UC-NRLF

STUDIES OF

INHERITANCE IN RABBITS

BY

W. E. CASTLE

IN COLLABORATION WITH

H. E. WALTER, R. C. MULLENIX, AND S. COBB

WASHINGTON, D. C.
PUBLISHED BY THE CARNEGIE INSTITUTION OF WASHINGTON

1909

STUDIES OF

INHERITANCE IN RABBITS

BY

W. E. CASTLE

IN COLLABORATION WITH

H. E. WALTER, R. C. MULLENIX, AND S. COBB

WASHINGTON, D. C.
PUBLISHED BY THE CARNEGIE INSTITUTION OF WASHINGTON

1909

CARNEGIE INSTITUTION OF WASHINGTON, PUBLICATION No. 114.

PAPERS or THE STATION FOR EXPERIMENTAL EVOLUTION, No. 13.

CONTRIBUTIONS FROM THE ZOOLOGICAL LABORATORY OF THE MUSEUM

OF COMPARATIVE ZOOLOGY AT HARVARD COLLEGE.

E. L. MARK, DIRECTOR. No. 199.

The Plimpton Press Norwood Mass. USA.

CONTENTS.

Page

PREFACE 5

PART I. — EAR-SIZE ". . . . 9-35

Introduction 9

Characteristics of lop-eared rabbits; sterility and its inheritance .... 9

Growth-rate of lop-eared and of short-eared rabbits in size and in ear-length . 12

Matings of short-eared rabbits inter se 14

Matings of lop-eared rabbits inter se 16

Cross i. — Lop-eared female X short-eared male 18

Cross 2. — Short-eared female X lop-eared male 18

Cross 3. — Belgian hare female X lop-eared male 20

Cross 4. — Lop-eared female X half-blood lop male 21

Cross 5. — Half-blood lop female X lop male 179 22

Cross 6. — Half-blood lops mated inter se 23

Other matings of half-blood lops and additional Cross 6 matings 26

Matings of three-quarter-blood lops 31

Cross 7. — Quarter-blood lop female X short-eared male 33

Cross 8. — Quarter-blood lop X three-quarter-blood lop 34

Limitations of the data studied • 34

Conclusions . . . 35

PART II. — WEIGHT 37-4°

PART III. — SKELETAL DIMENSIONS 41-44

PART IV. — COLOR 45-68

Color variation in relation to color factors 45

Development of the factor hypothesis 46

The general color factor, C 46

The specific pigment factors, B, Br, and Y 46

The intensity factor, I or D 46

The factor for a pigment pattern of the individual hair, A 47

The factor for uniformity of pigmentation, U, or spotting with white, S . 47
The factor for extended distribution of black or brown, E, alternative with R

(restricted distribution) 47

Interrelations of factors E and U 48

Interrelations of factors B, Br, and Y 49

Gametic structure and variation 49

Gametic and zygotic formulae 5°

Color varieties of the rabbit 51

Gray type 52

Black type 52

Yellow type 53

White type 53

Zygotic variation within each color variety 54

Gray 54

Blue-gray 60

Black 61

Blue 62

Yellow 63

Sooty 64

White 65

The material basis of heredity factors 68

BIBLIOGRAPHY 69

DESCRIPTION OF PLATES 70

3

PREFACE.

In this paper are recorded observations upon inheritance in rabbits
which were made in the Harvard Zoological Laboratory with the aid of a
grant from the Carnegie Institution of Washington. The authors desire
to express their appreciation of that aid, without which these observations
could not have been made.

The experiments described were planned by the senior author, and
this report also was written by him. Dr. H. E. Walter has made the
majority of the extremely laborious observations and computations con-
cerning the inheritance of ear-length and of body-weight. Dr. R. C.
Mullenix prepared and measured the rabbit skeletons as a foundation for
Part III of this paper; while both Dr. Walter and Mr. Cobb rendered
valuable assistance in connection with the study which has been made of
color inheritance. The senior author alone is responsible for the analytical
treatment of the observations.

STUDIES OF INHERITANCE IN RABBITS

BY W. E. CASTLE

IN COLLABORATION WITH

H. E. WALTER, R. C. MULLENIX, AND S. COBB.

PART I. — EAR-SIZE.

INTRODUCTION.

The inheritance of ear-size in rabbits has been characterized as blend-
ing, in certain preliminary publications, (Castle, 105, :o$a).1 The experi-
mental evidence for such a characterization is described in the following
pages. It consists of results obtained by experimental cross-breeding of
lop-eared rabbits with ordinary short-eared ones. A detailed account
of this evidence is of little interest to the general reader, who therefore
may advantageously omit pages 14-34. For consultation on the part of
the critical student of heredity, it has been thought essential to present this
evidence in some detail, even though it is intrinsically uninteresting.

CHARACTERISTICS OF LOP-EARED RABBITS; STERILITY AND ITS

INHERITANCE.

Lop-eared rabbits are distinguished from ordinary ones chiefly by the
enormous size of their ears, which are so large as to hang down, touching
the ground on either side of the head. (See plate i, fig. 2, and plate 2,
fig. 8.) This breed of rabbit is characterized also by a long tail and
unusual size, being one of the largest breeds known. The characters of
large size and long tail, however, have probably not been sought for their
own sake, but have been incidentally obtained in the production of the
breed as a result of selection for ears of large size; for among lop-eared
rabbits, as a rule, those of the largest size have longest ears.

In the winter of 1904 a pair of lop-eared rabbits was obtained from a
fancier and used in various breeding experiments. Matings of the two
together were for the most part fruitless, only one litter of 2 young being
successfully reared. These were similar to the parent rabbits in size and
ear-character. Of the two, one was a male, which was used extensively
in breeding experiments, including one successful mating with his mother,
from which came a good-sized litter. But only two out of this litter at-
tained the age of 20 weeks and they ultimately succumbed to disease under
conditions not unfavorable to other rabbits. The second of the two young
reared by the original lop-eared pair was a female. Only twice did this
rabbit bear young by any sort of mating. In one case she failed to rear
any of the young. In the other case she reared, when mated to her own

1 For complete titles see Bibliography, p. 69.

10 INHERITANCE IN RABBITS

brother, 2 young out of a litter of 3. Both were males. The larger one,
although apparently healthy, failed to breed; the smaller one was not
tested. Accordingly, out of 5 pure-bred lop-eared rabbits with which we
have experimented, 2 (a male and a female) were infertile — one of the
two largely so, and the other completely. Infertility has also been
encountered among a few of the female descendants of this lop-eared stock
produced by cross-breeding, but in no other stock of rabbits with which
we have experimented. Sterile individuals have not been observed among
half-blood lops of generation Ft, but a few have occurred in later genera-
tions. In the majority of cases, however, the sterile individuals have been
three-quarter-blood lops.

From these facts we conclude that a tendency to sterility is inherent in
the lop-eared stock used, and is transmitted, not to the immediate off-
spring (FJ if they are cross-breds, but to the next generation, when it
is produced by a back-cross between F2 and the pure lop-eared stock;
less frequently sterility reappears in F2, produced by breeding the half-
blood lops inter se. We should expect the infertility to occur only half
as frequently in this latter sort of mating as in the former, where it has
been oftenest observed. On the whole, it seems probable that a ten-
dency to sterility is inherited in rabbits, as in Drosophila (see Castle et al.y
:o6), after the manner of a Mendelian recessive character, i. e., skipping a
generation in crosses.

Why lop-eared rabbits more than other breeds should show a tendency
to sterility is not known; but as they are extensively inbred, it seems highly
probable that inbreeding is largely responsible for this sterility. The
lop-eared character is one which, from the manner of its inheritance, we
may be sure, has been built up slowly as the result of selection. In this
process inbreeding must have been continuously practised, for since every
out-cross would result in loss of half the ground gained by selection, it
would be practised only when absolutely necessary.

At birth rabbits have ears quite undeveloped, and the ears do not attain
their full growth until an age of 5 to 8 months have been reached. Ear-
growth is well advanced, however, at 20 weeks, after which time it becomes
very slow. Accordingly 20 weeks has been found a convenient age at
which to institute comparisons as to ear-character between different lots
of rabbits. Frequently, however, it is impossible to rear an entire litter of
rabbits to the age of 20 weeks, in which case an earlier determination
of ear-character becomes desirable. For this reason, after some experi-
mentation, we adopted the plan of making weekly measurements of the
ear dimensions at ages from 2 to 20 weeks inclusive. This process, while
laborious, fully eliminates errors due to observation, as well as those due
to temporary growth conditions.

The weekly observations upon each rabbit included taking its weight,
the maximum length and maximum width of its right ear, and finally the

EAR-SIZE

11

spread of the ears, i. e., the distance from ear- tip to ear-tip when the ears
are extended in a horizontal position and stretched slightly. Since the
measurements in nearly all cases were made by the same observer (Walter),
the personal equation is a fairly constant factor and may be disregarded

Ear length

in 777777. / 2 3 4 5 6 7 8 9 10 II /£ /3 14 /5 16 17 Id 19
IZO

//O

90 J

70-

60-

50

30

weigl
in
grarr

'1500
1400

•1300

-1200
1100
1000
900

-800
700
600
600
400
300
200
100

A9em I 2 3 4 5 6 7 8 9 10 II IZ 13 /4 15 16 17 /8 /9

weeks.

FIG. i. Chart showing growth in ear-length, and body-weight of a litter of six short-eared
rabbits between the ages of two and eighteen weeks. See table i.

12 INHERITANCE IN RABBITS

in comparing one set of observations with another. Records of this sort,
more or less complete, were made for 70 different litters of rabbits,
containing 341 individuals.

An inspection of figs, i to 3 shows that the growth-curve Jfor ear-length l
from 2 weeks after birth is of the same general form in the case of both
long-eared and short-eared rabbits. It is a curve convex above, indicating
a steadily diminishing daily increment in ear-length.

GROWTH-RATE OF LOP-EARED AND OF SHORT-EARED RABBITS IN SIZE
AND IN EAR-LENGTH.

The theoretical growth-curve of an organism in weight (Houssay, 107;
Robertson, :o8) is at first concave upward, but later becomes convex.
When the curve is concave upward the daily growth increment is increas-
ing. But when the growth-curve becomes convex upward, it is evident
that the growth increment is decreasing. Therefore the period of greatest
daily growth occurs when the growth-curve is changing from a concave
to a convex one. In rabbits this occurs at an age of from 6 to 8 weeks
after birth (see figs, i to 3). According to Robertson (:o8) the period of
maximum growth corresponds with the middle point of a growth-cycle
which in character resembles an autocatalytic monomolecular chemical
reaction. In the rabbit this growth-cycle probably has its beginning at
some time prior to birth and ends before puberty is attained.

It is possible that this same form of curve would be observed in respect
to ear-length also, if the measurements began at a period sufficiently early.
Growth of the ears is completed before increase in body-weight ceases,
and it is possible that the growth-curve for ear-length has already changed
from a concave to a convex form at the age of 2 weeks, when our measure-
ments begin. But it is, on the other hand, possible that the growth-curve
for ear-length would not show a convex form upward even if completed
for the period prior to 2 weeks of age; for ear-length is a linear dimen-
sion, whereas body-weight depends on volume, i. e.y size in three dimen-
sions, and a doubling of any linear dimension should be attended by an
eight-fold increase of volume.

A comparison of fig. i with fig. 2 shows a considerable difference between
ordinary short-eared (fig. i) and lop-eared (fig. 2) rabbits as regards size,
at corresponding ages; the difference is even more striking in regard to
ear-length. Crosses between the two varieties produce rabbits inter-
mediate in character as regards both weight and ear-length. But before
considering further the character of the cross-breds, it will be well to inquire
how each variety breeds by itself.

1 The measurements for ear- width and "spread" are closely correlated with those for ear-length.
For the sake of simplicity we shall deal with the statistics for ear-length only.

EAR-SIZE

13

far length
m mm

220

40-

30-

Aae in
v/eeks.

FIG

/ 234 56 7 8 9 10 II 12 13 14 15 16 17 18 13 20

2. Growth-curves for a litter of five lop-eared rabbits.
See table 2 and compare fig. i.

14

INHERITANCE IN RABBITS

MATINGS OF SHORT-EARED RABBITS INTER SE.

Several matings of short-eared rabbits inter se are recorded in table i.
They show great uniformity of result. The young differ little in ear-
length from their parents, which in no case differed from each other by
more than 5 mm.

TABLE i.

1

HATING i

MATING :

>.

Ear-
length.

Weight.

Age.

Ear-
length.

Weight.

Age.

Parents:

mm.

gms.

2.6^O

Adult.1

Parents:

9408

mm.

IIO

gms.
1,980

27 weeks.

c?497

no

2,045

7 mos.

( 1,9*5

Do.

Mid-parental
Offspring:
Litter i —
$768

112.5

IO7

2,347

I ,44 C

20 weeks.

c?497
Mid-parental
Offspring:
Litter i —
0782

no

IIO

/ 2,045
i,947

i,^6c

7 mos.
27 weeks.

20 weeks.

Q 760

I 4.8O

Do.

0787

106

1. 571

Do.

9 770

lie

1,780

Do.

9784

107

1,375

Do.

r?77l

113

I,74O

Do.

,^785

IIO

1,695

Do.

9 772

i

106

I, CCO

Do.

Litter 2 (see

*/?•

f?77?

in

»3OV'

I.OCO

Do.

fig. i) —

Litter 2 —

$84.0

112

1,450

1 8 weeks.

j\88i

no

I 780

14 weeks.

984.1

116

"2
1,460

Do.

$882

I 380

1 6 weeks.

9842

1 06

""
1,4.60

Do.

r?88t

I COO

14 weeks

r?843

in

I CIO

Do.

r?884.

lie

*ty~

I. COO

Do.

9844

lie

1,^40

Do.

9845. .

IIO

1,420

Do.

MATING

3-

MATING

*•

Parents:
9 268

IOC

2,280

Adult.

Parents:
9268

IOC

2,280

Adult.

c?497
Mid-parental
Offspring:

98CO

no

107-5

no

2,045
2,162

I 445

7 mos.
15 weeks

c? 56
Mid-parental
Offspring:
r?774

IO2

103-5

1 08

2,500
2,390

I 7l C

Do.
Do.

20 weeks

<3>86i

112

I.14O

Do.

^862

IIO

I,42O

Do.

j>863 . .

no

I,2Q5

Do.

By adult is meant i year or more old.

In mating i, the extreme deviations from the mid-parental 1 ear-length
are —6.5 mm. and +2.5 mm., the average deviation being only 2.5 mm.
The total range of variation is 9 mm. In mating 2, between brother and
sister, the extreme deviations are —4 mm. and + 6 mm., giving a total
range of variation of 10 mm. The average deviation from the parental
ear-length (no mm.) is, as in mating i, 2.5 mm.

1 By mid-parental, as we shall use the term in this paper, is meant a magnitude exactly halfway
between the magnitudes of the respective parents. It is the mean of the parental magnitudes.

EAR-SIZE

15

The growth-curves for litter 2, which were produced by this mating,
are shown in figure i. In mating 3, the deviations [are all plus in
character, but are small in amount, namely, 2.5, 4.5, 2.5, and 2.5 mm.

far length

• in mm.
180

no

160

ISO

140

130

120

no

100

BO-

70-

60-

50-

40-

30-

Age in

weeks.

766

Weight
m

2200

2/00

2000

1900

1800

1700

1600

ISOO

1400

1300

1200

1 100

1000

900

eoo

700
600
BOO
400
300
200
100

I 2 3 4 S 6 7 8 9 10 II 12 13 14 15 16 17 18 19 20 2/

FIG. 3. Growth-curves for a litter of five second-generation (F2) half-lop rabbits.
See table 9 and compare figs. I and a.

From mating 4, by the same female that was concerned in mating 3, a
single young one was reared, which showed a plus deviation of 4.5 mm.

16

INHERITANCE IN RABBITS

Another mating which falls in this category was made between the
Belgian hare (9 431) and the short-eared <? 56 (see table IA). It shows
a complete blending in the offspring of the parental ear-lengths, with a
very small range of variation, viz, 6 mm.

TABLE IA.

Ear-
length.

Weight.

Age.

Parents:

O All

mm.
118

gms.
2 4.00

Adult.

d1 56

IO2

2 >JOO

Do.

Mid-parental
Offspring:

1 10

102

2,95°

Do.

21 weeks

Q 232

Q 213

105
III

2,700

Adult.
21 weeks.

rr234.

1 08

Do.

r?23*C

( 108

Do.

$2*6..

( IIO

108

2,945

Adult.
21 weeks.

The mid-parental ear-length was exceeded by i of the young at 21
weeks of age; 3 others came within 2 mm. of the mid-parental ear-length
at 21 weeks of age, and i of these equaled it when adult. If the other 2
did as well they too must have attained the expected ear-length. Only
i individual (9 232), then, fails to attain the mid-parental ear-length.
This result is almost identical in general character with that shown by
table i.

We may conclude that short-eared rabbits breed true within a range of
fluctuating variability not exceeding 10 mm.

MATINGS OF LOP-EARED RABBITS INTER SE.

Our original stock of lop-eared rabbits consisted of a single pair. Both
of them gave vigorous young in matings with short-eared rabbits, but not
with each other. Consanguinity may have been the reason for this lat-
ter fact. They were obtained from the same source, and doubtless were
nearly related, as well as inbred. Nevertheless we did obtain from them
two good-sized and healthy young, rf 179 and 9 180. The former appears
in many of the crosses to be described, but the latter proved a very poor
mother, producing only occasional litters of young, none of which attained
maturity. Table 2 shows the only results obtained from mating lop-eared
individuals inter se.

Mating i produced 2 young, one (b* 179) very similar to the father, the
other (9 180) very similar to the mother, but not quite so large and with
ears 5 mm. shorter. The deviations from the mid-parental ear-length
are —7.5 and —2.5 mm., respectively.

EAR-SIZE

17

Mating 2 (between brother and sister) produced 2 young, which reached
the age of 20 weeks. Though they were not large, their ears attained
a good length, the deviations from the mid-parental ear-length being
— 5 mm. and —2 mm.

TABLE 2.

MATING

I.

MATING

3-

Ear-
length.

Weight.

Age.

Ear-
length.

Weight.

Age.

Parents:

mm.

gms.

Parents:

mm.

gms.

Old $ lop (pi.
i, fig. 2)..

225

4,600

Adult.

Old ? lop (pi.
i, fig. 2) . .

225

4,600

Adult.

Old tf lop ..

210

3.4501

Do.

c?i79 (pi. 2,

Mid-parental
Offspring:

217-5

4,025

Do.

fig. 8)
Mid-parental

2IO
215-7

3,410
4,005

Do.
Do.

d»i79(pl. 2,fig.
8)

•7 410

Adult.

Offspring (see

$180

22O

0,^"
3,765

Do.

fig. 2):
c?667

( 220
i 223

2,010
1,590*

14 weeks.
20 weeks.

MATING

$668
$669

200
21 ?

1,370
2,030

,4weeks.

f?67O

22O

I QOO

Do.

Parents:

#671

190

1,205

Do.

9i8o

2IO

34.IO

Adult

(5^170

,^AW
376?

Do

Mid-parental

215

3,587

- Do.

Offspring:

j»6i6

2IO

I 680

20 weeks

c?6i8

213

1,820

Do.

Estimated.

'Sick.

Mating^ (between"mother and son) produced a litter of 5 young, which
grew irf £ satisfactory manner until i4Veeks old (see fig. 2). Then,
as a result _ of disease, 4*of them died, andjhe fifth became greatly reduced
in flesh, so that at 20 weeks of age he weighed 400 grams less than at
14 weeks of age. Nevertheless his ears continued to grow slowly. At 14
weeks they measured 220 mm.; at 20 weeks, 223 mm.

The rabbit 671 was from the beginning much the smallest one in the
litter; we named him the "runt" and had hopes of securing from him a
race of small-sized but lop-eared rabbits. These hopes were ended by
the unfortunate illness which attacked the entire litter. The small size
of this rabbit accounts for the shortness of his ears (190 mm. at 14 weeks
of age). Leaving him out of consideration, the range of variation in ear-
length is 20 mm.; with him, it is 30 mm., at 14 weeks of age.

Two of the young produced by mating 3 had already at 14 weeks of
age exceeded the mid-parental ear-length, a third had almost reached
it, while the 2 others fell below it. This is a fluctuating variation around
the mid-parental ear-length, and indicates that the long-eared character
tends to breed true, within a range of variation of 20 (or possibly 30) mm.,
the minus variations, however, probably being greater than the plus ones.

18

INHERITANCE IN RABBITS

CROSS i. — LOP-EARED FEMALE X SHORT-EARED MALE.

The largest and longest-eared rabbit with which we have experimented
was a female obtained by purchase and of unknown ancestry. (See plate i,
fig. 2.) Her ear-length was 225 mm. and her adult weight 4,600 grams.
She was mated with a small-eared angora rabbit (c? 45, plate i, fig. 3),
whose ear-length was 105 mm. and adult weight 3,000 grams. A litter
of 8 young was obtained from this pair. All were reared to an age of
2 months, when 6 were discarded. The remaining 2 were reared to matu-
rity. One of them (^248) is shown in plate i, fig. i. The 6 discarded
rabbits had ears shorter than those of the rabbits which were kept. Their
ear-lengths are given in table 3 as estimated from the known relation of
their ear-lengths at 2 months of age to the ear-lengths of rabbits 247 and
248, the animals kept until adult. Table 3 shows that the young obtained
from this cross are, as regards ear-length, intermediate between the parents,
but stand nearer the short-eared than the long-eared parent. As regards
weight, 9 247 is smaller and c? 248 larger than the mid-parental condition;
the remaining 6 would probably not have exceeded 9 247 in weight had
they been reared to maturity. Accordingly as regards both size and ear-
length in this cross the resemblance is greater toward the smaller and
shorter-eared parent (father).

TABLE 3. — Cross i.

Ear-
length.

Weight.

Age.

Parents:

Q lop . .

mm.

22$

gms.

4 600

Adult

t.
cJ1 45 . . .

IOC

2 OOO

Do

Mid-parental
Offspring:
$ 247

I65
1*2

3,800

32QO

Do.

Adult

c?248
(^249

r53

14^ *

3,93°

Do.

(^250

147 l

^251 ...

n8 l

^252

I4O *

<?253
(^254

149

i45
142 l

Estimated.

CROSS 2. — SHORT-EARED FEMALE x LOP-EARED MALE.

This cross, the reciprocal to the foregoing, was made repeatedly. The
lop-eared male used (rf 179, plate 2, fig. 8) was a son of the lop-eared
female employed in cross i. He was, however, smaller than his mother,
and had shorter ears. The results of 4 different matings are shown in
table 4. In mating i there were only 2 surviving young, which there-
fore were probably the largest and strongest individuals in the litter and
received more than the average amount of nourishment. One of them

EAR -SIZE

19

surpassed at 18 weeks of age the mid-parental ear-length, while the other
one almost equaled it. Their weights at 18 weeks of age indicated that
the mid-parental weight would be attained at maturity.

TABLE 4. — Cross 2.

MATING

i.

MATING

3-

Ear-
length.

Weight.

Age.

Ear-
length.

Weight.

Age.

Parents:
$268

mm.

IQC

gms.

2 280

Adult.

Parents:
9 105

mm.

IOO

gms.

9 mos.

<?i79 lop ...
Mid-parental
Offspring:
(5*5 7 1

210
157-5

jee

3,410
2,845

2,^10

Do.
Do.

1 8 weeks

c?i79(l°P)--
Mid-parental
Offspring:
9626

2IO

i55

I«>O

3»4io
1,380

Adult,
20 weeks.

2
1 60

Do

r?627

I *\O

i 470

Do.

9620. .

ISO

1,48";

Do.

96*o. .

I4.q

I tA7O

Do.

96*2 .

148

I,77O

Do.

(^634 . . .

ICQ

I.72<

Do.

MATING

2.

MATING

4-

Parents :
$269 (pi. 2,
fig. 5) ....
0^179 (lop)..
Mid-parental
Offspring:
Litter i —

92

2IO

J51

1,935
3,410
2,672

Adult.
Do.
Do.

Parents:
9io8

<?i79
Mid-parental
Offspring:
c?6o7
(5»6o8

no

210
1 60

158
ice

2,520
3,4io
2,965

2,050
1,810

Adult.
Do.
Do.

20 weeks.
1 8 weeks.

Q c;74

140

i 0"\o

1 8 weeks

^609

158

2,080

Do.

$575

14?

1,040

Do.

Litter 2 (pi. 2,
fig- 6) —
c?64o
$641 . .

15°

I ^O

1,980
i 8«;o

20 weeks.
Do.

$642

I ^2

I Q3O

Do.

9643

144

I 670

19 weeks

(less at 20)

Under the head of mating 2 are given the results of 2 different litters,
litter i consisting of 2 rabbits, litter 2 of 4. The weight of the mother
was surpassed by that of the offspring in 4 out of 6 cases, at the early age
of 1 8 to 20 weeks. Three of the 6 young had ear-lengths very similar to the
mid-parental ear-length; the remaining 3 had ears somewhat shorter at
1 8 or 19 weeks old, and probably would not have attained at maturity
an ear-length equal to the mid-parental. Litter 2 is shown in plate 2,
fig. 6; the parents in figs. 5 and 8 of the same plate.

The 6 young produced by mating 3 were under-sized at 20 weeks of
age, which perhaps accounts for the fact that no one of them attained the
mid-parental ear-length, but all fell from 5 to 10 mm. short of it.

In mating 4, 2 of the 3 young came within 2 mm. of attaining the mid-
parental ear-length; the third came within 5 mm. of it.

20

INHERITANCE IN RABBITS

On the whole, the result of cross 2 is a fairly close approximation to
the mid-parental ear-length. In no case does the deviation from the mid-
parental value exceed n mm.; the average deviation from it is only 4.8
mm. But the differences between the respective parents ranged from 100
to 118 mm., and the least deviation of one of the offspring from either
parent was 45 mm. or more than four times the greatest deviation from
the mid-parental value. When deviation from the mid-parental value
did occur, it was oftener under than over the mid-parental value.

Accordingly, the results observed as regards ear-length may accurately
be described as a blend. As regards body-size, the data are insufficient,
since adult weights of the offspring were in no case obtained, but the
observed weights of the offspring in matings i and 2 indicate that a blend
might be expected, an intermediate condition having already been obtained
at 20 weeks of age.

CROSS 3. — BELGIAN HARE FEMALE X LOP-EARED MALE.

The "Belgian hare" (plate 3, fig. 9) used in this cross was larger and
had somewhat longer ears than the short-eared rabbits used in crosses i
and 2. The lop-eared male was father of the one used in cross 2, but had
about the same ear-length and body-size. A litter of 6 young was ob-
tained, five of which were reared to an age of 21 weeks or more. In size
the offspring exceeded either parent, approximating that of the female
lop used in cross i. Four of the 5 young also exceeded the mid-parental
ear-length by from 2 to 6 mm., but the fifth fell short of it by 8 mm. This
same individual ((? 177) showed the least deviation from either parental
ear-length, viz, 38 mm., or four and a half times the greatest deviation
from the mid-parental ear-length.

TABLE 5. — Cross 3.

Ear-
length.

Weight.

Age.

Parents:
Wfop8""111^--

mm.
118

2IO

gms.

3i4oo
* *.<Q( ?")

Adult.
Do

Mid- parental

l64

3»45°^;

•t A2C( ?}

Do

Offspring:
C?I74. •

I 7O

21 weeks

$17*

I7O

A 3Q C

Adult

<?i76...

\66

4I3O

Do

f?!77

ic6

$178

I7O

Adult

Accordingly, in cross 3, as in cross 2, the ear-length of the offspring is
approximately a blend of the ear-lengths of the respective parents. The
size of the offspring, however, is greater than that of either parent, though
it does not exceed the size of lop-eared individuals other than the father.

EAR-SIZE

21

CROSS 4. — LOP-EARED FEMALE X HALF-BLOOD LOP MALE.
This cross is a sequel to crosses i and 3, a male rabbit produced by
cross 3 being mated with the female lop used in cross i. This cross pro-
duced three-quarter-blood lops, the ear-lengths of which are indicated in
table 6.

TABLE 6. — Cross 4.

Ear-
length.

Weight.

Age.

Parents:
Old 9 lop

mm.

22?

gms.

A 6OO

Adult

<5liy6

166

4I3O

Do

Mid- parental

IO5 C.

§*y

4-ine

"Do

Offspring:
Litter i —

9 CO4. . .

1 86

* -A7O

f
• >••

i vear.

-x

d^eoc.

206

•7 770

* J*" ,

42 weeks

/-?eo6

IO2

3 I CO

30 weeks

9eo8

180

I OOO

1 8 weeks

O eoo

(185

16 weeks.

Litter 2 —

f?1IQ

/ 210
JI90'

3.910
3,465

i year.
25 weeks.

Q •» 20

j 200
l82

3,955

i year.
1 6 weeks

TO*"
$321

18*

1 8 weeks

9322 (pi. 3, fig. n)..

195

4,45°

i year.

1 23 weeks.

The range of variation in this mating is similar to that observed in a
mating of this same female with a lop-eared male (see table 2), viz, a
variation of between 20 and 30 mm. It is difficult to estimate it more
precisely, because the measurements recorded were made at such differ-
ent ages. Two of the offspring approximate the mid-parental conditions
both of ear-length and of weight, these two being ^319 and $322. The
same is measurably true of a third individual, 6* 506. But 9 504 and
9 508 fall considerably short of the mid-parental ear-length and the
mid-parental weight; while <f 505 and 9 509 considerably exceed the mid-
parental ear-length, and approximate the mid-parental size. The greatest
deviation from the mid-parental ear-length is a minus one of 15 mm.
(recorded at the age of 18 weeks), but a plus deviation of nearly the same
amount (14.5 mm.) is also observed, though not until the age of a year
had been attained. The average deviation from mid-parental ear-length
is 9 mm. The lowest measurement, 180 mm., stands almost exactly
midway between the ear-length of the father (166 mm.) and the mid-
parental ear-length 195.5 mm-J while the largest measurement (210 mm.)
stands midway between the condition of the mother (225 mm.) and the
mid-parental ear-length.

The result observed in this cross may be described as blending inheri-
tance, with fluctuation about the mid-parental ear-length, in about the
same degree as in the case of lop-eared rabbits purely bred.

22

INHERITANCE IN RABBITS

Two of the offspring produced by cross 4 were mated with each other,
viz, 9 504 and $ 506. They produced a litter of 3 young, the character
of which is shown in table 7.

TABLE 7.

Ear-
length.

Weight.

Age.

Parents:
9s°4
j»<o6

mm.
186

IQ2

gms.

3,43°
3,150

i year.
30 weeks.

Mid-parental. . . .
Offspring:

f?*74.O

I89
IQI

3.29°

2.2CC

3 Do.

20 weeks.

f?74.I

2O I

2, 2SIJ

Do.

9743

185

I,9OO

19 weeks.

The deviations from the mid-parental ear-length are + 2, — 12, and
— 4 mm. respectively, which lie within the limits of variation observed
among lop-eared rabbits purely bred.

The range of variation is chiefly upward (plus), doubtless because of
the small number in the litter, which would make the food supply of the
individual better than usual.

CROSS 5. — HALF-BLOOD LOP FEMALE X LOP MALE 179.

Female 167 was produced by a cross similar to cross 2, in which the
mother was a short-eared Himalayan rabbit (923), and the father the
male lop used in cross 3. She was mated with lop o" 179 and produced
3 litters of young, as indicated in table 8.

The range of variation in ear-length in these 3 litters of rabbits is wide,
extending from 165 mm. in a rabbit 7 months old (having full-grown
ears) to 194 mm. in one only 18 weeks old, or to 200 mm. in one 30 weeks
old, a total range of 35 mm. The largest minus deviation from the mid-
parental ear-length is 5 mm. ; the largest plus deviation, 30 mm. ; the least
deviation from the short-eared parent is 35 mm., but from the long-eared
parent only 10 mm. Hence in this cross the long-eared parent is approx-
imated more closely than the short-eared one. This, however, is not of
necessity evidence of Mendelian recessiveness of the character long-ear
in 9 167. Another and, we believe, better way of viewing the matter is
this: 9 167 transmitted a greater ear-length than she had herself attained.
It is known that conditions which influence general growth, during the first
20 weeks, influence also ear-length. But at 20 weeks of age ear-growth
is practically complete, although growth in other respects continues some
time longer. It is possible, therefore, for an animal to be stunted in ear-

EAR-SIZE

23

size and yet to attain a normal or nearly normal general size. It is not
to be expected, however, that such an animal will transmit to young reared
under normal conditions the diminished ear-size which it shows, but rather
the ear-size which it would have attained had it been reared under normal
conditions.

TABLE 8. — Cross 5.

Ear-
length.

Weight.

Age.

Parents:
9167

mm.
130

gms.

2, 55O

Adult.

<5*i79 (lop)

2IO

3,410

Do.

Mid-parental. . . .

Offspring :
Litter i —

f?437

170
184

2,980
2,550

Do.
20 weeks.

?4l8. .

200
177

S.MO
2,510

30 weeks.
20 weeks.

Litter 2 —
,51566

185
104

3,iio
i ,04. e

30 weeks.
18 weeks.

^568

ill

I.QI e

Do.

(-7>e6o

I 7O

i 86<;

Do.

9 CfQ

182

Do.

Litter 3 —
9644

i^4

1,655

20 weeks.

<5»645

1 165
I7O

2,43°
1,460

7 months.
20 weeks.

9 646

I7C

I 43O

Do.

9647

171

2 O3O

Do.

96^8

1 80

I 930

Do.

178

2 080

Do

CROSS 6. — HALF-BLOOD LOPS MATED INTER SE.

The same female half-blood lop already mentioned (9 167, cross 8),
was mated with a male produced by cross i (^248). Their young con-
stitute an F2 generation of half-blood lops.

In this litter the deviations from the mid-parental ear-length are all,
with one exception, positive (upward). This result accords with that
observed among the young of this same mother (9 167), in connection
with cross 5. She evidently transmitted a greater ear-length than she
manifested.

The range of variation, 35 mm., while high, does not exceed that found
among lop-eared rabbits mated inter se, as is clear from a comparison of
fig. 2 with fig. 3, the former showing growth-curves for lop-eared rabbits,
the latter for the litter of F2 half-blood rabbits under consideration. The
range of variation in this cross also agrees exactly with that observed in
cross 5, in which the same mother was mated with a full-blood lop. We
get, therefore, from this case no evidence of Mendelian splitting as regards
the character ear-length.

24

INHERITANCE IN RABBITS

TABLE 9.

Ear-
length.

Weight.

Age.

Parents:

Q 167

mm.
IT.O

gms.

2 «O

Adult

(^248

i ?3

7 Q3O

Do

Mid-parental ....
Offspring:

r?786

141-5

I4O

3,240
I 73O

Do.

20 weeks

^787

It*

I QIC

Do.

^788

I7O

2,O6O

Do.

0780 .

I7C

2.I7O

Do.

rT7Q2

I4O

2 I^i»

Do.

Two of the young produced by cross 5 were mated with each other,
viz, 6* 437 w^n % 438- Their young (table 10) vary closely about the
mid-parental ear-length.

TABLE 10.

Ear-
length.

Weight.

Age.

Parents:
94*8...

mm.
184

gms.

2.CCO

20 weeks.

^
Mid-parental ....
Offspring:

f?7lO

177
180.5

1 80

2,510
3,530

I O7O

Do.
20 weeks

Q 7 20

i8<

Do

vzo

0*721

n5
1 80

2,OIO

Do.

C?72'» . .

176

1, 82<

Do.

Another cross 6 mating was obtained between 9 247 and <? 248 (pro-
duced in cross i). The character of the young is shown in table u.

TABLE n.

Ear-
length.

Weight.

Age.

Parents:

Q 247

mm.

1^2

gms.
3 200

Adult

(^248

I C7

3Q7Q

Do

Mid-parental ....
Offspring:
Q 507 . .

152-5

122

3,610
1,77^

Do.

i o weeks

f?308

I7Q

i ,770

Do.

$4OO

I7C

2 460

8 months

94OI

J7t

I 860

10 weeks

f?4O2

I3.O

I 7^??

Do

Contrary to the result shown in table 9, the young obtained from this
mating all fall short of the mid-parental ear-length by from 17 to 29.5
mm., indicating probably conditions of nutrition below the normal, dur-
ing the period of principal growth of the ears, or of the transmission by
the parents of a condition of ear-length inferior to that which they mani-

EAR-SIZE

25

fested. The young vary in normal fashion about a mean ear-length of
130.4 mm. The total range is only 13 mm., indicating no Mendelian
heterogeneity among the gametes produced by the parents, though both
were F2 half-lops.

One of the young produced in this litter (9 400) was mated with the
lop-eared <? 179, table 2. The result is shown in table 12. Six offspring
were obtained from this mating; they vary rather closely about the mid-
parental ear-length, though chiefly below it, as we might expect from the
fact that the mid-parental value given is based upon measurements of
adults, and that of the young upon measurements at the age of 20 weeks.
The total range of variation is 15 mm.

TABLE 12.

Ear-
length.

Weight

Age.

Parents:

54.00

w/w.
I*e

gms.

2 460

8 months

(^179

2IO

3,4IO

Adult.

Mid-parental. . . .
Offspring:

f?7O3

172.5

162

2,935
1,680

20 weeks.

r?7O4

1 66

1.670

Do.

O 7o5

1 60

I ?2O

Do

c?7o6

171

I 880

Do

97O7

I

1 60

I.7I5

Do.

9 708 . .

175

I.72O

Do.

Female 400 was likewise mated with her father (0*248), producing a
litter of 4 young, all of which fell below the mid-parental ear-lengths.
(See table 13.)

TABLE 13.

Ear-
length.

Weight.

Age.

Parents:
9 400

fftfft*
J7C

gms.

2 060

8 months.

3*248

I C7

2 Q^O

Adult

Mid-parental ....
Offspring:
98i8

144

1 7<

3,445

I ?OO

20 weeks

r?8io

I 77

Do

9820

128

i,/iw
i 780

Do.

9821

126

*,/w

i soo

Do.

The deviations are — 9, —7, — 16, and — 18, average —12.5 mm. But
the total range of variation is only n mm., or scarcely greater than that
observed among short-eared rabbits. Certainly this result affords no
evidence of heterogeneity as regards ear-character among the gametes
formed by the parents, though one was an Ft and the other an F2 cross-
bred between the lop-eared and the short-eared races.

26

INHERITANCE IN RABBITS

OTHER MATINGS OF HALF-BLOOD LOPS AND ADDITIONAL CROSS 6 MATINGS.

Other matings in which the rabbits 9 247 and <? 248 were concerned
are recorded in tables 14 and 15.

In mating i (with <? 179) 9 247 gave a fully normal blending result.
Of the 5 young produced, 2 surpassed the mid-parental ear-length, i
equaled it, and 2 fell below it. All were intermediate, and the range of
variation was 14 mm., or about one-fourth of the difference between the
parents. In mating 2 (with ^319) 9247 gave a result similar to that
which she had given with rf 248. All the young were intermediate in
ear-length, but all fell short of the mid-parental ear-length, by from 3 to
1 6 mm. This was not due to consanguinity, for 9 247 and ^319 were
not closely related. It may, however, have been due to inferior condi-
tions of nutrition, perhaps resulting from the large size of the litter. The
whole litter seems to have been affected alike, the total range of variation
among the seven young being only n mm.

TABLE 14.

MATING

I.

MATING

2.

Ear-
length.

Weight.

Age.

Ear-
length.

Weight.

Age.

Parents:
9 247 ....

mm.
IS2

gms.
3,200

Adult.

Parents:
9 247

mm.

IC2

gms.
3,200

Adult.

cTiTg
Mid-parental
Offspring:
$635

210

181
170

3»4io
3.35°

74O

Do.
Do.

20 weeks.

c?3.i9
Mid-parental
Offspring:
9 731

2OO
I76

I7O

3,955
3,622

I O7O

Do.
Do.

20 weeks

9636
9637
^638

183
1 80

I7O

,620
,990

19 weeks.
20 weeks.

T /OX '
9732

9733

9 734.

170
1 60

2, IOO
2,O2O

Do.
Do.
Do

<5>(639

A
184

COC

20 weeks.

t/34
c?736 .

i6<

I 785

Do

(^737

**
1 60

I,7OO

Do.

9738..

171

2,O2O

Do.

Male 248 was mated with three different short-eared females, none of
which was nearly related to him. The results are shown in table 15.

In mating i, ^248 gives a result like that which he had given when
mated with his sister (9 247). All but i of the 6 young fell below the mid-
parental ear-length by from 8 to 19 mm. The shortest-eared one had
exactly the same ear-length (115 mm.) as the short-eared parent, a result
unparalleled elsewhere in these experiments except in one case, presently
to be noticed. The shortness in this case can not be attributed to the poor
condition or small size of the individual, for it was the largest rabbit but
one in the litter, a position which it maintained throughout the growth
period. Apparently this individual represents an extreme variate of a
fluctuating group. The extreme range of variation in this litter was 23
mm.; the difference between the parents, 38 mm.

EAR-SIZE

27

Mating 2 shows a result more nearly normal. Two individuals exceed
the mid -parental ear-length by 3 mm., 2 fall short of it 7 mm., and i by
9 mm. The total range of variation is 12 mm.

In mating 3, which produced 2 litters of young, the variations are again
chiefly below the mid-parental ear-length, but to no greater extent than
we might expect, in view of the difference in age between parents and
children, when measured. In litter i the deviations are —2, +3, — 4,
and —2 mm., a nearly normal result; but in litter 2, four individuals
show a deviation of —7 mm., and one a deviation of +3 mm. The range
in litter i is 7 mm.; in litter 2, 10 mm. There is no evidence of hetero-
geneity among the gametes.

TABLE 15.

MATING

I.

MATING .

J-

Ear-
length.

Weight.

Age.

Ear-
length.

Weight.

Age.

Parents:

9 2? ?

mm.

lie

gms.

2 6?O

Adult.

Parents:

9-2,80

mm.

IO?

gms.

2 OQO *

Adult.

(^248

I ei

1 Q-IQ

Do.

C?248 . .

les

7 Q3.O

Do.

Mid-parental
Offspring:
$619
9620

134

125
1 2O

3,290

,820
.7?o

Do.

19 weeks.
20 weeks.

Mid-parental
Offspring:
Litter i —
96?s,. .

I29

127

1,4.6?

Do.

so weeks.

^621

n8

,700

Do.

96?4. .

172

i. 84 <;

Do.

9622 .....

jje

.88 ?

Do.

96??

I2<

i 840

Do.

C?624

126

72O

19 weeks.

96?6

127

I 74.?

Do.

f?62?

12?

I OIO

20 weeks

Litter a —

cT793

i ao

1,700

20 weeks.

9 704. •

1 20

I.74O

Do.

970?

1 20

1,860

Do.

MATING

2.

c?7o6 .

I7Q

i 600

Do.

(?797

120

1,770

Do.

Parents:
9260

02

I O3?

Adult

c?248

I C3

3 O7O

Do

Mid-parental
Offspring:
9726 . .

122
lie

2,932
,77Q

Do.
20 weeks.

Q 727

113

68<;

Do.

c?728

lie

60?

Do

C?I72Q

I^e

eoo

Do

97-20

12?

,Q3O

Do.

i At 20 weeks.

Some other matings which fall in the category of cross 6, and consti-
tute a second generation (F2) from cross 3, are included in table 16.

The statistics contained in table 16 are not very satisfactory because
the observations are made at such different ages, and because, in one case
at least, that of #381, a remarkable increase in ear-length is recorded
subsequent to the age of 18 weeks. For observations made at the same
age, however, the variability in ear-length is considerable. The range
of variation in mating i, litter i, is 17 mm.; in litter 2 it is 15 mm.; in mating
2, it is 1 8 mm. In generation Fu cross 3, the range of variation was only

28

INHERITANCE IN RABBITS

slightly less, viz, 14 mm. So far, then, as table 16 is concerned, we get
no clear evidence of heterogeneity among the gametes formed by the
cross-breds produced by cross 3.

TABLE 16.

MATING

I.

MATING

2.

Ear-
length.

Weight.

Age.

Ear-
length.

Weight.

Age.

Parents:
$i75(pl-3,

fifir jo)

JHWl.
I7O

gms.

A •JQC

Adult.

Parents:
9178

mm.
170

gms.

4..O7O

Adult.

(^176

1 66

A I "1Q

Do.

(^176 ....

1 66

4I7O

Do

Mid-parental
Offspring:
Litter i —

1 68

4,218

Do.

Mid-parental
Offspring:

94.71

1 68

14-7

4,100

I 4.7O

Do.

c?37<» . .

I601

2,960

6 months.

J*»*
9472

1 68

1,865

Do.

c?176 . .

I721

2,07?

Do.

(^474

16*

i,?**

Do.

( jecl

2 8OO

22 weeks.

947C. .

ISO

I.eic;

Do

c?38i (pi. 3,

) 5S

3 I2C

6 months.

9476

I CO

1 ,7eo

Do

fig. 12) ...
Litter 2 —

c?759
(3>76o

( 168

1 80
170

3,800

2,220
2,2OO

i year.

20 weeks.
Do.

c?48o

150

•»/y

1,610

15 weeks.

?76l . ,

i8c

2.4IO

Do.

i 18 weeks.

The female half-blood lop 175 (plate 3, fig. 10) produced by cross 3
was mated with the three-quarter-blood lop c?437 (cross 5), and produced
a litter of 5 young, the character of which is shown in table 17.

TABLE 17.

Ear-
length.

Weight.

Age.

Parents:

9l7«;. .

mm.
170

gms.

A ,7O<

Adult.

e?4.17

(184

2,55°

20 weeks.

Mid-parental ....
Offspring:
9 847

} 200
185

176

3,140
3,722

1,920

30 weeks.
Adult.

1 8 weeks.

9848

188

i, 800

17 weeks.

9840

1 80

i ,070

Do.

(^851

174

1 ,7eo

1 6 weeks.

•5*852

IOO

I.7QO

1 8 weeks.

These young early (16 to 18 weeks) attained a large size, indicating
conditions favorable for growth. In ear-length they fluctuated about
the mid-parental condition, which was exceeded by 2 individuals, while
3 fell short of it. All had ear-lengths intermediate between those of their
respective parents. The range of variation among them at 18 weeks
was 14 mm., exactly the same as in cross 3, from which the mother was
derived. The greatest deviations from the mid-parental were — n (at
1 6 weeks) and +5 (at 18 weeks). No evidence is afforded of unusual

EAR-SIZE

29

heterogeneity among the gametes of either parent, although both were
cross-bred individuals.

Females 175 and 178 (cross 3) were also used in back-crosses with a
lop-eared male (179), resulting in the production of three-quarter-blood
lops. The character of these young is shown hi table 18.

TABLE 18.

MATING

I.

MATING s

r.

Ear-
length.

Weight.

Age.

Ear-
length.

Weight.

Age.

Parents:

9i75
<jti 70

mm.
170

2IO

gms.

4,305
3,410

Adult.
Do.

Parents:
$178
(^179

mm.
170

2IO

gnu.

4,070
3,410

Adult.
Do.

Mid-parental
Offspring:
$487 . .

I90
182

3,857

2-74O

Do.

6 months.

Mid-parental
Offspring:
QC46 .

190
IQO

3,740
2,242

Do.
20 weeks.

T*V|

(3*491

l82

I 7QC

16 weeks.

9 ^47

i8c

3, 1 60

26 weeks.

( 100

2,035

Do.

rfsJI

195

2,320

19 weeks.

C?492

/ 2OO

2 3 2O

Adult.

Q C4Q

JQ7

2 OOO

26 weeks.

f?4O3

1 7O

I OOO

1 6 weeks

* j^y

Q CCQ

2OO

^,yw

2 4.0O

Do.

Q CC.2

i8«;

2,360

Do.

The data for mating i are incomplete; but those recorded for mating 2
are entirely satisfactory. They show a total range of variation in ear-
length at 26 weeks of 15 mm. which is very similar to that found in cross
3, by which the mother was produced. The greatest minus deviation
from the mid-parental ear-length is 5 mm.; the greatest plus deviation,
10 mm.; the average deviation, 4.6 mm. The nearest approximation to
the short-eared parent is 15 mm., to the long-eared parent 10 mm. The
inheritance may fairly be described as blending, with no evidence of seg-
regation in F2.

The half-blood lop $ 176 was also employed in a back-cross with his
mother, the Belgian hare, 9431 (plate 3, fig. 9). Table 19 shows the
result obtained.

TABLE 19.

Ear-
length.

Weight.

Age.

Parents:
9431

7W7W.

118

gms.

34.00

Adult

^176

1 66

41 ?Q

Do

Mid-parental. . . .
Offspring:
(3^520

142

T4C

,i.5'-'

3,765

2 660

Do.

Q eji

2 87C.

Do

\-p

(V^22

Do

i^c.

•*O-*U
2 OOO

Do.

QC24

128

2 180

Do

Average

2 c8i

INHERITANCE IN RABBITS

The offspring fluctuate in ear-length about the mid-parental condi-
tion; 2 of them exceed it, 3 fall short of it. The minus deviations, how-
ever, are greater than the plus ones, precisely as in lop-eared rabbits bred
inter se (p. 17). The range of variation is 17 mm., which is greater
than that occurring among short-eared rabbits, but less than that occur-
ring among lop-eared rabbits.

Another son of £431, own brother to c? 176, was likewise mated with
his mother. This male (177, cross 3) had ears 10 mm. shorter than those
of his brother (0*176). The mid-parental ear-length, accordingly, was
only 137 mm. Two young only were reared to an age of 20 weeks, and
each of them had an ear-length of 125 mm.

Further tests of the half-blood lop females 175 and 178 are afforded
by the crosses recorded in table 20, with a related three-quarter-blood
lop male (319) produced by cross 4.

TABLE 20.

MATING

I.

MATING

2.

Ear-
length.

Weight.

Age.

Ear-
length.

Weight.

Age.

Parents:

9i75
(5*3 19

mm.

170
aoo

gms.
4,305

onec

Adult.
Do.

Parents:
9178
^319

mm.

170
200

gms.

4,070
•3 QCC

Adult.
Do.

Mid-parental
Offspring:
9674

c?67<; .

i8S

185
I7e

4,13°

2,400
2 2 CO

Do.

20 weeks.
Do.

Mid-parental
Offspring:
Litter i —
$660

185

IQC

4,012

Do.

20 weeks

cf677 . . .

IO2

2 4.IO

Do

c?66 1

Do

^678 ..

1 80

I 4.2O

Do

Litter a —

C?7S4. •

I SO

i 460

19 weeks.

t?*7ee .

III

i 860

1 8 weeks.

The young produced by mating i are all intermediate in ear-length
between their parents. One ( 9 674) exactly attains at 20 weeks of age
the mid-parental ear-length, a second ( o" 678) would doubtless have done
so had he not fallen into bad condition at about 13 weeks of age. Previous
to that he had been one of the largest and longest-eared rabbits in the litter.
Of the remaining 2, both of which developed normally and were of large
size at 20 weeks of age, one exceeded the mid-parental ear-length by 7 mm.
and the other fell 10 mm. short of it, approaching to within 5 mm. of the
ear-length of the short-eared parent. The range of variation (17 mm.) is
not excessive, and the result may be described as a fully normal blend, with
no indication of heterogeneity among the gametes of the cross-bred parents.

Mating 2 yielded 2 litters very different in character and illustrating
rather strikingly the influence of external conditions on growth. Litter
i consisted of 2 young only. They were born in summer and developed
under optimum conditions as regards food supply. At 20 weeks of age
they had attained large size and had ear-lengths exceeding by 6 and 10

EAR-SIZE

31

mm. respectively the mid-parental ear-length. Litter 2, on the other hand,
was born in the winter. It consisted originally of 8 individuals. The
2 weakest ones in the litter died, one previous to, the other subsequent
to weaning. The 4 largest ones were stolen, leaving 2 survivors, 6*754
and 0*755, both of which when last measured gave evidence of having
been permanently stunted in size and ear-length by the hard conditions
under which they had developed. They are too abnormal to throw any
light on the inheritance of ear-length in this cross.

MATINGS OF THREE-QUARTER-BLOOD LOPS.

The male 319, employed in the matings last described, was also used
in crosses with short-eared rabbits and with a three-quarter-blood lop,
his sister. The results of the crosses with short-eared females are shown
in table 21.

TABLE 21.

MATING

i.

MATING

2.

Ear-
length.

Weight.

Age.

Ear-
length.

Weight.

Age.

Parents:
$ 268

J13I9
Mid-parental
Offspring :
Litter i —

mm.

i°5
200

J52

gms.

2,280
3,955
3,"7

Adult.
Do.
Do.

i Parents:
9105

&3*9
Mid-parental
Offspring:
9747. .

mm.

100

200
J5o

I<O

gms.
3,955

I.9OO

9 months.
Adult.
Do.

20 weeks.

^657

ice

2.S3O

20 weeks.

c?7H ,

148

2,170

Do.

$658
c?659
Litter 2 —
c?662
9663
c?664
9665

*5«

156

141
142
1 60
141

2,460

2,45°

1,980
1,690
2,200
i,575

Do.
Do.

20 weeks.
Do.
Do.
Do.

c?745
9745A
c?746
d1746A

I4I
148

145
M7

1,990

1,840

1,990
1,900

Do.
Do.
Do.
Do.

The 7 young produced by mating i fluctuate about the mid-parental
condition of ear-length. The greatest minus variation is n mm., the
greatest plus variation 8 mm., giving a total range of variation of 19 mm.
This is not large, considering that the difference between the parents is
95 mm. The greatest deviation from the mid-parental, u mm., is 36
mm. removed from the nearest parental ear-length, that is, it is less than
one-third as great as the least deviation from either parent. The inheri-
tance is unmistakably blending. Even more clearly is this the case in
mating 2. The parents differ in ear-length by 100 mm. The young are
all almost exactly intermediate. The entire range of variation in the 6
young is only 5 mm., while the nearest approximation to the ear-length
of either parent is nine times this amount. A better example of fully
blending inheritance can scarcely be imagined. In neither mating do
we get evidence of heterogeneity among the gametes formed by the three-
quarter-blood father (6*319).

32

INHERITANCE IN RABBITS

It is of interest to note that both the mothers employed in matings i
and 2 were employed also in cross 2, with the lop $ 179. In that case,
also, they gave a distinctly and fairly uniform blending result.

Male 319 was mated also with his sister ($322), producing a litter of
only 2 young. These closely resembled their parents in ear-character.
(See table 22.)

TABLE 22.

Ear-
length.

Weight.

Age.

Parents:

Q 122 . .

mm.

IOC

gms.
4,450

Adult.

•H

(Ji^IO

200

3,955

Do.

Mid-parental. . . .
Offspring:

Q 770

197-5

208

4,202
2,680

Do.

20 weeks.

$780

IQO

I QCO

1 6 weeks.

The latest measurement recorded for one of them (?y8o) was made
at 1 6 weeks of age, but already she had attained an ear-length of 190
mm. At maturity she would doubtless have equaled or exceeded the mid-
parental ear-length. The other one (9 779) did exceed the mid-parental
ear-length at 20 weeks of age by more than 10 mm., and she exceeded by
8 mm. the ear-length of the long-eared parent. Her size also at 20 weeks
of age was very large, viz, 2,680 grams. This unusually great plus vari-
ation was doubtless due in part to extremely favorable conditions during
the growth period, especially during the period of lactation. During
that period the mother's milk was divided among 3 young only, but i of
these died soon after the young were weaned. At the last measurement
recorded, its ear-length was a little less than that of 9 780, while in size
it was inferior to both 9 779 and 9 780.

TABLE 23.

Ear-
length.

Weight.

Age.

Parents:

Q 322

mm.

IQC.

gms.

4.ACO

Adult.

V3 .
C?I70

2IO

3,410

Do.

Mid-parental ....

Offspring:
Qe8o .

202.5
J205

3.930
2,460

Do.

20 weeks.

9 <QO

/ 2IO
!2OO

2,785
2,220

25 weeks.
20 weeks.

<-?CQi

205

{ r95

2,510
1,970

25 weeks.
20 weeks.

Q coo

( 200
i 190

2,865
2,080

33 weeks.
20 weeks.

I 195

2,235

25 weeks. '

The same three-quarter-blood female (322) which was mated with
0*319 (table 22) was mated also with the lop $ 179, producing a litter of
seven-eighth-blood lops. (See table 23.) Two of the 4 young reared had

EAR-SIZE

33

at 25 weeks of age ear-lengths identical with those of the respective parents,
viz, of 195 and 210 mm. The other two had intermediate ear-lengths
of 200 and 205 mm. respectively. This is a fully normal blending result.
The total range of variation is 15 mm. In both ear-length and size the
young are similar to those produced by the mating with <?3i9 (table 22).

CROSS 7. — QUARTER-BLOOD LOP FEMALE X SHORT-EARED MALE.

Three different quarter-blood lop females, 521, 522, and 524 (table 18),

produced by a mating of the Belgian hare with her son (c? 176), were

mated with a son of the same Belgian hare by an unrelated short-eared

male. (See table 14.) The outcome of these matings is shown in table 24.

TABLE 24.

MATING

I.

MATING

3-

Ear-
length.

Weight.

Age.

Ear-
length.

Weight.

Age.

Parents:
$521
c?235

mm.
142

I IO

gms.
2,875

2 Q4 C

27 weeks.
Adult.

Parents:
?524
(^235 . . .

mm.
128

I IO

gms.
2,160

2 04 <C

27 weeks.
Adult

Mid-parental
Offspring :
^809

126

I^ae

2,910
2,080

20 weeks.

Mid-parental
Offspring:
(5*8^4..

119
lie

2,552
1, 600

20 weeks.

tfSio

I2<C

1,890

Do.

^83!

I2«C

1, 7OO

Do.

98ii

1 2O

1,820

Do.

9836..

121

1,820

Do.

98i3

IIS

2 O3?

Do.

$877

116

I.4<O

Do.

r?8l4

I2C

I 060

Do

98^8

134

2 OOO

Do.

$815

9

126

2,150

Do.

MATING

2.

Parents:
$522
c?235
Mid-parental
Offspring:
9823

!35
no

122.5

I2C

2,52O

2,945
2,732

2 3OO

27 weeks.
Adult.

20 weeks

r?824

I2C

Do

The offspring show, as regards ear-length, a rather wide range of vari-
ation, 20 mm., which is nearly two-thirds of the difference in ear-length
between the parents. The average ear-length of the offspring corresponds,
in each litter, closely with the mid-parental ear-length, the plus and minus
deviations being, except in mating 3, about equal in number and amount.
In mating i, 3 of the 6 young have approximately the mid-parental ear-
length, but 2 show minus deviations of 6 and n mm. respectively, and i
shows a plus deviation of 9 mm.

The 2 young produced by mating 2 were of large size at 20 weeks of
age, indicating conditions of nutrition above the average. The ear-length
of each exceeds by 2.5 mm. the mid-parental ear-length.

The 5 young produced by mating 3 show 2 minus deviations of 3 and 4
mm. respectively, and 3^>lus deviations of 2, 6, and 15 mm. respectively.

34

INHERITANCE IN RABBITS

CROSS 8. — QUARTER-BLOOD LOP X THREE-QUARTER-BLOOD LOP.
A single mating of this sort produced a litter of 3 young, all very sim-
ilar and close to the mid-parental ear-length. (See table 25.) The
observations were discontinued when the young were 14 or 16 weeks old,
but the mid-parental ear-length of the parents, when they were 20 weeks
old, had already been closely approximated. The deviations were —2,
— 5, and —5 mm. If growth progressed normally from the age of 14 or
1 6 weeks on, they would surely have attained the adult mid-parental
ear-length, viz, 167.5 mm.

TABLE 25.

Ear-
length.

Weight.

Age.

Parents:

Q C23

mm.
j 130

gms.
i,93o

20 weeks.

1 i35
i95

2,690
2,540

27 weeks.
20 weeks.

Mid-parental. . . .

Offspring:
884

1 200
(162.5
I 167.5

1 60

3,330
2,235
3,010

i,4CO

43 weeks.
20 weeks.
Nearly grown.

14 weeks.

885

I17

1,300

14 weeks.

886

1 57

1 ,4 CO

1 6 weeks.

LIMITATIONS OF THE DATA STUDIED.

In attempting to draw conclusions from the statistics presented in the
foregoing pages, one must bear in mind certain of their limitations and
imperfections.

(1) Ear-length is modified to some extent by external conditions. If the
young rabbit is well nourished up to the age of 20 weeks, its ear-length
will be greater than if it is poorly nourished, other conditions being equal.
While we have attempted to give our rabbits the best of care at all sea-
sons, it is inevitable that the quality of food supplied at different seasons
of the year should vary, and with variation in the quality of the food goes
variation in the growth rate. This renders it difficult to compare with
each other, as regards ear-character, rabbits reared at different seasons
of the year. But it has been impossible for us to rear enough rabbits at
any one season to afford adequate material for comparisons. Hence we
are forced to utilize material produced at different seasons of the year.

(2) Size of litter is of some consequence in determining the growth rate
of a rabbit. If there are several young in a litter each gets a smaller
amount of food during the period of lactation than it would have received
had the litter been smaller. Our material, however, is not extensive enough
to allow us to institute comparisons merely between litters of substantially
the same size.

EAE-SIZE 35

(3) It is the belief of fanciers that a warm, moist atmosphere, during
the period of active growth of the ears, favors the attainment of large
ear-size. This view we have not been able to put to an experimental
test, but we are inclined to think that the temperature and humidity are
much less important factors than abundant food supply.

(4) Rabbits of the small, short-eared races have a shorter growth period
than the larger races. Their ears are more likely to be full-grown at 20
weeks of age than are those of lop-eared rabbits. Therefore, in comparing
rabbits of different ancestry at the same age, say 20 weeks, one is in dan-
ger of underestimating the ear-length of the larger-sized rabbit.

(5) A cross between rabbits of entirely different races is likely to result
in young of unusual vigor, which causes them to attain a greater weight
and ear-length than the hereditary constitution of either race by itself
would result in. This is illustrated notably in cross 3, page 20. Supe-
rior size or ear-length, induced by crossing, we should not expect to be per-
manent in later generations.

(6) Disease frequently interrupts the orderly progress of a growth-curve
and necessitates the omission altogether of certain series of observations.

CONCLUSIONS.

Notwithstanding these limitations, which manifestly restrict the scope
of our conclusions, certain generalizations are clearly justified.

(1) A cross between rabbits differing in ear-length produces offspring
with ears of intermediate length, varying about the mean of the parental
ear-lengths.

(2) It is immaterial whether the larger parent was father or mother;
the result is the same in either case. As regards ear-length, then, we may
say, reciprocal crosses give the same result. This shows that ear-size is
a character inherited with equal intensity through father or mother.

(3) A study of the offspring of the primary cross-breds shows the blend
of the parental characters to be permanent. No reappearance of the grand-
parental ear-lengths occurs in generation F2, nor are the individuals of
that second generation as a rule more variable than those of the first gener-
ation of cross-breds. Fig. 3 shows the most extreme case of "scatter" in
F2 that we have observed. Yet the variation in this case is no greater
than among the young of lop-eared rabbits bred inter se.

(4) The extreme range of variation in ear-length among short-eared
rabbits is about 10 mm.; in lop-eared rabbits it is two or three times as
great, or from 20 to 30 mm. Among rabbits produced as crosses of vari-
ous sorts between short-eared and lop-eared rabbits the range of varia-
tion in ear-length is mostly intermediate in amount.

(5) The form of the growth-curve for ear-length from the age of 2 weeks
on is convex upward, indicating a steady diminution in the daily growth
increment.

PART II. — WEIGHT.

Our statistics for size inheritance are not very satisfactory, because we
were unable to keep any considerable number of rabbits until they were
full grown, owing to the smallness of our breeding room, so that a large
number of weighings of adults is not available for purposes of compari-
son. But the size of a growing rabbit varies greatly with the character
of its food, and this in turn is dependent upon a variety of conditions
which it was not possible for us fully to control. A comparison of the
weights of growing rabbits at corresponding ages is, therefore, not alto-
gether satisfactory, yet it is the best material we have.

In tables i to 25 the latest available weighing, or the heaviest weight,
is recorded for each rabbit. But since the weighings there recorded were
made at very different ages, it is necessary to select some particular age
at which to make comparisons. The age of 18 weeks has been selected,
because the weighings for that age are most numerous.

In table 26 are shown the average weights, at 18 weeks of age, of
different lots of rabbits, each lot containing those of like ancestry. The
number of individuals in each lot is also shown in the table, as well as
the greatest range of variation in weight found in any litter of each lot.
The statistics in table 26 are fullest for those crosses (left section) in which
ordinary short-eared rabbits were concerned. The average weight of such
rabbits, in a lot of 17 individuals, is seen to be 1,412 grams. For lop-
eared rabbits it is something over 1,743 grams, the weight given in the
table from observations on 2 rabbits. This weight, however, has been
exceeded at 14 weeks of age by a majority of the lop-eared rabbits which
we have reared, so that it is certainly too low.

The lots of rabbits, partly of short-eared, partly of lop-eared ancestry,
have intermediate weights, the weight tending to increase with increase
in the proportion of lop blood. The variability (range) in weight, which
was found to be twice as great in lop-eared as in short-eared rabbits, is
intermediate in the cross-bred lots, increasing with increase in the propor-
tion of lop blood.

^ Both the position of the average for each lot, and the amount of variation
within it, indicate that weight-inheritance, like the inheritance of ear-
size, is blending in character. Neither dominance nor segregation in
the Mendelian sense are recognizable.

The Belgian hare crosses and mixed crosses, recorded in the last sec-
tions of table 26, show, in general, results similar to those given by the
crosses with short-eared rabbits, but many of the averages are less reliable

37

38

INHERITANCE IN RABBITS

because based on too few individuals (in 4 cases, a single litter each time).
The Belgian hare was heavier than the short-eared stock used, and it
will be seen that, in all cases, her descendants exceed in size animals of
the short-eared series having a like amount of lop blood. Further, a
mixture of short and Belgian blood tends to produce a rabbit intermedi-
ate in weight between those of the short and of the Belgian series, respec-
tively. (See table 26, right section.) All these observations confirm the
idea that body-weight is a character blending in its inheritance.

TABLE 26. — Size at 18 weeks of age of rabbits of different proportions of lop
"blood" from crosses with short-eared, with the Belgian hare, or with both.

Lop blood.

Short-eared.

Belgian hare.

Mixed.

Average
weight.

Max.

varia-
tion in
weight.

Individ-
uals ob-
served.

Average
weight.

Max.
varia-
tion in
weight.

Individ-
uals ob-
served.

Average
weight.

Max.
varia-
tion in
weight.

Individ-
uals ob-
serv e.

None

gms.
1,412

gms.
3i5

i7

gms.

gms.

gms.

gms.

One-eighth

5

i,79i
1,580
1,888

490

205
575

i3
4
M

One-fourth
Three-eighths. . . .
One-half:
Gen. i

i,S92

345

20

1,788

627

1,463
1,700

i,58S
1,652

3*5
420
420

495

I?

18
35

i7

Gen. 2

2,076

1,965
1,940

!,954
i,936

260

820
890
890
59°

4

12
10
22
4

i,754

420

ii

Both
Three-fourths:
Gen. i
Gen. 2 ...

Both

8oo2

7

Seven-eighths . . .
All

I.7431

1 This is certainly too low, for in litter 70, table 2, mating 3, it was surpassed by three of the
five rabbits of the Utter, already at 14 weeks of age. The average given (1,743) is for the two ani-
mals, c?i79 and 9 180 (table 2, mating i).

2 At 14 weeks.

When the parents differ in size, the young are clearly of intermediate
size, but our observations are too incomplete to show in most cases whether
the size is midway between that of the respective parents or not. Prof.
W. C. Sabine has kindly pointed out that if linear dimensions give a mid-
parental condition (the mean of the respective parental conditions), then
we should expect the weights to be less than the mean of the parental
weights, provided the proportions of parts are the same in all cases. But
the proportions of the parts are different in the two parents, when rabbits
of different size are mated with each other, and the proportions in the off-
spring are unmistakably intermediate between those of the respective parents.

This, perhaps, accounts for some of the peculiarities observed in com-
paring weights of the rabbit <? 248 with those of his parents, and with
the mid-parental weights. All three rabbits were fully grown (2 or more
years old) when the observations were made, and these are fairly com-
plete. The maximum body-weight recorded for ^248 was somewhat

WEIGHT

39

in excess of the mid-parental (table 27), but since he shows a less per-
centage of bone to total body-weight than either parent it is probable
that the excess is due in part at least to some temporary condition (fat-
ness). If he showed a mid-parental percentage1 of bone to body-weight,
and this would possibly be the case if all 3 rabbits had been in like con-
dition, as regards fatness, then his weight should be something less than
the mid-parental weight, or about 3,326 grams, instead of the mid-parental
weight (3,800) or the observed maximum weight (3,930). But in the
absence of more extensive observations, we can not be certain that the
percentage ratio of bone to body-weight is a mid-percentage. The ques-
tion must remain an open one until further data can be accumulated.

TABLE 27. — Relation between bone-weights and total body-weight in the
rabbit tf 248 and in his parents.

Bone-
weight.

Body-
weight.

Per cent
bone-
weight.

Old?lop

c?45

gms.
77-15

1Q 7C

gms.
4,600

•7 OOO

1.67
i 31

Mid-parental
Son, 6^248

S8-25
40-7

3,800

2 ,020

1.26

As regards bone-size, however, we can reach more satisfactory conclu-
sions, for this character is unaffected by temporary conditions of the flesh.
In table 29 are recorded bone measurements of this family of rabbits,
which show the measurements of the son ( <? 248) to be close to the mid-
parental as regards both absolute measurements and proportions of parts.

In table 28 are recorded observations upon the weight and volume of
certain bones of these same rabbits. Both in weight and in volume the
bones of the son are less than the mid-parental. This is what we should
expect if the bones of the son correspond with the mid-parental in linear
dimensions and in the proportions of parts; for linear dimensions should
be to each other about as the cube roots of the volumes and weights (pro-
vided the specific gravity is alike in all three cases).2 On this hypothesis
"expected weights" and "expected volumes" have been calculated for
the son, and these are entered in table 28 in a parallel column, along with
the observed weights and volumes. It will be noticed that the "expected"
uniformly exceed the observed weights and volumes. The expected, to
be sure, is less than the mid-parental, but the observed is still less. As
regards the total weights of the parts observed, a graphic presentation is
made in fig. 4 of the relation of mid-parental to expected and observed.
The expected falls below the mid-parental by a certain amount, but the

1 The percentages given are based upon the combined weights of particular bones, not of all the
bones of the body.

3 A comparison of the weights and volumes of corresponding bones in table 28 indicates that the
specific gravity of the bones of the son (^248) was slightly less than that of either parent, viz,
about 1.19 for the son, 1.20 for the mother, and 1.26 for the father.

40

INHERITANCE IN RABBITS

observed falls below the mid-parental by about three times that amount.
It is in fact removed from the mid-parental only a little less than from
the weight of the smaller parent. It is difficult to explain this extensive
deviation, but it undoubtedly exists and is apparently fairly uniform,
though possibly the method of computing the "expected" magnitudes is
faulty. The computation is made in the following way. The cube
root of the weight for each parent was found. These two roots were then
added together, and their half -sum found, which was then cubed.

FIG. 4. Relation of the bone-weights of rabbit 248 to those of his parents and to the mid-
parental bone- weights. If distances are laid off from a point at the left proportional to
the bone-weights, they bear to each other the spacial relations of the following points :
A, bone- weights of the mother; B, of the father; D, of the son, cT 248; C, the mid-parental;
E, the expected bone- weights of the son.

Observed and expected deviate in the same sense from the mid-parental,
that is, are less, but the uniform difference in amount between observed
and expected is something requiring fuller analysis.

TABLE 28. — Weights and volumes of skeletal parts of <? 248 in relation

to those of his parents.
[Mother, old female lop; father, 6^45 ; son, (^248. Plate i, figs. 2, 3, i.]

Part.

Weights of bones (grams).

Mother.

Father.

Mid-
parental.

Son.

Ob-
served.

Ex-
pected.

Devia-
tion of
observed
from ex-
pected.

Devia-
tion of
expected
from
mid-
parental.

Devia-
tion of
observed
from
mid-
parental.

Humerus
Femur
Tibia-fibula
Innominate (of one
side)
1 2 ribs (of one side)

Vertebrae:
i to 6 ...

5-9
10.8
9.1

8.4
6-95

3-2

5-9

5-45

4.1
3-2

4-55
8.35
7.27

6.25
5-°7

3-7
7-5
6.15

5-4
3-9

4-4
7.8

7.13

5-9
4-7

-0.7

-0-3
-0.98

-o-5
-0.7

-O.I

-o-5
-0.14

-0-35
-o-3

-0.85
-0.85

-1. 12

-0.85
-I.I7

7-4
9-9
18.7

3-3
4-35
9-85

5-35
7.12
14.27

4.6

5-95
12.5

5-04
6.6

13-7

-0.44
-0.65

— 1.2

-0.31
-0.52
-0-5

-0-75
-I.I7
-1.77

7 to 12

13 tO 20

Total, i to 20 .

Total, counting
weight of verte-
brae once only .

36.0

17-5

26.75

23-05

25-5

-2.4

-1.25

-3-7

77-15

39-35

58.24

49-7

55-43

5-58

2.64

8-54

Volumes of bones (cubic centimeters).

Humerus
Femur

5-2

9.1
7-i

2-5
5-o
4.0

3-85
7-05
5-55

11

5-o

3-65
6.94

5-40

-0-55
-0.44
—0.40

-0.20
-O.II

-0.15

-0-75
-0-55
-0-55

Tibia-fibula

PART III. — SKELETAL DIMENSIONS.

Skeletons were prepared of certain of the rabbits concerned in this
series of experiments, and upon these several series of measurements were
made. The most complete series are recorded in tables 29 and 30.

In one case (cross i, table 3) the skeletons of both parents were pre-
served, as well as that of one of the fully-grown young, viz, c? 248. The
measurements of this animal (recorded in table 29) are approximately
intermediate between those of his respective parents. They include 7
different skull measurements and 7 of other parts, chiefly bones of the
appendages.

The skull of the lop-eared rabbit is relatively much longer and more
slender than that of short-eared rabbits. (See plate 4.) The proportions
of half-blood lops (like their absolute dimensions) are intermediate, cor-
responding closely with the mid-parental or mean of the parents in this
respect. (See table 29, ratios.)

The limb bones are shorter in proportion to the length of the innominate
bone in lop-eared than in short-eared rabbits. In this particular also
part-blood lops are intermediate. (See tables 29 and 30, ratios.)

In the case of the rabbit c? 248, table 29, the deviation from the mid-
parental measurements or proportions in no case equals one-fifth of the
difference between the parents; in most instances it is much less. In this
animal the inheritance of skeletal dimensions and proportions is unmis-
takably blending.

Measurements of another half-blood lop (9 167) are recorded in table
30. The mother's skeleton was not preserved. She was a short-eared
rabbit similar to <? 45. If, then, the inheritance was blending also in the
case of 9 167, her measurements and proportions should resemble those
of c? 248, table 29. This, it will be observed, is the case.

Measurements of a third half-blood lop (9 178) are recorded in table
30. The father of this rabbit also was the old male lop, table 30. The
mother was the Belgian hare (9431, table IA). In size and proportions
of parts the Belgian hare occupied an intermediate position between the
lop-eared and the small short-eared races used. Accordingly it is not
surprising to find that the half-blood daughter (9 178) deviates from the
other half-blood lops examined, both in absolute measurements and in
proportions of parts, being more like lop-eared rabbits than they are.

41

42

INHERITANCE IN RABBITS

A sister of 9178, viz, 9175 (table 5), had a son (^492) by the lop-
eared male 179. This last-named rabbit was a son of the old female lop
whose skeletal measurements are recorded in table 29 and of the old male
lop whose skull measurements are recorded in table 30. His own skele-
ton was not preserved, nor was the skeleton of 9 175 preserved, but if
each was in skeletal character the mean of its parents, and if their son
(0*492) was intermediate between them in character, we should expect
his measurements to resemble the dimensions entered in column 4 of table
30. A comparison of this column with the next one shows that such was
the case.

TABLE 29. — Bone measurements of $ 248 and of his parents.

Old

Differ-

Devia-

Per cent

Measurement.

female
lop

<?45
(father).

ence
between

Mean of
parents.

C?248

(son).

tion
from

ofdif.

between

(mother).

parents.

mean.

parents.

mm.

mm.

mm.

mm.

7W7W.

mm.

i. Total length of skull . .

111.3

86.2

25.1

98.7

98.1

-0.6

2.6

2. Length, incisor to.palate

inclusive

<u.8

36.

!(• g

A 7 n

A3 O

— o o

S>T

3. Length, occipital to

O A"t*

O"'

13.0

T-O'V

16'^

u.y

•/

palate inclusive

73-o

57-8

15.2

65-4

63.0

-2.4

I5.8

4. Width, anterior to orbit .

46.8

40.8

6.0

43-8

44-3

+ 0-5

8-3

5. Width, posterior to orbit
6. Width, at auditory bullae
7. Length jugal arch ....

25-3
39-8
43-9

26.9
34-0
37-2

1.6

5-8
9-7

26.!

36.9
42.1

25.8

36.7
42.8

-o-3

— O.2

+ 0.7

18.8
3-4

7-2

8. Length lower jaw ....

88.3

68.2

20. i

78.2

80.9

+ 2.7

i3-4

9. Length femur

95-8

85-

10.8

90.2

90.2

0

0

10. Length tibia

116.4

98.1

18.3

107.2

108.7

+ i-5

8.2

ii. Length humerus . . .

77-3

67-3

IO.O

72-3

71.7

— 0.6

6.0

12. Length ulna
13. Length radius

88.5
74.0

74-5
62.5

14.0
"•$

81.5

68.2

82.7
67.8

+ 1.2
-0.4

8.6
3-5

14. Length innominate . .

103.0

81.7

21.3

92-3

92-3

0

0

Ratio:

4 to i

0.420

0.473

O.O53

0.446

0.4^1

+ O.OO5

0.4

<? to i . .

.233

v/**r / O

.3IC

JO

.082

• 274

*»•««

.262

— .OI2

146

ii to i

oo
.604

•o^D
.78l

087

»• i if.
.737

731

— .006

±T.. v

6 o

13 to i ....

kVU^

66 *

• fUl

72C

.WU I

.060

/ o /
60 e

" / O

60 1

— OOA

66

ii to 14

•\j\j$
7^o

• / ^j
824

O74

•"VJ

787

.uyj.

*7*7*7

.wvjif.
— .OIO

T 7 C

•/i"

•WQ

.w 1 1).

* / ° /

•III

*•$* J

A brother of 9 178, viz, c? 176 (table 5), was mated with the old female
lop and had young which are described in table 6. One of these was the
three-quarter-blood lop 9 504. Certain of her skeletal measurements are
recorded in the last column but one of table 30. Her skull unfortunately
was accidentally destroyed in preparation. If c? 176 had skeletal measure-
ments like those of his sister (9 178) we should expect the daughter (9 504)
to approximate the dimensions entered in column 6, table 30, which is
the case. In fact, however, ^176 had ears less long than those of his
sister, and it is probable that his skeletal dimensions also were less, which
would account for the fact that $ 504 falls somewhat below the skeletal
dimensions given in table 30, column 6.

SKELETAL DIMENSIONS

43

The measurements made upon the 2 three-quarter-blood lops (0*492
and 9 504) indicate that in their production, as in that of half-lops, the
inheritance of skeletal dimensions is blending.

It would be premature to conclude that such is the case in all mammals.
Farrabee (105) has shown that in man hypophylangia (2-jointed fingers and
toes) is associated with an abnormal shortness of the arms, legs, and trunk.
It would seem that all the skeletal parts are abnormally shortened. In-
heritance in this case is clearly not blending, but alternative. Some dis-
continuous alteration has evidently occurred in the growth-character of
cells that form the skeleton, just as in the activities of the follicle cells in
long-haired mammals (see Castle and Forbes, : 06) . It would be of inter-
est to know whether such is the case also in bantam fowls and Shetland

ponies.

TABLE 30. — Bone measurements of rabbits.

Measurement.

Old

tflop.

Half-
blood
lop
9178.

Expected
mean of

c?i79
and
c?i78.

Three-
quarter-
blood
lop
c?492-

Mean of
old 9 lop
(table
29) and
9178.

Three-
quarter-
blood
lop
9504-

Half-
blood
lop
9167.

i. Total length of skull . .
2. Length, incisors to pal-
ate inclusive

mm.
103.7

48.0

mm.
104.5

48.4

mm.
106.0

40. i

mm.
I07-5
49.0

fWTW.

mm.

mm.
98.S

AC. 7

3. Length, occipital to
palate inclusive
4. Width, anterior to orbit .
5. Width, posterior to orbit
6. Width, at auditory bullae
7. Length iugal arch ....
8. Length lower jaw
9. Length femur
10. Length tibia

67.4
46.3
27.4
36.4
45-9

66.5
45-4
27.1

37-o

87.6
99.0

68.3
46.6
26.7
38.1

68.6
47-4
27-5
39-6
46.0
86.4

IOO.

no. 6

87.9

97-4

84.0'
95-7

IOQ.O

63.4
43-4
25-1
35-9
41.7

89.1'

IOC. I

ii. Length humerus
12. Length ulna

....

79.6

QO 2

78.9

QI.8

78-4
80 i

slS

Aw3.i
72.6

8l 7

13. Length radius
14. Length innominate . . .

75-9

IOO

•

yi.u
76.7

97-4

74-9
101.5

73-5
97-3

68.0
90-5

Ratio:
4 to i

O 446

O 4.1$

o 440

O 441

O 443

e to I

2<CO

2<C2

248

2ce

1 1 tO I

.76l

• 734

.737

13 to i

.726

.713

.600

II tO 14

' "
.706

.810

0.772

O.77C

!S02

Aside, however, from such unusual cases, it seems probable that skel-
etal dimensions, and so proportions of skeletal parts, behave in general
as blending characters. The linear dimensions of the skeletal parts of
an individual approximate closely the mid-parental dimensions.

Volume and weight magnitudes, however, follow a different law, which
has not yet been clearly made out. It is plain that they are less than the
mid-parental magnitude. (See Part II.)

44 INHERITANCE IN RABBITS

It is probable that in plants, as well as in animals, linear dimensions
are in general blending in their inheritance. In regard to the height of
maize, Lock (:o6, p. 130) says:

Some of the strains which were made use of were uniformly much taller than others. In Fi
the height of the cross-breds between such strains was obviously intermediate. In a number
of cases the cross was made between Fi plants and the shorter of the parental types. The
offspring of this cross showed no such segregation into short and intermediate plants as was
to be expected if Mendel's law held good. On the contrary, the plants produced were re-
markably uniform in height.

This account agrees precisely with our observations upon the inheri-
tance of linear dimensions in rabbits.

The obviously blending inheritance of height in this case does not con-
tradict the known Mendelian behavior of the growth-habit in such plants
as the sweet pea, where Bateson (confirming Mendel) has shown dwarf ness
to be alternative with tallness. Dwarfness is plainly such a discontinuous
variation in plants as is hypophylangia in man, and its inheritance is quite
different from that of ordinary variations in height. The former is a
discontinuous variation, Mendelian in its inheritance; the latter belongs
to a series of continuous variations, and is blending in its inheritance.
In a dwarf plant the internodes are shortened throughout the entire
plant, just as in a case of hypophylangia there is a general shortening
of the skeletal parts.

PART IV. — COLOR.

COLOR VARIATION IN RELATION TO COLOR FACTORS.

A preliminary discussion of color variation in the rabbit was made by
Castle (loja). Since that paper was written several obscure points have
been cleared up. In the light of our present knowledge an attempt will
be made to describe, in terms as simple as possible, the color varieties
of rabbits and the mode of their production.

The gray pigmentation, common to wild rabbits, is complex in its nature,
and all other color varieties are relatively simpler. The gray coat results
from the joint action of several independent color factors; all other types
of pigmentation result from a weakening or entire loss of one or other of
the several factors concerned in producing a gray coat. In other words,
color variation in the rabbit is wholly retrogressive. We are able to recog-
nize the existence in the gray coat of the rabbit of 8 independent factors.
To assume the existence of so many factors will probably seem to some
absurd; at first it seemed so to us; but we have been forced step by step
to the assumption that they exist as the simplest way of explaining the
observed facts.

The factor hypothesis was first introduced by Cue*not ( : 03) to explain
the latent transmission of pigment characters through albinos; it was devel-
oped independently by Tschermak ( : 03) to explain similar phenomena
(kryptomerism, the existence of hidden factors) in beans; and has been
further extended by Bateson (:o6) and his associates.

The 8 color factors which are recognizable in the case of the gray rab-
bit, and the symbols which we shall use to designate them, are as follows:

Symbol C. A common color factor necessary to the production of all
pigment, wanting only in albinos.

B. A factor for black, some substance which acting upon C

produces black pigmentation.

Br. A factor for brown, some substance which acting upon C
produces a chocolate-brown pigmentation.

Y. A factor for yellow, some substance which acting upon C
produces yellow pigmentation.

I. An intensity factor, which determines whether the pigmenta-
tion shall be intense (as in black and in yellow), dilute
(as in blue and in cream), or of some intermediate degree
of intensity.

A. A pattern factor which causes the black or brown pigments
to be excluded from certain portions of the individual

45

46 INHERITANCE IN RABBITS

hairs, where yellow then shows. A " ticked" gray coat
results. When this factor is present the under surfaces
of the rabbit (tail, belly) are unpigmented (white). The
symbol, A, stands for agouti, this factor having first been
demonstrated in the "agouti" guinea-pig. (See Castle,
:o7.)

U. A factor for uniformity of pigmentation (in distinction from
spotting with white, S).

E. A factor governing the extension of black and of brown pig-
mentation, but not of yellow. When most restricted in
distribution the black or brown pigments are found in the
eye and in the skin of the extremities only, but not in the
hair, when more extended they occur also in the hair
generally.

DEVELOPMENT OF THE FACTOR HYPOTHESIS.

Scientific hypotheses, to be of service, should be as simple as possible.
Therefore no unnecessary assumptions should be made. To assume
the existence in gray rabbits of eight independent color factors requires
justification.

THE GENERAL COLOR FACTOR, C.

The existence of a color factor (C) was first suggested by Cuenot (: 03)
to explain how it is that albinos transmit in crosses the particular colors
which were borne by their pigmented ancestors. This common color
factor being acted upon by specific substances (perhaps color enzymes)
produces specific pigments, such as black, brown, or yellow. No hypoth-
esis simpler than this has been suggested, nor any other which adequately
accounts for the observed facts.

THE SPECIFIC PIGMENT FACTORS, B, Br, AND Y.

The existence of separate factors for black and for brown pigmentation
is shown beyond question by the results of crossing black with brown
varieties, in guinea-pigs and in mice. In rabbits a brown variety is not
known to us personally, though we have been informed that such a vari-
ety exists in continental Europe.

The existence of a separate factor (Y) for yellow pigmentation can
scarcely be questioned, in view of the fact that the yellow pigmentation
is as regards distribution quite independent of both black and brown,
remaining extended throughout the fur when they are restricted to the
eyes and the skin of the extremities.

THE INTENSITY FACTOR, I OR D.

The existence of an intensity factor was first announced by Bateson
(: 06) as having been demonstrated by Miss Durham in the case of mice.

For guinea-pigs and rabbits we are able to confirm completely Miss
Durham's discovery. Since dilution or concentration of pigment is a

COLOR 47

property transferable from one pigment (as black) to another (as yellow),
it is evidently due either to some modification in C, or else to an indepen-
dent factor. But it can not be due to C, since it is transmissible through
an albino, which by hypothesis lacks C. We are forced to conclude that
it is transmitted through some independent factor, which we shall desig-
nate I, intensity; it is alternative with D, a state of dilution (as in the blue
modification of black, or the cream modification of yellow).

THE FACTOR FOR A PIGMENT PATTERN OF THE INDIVIDUAL HAIR, A.

Evidence for the existence of a factor (A) governing the pigment pat-
tern of the individual hair has been presented elsewhere (Castle, 1070).
It was first recognized in the case of the guinea-pig (Castle, :o6) as an
essential factor of the " agouti " coat, indeed as the only feature which
differentiates the agouti variety from black. Hence the symbol A (agouti)
was adopted to designate it. Cuenot (104) employed the symbol G to
designate in mice the agouti or gray coat, and designated black by a differ-
ent symbol, but he failed to recognize that gray is simply black plus a
second factor. Hence his G equals B (black) plus A. Hurst has inde-
pendently discovered the existence of the A factor in rabbits (Proceedings
Seventh International Zoological Congress, unpublished). In the guinea-
pig, a new color variety, cinnamon-agouti, has been deliberately produced
through the agency of the independent factor A. (See Castle, : 08.)

THE FACTOR FOR UNIFORMITY OF PIGMENTATION, U, OR SPOTTING WITH WHITE, S.

The factor U (uniformity of pigmentation) is alternative with spotting
with white, S. Its existence was first established by Cuenot (104). Like
I, the intensity factor, it may be regarded as a modifier of C, though not
identical with it; for U and S are transmissible through albinos, which
themselves have no pigmentation and which by hypothesis lack the fac-
tor C. U is also demonstrably independent of any particular color, for
spotting with white is transferable in crosses from one color variety to
another, as, for example, from black to yellow.

THE FACTOR FOR EXTENDED DISTRIBUTION OF BLACK OR BROWN, E, ALTERNATIVE
WITH R (RESTRICTED DISTRIBUTION).

The assumed factor E is a modifier of black and brown, but not of
yellow pigmentation. It is alternative to R, a restricted distribution of black
and of brown pigments, in which distribution they are confined to the eyes
and to the skin of the extremities. The distribution of yellow pigment
(Y) is wholly unaffected by this factor. When black and brown are
restricted, yellow remains as the principal or even as the exclusive pig-
mentation of the hair (yellow varieties).

That E really exists as an independent factor, and not as a condition
merely of black or of brown, is shown by the following experiment. If
one crosses a brown ("chocolate") guinea-pig with an ordinary yellow

48 INHERITANCE IN RABBITS

one (black-eyed), the young are black pigmented, but in F2 4 varieties
are obtained, viz, black, brown, black-eyed yellow, and brown-eyed
yellow. The case, at first thought puzzling, is entirely plain if we con-
sider the distribution independent of the kind of pigment. In the original
cross extended brown was combined with restricted black. Extension
dominated restriction, and black dominated brown, but in F2 black and
brown each occurred both in the extended and in the restricted condi-
tion. Plainly the case is one of Mendelian dihybridism, in which two
independent pairs of alternative characters are concerned.

The extension factor (E) may be replaced, not merely by the extreme
condition (R) in which black and brown pigment are absent from the fur,
but also by conditions of restriction less extreme, in which spots of black
(or brown) occur on a background of yellow. Such intermediate con-
ditions (E', E", etc.) are heritable, and are alternative with E and R,
respectively. In some of these intermediate conditions the spots are of
large size and sharply limited, in others the spots are numerous and small.
Each condition has a tendency to breed true, i. e., is alternative to other
conditions of E.

INTERRELATIONS OF FACTORS E AND U.

Spotting with black or brown on a yellow background is independent of
spotting with white, though the two may coexist. The one is due to a
modification of E, the other to a modification of U. When E and U are
both unmodified the animal is of course black (or brown) pigmented all over.
When U alone is modified (and occurs in condition S) , the animal is black
(or brown) but spotted with white. When E is modified (to E' or E")
but U is unmodified, the animal is spotted with black (or brown) on a
yellow background, but is devoid of white. When both E and U are
modified (to E' or E" and to S, respectively) the animal bears two differ-
ent sorts of colored spots on a white background. The spots are either
black and yellow or brown and yellow, and constitute with the white
background on which they lie the so-called " tricolor " condition, well
known in the case of guinea-pigs, dogs, cats, and mice.

It is a singular fact that spots of black and of brown do not occur on
the same animal, so a 4-colored condition is never attained. The reason
for this is apparent, if the hypothesis stated in this paper is correct. The
distribution of black and of brown is controlled by the same factors, E and
S, so that when black and brown are present together, their distribution
is the same, and black because of its greater opacity covers up the brown.

The "black-and-tan" dog is, we believe, an apparent, not a real, excep-
tion to this generalization; for the " tan' ' is not a chocolate-brown pigment
such as is found in the brown water-spaniel, but merely a yellow pigment.
The black-and-tan dog is not a spotted dog, but is a black dog plus a color-
pattern, similar to the agouti-pattern of guinea-pigs and rabbits. In

COLOR 49

this pattern black is largely excluded from the lower surfaces and from
a spot over each eye, where yellow then shows. The correctness of this
hypothesis is shown by the existence of this same pattern among brown-
pigmented dogs. The brown-and-tan has chocolate-brown pigment above
and tan (yellow) below, as well as a spot over each eye. It bears the same
relation to self-brown that black-and-tan does to self-black. On this
interpretation brown-and-tan is brown plus pattern, and black-and-tan
is black plus pattern. If, then, brown-and-tan is crossed with self-black,
black-and-tan offspring should result in FM and in F2 there should be
obtained black-and-tan, brown-and-tan, self-black, and self-brown, in
the proportions 9:3:3:1. The experiment is commended to dog breeders.

INTERRELATIONS OF FACTORS B, Br, AND Y.

Returning, after this digression, to a consideration of the interrelations
of the three pigments, black, brown, and yellow, the fact seems clearly
established that black and brown are closely related but alternative con-
ditions dependent for their distribution upon two factors, which we may
designate E and S, whereas yellow is dependent for its distribution solely
upon one of these two factors, S. It would seem probable, therefore,
that in the genesis of the hair pigments, yellow is a first product of the
interaction of C and Y, which may or may not be further modified to pro-
duce brown or black, depending upon whether certain other factors (B and
Br) are or are not present. The amount and distribution of the yellow
pigment produced is conditioned by a factor which may assume phases
U, S, S', etc. The amount of the yellow pigment which is converted into
black or brown and its distribution is conditioned by another factor which
may assume phases E, E', etc., to R.

GAMETIC STRUCTURE AND VARIATION.

A diagram like those employed by the organic chemist may help to show
the relationships to each other of these 8 assumed pigment factors.

C, the general color factor, is indispensable y B

to the manifestation of any of the others. All / \

the others may be represented as linked A <p - - Y E

directly or indirectly with it. E, however, is Br

a modifier of B and Br alone, and is there-
fore joined with them alone in the diagram; and since B and Br are
assumed to act only after Y has acted, they are represented as joined
with it.

Homozygous gray rabbits, wild ones for example, possess and transmit
all these 8 factors in each of their gametes. The diagram, therefore,
expresses their gametic composition. A homozygous black rabbit lacks,
of all these 8 factors, A alone. A yellow rabbit has R (restricted) in place
of E (extended black or brown), but otherwise is like the gray, or else the

50 INHERITANCE IN RABBITS

black rabbit. Those with A and those without A are, however, visibly
different.

Theoretically, if each factor is capable of independent variation, 256
different gametic combinations should be possible. In reality we are
acquainted with 18 visibly different color varieties, and we have evidence
that 48 different gametic combinations are capable of realization. This
leaves still a wide discrepancy between theoretical and known, and leads
to the conclusion either that many as yet unknown mutations are possible
in the rabbit, or that couplings may exist among these factors which pre-
vent their independent action.

We have evidence of independent variation on the part of the factors

A, C, I, U, and E, each of which has in one case or another either been
lost or been replaced by the alternative condition already described; but

B, Br, and Y are un variable; at least we have not ourselves seen evidence
among rabbits of independent variation on the part of these factors.
There can be no question, however, that both in the guinea-pig and in
the mouse such variation has occurred, resulting in the complete loss of
B from the gamete, and it is possible, as elsewhere stated, that such a
change has already occurred among European rabbits. Supposing, how-
ever, that B, Br, and Y are all constant constituents of the rabbit gamete
and that each of the five others may be either present or absent, the num-
ber of different gametic combinations theoretically possible becomes 32.
We have reason to believe that this entire assortment is produced and
that 1 6 other ones also occur owing to a second and different sort of vari-
ation in factor C.

GAMETIC AND ZYGOTIC FORMULAE.

The diagram given on page 49 was intended to express the known aggre-
gate of independent factors which a pure gray rabbit transmits in each of
its reproductive cells (gametes). In producing a new individual each re-
productive cell must unite with another reproductive cell, the two together
forming a zygote. An individual resulting from the union of two gam-
etes of like constitution will be double as regards each hereditary factor.
It is known as a homozygote (Bateson). This double condition we might
express by a subscript 2 following the symbol for each factor indicated.
We should then have a zygotic formula for the individual.

But it sometimes happens that a gamete unites with another gamete
having a composition slightly different from its own — one which, for
example, lacks one or more factors found in itself. The zygote produced
is then a heterozygote and will be double as regards certain factors, but
single as regards others. But in sexual reproduction, as is well known,
there is a return from the double to the single condition. So that when
a heterozygous individual attains sexual maturity, it forms gametes each
of which contains the factor double in the zygote, but as regards those
which were single in the zygote, half the time they will be present, half

COLOR 51

the time absent from the gamete (or if not absent, then represented by
an alternative condition) . This is simply another way of stating the funda-
mental Mendelian principle that heterozygotes do not breed true, but
form at least two different kinds of reproductive cells.

The breeder has to deal always with individuals, and only indirectly
with gametes. Therefore zygotic formulae are to him quite as important
as gametic formulae. Accordingly in what follows we shall endeavor to
give the zygotic formula of each variety described. Its breeding capac-
ity may quickly be inferred from an inspection of its zygotic formula. Each
factor which is double in the zygote will be represented in every gamete
formed, each factor which is single in the zygote will be present in only
half the gametes formed, or will be represented by the alternative (reces-
sive) condition expressed in the zygotic formula by a symbol in paren-
thesis.

The zygotic formula of a gray rabbit which breeds true (an ordinary
wild one, for example) is B2Br2E2A2C2I2U2Y2, and the interrelations of
these factors, as at present understood, may be expressed in a diagram.

Other gray rabbits are single (or heterozygous) as regards one or more
of the factors enumerated in this formula, though none of them lacks
altogether any one of these 8 factors. When a factor drops out altogether
a new color variety is produced. New color varieties have undoubtedly
originated in this way in the past, and are still doing so at the present
time. A maturation division in which the two components of a double
factor should fail to separate (as they do normally) might be the starting-
point of a new color variety, since it would result in the production of a
gamete which lacked a particular factor. Abnormal maturation divi-
sions, therefore, may be the immediate cause of color variations.

COLOR VARIETIES OF THE RABBIT.

It is impossible to make a scientific classification of the color varieties
of the rabbit without discarding or modifying some of the names now
in use; for many of these names are either without significance or are
misleading. From a perusal of the literature of the rabbit-fancy, we are
unable to decide what certain named varieties are, and it is more than
likely that we are not acquainted at first-hand with many varieties known
to the fancy in Europe. All such cases must necessarily be omitted, for
the present, from our classification.

For convenience we may recognize 4 general color types, viz, (i) gray,
(2) black, (3) yellow, and (4) white. Each of the pigmented varieties

52 INHERITANCE IN RABBITS

(gray, black, and yellow) may have either intense or dilute pigmentation
(disregarding intermediate shades, which, however, exist and are heri-
table) . Further, each may either have uniform pigmentation or be spotted
with white (disregarding differences in the fineness of the spotting, which,
however, exist and are heritable) . Further, the yellow may have either
pigmented or white under surfaces. Even with categories so inclusive
as these, the number of visibly different pigmented varieties rises to 16,
and since albinos may either have or not have pigmented extremities, the
total number of visibly different varieties mounts to 18.

There is every reason to suppose that each of these 18 varieties may be
obtained in a homozygous condition. Most of them, indeed, have been
so obtained in our experiments. But for each homozygous condition
there are possible several heterozygous conditions. An enumeration of
all these is unnecessary, as the number is truly stupendous. With 5 inde-
pendently variable characters (the number known to be independently
variable in the rabbit) the number of different zygotic combinations theoret-
ically possible is 243.

We shall content ourselves with enumerating the 18 different known
gametic combinations, and in giving examples of a few of the different
zygotic combinations.

GRAY TYPE.

(1) Gray, found in wild rabbits; gametic composition —

? /N

A C Y E

I \ /

I Br

(2) Blue-gray, same as the foregoing, with the substitution of D (dilute)

for I (intense).

(3) Spotted gray, same as i, with the substitution of S (spotted) for U

(uniform pigmentation).

(4) Spotted blue-gray, same as 2, with the substitution of S for U.

BLACK TYPE.

(5) Black, same as i without A, namely,

•••'.. ' .! '•' i /'\

C Y E

(6) Blue (i. e., dilute black), same as 5, with the substitution of D for I.

(7) Spotted black, same as 5, with the substitution of S for U.

(8) Spotted blue, same as 6, with the substitution of S for U.

COLOR

53

YELLOW TYPE.

(9) Yellow (with white belly and tail), same as i, with R (restricted)
substituted for E (extended black or brown pigmentation), namely,

B.

Br'

R

(zo) Cream (i. e., dilute yellow), same as 9, with D substituted for I.
(n) Spotted yellow, same as 9, with S substituted for U.

(12) Spotted cream, same as 10, with S substituted for U.

(13) Sooty (yellow with pigmented belly and tail), same as 9 without

A, or as 5 with R substituted for E, namely,
U ^B,

R

(14) Pale sooty, same as 13, with D substituted for I.

(15) Spotted sooty, same as 13, with S substituted for U.

(16) Spotted pale sooty, same as 14, with S substituted for U.

WHITE TYPE.

(17) White (wholly unpigmented) , in any of the foregoing 16 varieties

with C omitted.

(18) Himalayan white, a pink-eyed albino variety differing from 17 in

appearance, in having black pigmented extremities (nose, ears,
feet, and tail) and in having fur of a creamy white, not of a snowy
white as in 17. Those with which we have experimented seemed
to be of the formula1 —

That is, they were black pigmented rabbits (see 5) in all points except C. It
would seem that we must assume the presence of C in some form in an animal
which like these does bear a certain amount of pigment. Nevertheless this C
is not the same as the C found in dark-eyed pigmented varieties, for a cross of
Himalayan with other albinos produces no dark-eyed offspring, and gives no
increase of pigmentation over that found in the Himalayan parent, but rather a
diminution of it (see Castle, 105). If, then, we assume C to be present hi the
Himalayan, it must be in a greatly modified form, as compared with its condi-
tion in dark-eyed animals. This is why we use C' rather than C in the formula.
The factors E, I, and U, were all found to be present in our Himalayan rabbits,
but not A, for crosses of Himalayan with homozygous gray gave only gray in

1 April, 1909. Himalayan rabbits have now been produced which contain also factor A. They
have extremities less heavily pigmented than ordinary Himalayans, and the tail is white
underneath, as in gray and in yellow rabbits.

54

INHERITANCE IN RABBITS

F,, and in F2 gray, black, and Himalayan, but no other varieties. Whether it
is possible to associate A with the other factors found in a Himalayan rabbit
remains to be demonstrated.

The reader will naturally expect some concrete evidence in support of
the gametic composition ascribed to the various color varieties in the fore-
going enumeration. To a consideration of this we may proceed immedi-
ately. It would be wearisome to describe in detail all the experiments
which have been made in the investigation of this matter. They have
involved the production in various sorts of matings of some thousands of
rabbits. It will suffice, we think, to cite from our experiments certain
matings which were of such a nature as to test the validity of our hypotheti-
cal formulae.

ZYGOTIC VARIATION WITHIN EACH COLOR VARIETY.
GRAY.

The formula has already been given of a gamete which transmits the
coat characters of a wild gray rabbit. It contains, as we have seen, 8
distinct factors. Such a gamete might be produced by gray rabbits of
many different sorts, all of which look alike but breed differently, i. e.,
which have a different zygotic composition.

(i) The first sort which we will consider is homozygous (double) as regards
each factor which enters into the composition of the gamete transmitting gray.
Its zygotic formula is B2Br2E2A2C2I2U2Y2 (compare diagram, p. 51). Every
gamete which it forms transmits, therefore, all the components of a gray coat.
This is the condition found in ordinary wild rabbits. One of our original stock
of rabbits (9 431), a Belgian hare, was of this sort. In a variety of crosses she
produced only gray offspring.

TABLE 31. — Matings and young of 9 431, the Belgian hare.

Mating.

Gray
young.

With (^56, an albino .

With (5*8, a Himalayan albino

With old lop male yellow

With c?i76, her son, gray

7

Total

2?

The matings with <? 56 and <? 8 indicate that she did not carry albinism as
a recessive character; the mating with the yellow rabbit shows that she did not
carry yellow as a recessive character. The yellow rabbit in question was found
by other tests to be heterozygous in the pattern factor A. Consequently the
Belgian hare was almost certainly homozygous in that factor; otherwise half
the young produced in this mating should have been black instead of gray.

(2) A second sort of gray rabbit produces (when mated with animals like
itself) gray offspring and black ones, but produces none of other color varieties
in any kind of mating. It differs from variety i only in regard to the factor
A, in which it is heterozygous (single). Its zygotic formula accordingly is
B2Br2E2AC2I2U2Y2. In half its gametes the A factor is transmitted along with
all the other 7 factors; in hah" its gametes the factor A alone is wanting. Gray

COLOR

55

rabbits of this sort are readily produced by mating a gray rabbit with a black
one. A rabbit of this sort produced in a slightly different way was our gray
o" 2005, which, when mated with a sooty yellow (9 1471), produced a litter of
3 black and 2 gray young; likewise our gray rf 2004, which, when mated with
sooty yellow 9 1491, produced 5 black and 5 gray offspring (exactly the expected
equality of blacks and grays). Many Belgian hares are of this second variety
("throwing blacks," as well as grays, but not other colors).

(3) A third sort of gray rabbit produces (when mated with animals like itself)
albino offspring as well as gray ones, but none of other colors. It is heterozygous
(single) in C, but otherwise homozygous. Its formula is B2Br2E2A2CI2U2Y2.
We have at the present time some rabbits believed to be of this kind, but they
are as yet not fully tested. Hurst (105) also describes rabbits of this variety.

(4) A fourth sort of gray rabbit produces * young of the varieties gray, black,
and white only. It is heterozygous (single) in A and C, but not in other factors.
Its formula is B,Br2E2ACI2U2Y2. It is represented in our rabbits (9 1161 and
c? 1172) produced by a cross between a white rabbit (of gray ancestry) and a
heterozygous gray one. Mated with each other these two produced 17 young,
distributed as follows: 10 gray, 3 black, and 4 white (expected 9:3:4). Male
1172 was mated also with a white female of black parentage (9 789), and pro-
duced 3 gray, 5 black, and 2 white offspring (expected 1:1:2).

Rabbits differing from variety 4 only in the respect that the albinos which
they produce are of the Himalayan type are represented in our gray 6" 48, and
9 49 and 9 50. They were produced by the mating of the Belgian hare 9 431
with a Himalayan (<? 8). The result of their matings inter se is given in table 32.

TABLE 32. — Matings and young of c? 48, gray.

Matings

Young.

Gray.

Black.

Himalayan.

With $49, gray
With ? 50, gray

9

je

5

c

7
6

Total»

24.

IO

mm

1 Expected ratio of 9: 3 : 4, giving in this case 27: 9: 12.

The formula of such rabbits may be expressed by adding the symbol C' to
the formula as given for variety 4, viz, B2Br2E2AC(C/) I2U2Y2.

TABLE 33. — Matings and young of gray 9 175.

Young.

Gray.

Yellow.

Black.

With (^176, gray

24

i»

o

With c?i77> gray
With c?437*, gray

ii
4

5

2

0
0

Total

3O

2O

o

Expected

I

o

(5) A fifth sort of gray rabbit produces young of the varieties gray and yel-
low, but none of other colors. It is heterozygous (single) in the extension factor
E, carrying as an alternative (recessive) character R (restricted distribution of
black and brown pigments). Its formula accordingly is B2Br2E(R)A2C2I2U2Y2.

1 When on this and subsequent pages the nature of the mating is not specified it will be under-
stood that the mating is with animals of its own variety or of varieties recessive to its own.

56

INHERITANCE IN RABBITS

It is represented in our gray ^2071, which, when mated with a sooty yellow
female (9 1414), produced 4 gray and 3 yellow young. This variety is repre-
sented likewise by 9 175 and <? 176, borne by the Belgian hare (9431) in a
mating with the old yellow lop male.

Female 175 was not actually mated with a black rabbit, but, had she been
capable of producing black offspring, she should have done so in the matings
with c? 177, a rabbit that did produce black young. Male 176 was mated with
two does known to be capable of producing black offspring, with the result
shown in table 34.

TABLE 34. — Matings and young of c? 176, gray.

Mating.

Gray.

Yellow.

Black.

With old $ lop, sooty ....
Expected

ii

3

i

0

o

With $ 1 78 gray

g

o

Expected

i

o

(6) A sixth variety of gray rabbit produces gray, black, yellow, and sooty
yellow offspring, but none of the other colors. It is heterozygous both in E
and in A. It differs from variety 5 only in being heterozygous instead of
homozygous in A. It may readily be produced by crossing yellow with black,
or gray with sooty yellow, the result of these two crosses being identical. The
formula of this variety is B2Br2E(R)AC2I2U2Y2. This variety is represented in
our gray rabbits (^ 177 and 9 178), which were borne by the Belgian hare (9 431)
in the same litter as the rabbits of variety 5 already described, viz, 9 175 and
c? 176. Another rabbit of this variety was the gray <? 505.

TABLE 35. — Matings and young of <? 177, gray.

Mating.

Gray.

Black.

Yellow.

Sooty.

With 9i75, gray (variety 5) .
Expected . .

5

0

o

2

I

0

o

With 9178, gray (variety 6) .
Expected

i
9

2

3

0

3

o

I

Why rabbits 177 and 178 should differ in breeding capacity from their brother
and sister, 176 and 175, is readily explained. The father was heterozygous as
regards factor A. To 175 and 176, he transmitted it; to 177 and 178, he did
not. But all four received this factor from their mother, a homozygous gray
rabbit (9 431)- Hence 175 and 176 were double in A, but 177 and 178 were
single as regards A. Evidence for the classification given of rabbits 177 and
178 is shown in tables 35 and 36.

(7) A seventh variety of gray rabbit should produce young of the varieties
gray, yellow, and white, but none of other colors. It should differ from variety 5
in being heterozygous (single) in C. Its formula would be B2Br2E(R)A2CI2U2Y2.
We are unable to cite an undoubted example of this variety, though it could
probably be produced by crossing variety 3 with variety 5, as well as in several
other ways.

(8) An eighth variety bears the same relation to variety 6 that 7 does to 5.
It is of the formula B2Br2E(R)ACI2U2Y2, and it produces young of the 5 visibly
different classes — gray, black, yellow, sooty yellow, and white. This variety

COLOR

57

is represented in rabbits 3 1160 and 9 1171, bora in the same litter with 9 1161
and $ 1172 of variety 4 (p. 55). When mated with each other they produced 7
gray, i black, 2 yellow, i sooty, and 5 white young. Other rabbits of this variety
were produced by the Belgian hare (9431) in a mating with an albino rabbit
(c? 56). These gray rabbits (2 males and 3 females, 232 to 236), when mated
inter se produced 17 gray, 5 black, 6 yellow, 2 sooty yellow and 6 white young,
the expected Mendelian proportions being 27 : 9 : 9 : 3 : 16. When mated with
albinos these same gray rabbits produced 9 gray, 6 black, 2 yellow, and 16
white young. The zygotic composition of the albino mates in this case is not
fully known, so that the theoretical proportions of the pigmented young can
not be stated. The albinos are as expected approximately half the total young,
and all expected color varieties are represented except sooty yellow.

TABLE 36. — Matings and young of 9 178, gray.

Mating.

Gray.

Black.

Yellow.

Sooty.

With (^176, gray (variety 5)

8

o

2

o

Expected ...

•i

o

I

o

With <5*i 77, gray (variety 6)

i

2

o

o

With ^505, gray (variety 6V

Total for last two matings
Expected

6

5

I

(?)

I

With (^179 yellow

i

With (j^3 1 9 yellow

8

6

Total for last two matings
Expected

9

i

6

i

7

i

6

1 Plus 3 yellow or sooty, died young.

For simplicity the young of all 5 gray rabbits are grouped together in the
foregoing account. In reality, however, one of the males was heterozygous in
factor U, and at least i male and i female were heterozygous in factor I, as is
shown by the facts (i) that several of the young produced in matings with white
individuals bore spots of white, the whitest of all being belted with white ("Dutch
marked"); and (2) that one of the gray offspring of the gray parents was of a
pale blue-gray color.

(9) The gray rabbits just described, which produced a blue-gray young one,
in reality belong to a ninth variety of gray rabbit, indistinguishable in appear-
ance from the other eight, but producing a different assemblage of young, viz,
dilute-colored as well as intensely colored young in each of the pigmented vari-
eties, and also albinos. The whole assemblage of visibly different varieties is
gray, black, yellow, sooty yellow, blue-gray, blue, pale yellow, pale sooty yellow,
and white. Not all of these varieties were obtained directly from the pair
of rabbits in question, doubtless because too small a number of young was
produced, but in later generations all were obtained. Thus from the single
blue-gray individual, when mated within the same family, were obtained blues,
pale sooties, and pale yellows, as well as individuals of normal intensity. The
zygotic formula of this ninth variety of gray rabbit is B2Br2E(R)ACI(D)Y2U2.
Since it indicates a heterozygous condition in 4 character-units, we should
expect a pair of individuals of this formula to produce 16 different gametic
combinations. Eight of these are represented in the enumerated 8 classes of
pigmented young, 8 others would occur among the albinos which would differ
from the pigmented classes in the absence of C, but all of which would look
alike, though breeding differently in crosses with pigmented animals.

58

INHERITANCE IN RABBITS

(10) The gray rabbit which produced spotted offspring in crosses with albinos
was undoubtedly heterozygous in regard to the factor U, for experience has shown
(in agreement with Hurst, 105) that uniform pigmentation is in the main domi-
nant over spotting. We might then recognize as a tenth variety one of the
formula B2Br2E(R)ACI2U(S)Y2.

(n) Had the rabbit in question produced pale-pigmented as well as spotted
young (and such we have since derived from this stock of rabbits), we should
need to modify the formula as given by writing I(D) instead of I2, i, e., intense
(dilute recessive). The formula of such a rabbit would be

B2Br2E(R)ACI(D)U(S)Y2.

This indicates a heterozygous condition as regards 5 character-units, and
rabbits of this formula should be capable of producing 32 different gametic
combinations, 16 of which would be visibly expressed in different pigmented
varieties, while an equal number lacking the factor C would produce albinos
visibly alike but gametically different.

If, then, we were to carry to its logical conclusion the enumeration of
the conceivable different varieties of gray rabbit, all alike in appearance
but all different in breeding capacity, i. e., of different zygotic formula,
we should need to mention 32 varieties: 8 of these would correspond with
the first 8 which have already been enumerated and the existence of which
has (except for variety 7) been demonstrated, namely:

(1) Gray producing gray only.

(2) Gray producing gray and black.

(3) Gray producing gray and white.

(4) Gray producing gray, black, and white.

(5) Gray producing gray and yellow.

(6) Gray producing gray, black, yellow, and sooty.

(7) Gray producing gray, yellow, and white.

(8) Gray producing gray, black, yellow, sooty, and white.

Eight other varieties would produce the same sorts of young as these 8,
but would produce in addition dilute pigmented ones of the same color
types, i. e., blue-grays as well as grays, blues as well as blacks, pale yellows
(cream) as well as yellows, and pale sooties as well as sooty yellows.

The 1 6 remaining varieties would produce the same sorts of young as
the 1 6 varieties already described, but would produce spotted as well as
uniformly pigmented (self) individuals.

TABLE 37.

Color.

Observed.

Expected.

Gray

24.

27

Black

g

Yellow
Sooty

16

2

9

Blue-gray

8

Blue

2

Cream

Pale sooty. .

2

i

COLOR

59

Not every one of these 32 varieties of gray rabbit has actually been
demonstrated to exist in the course of our experiments; 10, however, which
we have shown to exist, have already been mentioned, and several others
are known. For example, by crosses of black with pale yellow (cream)
or of blue with yellow, we have obtained grays which produced the same
sorts of young as variety 6, and in addition blue-grays, blues, creams, and
pale sooties. Such was the character of our gray females 1413, 1457, 1525>
1526, and 2009. By a male of like character they have produced young
as shown in table 37.

Other gray rabbits produced by the same crosses, black X cream, or
blue X yellow, produce the same assortment of young, and in addition
albinos. That is, they are like variety 8, but heterozygous in intensity of
pigmentation. These gray rabbits, females 1423, 1443, and 1505, and
males 1351 and 1458, mated inter set have produced young as indicated in
table 38. .

TABLE 38.

Color.

Observed.

Expected.

Gray

20

27

Black

8

Yellow

12

Sooty

I

Blue-gray

Blue

Cream

(?)

*

Pale sooty

I

i

White

8

21

The category yellow is probably too large because of a failure on our
part to discriminate between yellow and cream, a difference which at first
we failed to record. It is possible also that albion young were not enu-
merated in all the records which we have combined, and so albinos are
apparently deficient in number.

It is needless to go farther in the enumeration of zygotic varieties of gray
rabbits. There is little doubt that the entire 32 varieties theoretically
possible could readily be produced; or we have found that a spotted
coat may be transferred from one color variety to another by means of
crosses, and the same is true of a dilute condition of the pigmentation in
contrast to intense pigmentation. It is known also from a variety of
sources, including besides our own observations the valuable experiments
of Hurst ( : 05) , that albinism may occur as a recessive character in any
and all color varieties of rabbits. Additional evidence seems to be desir-
able chiefly as concerns the assumed factor E; therefore, we may proceed
to the consideration of color varieties other than gray, in the course of
which this evidence will be produced.

60

INHERITANCE IN RABBITS

BLUE-GRAY.

A blue-gray rabbit differs from a gray one only in the intensity of its
pigmentation, which is always dilute. As regards the intensity factor,
therefore, it is invariably homozygous, D2, since D is recessive to I, whereas
a gray rabbit may be either homozygous, I2, or heterozygous, I (D). Con-
sequently only half as many zygotic combinations are possible among blue-
gray as among gray rabbits, 16 instead of 32 being the maximum.

The 1 6 conceivable varieties of blue-gray rabbits, all of which should
be similar in appearance but different in breeding capacity, are:

(1) Blue-gray producing only blue-gray; formula, B2Br2E2A2C2D2U2Y2.

(2) Blue-gray producing blue-gray, and blue; formula, B2Br2E2AC2D2U2Y2.

(3) Blue-gray producing blue-gray, and white; formula, B2Br2E2A2CD2U2Y2.

(4) Blue-gray producing blue-gray, blue, and white; formula, B2Br2E2ACD2U2Y2.

(5) Blue-gray producing blue-gray, and cream; formula, B2Br2E(R)A2C2D2U2Y2.

(6) Blue-gray producing blue-gray, blue, cream, and pale sooty; formula,

B2Br2E(R)AC2D2U2Y2.

(7) Blue-gray producing blue-gray, cream, and white; formula,

B2Br2E(R)A2CD2U2Y2.

(8) Blue-gray producing blue-gray, blue, cream, pale sooty, and white; formula,

B2Br2E(R)ACD2U2Y2.

The 8 remaining varieties would be identical with these, except for the
factor U, in which they would be heterozygous, U (S) , producing spotted
as well as self-pigmented young.

Three blue-gray rabbits, all females, have been tested, and each of
these is of a different zygotic formula.

Female 389, the original blue-gray individual, proved to be of variety
4. When mated with J 248, a black animal heterozygous in E, C, and
I, i.e.,ot formula B2Br2E(R)CI(D)U2Y2, she produced gray, blue-gray,
blue, and white young, all the expected classes except black being produced.
The observed numbers of the young and the expected proportions are
given in table 39.

TABLE 39.

Color.

Observed.

Expected.

Gray ... .

27

Blue-gray

Black

o

Blue

White

16

Female 656 was of variety 2, heterozygous in A only, as 'is shown by
table 40.

Of the 4 males with which 9 656 was mated, all but c? 1340 produced
albino young in other matings. This indicates clearly that $656 was

COLOR

61

not heterozygous in C. The matings with males 402, 1340, and 248
show that she was homozygous in E.

TABLE 40. — Matings and young of blue-gray 9 656.

Mating.

Gray.

Blue-
gray.

Black.

Blue.

With sooty cjfyoao

5

o

2

4.

With sooty c?I34

i

o

I

o

With black ^248

4

o

o

With blue ^1228

o

o

Female 1437 was either of variety 6 or else of variety 8, i. e., she was
known to be heterozygous in E and in A, but was insufficiently tested as
regards C. Mated with blue <? 1434 she produced 2 blue-gray and i pale
sooty young and i pigmented animal of uncertain character.

BLACK.

Black rabbits, as we have already observed, differ from gray ones only
in the factor A, which they lack completely. 16 different zygotic com-
binations are theoretically possible among them.

(1) Black producing nothing but black; formula, B2Br2E2C2I2U2Y2 (compare

diagram, p. 52).

(2) Black producing black, and white; formula, B2Br2E2CI2U2Y2.

(3) Black producing black, and sooty; formula, B2Br2E(R)C2I2U2Y2.

(4) Black producing black, sooty, and white; formula, B2Br2E(R)CI2U2Y2.

(5) Black producing black, and blue; formula, B2Br2E2C2I(D)U2Y2.

(6) Black producing black, blue, and white; formula, B2Br2E2CI(D)U2Y2.

(7) Black producing black, blue, sooty, and pale sooty; formula,

B2Br2E(R)C2I(D)U2Y2.

(8) Black producing black, blue, sooty, pale sooty, and white; formula,

B2Br2E(R)CI(D)U2Y2.

The 8 remaining varieties would be identical with these 8, except that
they would be heterozygous as regards U, viz, U(S) instead of U. Con-
sequently they would produce spotted as well as self-pigmented young.

All the black rabbits (with one exception) which we have used for breed-
ing purposes were produced in the course of our experiments from animals
of other color varieties. All were in one or more respects heterozygous,
except possibly the recently purchased black rabbit, not yet fully tested,
but apparently homozygous in all particulars and so of variety i. Hurst
(105) obtained rabbits of variety 2, but we do not happen to have had any
of this variety, nor of variety 3.

Variety 4 is represented hi a modified form in our rabbits 104, 105, 167,
247, and 255. The modification consists in this: the albino offspring are,
at least in part, of the Himalayan type, having pigmented extremities.

62

INHERITANCE IN RABBITS

This we might express by adding the Himalayan factor (C') to the for-
mula as given for variety 4. Variety 5 is represented in our black X,
which when mated with blue o* 1434 (variety 3) produced 4 black and
4 blue young.

Variety 7 is represented in our rabbits 1230 and 1231, 2011, and 2038;
and variety 8, in a modified form, in our c? 248, which has sired black, blue,
sooty, pale sooty, white, and Himalayan white offspring by black, sooty-
yellow, or blue-gray mates. He therefore differs from variety 8 as previ-
ously described in that he is heterozygous in the Himalayan factor C'.
His formula accordingly is B2Br2E(R)C'(C)I(D)U2Y2. Some evidence for
this classification of our black animals will be found in the table 41. Other
evidence is derived from matings with yellow or gray animals.

TABLE 41. — Matings of black rabbits with black or sooty individuals.

Mating.

Black.

Sooty.

Hima-
layan.

White.

Blue.

Pale
sooty.

9 io<; black withe?* 04 black

i

i

9 167 black with ^248 black

2

7

2

9 247 black with ^248 black

i

$ 255 black with (^248 black

IO

I

2

Total

24

7

4

2

Expected proportions

7

7

I

9 1230 black with ^1340 sooty

7

4

Expected

I

i

9 1230 black with (^1414 sooty
Expected

4

7

4
3

i

i

i

BLUE.

"Blue" pigmentation in rabbits and other rodents is merely a dilute
condition of black. The zygotic formula of a blue rabbit is the same as
that of a black one, if we substitute D2 for the I2 or I(D) of the black vari-
eties. Blue rabbits may occur theoretically of 8 different sorts, viz:

(i) Blue producing blue only; formula, B2Br2E2C2D2U2Y2.

(a) Blue producing blue, and white; formula, B2Br2E2CD2U2Y2.

(3) Blue producing blue, and pale sooty; formula, B2Br2E(R)C2D2U2Y2.

(4) Blue producing blue, pale sooty, and white; formula, B2Br2E(R)CD2U2Y2.

TABLE 42. — Matings and young of & 1434, blue.

Mating.

Black.

Blue.

Sooty.

Pale
sooty.

With 9647, sooty, variety 2

7

7

Expected

I

I

With 9 1471, sooty, variety 3 or 4.
Expected

4

i

2

I

5

i

2

I

With 9 black, variety 5 or 6
Expected

4

i

3

i

COLOR

63

The 4 remaining varieties would be identical with these 4 except as
regards the factor U, in which they would be heterozygous, U(S), instead
of homozygous, U2.

We have determined the zygotic formulae of 2 blue rabbits only, both
of which were produced in the course of our experiments. One (c? 1434,
table 42) was of variety 3, the other (<? 1228, table 43) was of variety 4.

We shall pass by the spotted black and spotted blue varieties of rabbit,
of both which sorts a certain number of individuals have been produced
in our experiments, but which have not been thoroughly tested.

TABLE 43. — Matings and young of 3 1228, blue.

Mating.

Blue-
gray.

Black.

Blue.

Sooty.

Pale
sooty.

White.

With 9 647 sooty variety 2

I e

17

Expected

•z

-3

2

With 9 1 280, sooty, variety 3 or 4 ...
Expected

2

I

2

I

2
I

0

With 9656 blue-gray variety 2 . .

Expected

i

I

YELLOW.

Yellow rabbits differ from gray ones only in the factor E (extended
black or brown pigmentation), in place of which they bear the alternative
condition R (restricted black or brown pigmentation). Since R is reces-
sive in relation to E, yellow rabbits are invariably homozygous, R2, as
regards this factor. Theoretically sixteen different varieties are possible,
as follows:

(1) Yellow producing yellow only; formula,

(2) Yellow producing yellow, and white; formula,

(3) Yellow producing yellow, and sooty; formula, B2Br2R2AC2I2U2Y2.

(4) Yellow producing yellow, sooty, and white; formula, B2Br2R2ACI2U2Y2.

Four other varieties should differ from these 4 in factor I only, being
I(D) instead of I2, and producing dilute as well as intensely pigmented
individuals. Eight others should differ from these eight in producing
spotted as well as uniformly pigmented individuals. We shall content
ourselves with giving examples of the first four varieties enumerated.

Variety i produces only yellows when mated with other yellows or
with sooties, and only yellows and grays when mated with blacks or blues
of any sort whatever. It is represented in our yellow c?38i, a son of 2
gray rabbits of variety 5, viz, 9 175, and c? 176. He was mated with 6
different yellow females, 3 of which had produced sooty offspring by other
mates, and there resulted 61 young, all yellow. In matings with 2 differ-
ent sooty females he produced 12 young, all yellow; and in a mating
with a gray female of variety 6 ( 9 178) he produced 3 yellow and 6 gray

64 INHERITANCE IN RABBITS

young; expected i: i. We have had several other yellow rabbits which
were probably of this same variety, but they were less extensively and
inconclusively tested.

Variety 2 is represented in a yellow rabbit obtained by purchase
(c? 1256). He was mated with 9 547 yellow, variety 3, and produced
9 young, all yellow (as expected) ; by yellow 9 745 J, variety 4, he produced
4 yellow and 3 white young (expected 3:1); and by black 9 1230 and
9 1231, variety 7, he produced 8 yellow, 3 gray, and i blue-gray young;
expected 4:3: i.

Variety 3 is represented in yellow females 547, 714, and 1115, which
in matings with yellow males of variety 4 produced 24 yellow and 2 sooty
young, but no white ones. Had these females been of variety 4 they
should have produced 2$ per cent of white young in the mating mentioned.
It is possible that the recorded number of sooties is too small, owing to
a failure in our earlier records to discriminate sooty from yellow. No
such possibility exists in the case of the records for albinos. One of the
females already mentioned, of variety 3 (9 1115), when mated with sooty
cf 1340 produced 2 yellow and 6 sooty young.

Another yellow rabbit of variety 3 was the lop c? 179 (plate 2, fig. 8).
When mated with the sooty "old female lop" (plate i, fig. 2) he produced
4 yellow and 4 sooty young (expected i : i) ; and when mated with black
females of variety 4 (9 9 105, 167, and 247) he produced 7 gray, 5 black,

6 yellow, and 7 sooty young (expected i: i: i: i). Notice in the matings
with black females the total absence of albinos, though all these females
had produced albinos by other mates.

Still another male of variety 3, <?3i9, son of the sooty old female lop
by gray $ 176, variety 5, when mated with the black 9 247 (variety 4)
produced 2 gray, 2 black, 2 yellow, and i sooty young (expected i : i : i : i).

Variety 4 is represented in our "Cutler's yellow" and in 9 7454 pro-
duced by black 9105 (variety 4) mated with yellow ^319 (variety 3).
When "Cutler's yellow" was mated with the above female, 745^, he pro-
duced 4 yellow, 3 sooty, and i white young (expected 9:3:4). When
mated with sooty females 632 and 647 (variety 2), he produced 7 yellow,

7 sooty, and 2 white young (expected 3:3:2).

SOOTY.

Sooty rabbits differ from yellow ones only in the factor A, which they
lack. Theoretically 8 varieties are possible, viz:

(1) Sooties producing sooties only, when mated inter se; formula, B2Br2R2C2I2U2Y2.

(2) Sooties producing sooty, and white; formula, B2Br2R2CI2U2Y2.

(3) Sooties producing sooty, and pale sooty; formula, B2Br2R2C2I(D)U2Y2.

(4) Sooties producing sooty, pale sooty, and white; formula, B2Br2R2CI(D)U2Y2.

The 4 remaining varieties would be like these, except as regards the
factor U, in which they would be heterozygous, U(S), instead of homozy-

COLOR 65

gous, U3. They would produce spotted as well as uniformly pigmented
young.

An example of variety i is the "old female lop" (plate i, fig. 2). When
mated with an albino of black ancestry, c?45 (plate i, fig. 3), she produced
a litter of 8 black young (plate i, fig. i). This experiment shows her to
have been homozygous in C, i. e., to have been C2 in character, and to
have lacked factor A. When mated with yellow c?i79 (plate 2, fig. 8),
variety 3, she produced 4 yellow and 4 sooty young, exactly the expected
equality of yellow and sooty. Another individual probably of this same
variety was 9 1472, which when mated with a sooty male, 1414, produced
12 young, all sooty. The male just mentioned belonged apparently to
variety 3, for when mated with the black 9 1230 (variety 7) he produced
5 black, 6 sooty, and i blue young (expected 3:4:1; or if pale sooties
were differentiated from sooties, which we probably failed to do hi making
the records, 3 black : 3 sooty : i blue : i pale sooty).

Variety 2 is represented in our ^402 and 99632 and 647. When
3 402 was mated with 9 632, they produced a litter of 4 sooty and i white
young (expected 3:1). Female 647, when mated with "Cutler's yellow"
(variety 4), produced 5 yellow, 4 sooty, and i white young (expected
3:3:2). The white individual produced by 9632 and #402 was a
Himalayan albino. This shows one or both of the parents to have been
slightly different from typical variety 2, and to have carried C'.

Variety 3 is represented probably by our 9 1471 which, when mated
with blue <? 1434, produced 4 black, 2 blue, 5 sooty, and 2 pale sooty young
(expected i : i : i : i) . The possibility is not excluded that this female
was of variety 4 (capable of producing also albino young) , but she can not
have been of either variety i or variety 2. Another probable example of
variety 3 is 9 1280. (See matings of c? 1228, blue, p. 63).

Variety 4 we have not identified with certainty. Neither have we
made a detailed study of pale yellows, pale sooties, or spotted rabbits of
any color variety. We have observed, however, that dilute pigmentation,
as well as spotting, occurs in all the fundamental color varieties and are
entirely satisfied of the independent inheritance of both.

WHITE.

Albino rabbits differ from pigmented ones only in regard to the factor
C, which they either lack, or possess only in a greatly modified form, C'.
If C is absent, there are possible 16 different combinations of the 4 remain-
ing variable factors, which combinations correspond with gametes of the
1 6 visibly different pigmented varieties of rabbit, minus C. But if C is
present in the modified form, C', found in Himalayan albinos, 16 other
gametic combinations should be possible, only slightly different from the
foregoing, making in all 32 different gametic possibilities, or 232 zygotic
possibilities.

66 INHERITANCE IN RABBITS

Plainly it is unprofitable to attempt to find illustrations of all these con-
ceivable variations. We shall content ourselves with noticing some of
the more important varieties of albinos and presenting evidence that each
of the 4 variable factors, A, E, I, and U, is transmitted through albinos.

The following albino varieties may be expected to occur:

(1) White producing gray only (in crosses with any pigmented variety); for-

mula, B2Br2E2A2I2U2Y2.

(2) White producing black only (in crosses with black or any pigmented variety

recessive to black); formula, B2Br2E2I2U2Y2.

(3) White producing yellow only (in crosses with yellow or sooty individuals) ;

formula, B2Br2R2A2I2U2Y2.

(4) White producing sooty only (in crosses with sooty); formula, B2Br2R2I2U2Y2.

(5) White producing gray, and black (in crosses with black or any pigmented

variety recessive to black); formula, B2Br2E2AI2U2Y2.

(6) White producing gray, and yellow (in crosses with yellow or sooty) ; formula,

B2Br2E(R)A2I2U2Y2.

(7) White producing gray, black, yellow, and sooty (in crosses with sooty) ;

formula, B2Br2E(R)AI2U2Y2.

(8) White producing black, and sooty (in crosses with sooty); formula,

B2Br2E(R)I2U2Y2.

(9) White producing yellow, and sooty (in crosses with sooty); formula,

B2Br2R2AI2U2Y2.

Another set of 9 varieties, quite similar to these, would produce only
pale-pigmented offspring. As regards the intensity factor they would
be D2 instead of I2. Another set of 9 varieties would produce both dilute
and intensely pigmented offspring, being heterozygous, I(D), as regards
the intensity factor.

Nine other varieties, in which S2 replaces U2, would produce only spotted
young; and another set of 9 would produce both spotted and self-colored
offspring; in these U(S) would replace U2. Another set of 9 varieties
would produce only pale-pigmented spotted individuals, another would
produce pale-pigmented individuals, both self and spotted; and lastly a
set of 9 varieties would produce both dilute and strongly pigmented indi-
viduals, both spotted and self-colored.

It is probable that the foregoing list of 72 varieties could be duplicated
in varieties having the Himalayan modification, and duplicated a second
time in varieties heterozygous in the two sorts of albinism.

A few examples will now be mentioned of some of the 9 varieties of
albinos first enumerated, or of animals differing from those 9 varieties
in one or two characters only.

Variety i is represented in our c? 1425, which when mated with black

9 1541 produced n young, all gray, and when mated with yellow 9 547

(variety 3) produced 4 young, all gray. Variety 2, but heterozygous in

the Himalayan modification, C', and in spotting with white, U(S), is rep-

COLOR 67

resented in our c?45 (plate i, fig. 3), which when mated with the sooty
lop (plate i, fig. 2) produced 8 young, all black. One of these is shown
in plate i, fig. i. When mated with black 9 105, he produced 8 black pig-
mented and 3 Himalayan albino young, but several of the pigmented
young were spotted with white, this character being recessive in 9 105,
which had a Dutch-marked father.

Variety 5 is represented in 9 269, which when mated with sooty <? 402
produced i gray and 3 black young (expected i: i). When mated with
yellow <? 179 (variety 3) she produced in one litter 3 black young, and in
a second litter, 3 gray and i black. This second litter, in which the
expected proportions of gray and of black young are exactly realized, is
shown in plate 2, fig. 6, the parents being shown in figs. 5 and 8 of the same
plate.

Variety 8 is probably represented in $ 268 which when mated with yellow
<?3iQ (variety 3) produced 2 gray, i yellow, 2 black, and 2 sooty young
(expected i: i: i: i). The only other possibility is that this female was
of variety 7, which should in this mating produce the same varieties of
young, but in the proportions, 9:3:3:1.

Variety 8 (but heterozygous in C', the Himalayan factor) is represented
in 9 108, which when mated with black c? 104 (variety 4) produced 3
black, 3 sooty, and 6 Himalayan albino young, exactly the expected pro-
portions. By yellow <? 179 (variety 3) she produced i gray, i yellow,
and i sooty young (expected i : i : i : i black).

The foregoing cases would afford confirmation (if confirmation were
necessary) of the discovery by Cuenot (: 03) and by Hurst (: 05) that albino
mammals transmit color factors, and that they vary in zygotic composi-
tion as regards color factors. That albinos transmit the factor A is shown
by the observation that some of them (which bear A) produce gray off-
spring in crosses with black pigmented animals, while others (lacking A)
never produce gray offspring, though mated to the same black animals.

That albinos transmit the factor E is shown clearly by extensive experi-
ments with guinea-pigs carried out by one of us. One family of albino
guinea-pigs has been found invariably to produce black offspring in mat-
ings with any pigmented variety devoid of factor A, whether that variety
has the extended or the restricted distribution of black or brown pigment;
a second family of guinea-pigs, with equal uniformity, produces colored
offspring having a restricted distribution of black pigment, if crossed with
colored individuals having the restricted distribution. This second vari-
ety produces black-eyed yellows, if crossed either with black-eyed yellow
or with brown-eyed yellow individuals. Of the 2 albino varieties men-
tioned, the first evidently carries B with E, the second B with R.

These same two families of albino guinea-pigs likewise differ in factor
I, which is present in the first family, but replaced by D in the second.
If each is crossed with pale yellow (cream) individuals, the former produces

68 INHERITANCE IN RABBITS

in Ft black offspring, and in F2 black, blue, red, and cream, as well as
albinos; whereas the latter produces in Fx cream, and in F2 cream and
albino offspring only.

As regards the factor U, Hurst ( : 05) has shown clearly that some albino
rabbits transmit a uniformly colored coat, others a spotted coat, in crosses
with colored rabbits. The former we may regard as carrying U, the latter
S. In rabbits we have not made an extensive study of this matter. We
have found, however, in agreement with Hurst, and Woods (:o3) that
spotted rabbits in general produce only spotted young, when mated with
each other, i. e.} that spotting with white is recessive to uniform pigmenta-
tion, and the case has been mentioned of an albino (<? 45) which produced
spotted young when mated with a black rabbit that had a spotted father.

In guinea-pigs also, spotting is in the main recessive, and spotting is
clearly transmitted by albinos. Thus the #2002, figured and described
by Castle ('.05), produced spotted young when mated with spotted females,
and among his grandchildren spotted animals were very common, no matter
whether the female grandparent and the parents were spotted or not.

All the varied evidence which has been obtained from the study of
rabbits, guinea-pigs, rats, and mice, by others as well as by ourselves,
supports the hypothesis that albinos differ from pigmented individuals,
by a single factor only, which factor we call C.

THE MATERIAL BASIS OF HEREDITY FACTORS.

In what form, it may be asked, are we to suppose that the various
assumed factors exist. Do they occur as so many different substances
lying side by side but unmixed in every reproductive cell? To this ques-
tion we may give at present no satisfactory answer.

It is, however, we think, not necessary to suppose that there exist in
the minute germ-cell as many complex organic substances as there are
activities of the cell; neither is it necessary to suppose a different substance
present for every independent factor identified. The various indepen-
dent factors may have a basis no more complicated than that of so many
atoms attached to a complex molecular structure. Experiment shows that
the factors may be detached one by one from the organic complex. The
discontinuity of their coming and going is entirely in harmony with the
conception of them as components merely of complex molecular bodies.

BIBLIOGRAPHY.

BATESON, W.

: 06. The progress of Genetics since the rediscovery of Mendel's papers. Progressus

rei botanicae, i, pp. 368-418.
BATESON, W., SAUNDERS, E. R., and PUNNETT, R. C.

: 06. Experimental studies in the physiology of heredity. Report III to the Evolution

Committee of the Royal Society. 53 pp. London.
CASTLE, W. E.

: 05. Heredity of coat characters in guinea-pigs and rabbits. Publication No. 23

of the Carnegie Institution of Washington, 78 pp., 6 pi.
: 050. Recent discoveries in heredity and then- bearing on animal breeding. Popular

Science Monthly, vol. 66, pp. 193-208, 14 fig., July 1905.
107. On a case of reversion induced by cross-breeding and its fixation. Science, n.s.,

vol. 25, pp. 151-153-
: 070. Color varieties of the rabbit and of other rodents; their origin and inheritance.

Science, n.s., vol. 26, pp. 287-291.

:o8. A new color variety of the guinea-pig. Science, n.s., vol. 28, pp. 250-252.
CASTLE, W. E., and FORBES, A.

: 06. Heredity of hair-length in guinea-pigs and its bearing on the theory of pure gam-
etes. Publication No. 49 of the Carnegie Institution of Washington,

pp. 1-14.

CUENOT, L.

103. L'heredite de la pigmentation chez les souris. (2me Note.) Arch, de Zool.

exper. et gen. (4), tome i, Notes et revue, pp. 33-41.
:o4. L'heredite de la pigmentation chez les souris. (3^6 Note.) Arch, de Zool.

exper. et ge*n. (4), tome 2, Notes et revue, pp. 45-56.
DONALDSON, H. H.

: 06. A comparison of the white rat with man in respect to the growth of the entire

body. Boas Memorial Volume, New York. pp. 5-26, 2 pi.
FARABEE, W. C.

:o5. Inheritance of digital malformations in man. Papers of Peabody Museum,

Harvard Univ., vol. 3, No. 3, pp. 69-77, P1- 23-27.
HOUSSAY, F.

107. Variations experimentales. Etudes sur six generations de poules carnivores.

Arch, de Zool. exper. et gdn. (4), tome 6, pp. 137-332, 47 fig.
LOCK, R. H.

: 06. Studies in plant breeding in the tropics. III. Experiments with maize. Annals

Roy. Bot. Gardens, Peradeniya, vol. 3, pt. 2, pp. 95-184.
ROBERTSON, T. B.

: 08. On the normal rate of growth of an individual, and its biochemical significance.
Arch. f. Entwicklungsmechanik der Organismen, 25, pp. 581-614.

TSCHERMAK, E.

: 03. Die Theorie der Kryptomerie und des Kryptohybridismus. Beihefte zum Botan.

Centralblatt, Bd. 16, pp. 1-25.
WOODS, F. A.

: 03. Mendel's laws and some records in rabbit breeding. Biometrika, v. 2, pp. 299-306.

69

DESCRIPTION OF PLATES.

PLATE i. — Photographs from life of rabbits described in the text.

FIG. i. (^248, son of the rabbits shown in figs. 2 and 3; ears of intermediate length, hair

short, color black.

2. "Old female lop," sooty yellow in color.
3- cT45j a short-eared, Himalayan albino, angora rabbit.
4. 9 400, daughter of cT 248, fig. i, and of his sister $ 247, a rabbit of like character.

PLATE 2. — Photographs from life of rabbits described in the text.
FIG. 5. 9 269, a short-eared albino rabbit.

6. A litter of four rabbits (640 to 643) borne by 9 269, fig. 5, when mated with c?i79,

fig. 8. Three are gray, one is black; all have ears of intermediate length,
as compared with the parents.

7. Gray quarter-blood lops borne by 9 431, plate 3, fig. 9, in a mating with her son

,5*176, a half-blood lop similar in appearance to his sister 9 175, plate 3, fig. 10.

8. c?i79, a full-blood yellow lop, son of the "old female lop," plate i, fig. 2, and of the

"old male lop," a yellow rabbit.

PLATE 3. — Photographs from life (except fig. 9) of rabbits described in the text.
FIG. 9. Mounted skin of 9 43 J> the "Belgian hare," a gray rabbit with short ears.

10. ? 175, a gray half-blood lop, daughter of 9 43 1, fig- 9, and the old c? lop, a yellow

rabbit similar in appearance to his son (^179, plate 2, fig. 8.

11. 9322> a gray three-quarter blood lop, daughter of old female lop, plate i, fig. 2,

and the half-blood lop 1^176; compare fig. 10, which gives a good idea of
the appearance of (^176.

12. 6^381, son of 9 i7S> fig- i°> and her brother, 6^176; an F2 half-blood lop, with the

same general ear-character as his parents, but yellow in color, like his grand-
father.

PLATE 4. — Dorsal and ventral views of the skulls of 3 rabbits.

In the middle the skull of (^248 (compare plate i, fig. i); at the right the skull of his mother
"old female lop" (plate i, fig. 2); and at the left the skull of his father
C?45 (plate i, fig. 3).

70

CASTLE

PLATE I

CASTLE

PLATE 2

. CASTLE

PLATE 3

CASTLE

PLATE 4

8 3

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