THE EVOLUTION OF A BURSA.
A bursa exercises a function in the animal body which is the direct
opposite of that shown to belong to the flexures of the hand and foot.
Whereas the latter are adapted to the prevention of slipping in the
act of prehension, bursæ are delicate contrivances for producing the
maximum effect of sliding, within certain limits, between two opposed
surfaces, either between the skin and a hard surface beneath it,
between two muscles, or a tendon as it moves over a bone. As they are
very variable and most of them are inherited and congenital, while
some are produced only in the lifetime of the individual, they are
useful for consideration in regard to the questions of transmission of
modifications and of the origin of initial variations. Their degree
of utility ranges, for example, in man, from that of the prepatellar
bursa without which no useful movement of the knee-joint is imaginable,
to the insignificant bursa which may or may not be found on the
dorsal surface of a phalangeal joint of the foot. The principle laid
down by Lyell, to which allusion has been made elsewhere, that is,
of “explaining changes in the surface of the earth by reference to
causes now in action,” is applicable in this small department of the
evolution of a minor structure of the animal body. As man furnishes the
largest of all collections of these lubricating organs, his skeleton
and skeletal muscles will form the main subject of this chapter, and I
venture, if one may say so, to “Lyell” them. None of the sections of
this book except that on the mammalian hair affords so simple and easy
a field for watching in operation certain mechanical forces. We may
here go down to the potter’s house and watch him moulding his clay, or
the cobbler his leather. So much are bursæ in the human body under the
power of extraneous forces that I venture to say that if some young
surgeon of an inquiring mind were to choose a place and time when the
Honourable and Vigilant Stephen Coleridge was out of the way, and were
to produce in a young _chimpanzee_ under an anæsthetic a “greenstick
fracture” of his radius and ulna, immobilising it at a right angle
for a month, the animal would exhibit at his death some years later a
highly developed bursa over the bony protuberance nearly as good as
the olecranon bursa on the uninjured side, and better than that of the
injured limb. As I have reason to know the meticulous vigilance of this
professional and expert humanitarian I hasten here to say in advance
that I do not recommend this experiment, not because it would not be
entirely justifiable, but because nature herself in the highest Primate
has produced many undesigned experiments of nearly equal value, as I
hope to show.
Bursæ Described.
Broadly considered a bursa is a sac lined by synovial membrane, and an
extreme example of the simplest form in which it is found may be said
to be that of the condition found in a domestic dog. Under its skin,
except on such regions as the snout, the tail and the feet, there is
hardly a place where a bursal surface does not exist. Here and there
trabeculæ may divide the great sac imperfectly, but from the protective
and selective point of view this mechanism under a dog’s skin may be
compared to the oil with which an Indian criminal lubricates his naked
body so as to elude capture. To us who are too familiar with dog-fights
(to which the Hon. Bertrand Russell likened the recent Great War, as we
all remember) and who know how much noise and ferocious attempts are
made by the warriors to bite one another, and how little success they
achieve, the beautiful adaptation of nature in the dog far surpasses
that of the Indian criminal. Indeed the latter may well have been
suggested by the former.
Between such a simple and undifferentiated bursal surface as this and
another such as the small but essential bursa under the tendo achillis
there are endless variations adapted to particular uses and regions.
The description of bursæ given by Macalister is too clear and good not
to be given in his own words.[67]
“Synovial membranes are found either as the lining of joints, or as
_Bursæ_, which are closed sacs (_a_) between contiguous soft parts,
or (_b_) beneath soft parts which glide tensely over a bone. Bursæ
are formed around and beneath tendons in the neighbourhood of joints;
and the hard part on which the tendon plays is often invested with a
layer of cartilage over which the synovial membrane does not extend.
When they completely surround tendons, as in the finger and toes they
are called _thecæ_ or sheaths, and the tendons are connected to the
sheaths by synovial reflections. Sometimes bursæ lie between exposed
areas of skin and projecting bony points, such as the patella,
olecranon, ankles, etc.
“Their (synovial) membrane differs from the synovial membrane of
joints in not having so continuous or definite an endothelial lining;
indeed, while some bursæ, such as that beneath the ligamentum
patellæ, have a more or less regular lining of regular endothelium,
others have only elongated connective cells forming an imperfect
lamella, and there are all possible gradations met with between
the regular saccular bursa, and a loose meshwork of areolar tissue
of which the bursa is only a specialisation. Bursæ may be (1)
subcutaneous, (2) subfascial, (3) between two tendons, or (4)
between tendons and subjacent ligaments or bone. Of these, some
communicate with the neighbouring joints always, some occasionally,
and some never. Bursæ underlying parts which have an extensive range
of motion are _unilocular_, with a single cavity. Bursæ spread
over an extensive surface, and whose walls move but little on each
other, are often divided by imperfect fibrous septa, and are called
_multilocular_. Almost all the lesser bursæ are unilocular, most of
the subcutaneous bursæ are multilocular.”
[67] _Text Book of Human Anatomy._ A. Macalister, 1889, p. 48.
Now if one were not engaged upon such a problem as that of initiative
in evolution and in trying to give examples of it there would be no
Gordian knot to cut, and the condensed statement of Macalister might
be simply taken as an accepted account of the manner in which reading
between the lines a bursa is formed in the animal body. But, when an
hypothesis such as the present is in question, one may not cut the
Gordian knot in this way, and must produce briefly certain observations
of the process, not only those known in man by anatomists and surgeons
but also some found in lower Primates.
Human Bursæ Enumerated.
The following is a list of bursæ in man of which some are normal
or always present, and others which are both occasional in their
appearance and often imperfectly developed.
_Front of Neck._
(A) One in front of the pomum adami.
(B) One in the thyro-hyoid space extending to the under surface of
the hyoid bone.
(C) One beneath the stemo-hyoid muscle.
(D) One above the hyoid bone.
_Pharynx._ A small central pit constituting a single bursa the _bursa
pharyngea_.
_Behind the angle of the lower jaw._ One.
_On the symphysis of the chin._ One.
_On the Acromion process._ One.
_Beneath the deltoid_ and the acromion process, one large bursa often
opening into the shoulder-joint.
_Elbow._
(A) One over the olecranon.
(B) One occasionally over the inner epicondyle.
(C) One over the internal condyle of the humerus.
(D) One over the external condyle of the humerus.
(E) Small one between the biceps tendon and the head of the radius.
(F) Often a second bursa which separates the tendon from the oblique
ligament crossing it.
_Wrist._
(A) One over the styloid process of the radius.
(B) One over the styloid process of the ulna.
_Hand._
(A) One over each of the metacarpo-phalangeal joints.
(B) One over each of the phalangeal joints.
_Region of hip._
(A) One over the anterior superior spine of the ilium.
(B) Large one between the great trochanter and the gluteus maximus
muscle.
(C) One between the gluteus medius and the bone.
(D) One between the gluteus minimus and the bone.
(E) One between the psoas and iliacus muscles often opening into the
hip-joint.
_Thigh._
(A) One over external condyle of the femur.
(B) One over internal condyle of the femur.
_Knee-joint._ The prepatellar bursæ.
(A) Between the skin and superficial fascia at the lower edge of the
patella there is often a small subcutaneous bursa.
(B) Beneath the superficial fascia over the fascia lata there is
always a large interfascial bursa, intersected by smooth fibrous
bands extending downwards over the upper part of the patellar
ligament.
(C) One still deeper between the deep fascia and front of the bone
there is a layer of lax connective tissue.
(D) Sometimes a third or deep subfascial bursa.
“These bursæ over the knee-joint appear in fœtal life and vary in
size in persons of different occupations, being often large in
housemaids and carpet-nailers, and often communicating with each
other.”[68]
[68] Macalister, p. 488.
(E) Occasionally the upper part of the synovial pouch of the
knee-joint is shut off from the general cavity and forms a separate
bursa beneath the extensor muscles. It always communicates with the
knee-joint though originating independently.
_In the Ham._
(A) Large bursa between the inner condyle of the femur and the
gastrocnemius muscle, often opening into the joint.
(B) A smaller one on the outer side.
(C) One between the biceps tendon and the external lateral ligament.
(D) One between the semimembranosus }
(E) One between the popliteus }
(F) One between the sartorius } and the bone.
(G) One between the gracilis }
(H) One between the semitendinosus }
_Tibia._
(A) One over the tuberosity.
_Ankle._
(A) Over both malleoli.
(B) Between the tendo achillis and the os calcis.
_Foot._
(A) Over plantar surface of the great toe.
(B) Over plantar surface of the little toe.
(C) Over the dorsal surfaces of all the phalangeal joints of the toes.
(D) Over the dorsal surface of metatarso-phalangeal joint of the great
toe.
(E) Over the plantar surfaces of the metatarso-phalangeal joints of
all the toes.
I calculate that there are at least fifty-two separate bursæ (about one
hundred on the two sides of the body) in the normal or fully developed
state, though of these many will be found either absent or with very
little of the full structure of a bursa. One small but significant
point may be referred to here. We are all familiar with the prominence
of the knuckles of the hand and the very efficient bursæ which cover
them, but most persons do not recognise that the foot has no such
knuckles (or prominent metatarso-phalangeal joints) and no bursæ over
these joints, except that of the great toe which happens to be very
much more exposed to friction and has a much greater range of action
than the other four metatarso-phalangeal joints. This might be called
by some persons a beautiful bit of adaptation _for_ locomotion and by
others an equally admirable bit of adaptation produced _by_ locomotion.
Examination of Two Still-born Children.
Some further light may be thrown upon the human bursæ by an examination
of two still-born children I dissected in 1908 in Lewisham infirmary,
and give here the results as to the more important subcutaneous bursæ.
_Male Child_: full term.
_Shoulder_: bursæ under acromion processes absent.
_Elbow_: bursæ over outer condyle of humerus present.
" " inner " " absent.
" " olecranon both present.
_Wrist_: bursæ over styloid process of ulna present.
" " " " of radius present.
_Hand_: bursæ over metacarpo-phalangeal joints D 1 absent, D 2, 3,
4, 5 present.
bursæ over phalangeal joints, first set present, second set
absent.
_Hip_: bursæ over anterior superior spine of the ilium both absent.
_Knee_: prepatellar bursæ well-developed.
_Ankle_: bursæ over both malleoli present:
bursæ beneath tendo achillis well-developed.
_Great toe_: plantar bursa present.
_Little toe_: plantar bursa absent.
_Toes_: D 1 (great toe) bursa over metatarso-phalangeal joint
present.
D 2, 3, 4, 5 bursæ over metatarso-phalangeal joints absent.
Bursæ over _Phalangeal joints_.
D 1 present.
D 2, 3, 4, none over either of the phalangeal joints.
D 5 bursa present over the first and absent over the second
phalangeal joint.
This example of a still-born, but otherwise normal infant illustrates
well the previous statement that certain bursæ are congenital and
others of less functional importance are formed after birth. Whereas
the olecranon, wrist, patellar, ankle and tendo achillis bursæ are
fully formed, those under the acromion processes, one of those of the
condyles of the femur, and the digits of the hand, those over the
superior anterior spines of the ilium and those of the foot are little
if at all developed in this case.
Another still-born child at seven months was also dissected and this
had well-formed prepatellar bursæ, scanty ones over the olecranon
processes, also over the small joints of the hand and foot where they
were difficult to isolate and over the malleoli they were only slightly
developed.
A fœtus in spirit I examined and found no commencement of a prepatellar
bursa.
Examination of Living Primates.
_Anthropoid Apes._
Eight of these I examined during life at the London Zoological
Society’s gardens in 1908, four _chimpanzees_, two _orangs_ and two
_gibbons_. These afforded the opportunity of ascertaining by means
of touch the presence, and in a minor degree the size and efficiency
of the main subcutaneous bursa, just as one can do this in a human
subject. The _chimpanzees_ were A, aged thirteen; B, aged seven; C,
aged three; and D, aged two-and-a-half years; the orangs E, aged
thirteen; F, aged three years; the gibbons G and H both two to three
years.
These eight specimens possessed good examples of the leading
subcutaneous bursæ over the olecranon process, the styloid process of
the ulna, the patella and both malleoli.
The smaller and less definite bursæ gave the following results.
_Chimpanzees._
A. _Hand._ Bursæ on all the metacarpal and first phalangeal
joints; none on the second phalangeal joints of D 2, 3,
4, 5.
_Foot._ Bursæ well marked on the five metatarsal first phalangeal
joints; none on D 2, 3, 4, 5 joints, but one on that of D
1. None found on second row of phalangeal joints.
B. Moderate development of bursæ on metacarpo- and
metatarso-phalangeal joints of D 1; doubtful on those of D 2, 3,
4, 5.
On hand and foot first phalangeal joints, bursæ present, on second
row absent.
C and D were similar. Metacarpo- and metatarso-phalangeal joints,
none in C and scanty in D.
No bursæ on any phalangeal joints of hand or foot.
_Orangs._
E. Metacarpo- and metatarso-phalangeal joints, bursæ ill-developed,
first row of phalangeal joints of hand and foot moderate, second
row none.
F. Metacarpo- and metatarso-phalangeal joints more marked than in E.,
and well developed on all phalangeal joints.
_Gibbons._
G. Metacarpo- phalangeal and metatarso-phalangeal joints poorly
developed on D 2, 3, 4, 5, and none on those of D 1. Absent on all
phalangeal joints.
The digits of the gibbons were very long and evidently efficient
in action, but were never flexed to any great degree.
Dead Specimens.
I also examined the hands and feet after death of certain lower
Primates in 1909:--
H. Hapalemur Griseus.
I. Hapale Jacchus.
J. Cercopithecus Callitrichus.
K. Cercopithecus Mona.
L. Macacus Rhoesus.
_Hapalemur Griseus_ H. _Hands._ No bursæ on styloid processes of radius
and ulna, and no localised bursæ on any metacarpo-phalangeal or
phalangeal joints.
_Feet._ Bursæ under tendo achillis small but distinct. Present over
both malleoli.
Metatarso-phalangeal joints D 1, 2, 3, rudimentary D 4 and 5
absent.
First phalangeal joints of D 1, 2, 3, 4, rudimentary absent
over D 5.
Second phalangeal joints absent on all digits.
_Hapale Jacchus_ I.
_Hand._ Lower end of ulna, which is very prominent, a bursa present,
over end of radius, _which is much less prominent_, absent.
Metacarpo-phalangeal joints, present in all.
First phalangeal joints, which are prominent, present in all
digits.
Second phalangeal joints absent in all.
_Foot._ Bursa under tendo achillis and over both malleoli.
Metatarso-phalangeal joints absent on D 1; present on D 2,
3, 4, 5.
First phalangeal joints, present in all.
Second phalangeal joints, absent in all.
_Cercopithecus Callitrichus_ J.
_Hand._ Dorsal surface of the whole hand shows no localised bursæ,
only a loose areolar tissue under the skin. Styloid
processes of radius and ulna no bursæ.
_Foot._ Dorsal surfaces over the whole foot similar to that of the
hand.
Bursæ present over both malleoli.
Well-formed small bursæ under tendo achillis.
_Cercopithecus Mona_ K.
_Hand and Foot._ Dorsal surfaces similar to those of J and similar
loose areolar tissue over styloid processes of ulna and
radius.
Bursæ over both malleoli.
Well-formed bursa under tendo achillis.
_Macacus Rhoesus_ L.
This specimen showed more examples of bursæ than the two
of Cercopithecus.
Bursæ present over styloid processes of ulna and radius,
also over metacarpo-phalangeal joints.
Bursa well-marked over malleoli and under tendo achillis.
Bursæ present over metacarpo-phalangeal and
metatarso-phalangeal joints.
No bursæ over phalangeal joints.
Further Undesigned Experiments.
The preceding facts as to the natural history of bursæ in man and
some lower Primates, even if they stood alone, are enough to produce
conviction as to the manner in which bursæ of all degrees of perfection
are formed by function, and point to the origin of the initial stages
of these structures. But they do not stand alone, for in man there
have been carried out certain undesigned experiments in a similar
direction, comparable to those described in the sections on direction
of hair and arrangement of papillary ridges. These demonstrate the
fact that frequent friction of skin over a hard surface has the power
of producing adventitious bursæ in regions where they are not found in
the normal state.
These adventitious bursæ are the following:--
In the first place certain normal bursæ in important situations are
frequently so much enlarged by the constant irritation of pressure and
friction that they become considerably enlarged. This enlargement may
go on to definite pathological changes and thus come under the care of
surgeons.
They are Prepatellar bursæ--“housemaid’s knee.”
Olecranon bursæ--“student’s elbow” and “miner’s elbow.”
Tuber ischii bursæ--“weaver’s bottom.”
These may be called “occupation-bursæ” and may be classed with three
other well-known adventitious bursæ which are formed on the shoulder
in “deal runners,” on the scalp in “fish porters” and in the back of
the neck in Covent Garden porters, known as a “hummy.” Entirely new
bursæ are formed also over the cuboid bone in talipes equino-varus,
over the internal condyles of the femur in bad cases of knock-knee from
friction of one joint against the other, over the prominent vertebrae
in a humpback. A structure closely resembling a bursa and arising
from similar causes to those producing adventitious bursæ is found in
unreduced dislocations or ununited fractures.
A small example of an adventitious bursa came under my notice. A woman,
E. L., aged 49, had remarkable enlargement of the metatarso-phalangeal
joint of her great toe of the left foot, and over this joint was formed
a well-marked bursa on the dorsal surface. The right foot showed a
much less prominent joint and only a very slight development of the
corresponding bursa.
This instance of a bursa-like structure being produced in unreduced
dislocations and ununited fractures suggests the conception which I
here propose, but do not attempt to verify that _all joints in all
animal forms from the lowest up to man have been evolved in a manner to
which this pathological experiment may give a clue_.
A remarkable case reported by Sir William MacEwen in the Royal
Society’s _Phil. Transactions_, Series B, Vol. 199, pp. 253, 279, is
worth referring to in this connection. It was a case of a growth of
bone in muscle connected with an old injury to the thigh of a man 38
years old, and healthy. At the operation performed by the author of
the paper the tumour was found to be movable, partly attached to the
fascia lata of the thigh, and the upper part of the tumour moved on
the lower. It was found that the tumour consisted of two parts, the
upper three-and-a-half and the lower seven inches long, altogether
a mass about ten inches in length. Muscular bundles of the vastus
externus were included in this ossific formation, one passed through a
tunnel in the bone through which it worked, and the sides of it were
polished. _At the point where the newly formed bone came in contact the
surfaces fitted each other and were polished as if they were covered
with cartilage, and were here surrounded by a capsule._ (Italics not in
original.) This fibrous covering when opened was seen to contain a thin
serum, which, though not of the consistence of synovial fluid, still
aided in lubricating the polished surfaces as they played over one
another.
A similar case was reported also by Dr. C. Paterson, surgeon to the
Glasgow Royal Infirmary.
A very interesting address by the Hunterian Professor, Mr. Jonathan
Hutchinson, was given in February, 1917, on Dupuytren’s work,
especially in the discovery of the cause and treatment of the
contraction of palmar fascia known by his name. Professor Hutchinson
described his method of curing this by the removal of the head of
the first phalanx, and showed excellent results and evidence of the
formation of a perfect new joint to take the place of the old distorted
one, and the fingers were as efficient as in the normal state in the
exercise of flexion. He gives photographs of the hand some months after
the operation showing it to be capable of easy and full extension as
well as of flexion. This again agrees well with the cases of Sir W.
MacEwen and Dr. Paterson of the formation of a functional joint _by_
use and habit.
Another distinguished Hunterian Professor A. Keith, also gave two
lectures in January, 1918, on the “Introduction of the Modern Practice
of Bone-grafting,” which, in its modern form, he assigns to the credit
of Sir William MacEwen. He lays great stress on the important work
performed in such cases by the osteoblasts without whose living and
formative action these results could not be obtained. He explains how
necessary it is that these living elements should be stimulated into
action by _work_. They thrive only so long as they have work to do.
Another surgeon, Ollier, “wondered why the fragments of bone which he
had succeeded in raising from slips of periosteum planted beneath the
scalp or amongst muscles ceased to grow and tended to disappear. These
bony grafts withered because they were not subjected to the strains
and stresses which rouse the activity of osteoblasts.” MacEwen, “by a
fortunate chance, planted his tibial grafts in a situation where they
soon became subjected to muscular strains and stresses. In a short
time bony fragments gathered from the legs of six boys became intrinsic
parts of the humerus of a seventh; from the moment of primary union the
bone cells of the graft were brought under the stimulating impulses of
the biceps and triceps. Osteoblasts are the obedient slaves of muscles;
_muscular dominance is their breath of life_.” (Italics not in the
original.)
“Wolff was the first to devote thirty years of constant work and
observation to prove that the shape and structure of growing bones
and adult bones depend on the stresses and strains to which they are
subjected. By altering the lines of stress the shape of a bone can be
changed.”
Wolff’s law is simply this: “Osteoblasts at all times build and
unbuild, according to the stresses to which they are subjected.”
Professor Keith says further: “We are driven, as I have pointed out in
a previous lecture, to look for the primary cause, not in the bones,
but in the muscles, particularly in those which are tonically and
constantly in action so long as we are standing.”
A terse expression of Wolff’s law is quoted from Dr. John B. Murphy, of
Chicago: “The amount of growth in a bone depends upon the need for it.”
A remarkable illustration of a similar process is given in the
construction of sponges by the scleroblasts and it is stated: “The
soft walls of this sponge are constantly exposed to the force of
moving waters, and we shall see that the spicule-builders--the
scleroblasts--are endowed with the same properties as osteoblasts--the
powers of fashioning and depositing the elements of the skeleton so
that the sponge can best resist the forces to which it is habitually
exposed.”
One more important quotation from this lecture will suffice. “No
one who has watched the behaviour of scleroblasts and marked the
design in their workmanship can doubt that they have acquired certain
characteristic qualities, chief of which is a sensitiveness to
vibrations--to stresses. We see them build the same form of spicules as
their ancestors, and therefore must suppose that their building quality
is a gift of inheritance. We see them alter their mode of building as
stresses change; we must therefore suppose that their inherited powers
can be changed by the circumstances under which they work.”[69]
[69] Hunterian Lecture on “The Introduction of the Modem Practice of
Bone-grafting.” Royal College of Surgeons of England, January, 1918.
Reported in the _Lancet_, February 9th and 16th, 1918.
In regard to the action of the scleroblasts of sponges I have only
to point out that the cautious words of Professor Keith on the
treacherous ground of inheritance amount to the very same conception
of personal selection and inheritance as are involved in the term
“educability” of Sir E. Ray Lankester. Whether or not in the case of
sponges this be a complete account of the matter it at any rate is a
very important piece of evidence, if valid, for selection. Whether or
not further it is a piece of evidence for a Mendelian factor implicit
in the primordial sponges and released by some loss of inhibiting
factors, as Professor Bateson would probably claim, is another and
far more imaginative conception. The mere neo-Lamarckian with the aid
of personal selection fails to see any difficulty in realising the
wonderful process described by Professor Keith.
An apology must be offered here to the patient reader for the
introduction under the heading of the “Evolution of a Bursa” of the
apparently alien subjects of bone-grafts, artificial new joints and
sponge-spicules, but I have hazarded the guess that all joints in all
animals have been fashioned--“forged by the incident of use,” to employ
a fine phrase of Professor Macdonald’s in another connection--in slow
but intelligible ways by use, and that in them, as elsewhere, function
has preceded structure. This arose so simply out of the story of the
bursæ that I ventured to digress as aforesaid rather than make it the
subject of a separate section.
The Significance of the Proceeding.
The foregoing slender contribution to the comparative anatomy and
physiology of bursæ is sufficient to show that at certain important
and “critical” points in the mammalian anatomy, efficient bursæ are
always present. One cannot indeed conceive the function of the parts
involved being carried on at all without these ingenious contrivances,
and no doubt can exist that in certain of the leading bursæ selection
guides and guards, while use and habit maintain them. Over such as
these “dominance” or the appearance of mutations might perhaps be
supposed to preside, and possibly some useful statistical results might
arise from their study from these points of view. But, between these
major bursæ in man and lower Primates and the undifferentiated sacs
which hardly deserve the name of bursæ, there is a perfect little host
of insignificant structures, which at the first attempt at dominion
over them on the part of Mendel or de Vries would hoist the standard
of revolt. These would even refuse allegiance to Personal Selection
under the persuasive banner, “Educability,” which however valuable
elsewhere, must stand aside in this little province of Nature. I have
thus attempted to “Lyell” this body of facts. Basing the statement on
an analysis of a considerable mass of small facts which no one disputes
I claim that the modifications drawn from normal anatomy on the one
hand and on the other adventitious structures, produced by acknowledged
mechanical forces, are examples of the transmission of modifications,
and illustrate the mode of formation of certain initial variations. In
other regions where Plasto-diēthēsis, as I conceive it, is at work in
producing adapted organisms, there may be included in the hyphenated
area certain factors of heredity, Mendelian, mutational and others, but
not in this group. This is merely an assertion of an opinion though I
submit that there is good evidence for it. Not even the hardest hearted
Weismannian, Mendelian or mutationist, and not even the biometrician
can refuse to this poor little province the required time and
mechanical forces, and, unless an opponent can offer some explanation
more consistent with the facts than that here offered, the proof of
causation is as sound as that shown in the larger one of the direction
of hair.