CASTLE & FORBES

HAIR-LENGTH

IN

&UINEA-PIGS

CASTLE

POLTDACTYLOUS RACE

OK

GUINEA-PIGS

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5. Heredity of Hair-Length in Guinea-Pigs

AND ITS Bearing on the Theory

OF Pure Gametes

BY

W. E. CASTLE AND ALEXANDER FORBES

6. The Origin of a Polydactylous Race
OF Guinea-Pigs

BY

W. E. CASTLE

Published by the Carnegie Institution of Washington

May, 1906

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Carnegie Institution of Washington

Publication no. 49

4 SI

Press of
The New Eia* PfliNTi<io COhpaht

UM CAS TEH, P*.

5. HEREDITY OF HAIR-LENGTH IN GUINEA-PIGS

AND ITS BEARING ON THE THEORY

OF PURE GAMETES

\V. E. CASTLE AND ALEXANDER FORBES

Papers of the Station for Experimental Evolution at Cold Spring Harbor,

New York, No. 5

Contributions from the Zoological Laboratory of the Museum of Comparative
Zoology at Harvard College. E. L. Mark, Director. No. 175

5. HEREDITY OF HAIR-LENGTH IN GUINEA-PIGS, AND ITS
BEARING ON THE THEORY OF PURE GAMETES.

By W. E. Castle and Alexander Forbes.

1. INTRODUCTION.

In earlier papers (Castle, :o3, :o5) it has been shown by one of us that
long or " Angora " coat in guinea-pigs and rabbits is alternative in heredity
to normal or short coat. It has been shown further that in cross-breeding
normal or short coat dominates over long coat, in conformity with Mendel's
law of heredity, but that the purity of the gametes formed by cross-breds
is not absolute. Impurity of the gametes is indicated by two facts: First,
the number of long-haired young produced by cross-bred parents is in
excess of the Mendelian proportion, one-fourth ; secondly, many of these
long-haired young show an imperfect development of the long-haired char-
acter, as compared with their long-haired ancestors. Both these facts may
be explained by supposing that the alternative characters, short and long
hair, which are present in the cross-breds — one seen, the other unseen — have
in many cases failed to segregate, or have segregated only imperfectly, when
gametes have been produced by the cross-breds. Accordingly the con-
formity with Mendel's law is a qualified one.

More extended and detailed studies made by us during the past year
confirm these conclusions and add several new facts concerning the behavior
in heredity of these alternative characters. The idea advanced tentatively
by Castle (:05), that the hair-lengths of guinea-pigs form a discontinuous
series of two, three, four or more times the length of short or normal hair,
is found to be incorrect. Careful examination of the hair of several hundred
guinea-pigs (mostly cross-breds) shows that there occur hairs of practically
all lengths from 3.3 cm. up to about 23 cm. The series of supposed maximal
hair-lengths of twice 4 cm., thrice 4 cm., etc., resulted from an insufficient
number of observations.

HEREDITY OF HAIR-LENGTH IN GUINEA-PIGS.

M--

2. CHARACTER OF FOLLICLE ACTIVITY IN THE PRODUCTION OF SHORT

AND OF LONG HAIR.

If an examination is made of the longest hairs plucked from the back
of an adult short-haired guinea-pig, it is found that the hairs narrow toward
the base, owing to a diminution in the diameter of the medulla, which is
entirely wanting in the follicle (see Fig. i, A). Such a hair has ceased to

grow, having completed a definite growth cycle,
and will sooner or later be shed. This growth
cycle was nearing its end when the follicle ceased
to form medullary substance. In ordinary
guinea-pigs the hair stops growing when it has
reached a length of about 4 cm. In long-haired
guinea-pigs the hair-follicle does not cease form-
ing medullary substance when a definite hair-
length has been attained. The growth of the
hair is indefinite, ending only with the degenera-
tion of the follicle itself. The time when this
occurs is determined to some extent by the physi-
cal condition of the animal. The lifetime of a
hair-follicle of this sort may be prolonged by
good care, as fanciers well know. Pregnant or
nursing mother guinea-pigs frequently lose their
longest hair. Insufficient or improper food is
likely to have a similar result in either sex. Ac-
cordingly, the fancier gives careful attention to
the diet of long-haired animals intended for ex-
hibition, and often protects the hair in special
ways from mechanical injury. There is, how-
ever, no reason to suppose that such care induces
indefinite activity of the hair-follicle. On the
contrary, animals of a short-haired race, under
the best of care, will form only hairs of deter-
minate length, whereas animals of a long-haired
race, however much abused, will, so long as they
live, continue to form hairs of indeterminate
growth. The two types of hair-growth are quite distinct, and are sharply
alternative in heredity. They are probably paralleled in our own head- and
body-hairs respectively, the fonner being of indeterminate, the latter of
determinate growth.

FlG- I. — A, Base of hair, fully
grown, of determinate growth type.

B, Base of hair of indeterminate
growth. From camera drawings, same
magnification.

C, Cortex ; /s follicle ; Af, me-
dulla.

HEREDITY OF HAIR-LENGTH IN GUINEA-PIGS. 7

Like ourselves, long-haired guinea-pigs bear hairs of both sorts, whereas
short-haired guinea-pigs, like our probable simian ancestors, bear only hairs
of determinate growth. Similar alternative conditions of hair-growth occur
in many different kinds of mammals, as for example in rabbits, goats, cats
and horses, the indefinite growth in the latter case occurring in the hairs
of the mane and tail only (see Davenport, 104) . The long-haired (or indefi-
nite growth) condition is doubtless coenogenetic in all cases, and has probably
arisen independently in each case as a discontinuous variation or mutation.
By selection the long-haired condition is easily made a racial character,
for long-haired animals produce only long-haired offspring. But without
selection, what would be the fate of this mutation? This our breeding
experiments may perhaps indicate.

Let us consider first the extreme conditions of follicle activity which
we have encountered in the guinea-pigs studied. In long-haired guinea-pigs
the hairs all over the body attain a greater length than in short-haired ones,
but only hairs of the back and sides grow indefinitely. Accordingly, in com-
parative studies of the hair-length, it has been found convenient to select a
few of the longest hairs to be found on the back or rump of the animal and
to use these as a standard of comparison. Following this method the hair
of guinea-pigs of different races has been measured at frequent intervals
from birth to an age at which the hair had attained full growth.

A litter of four guinea-pigs of a pure short-haired race yielded measure-
ments which combined are expressed graphically in Fig. 2, A. Starting at
birth with a length of about 18 mm., the hairs grew very rapidly during the
first week, at the end of which they measured about 25 mm., an average
increase of a millimeter a day; in the second and third weeks they grew less
rapidly, measuring about 29 mm. at the end of two weeks, and 33 mm. at the
end of three weeks. At this time the growth of the hairs which the animals
bore at birth was practically complete ; the hair- follicles now ceased to form
medullary substance, and consequently the hair narrowed to a base of solid
cortex (Fig. i). From this time on the measurements show from week
to week only slight deviations from a length of 33 to 35 mm.

Several series of measurements of the hair of long-haired guinea-pigs,
when combined and expressed graphically, are shown in D, Fig. 2. The
hair-length at birth is about the same as in animals of class A, or even a little
shorter, but the growth rate does not show the series of rapid changes seen
in class A. Growth progresses very steadily at an average rate of about 0.83
mm. a day during the first one hundred days and at about 0.75 mm. a day
•during the second one hundred days. But a glance at Fig. 2, D, shows
that the slowing up of the growth rate is a very gradual process.

After the hairs have attained a length of about 170 mm. they begin to fall
out, but without previously narrowing to a base, as in class A. Rarely does a

HEREDITY OF HAIR-LENGTH IN GUINEA-PIGS.

hair attain a length of 200 mm. As the longest hairs are shed, new ones
are found to be growing up to take their places, but there is usually a period,
after shedding sets in and before the second-growth hairs are fully devel-
oped, when the maximum coat length shows a decided falling of?. This
is indicated in D, Fig. 2, as occurring at about the age of two hundred and
and fifty days. The type of hair growth found in class D may be called
indeterminate or continuous ; that of class A, determinate.

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Fig. 2.

Our original stock of short-haired guinea-pigs showed considerable
variation in maximal hair-lengths, ranging from 35 to 50 mm., but all of
the determinate type of growth (classes A and B, Fig. 2). The great
majority of them would probably have fallen in class B, but we can not be
sure of this, as many of the animals were no longer available for record
when we began making hair-measurements systematically. The hair-lengths
recorded for class A are certainly exceptionally short, even for animals of
determinate hair growth.

HEREDITY OF HAIR-LENGTH IN GUINEA-PIGS.

3. THE EFFECTS OF CROSS-BREEDING BETWEEN SHORT-HAIRED AND
LONG-HAIRED ANIMALS.

(a) Mating BXD.

When the short-haired animals were mated with animals of class D,
only young of the former sort were obtained, which result shows clearly
the dominant character of short hair. However, the maximal hair-length
of the cross-breds was usually 40 to 50 mm. (class B, Fig. 2) ; that is, it fell
within the tipper half of the range of variation of the pure short-haired
stock, though in no case, we think, did it transcend the range of variation
of the short-haired stock itself. We are warranted, then, in concluding
that the dominance in the offspring of the short-haired type of growth is
complete. Type B mated with type D gives only type B.

(&) Mating B{D) XB(D).

But when cross-breds between the types B and D were bred together,
not only did type D reappear after skipping a generation, but a new and
intermediate type was found also, which we may call type C. This is not
sharply separated from types B and D, but is made up of individuals scat-
tered all the way between those parental types. It contained, first, indi-
viduals whose hair grew continuously from the age of twenty days on, but
much more slowly than does the hair of class D, with a tendency to break
off at lengths much less than those attained in class D. The long hairs of
such animals were also frequently less numerous than in typical class D,
as if part of the hairs only were continuous in growth, while the others
were determinate. In these individuals we see the alternative characters
coexisting as in a mosaic. In other cases animals placed in class C appeared
to have only hair of determinate growth, but growth continued until a
length of 60 to 80 mm. had been attained before the hair narrowed to a base.
Such animals represent an intimate blend rather than a mosaic of the con-
trasted types B and D, yet with a closer approximation to type B; but no
sharp line could be drawn between the blends and the mosaics, as the two
graded into each other and into the two parental types. These intermediates
(class C) were about as numerous as the animals of type D, but only about
half as numerous as those of type B. Thus, in a particular experiment there
were produced twenty-nine B, twelve C and ten D individuals, together with
eleven of unknown character, because they died or were disposed of before
their hair was fully grown. The grandparents of this lot were the long-
haired male, 2002 (Castle, :o5, PI. i, Fig. i), and several different short-
haired females ; the parents were all of class B. The Mendelian expectation

lO HEREDITY OF HAIR-LENGTH IN GUINEA-PIGS.

from an experiment like the foregoing is that there will be produced three
times as many animals of type B as of type D, or in a total of fifty-one
voung (the number whose character was definitely ascertained) thirty-eight
B to thirteen D. Comparing with these numbers the observed ones (twenty-
nine B and ten D), we notice that there are fewer individuals than expected
in each of the classes B and D, from which we may conclude that each has
contributed to the formation of the intermediate class C. The hypothesis
on which the IMendelian expectation rests is this, that each cross-bred animal
will form in equal numbers gametes bearing the pure B character and the
pure D character respectively. Evidently from this experiment it follows
either that some of the expected B and D gametes are not pure, or else that
in this generation the result of a union between a B and a D gamete is not
the same as in the original cross. In either case we are forced to admit
modification of gametes from their original pure condition.

In two other families of guinea-pigs, cross-breds mated inter se pro-
duced a result similar to that already described. The young in these families
were classified as nine B, five C, two D and four undetermined. These cases
differ from the foregoing in that, in producing the cross-breds used, two
crosses with the B race were made to one with the D race, whereas in pro-
ducing the first mentioned lot of cross-breds the B and D stocks were
equally represented in the ancestry. It is perhaps significant that the D
young are relatively fewer in these families, though the total is too small
to allow one to attach much importance to the proportions observed among

the young.

(c) M.\TiNG B(D) XD.

On the Mendelian hypothesis of pure gametes, this cross should yield
classes B and D in equal numbers. In matings in which three different D
males were employed, it has produced fourteen B. seventeen C and nineteen
D offspring, together with five of undetermined character. Since D parents
are known to form only D gametes, it seems clear from this experiment that
the hybrid, or B(D) parent, formed gametes a considerable number of
which bore the intermediate or C character. If a single cross of D with B
has such an effect in modifying gametes, a repetition of the cross should
have a still more marked influence, producing a still larger proportion of
intermediate or C gametes. The mating next to be described bears on
this question.

(rf) Mating B{D) [from Two Successh-e Crosses wrrn B] X D.
Three diflFerent D males were used in making this cross. They pro-
duced thirty -two B, thirty-seven C, and twenty-nine D offspring, together
with seven of undetermined character. Here the C class is actually larger

HEREDITY OF HAIR-LENGTH IN GUINEA-PIGS. II

than either of the others, though the increase over the last mating is not a
very striking one, and several individuals were just on the line between
C and D, so that little stress can be laid upon the classification made.

(f) Mating 5(0) [from Two Successive Crosses with L>\ XD.

If two crosses introduce greater contamination of the gametes than
one, then the C and D classes resulting from this cross should be high at
the expense of the B class. The observed result accords with this inter-
pretation, though the number of young produced is not large. There were
one B, three C and five D offspring.

The question now arises, What is the nature of the C individuals? Are
they the result of partial reversal of dominance, so that when B meets D in
fertilization there is produced an intermediate condition, or have B and D
actually fused to form something different from either? Matings of C indi-
viduals among themselves or with B and with D respectively should throw
light on this question. If C individuals result merely from partial arrest
of the dominance of B, we shall expect C to split at gamete formation into
B and D. If it does not do this we may conclude that it represents a firm
union of B and D to form a new character, C.

if) Mating C{D?)XD.

This mating produced a mixture of C's and D's, in all eight C, nine D
and five individuals of undetermined character. Apparently the C's used in
this experiment were heterozygous, producing some C and some D gametes.
The former, combined with D gametes, produced C individuals ; the latter
similarly united produced D individuals. The fact that no B offspring were
produced indicates that the C parents did not form B gametes. We con-
clude that the C gamete is probably a new creation due to a partial and
permanent blend of B with D.

(g) Mating C(D) [produced by Two Successive Crosses with D] X D.

This mating should yield a larger proportion of D offspring than the
last, if cross-breeding introduces contamination of gametes. The observed
result was nine C, eighteen D and six young of undetermined character,
which result supports the hypothesis stated. It should be said that in this
mating, as in those previously described, the classification of the offspring
was wholly unbiased, as it was in each case made before the animal's ped-
igree was looked up to see in what group of offspring the individual in
question should be placed. It must be borne in mind, however, that the C
group it not a natural one to be considered a unit-character by itself. It

12 HEREDITY OF HAIR-LEXGTH IX GUINEA-PIGS.

is simply a poorly developed D, C and D being grades, arbitrarily fixed, of
the continuous growth type of hair. Or, if we think of C as a synthesis of
B with D, then this C partakes more largely of the character of D the
more often the parental B has been crossed with D in its production. From
the fact that the C parents give off gametes partaking in different degrees
of the D character, it seems probable that the synthesis of B and D to form C
has been as yet imperfect, as of two ingredients incompletely mixed together,
so that different samples contain different proportions of the D character.
This view is supported by the result obtained by mating C with C, but as
the parents used in this experiment were of different origin from those men-
tioned in the foregoing pages, it may be well first to describe the source
from which they came.

4. ORIGIN OF RACE C, AND RESULTS OF CROSSES IN WHICH IT WAS

EMPLOYED.

A family of short-haired guinea-pigs (class B), when inbred, produced
a few individuals with hair about twice as long as that of their parents.
The long hairs were not very numerous. It seems probable, in the light of
subsequent studies, that only a few of them were continuous in growth,
the others being determinate, but attaining a greater length than do ordi-
nary hairs before growth ceased. \\'hen two of these long-haired animals
were mated together, all the offspring were long-haired, though some of
them had more numerous long hairs than others, or hairs of a greater maxi-
mum length. By selecting the best long-haired individuals for two genera-
tions a race of imperfectly long-haired individuals was produced, of about
the same degree of excellence as the group C already described, which was
produced by cross-breeding between B and D. To distinguish it from the
latter, we may call this race C

The C race, bred by itself, produced no B individuals, though the
maximal hair lengths obtained varied considerably. This result shows
clearly that no B gametes were produced by the C race, though it had just
arisen from the B race, in which very likely it had previously been present
as a recessive character.

A mating between C and D gave a mixture of C and of D individuals,
the two classes being about equally numerous (seven C to five D in one
set of experiments). Certain individuals, however, were just on the line
between the two classes, so that no great importance can be attached to
the proportions observed. But the result does show, what breeding C or C
individuals inter se had shown, a considerable degree of variability among
the C and C gametes, this variability being a sufficient basis for selection
for increased hair-length. While, accordingly, the variation made its appear-
ance as a discontinuous one, it showed itself amenable to selection.

HEREDITY OF HAIR-LENGTH IN GUINEA-PIGS. 1 3

5. GAMETES OF CROSS-BREDS OFTEN IMPURE.

From the foregoing observations it is clear that, while the long-haired
and short-haired conditions are sharply alternative to each other in heredity,
the gametes formed by cross-breds are not in all cases pure. Frequently
they consist of a blend or a mixture of the two alternative conditions, con-
stituting in effect a new condition intermediate between the other two. A
study of other characters alternative in heredity yields results somewhat
similar.

Albinism is, in heredity, the most sharply alternative of characters, yet
cross-breeding between albino and pigmented guinea-pigs may modify the
character both of the albino race and of the pigmented one. This modifi-
cation may take on a variety of forms, as has elsewhere been pointed out
(Castle, :05). It may result in the production of mosaics (pigmented
animals spotted with white), or of albinos with a modified peripheral
pigmentation, or of albinos visibly like their ancestors but transmitting a
different set of latent characters. Again, the rough or rosetted coat of certain
races of guinea-pigs is sharply alternative to smooth coat, yet cross-breeding
of rough with smooth races may induce curious modifications of the rough
character or produce smooth individuals bearing the merest trace of the
rough character.

All these facts are in harmony with the hypothesis, for which there is
strong evidence on the cytological side, that each separately heritable char-
acter is represented by a different structural element in the germ (egg or
spermatozoon). In fertilization the paternal and maternal representatives
of a character become more or less closely united, this union persisting
through all subsequent cell-generations until the new individual forms its
sexual elements. At that time the paternal and maternal representatives of
a character separate from each other and pass into different cells.

But the paternal and maternal representatives of a character may in
the meantime have exercised on each other a considerable influence. In
the case of some characters, as ear-length in rabbits (Castle, :0s''), they com-
pletely blend and intermingle, so that a new character is produced strictly
intermediate between the conditions found in the respective parents.

In other cases the modification may be slight, as if the paternal and
maternal representatives of a character had been scarcely more than approx-
imated. Sometimes in cases of alternative inheritance no influence of the
cross is observable in certain of the " extracted " individuals, but if any con-
siderable number of individuals is examined, others will be found in which
the cross-breeding manifests its influence. From this we conclude that
gametic purity is not absolute, even in sharply alternative inheritance.

14 HEREDITY OF HAIR-LENGTH IX GUINEA-PIGS.

6. BIBLIOGRAPHY.

Castle, W. E.

■.03. The heredity of " Angora " coat in mammals. Science, n. s., vol. 18, no.
467, pp. 760-761. December 11, 1903.

Castle, W. E.

:os. Heredity of coat characters in guinea-pigs and rabbits. Carnegie Institu-
tion publication no. 23, 78 pp., 6 plates and 8 text-figures. February, 1905.

Castle, W. E.

:05". Recent discoveries in heredity and their bearing on animal breeding.
Pop. Sci. Monthly, July, 1905 ; pp. 193-208, 14 figs.

Davenport, C. B.

:04. Wonder horses and Mendelism. Science, n. s., vol. 19, no. 473. PP- I5i-I53-
January 22, 1904.

6. THE ORIGIN OF A POLYDACTYLOUS RACE OF

GUINEA-PIGS

BY

W. E. CASTLE

Papers of the Station for Experimental Evolution at Cold Spring Harbor,

New York, No. 6

Contributions from the Zoological Laboratory of the Museum of Comparative
Zoology at Harvard College. E. L. Mark, Director. No. 176

6. THE ORIGIN OF A POLYDACTYLOUS RACE OF GUINEA-PIGS.

By W. E. Castle.

1. FIRST APPEARANCE OF THE POLYDACTYL CHARACTER.

Normal guinea-pigs have four toes on each front foot but only three
on each hind foot. The race of guinea-pigs whose origin is to be described
diflfers from ordinary guinea-pigs in possessing four digits on each hind
foot instead of three. The four digits of the front foot apparently corre-
spond with digits 2-5 of the typical five-fingered appendage, while the three
digits of the hind foot correspond with digits 2-4 of the typical appendage.
In other words, all guinea-pigs have lost from the front foot the digit which
corresponds with our thumb, and normal guinea-pigs have lost from the hind
foot two digits, which correspond respectively with our " great " and
" small " toes. In the race to be described the " small toe " is present on
the hind foot as well as on the front.

In October, 1900, several ordinary guinea-pigs were obtained from a
nearby breeder, one pair of which produced in the following June three
male young. The largest of these young bore an imperfectly developed
digit on its left hind foot, in the position of a " small toe." This toe bore
a well-developed claw and apparently contained the bony phalanges, but
these evidently were not joined with the foot by appropriate muscular and
tendinous connections, for the toe hung limply down from the side of the
foot like a bag of skin. It remained attached to the foot until the animal
was fully grown, but was then lost, probably having been accidentally
pulled off.

Previous to the birth of this polydactylous guinea-pig, I had never seen
a fourth digit on the hind foot of a guinea-pig, nor heard of its occurrence
either among wild or domesticated Caviidas, and I am unable to find any
reference to such a character in the literature of the group. But I have
since found that the " extra-toe " does occur not infrequently among guinea-
pigs in an imperfectly developed condition, and I have twice since obtained
animals of this sort from breeders. From the progeny of the single poly-
dactylous individual born in June, 1901, a well-established race of four-toed
guinea-pigs has now been formed. An account of how this was done, it
is hoped, may be of value as bearing on the origin of breeds.

17

1 8 ORIGIN OF A POLYDACTYLOUS RACE OF GUINEA-PIGS.

The original polydactylous guinea-pig (J' 23, table i, p. 18), was born to
parents of almost unknown ancestry, but certainly not closely related to
each other. The mother, a spotted animal (5 51) produced no other poly-
dactylous offspring, though she bore in all thirty young. But the father
((^41.1"), who sired in all one hundred and forty-seven young, had five
other polydactylous offspring. These were all, except one,* borne by females
descended from himself, and that one was borne by his half-sister (942.1*),
so that it seems certain that the polydactylous character came in every case
from the same individual, ^ 41.1^. A son of (^ 41.1^ — viz, 1$ 243 (table i) —
whose mother was half-sister to J* 41.1^, produced, like his father, a certain
number (five) of polydactylous offspring, which were used in building up
the polydactylous race.

Table I. — Ancestry of thi various males used in building up the polydactylous race.

The character of the extra-toe is indicated by letters following the numeral which designates each individual
These letters signify ; (P, good ; /', fair ; ^, poor; A', normal, i. ^., three-toed. Reid downward,

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*In an earlier paper (Castle, :05) this e.xception was inadvertently overlooked,
and the total number of young sired by d'4i.i=' was given as 139 instead of 147.

ORIGIN OF A POLYDACTYLOUS RACE OF GUINEA-PIGS. 19

2. PROGENY OF THE ORIGINAL POLYDACTYLOUS INDIVIDUAL.

The polydactylous male {(^2^), in a total of seventy-seven young,
produced fifteen polydactylous ones. The proportion of polydactylous
individuals varied much among the offspring by different mothers. (See
table 2.) Unrelated mothers, from families in which polydactylism had
not been observed, gave only two polydactylous young to thirty normal
ones, or 6.25 per cent, polydactylous. Related normal mothers — that is,
mothers descended from (^ 41.1^ — gave nine polydactylous to twenty-seven
normal, or 25 per cent, polydactylous, while polydactylous mothers gave
four polydactylous to five normal young, or 44 per cent, polydactylous young.

Many of the young of (^ 23 had the extra digit present on both the
right and the left hind feet, and in several of these the digits were better
developed than in the father, evidently with all the appropriate muscles
necessary for functional toes. One of the best of these young was (^ 193
(table i). His mother was a normal individual descended from ,^ 41.1^, but
gave a larger proportion of polydactylous young than any other mother had
done up to that time. By (^ 23 she had five polydactylous and six normal
young.

To express the degree of development of the extra digit a series of
three grades was now established, good, fair and poor, which will be abbre-
viated to G, F and P respectively. Good means a fully developed and func-
tional fourth toe; fair means a fourth toe rather smaller than the others,
often turned upward a little in walking, as if its muscular equipment were
imperfect ; poor means a loosely hanging toe, either with or without a nail ;
sometimes the toe described as poor is represented by only a soft fleshy bag
of skin attached to the side of the foot, without either n^il or hair, destined
to shrivel up and drop off within a few days after birth. In distinction from
these three classes, the term normal (N) will be used to describe a three-
toed foot. In describing the condition of the toes of an individual, the left
foot will always be named first ; thus GP will mean an individual having a
good fourth toe on the left foot, a poor one on the right foot.

3. PROGENY OF THE SECOND POLYDACTYLOUS GENERATION.

Male 193, as regards the extra-toe, was classed as FF, but he pro-
duced about fifteen young with well-developed e.xtra-toes, which placed
them in class G. About one-fourth of all his polydactylous young were of
this sort, the polydactylous including more than half his young (forty-
eight out of eighty-six).

20 ORIGIN OF A POLYDACTYLOUS RACE OF GUINEA-PIGS.

Method of Grading the Progeny.

Of course the female mates of J* 193 were in many cases superior in
toe development to the mates of his father, ,^ 23, though in many cases they
were identical. Accordingly it is necessary, in estimating the respective
potency of the two animals, to separate the mothers into groups of similar
toe development, or of similar pedigree when no extra-toes were present.
Consequently, five groups of mothers have been made (see tables 2-14),
viz, G, F, P, N and A''. Group G includes only mothers having well-devel-
oped extra-toes on both feet (GG) ; F includes mothers having fairly well-
developed toes, or with one good toe only (FF, FG, or GF) ; P includes
mothers with at least one toe poorly developed (P) ; the other toe may be fair
or poor, or the foot may be normal. A' includes normal (A'.Y) mothers
descended from (^ 41.1^ or nearly related to him ; N' includes normal mothers
not descended from (^4i.i-'', but belonging to families in which, when
inbred, polydactylism does not occur.

When the mothers are grouped in this way, we find, first, that the pro-
portion of polydactylous young produced by a male decreases in the suc-
cessive classes from G to A''; and secondly, that the degree of development
of the toes produced on those polydactylous young diminishes in the same
order.

It would manifestly be unfair in estimating the potency of transmission
in a given case to omit either of these considerations — the proportion of
polydactylous young, or the degree of development of their extra-toes. So
an attempt has been made to combine the two into a numerical grade for
the young by each group of mothers. This grade is given in the last
column of tables 2 to 14. In making up the grade each G toe counts
100, each F toe 80, and each P toe 50, while A' counts o. The total thus
obtained for a group of young is divided by twice the total number of
young in that group, that is, by the whole number of feet which might bear
the extra-toe. Measured by this standard, a group of young, all of which
had the extra-toe well developed on both hind feet, would be graded 100;
a group of young with no extra-toes would be graded o, while mixed groups
would come in between o and 100. A group of young averaging P would
be graded 50, and a group averaging F would be graded 80.

Comparing the young of ^ 193 (table 3) with those of his father, c? 23
(table 2), we see a marked increase in the potency of transmission of the
extra-toe within the same group of mothers. The grades of the young
produced by the last four groups of mothers (F to A'') in the case of (^ 23
are 29, 19, 17, and 1.5, respectively, while in the case of ^ 193 they are
58, 57.5, 22 and 18.5.

ORIGIN OF A POLYDACTYLOUS RACE OF GUINEA-PIGS. 21

4. PROGENY OF THE THIRD POLYDACTYLOUS GENERATION.

Continuing to trace the history of the polydactylous race in the male
line, we may notice that the sires of the next generation of young consisted
of four sons of t^ 193. Two of these were own brothers, viz, ^ 628 and
c^8o9 (see table 1). The other two were half-brothers to these two and to
each other. They were (^635 and ^711. The young of these four males
are classified as regards the extra-toe character in tables 4 to 7 (see pages
25 and 26). An examination of these tables shows for (^ 809 an increase of
potency over that of his father, J* 193; c? Z" shows a potency very similar
to that of his father, but the other two males, ,^ 628 and J" 635, are clearly
inferior to their father in potency, their average grades being about 34 and
38 respectively, while that of their father was 46.

5. PROGENY OF THE FOURTH POLYDACTYLOUS GENERATION.

The sires of the next generation consisted of six males, four of which
were sons of the best male (809) of the previous generation, while one was
a son of (^628, and one a son of c?7ii. Considering first the four sons
of ^ 809 (table 7), we notice that three of them, ^ 954 (table 8), ^ 1140
(table 10) and ^ 1264 (table 13), are clearly inferior to their father in
potency. The fourth male, J* 1263 (table 12), makes a somewhat better
showing than his father, though two of the possible groups of mothers (F
and A'^) are wholly unrepresented among his mates and their values can
only be roughly estimated by interpolating values, while two other groups
are very inadequately represented. Yet all the data available agree in
showing high potency on the part of this male. Out of a total of eighty-three
young which he has sired, only one has been three-toed.

Male 1003 (table 9) has a record similar to that of his father, ^ 628
(table 4), but on the whole better. It is noteworthy that neither of these
animals gave polydactylous oflfspring when mated to unrelated normal
females, though by polydactylous mothers they had a good proportion of
polydactylous young.

Male 1 165 (table 11), also, has a record like that of his father, ^ 711
(table 6). By unrelated normal (iV) females he produced offspring grad-
ing ID as regards the extra-toe character, his father's offspring by the same
group of mothers grading 8. By matings with polydactylous mothers, both
father and son have produced offspring grading below those of c? 809 and
of ^ 1263.

2 2 ORIGIN OF A POLYDACTYLOUS RACE OF GUINEA-PIGS.

6. PROGENY OF THE FIFTH POLYDACTYLOUS GENERATION.

Only one male, (^3046 (tables i and 15), in this generation, has been
extensively tested, and he has been used almost exclusively in matings with
the best four-toed stock. His young in that sort of mating grade very
high, viz 97, the standard of perfection being 100. He was mated to a
female of class A" a single time, and produced three normal offspring and
one polydactylous ; these three were the only normal young he ever got in
a total of ninety-two young. But this same mother bore to ^ 1263 a litter
of four young, all polydactylous. Facts like this indicate characteristic
potencies on the part of gametes furnished by certain individuals, these
potencies perhaps being handed on from generation to generation, as from
(^ 628 to his son, (^ 1003.

7. INDIVIDUAL POTENCY VERSUS ANCESTRY.

The factor of gametic potency is evidently much greater than that of
ancestry. For if we arrange the various sires in the order of the respective
amounts of polydactylous ancestry which they possess (table 15), we see
at once that this is not the order of their potencies, for those having the
same amount of polydactylous ancestry often differ much in the potency with
which they transmit the polydactyl character.

8. EXTRA-TOE OF LEFT SIDE BEST DEVELOPED.
The extra-toe, in the case of J* 23, was found on the left side of the
body only. Among his descendants, also, the extra-toe, if present on one
side of the body only, is more apt to be found on the left side. When the
extra-toe is found on both sides of the body, but unequally developed, it is
oftener the left side which has the better developed toe. The difference is
not great, but is quite constant. Thus there have been recorded the follow-
ing numbers of polydactylous feet: Left, 630; right, 589. These totals are
made up from various partial sums given by different generations or families
as follows:

Total.

Left 61 66 51 41 18 98 112 21 51 34 24 S3 630
Right 56 65 50 39 15 86 III 20 so 32 20 4S 5S9

In ever}' case the left toes slightly outnumber the right ones.

The extra-toes, as regards both degree of development and position on
the body, are distributed as shown in table 16.

The extra-toe, it will be observed, occurs oftenest on the left side of
the body, particularly in its higher grades of development (G and F).

ORIGIN OF A POLYDACTYLOUS RACE OF GUINEA-PIGS. 23

The attempt has twice been made to increase by selection the asymmetry
of the two sides of the body with reference to the extra-toe, but without
success. The slightly superior development of the extra-toe on the left
side of the body apparently remains unaffected. The cause of the asymmetry
is unknown. The case doubtless belongs in the same category as the
unequally developed right and left sex-glands of certain birds and mammals.

9. RESULT OF MATING NORMAL WITH POLYDACTYLOUS INDIVIDUALS.

Alatings between normal females and polydactylous males have repeat-
edly been made, as will appear from tables 2 to 14. Crosses reciprocal to
these have yielded the results indicated in table 17. The results, it will be
seen, are not uniform. The offspring have, in some cases, a greatly weak-
ened condition of the extra-toe, in other cases no extra-toe at all ; in still other
cases, the extra-toe may be present in a fairly well-developed condition.
The inheritance is evidently neither sharply alternative (i. e., Mendelian)
nor completely blending. The result of a cross involving the extra-toe
character is influenced by the individual potency of the respective parents.
Fewer polydactylous young are produced if the normal parent comes of a
stock in which the polydactylous character does not occur.

10. INHERITANCE NEITHER ALTERNATIVE NOR BLENDING.

It is very evident that in the inheritance of the extra-toe we are not
dealing with a case of simple Mendelian dominance. An occasional case,
like the matings of c?628 with normal females (see table 4), would indicate
that the extra-toe is a recessive character, but most polydactylous parents,
in matings with normal individuals, give a mixture of normal with poly-
dactylous offspring. Further, in these mixed lots of offspring, the polydac-
tylous and normal individuals are rarely equal to each other in number, as we
should expect if one parent were a Mendelian heterozygote, the other pure.

On the other hand, when two of the offspring produced by a cross
between polydactylous and normal parents are mated together, we do get
some evidence of Mendelian segregation. The offspring are highly variable
as regards the character, extra-toe. Some are normal, some have poorly
developed toes, and some have very well developed toes. The experiments
are still incomplete as regards this matter, but so far do not indicate the for-
mation of sharply separated Mendelian classes.

On the whole, it seems probable that the extra-toe is inherited in a
manner intermediate between blending and alternative inheritance. The
gametes only partially blend in the zygote, producing a variable result, most

24 ORIGIN OF A POLYDACTYLOUS RACE OF GUINEA-PIGS.

often an intermediate condition. The gametes formed by the cross-breds
are not homogeneous, as would be the case if complete blending occurred
in the zygote, but are highly variable as regards the extra-toe. If the inher-
itance were sharply alternative, we should expect to get, not a series of
graduated forms, but two or at most three sharply distinct groups, but this
is not the observed result. If, on the other hand, inheritance were fully
blending, all the offspring of two pure parents, or of two cross-bred parents,
should be alike, but this is not the observed result. We are forced to
conclude, therefore, that there occurs a partial blending of gametes in the
zygote, and a partial segregation as the zygote gives off gametes.

Not improbably more characters fall in this category than in any other.
Sharp alternative inheritance is comparatively rare, so is fully blending
inheritance ; most characters appear from generation to generation in a more
or less well developed condition, not always strictly intermediate between
the conditions found in the respective parents, nor always corresponding
closely cither with the condition found in one parent or with that found in
the other. In dealing with such characters, selection must be the breeder's
method of working. If he wishes either to eliminate or to " fix " a par-
tially blending character he must make an appropriate choice of parents,
not once nor twice, but many times over until the undesired condition ceases
wholly to reappear.

It would be interesting to know in what condition characters like the
extra-toe exist in the germ. It can not be in a state of simple recessiveness,
for in that case the character should reappear in a Mendelian proportion
of the offspring formed, but this, as we have seen, is not the case. More
probably the character is present, active or inactive, in every gamete, but
the conditions under which it may become active are too complex for present
analysis. On the other hand, it is possible that nothing in the germ of a
normal guinea-pig stands for the character, extra-toe, and that when this
character is formed, it is formed de novo. But if so, we must account
for the appearance of a new digit in the precise position of a lost one,
and with all the appropriate nervous and muscular connections. This
it seems quite as hard to do as to suppose an antecedent state of latency or
inactivity of the character throughout certain generations of ancestors.
Moreover, we have strong reasons for believing that, in color inheritance,
specific pigments and specific color patterns may be transmitted unseen in
a latent condition, often through long series of generations. If color char-
acters are subject to transmission in a latent condition, it seems reasonable
to suppose that other characters also may be transmitted in the same way.

ORIGIN OF A POLYDACTYLOUS RACE OF GUINEA-PIGS. 25

Table 2. — Character of the extra-toe in young 0/ $ 2^ [FN), of polydactylous generation I.

Character of

G

F

P

N

Total polydac-
tylous indi-
viduals.

Total normal
(.V.V) indi-
viduals.

Per cent,
polydac-
tylous.

Grade

mother.

L R

L R

L R

L R

G
F
P

N
N'

2 — I

4-S

— 1
2 — I
2 — 2
2 —

— I
3—2

— 2

2
2

9

2

J

30

40

so

25

6

29

1? ^ n

1.5 /,

6-6

6-4

3-S

15

62

19s

22(?) IL

Table 3.* — Character of the extra-toe in young of $ 193 {FF), generation II.

Character of

G

F

P

N

Total polydac-
tylous indi-
viduals.

Total normal
(A7V) indi-
viduals.

Per cent,
polydac-
tylous.

L R

L R

L R

L R

G
F
P

N
N'

I — I

6— 5
5-4

I —

5— 4
10— 7

3— 2
I — I

— I

2— 4

4-7
1— I

— I

1—3
4—3
I — I

2

1?

16

3

I
2

5
26

4

[67]
80

79
38
43

[55]
58 U\i
5J.5 i.-^
22
18.5

15—13

20 — 14

7—13

6-8

48

38

56

46

Table 4. — Character of the extra-toe in young of $ 628 ( GF), generation III.

Character of

"^

F

p

N

Total polydac-
tylous indi-
viduals.

Total normal
(A'A') indi-
viduals.

Per cent,
polydac-
tylous.

Grade

L R

L R

L R

L R

G
F
P

N
N'

I — 1

7-5

I — I

1—3

1 — I

4—3

1—2

2

I

13

2

2

4
8

[100]

87

0
0

[90]

[ir

62
0
0

8-6

2—4

5-4

I — 2

26

16

62

34

Table 5. — Character of the extra-toe in young of $ 635 (FF), getieration III.

Character of

G

F

P

N

Total polydac-
tylous indi-
viduals.

Total normal
(AW) indi-
viduals.

Per cent.
polydac-
tylous.

Grade '

L R

L R

L R

L R

G

F
P

N

N'

3-'

3—2

2 — 2

4— 4
8— 8

3— 3

2 — I

4— 4
II— 7

7— 3

— I

— 2

I— 6

— 4

2

3
II

23
10

I

7
32

24

[67]

[100]

78

42

29

[25]

[87'

40

24

14

7—4

17—17

24—15

I— 13

49

64

43

38

*In the last two columns of tables 3 to 13, per cent, or grades of the young, when based on a

smaller number of individuals than four, are placed within brackets.

26 ORIGIN OF A POLYDACTYLOUS RACE OF GUINEA-PIGS.

Table 6. — Character of thi txira-toe in young of $ 71 1 [GG), generation III.

Character of

G

F

P

N

Total polydac-
tylous indi-
viduals.

Total normal
(i\W) indi-
viduals.

Per cent.

polydac-

tylous.

mother.

L R

L R

L R

L R

G

7- 7

I — I

8

100

97.5

F

2— 3

2 — 2

3—2

7

2

78

60

P

I — I

2 — I

I — 2

4

3

57

43

N

I— 4

3-1

3—2

I — I

8

8

50

33

N'

i-i

4—3

— I

5

27

12.S

8

II — 15

9-6

11-9

I — 2

32

40

44

48

Table 7. — Character of the extra-toe in young of $ 809 ( GG), generation III.

Character of

G

F

P

N

Total polydac-
tylous indi-
viduals.

Total normal
(A-A-) indi-
viduals.

Per cent,
polydac-
tylous.

mother.

L R

L R

L R

L R

G

F
P

N
N'

28—24
4— 4
4— 3
I —

I —

4—7

3-3

— I

I — 2

1-3

2—1

5-4

I —

—2
I — 2

34
4
9

I

8

I
I

I

6

97

80

100

[50]

57

90

80
74

[45]

28

38-31

8-13

8-8

8—8

56

9

86

63

Table 8. — Character of the extra-toe in young of $ 954 ( GG), generation IV.

Character of

G

F

P

N

Total polydac-
tylous indi-
viduals.

Total normal
(AW) indi-
viduals.

Per cent.

pulydac-

tylous.

Grade,

mother.

L R

L R

L R

L R

G
F
P

N
N'

9— 9
3— 2
7— 5

I— 3

3— 3
3— 4
7— 9

2 —

6-5

I — I

10 — 10

4— S
I —

1—2

2—2

I —
— I

19

7

26

8

2
I

3

I
I

90

87.5

90

89

[SO]

67

73

60
36

[12.5]

20 — 19

1S-16

22 — 21

4-S

61

8

88

54

Table 9. — Character of the extra-toe in young of $ IO03 (^FC), generation IV.

Character of

G

F

P

N

Total polydac-
tylous indi-
viduals.

Total normal
(N.W) indi-
viduals.

Per cent.

polydac-

tylous.

Grade

mother.

L R

L R

L R

L R

G

6—6

6

100

100

F

P

N

3-3

2 — I

— 2

I —

6

3

67

58

N'

20

°

0

9-9

2 — I

— 2

I —

12

23

34

51

ORIGIN OF A POLYDACTYLOUS RACE OF GUINEA-PIGS. 27

Table 10. — Character of the extra-toe in young of $ \ 140 ( GN), generation IV.

Character of

G

F

p

N

Total polydac-
ty1oU5 indi-
viduals.

Total normal
KNN) indi-
viduals.

Per cent.

polydac-

tylous.

mother.

L R

L R

L R

L R

G
F
P

N
N'

3—2
4—2
3-2

2—3
3—3
2 — 2

4—1

4—4

— I

—3
1-3

9
12

S

2

3

I

82
80
83

18
19
72.5

10 — 6

7-8

8—6

1—6

26

6

81

56.5

Table ii. — Character of the extra-toe in young of $ 1165 [^GG), generation IV.

Character of

G

F

p

N

Total polydac-
tylous indi-
viduals.

Total normal
(NN) indi-
viduals.

Per cent,
polydac-
tylous.

mother.

L R

L R

L R

L R

G
F
P

N
N'

32-30
4— 4

5- 7
5— 5

2 — 2
I — I
I — 2

1—3

2 —

39
10

3
3

2
2

7

95

100

60

30

90
85
15

10

38-35

10 — 12

5-8

2 —

55 1 "

83

50

Table 12. — Character of the extra-toe in young of $ 1263 ( GG), generation IV.

Character of

G

F

p

N

Total polydac-
tylous indi-
viduals.

Total normal
(NN) indi-
viduals.

Per cent.

polydac-

tylous.

mother.

L R

L R

L R

L R

G
F
P

N
N'

66—61
2 — 2
I —

6— 6

3— 4

5—9

— I

77

2

4

I

100

[67]
100

93
[6T]
82.5

69-63

9—10

5—9

— I

83

I

99

81

Table 13. — Character of the extra-toe in young of $ 1264 (C7C), generation IV.

Character of

G

F

P

N

Total polydac-
tylous indi-
viduals.

Total normal
(A^V) indi-
viduals.

Per cent.

polydac-
tylous.

Grade

mother.

L R

L R

L R

L R

G
F
P

N
N'

21 — 22
I — I

6-5

12 — 9
I —
3— 2

2— 4
— I

3— 4
2 — 2

I — I
— I

36

2

12

2

I

2
4
3

97-5

85

[82.5]
66
17
[0]

28—28

16 — II

7— II

1—2

52

10

84

50

28 ORIGIN OF A POLYDACTYLOUS RACE OF GUIXEA-PIGS.

Table 14. — Character of the young of $ 3046 (GG), generation V.

Character of

G

F

P

N

Total polydac-
tylous indi-
viduals.

Total normal Per cent.

(A7V) indi- , polydac-

viduals. tyloos.

Grade.

mother.

L R

L R

L R

L R

G

19— n

8—8

I — 2
I —

— I

88

I

0 100
3 25

8

79—77

8—8 2-2

-I I 89

3 97

Table 15. — Sires grouped by generations of polydaciylotts ancestry.

[The numerals in parenthesis indicate rank in potency of transmission of the polydactylons character. I>escent is

shown in the male line only.]

Poiydactyloos
generations
in ancestr>'.

0 y.

I

^%

2

^%

3

2-jf.i^ iQifa\ .

628(4) 1 1003(3)
635(4)

1

7"{3)

ii6s(3)
""i 140(3')

^046(1)

809(2)

954(3)

L 1264(8)

Generation (in
male line).

I.

II.

III. IV.

V.

Table i6. — Frequency of ocntrrence of tht extra-toe in its several
degrees of development on the two sides of the body.

Left side.

Right side.

Per cent. left.

G
F
P

335
148

147

310

133
146

5«-9
52.6
50.2

630

589

SI.6

Table it .—Character of the young produced by a mating between a normal {N') male and
females either polydactylous or of polydactylous ancestry.

Character of
mother-

F

P

A-

Total polydac-
tylous indi-
viduals.

Total normal
(A'.V) indi-

Per cent.
Dolvdac- Grade.

L R

L R

L R

riduals. tyl'ous. j

S 482
S 981

S 2054

$ 2060

S 3609

P

F
P

P

N

G
G

1 —
2 — 1

I —
2 — I

2—3

—I

4-5
2 — I

— 2

I — 2

I —

I—
— I

2

3
3

0

I

6

2

0

2
0

4

5

10
6

[100] [sa^]

60 15

[100] [25]

0 0

17 j 4

37-5 M
25 9

ORIGIN OF A POLYDACTYLOUS RACE OF GUINEA-PIGS. 29

POSTSCRIPT.

A critical examination of the results obtained by Bateson, Punnett and
Hurst (see Bateson, Saunders, Punnett and Hurst, :o5) in poultry indicates
that there too the inheritance of extra-toe is not strictly Mendelian, but cor-
responds with what I have found to be the condition in the most [>otciit poly-
dactylous sires. In poultry the extra-toe has been for a long time an estab-
lished character of certain breeds. During all that time selection has un-
doubtedly been exercised in its favor, so that it is not surprising to find the
character more strongly dominant than in my four-toed race of guinea-pigs.
Nevertheless both Bateson and Hurst record cases in which polydactylous
chicks are produced by normal parents of polydactylous ancestry.

Similar observations have repeatedly been made concerning the inher-
itance of polydactylism in man. See Ballowitz ( 104) and Davenport ( :o4).
Polydactylism usually makes its (recorded) appearance in some note-worthy
form, is transmitted more or less strongly through two to five generations
and then disappears, doubtless so weakened by repeated out-crosses that its
manifestations, if any occur, are no longer observed.

Apparently it is only when selection is exercised for the polydactyl
character and like individuals are mated to each other that a polydactylous
race can be established. In its origin, polydactylism is a discontinuous varia-
tion or mutation, but without the aid of selection it would probably never
become a racial character. Is not the same thing true of a great many of
the characters which serve to distinguish the various races of domesticated
animals and plants?

11. BIBLIOGRAPHY.
Ballowitz, E.

:04. Ueber hyperdactyle Familien und die Vererbung der Vielfingerigkeit des
Menschen. Archiv. f. Rassen-und-Gesellschafts-BioI. Bd. i, pp. 347-
365, II Fig.
Bateson, W., Saunders, E. R., Punnett, R. C, and Hurst, C. C.

:0S. Experimental Studies in the Physiology of Heredity. Report II. to the
Evolution Committee of the Royal Society. 154 pp.
Castle, W. E.

:0S. The Mutation Theory of Organic Evolution, from the Standpoint of Animal
Breeding. Science, n. s., vol. 21, no. 536, pp. 521-525.
Davenport, C. B.

:04 Wonder horses and Mendelism. Science, n. s., vol. 19, no. 473, pp. 151-153.

i

OH Castle, Willis^ Ernest
431 Heredity of hair-lenght
C383

BioMed.

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