PIEBALD RATS AND SELECTION

AN EXPERIMENTAL TEST OF THE EFFECTIVENESS OF

SELECTION AND OF THE THEORY OF GA^IETIC

PURITY IN MENDELIAN CROSSES

QH\Ay

'?3N5

BY
W. E. CASTLE AND JOHN C. PHILLIPS

0H431
C37

WASHINGTON, D. G.
Published by the Caknegie Institution of Washington

1914

SH^p S. M. 'Ml IGtbrarg

North (Earolina ^tate HniuprHitg

QH431
C37

Nortri^rolin^tat^ibrary

PIEBALD RATS AiND SELECTION

AN EXPERIMENTAL TEST OF THE EFFECTIVENESS OF

SELECTION AND OF THE THEORY OF GAMETIC

PURITY IN MENDELIAN CROSSES

BY
W. E. CASTLE AND JOHN C. PHILLIPS

WASHINGTON, D
Published by the Carnegie Institut

1914

D. H. HILL LIBRARY
lie. STrtiE UNIVERSITY

THIS BOOK IS DUE ON THE DATE
INDICATED BELOW AND IS SUB-
JECT TO AN OVERDUE FINE AS
POSTED AT THE CIRCULATION
DESK.

lOOM/5-79

j0m*'m 'MT^^vV^'

Carnegie Institution of Washington, Publication Xo. 195

Paper No. 21 of the Station for Experimental Evolution
AT Cold Spring Harbor, New York

From the Laboratory of Genetics
of the Bussey Institution

■' t ?'$ Book
issued

^E8 181914

PRESS OF GIBSON BROTH KRS. INC.
WASHINGTON, D. C.

CONTENTS.

Page.

Introduction .5

Material and methods 7

Plus selection series 9

Minus selection series 12

Return selection 13

Crosses with wild rats IG

Crosses with black "Irish" rats 18

Plus selection of "extracted hooded" rats 20

Crosses of the plus race with the minus race 22

Summary of results 22

Discussion 23

The "mutant" series 25

Bibliography 31

Tables 32-54

Explanation of plates 56

PIEBALD RATS AND SELECTION.

INTRODUCTION.

The fundamental importance of Mendel's law of heredity is generally
recognized among biologists. It is a working hypothesis whose utility
is fully substantiated by abundant results daily increasing in amount.
But biologists are not in agreement as to how much this law includes.
All perhaps would agree that it implies the existence in the germ cell of
specific determiners essential for the production of particular character-
istics in the offspring. Further, no one probably will object to the
statement that it implies a dual or duplex condition of the zygote as
regards determiners and a simple or simplex condition of the gamete.
Thirdly, the fact will be admitted by all that most mendelizing char-
acters are wholly independent of each other in heredity, for which
reason we are forced to suppose that their determiners are distinct
within the germ-cell.

But beyond these few generalizations great diversity of opinion
exists. As regards the very nature and function of the determiners,
some consider them unvarying, and explain the observed variation of
mendelizing characters in organisms as due to a modifying action of
other determiners. At one time even a modifying action of other
determiners was denied, and the theory was advanced that the gametes
extracted from a mendelian cross are pure as regards the single char-
acters which may have been concerned in that cross. Investigations
carried out by Castle have done something to dispel this idea. In
particular it was shown (Castle, 1905, 1906; Castle and Forbes, 1906)
that in guinea-pigs, polydactylism, long-hair, and rough coat are men-
delizing characters which are affected in the degree of their develop-
ment by crosses — that is, when these characters are "extracted" from
crosses the characters are not exactly the same as before; hence the
gametes are not ''pure."

The experimental result is not denied, but in order to save the sub-
stance of the theory its advocates now suppose that the determiners
have not changed, but in consequence of the cross certain modifiers
have become associated with them which change their appearance in
the organism. The real unchanging thing is now called the "geno-
type," its appearance the "phenotype. "

In this genotype theory we are dealing only with a new and more
refined aspect of the "theory of pure gametes." It is not a necessary
part of mendelism, not even an original part; but it is ver}^ important
for us to know whether it is true or not. For if it is true, selection
unattended by hybridization is largely a waste of time, as De Vries and
Johannsen have maintained, and Jennings and Pearl have reiterated.

5

6 INTRODUCTION.

The investigation which we are about to describe was started six
years ago to test the vahdity of the theory of pure gametes which was
then current. Pure "genes" had not yet been invented. The inves-
tigation has been in continuous progress ever since, and while we expect
to continue it further, it seems to us desirable that the results already
obtained be presented for criticism.

Some conception of the work entailed in the investigation may be
gathered from the statement that we have during its progress reared
and studied the color pattern of over 25,000 rats. A long and arduous
investigation of this kind has been made possible by a series of grants
from the Carnegie Institution of Washington made to the senior author,
for which he here makes grateful acknowledgment. Thanks are also
due to Dean W. C. Sabine, of Harvard University, for encouraging and
supporting the work in a variety of ways.

MATERIAL AND METHODS.

MATERIAL AND METHODS.

In June 1906 Dr. Hansford MacCurdy completed, under the direc-
tion of the senior author, a study of the inheritance of color in rats.
His studies had shown that the piebald pattern of "hooded" rats
behaves as a mendelian recessive character in relation to the uniform
or nearly uniform coloration of wild rats, but that the hooded pattern,
when extracted from a cross with wild stock, shows a different vari-
ability, the pigmentation of the extracted recessives being increased in
extent. This result was interpreted as showing the unsoundness of the
current doctrine of ''purity of the gametes" in mendelian crosses.

Upon the conclusion of Dr. MacCurdy's experiments, the pedigreed
stock which he had used was not entirely discarded. A certain portion
of it was utilized for new experiments designed to show whether the
"hooded" coat-pattern can be modified by selection unattended by
cross-breeding.

Two series of selections were started in October 1907, in one of which
animals w^re chosen as parents which had pigmentation as extensive as
possible. This we may call the pZws series. In the other series animals
were chosen as parents which had pigmentation as restricted as pos-
sible. This we may call the minus series.

During the academic year 1906-7, the experiments were in immediate
charge of Mr. W. G. Vinal; during 1907-8 the plus series was in charge
of Mr. H. S. Rand, while the minus series was in charge of Mr. F. C.
Bradford. Throughout this time the experiments were closely super-
vised by the senior author, w^ho assisted in the "grading " of every litter
of young. In October 1908 the junior author began his association in
the experiments, which has continued up to the present time. Through-
out these five j^ears he has looked after the details of the experiments
almost continuously, but both authors have in most cases taken part
together in the grading of the young, and in no case has the grading
been done except under the immediate supervision of one or the other
of the authors. This fact is stated to show that the personal element
in the grading has been kept as constant as possible. In the tabulation
of results and computation of statistical constants, the authors have
worked together. This statement of results is written by the senior
author.

During the year 1906-7 the young rats were graded by the method
used by MacCurdy and Castle (1907) that is, the back-stripe was
measured and a calculation made of the percentage of the dorsal sur-
face posterior to the hood which was pigmented. But on account of the
irregular outline of the back-stripe in many individuals the method of
measurement was found to be at best a rough one, as well as extremely
laborious. Accordingly in the summer of 1907 a set of arbitrary grades

8 PIEBALD RATS AND SELECTION.

was adopted, which is shown at the top of Plate 1 . Each j^ouiig rat was
classed in that grade which it most nearly approached in amount of
pigmentation. Skins of rats graded from — 3^ to +4f are shown in
the middle and lower rows of Plate 1. The grading was done when
the rats were about three or four weeks old, at which time selected
individuals were reserved as the parents for a later generation, the
remainder being discarded. This method has been followed ever
since its adoption and the data thus obtained are summarized in the
tables, which cover the breeding operations of a little more than six
years, 1907-1913.

The grouping of the young in a series of generations is only approx-
imately accurate, for practical considerations have often led us to mate
together animals which belonged to different generations of offspring.
When, for example, an animal of generation 2 was mated with one of
generation 4, the question would arise: To what generation do the off-
spring belong? In deciding this question we simply added one to the
mean of the generations to which the respective parents belonged. In

2 -)- 4

the foregoing case this would be — - — 1-1 = 4.

In case one parent belonged to generation 2 and the other to generation

2 -f- 3

3, a fractional result would be obtained, thus — - — 1-1=35. In making

up the summaries of the generations as given in the tables, offspring
like the foregoing, of generation 3§, were divided equally between gener-
ations 3 and 4, alternate litters of young as recorded in the ledger being
assigned to each. Offspring belonging to generations 2f and Z\ were
tabulated in generation 3; those belonging to generations 3f and 43:
were tabulated in generation 4, etc. While, therefore, the genera-
tions as tabulated overlap, it is clear that they include groups of off-
sprmg of selected parents each the result of one additional selection over
the preceding group.

The early generations include too few individuals to be of much
statistical value, but where the number of offspring rises to 500 or over,
the statistical constants acquire undoubted value. The datahavebeen
given in the form of correlation tables which will repay careful study.
In the tables a single entry has been made for each individual offspring
in that row which corresponds with the mean grade of its two parents.
Thus, if one parent were of grade 2 and the other of grade 2|, the off-
spring would be entered in the row 21 along with the offspring of parents
both of grade 2j. Offspring of parents whose mean grade fell betwee?i
the rows given in the tables were divided equally between the adjacent
rows, alternate litters being assigned to each. Thus, if the mean grade
of the parents were 21^,, alternate litters of offspring would be entered in
row 2 and in row 2|.

PLUS SELECTION SERIES. 9

PLUS SELECTION SERIES.

This series begins with pairs ranging in average grade from +1-87
to +3. From these parents were obtained 150 young, which range in
grade from + 1 to +3, as is shown in Table 1. It will be observed that
the lower-grade parents have on the average lower-grade offspring than
the higher-grade parents. But in no case is the average grade of the
offspring as great as that of their parents. Thus L87 parents had 1.82
offspring (average grade) ; 2.00 parents had 1.76 offspring; 2.25 parents
had 1.87 offspring; and so on to 3.00 parents, which had 2.35 offspring.
There is a falling back in grade or "regression" of the offspring as com-
pared with their parents, which increases in amount as the grade of the
parents becomes higher. (See column "Regression" in Table 1 .) The
parents of this first generation were chosen because of their high grade.
They were all probably in grade above the general average of the popu-
lation from which they were selected. In the case of those which
deviate most from the general average the regression is greatest, as we
should expect.

This phenomenon of regression, which is a very general one in cases
of selection, was first observed by Galton in selecting sweet-peas of
varying size from a mixed population. Later Johannsen, who repeated
the experiment with beans, found that by pedigree culture he was able
to break the mixed population up into pure lines within which, con-
sidered singly, no regression occurred. We shall need later to return
to this subject and consider whether pure lines free from regression
exist or can be produced as regards the hooded pattern of rats.

Returning to the examination of Table 1, since the high-grade parents
produce higher-grade offspring than do the low-grade parents, it is
evident that we might hope by further selection either to isolate a pure
line of high-grade rats which would be free from regression and therefore
stable, or else to advance the grade of the offspring still higher, even
though regression persists. As a measure of the extent to which high-
grade parents have high-grade offspring and vice versa, in each genera-
tion, we may employ the well-known correlation coefficient. This for
Table 1 is 0.30.

The second generation in the plus series (Table 2) includes the off-
spring of parents which appear as offspring of the higher grades in
Table 1, together with a few individuals which appear in Table 2 both
as offspring and as parents of other offspring, by reason of their having
been mated with generation 1 individuals and so having produced
generation 1| offspring, as explained on page 8. To obtain larger
numbers of offspring, several new pairs were added to the experiment
in this generation, which do not appear in Table 1 either as offspring or
as parents, but which were derived from the same general stock as the
parents of generation 1. Their inclusion here accounts for the very

10 PIEBALD RATS AND SELECTION.

low range of the offspring in Table 2, which extends from —1.00 to
+ 3.75. The parents' range (means of pairs) extends from 2.00 to 3.12.
The grand average of the parents is 2.52, that of the offspring is 1.92.
The correlation between grade of parents and grade of offspring is 0.32.

From this point on in the series no new stock was added and each
generation of offspring furnished the parents for the following genera-
tion, except for the slight overlapping of generations when parents of
different generations were mated with each other, as has already been
explained.

In generation 3, Table 3, the parents ranged from 2.12 to 3.37 in
grade, the ofTspring from 0.75 to 4.00. The mean of the parents was
2.73, that of the offspring 2.51. The degree of correlation between
parents and ofTspring is expressed by the coefficient 0.33 (a perfect
correlation would give 1.00).

In generation 4, Table 4, the selection of parents became considerably
more rigid ; most of the parental pairs were of grade 3 or higher, their
average being 3.09. The average grade of the offspring was 2.73, their
range extending from 0.75 to 3.75. The correlation in this generation
fell very low, to 0.07, not because of a lessened regression but rather
because of a very high regression on the part of the offspring of high-
grade parents.

In generation 5, Table 5, the grade of the selected parents ranged
from 2.75 to 4.12, its mean being 3.33. The offspring, showing the
usual regression, ranged from 0.75 to 4.25, their mean grade being 2.90.
The correlation between parents and offspring in this generation was
0.16. The number of individuals comprising this generation of off-
spring was 610.

It is scarcely necessary to discuss separately the correlation table
for each of the next eight generations, Tables 6 to 13. The number of
offspring rises to a maximum (1,408) in generation 8, Table 8; then
declines to less than 200 in generation 13. But as this generation and
the preceding one are still iDeing produced, it is probable that the num-
ber recorded will be considerably increased before the generation is
complete. The means of parents and offspring and the other statistical
constants for the several generations can be most easily compared by
reference to Table 14. Leaving out of consideration the exceptional
generation, 2, the following will be observed:

(1) The mean of the selected parents has steadily advanced through-
out the series, as has also that of their offspring.

(2) The variability (standard deviation) of the parents as a group
has decreased somewhat as increase in numbers made a more rigid selec-
tion possible; that of the offspring has undergone a similar change.

(3) The correlation between parents and offspring has not materially
changed. The average of the correlation coefficients for the entire
series is 0.194, for the last three generations it is 0.175, for the three pre-

PLUS SELECTION SERIES.

11

ceding generations it is 0.141, for the three which precede those it is
0.185, while for the first four generations it is 0.253. In every case the
correlation is positive— that is, the higher-grade parents have higher-
grade offspring and vice versa.

(4) The offspring as a group average lower in grade than their
parents — that is, their mean regresses on that of the selected parents,
but because of the higher mode about which variation occurs in each
generation certain of the offspring are of higher grade than their parents.
Thus an elevation of the grade of the parents in the next generation is
made possible.

(5) With the selection of more extreme parents, the absolute regres-
sion of the offspring has not increased, but on the contrary has slightly
diminished — that is, the advance made by the parents is retained by
their offspring.

In Table 15 have been brought together for comparison the means of
the several horizontal rows of Tables 1 to 13. By examining the vertical
columns of Table 15 the mean grade of the offspring of parents of a
particular grade in any generation may be compared at a glance with
that of parents of the same grade in any other generation. By running
the eye dow^n the columns, it will be observed that the mean grade of
the offspring tends to increase upon repeated selection. Thus parents
of grade 3.75 appear first in generation 4, the grade of their offspring
being 2.75; the offspring of such parents in subsequent generations
grade in order, 3.07, 3.22, 3.35, 3.49, 3.50, 3.69, 3.75, and 3.83 (twelfth
generation not complete) . The difference between parents and offspring
in this series grows less and less and finally disappears altogether. If
the grade of 3.75 parents in this series is compared with the grade of all
offspring in the corresponding generations we have the following:

Table A.

Genera-
tion.

Mean of
offspring of
3.75 parents.

Mean of
offspring of
all parents.

Genera-
tion.

Mean of offspring
of 3.75 parents.

Mean of offspring
of all parents.

4
5
6

7
8

2.75
3.07
3.22
3.35
3.49

2.73
2.90

3,n

3.20
3.48

9
10
11
12

3.50
3.69
3.75
3.83 (35 individuals)

3.54

3.73
3.77
3.94 (590 indiWduals)

In generation 4 the 3.75 parents represented the most advanced indi-
viduals of the series, a whole grade in advance of the general average
of the race. Their offspring showed a correspondingly large regression.
The general average of the race steadily advanced in later generations
until in generation 11 it equaled that of the 3.75 parents; then the
regression vanished. In the following generation, 12 (which is still
incomplete, but in which the average of the offspring thus far is 3.94),
the 3.75 group of parents, which are now below the average of the race,

12 PIEBALD RATS AND SELECTION.

actually produce offspring of higher grade than themselves, viz, 3.83.
It will thus be seen that the regression is uniformly toward the mean of the
race and changes its direction ivhen that mean changes its position with
reference to a particular grade of parents. This conclusion is supported
by other columns of Table 15, but is best illustrated by this particular
case because here the selection has extended over a greater number of
generations than elsewhere in the series.

If one examines the horizontal rows of Table 15, he finds in general
that numbers increase toward the right. Exceptions are coimiionest
toward the ends of the rows Vv^iere fewest individuals are represented.
This increase means that, within any generation, as the grade of the
parents rises, that of their offspring rises also. Since in general the
selected parents are above the general average of the race for the time
being, regression is naturally downward in nearly all cases.

From what precedes we may conclude (1) that in this series of rats
the somatic character (appearance) of an individual is in general a true
indication of its germinal character, since the higher the grade of the
parents the higher the grade of the offspring, and vice versa; but that (2)
the somatic character of an individual is not a j)erfect index of its ger-
minal character, since the offspring of aberrant individuals are less
aberrant than themselves, i. e., the offspring regress toward the mean of
the race; yet that (3) by selection of plus variations we can displace, in a
plus direction, not only the mean of the race, but also the upper and
lower limits of its variation, the total amount of variability (standard
deviation) being thereby only slightly decreased.

MINUS SELECTION SERIES.

This series begins with selected parents ranging in grade from — 1.25
to — 1.87. Their average, if each pair is weighted in proportion to the
number of its offspring, is —1.46. The offspring (Table 16), like the
offspring of the original plus selections, regress toward grade 0. They
range in grade from +0.25 to —2.00, their mean being —1.00. The
total number of offspring recorded in this generation is only 55, this
being too small to warrant the calculation of a correlation coefficient.

Generation 2 (Table 17) is somewhat larger, but still too small to
make statistical constants based upon it of much consequence. The
offspring show substantially the same range of variation as in the pre-
vious generation, but with a slightly higher average ( — 1.07). The
coefficient of correlation ( — 0.03) is negative, but too small to be signifi-
cant. The record of the next eleven generations will be found summar-
ized in Tables 18 to 28, or in more condensed form in Tables 29 and 30.
Generation 13 (Table 28) is still incomplete.

The mean of the parents steadily rises from — 1.56 in generation 3 to
— 2.50 in generation 13. The mean of the offspring rises by like incre-
ments from — 1 . 18 in generation 3 to — 2.39 in generation 13. There is

MINUS SELECTION SERIES. 13

throughout these generations a positive correlation bet^veen parents
and offspring. This amounts on the average to 0. 137 as compared with
0.193 observed in the phis selection series. The absolute change in
amount of pigmentation is no doubt less in the minus selection than in
the plus selection series, but if the change were recorded as percentage
decrease of pigmentation in one case and percentage increase in the other,
the change indicated would probably be as great in one as in the other.
In the minus as in the plus series we observe :

(1) The character of the offspring varies with that of the parents;
high-grade parents have high-grade offspring and vice versa.

(2) The variability of the race (as indicated b}^ the standard devia-
tion) undergoes some reduction and the limits of variation, both upper
and lower, are displaced in the direction of the selection.

(3) The regression from a new and extreme class of parents is at first
large, but decreases as the selection is repeated and finally disappears
altogether when the average of the race becomes equal to the particular
grade under discussion.

RETURN SELECTION.

The plus and minus selection series already described make it clear
that one can, in a race of hooded rats, either increase or decrease the
average pigmentation at will, and at the same time secure more advanced
stages either of pigmentation or of depigmentation than those pre-
viously occurring in the race. The question now arises, are these
changes permanent; will these displaced means retain their new posi-
tion, if the race is left to itself; or will the newly obtained stages vanish
as soon as selection is suspended? A presumption that the changes
will prove permanent is afforded by the gradual decrease of regression
and its final reversal in the case of offspring of a particular grade, upon
repeated selection made in the same direction. (See page 12.) But in
order to test the matter more directly and thoroughly, the experiment
has been repeatedly made of reversing the course of selection, after it had
been in progress for several generations, with a view of ascertaining
whether the return toward the former condition of the race would be
made more speedily and easily than the original departure from it had
been.

The first experiment of this sort was a return selection from genera-
tion 6 (and 6|) of the minus selection series. The parents of generation
6 (Table 21) averaged —1.86 in grade; the average grade of their off-
spring w^as —1.56, a regression of 0.30. The range of the offspring
extended from 0 to —2.50. Some low-grade offspring were chosen foi-
a return selection series (Table 31). The mean grade of the selected
pairs ranged from —0.37 to —0.87, their mean being —0.60. These
parents produced 118 offspring, whose average grade was —1.28, a
regression of 0.68 in a direction contrary to that of the regression in the

14 PIEBALD RATS AND SELECTION.

miJius selection series. The large amount of the regression might seem
to imply that it was even more difficult to return toward the former
state of the race (in the neighborhood of 0) than it had been to depart
from it, but this can not be insisted on, because the number of indi-
viduals under observation is not sufficiently large. To test the reality
and permanenc}^ of the reversed regression, the selection was repeated
five additional times, altogether six successive return selections being
made with the idea of undoing what had been effected by six original
selections in an opposite direction. The result of the second successive
return selection is shown in Table 32. The parents here were of grade

— 0.50 and they produced 19 offspring of the average grade —0.95, a
regression of 0.45 away from 0 as before.

Table 33 shows the result of the third return selection. Individuals
entered in Table 32 as offspring appear here as parents. Only those
pairs which were of mean grade, —0.25 or —0.37, should really be
regarded as a third return selection. They gave offspring with mean
grades of —0.63 and —0.86 respectively, which show regression of 0.38
and 0.49 aumy from 0.

But Table 33 shows also the character of 3'oung produced by —1.12
and —1.25 parents in this same third return-selection generation, i. e.,
by unselected parents of the generation in question. Their young also
regress aivay from 0 — that is, in the direction of the original selection.
The — 1.12 parents produced — 1.61 offspring, a regression of 0.49, while
the — 1.25 parents produced — 1.35 offspring, a regression of 0.10. For
Table 33 as a whole the regression au'a?/ /row 0 averages 0.31.

A fourth generation in the return-selection series is summarized in
Table 34. The parents are of mean grade —0.63; their 50 offspring are
of mean grade —1.17, a regression amounting to 0.54 away from 0 and
in the direction of the six generations of original selection.

Table 35 contains the results of the fifth generation of the series.
The parents are here of mean grade —0.65. The number of offspring
is very small (13), but they nevertheless show the reversed regression
which characterized the four preceding generations. Their mean was

— 0.75, a regression of 0.10 away from 0.

A sixth and final generation in this return-selection experiment is
summarized in Table 36. It includes 36 offspring of mean grade — 0.39,
the mean of the parents being —0.26, a regression of 0.13 away from 0.
It will be seen, therefore, that the effect of the six original selections had
not been entirely overcome by an equal number of return selections.
The reason for this is obvious. Much smaller numbers are concerned in
the return selections than in the original minus selections. The return
selections are accordingly less eflficient. Nevertheless, after the sixth
return selection we find that 1 in 6 of the offspring have plus grades and
their average is lower (that is, less minus) than the offspring in the minus
series after a single generation of selection. (Cf. Tables 16 and 36.)

RETURN SELECTION. 15

The amount and persistency of the reversed regression in this series
show clearly that return selection is not easier or more rapid than the
original modification of the race by selection, but that selection in either
a plus or minus direction has cumulative and permanent effects.

Further support for this conclusion is furnished by return selections
(one each) made from the seventh generation, from the eighth genera-
tion, and from the eleventh generation of the minus selection series.
(See Tables 37, 38, and 39.) Generation 7 (Table 22) was produced by
parents of average grade —2.01. Their offspring were of average
grade — 1.73, a regression (toward 0) amounting to 0.28. Certain pairs
of these offspring of grade —0.75 and —0.87 (mean —0.78) constitute
the return selection from generation 7 (Table 37). They had 33 off-
spring of average grade —1.15, a regression away from 0 amounting to
0.37.

Generation 8 of the minus-selection series (Table 23) was produced
by parents of mean grade —2.05. Their offspring were of mean grade
— 1.80, a regression (toward 0) of 0.25. Certain pairs of these offspring
of grades —0.50, —0.62, and —1.00 (mean —0.72), when chosen as
parents, produced 41 young of mean grade —1.51, a regression away
from 0 amounting to 0.79. (See Table 38.)

Generation 11 of the minus series (Table 2G) was produced by parents
of mean grade —2.30. The offspring were of mean grade —2.15, a
regression of 0.15 toward 0. A pair of the offspring of mean grade
— 1.62 (Table 39) produced 16 young of mean grade — 1.95, a regression
of 0.32 away from 0. This result shows that the selected race had now
passed the point represented by the grade of the parents ( — 1.62) and
the offspring regressed toward a racial mean as advanced as the most
extreme individuals obtained previous to selection.

To show that, in the plus selection series, a return selection has a
result similar to that just described, two experiments may be cited:

The sixth generation of the plus selection series was produced by
parents of mean grade 3.52, and their offspring were of mean grade 3.11,
a regression toward 0 amounting to 0.41. Certain of these offspring
of mean grade 2.00, when chosen as parents, produced 17 young of
mean grade 2.36, a regression away from 0 amounting to 0.36. (See
Table 40.)

The eleventh generation of the plus selection series (Table 11) was
produced by parents of mean grade —3.97; their offspring were of mean
grade —3.78, a regression of 0.19 toward 0. Certain of these offspring,
ranging in grade from —2.62 to —3.25 (Table 41), mean —2.79, pro-
duced 53 young of mean grade —3.32, a regression away from 0 amount-
ing to 0.53. The regression in this case, as in all those previously
described, was toward the racial mean of the previous generation, which,
however, it has in no case reached.

16 PIEBALD RATS AND SELECTION.

This can have but one meaning. The genetic character of the hooded
rat is in a general way correctly indicated by its somatic character.
Selection is therefore immediately effective, whether plus or minus in char-
acter and whether or not preceded by selection in the same direction or in an
opposite direction. But regression may be expected from the character
of aberrant parents back toward the normal of the previous generation,
yet this regression will in general be less than the departure of the
aberrant parents from the normal of their generation. If one desires
in such a case to obtain continuous and progressive departure from the
normal in either a plus or a minus direction, he need only select con-
tinuously in the desired direction.

CROSSES WITH WILD RATS.

As a further test of the permanency of the modification effected by
selection in the hooded pattern of rats, crosses have from time to time
been made of the selected races with a pure wild stock, i. e., with ordi-
nary wild animals caught in traps. In all cases the wild animals used
were known to be homozygous as regards gray coat and self pattern,
since when crossed with black-hooded animals they produced only gray
self offspring. In such crosses the hooded pattern is recessive, the Fi
offspring being indistinguishable from ordinary wild gray rats except
for the possession of a white patch of varying size upon the belly, but
even this may be lacking. (See Plate 2, d^ 8000, 8018, and 8021.)

The grade of the hooded young extracted from a cross with wild ani-
mals corresponds in a general way with the grade of the hooded animal
used in making the cross, as the following cases will show. (Compare
also Plates 2 and 3.)

A female of grade —1.87, belonging to generation 2\, minus series
(compare Tables 2 and 3), was crossed with a wild male. (See Plate 2,
9 6176.) Among her Fo descendants (cf . Plate 2, 8070 to 8078) occurred
62 hooded individuals, whose grade distribution is shown in Table 42,
first row. Their mean grade is +0.31, although the uncrossed race of
the same grade and generation gave offspring of mean grade about
— 1.20. The cross, therefore, had apparently increased the pigmenta-
tion of the extracted hooded recessives. This idea is supported by the
result of a control mating of the particular female used in making the
cross. When she was mated with a hooded male of the same grade as
herself, she produced three young, all of grade —1.00. The extracted
recessive grandchildren, as a group, show greatly increased pigmenta-
tion as compared with this, but vary greatl}^ in the extent of the increase.
Some show very little modification, others very much, the most extreme
individual being of grade 4-3.50. It was undoubtedly out of just such
modified recessive individuals as this that the material for our initial
plus selections arose; to this point we shall return later.

CROSSES WITH WILD RATS. 17

The F2 (or second generation) offspring, however, include about 1
hooded individual in 4. In a total of 962 F2 young, 230 were hooded,
or 24 per cent. This summary includes only those litters in which
dominants as well as recessives were recorded. In many litters only
the hooded young were recorded, as the special object of the investiga-
tion was to ascertain whether the extracted recessives were like the pure
hooded race in grade or not. In the above summary also the hooded
grandparent was in every case a female. The reciprocal cross is more
difficult to obtain, but one wild female rat, caught in 1911, has bred
quite regularly in captivity, though each time she has murdered her
hooded mate prior to the birth of the young. Her Fo grandchildren
derived from matings with males of the minus series include 32 hooded
and 96 non-hooded individuals, exactly 25 per cent hooded.

A second cross of selected animals of the minus series was made
between a wild male and four females of grade — 2 derived respectively
from generations 5j, 5^, 6^, and 7. As a group these mothers are more
nearly comparable with generation 6, Table 21, than with any other
uncrossed group. As the Fi progeny of these four mothers by a wild
male were mated i?iter se, it is possible to deal with their hooded grand-
children only as a group. The character of these is indicated in the
second row of Table 42. They number 48 individuals and have a
mean grade of +0.25, showing a modification in a plus direction similar
to that observed in the previous case.

A third cross in which the minus series is concerned was made between
females of grade —2 and —2.25, generation 10, and wild males. The
F2 offspring include 91 hooded individuals classified as to grade in the
third row of Table 42. Their mean grade is +0.24, confirming fully
the results obtained in the two previous experiments.

With these three cases we may compare three cases in which animals
of the plus selection series were crossed with a wild male. (See the last
three rows of Table 42.) Females of grade +3.00, generation 3, were
crossed with a wild male. From this mating resulted 21 hooded grand-
children, ranging in grade from +1.75 to +3.50, mean +2.56. These
grandchildren, it will be observed, in no case are of minus grade, as are
about half the grandchildren when the grandparent is of minus grade.
There is also no clear evidence of modification of the hooded character
by the cross in this case. The grade of the extracted hooded individual
is just about what uncrossed individuals of grade +3.00 produced in the
corresponding generation of the plus series.

In the next case two females of the plus series, belonging to genera-
tions 5 and 6, respectively, were crossed with a wild male and their
children were bred inter se. There resulted 38 hooded grandchildren,
as shown in the next to the last row of Table 42. The range of the
grades of these hooded grandchildren was similar to that of the grand-
children in the foregoing case, but their mean was somewhat higher, as

18 PIEBALD RATS AND SELECTION.

we should expect, since they are descended from more highly selected
individuals; for the hooded grandparents in this case were of grade
+3.25 (generation 5) and +3.50 (generation 6), whereas the grand-
parent in the foregoing case was of grade +3.00 (generation 3).

It is a noteworthy fact that in both these cases the wild cross does not
seem to have increased the pigmentation in extracted hooded indi-
viduals, as it did when the minus series was crossed, but rather to have
diminished it; yet the difference between observed and expected is not
great. We might disregard it altogether, if a similar but more striking
result were not observable in the third case as well as in another series of
crosses presently to be described.

The third case (last row of Table 42) involves a cross between a
female of grade +4.25, generation 10, plus series, and a wild male.
The F2 offspring include 16 hooded individuals of mean grade +3.15.
Animals of this grade in the uncrossed race in this generation produced
young of mean grade +3.84.

Before leaving this subject it is important to observe the considerable
difference between the extracted hooded grandchildren of the minus-
series rats, as a group, and those of the plus series. The latter is unmis-
takably a plus-series group; the former is on the border line between
the two series. (Cf. Plates 2 and 3.)

CROSSES WITH BLACK "IRISH" RATS.

As a control on the results given by the wild crosses, we may examine
the results obtained by crossing the plus and minus selected races with a
black Irish race. The Irish race used for this purpose consisted of ani-
mals black everywhere except on the bell}^ On the system of grading
used in this paper they would range from +4f to +5f , +6 being an all-
black rat.

Crosses of minus-series hooded rats with Irish produced Irish Fi off-
spring with rather more white on the belly than the Irish parents
possessed. In the F2 generation hooded individuals reappeared in
approximately the expected 25 per cent. In a total of 764 second gen-
eration young, 171, or 22.4 per cent, were hooded. The grade of pig-
mentation of these extracted recessives as compared with that of their
hooded grandparents we may now consider, as was done in the case of
the wild crosses. (See Table 43.)

Six individuals of the minus selection series, of generation 3h, and of
mean grade -1.50, were crossed with Irish rats producing Irish off-
spring which were mated inter se. Among the grandchildren appeared
the usual proportion of recessives (hooded), 90 in number. The dis-
tribution of these as regards grade of pigmentation is shown in the first
row of Table 43. Their mean grade is - 0.62, that of uncrossed hooded
rats of the same grade as the hooded grandparents being - 1.31 in gener-

'i T^,T.,T^ "

CROSSES WITH BLACK " IRISH ' RATS. 19

ations 3 and 4. In other words, the cross has considerably increased the
pigmentation in the hooded grandchildren over what was to be expected
had the cross not taken place. Nevertheless the increase in this case is
less than in the similar cross with wild rats. (Compare Table 42.)

In the second row of Table 43 is shown the grade distribution of
extracted hooded grandchildren of two mothers of grade —1.87 and
generation 4. The mean of the 53 hooded grandchildren is in this case
— 0.73, that of uncrossed hooded parents of the same grade and genera-
tion being 1.18. This average is probably too low. An examination
of the means of adjacent classes (Table 19) indicates that it should be
about 1.35.

In the third row of Table 43 is shown the grade distribution of the
extracted recessive grandchildren of a —2.00 male, minus-series rat, of
generation 7|. The 66 grandchildren are of mean grade —0.94, ex-
pected — 1.75.

Comparing the three experiments (first three rows of Table 43), we
see that the more advanced grandparents, in grade and generations of
selection, have the more advanced grandchildren; but in every case
these are less advanced than grandparents of the same sort would have
given had they not been crossed. Hence crossing with Irish has clearly
had the effect of increasing the pigmentation in the minus series in the
same waj'- (but in lesser degree) as did crossing with wild animals.

The results of crossing hooded rats of the plus series with Irish ones
are shown in the last two rows of Table 43. Several rats of mean grade
+2.25 and of generation 2 were crossed with Irish, and their Irish
young were then bred inter se, producing 239 hooded grandchildren.
These ranged in grade from —1.00 to +3.25, their mean being +1.27.
The grade of uncrossed rats of like grade and generation to the hooded
grandparents is +1.80. Hence here, as in the cross with wild rats, the
pigmentation has not been increased, but decreased by the cross, con-
trary to what we should expect. Further, the departure from expecta-
tion is greater in this cross than in the wild cross. These conclusions
are supported by the results shown in the last row of Table 43. In the
experiment here recorded a + 3.00 rat of generation 3 was mated with
an Irish rat. The hooded grandchildren derived from this cross were,
as shown in the table, of mean grade +0.95, expected about +2.50.
Since the number of animals recorded in this experiment is compara-
tively^ small, the quantitative result is less important than that of the
foregoing experiment, but qualitativeh^ the two are in entire agreement.

The various crosses of the selected minus and plus series v.'ith wild
rats and with Irish rats respectively are consistent with each other.
In every case the cross increases the pigmentation of the minus series and
decreases that of the plus series; in other words, it undoes the work of
selection to some extent. Does this mean that the condition created by
selection was in reality an unstable one, so that an outcross tends to do

20 PIEBALD EATS AND SELECTION.

away with it? We do not think so, but to this question we shall return
again.

The question might be asked whether the modifications produced in
the selected races by a cross with wild or Irish stock are likely to be
more or less permanent than those produced in unselected races by the
same means. A single experiment was made which bears on this
question in relation to the Irish cross. One of the —2.00 grandchildren
recorded in the third row of Table 43 was mated with —2.00 individuals
of the uncrossed stock of the minus series and produced nine young of
mean grade —0.63, the expectation for the uncrossed race of the same
grade and generation being about —1.90. In other words, this ex-
tracted — 2.00 individual regressed (in breeding) as if it really had been
affected by the cross, even though it did not show it, but the number of
young is so small that no emphasis should be placed upon this result.

From the experiment recorded in the last row of Table 43 were
obtained extracted individuals of mean grade +1.37, which as parents
produced 16 young of mean grade +1.68, or, in other words, offspring
about like themselves. Hence the changes effected by a cross are per-
manent, like those effected by selection.

PLUS SELECTION OF "EXTRACTED HOODED" RATS.

It has been suggested that the original material out of which the
plus series came consisted of rnodified individuals produced by a cross
with the wild race. This was not known positively to be so, because
part of the original stock (with which MacCurdy worked) consisted of
hooded black and hooded gray rats captured in company with gray self
and black Irish rats and albinos. Subsequent experiments showed that
ordinary albino rats, if crossed with wild gray ones, will produce in F2
all these classes of individuals. This indicated pretty clearly that the
particular colony which had fallen into our hands had probably arisen
by the crossing of an escaped albino rat with wild ones. But it still
remained uncertain what sort of hooded pattern the escaped albino had
transmitted and w^hether or not this had been influenced by the wild
cross. We therefore determined to ascertain whether out of our minus
series crossed with wild a plus series could be derived. To this end
certain of the F2 extracted hooded indi\dduals (entered as grandchildren
in Table 42, row 1, and descended from a single hooded individual of
grade —1.87, generation 2^) were mated inter se, thus producing an F3
generation. Table 44, second row. The selected individuals were the
aberrant male of grade +3.50 and females of grade +1.50, so that the
mean grade of the chosen parents (extracted from the crossed minus
series) was +2.50. They had 34 young ranging in grade from 0 to
+3.50, mean +2.06, a regression of 0.44 toward 0, repeating the phe-
nomenon regularly found in both selection series.

In this same experiment some Fo parents of mean grade -0.75 had
19 young (first row of Table 44), whose mean grade was -0.04, a

PLUS SELECTION OF '^EXTRACTED HOODED" RATS.

21

regression of 0.71 toward 0. We should expect the regression of the
offspring of such parents to be less than that of the offspring of the
— 2.50 parents, and so it would be if it were not for one aberrant indi-
vidual. Larger numbers of offspring would undoubtedl}- have given
the expected result.

From among the F3 offspring were chosen parents for the next gen-
eration (F4). The chosen parents ranged in mean grade from +2.25 to
+3.12 (Table 45), average +2.52. They produced 205 young ranging
in grade from —0.25 to +3.50, mean +1.86, a regression of 0.66.

The parents for the next generation (Table 46) ranged in mean grade
from +2.00 to +3.00, the mean being +2.27. They produced 119 off-
spring of mean grade +2.06, a regression of only 0.21.

Table B. — Comparison of the present scries with the more general plus selection series.

Selection.

Present series.

General (plus) series.

Mean
parents.

Mean
offspring.

Regres-
sion.

No. of
offspring.

Mean
parents.

Mean
offspring.

Regres-
sion.

No. of
offspring.

1

2.50

2.06

0.44

34

2.51

2.05

0.46

150

2

2.52

1.86

.66

205

2.52

1.92

.60

471

3

2 27

2.06

.21

119

2.73

2.51

.22

341

4

2.69

2.41

.28

194

3.09

2.73

.36

444

5

2.77

2.32

.45

97

3.33

2.90

.43

610

6

3.08

2.67

.41

45

3.52

3.11

.41

861

The parents chosen from among these offspring ranged in mean grade
from +2.37 to +3.25, average +2.69. They produced 194 offspring
of grade +0.50 or higher (Fe, Table 47), the range for the first time
lying wholly in the plus direction. The mean grade of the offspring was
+2.41, a regression of 0.28.

The parents of the next generation (F^, Table 48) range in mean grade
from +2.62 to +3.37, their average being +2.80. Their 154 offspring
range from +0.75 to +3.75, mean +2.46, a regression of 0.34.

The parents of the last generation in this experiment (Fg, Table 49)
were of mean grade +3.08. They produced 45 offspring of mean grade
+2.67, a regression of 0.41.

As a result of a single cross with a wild race followed by six successive
selections, a narrow-striped or minus family has thus been converted
into a wide-striped or plus family. Considering the smaller number of
offspring from which selections could be made, progress was quite as
rapid in this series as in the larger plus selection series. The regression
is surprisingly similar, generation by generation, in the two series. (See
Table B.) But it seems improbable that the closeness of the agreement
has any significance. This series has the theoretical advantage of being
derived from a single individual of the minus selection series.

22 PIEBALD RATS AND SELECTION.

CROSSES OF THE PLUS RACE WITH THE MINUS RACE.

When animals of the plus selection series are crossed with animals of
the minus selection series, an Fi generation of offspring is obtained
which varies about a mean intermediate between those of the respective
uncrossed races. Thus, from an examination of Table 50 it will be seen
that when — 2 animals of generation 6, minus series, were crossed with
+3.50 or +3.75 animals of generation 5, plus series, an Fi generation
(Series 1) was obtained consisting of 93 animals of mean grade +0.0G.
This generation is rather more variable than either uncrossed race,
its standard deviation being 0.71. The same is true of a second set
(Series 2) of crosses made between a male of grade —3.25, generation
10, minus series, and females of grade +3.75, generation 10, plus series.
The 14 Fi offspring are of mean grade + 1 and have a standard deviation
of 0.60. (See Table 50, Series 2.)

In both the series of crosses summarized in Table 50, the Fo genera-
tion is more variable than Fi. In Series 1, 305 Fo animals are recorded,
having a standard deviation of 1.01 as compared with 0.71, the standard
deviation of the Fi generation. In Series 2, the F^ offspring number 73
and have a standard deviation of 0.87, that of the previous generation
being 0.60.

The mean of the F2 generation is very similar to that of the Fi genera-
tion. In Series 1, the mean of Fi is 0.06, and that of F2 is 0.24; in Series
2, the mean of Fi is 1.00, and that of F2 is 0.72.

It may also be seen from an examination of Table 50 that among the
Fi offspring produced by crossing the plus and the minus series there are
differences in transmission, as there are in the expression of the hooded
pattern. In general those Fi individuals which are of high grade pro-
duce offspring of higher grade than do their low-grade brothers and
sisters. This is exactly what has been observed in both uncrossed races.

SUMMARY OF RESULTS.

The experiments which have been described in the foregoing pages
have shown that :

1. The hooded pattern of rats behaves as a simple Mendelian char-
acter in crosses with either the Irish pattern or the wholly pigmented
condition of wild rats.

2. Though behaving as a unit, the hooded pattern fluctuates— that
is, it is subject to plus and minus variations.

3. Selection, plus or minus, changes the position of the mean and
mode about which variation occurs.

4. The results of such plus or minus selections are permanent, for
return selection is not more effective than the original selection, and
during return selection regression occurs away from the original mode,
that is, toward the mode established by selection.

SUMMARY OF RESULTS. 23

5. During the progress of the original selection (thirteen successive
generations) variability as measured by the standard deviation was
somewhat diminished.

6. Upon crossing the selected plus and minus races with each other,
the variability was somewhat increased in Fi and was further increased
in Fa. The extreme conditions (plus or minus) of the grandparents
rarely, if ever, recur in this generation. Only one individual among 378
F2 young has been recorded in a grade as extreme as either grandparent.

7. Hooded animals extracted in F2 as recessives from a cross with
either Irish or wild rats are as a rule more variable than the selected race
used in making the cross. In crosses with an Irish race the minus series
was affected in like measure. In crosses with wild rats the variability
of the plus series was not appreciably affected (in two experiments it
was slightly reduced, and in one experiment it was slightly increased).
But the variability of the minus race was more than doubled by crosses
with wild rats.

8. The mean of the minus race was lowered by a cross with either the
Irish race or with wild rats, but more extensively by the latter. The
mean of the plus race was lowered a very little by a cross with wild rats,
but considerably by a cross with the Irish race.

DISCUSSION.

It Y\^ould be possible to suppose, as the senior author has elsev/here
suggested (Castle, 1912), that the Mendelian unit character involved in
these experiments is subject to quantitative variation and that such
quantitative variations have a tendency to persist from generation to
generation. This would account for the effectiveness and permanency
of selection when brought to bear upon the variations. It might also
form a basis for explaining the increased variability which follows cross-
ing, this being regarded as due to contamination in the heterozygote, but
there are certain other observed effects of crossing which it seems impos-
sible to account for on this basis. In particular it is observed that while
crossing the minus series makes it less minus as the hypothesis of con-
tamination would demand, crossing the plus series makes it less plus, the
opposite of what a contamination theory would demand. For we can
readily understand, on the basis of contamination, how a +6 gamete
being combined with a —2 gamete might change the latter in a plus
direction; but if the same +6 gamete is associated with a +4 gamete
we should expect it, if it has any influence at all, to make this also more
plus, but the observed effect is the opposite; the extracted gametes are
less plus in character.

This difficulty is met by an alternative explanation, the main feature
of which was first suggested by our colleague, Dr. E. M. East, viz, that
although we seem to be deahng with a single unit character as evidenced
by the monohybrid ratios obtained, nevertheless the modifications

24 PIEBALD RATS AND SELECTION.

which form a basis for selection are due (in part at least) to agencies
transmitted independent!}^ of the hooded pattern (not forming a part of
the same unit character), and which may be present in Irish as well as
in wild rats. By crosses with such rats the supposed modifiers may
become associated with the hooded pattern in extracted recessive indi-
viduals and so increase its extent. Such increase does actually occur in
experiment.

The hypothesis of modifiers independent in transmission of the
hooded unit will account for the fact that F2 is more variable than Fi
when crosses are made, on the familiar principle of recombination of
independent factors. It w411 account for the observed effectiveness of
selection on the ground that what selection accomplishes in the plus
series is the isolation of homozygous conditions of modifiers at first
present only in heterozygous form, and that what it accomplishes in
the minus series is the isolation of conditions homozygous for lack of
modifiers (or for inhibitors) of pigmentation. This same hypothesis
will account also for the observed reduction of variability during the
progress of selection, for as soon as any particular modifier attains a
homozygous condition in the race it will cease to occasion variability,
and as more and more factors become homozj-gous the variability
should accordingly diminish and finally disappear altogether, so far as
it is due to internal and heritable causes.

At this point the hypothesis of modifiers encounters serious diffi-
culty, if one holds the prevalent or ''genotype" conception as to the
nature of Mendelian factors, viz, that they are fixed and unchangeable
and not subject to quantitative variation, but only to combination in
different ways with other factors. This conception has been presented
very clearly by Dr. East (1912). Some objections to this view had
previously been stated by Castle (1912) and need not here be repeated.

If we assume that there exists at the outset a definite number of
modifiers and that these possess a definite and unchanging power to
modify, then it is evident that selection can do nothing but secure homo-
zygous conditions as regards the presence or absence of these modifiers.
When such homozygous conditions are secured, selection will cease to
modify the race. The experiment has progressed far enough to show
that extensive modification through selection is possible without any
marked falling off in variability. No indication is observable that
selection will become ineft"ective before an all-black rat is obtained in
the plus series and an all-white rat in the minus series. A nearly all-
black race of rats has already been secured. We propose to continue
the experiments until demonstrative evidence is obtained.

If the fixed-factor idea as regards modifiers of the hooded pattern is
rejected, there remain still tw^o possible alternative views regarding
them. Either we may consider that the modifiers vary in strength,
that is, in power to modify, or we may consider that new modifiers arise

DISCUSSION. 25

from time to time, which selection may either add in homozygous form
to the germinal complex or reject altogether from it. If we assume
that the modifiers vary in strength, we shall have to grant also the pos-
sibility that the character modified, the hooded pattern, may itself vary
in strength independently of its modifiers. For evidence see the de-
scription of the "mutant" series, page 30. This assumption, I under-
stand, would be unacceptable to those who hold a genotj'pe conception
of heredity, though we ourselves can offer no valid objection to it.

If, on the other hand, we admit that new modifiers or inhibitors are
from time to time coming into existence spontaneously, and that selec-
tion can use these to modify the pattern either in a plus or in a minus
direction, then we must admit that selection is an agency of real creative
power, able to modify unit characters indefinitely so long as physio-
logical limitations are not reached.

Now it seems to us probable that what we call the unit-character for
hooded pattern is itself variable ; also that ' ' modifiers " exist — that is, the
extent of the hooded pattern is not controlled exclusively by a single
locahzed portion of the germ-cell; otherwise we should be at a loss for
an explanation of the peculiar results from crossing plus series hooded
rats with those which are still more extensively pigmented; for by
such crosses the pigmentation is rendered not more extensive but less so.
This result we can explain on the supposition that the selected plus
series has accumulated inoi^e modifiers of the hooded pattern than the
wild race contains, so that a cross tends to reduce the number of modifiers
in the extracted hooded individuals. No other explanation at present
offers itself for this wholly unexpected but indubitable result. If a
different one can be found we are quite ready to discard the hypotheti-
cal modifiers as a needless complication, contenting ourselves with the
supposition that the unit character for hooded pattern is itself variable,
and that for this reason racial change in either plus or minus directions
may be secured at will through repeated selection.

We have been led to adopt tentatively an hypothesis that modifying
factors exist independent of the single factor for hooded pattern (though
both the factor for hooded pattern and its modifiers may, so far as we
can see, be quantitatively variable) by another series of observations,
which will now be described.

THE "MUTANT" SERIES.

In the tenth generation of the plus selection series there appeared two
individuals, a male and a female, of considerably higher grade than any
previously recorded in this series. They are not included in Table 10
because we have been and still are in doubt as to their exact nature and
think it best to give a separate account of them. If entered in Table 10
one would appear as a 5^ individual born of 3| parents (mean grade),
the other as a 5f individual born of 3f parents (mean grade). The

26 PIEBALD RATS AND SELECTION.

nearest individuals in grade to these two produced by the same group
of parents are of grade 4^, but some 4^ parents of the same generation
produced two offspring of grade 5. (See Table 10.) Because of the
marked advance in grade of these individuals beyond the ordinary
range of variation in the series we called them ''mutants," without
wishing then or now to commit ourselves to any particular theory as to
their nature or origin. We have used the term and now use it as one of
convenience merely. The two ''mutant" individuals had the same
father and their mothers were sisters. Their pedigree for two genera-
tions is as follows:

Mutants. Parents. Grandparents.

9 295G, +3|1 |91939, +3i

9 2957, +3^1 lcfl817, +3f

c?47G3, +5^
9 5153, +51

(91162, +3^
cf2963, +4 Lisio, +31

The mutant male was mated with the mutant female and also with
other females of the plus series, with the results shown in Table 51. In
every case the young fall into two distinct groups, one of which varies
about the general mean of the plus series (approximately 3|), while the
other varies about the father's grade as a mean (approximately 5^).

The mutant female had IG young, 6 in the lower group, mean 3.87,
and 10 in the upper group, mean 5.60. (See Table 51, lowest row.)
The other females had in all 114 young almost equally divided between
the two groups, 58 in the lower group, mean 3.73, and 56 in the upper
group, mean 5.45. This result indicates clearly (what the sequel also
confirms) that the male mutant transmitted in half his gametes the high
grade of pigmentation which he himself manifested, while in the other
half of his gametes he transmitted the ordinarj- condition of the plus
race at that time. In other words his "mutant" character behaved as
a dominant unit in relation to the ordinary condition of the plus race.

It is evident that the female mutant was of similar constitution.
This being the case, we should expect three-fourths of the offspring of
the two mutants to be in the upper group. In reality 10 of their 16
young were of this sort.

The male mutant was mated also with females of the minus series
with the results indicated in Table 52. Again, the offspring fall into
two distinct groups, a lower and an upper. The lower group should be
comparable with the result obtained in Fi when the plus and minus
races are crossed with each other. (Compare Table 50.) Such it
proves to be. It includes 35 individuals of mean grade —0.49 and
standard deviation 0.77. Series 2 of Table 50 is nearly contempo-
raneous with this experiment. The Fi offspring in that series were of
mean grade - 1 and standard deviation 0.60.

THE '' mutant" series. 27

The upper group of offspring (Table 52) result, we may suppose,
from a mutant gamete (grade about 5|) united with a narrow series
gamete (grade about —2). This group includes 31 individuals varying
closely about grade 4|, and with a standard deviation of only 0.31.
The lower average grade of this group (4.43) compared with the similar
group of Table 51, which had a mean of 5.47, shows the influence of
the minus-series gamete upon the heterozygote in lowering its grade by
about 1. Whether the plus-series gametes have any effect upon the
grade of the heterozygotes recorded in the upper group of Table 51 is
not certain, because a homozygous group of mutants has not yet been
established. It may be observed, however, that one individual in the
upper group of Table 51 was of grade 6 (colored all over), and it is pos-
sible that homozygous "mutants," when obtained, will approximate
that grade, as most wild rats do. Further, a comparison of Tables 51
and 53 shows that mutant heterozygotes formed by crosses with the
plus series are of slightly lower mean grade than the offspring of the two
mutants, among which should occur both homozygous and heterozygous
mutants. It seems probable, therefore, that homozygous mutants will
be found to be of somewhat higher grade than heterozygous ones.

The question early suggested itself to our minds, will these "mutants"
prove to be mutants in the sense of De Vries? Will they prove to be
more stable than the modifications ordinarily secured b}^ selection in our
experiments? To test this matter, we have raised two additional gen-
erations of offspring from the two mutants and have bred a second
generation of offspring from each of the four groups of Fi offspring
recorded in Tables 51 and 52, derived from matings with the plus and
minus races respectively.

The F2 descendants of the two original mutants proved very similar
to the Fi descendants. (See Table 53.) They fall as before into two
groups, an upper and a lower. The former includes 30 individuals of
mean grade 5.52, the latter 2 of mean grade 3.37. As the parents of
this generation were taken wholly from the upper group of offspring of
generation Fi, and as theoretically that group should contain 2 hetero-
zygous individuals to one which is homozygous for the "mutant" char-
acter, it is to be expected that in F2 more than three-fourths of the
offspring will fall in the upper group. For any pair, one member of
which is homozygous for the mutant character, should produce only
offspring falling in the upper group; and offspring falling in the loiver
group should be produced only by pairs both members of which are
heterozygous.

The upper group in Fo should contain a larger proportion of homozy-
gous mutants than in Fi, and since the parents of F3 were chosen from
this upper group of F2 offspring, it is not surprising that the 11 F3 off-
spring recorded up to this time all fall in the upper group. The mean
of this upper group is remarkably constant through the three genera-

28 PIEBALD RATS AND SELECTION.

tions, and the variability of the group as measured by its standard
deviation is also low, namely, 0.19. This indicates that the mutant
character is a strongly dominant unit in relation to the ordinary condi-
tion of the plus series.

Table 54 shows the character of the F2 offspring of the original male
mutant mated with females of the plus series. The lower group
parents, those into which the mutant character did not presumably
enter at all, produced 59 offspring recorded in the first part of Table 54.
Their mean grade is 3.78 and their standard deviation 0.33. These are
very close to the constants of the general plus series, which for genera-
tion 10 were 3.73 and 0.36, respectively.

The second division of Table 54 shows the character of the young
produced by the Fi parents of the upper group (Table 51). Such
parents are supposed to have received a ''mutant" gamete from their
father, grade about 5.50, and a plus -series gamete from their mother,
grade about 3.75. If they produce gametes of these same two sorts, their
offspring should also fall into two corresponding groups; in fact they do.
There are 11 offspring of mean grade 3.86 and 79 offspring of mean
grade 5.50. As in the previous generation, the two groups do not
approach each other in grade. The mean and standard deviation of
the lower group of offspring are similar to those of the plus race. The
mean of the upper group is about the same as that of their parents
(upper group of offspring. Table 51), namely, 5.50, as compared with
5.45; their standard deviation is somewhat lower, namely, 0.15, as com-
pared with 0.23. This result indicates that the "mutant" character
and the hooded character of the plus series segregate from each other in
a simple way without modifying each other appreciablj^ It seems
possible that they contain the same modifiers (if modifiers are present)
and differ merely by the main unit which we called the hooded character
in the early part of this paper. Each contains a different condition of
that main unit. Consequently there is no increase of variability in F2
when these two conditions are intercrossed. This we should expect to
happen, if they differed by more than a single factor.

A very different result is obtained from the cross between the mutant
and narrow races. Although Fi from that cross was quite variable
(see Table 52), F2 is still more variable (see Table 55). The lower
group Fi individuals, which resembled Fi between the plus and minus
races, produced 61 young (first division of Table 55), which resemble
F2 between the plus and minus races. They range in grade from —2
to +31, mean +0.58, standard deviation 1.17. In the two series of
crosses between the plus and minus races (Table 50) the means were
+0.24 and +0.72, respectively, and the standard deviations 1.01 and
0.87. This indicates, as did the cross with the plus series, that the
"lower group" gametes produced by the original mutant male did not
differ materially from gametes produced by the ordinary plus race from
which the mutant sprang.

THE "mutant" series. 29

The second division of Table 55 shows the character of the F2 young
produced by the upper group of Fi offspring recorded in Table 52. It
consists of two groups, a lower and an upper. The lower represents
the extracted minus race, the upper represents the extracted dominants
or mutants, whether homozygous or heterozygous. The former group
lias an average of +0.75 and a standard deviation of 1.03, which values
are close to the corresponding constants of Series 2, Table 50, the
latest of the plus-minus crosses, in which the mean was +0.72 and the
standard deviation 0.87.

The upper group offspring of Table 55, second division, the homo-
zygous and heterozygous mutants, number 68; they have a mean grade
of 4.77 as compared with 4.43 in Fi, which consisted exclusively of
heterozygotes. This shows the extracted homozygotes to be of higher
grade than the heterozygotes. The highest grade mutant among the
31 Fi young, all of which were heterozygotes, was of grade 5, but among
the 68 Fo young are 16 of higher grade than 5. We expect one-third of
these 68 individuals to be homozygotes. Now all of the F2 mutants
from the cross of mutant with plus race (Table 54) were of grade 5 or
higher, only 2 in 79 being as low as 5, and 13 of the 79 being of grade 5f ,
a grade not attained at all in Fo from the mutant-minus cross (Table 55) .
This result shows us that the cross with the minus race does affect per-
manently the mutant character, lowering its grade even in homozj^gous
mutants extracted from the cross. It also increases the variability of
the mutants, for the standard deviation of the mutant group in Table
55 is 0.44, whereas in Table 54 (mutant-plus F2), in a like number of
individuals, it was 0.15, or only about one-third as great.

That the variability of the mutants is unaffected by a cross with the
plus race, but that it is increased by a cross with the minus race, and
that, further, the mean of the mutants is affected little or none by a cross
with the plus race, but that it is lowered by a cross with the minus race —
these several facts are all conformable with the hypothesis that the
change in variability due either to crossiiig or to selection results from 77iodi-
fyiiigfactorswliich, as they are independent of the main factor concerned,
are probably transmitted in a different part or component of the germ-
cell than that factor. For if the mutant and the plus race are alike as
regards the modifiers, but differ only in the main factor, then no change
in variability should result from intercrossing them, but only alterna-
tive conditions as regards the main factor. This is the observed result.
But if the mutant and the minus race differ not onh^ in the main factor,
but also in modifiers which are independent of it, then, when they are
crossed, we may expect that through independent segregation of main
factor and modifiers the extracted minus race will be raised in grade,
while the extracted mutants are lowered, and both will become more
variable. This also is the observed result.

One objection may be offered to this interpretation, namely, that
the increased variability is not delayed until Fs, but is already in evi-

30

PIEBALD RATS AND SELECTION.

dence to some extent in Fi. The same thing was observable in the
crosses of the plus and minus series (Table 50). From that table,
Series 1, it will be observed that when the plus and minus races had
standard deviations of 0.49 and 0.50, respectively, their Fi offspring had
a standard deviation of 0.71, an increase by nearly one-half; F2 showed
a further increase to 1.01. In series 2, Table 50, the uncrossed races
(generation 10) had standard deviations of 0.36 and 0.24; their Fi off-
spring had a standard deviation of practically twice this, namely 0.60;
F2 showed a further increase to 0.87.

At the time of the mutant-minus race crosses, the minus race (genera-
tion 10) had a standard deviation of 0.24, the plus race of 0.36. Fi
(lower group) had a standard deviation of 0.77, and F2 of 1.17. F^
mutants (upper group) had a standard deviation of 0.31 which rose in
F2 to 0.44. These various facts will perhaps be better grasped if pre-
sented in tabular form:

Table C.

Standard deviation of

races crossed, same

generation.

S. D.
Fi

S. D.
F2

Plus-minus cross, series 1 (Table 50)

Plus-minus cross, series 2 (Table 50)

JIutant-minus cross, lower group

Mutant-minus cross, upper group

Mutant-plus cross, lower group

0.49
.36
.25
.19
.25
.19

0.50
.24
.24
.24
.36
.36

0 71
.60
.77
.31
.24
.23

1.01
.87

1.17
.44
.35
.15

Mutant-plus cross, upper group

The mutant-plus cross, it will be observed, shows no increase of vari-
ability either in Fi or in F2, but crosses involving the minus race show
increase of variability both in Fi and in Fo. Interpreted on a jVIendelian
basis, this means that the mutant and plus races on the one hand and the
minus race on the other hand differ by more than a single factor. If
they differed by only a single factor, then crosses between them should
bring no increase of variability, either in Fi or in F2. This appears to
be true as regards the mutant and plus races when crossed with each
other. But if the races crossed differ by more than one factor, and if,
further, neither parent is homozygous as regards the factors in which
they differ, then we may expect an increase in variability both in Fi and
in F2. This is exactly what we observe when the minus race is crossed
with either the plus race or its derivative, the mutant race.

If we suppose that the plus race and the minus race differ from each
other by certain "modifiers," we can not suppose that the plus and the
mutant races differ by these same modifiers. They differ in some other
single respect; perhaps that in which they differ is the mai7i hooded
factor. Ai-e we, then, to suppose that the plus and the minus races do
not differ as regards this same main factor? This can not be stated, but

THE ''mutant" series. 31

we see no reason for considering them identical as regards that factor.
It appears that the mutant race arose from the plus race by a single
large plus variation, which seems to have its determiner in some single
component of the germ-cell. But the fact that this change came as a
large quantitative variation does not show that small variations are
impossible in that same cell component. It seems to us quite improb-
able that the plus mutation could have arisen in the minus selection
series. We believe that the repeated selection which was practised had
something to do with inducing this change in the plus direction. If
one can increase at will the "modifiers" which make the pigmentation
more extensive, it does not seem strange that after a time a readjust-
ment should occur within the cell which should incorporate modifiers
in that part of the cell which is responsible for the unit-character
behavior of the hooded pattern. This would amount to a quantitative
change in the unit-character for hooded pigmentation.

BIBLIOGRAPHY.

Castle, W. E.

1905. Heredity of coat characters in guinea-pigs and rabbits. Carn. In.st. Wash.

Pub. 23.
1900. The origin of a polydactylous race of guinea-pigs. Carn. Inst. Wash. Pub. 49.
1912. The inconstanc}'^ of unit-characters. American NaturaUst, vol. 46, pp. 352-362.
Castle, W. E., and Alexander Forbes.

1906. Heredity of hair-length in guinea-pigs and its bearing on the theory of pure

gametes. Cam. Inst. Wash. Pub. 49.
DeVries, H.

1901-1903. Die Mutationstheorie. Veit & Co., Leipzig.
East, E. M.

1912. The mendelian notation as a description of physiological facts. American

Naturalist, vol. 46, pp. 633-655.
Jennings, H. S.

1909. Heredity and variation in the simplest organisms. American Naturalist, vol.

43, pp. 321-337.

1910. Experimental evidence on the effectiveness of selection. American Naturalist,

vol. 44, pp. 136-145.
Johannsen, W.

1909. Elemente der exakten Erblichkeitslehre. G. Fischer, .Jena.
MacCurdy, H., and W. E. Castle.

1907. Selection and cross-breeding in relation to the inheritance of coat-pigments and

coat-patterns in rats and guinea-pigs. Carn. Inst. Wash. Pub. 70.
Pearl, R.

1913. Genetics and breeding. Science, n. s., vol. 37, pp. 539-546.

32

PIEBALD RATS AND SELECTION.

TABLES

Table l.—Classifwalion of the first generation of offspring in the plus selection series. At
the head of each column is indicated the grade of the individuals recorded in that coiuinn.
The figures in the body of the table indicate the manbcrs of offspring of the several grades
indicated.

Grade of parents.

Gr:

<le

of offspring.

Totals.

Means.

Regres-
sion.

+ 1

u

H

If

2

2\

2i

2|

3

n

1
4

1

3

1

1

1
4

1

4

1

7
18

1.82
1.76

.05
.24

2

2i

21

3

1

3

1

6

2

2

20

1.87

.38

2J

2i

5

3

1

13

8

1

12

5
2

1

1

9

37

5

51

2.06
2.15
2.12

.44
.47
.63

21

2f

7

....! 3

2

17

25

3

1

2

3

3

3

12

2.35

.65

Totals or means, 2.51

i'"1

20

3

13

8

44

7

31

10

14

150

2.05

.46

Table 2. — Classification of offspring in generation 2, plus selection series.

Grade of parents.

Grade of ofif.spring.

r/1

■A

a
a

1

c
_o

"co

T.

;->

-1

-1

1

— 2

-io+i

h

3

4

1 li

Ul|2

[
2i2i2| 3

3^3^

1 '^

31; 1

2

1

1

.... 1

.... 2

:

2 5
12 : 8

7 ; 3
5 1 5

7
8

1
7

91

8
2
1
8

2

1

7

3 2

1

... 40

...' 56

1.70
1.28
1.87
1.92
1.80
2.11

1 92

2 41
2.47
2.50

.30
.83
.38
.45
.70
.51
.83
.46
.53
.62

.60

21

2\

1

1

3

2
1

1

1

4
19

6

3

3

9

11

g

2f

1

22

3

1
16

1

1

45

21

4
3

13 i 7 in i 0

1
1

... 133

...1 44

52

21

1

2

6 5
5 9

1 3

7

Q i fi

3

1
1

11

2| i

1

4 2

6 9 6
5 3 9

7 4 ' ■;

2|

1

i 93

3.

1

1
1

1

2

14

2

6

3

1 59
5

31

1

1

1

... 1 ... 2

i ' i

Totals or means, 2.52

1

1

2

! t
14 1

5

16

45 37

45 45 77

1 1

57 44 148 30

1 1 1

9

3

1 471

1.92

TABLES.

33

Table 3. — Classification of offspring in generation 3, plus selection series.

Grade of parents.

Grade of offspring.

03

•— »
c3

O

03

i

d
o

OJ

bO

f

1

u

u

If

2

2i

2i

2f

3

3i

3f

3f

4

21

1

1

1
5 6

4

1

3

21

2.06 ;

.06

21

21

1
5 j 5

5 j 5

3 9

1 7

1

10
4

4
10

2
9

1

17 i 9. l.n

.22
.18
-.02
.29
.17
.51
.05
.08
.46

2\

1

2 1

2

1
2

1

7 1 2

54
64

2.32 j

2.64 '

2.46

2.70

2.49

3.07

3.17

2.91

2f

2

15 j 18
7 7

3
1
9
3

1

1
1

1

2

21

10 7

43

47

21

1

7
1?

3

14

7
U

9

17

2

1
1

10

11

5

2

3

2

1

71

3J

!

2
1

10
6

31

2

31

1

2

3 2

8

1

Totals or means, 2.73 . .

1

2

5

8

20 j 51

46 47

66

63

21

6

4

1 341

2.51

.22

Table 4. — Classification of offspring, generation 4, plus selection series.

Grade of parents.

Grade of offspring.

Totals.

Means.

d
.2

o

f i 1

'-'

11

If

2 2i

21

2f

3

3i

3§

3f

2|

2

2

....

4

3

11

2.55

.05

2

21

2f

3

3
3
25
14
6
6
1

2
3
19
21
8
2

3
7

35

28

13

3

2

4

15
29
30
17
9
3

3
4
9
11
9
2
6

18
37
143
122
64
23
20

2.65
2.97
2.60
2.69
2.89
2.70
3.02

.10
.10
.40
.43
.36
.67
.48

*

21 . .. ..i----

1

3
4

2
8

2
1

2

1

3

1

1
1

7 12

2 11

.... 1
1
1 ..

31

31

!

31

1
1

3*

3

2

3

31 . . .

31

3

1

1

1

6

2,75

1.00

1

j

Totals or means, 3.09

1 1

1 2

12

30

60

58

96

110

45

19

9

444

2 73

36

34

PIEBALD RATS AND SELECTION.

Table 5.— Classification of generation 5, plus selection series.

Grade of parents.

Grade of offspring.

Totals.

03

a

d
0

'm
m

V

bO

<u

3

1

li

u

n

2

2i

^2

2f

3

3i

3A

31

4

41

93

1

2

4

3

20

24

25

14

7

2

4

2

2

29

39

36

16

16

4

1

1

1

4

3

3

18

27

28

8

12

3

1

1

2
1
8
7
15

11
2

1
1

12
21

15
114

3.00

2.87
2.81
2.81
2.81
2.94
2.86
3.08
3.07
3.35
3.36
3.00

-.25
0
.19
.31
.44
.43
.64
.54
.68
.52
.64
1.12

^*

9'

4
1
9
6
14
5
3

4

5

14

14

11

10

5

2

1

2

•^8

3-i- i...

1
1

1

1

1
1

1
4

1

8
12
5
5
3

3

"7
3
4

1
2
2

1

1
2

3i

1

138

145

69

3f

iih

"2

3|

3

64

14

8

34

31

1

1

2
1
1

4

... 7

...; 3

4i

1

Totals or means, 3.33

1

2

2

2

6

35

44

66

101

151

107

57

24

U

1
1 610

2.90

.43

Tabi^ 6. — Classification of generation 6, plus selection series.

Grade of parents.

Grade of offspring.

Means.

Regression.

1^

If

2

2i

21

2f 3

3i

31 3f

4

4i

41

0

1 i

1

1

2
28
143
123
212
181

31

3

1

4
3
5
9
4
2
4
1

1

3
6
4
8

1

1

8

13

15

11

3

3

4 7
26 31

5

3

24

2.84
3.10
2.96
3.10
3 16
3.22
3.26
3.41

.28
.15
.41
.40
.46
.53
.61
.59

3i

1
1

1.<>

1

3|

20 28 30
29 i 49 41

24 34 1 36

14 \ 5
43 j 17
37 21
20 13
20 j 14
6 4

3i

4
5
2
4

1

3f

1

31

1

1

4 18

5 14
... i 2

13
15

1

78
80
14

3|

4

. . . .

Totals or means, 3.52. . .

3

6

32

24

54

113 183

172

166

90

17,.... 1 ' 861

3.11

.41

TABLES.

Table 7. — Classification of generation 7, plus selection series.

35

Grade of parents.

Grade of offspring.

m

CO

§

2

H

If

2

21

2i

2f

3 I3J

3h

3|

4

1
4i 41 4|

1 1

31 1. ..!....

2 2
6 1 11

4 5

7 i 11

7 4
30 i 19

3

22
21

35
75
58
23

2

1

1

23
20
34
49
42
21
4
1

1

28 ' "7

.25
.16

.40

.32

.39

.40 1

.38;

.47 1

.50

3^ l!....

31 1 2

3J

1
4
3

7
4
6
3
1

;

131

160

177

289

184

90

15

3

3.09
2.97
3.18
3.23
3.35
3.49
3.53
3.75

10
6

18
9

11
3
1

17
9

13
3
2
1

21
21
26
16
4
1

27
31
47
28

18
28
53
9'i

1

1
1
3
6

1

1

3f

7

. . . .|. . . .

i

3^

2
2

i

3| '

10 12

3 1....

2

1
1

4 !

41 ! !

i \

...

Totals or means, 3.56. .

1 1 ! 1 1
2 1 2 35 1 55 56 103 il83 159 i240

195

29

12

2 1 2

1077 3.20

.36 '

Table 8. — Classification of generation 8, plus series.

Grade of parents.

Grade of offspring.

CQ

J

1
m

K

Qi

0

If

2

01 01 03

i !

3 3J 3^

3f

4

4i

4i

0
H

31

i
1

1

2

2

4

3.23

0

31

3^-

1

1 4

6

69
55
23
10

6

9

IS

17
149
173

2
22
23
19

1
14

2

5

2
1

2
1

10
1

....| 59
.... 484

3.46
3.50
3.49
3.53
3.31
3.72
3.69
3.96

.04
.12
.26
.34
.69
.40
.56
.41

3f

1 5

s

44 ' 144

3|

Vt

1
1 j 9 12

2.... 4

1 9 i ."?

39

18

6

15.

469

3| ! .

1

08

fiO

1

23S

64

4

34

6
13
11

2

4|

....1 2

2

14

.... 60
. . . . ' 22

U

1
1

1 7

4f

\

1

8

Totals or means.. 3.75

' :

1

1
f

1 5

13 25 1 55

1
170 119 { 469

440

88

20 ' 1 1408

3.48

.27

36

PIEBALD RATS AND SELECTION.

Table 9. — Classification of generation 9, plus series.

Grade of parents.

Grade of offspring.

03

0

c
0

'to

03

bC

If

2

2i

21

2f

1

3 31

31

3f

4

4i

H

31

1
6
7
2

1

1 1

26 96

63

212

177

15

4

1

4

244

591

424

45

4

3

3.25
3.43
3.50
3.65
3 57
3.57
3.75
3 50

.25
.19
.25
.22
.43
.55
.50
.87

3|

1
2

1
4

12

19

5

2

28

10

25

64

7

1

2

1

2
8

6
1

31

1

i
45 1 50 ! 224

31

25

32

105
14

4

5

4i

?,

41

1

3

41

Totals or means, 3.78. . . .

1

1

3

5

17

38

105

110

443

473

109

11

7

1322

3.54

.24

Table 10. — Classification of generation 10, plus series.

Grade of parents.

Grade of ofT.spring.

Totals.

Means.

c
_o

'a-

<n

<u
(-<
fcO

«

0^

21

2i

2f

3

3i

31

Q3

4

^

A\ 5

3a 1 ..

1

1
3

9
3

13

16

4

63
95
26
11
1

117

142

61

17

4

44

62

20

7

2

8

20

7

6

1

1 ....

1

5 ....

2 ....
1 2

257

347

120

44

8

3.69
3.72
3 76
3.87
3.84

.06
.15
.24
.25
.41

3|

1

4

41 !::::

41 L---

Total or means, 3.88

1

1

4 12 .^3

196 Ml

135

4'>

.1 ,

776

3.73

.15

Table 11. — Classification of generation 11, plus series.

Grade of parents.

Grade of offspring.

2f

3^

3f

4 Ul

4f

Totals.

Means.

Regres-
sion.

4|.
4i.

2
87
25
16

2

7

162

87

49

13

1

2

41

65

27

5

3

Totalsor means, 3.97. . 2

13

22 132

319

143 1 46

12

11

333

214

110

25

4

3.75
3.70
3.87
3.81
3.91
3.94

0
.17
.13
.31
.34
.43

697

3.78

.19

TABLES.

37

Table 12.

—Classification of general'

'on 12, plus

series.

Grade of parents.

Grade of offspring.

TO

C

d
.2

2§

3

H

31

31

4

H

4*

41

5

5i

3f

4
2
10
17
7
3

20

19

53

86

19

6

1

3

7
17
59
83
29
14
4
4

4

6

12

28

11

10

1

3

35

48

145

227

71

45

7

11

1

3.83
3.96
3.93
3,91
3.94
4.13
4.14
3.90
4.75

-.08
.09
.07
.21
.31
.24
.36
.72
.25

3|

2
5
6
3

7

2

3

1
3

4

1
1

1
2

1

1

2

U

1

4i

4f

1
1

1

4*

4f

1

5

1

Totals or means, 4.09

1

3

3

43

207

217

75

23

11

4

3

590

3 94

.17

Table 13. — Classification of generation

13,

plus selection series.

Grade of parents.

Grade of offspring.

Totals.

Means.

Regres-
sion.

2f

3i

31

3i

4

4i

4i

4f

4

4

17
1
5

1

1

23
8

10
5

14
3

19
16
20

3
15

1

2
5

6

1
7

65
31
43
10
40
5

3.67
3.97
3.93
3.85
3.98
3.95

.31
.15
.32
.52
.52
.80

4i

....

1

4J

1

41

4^

....

4i -

Totals or means, 4.22

1

4

25

63

74

21

5

1

194

3.88

.34

Table 14. — Summary of the results of thirteen generations of plus selection based on Tables 1-13.

Gener-
ation.

No. of
offspring.

j

Moan j Mean
parents, j offspring.

I

Standard

deviation,

parents.

Standard
deviation,
offspring.

Correlation,
parents-
offspring.

Absolute
regression
of offspring
on parents.

Advrnce

of
parents.

Advance

of
offspring.

1...

2...

3...

4...

5...

6...

7...

8...

9...
10...
11...
12...
13...

150

471

341

444

610

861

. 1,077

. 1.408

.' 1,322

776

.! 697

.1 590

194

2.51
2.52
2.73
3.09
3.33
3.52
3.56
3.75
3.78
3.88
3.98
4.09
4.22

2.05
1.92
2.51
2.73
2.90
3.11
3.20
3. 43
3.54
3.73
3.78
3.94
3.88

.313

.307
.285
.215
.240
.209
.212
.246
.112
.112
.113
.176
,433

.541
.732
.531

.468
.505
.490
.555
.439
.346
.362
.289
.302
.270

.298
.317
.331
.066
.160
.180
.215
.099
.210
.116
.233
.161
.132

.46
.60
.22
.36
.43
.41
.36
.27
.24
.15
.20
.15
.34

.01
.21
.36
.24
.19
.04
.19
.03
.10
.10
.11
.13

-13
,59
.22
.17
.21
.09
.23
.08
,19
.05
,16

-.08

Total

. 8,941

1 i

i

38

PIEBALD RATS AND SELECTION.

Table 15.— Mean grade and number of offspring produced by parents of a particular grade in each
generation of the plus selection series, based on Tables 1-13. The grade of the parents is indicated
at the head of each column. In the body of the table is recorded the grade of the offspring {in light-
faced figures) and the number of offspring {in heavy-faced figures) .

Genera-
tion.

Grade of parents; below, grade and number of their off.spring.

Total
number
of off-
spring.

2

2\

91 1 2^

1

2\ 2f

2f

1

21

3 \

2.35

12

2.4?!

31

3i|3|

31

31

31

3|i4i

1

41

4i 41

4i 4f 4f

1

2

3

4

5

6

1.76....
18....

1.701.28
4C

1.87....

20|....

1.87|l.92

2.06
37

1.80
133

2.32
54

2.55
11

....

2.15

5

2.11

44
2.63

64

1 i
2.12....

51

1

'

150

1.92*
52

2.46
43

■■■'1

2.41
23

2.70

2.50'

1

1 ■

471

341

444

610

861

1,077

1,408

1.322

776

697

590

194

2.06
21

. . . .

2.15
17

2.49:

2.60

143

2.81

15

3.25

2

3.07

2.69
122

2.81
114

3.172.91
6 8

1

2.652.97
is! 37

3.002.87
12 21

2.892.703.02
64 23 20

3.08

64

3.16

181
3.23

289
3.50

484
3.43

244

2.75

3.07

14

3.22

78

3.35

184

3.49

469

3.50

591

3.69

257

3.75

11

3.82

35

2.812.942.87

3.35

8

3.26

80

3.49

90

3.53

238

3.65

424

3.72

347

3.70

333

3.96

48

3.36

7

3.41

14
3.53

15
3.31

64
3.57

45
3.76

120
3.87

214
3.93

145
3.67

65

3.00
3

2.843.102.963.10
28 143 123 212

2.873.092.973.18

7

3.72

60

3.57

7

3.87

44

3.81

110

3.91

227

3.97

31

3.75

8

28

131 IRn 177

3

3.69

3.46

59

3 25

3.96

9 .

22 8

3.75

4

3.84

3.91
25

3.94
71

3.93
43

3.50
3

10

4

11 ...

3.94
4

4.12
45

3.85
10

12 ...

4.14

3.98
40

3.90
11

....

. . . .

4.75
1

3.95
5

1
13

8,941

Table 16. — Classification of the offspring in generation 1 of the minus selection series.

Grade of parents.

Grade of offspring.

Totals.

Means.

Regres-
sion.

+1

0

1
— *

1

3

4

1

U

n If

2

-li

1

1

2

8
1
2

3
1

3
1

1
3

2
1

1
1

1

1

8

31

6

10

134

.86
1.37
1.05

-.09
.61
.13
.82

If

1

3

12
1

2

u

1|

1

1

Totals or means, 1.46. . .

2

1

4

1

15

13

4

8 6

2

55

1.00

.46

TABLES.

Table 17. — Classification of generation 2, minus selection series.

39

1

1

j Grade of parents.

1

Grade of offspring.

Totals.

Mean.s.

Regrcs-
i sion.

+ 1

0

i

_1 1 1 3 1 1

li 1^

li

2

3

'

2
2

2 j 1 6
2 L... 2

2 3
2 S

2

3
1
4

....! 19

1 12

1 17

2 ! 5

1 57

1.04
1 05
I.IS
1.45
1.11
.67
1.09
1.10

-.29
i .07
.07
-.08
.39
.95
.66
.77

11

li-

1

1 4

6

i.

3

1
J.

If

1

n

1

....lio

.... 1

13

1

i<

4

....

If

1

3

97

If

1

1 2

14

3
1

1
3

1

11

Totals or means, 1.41 .. .

1 1

1

1 2 3

....I 12

2 ; si el C I 20 44

1 ' I : >

13 19

14

5 132

1.07

1 .34

Table 18. — Classification of generation 3, minus series.

Grade of parents.

Grade of off.spring.

Totals.

Means. ^,«.f ^«-
sion.

0 -i

1

2

3

4

1

li

11

If

2

-n

2 i 1 1 1

! 1

5 .85 .27

20 1.05 ' .20
28 1 1.03 .34
28 1.31 .19

48 1 1 22 j .40

03 1 1 26 : 49

3 i 1 GO 87

U

1
1

3
3

1
1
2

6 a

1

3
1

5
9

2
4
5

8

0

1
2
5

8
7

1

3

4
8

If 1....

4
1

10
8

11
12
10
19
3

n !....

If i 1

n ! .

1
1

i| L.

1

:

Totals or means, 1.56

1 4

1

12

30 61

1

19

29

23

16 195 I 1.18 .38

1

Table 19. — Classification of generation 4, minus selection series.

Grade of parents.

Grade of offspring.

32

O

m

Q

§

C

CO

o

+-I

+i

0

1
i

i

2

f 1

li

li

If

2

2i

-li

i
i

1

1

8

1

1 ; 1

2 : 6

16 8

7 6

22 11

19 5

1 2

. . . .

1

4
29
59
40
93
95

9

1.56
1.16
1.31
1.36
1.34
1.18
1.36

-.31
.21
.19
.26
.41
.69
.64

If

2
3
1
4
6

3 i 4

3 1

li

1

1

1 1
21717

6

8

If

1

.... 4

10

19

19

2

4

11

12

3

8
16

17

If

4
1

6

n

2

1

2

2

1

Totals or means, 1.69

2

2

3

16

10 32

1

66

39

51

68 39

1

329

1.28

.41

40

PIEBALD RATS AND SELECTION.

Table 20. — Classification oj generation 5, minus series.

Grade of parents.

Grade of offspring.

CO

G
03

c

o
o

0 -i

1

1

1 U

n

If

2

2i

2§

7

2 1

4

5

1

6

9

12

50

11

14

3

4

:

11

5

4

24

31

25

2

?0

1.09
.99
1.50
1.25
1.35
1.30
1.64
1.52
1.80

-.22
.01

-.13
.25

.27
.45
.23
.48
.32

1

2
4

1
9

4
51
53

54
202
143
109

5

If

1
3

1
14

1
9
5

29
4

11

2

10

14
7
12
55
50
28
2

2

1

u

1

11 i 7

If

4

30
12

5

15

50
29
19

1

If

1
6
4

....

ll

2

1

2

21

Totals or means, 1.73

3

18 21

!

63

108

64

134

172

104

13

1 701

1.41

.32

Table 2L — Classifix:ation of generation 6, minus series.

Grade of parents.

Grade of off.spring.

CO

o

02

o

d
o

03

0

_1

4

h

f

1

u n

If

2

2i 2h

-11

1

1

....

2

1

4

16

39

61

32

4

1
4
3

20

40

22

4

1

1

4

17

39

71

37

5

2

5

4

24

94

244

502

283

85

11

.85
1.44
1.34
1.45
1.49
1.59
1.58
1.52
1.82

.27
-.07
.16
.17
.26
.28
.42
.60
.68

If

6
28
77
156
89
35

6

1

2
17
43
127
76
31
3

H

1
1

8
9
5

3
11
14
24
12

2

If

1

1

1
1

3

If

12
5
4
1

2
1

1

n

2

1

2i

2i

1

Totals or means, 1.86

3

6

25

66

159

94

177

396

300

22

4

1252

1.56

.30

TABLES.

41

Table 22. — Classification oj generation 7, minus series.

\

Grade of offspring.

-I-'
O

^ i

U3

i

§

.11

10

CO

to
Pi

.07
.08
.22
.28
.38
.32
.74
.62
.09
.88

.28

0

i

1

h

3

4

1

U

U : If ' 2 21 2i

1 1 j

2f

If

1

2

4

14

3

1
6

2 1

5

32

330

969

260

41

4

15

18

6

!

1.55
1 67
1.65
1,72
1.74
1 93
1.63
1.88
2.28
1.87

If

1

6

8

1

35
51
14

1

20
60

11

9
106
S';>4

1
1

n 1 2

2

1

47 1 99
119 36S

6

22

4

6

3
3

21

1

9,

10 30 ! 02 104

2i

i

5 :0 19

1 ....

21 ::::::::::::

2 2

2i . 1

1

' 4

1 3

10
8
3

!"'"'

2| 1 ...L...L..

5

1

21

1
1

1

3

1

Totals or means, 2.01 .... 2

2

17

24

101

92

211 594

584 ! 43

8

2

1680

1.73

Table 23. — Classification of generation 8, minus series.

Grade of parents.

-

Grade of offspring.

0

H

to

i

d
0

03
V
U
M

m

0 §

1

1

U

11 13

1

2

2i

2h

21

n

1
38
12

1

1

9

550

118

19

11

8

423

187

20

17

15

1

1

21

32

3

19

1202

396

45

30

28

6

1.84
1.81
1.86
1.87
1.87
1.93
1.67

.03
.19
.26
.38
.50
.57
.95

2

6

8
3

22

7

1

122

31

2

1

9
2

3
3

2i

1

2i

2|

2h

3 i 6
3 1 2

4

2|

Totals or means, 2.05

1

6

11

52

30 162 1 715

671

61

11

6

1726

1.80

.25

Tabi.^ 24. — Classification of generation 9, minus series.

Grade of parents.

Grade of offspring.

Totals.

Means.

Regres-
sion.

-h

3

4

1

H

u

If

2 2J

2^ 2f

2 .

13

1

4
1

1
1

36

268 420

55
40
12
12
14

811
403

1.90
1 93
1.93

1.91
2.07
2.00

.10
.19
.32
.46
.43
.62

2|

25

7

110
43

218

77
89

7
6

1

2i

1

1

148

2f

1

1

7 61

i

4!.... 175

5 2 53

....!.... 1

i

2h

2

6 23

2f

1

1 Totals or means, 2.11. .

1

l|l6

7 j 77 1 488 828 ' 133

! !

32 8 ; 1,591

1.92

.19

42

PIEBALD RATS AND SELECTION.

Table 25. — Classification of generation 10, minus series.

Grade of parents.

Grade of offspring.

Totals.

Means.

Regre.s-
sion.

-1

U

n

If

2

2i

2h

2f

3 31

1

2

1
2

2
1

13

15

5

4

120

100

45

16

9

5

8

2

287
251
92
60
27
17
10
3

43

78
58

7
22
13

2
4
4
3
2

2

1

1

473

474

217

119

67

49

27

20

1.96
2.00
2.05
2 05
2.13
2.15
1.95
2.19

.04
.12
.20
.32
.37
.47
.80
.68

2i

2i

2i

. . . .

1

29 I 6

,

2i

19

17

6

6

10
8
1
5

21

1

1
1

1

. .. .

21

1
1

2i

2

Totals or means, 2.18. .

3

6 40

I

305

747

256

72 17

3

1
2 1,451

2 01

.17

Tabijj 26. — Classification of generation 11, minus series.

Grade of parents.

Grade of offspring.

o
H

a

1

CO

M

o

-1

U

n

If 2 2i

i

21

2|

3

3§

2

7

20

49

24

i

3

4

2

26

75

111

112

32

18

9

2

14
45
76
66
21
15
4
1

3

26
50
33
16
11

3

12

13

20

7

4

1

1

6
4

51

2.08
2.15
2 13
2.16
2 21
2.20
2.23
1.95

.08
-.03
.12
.21
.29
.42
49
.92

2i

4

12

4

1

1

1

183
318
?fi8

21

1

2f

2 1

2

2h

81

52

?6

21

1

21

1

2

1

2|

5

Totals or means, 2.30

1

3

2

22

113

385

242 142

57

12

5

1
1 084

2.15

.15

Table 27.-

■Classification of generation 12,

minus series

•

Grade of parents.

Grade of offspring.

Totals.

Means.

Regres-
sion.

1

n

If

2

2|

21

2f

3

3i

3^

2

2

6

16

21

12

5

7

65

67

116

81

38

8

7

5

1

26

45

64

75

36

7

9

5

10

1.98
2.14
2.15
2.11
2.24
2 32
2.45
2.51
2.65

.02
-.02
.10
.26
.20
.30
.30
.36
.35

2i

1

2
1

1

15

29
24
53
37
14
14
5

4

6
3
17
12
10
15
16

1

118
166
230

2i

1

2f

1

3

8
3
7
5

2|

242

137

42

53

21

1 ....

2f

2i

1

3

2 1

35

Totals or means, 2.44. .

1

5 63

394

268

191

83

27

1
3 2

1.037

2.23

.21

TABLES.

43

Table 28. — Classification of generation 13, minus series.

Grade of offspring.

Grade of parents.

Totals.

Means.

Regres-
sion.

If

2

2i

21

2f

3 31 3|

2J

3

30

12

17

4

1

.,..'....

67

2.22

.03

21

4

40

50

46

24

5

iL...

170

2.35

.02

21

4

32

34

47

26

8

2j 1

154

2.40

.10

2f

2

11 j 21

41

28

3

2 |....

108

2.47

.15

21

1

5

2

9

7

6

1 ....

31

2.56

.19

21

4

7

8
5

6
5

1
1

i

26
15

2 43
2 50

.44
.50

3

3 i 1

Totals or means, 2.50

14

125 127

173

100

25 6 1

571

2.39

.11

.....

Table 29. — Summary of the results of thirteen generations of minus selection, based on

Tables 16-28.

Genera-
tion.

i 1

No. of
offspring.

Mean,
parents.

Mean,
offspring.

Standard

deviation,

parents.

Standard
deviation,
offspring.

Correla-
tion,
parents-
offspring.

Absolute

regression

of offspring

on parents.

Advance

of
parents.

Advance

of
offspring.

1...
2...

55
132

1.45
1.41

1.00
1.07

.208
.342

.515
.493

.46
.34

-.033

-.05

.07

3...

195

1.56

1.18

.196

.484

.206

.38

.15

.11

4...

329

1.69

1.28

.190

.460

.020

.41

.13

.10

5. . .

701

1.73

1.41

.233

.500

.184

.32

.04

.13

6...

1,252

1.86

1.56

.185

.438

.164

.30

.13

.15

7...

1,680

2.01

1.73

.132

.352

.143

.28

.15

.17

8...

1,726

2.05

1.80

.107

.283

.094

.23

.04

.07

9...

1,591

2.11

1.92

.184

.285

.059

.19

.06

.12

10...

1,451

2.18

2.01

.255

.242

.158

.17

.07

.09

11...

984

2.30

2.15

.229

.349

.081

.15

.12

.14

12...

1,037

2.44

2.23

.310

.372

.406

.07

.14

.08

13...
Total

571

2.50

2.39

.177

.317

.235

•"

.06

.16

11,704

! i i 1

■ 1 i i 1 :

44

PIEBALD RATS AND SELECTION.

Table 30.— Mean grade and number of offspring produced by parents of a particular grade in
each generation of the minus selection series, based on Tables 16-28. The grade of the
parents is indicated at the head of each column. In the body of the table is recorded the
mean grade of the offspring {in light-faced figures) and the number of offspring {in heavy-
faced figures) .

Generation.

Grade of parents; below, grade and number of their offspring.

Total
number
of off-
spring.

li

1^

U

If

11

11

2 2|

2i

2| 21

2f 2i

2| 3

1

1.34

8

1.17

1.05
20

.85

31

1.45

5

1.04

9R

1.37

6

1.11

37
1.31

28
1.31

59
1.25

53
1 34

1.05

in

"

55

132

195

329

701

1.252

1,680

1,726

1.591

1.451

984

1.037

£71

2

0.67

3

1.22

48
1.36

40
1.35

54
1.46

94

1.55

5

1.09

27
1.26

63
1.34

93

1.30

262

1.49

244

1.67

32

1.10

i

3

12

1.96

I

4

3

1.18

1.561 1.16

1.36

5

4

29

1.50
51

95 9
1.64 1.52

1.80:

6

143 109 5

1.59, 1.58 1.52

502 283 85

1.65 1.72 1.74

1.82 ;

7

24

11

1.88

1.93

1.62

2.28' 1.87

8

330 969 260 41

4 15 18

6

1.84 1.81 1.86

1.87

36

1 93

1.87

30

1.91

1.92 1.67
17 6

2.07

9

19 1170

377

1 93

1.90

10

811 403 148

1.96; 2.00 2.04

478 474 217

2.081 2.15 2.13

175 53

1

2.05

2 13

\""

2.15 1.95

2.18
20

1.95
5

2.51
53

2.43
26

2.65
35

2.50
15

11

119 67 49 27
2.16 2.21 2.20 2.26

12

51, 183 313
I.QSJ 2.14 2.15

268 81
2.11 2.24

52 26
2.32 2.45

13

10 118

166

230 242 137 42

2. .35 2.40 2.47 2.55

\

67

170

154

108 31

11,704

Table 31. — Results of a first return selection from generation 6, minus series.

Grade of parents.

Grade of offspring.

Totals.

Means.

Regres-
sion.

0

1

-f-i-U-ii-if-2

i 1 ' : !

_|

2

1

4

1

3 4 j 8 ] 8 ... 26
10 > 14 17 i 20 i 4 i 7fi

1.08
1.33
1.30
1.41

-.60
-.71
-.65
-.63

-1

2

3

' 1
1 ...1 2 1 1

5
2 11

-1

2

2

1 4

Totals or means, .GO

2

4

6 5 16 { 18

1
28 33 6

118

1.28

-.68

TABLES. 45

Table 32. — Results of a second return selection from generation 6, minus series.

Grade of parents.

Grade of offspring.

Totals.

Means.

Regres-

0

1

2

_3

-1

-n-n

sion.

1

1

3

4

5 2 4

19 0."; I AK

" 1

Table 33. — Results of a third return selection from generation 6, minus series.

Grade of parents.

Grade of offspring.

Totals.

Means.

i

Regres-
sion.

0

-i

1

— '2

1

-f-l-lij-U

-lf-2

1

5
?,

2
3

2 3

5 4

.... 1

1

2

. . . . 1
3

13
21
13
31

.63

86

1.61

1 35

j -.38

' -.49

-.49

1 -.10

3

7

-1|

2
9

3

8

4
5

3
2

-U

1

2 4

Totals or means, .83 ... .

2 7

C

1

0 ; 12 11 14

1
12 5 i 78

1.14

1 -.31

Table 34. — Results of a fourth return selection from generation 6, minus series.

Grade of parents.

Grade of offspring.

-2

Totals.

Means.

Regres-
sion.

0

_i

4

_i

2

3
4

-1

-u

-H

-If

1

1
1 1

2
1
1

2

1
1
1

1

2

2
2

4

2

3 2

16
5
3

10
10

1-34

.70

.83

.83

1.17

1.42

-1.09

- .20

- .21

- .08

- .30

- .42

-1

1

5

1
2
2
1

3

1

1
2
3

-I

3

1
1

-1

Totals or means, .63

3 1 2 6 4

7

10

8

6 4 50

1.17 - 54

[

Table 35. — Results of a fifth return selection from generation 6, minus series.

Grade of parents.

Grade of offspring.

-H

Totals.

Means.

Regres-
sion.

0

4

1

— 2

3
■~4

-n

-1

2

2

3

4
4
5

1.00

1.12

.25

-.75
-.37
+.62

_3

1

7

— -o

2

2

1

Totals or means, .65

3 2

,

2

2

3

13

.75

-.10

46 PIEBALD RATS AND SELECTION.

Table 36. — Results of a sixth return selection from generation 6, minus series.

Grade of parents.

Grade of offspring.

1 .

1 "

tA

.
c

(0

+U

+ 1

+f

\

0

1

-H

-u

-n^i\

Mean
Regrc

-1

1

1

3

1

2
4

1

1

....| 2
1 i 4

2

!H

... '13

.29
.35
.87
.25

-.17
-.10
-.50
+.25

_i

2

1

-f J ..

....i 2\ 1
1 2 1 1

1

1

1 6 :
1
s

-i

1

I

Totals or means, .26 . .

i

1

1

1 1 2 1 1 j 4 1 7

2 5

8

2

1

1 36 i

1

.39

-.13

Table 37. — Results of a return selection from generation 7, minus series.

Grade of parents.

Grade of offspring.

Totals.

Means.

Regres-
sion.

2

3

-1 -\\

-H -If

-2

-1

1

2

3
2

3

7 1 8

3

25

1
t

-1

2

(
1 2

! 8

Totals or means, .78

1

1

2 3 ! 5 3 7 10

3 33 1 1.15 -.37

Table 38. — Results of a return selection from generation 8, 7ninus series.

Grade of parents.

Grade of offspring.

1

Means.

Regres-
sion.

-f -1 -U -H

1

1

i

1 lotals.

-2|

-1

1

3

9

3 1 13
3 i 13
2 ■ 2
2 1 13

1.69
1.21
2.00
1.56

-1.19

- .59

- .13

- .56

-f

1

7

-1

1

2

4 4 i

Totals or means, .72

■i ^

2

7

1
19

10 41

1.51

- .79

TABLES. 47

Table 39. — Results of a return selection from generation 11, minus series.

Grade of parents.

Grade of offspring.

T^^+r.lrt

T^T.

Reeres-

1

4

-If

-2

-2i

-21

lOtalS. ivieaiis. •"

1 sion.

-If

1

3 7

3

2

16 ! 1 OS _ 35

1

Table 40. — Results of a return'jelection from' generation 6, 'plus series.

Grade of parents.

Grade of offspring.

Totals.

Means.

Regres-

+1+1+2

2i 2| 2f

3

31 31

sion.

2..

I i 1 i

1113 3 12 12

3

11 17 1 2 36 - HB

■

1

Table 41. — Results of a return selection from generation 11, plus series.

Grade of parents.

Grade of offspring.

Totals.

1

Means.

1

Regres-
sion.

21

2§

21

3 31

3i

31

4

4i

2f

2f

3

1

2 3

9 a [ a

1

8

1

22

17

9

5

2 4 1

1 . L..

2
4
3

4 1
1 ....

'^ i ••■

3} !

.■' 1 '....!.. ..

Totals or means, 2.79

2

1 ' 1

1 t

3

1 1

4 1 10 6 12

1 1

10

5| 1

53

3.32

-.53

48

PIEBALD RATS AND SELECTION.

'a
e

o

o
o

s:

u

9

•a

o

o

c
o

o
(^

v>

c

c
o
u

•9

o

■a
s

"a
o
o

-a

<»

u
O

H

o

c

o

•**

a

V)

•'&
a

a

•90UJ

passojoun

o

•^

?;

^2 2 S

'U0l?BIA9p

p-repuB^jg

•noi'j'BiAap

05

00

O « 00

pjepn^^g

■aovi

o

O

«3

o "* 3;

e<

IC.

c

passojo
-an 'DBOfxr

1

1

1

M CO CO

+ + +

CO

S

■.**
c^

CO 1^ lO

•u-BOj^

d C^ CO

4

+ + + +

+ 1

•|B^ox

CI

CO

00

5

d cc

1-1

K!1i

»-l CO

CO

*

rHiN

9M

i-t CO CO

cc

rfW

W CO M

CO

CO

-^ o> -^

wh*

1>H

1^

CO CD CO

<N

HIN

•*

w ^

c

M

r<I-#

v-t

»]

CO ^^

(M

C<l

C4

m

lO F-« 1— t

M

n\-f

-*

IH

ec

»-l CI

T3

»— 4

f5

HIM

CO

■*

-J

»-H

T-H

rtl*

lO

«

«-4

oded

1-4

1—1

^^

■<JI

00

^

nl-«

CO

o

t~

fH]C4

w

■^

t3

W

r-|-»

M

C^l

o

«

t-

•>*

—1

s

H*

to

<o

oo

1

1

uz

to

O!

■73

1

(S

f^

1

<a

eg

M

o

1

1

■<t

T^

<=

H-»

1—1
1

.H

t-H

r<lN

I— (
1

C3

»-<

M

nl-d

T-l
1

tH

M

1

-^

M

tc

w

'^ GO

<n

D

3

2

C 3

D

a

a

bfl

"E

S-a

a

TS

S

a

O •

«!■«

^

LCI

-o i2

^

^

(N

CO

o

c

h;n

cc

1 °

I— (

(N

1 c

'

a

C

c

a.

, bO

So?,

d

^"^

o

bC

S "

bC

bC

"■^ c

bO

rt -

^ rt

c

>^

c3 tJ

>-> to

O

: 1 T

!7"^

CO t*^ CO ^ ^ .. w

< _L. .2i _L HN 2 «i;; «
. -r n T" CO n ■* "E

1 <^

' o.- .^

;oHi=

iof ^o^+ ^ + ^

Oi-

Oi-

c+

O Oi- 0^

o

00

-a
e

a

"a

<a
O

o
e

0^

CO
09

o

-^

e
o

a
c

-a

s

o
<»
oa

«)

e
a

o

a
a
■a

-a

■a
"a
o
o
-«;

-a
a

H
%i

8

o

a

CO

m

•ao'Bj
passojoun

UOI^'BIAap

pj'Bpii'B^^g

•UOI^^'BTAap

pa^puB^g

•ao'Bi
passojDim
'aoi^'BjauaS
ptre apxjjg
auiT;s'uuaj,\[

a

a
a
u<

bC

o
-a
o
o
ja

T3

<u
-u
o
c3
t-i

X

o

o

<o

'T3
c3
I-,

o

•aT3aj\[

"F^ox

CO

+

CO

+

+

(M
+

+

+

+

+

+

+

+

+

+

N

CO

c3

O &,

^^ a

QO CO U9 to CO
■* '•< CO t~ US

I I I + +

I I I + +

CO fl CO «0 CO

O 1-t CO CO f-(

OS 00 -^ -^

oa -^ ■* "3

C4 IC CO CI CI

CO -^ ^

CO CO CJ

to CO CO

t^" ^ ?r ^ CO

(3 c • d •
53 Qj c 4) c

fcn bC <a bC <y
^ , bC „ bO

»<« ^+» .^ hN" -
i-l rH (M (M CO

+ +

TABLES.

49

Table 44. — F3 generation produced by extracted hooded rats (F2 generation) derived from a
cross of a hooded rat of the minus series with a wild rat. {See Table 42.)

Grade of (F2) parents.

Grade of (F3) offspring.

Means.

d
'to

CO

tt

Ph

.71
.44

+1

3

4

1

U

n

If

2 2i 21 2f

3

3J

3^

f

2 1 ! 4 3 14

...J....' 1

1

1

2
1

1
3

I

1

1 ...
1 1

19
34

+ .04
+2.06

2*

2 9.

1 al a . R

■■■,■■■

4 i 3

1

1 i

Table 45. — Ft generation from minus series crossed with wild.

Grade of (Fs) parents.

Grade of (F4) offspring.

CO

r^

0

'co
M

1

0

+i

1

3

4

1

n

1^

n

2

21 2§ 2f

3

31 3^

'3
0
H

2i

2

1

2

1

1

1
2
1

3
1

1

6
6

1

3

6
3

1

9
8
1
3

1

16
3
1
5
1

25
9
1
9
3

14

10
4

2

4

1
1

96
44
8
39
18

1.78 .47

1.67 .70

91 1 nn

2f

—

1

....

2|

1

3

■
5

1

7
2

3
5

2.02
9. R9.

.98
.30

3|

Totals or means, 2.52.

1

1

1

3

3

1

5

5

16

10

22

26

47

20

23

14

7 I 1 i 2 ; 205; 1.86 .66

1 1 1 1

Table 46. — Fs generation from minus series crossed tviih wild.

Grade of (F^) parents.

Grade of (Fj) offspring.

0

CO

d
0

CO

0

0

+h

+1

1 n n

If

2

2i

2h

2f

3

3J

2

2i

]

1

3

1

3

2
1
1

12

1
3

1

15
4
4

2

10

1
1

3

2

—

2

2

3
4

2
4

1

1

53
13
13
14
7
5
14

2\

^*'

....

9

. . . .

2f

1

2

1

2

1

21-

2

1

1

1

1

7

3 .

1

^^•■1 '

4

_

Totals or means, 2.27

2

1

3

6

3

6

17

28

16

8 21

1

7

1

119

2,06

21

50

PIEBALD RATS AND SELECTION.

Table 47. — F^ generation from minus series crossed with wild.

Grade of (Fs) parents.

Grade of (Fe) offspring.

-(J

o

.2

o

;-!

P5

+1

i 1

li 14

If 2

2J

21

1

2i! 3

?'l

34

3i

2'

1 1

1

3

1

4
2
2

4

1

8
2
1
2

1

13
4
2
5
4
4

4 4
3 3
3!....

7
5
3

8

3
4
4
2

2

48
26
20
31
40
29

2.14

2.24
2.26
2.35
2.74
2.73

.23
.26
.36
.40
.13
.39

•^8

2i

21

1,
1

!

1
5
5
2

1

....

*
21

1
2

3
9
4

2f

7 6

2
1

2

3i

Totals or means, 2.G9. .

i 1

2

1 2

6 13

14

32 13 i 23

1

1
37 1 30

1

15

4

2

194

2.41

.28

Table 48. — F^ generation from minics series crossed with wild.

Grade of (F7) ofTspring.

Grade of (Fc) parents.

!+f

21 .
21 .

3L

+1 14

1^

0 91 i 91 93 I q qi I 01

3f

m

-t->
o

— I

12

2

9

6

5 \

4

1

2;

3

1

3

4

3

3

Ij

3

1

2

12 ! 1

3L..

3 ; 1

5 1 2

13
2 2

53
28
10
12
27
14
10

1 o !

Totals or means, 2.80 3 2 ' 8 j 9 . 26 12 j 28

23 I 26 ^ 10 I 6

1 154

a

o

2.41
2.14
2.32
2.93
2.59
2 63
2.85

.21
.61
.55
.07
.53
.62
.52

2.46 .34

Table 49. — F^ generation from minus series crossed with wild.

Grade of (F7) parents.

Grade ot (Fs) offspring.

Means.

Regres-
sion.

+1| 2

1

2i 2||2f

1

3

31

34

AUtcli6.

31

2|

1

.... 1

i 2

2 75
2.74
2.60

0
.01
.52

3

11 3

5

1

1 24
19

3i

12 3 3 5 3

Totals or means, 3 08

2 3 6 9 13 4 6 1

1 45

5.07

.41

TABLES.

51

so

v>
S

CO

fa-

c

CO

CO

o
J-
o

0^

O

•UOIC^■BlA^p

pj'bpu's^g

t-; CO CO <3> O O C- O 0> OO O

1.01
.60

to
0

•SUT38I,\[

§ Lo S ^ S 2 g !3 8 S ^ ^
+ 1 I+ + + + + 4- + +

rM O ^^ O Ol CO <D
C»» O CO -^ 00 o »o

+ + + + + + +

-t-

•sjB^ox

CO coccoo»-Hooooeococo«

«i» HJt CO CO t* CI «0
O « rH CT M

CO

CO
+

TH

-

raw
+

1-^

rH

rH :

Cv)

+

> 1-4

• <D

l>-

1-1

-

+

C<l

■ CO

lO rH

rH

1— t

C>1

+

TH

. t-1

^. (M rH rH

t~ '.

rH

i -

CI

+

: IH rH CO _

; CO rH CO :

CO wt
rH

rH •*

•.-5

+

-

.H W CO CO

rH •* M •^h :

O r.

rH ^

rH

CO

to
a

1— (

+

CO

iH rH

.—1

rH T}<

iM :

S <N

^ rr

»o

Grade of offspri

1— t

+

<o

ro M 5D .H e^ u5

■*

S '»- rH

rH e« CJ

®

+

t-1

rHCOO<»H?5ir5COCO •

Cd -^i CO

o>

+

00 tHt-(i-ItHC^i-H •00»-llO •

■* rH rH CO

CO 10

<M

+

C^l »-( -f-ti-li-l .*M<i34 '^H I

O rH

CO

rH ■*

QO

o
I

OS •

CO « W rH

00
CO

1— t

O rH 1

rH

rH rH

1

^

N

rH rH

■*

1

00 •CO'Tl«C0CO*-<C^Jt^»-t

S

r-t

1

1-1 ■^ 1-1 *-* CO !>• T-»

u5 :

a

C4

M

1

M

M

^H C^ tH

rH

t>-

rt|C«

1— 1

1

;

i-H 1-1 T-* Cq T-4

rH rH

rH

Cb

-

I— 1
1

N 1-t

rH eq

rH

1

;

rH

t-H

J

m

o

o

6

Series 1, Fj generation; 9 9 —2, gen. 6;
d'c? +34 or 31, sen. 5

T-H

1

a

0)

rH

tn

o:

'tH

ID

CO

:

Hie
1

•

d

Hioe
+

+

+

+

• rH|0C
1-H rH

+ +

1-H
+

r<:N

rH
+

Series 1, totals or means, F2 generation. . .

Series 2, F] generation; 9 9 +3|, gen.
10; cf -3L gen. 10

0"
.2

'■*3

(H

oQ
Ph

ci

m

<D

'S
0

02

•
+

rH
+

rllOO

+

Hi-*
1-1

+

C
0

c

c

cS

6

S-.

0
re

0
•t->

(N

CD

IV

52

PIEBALD RATS AND SELECTION.

Table 51. — Fi offspring of the original "tnutanl" viale, 4763,-^6^, mated with females of the
plus series, and with the "mutant^' feynale, 5153,-\-5\.

Grade of mother.

Grade of offspring.

Lov?er group.

Upper group.

+3

3i

31

31

4

4i

41

o

Standard
deviation.

5

5i

5§

5f

6

Standard
deviation.

+31

ii

4
3
4

11
7
9
3
1

1
3
4

1

1

....

18

13

20

5

2

3.69
3.75
3.78
3.90
3.62

1

. .. 1

3
4

7
2

9 ! 4

5 1 ^

.

18
13

18

7

5.51
5.44

5.40
5 46

31

4

4i

.... 1

1
1

1

2

5
4

4
1

A\

1

Totals or means. . . .
Mutant 9 5153, +5|.

1

1

12 31

9

3 1

58

3.73

.24

4 16

23

12

1

56

5.45 .23

3

3

6

3.87

6

4!

10

1
5 60 '

1

Table 52. — Fi offspring of the original mutant male, 4763, -\-5\, viated with females

of the minus series.

Grade of

Grade of offspring, lower group.

mother.

_i _a_i_ii n '4-1

+\

+1-

fl

+u

+U

+lf-

f 2J Totals.

i i

INIeans.

Standard
deviation.

-2 1

1

!

1 ....
....■ 1

3
1
2
1

1
2

2

1
2

1 .

2

1

1
1

1
1 .

1 13 1

+ .40
+ .70
+ .31
+ .18

-2\

1

2

3

-2h

1

4

4

-2|

.... 1

Totals or means

■

2

1

l| 2

7

3

5

4

2

3

2

2 j 1 35

+ .49

.77

Grade of mother.

Grade of offspring, upper group.

1 (
+4+4i+4^l+4^

! i

+5

Totals.

Means.

Standard
deviation.

-2

• 2

e 1 1 9

2

12

9
6

4

4.46
4.53
4.21
4 19

-21

2

1 Q

-2\

>>

1

!

-2|

0

\""

Totals or meant

3

' A ! 1* ' * ' '

31

; 1

■l

i "

1 "

! "

1 2

4.43 "' '

iV

TABLES.

53

Table 53. — Classification of the descendants through three generations of the two original
mutants, &J!i763,-\-5^, and 9 5153, -]r5\. The -parents are in every case of grade 6^ or 5j.

Generation.

Grade of

offspring.

Lower group.

Upp

er group.

3i

3^

3f

4

O

CO

Standard
deviation.

5

5i

5^

O

1

a

Standard
deviation.

Fi

3

3

6
2

3.87
3.37

6

15

7

1
4 1 10

10 1 30

3 11

5.60

Fo

1

1

2

3

1

5.52

5.55

F,

Totals or means

1

1

3

3

8

3.75

.25

2

4

28

17 51

5.54 .19

Table 54. — F2 descendants of the original mutant male, 4763, +5^, mated with females of the

phis series. {Compare Table 51.)

Grade of (F2) offspring.

Grade of Fi parents.

Lower group.

Upper group.

3

3i

3^

31

4

4i

41

m
-a
0

H

<u

u 0

02 T)

5

H

H

H

03

CO

Standard
deviation.

Lower group
parents . . .

'3|....

03

2 '

10
2
2

8
2
6

1
7
8

22
15
22

3.56
3.92
3.92

1

2
4

1

1

31

1

2

49
5

13

2

5.50

Totals or mea

Upper group
parents. . .

ns . . . .

2

3

14
1

16

16

6

2

59

1

10

3.78
3 50

.33

51....

1

1

3

1

4

....

3.90

2

6
2

70 j 5.51 1

51....

!

7 I 5.43

ms . . . .

1

'■

Totals or me?

.... li 2
i

3 1

1

4 .... 11

1

1

3 86 .35

i

2

8

56

13

79 5.50

.15

54

PIEBALD RATS AND SELECTION.

01

^.2

t3

-^

J'?

fi

CO

§

^^

t^

: o

^

■S

^

1

fO

>

fc« C<l CO 00 »-H t^

^

; t*

r

i

P

OO CD U5 t^ a> t-

w

T3

^

f-i

I

h

e

P,

c3

a

S

iH

s

h-?

: +

>
3

o

1

u

C-1

I

p

j U5 O -.»< »H O) 1 OO

K<*

02

—I •*

CO

+

00

e8
o
H

o

h4

CO rH <M C£> C

^

CO

^

m

H-

K

a 00

r>.

o

<M O <M CT

00

to

c3 «

u:

CC

t^ O '^ .-

ta

a

1 ^^

i «

• t^

1

^-i

,

■HlV

»-l rH

■Tt* CC

o>

o

*^

Ui

lO

c

O^

=3 «

»-H

3

O

.S

O

CI

CO

"3

• rH rH 00 e<l

9~*

&

s

rH rH rH CD r-l

o

rH

?^

1

a

'>-.

-li-

»-

*-H

ce 1

rtiN

rH CO • O —

CO

c«

•*-fc

1

■*

• rH

3

.:.

CI

CO

ra

rH rH C« Hll rH

o>

•^

oi

c

^

a

'i*

c*

n

e

R

Oi

1— 1

f-4

-^JM

»-^

rH

a

2

bO

CO

+

(N

w

CO

la

c^

*H ^H rH

CO

eo

nW

T-1

N

"W

i-H

■to

T— 1

b

i-H

o

u

•-l|N M

T-a

*-H

■»«<

H|«

T— H

^

rH

'-«*

o

u

-<!■•

rH

(M

• I— (

■*

.HNi

s

1—1

•

^

i—l

b)D

.s

'u

o

,-H

»-t

Tf< T-H tH d

o>

1
to

T-H

w

Ol

>

1

n|*

• «-l

«-H

(M

nl^

^

C4

"«<

s

•

o

fa

o

e

•-i[N '

»-i

*-t

C4

HN

j

+

+ :

j \

1—1

1-1

r<l-«
+

i

s

fa

o -^

CO

T-i

CO

^H

a>

a>
c3

o

o

1-1

f-H

o

1

-«

I-H|^

ihHi

u

••^

t4-l

r '"'

•*

•5>

o

CI

O

1

1

•c

Q.

I-liM

Hn

CO

•**

c

1 :

r-^

rH

*H

1

rH

r-i

la
o

1 i

»^

»-t

1

a.
3

2

rH

1— H

M

1 :

»-(

«-*

f— I i-t

■*

1

60

1

O

C4

tH

^1

I— 1

1-1

rH

k,

1 :

1

t

1 :

*-l 1-1

«

1

i

<

c

fa .

■73 t<

Q

c3

a

o

t-i

o

rt rt

03

CD

It Wlo

5 r-tl-a

3

P r-.'^

» 1— i

c3

-3

o

1

1

1

d + + -f

Hoo rtl'* c5|<r Hc« i.*:qc

North Carolina State Library

EXPLANATION OF PLATES.

Plate 1.

Top row, a set of arbitrary grades used in the classification of the rats studied.

Middle and bottom rows, skins of rats graded as indicated by the numerals above each

skin. The animals graded, +4, +4| and +4f , being entirely dark above, are shown

in ventral view.

Plate 2.

cf 8000, a wild gray rat caught in Cambridge, Mass., October, 1908.

96176, a black hooded rat of grade —If which was mated with cfSOOO.

(^8021 and 6^8018, Fi offspring of the pair just described, cfSOOO and 9 6176. c:f^8021 is

of grade +4|, with considerable white below. Notice also liis white legs and compare

with those of his father. c?8018 is of grade +5^. Notice white areas on belly and

front legs.
8075-8078, four Fo progeny (grandchildren) of cfSOOO and 96176. 8075 is black hooded,

grade -f ; 8076 is gray, +4 J; 8077 is gray, +4; 8078 is black, +5^.

Plate 3.

cflOS, an evenly marked rat of grade +3. A female rat of this same grade was mated

with c?8000 (the wild gray male). Two litters of F2 grandchildren are shown in

8062-8067, and 8070-8074.
8062 is black hooded, +2; 8064 is gray hooded, +2; 8065 is black, +5; 8066 is gray,

+5j; 8067 is gray, +4:^ (notice white feet).
8070 is black hooded, +2; 8071 is gray hooded, +lf; 8072 is gray, +5; 8073 is black,

+5§; 8074 is gray, +5f.

56

PLATE 1

— 1

+ 1

+ 3

+ 4-

PLATE 2

$ 8000

ri'^.

J" , /^'^

$ 6176

i^

5 8021 F^

8078

2

QQll

Jt

5 801 8 F,

J^., ^ gg||^_^

8076

8075

A(

PLATE 3

3 103 -f 3

806'7 8066

8073 8074-

V\

V

pp^^

.•n« state university Libraries
»4orthCaro\.na State