phylo-genetically older one. Many have suggested
that it is the positive or present character that dominates over the
negative, latent or absent. This last idea has become the prevailing
one and its history is worth summarizing.
As early as 1902, Correns used as Mendelian pairs, presence of
coloring material and absence; also modification into yellow and _no_
modification. In 1905, he extended somewhat this use of present and
absent characters, _k_ (keine) preceding the symbol of a character as
a negative. Still he did not pretend to generalize the relation of
dominance and recessiveness to be that of presence and absence. In
1903 (p. 146) de Vries stated that in very many cases Mendel's law
held when one quality is active and the other latent, and that the
active quality is dominant. His illustrations show that by activity
he meant essentially presence, by latency absence from the visible
soma. Bateson's third report (1906) applies presence and absence
to several additional cases, and, at the International Genetics
Conference of that year, Hurst developed the presence-and-absence
hypothesis, favoring the view that the factor for absence is nothing
at all, but finding that certain cases, such as Angora coat, offer a
difficulty. At the same meeting I suggested that "a variation * * *
that is due to abbreviation of the ontogenetic process, which depends
on something having dropped out, will be recessive," a progressive
variation dominant; and in 1908 I expressed the conclusion that
"dominance in heredity appears when a stronger determiner meets a
weaker determiner in the germ. The extreme case is that in which a
strong determiner meets a determiner so weak as to be practically
absent, as when a red flower is crossed with white." I suggested that
in some cases of recessiveness of an apparent advanced condition,
like Angora hair, the dominant factor is an inhibitor. In the
last year or two the presence-and-absence theory has gained wide
acceptance, but I still think the cases where there is dominance of
the advanced condition over the less advanced--of the quantitatively
well-developed over the quantitatively less well-developed--have
not been sufficiently considered. In human hair-color any other
hypothesis demands that there are many units in the higher grades
of pigmentation and fewer in the lower grades and that the presence
of the surplus factor in any other higher grade dominates over
its absence in the next lower grade; but there is no evidence in
human hair-color of distinct, discontinuous units in the common
yellow-brown series. And, in ontogeny, the different grades of color
form a _continuous_ series whose development proceeds throughout
early life and may even be stimulated to an advanced stage of
darkening by disease. The _cessation_ of color development may take
place at any point, and this seems incompatible with the theory of
unit-characters for the different grades of human hair-color. In
the present paper, on the other hand, the characters dealt with are
mostly unit-characters and their quantitative variations mostly
heterozygotic. Even the case of the Silkie boot (table 31, C)
referred to in an earlier paper[14] as illustrating recessiveness
of the less advanced condition proves, on further analysis, to be
a case of heterozygotism. It seems highly probable that the future
will show that many more advanced or progressive conditions are
really due to one or more unit-characters not present in the less
advanced condition. In that case it will appear that there is perfect
accord in the two statements that the progressive condition and the
"present" factor are dominant.
[14]: Davenport, 1908, page 60.
The definition of dominance on the ground of results meets at the
outset with a difficulty the germ of which is observable in Mendel's
cautious statement "ganz oder _fast_ unverändert." Even Mendel
observed that the hybrids between white-flowered and purple-red
flowered peas have flowers less intensely colored than the darker
parent. The experiments of the last seven years have shown that
the "dominant" character is often very greatly changed--indeed,
in extreme cases a blending of characters may occur--in the first
generation. Correns (1900 _b_, p. 110) very early stated that in a
certain set of crosses between good species the hybrids showed the
character of _both_ parents, only reduced, but in varying degrees.
Bateson and Saunders (1902, p. 23) found in crossing two forms of
_Datura_ that--
Although the offspring resulting from a cross between any two
of the forms employed are usually indistinguishable from the
type which is dominant as regards the particular character
crossed, yet in other cases the intensity of a dominant
character may be more or less diminished either in particular
individuals or in particular parts of one individual. In
_Tatula-Stramonium_ cross-breds the corolla is often paler in
color than that of the dominant parent (as has already been
noticed by Naudin), but even in the palest specimens the deep
blue color of the unopened anthers leaves no doubt as to the
presence of the dominant color element. * * * The occurrence
of intermediate forms was also occasionally noticeable in the
fruits. Among the large number of capsules examined, there
were some of the mosaic type, in which part of the capsule was
prickly and the remainder smooth, while others, suggesting a
blend, were more or less prickly all over, but the prickles
were much reduced in size, and often formed mere tubercles.
Bateson and Saunders further showed (1902, p. 123) that in the case
of comb and extra-toe in poultry "the cross-bred may show some
blending and * * * the intensity of the dominant character is often
considerably reduced."
Correns (1905, p. 9) pointed out that there was known, even at
that time, a complete series of cases at one extreme of which one
determiner completely hindered the appearance of the other, while
at the opposite end of the series the hybrid showed an intermediate
condition, both determiners appearing with equal strength.
The following year, in my first report on Inheritance in Poultry,
I laid great stress on the imperfection of dominance, and this
phenomenon has become more striking and clear in the subsequent
years, until in the present paper it is recognized as the key to the
explanation of many apparently anomalous types of heredity.
The first case in the present work in which imperfection of dominance
is considered is that of the hybrids between I and oo comb.
Here median comb is mated with no-median. Each somatic cell of
the hybrid--at least in the comb region--has only half the full
determiner for median comb. The determiner is weakened, and so the
median comb is imperfectly developed, namely, at the anterior end of
its proper territory. The weakening varies much in degree in the
heterozygote. The median comb may be reduced to 70 per cent of its
normal length or it may not develop at all.
The second case of imperfection of dominance is that of
polydactylism. Extra-toe mated to normal gives extra-toe in 73 per
cent only of the offspring in the case of the Houdans. Any trace of 6
toes (on one or both feet) is found in only 12 per cent of the hybrid
offspring from a 6-toed Silkie parent. Certainly dominance here is
very like blending.
The third case of imperfection of dominance is that of syndactylism.
No syndactyls were noticed in F1. My first conclusion was that
syndactylism is recessive; but later studies have shown that it is
dominant and that all matings of two syndactyl parents yield about 56
per cent syndactyl offspring.
Rumplessness gives an illustration of how dominance may be so weak
as to be absent altogether; so that from F1 alone the erroneous
conclusion is drawn that it is recessive; indeed, in one strain, only
faint traces of the character made their appearance in successive
generations.
Finally, winglessness is a character which _appears_ not to be
inherited at all. Nevertheless our experience with rumplessness
leads us to suspect that winglessness also is an impotently dominant
character.
Looking at the matter frankly and without prejudice, the question
must be answered: Has not the whole hypothesis of dominance become
_reductio ad absurdum_? What visible criterion of dominance remains,
where dominance fails completely? All the usual statistical landmarks
of proportional appearance in successive generations being lost, can
one properly speak of dominance and recessiveness at all?
Amid the general ruin of criteria, however, one means of detecting
dominance remains. That extracted character which in F2 or subsequent
generations shows in homologous[15] matings in some families a wide
range of variability is dominant, while that extracted character
which constantly, in all homologous matings, shows no or very little
variation is recessive.
[15] By homologous matings I mean those in which the
germ-plasms of _both_ parents are in the same condition with
reference to the unit-character; _i. e._, both either possess
it pure or lack it altogether.
The reason for this difference in the inheritableness of the two
conditions is easy to understand on the principles enumerated in
the last section. A positive character has a real ontogeny. But, as
we have seen, the development of any character may be _interrupted_
at any stage. Most aberrations among organisms are due to a
retardation or failure of normal development. In human affairs we
recognize this tendency in the terms "degenerates" and "defectives"
(constituting from 2 to 4 per cent of the population). Indeed, there
are few persons who are not defective in some physical or psychical
character. In cases where the commonest form of abnormality is due to
a development _in excess_ it seems probable that a normal restraining
or inhibiting factor is defective or absent. On page 88 I tried to
show how common in ontogeny such restraining and inhibiting factors
are. Since ontogenetic processes are so often cut short by external
conditions, we can understand the _variability_ in the degree of
development of positive characters.
On the other hand, whenever the fundamental hereditary stimulus or
the material for a character is absent from the germ-plasm of both
parents, then it can appear in none of the offspring; they will
be practically invariable in respect to this condition. Only the
ontogenetic fluctuations of other real characters may influence
the defect. Consequently the absent state reproduces itself, the
"recessive breeds true."
The considerations here presented bear upon the hypothesis of change
of dominance. Bateson and Punnett (1905, p. 114) say of poultry: "The
normal foot, though commonly recessive, may sometimes _dominate_ the
extra-toe character." This idea of occasional _change_ in dominance
has been expressed more than once in the literature. I think the
phrase an unfortunate one. In my earlier report[16] I urged that a
characteristic that is anywhere dominant is so without regard to race
or species involved. If this is so it is clearly improbable that it
should vary from individual to individual, or in the same individual
at different times. Rather in view of the imperfection of dominance
we should say that a dominant character sometimes fails to develop,
in which case it is absent from the progeny; that is all. It is
particularly apt to fail of development when dilute--_heterozygous_.
[16] Davenport, 1906, page 86.
C. POTENCY.
Perhaps an apology is needed for introducing the much-abused word
"potency"; but there is hardly another that can be so readily adapted
to the precise definition I desire to give to it. The potency of a
character may be defined as the capacity of its germinal determiner
to complete its entire ontogeny. If we think of every character as
being represented in the germ by a determiner, then we must recognize
the fact that this determiner may sometimes develop fully, sometimes
imperfectly, and sometimes not at all. When such a failure occurs in
a normal strain a sport results.
Potency is variable. Even in a pure strain a determiner does not
always develop fully, and this is an important cause of individual
variability. But in a heterozygote potency is usually more or less
reduced. When the reduction is slight _dominance_ is nearly complete;
but when the reduction is great dominance is more or less incomplete
and, in the extreme case, may be absent altogether. The series of
cases of varying perfection of dominance described in this work
illustrate at the same time varying potency. The extreme case is that
of the rumpless fowl. The character in this case is an inhibitor
of tail development. This character has arisen among vertebrates
repeatedly and has become perpetuated in some amphibia and primates,
including man. In the case of our cock No. 117, the action of the
inhibitor is very weak, so that in the heterozygote the development
of the tail is not interfered with at all and even in extracted
dominants it interferes little with tail development, so that it
makes itself felt only in reduced size of the uropygium and in bent
or shortened back. But in No. 116 the inhibiting determiner is
strong. It develops fully in about 47 per cent of the heterozygotes
and 2 extracted dominants may produce a family in _all_ of which
the tail's development is inhibited. In the case of the rumpless
condition that arose apparently _de novo_ in my yards, the new
inhibitor showed an intermediate potency completely stopping the tail
development in 1 out of 25 heterozygotes. These three cases afford
a striking illustration of a variation in the potency of the same
inhibiting character in different strains.
Not only is potency variable, but its variations seem, in some cases,
to be inheritable. This we have seen to be the case with the Y-comb
(p. 15); with the extra-toed condition of Houdans (p. 23); and with
rumplessness (_cf._ offspring of No. 117 as compared with No. 116,
p. 40). On the other hand, the extra-toed condition of Silkies, the
grade of clean shank, and the degree of closure of nostril seem not
to be inherited.
D. REVERSION AND THE FACTOR HYPOTHESIS.
The brilliant development of the factor hypothesis, only dimly
fore-shadowed by Mendel[17] (1866, p. 38), clearly expressed by
Correns (1892), applied to animals by Cuénot, and further elaborated
by Bateson and Castle and their pupils, has quite changed the
methods of work in heredity. More forcibly than ever is it brought
home to us that the constitution of the germ-plasm--not merely
the somatic character--is the object of our investigation. With
this principle fully grasped the existence of cryptomeres and the
resolution of characters have become clearer. But the most striking
result accomplished has been that of clearing up the whole range of
phenomena formerly placed in the category of "reversion." No idea
without a semblance of inductive explanation has been more generally
accepted in the Darwinian sense both by professed biologists and
practical breeders than this. Not only was the fact of recurrence
of ancestral types in domesticated organisms accepted, but the idea
that, in some way, hybridization _per se_ destroyed the results
of breeding under domestication was maintained.[18] Now we know
that, under domestication, many races have been preserved that are
characterized by a deficiency of a character or by a new, additional
one, and that hybridization, by bringing together again those
characters that are found in the ancestral species, may bring about
again individuals of the ancestral type. There is nothing more
mysterious about reversion, from the modern standpoint, than about
forming a word from the proper combination of letters.
[17] Mendel's expression on this subject is translated
by Bateson (1902, p. 84) as follows: "Whoever studies the
coloration which results in ornamental plants from similar
fertilization can hardly escape the conviction that here also
the development follows a definite law which possibly finds its
expression _in the combination of several independent color
characters_. (The italics are Mendel's.)
[18] "An inherent tendency to reversion is evolved
through some disturbance in the organization caused by the act
of crossing." (Darwin, Animals and Plants under Domestication,
Chapter XIII, section, "Summary on proximate causes leading to
reversion.")
E. THE LIMITS OF SELECTION.
In the last few decades the view has been widespread that characters
can be built up from perhaps nothing at all by selecting in each
generation the merely quantitative variation that goes farthest in
the desired direction. I have made two tests of this view, using the
plumage color of poultry.
(1) _Increasing the red in the Dark Brahma × Minorca cross._--The
Dark Brahma[19] belongs to the group of poultry that contains a
majority of characters derived from the Aseel type. Nevertheless, its
plumage is closely related to that of the Jungle-fowl, from which it
may be derived on the assumption that the red part of the pattern has
become, for the most part, white. However, a little red remains on
the middle of the upper feathers of the wing-bar. I crossed such a
bird with a Black Minorca, and, as reported in my earlier work,[20]
the offspring were all black, except that the males showed some red
on the wing-bar. The amount of red varied in the different males,
and I decided to test the possibility of much increasing the amount
of the red by selection in successive generations. So I chose the
reddest cock to head the pen. In this pen (No. 632) 222 chicks were
produced and grew to a stage in which their adult color could be
determined. Of these 222 chicks, 160, or 72 per cent, were black,
without red; 24, or 10.8 per cent, were black with some red; 38, or
11.7 per cent, were typical Dark Brahmas, and 9 others, or 4.5 per
cent, were modified Dark Brahmas.
[19] Plate 11.
[20] Davenport, 1906, page 35.
The following year (pen 732) I bred a cock derived from the last
year's pen, a bird that resembled much the male Dark Brahma (except
that it was somewhat darker), to sundry hens, hybrids between the
Dark Brahma and Minorca--some of the first and some of a later hybrid
generation, but all black except that some of the 1906 birds had a
little buff on the breast and the primaries. The F1 (black) × F2
(Dark Brahma) gave 51 per cent black offspring, 27 per cent with a
black-and-red Game pattern, and 22 per cent with the Dark Brahma
pattern devoid of red. Thus the third generation suddenly gave me a
red-and-black Game-colored bird (plate 12)!
My interpretation of the foregoing results is as follows: The Dark
Brahma gametic formula proves to be _CIrnwx_, whereas the Black
Minorca is _C(IR)Nwx_, where (_IR_) is equivalent to, and merely
a further analysis of, the _J_ of the formula of the Minorca as
given in earlier sections. The _I_ stands for the Jungle pattern
without red and _R_ is the red element in that pattern. Obviously
_N_ and _R_ are the differential factors, 4 kinds of gametes occur
in F1, and in every 16 offspring these factors are combined in the
following proportions: 9 _NR_, 3 _Nr_, 3 _nR_, 1 _nr_ (compare the
distribution of color types in the 222 offspring of pen 632). The F2
male selected as father of the next generation (in pen 732) was an
extracted Dark Brahma in coloration and probably formed only 1 kind
of gamete, _nr_; but the hens were heterozygous in respect to _N_
and _R_. Consequently 4 kinds of zygotes are to be expected in F3;
and expectation was realized as indicated in table 68.
TABLE 68.
+-------------+------------+------------+-----------+-----------+
| | _NnRr._ | _Nnr2._ | _n2Rr._ | _n2r2._ |
| | | | | |
| | Black with | Black. | Game. | Dark |
| | traces of | | | Brahma |
| | red in | | | (without |
| | male. | | | red). |
+-------------+------------+------------+-----------+-----------+
| | _P. ct._ | _P. ct._ | _P. ct._ |
| | | | |
| Expectation | 50 | 25 | 25 |
| Realization | 51 | 27 | 22 |
+-------------+-------------------------+-----------+-----------+
In the case where both parents are F2 or F3 it is impossible to
summate results, since the gametic formulæ of the different parents
are so diverse; but the same types of solid blacks, black with trace
of red in the males, Game-colored males and females, and Game with
red replaced by white repeatedly occur. My plan of increasing red in
the Dark Brahmas met with wholly unexpectedly prompt success, but not
in the way anticipated. The result was not due to selection, but to
the recombination of the factors necessary to make the Game plumage
coloration.
(2) _Production of a buff race by selection._--The second test was
directed toward the production _de novo_ of a new buff race from a
Game fowl.
As is well known, all of our red and "buff" races, like the Buff
Leghorn, Rhode Island Red, and others, have been derived from the
Buff Cochin that came to us from China. The fact that a buff bird
has, so far as I have been able to learn, not been produced in
western countries indicates the probability that it can not be so
produced at will; but the attempt seemed worth while.
I began with a Black Breasted Red Game because its plumage color
is that of the primitive ancestor of domesticated poultry, and on
that hypothesis the ancestor of the buff races. If these buff races
were produced by extending the red through selection of the reddest
offspring, that should be possible now as in the past.
A start in the direction of creating a buff bird would seem to
require the elimination of the black. By crossing a black and red
Game with a White Leghorn I got, in 1905, 2 white pullets with red on
breast and some black specks. By crossing a Game Bantam (wingless)
with a White Leghorn I got white birds with red present on wing-bar
of male and breast of females and also some black spots.
In 1906 I mated 2 of these white (+ red) bantam hybrid hens with a
hybrid cock and obtained again red on the wing-coverts of some white
hybrids, while some were without red. From one of the hens I got 4
offspring, or 20 per cent of all, with buff on hackle-lacing, breast,
and wing-coverts.
In 1907 I mated a prevailingly white male of the preceding year,
that had red wing-bar, hackle, and breast, with the reddest females
and obtained, along with pure whites and blacks and barred birds,
these colors combined with red in various degrees, but not clearly
in advance of the reddest of 1906. In 1908 I mated a white male,
having red as in the Game, with my reddest hybrids. Again, white and
white-and-buff birds appeared, but they showed no advance, except in
one instance, among 138 young. This individual (No. 7950), derived
exclusively from the Black-red Game and White Leghorn on one side and
on the other from the White Leghorn-Game Bantam cross, had a _uniform
buff_ down. Unfortunately the chick quickly died.
The conclusion is that after three years of selection of the reddest
offspring no appreciable increase of the red was observed--except for
the remarkable case of one undeveloped chick with completely buff
down. This, indeed, looks like a sport, or, perhaps, it is due to
unsuspected factors. The experiment will be continued.
F. NON-INHERITABLE CHARACTERS.
So well-nigh universal is heredity that it is justifiable to
entertain a doubt whether any character may fail of inheritance. So
far as my experience goes, non-inheritable characters are such as
are weak in ontogeny, so that they may readily fail of development
even when conditions are propitious; or else they are so complex--so
far removed from simple unit-characters--that their heritability
in accordance with established canons is obscured. The first case
is apparently illustrated by the rumpless cock (No. 117) and the
wingless fowl; the second case by lop-comb and by right-and-left
alternatives in general.
Apart from the distinct _characters_ that fall under these two
categories there are the fluctuating quantitative _conditions_. These
depend for the most part, as already pointed out, on variations in
the point at which the ontogeny of a character is stopped; and the
stopping-point is, in turn, often, if not usually, determined by
external conditions which favor or restrict the ontogeny. Whether
or not such quantitative variations are transmitted is still
doubtful. Our experiment in increasing qualities, such as redness in
plumage-color, by selection of quantitative fluctuations have not
been successful in the sense anticipated; neither have selections of
comb, polydactylism, or syndactylism. Recently, prolonged attempts
at the Maine Agricultural Experiment Station to increase egg-yield
of poultry by selection have been without result. Apparently,
within limits, these quantitative variations have so exclusively an
ontogenetic signification that they are not reproduced so long, at
least, as environmental conditions are not allowed to vary widely.
The conclusions which others have reached, and upon which de Vries
has laid the greatest stress, that quantitative and qualitative
characters differ fundamentally in their heritability is supported by
our experiments.
G. THE RÔLE OF HYBRIDIZATION IN EVOLUTION.
The criticism has often been made of modern studies in hybridization
that they are really unimportant for evolution because hybridization
is uncommon in nature. Even at the beginning of the new era it could
be replied that, first, we did not know how common hybridization
might turn out to be in nature, and, second, that certainly in human
marriage and among domesticated animals and plants, intermixing of
characters played a most important part, and, finally, the laws of
inheritance of characters were of such grave physiological import
as to deserve study wholly apart from any question of the rôle of
hybridization in evolution.
The last decade of work has made clear many things that were before
uncertain. We now realize that in nature hybridization may and
actually does proceed extensively. Dr. Ezra Brainerd has shown how
many wild "species" of _Viola_ have arisen by hybridization, as may
be proved by extracting from them combinations of characters that
are found in the species that are undoubtedly ancestral to them.
In such highly variable animals as _Helix nemoralis_ and _Helix
hortensis_ it is very probable that individuals with dissimilar
characters regularly mate in nature and transmit diverse combinations
of characters to their progeny. Indeed, if one examines a table of
species of a genus or of varieties of a species one is struck by
the paucity of distinctive characters. The way in which species, as
found in nature, are made up of different combinations of the same
characters is illustrated by the following example, taken almost at
random. Among the earwigs is the genus _Opisthocosmia_, of which the
5 species known from Sumatra alone may be considered. They differ,
among other qualities, chiefly in the following characters (Bormans
and Kraus, 1900):
Size: _A_, large; _a_, small.
Wing-scale: _B_, brown; _b_, yellow.
Antennal joints: _C_, unlike in color; _c_, uniform.
Forceps at base: _D_, separated; _d_, not separated.
Edge of forceps: _E_, toothed; _e_, not toothed.
Fourth and fifth abdominal segments: _F_, granular; _f_, not granular.
The combinations of these characters that are found are as follows:
_Opisthocosmia ornata_: _AbcDEF_.
_insignis_: _ABcDEf_.
_longipes_: _AbCDEf_.
_tenella_: _AbCdef_.
_minuscula_: _aBCDEf_.
Other species occur, in other countries, showing a different
combination of characters, and there are characters not contained in
this list, which is purposely reduced to a simple form; but the same
principles apply generally.
The bearing upon evolution of the fact that species are varying
combinations of relatively few characters is most important. Combined
with the fact of hybridization it indicates that the main problem
of evolution is that of the origin of specific characteristics.
A character, once arisen in an individual, may become a part of
any species with which that individual can hybridize. Given the
successive origin of the characters _A_, _B_, _C_, _D_, _E_, _F_,
in various individuals capable of intergenerating with the mass of
the species, it is clear that such characters would in time become
similarly combined on many individuals; and the similar individuals,
taken together, would constitute a new species. The adjustment of the
species would be perfected by the elimination of such combinations as
were disadvantageous.
COLD SPRING HARBOR, NEW YORK,
_May 20, 1909_.
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[Illustration: PLATE 1
Jungle Fowl, male, showing distribution of black and red elements
of pattern.
A. Hoen & Co. Baltimore. Kako Morita, pinx.]
[Illustration: PLATE 2
Jungle Fowl, female, showing coloration and pattern.
A. Hoen & Co. Baltimore. Kako Morita, pinx.]
[Illustration: PLATE 3
White-faced Black Spanish, male.
A. Hoen & Co. Baltimore. Kako Morita, pinx.]
[Illustration: PLATE 4
First generation hybrid between White-face Black Spanish Cock and
White Silkie Hen.
A. Hoen & Co. Baltimore. Kako Morita, pinx.]
[Illustration: PLATE 5
First generation hybrid between Black Minorca Cock and White Silkie
Hen.
A. Hoen & Co. Baltimore. Kako Morita, pinx.]
[Illustration: PLATE 6
Second hybrid generation between Silkie and Spanish Minorca, (No.
3898) female.
A. Hoen & Co. Baltimore. Kako Morita, pinx.]
[Illustration: PLATE 7
Buff Cochin, (No. 545) male.
A. Hoen & Co. Baltimore. Kako Morita, pinx.]
[Illustration: PLATE 8
Cock of first hybrid generation between Black Cochin and Buff
Cochin.
A. Hoen & Co. Baltimore. Kako Morita, pinx.]
[Illustration: PLATE 9
Cockerel (No. 6094) of first hybrid generation between Buff Cochin
Cock and Silkie Hen.
A. Hoen & Co. Baltimore.]
[Illustration: PLATE 10
Cockerel (No. 2561) of second hybrid generation between Buff Cochin
and White Leghorn.
A. Hoen & Co. Baltimore. Kako Morita, pinx.]
[Illustration: PLATE 11
Dark Brahma, (No. 122) male.
The detailed feathers are in order from right to left from first,
third and fourth wing coverts.
A. Hoen & Co. Baltimore. Kenji Toda, pinx.]
[Illustration: PLATE 12
A cock (No. 5257) of the third hybrid generation between a
single-comb Black Minorca and a Dark Brahma shown in plate 6.
The detailed feathers are in order from right to left from the
first, second, fourth and third wing coverts.
A. Hoen & Co. Baltimore. Kenji Toda, pinx.]
Transcriber's notes.
Words in italics are surrounded by _underscored-.
Clear printer's errors were corrected, but original spelling was not
modified or harmonized.