VARIETIES OF EPIDERMIS.
Passing now to the smaller trenches of the front line I have chosen as
the first of them a small study of the varieties of epidermis found
in mammals. With the exception of aquatic mammals so few of this, the
greatest vertebrate class, are not clothed with hair that it is only
on the comparatively hairless body of man, with its third of a million
fine hairs, that the varieties of epidermis can be broadly studied.
Much of this chapter will resolve itself into a consideration of the
palmar and plantar surfaces of certain mammals, where no hairy covering
obscures the operation of stimulus and response.
I assume that the foregoing phenomena of hair-direction have chosen
and raised on his shield their own king. But here I must ask of the
succeeding groups when they say, “I am, Sir, under the King, in some
authority,” the question, “Under which King, Bezonian, speak or die”--
Shall it be Darwin’s Personal Selection?
Roux’s Cellular or Histonal Selection?
Wallace’s and Romanes’ Sexual Selection?
Weismann’s Germinal Selection?
The rule of Mendel?
Selection of mutations according to de Vries?
Or shall it be the barbarian king _Plasto-diēthēsis_?
Which indeed of the seven kings will they choose, if I may thus
personify them? I may, perhaps, urge on them the mild and tolerant
rule of Lamarck and Darwin rather than that of the other anointed
sovereigns, hoping this cannot be taken as an attempt to influence the
jury through the Press in a case which is still _sub judice_.
Stimuli and Response.
The skin over the trunk and limbs of man is exposed to stimuli of
pressure, friction, heat, cold and wind in very different degrees,
according to the part which it covers. I do not here refer to nocuous,
or so-called noci-cipient stimuli, as being too casual in their
incidence for the question in hand. Broadly the ventral surface of
the neck and trunk differ much, in respect of the qualities of their
epidermis, from the dorsal. The skin over the former is softer, thinner
and more flexible than the latter, which is in adult life thick, hard
and with larger openings of the sebaceous glands. As the two main
layers of the skin are so closely united it is impossible to state
any general rule as to the parts played in this manufacture by the
epidermis and dermis respectively. Altogether the skin from the dorsal
surfaces of mammals provides a much denser fabric than the latter, and
different qualities of leather are obtained from different regions.
Corresponding differences of texture are found on the extensor and
flexor surfaces of the limbs, especially on the hands and feet. In the
course of his long evolution from a hairy stock, whether simian as we
thought yesterday, or a lower one as Professor Woods Jones suggests
to-day, these dorsal surfaces of neck, trunk and extensor surfaces of
limbs have been exposed through countless generations of men to vastly
more stimuli of friction, pressure, and response, than those of the
ventral and flexor regions. As man’s hairy covering diminished, through
some mysterious and at present unrecognised cause, these stimuli became
increasingly potent in producing a tissue denser than that of the more
protected ventral parts where all forms of these stimuli are slight. I
do not claim that this was a phenomenon that began with man, for in a
measure it was present in those forms which preceded him, and in many
related mammals under the cover of their hairy covering.
When we remember, or conceive what a large portion of each of his
24-hours even in his earliest form throughout life man must have spent,
as he still does, in lying on his back or sides, and in sitting with
his back against a supporting object, and with his gluteal and ischial
regions pressed hard against whatever seat he has selected in cave
or drawing-room, we need not travel far in thought to understand how
great has been the preponderance of stimuli from friction and pressure
on the dorsal and extensor surfaces over those on the ventral and
flexor--and here comes in our familiar “total experience” with stimulus
and response spread over a vast stretch of time. It must be borne in
mind that from the facts of the case a very large number of individual
men and women were exposed to similar, but not the same stimuli at each
stage of the process involved. It is matter of common knowledge that
not only on the palm and sole of man, but on regions where the skin
is not specialised in that remarkable manner that is found in those
regions, but also in others, that increased pressure and friction will
very soon cause a harder and thicker growth of epidermis, as on the
skin over a projecting bone in club-foot, over the shoulder where a
weight is constantly carried, on the knuckles of many manual workers,
and over the patellæ of a devout Roman Catholic, as I have often seen.
On the other hand what conditions more calculated to thin and soften
the skin could exist than those operating on the ventral and flexor
surfaces, axillæ, groins, external genitals and the bends of the elbow
and knee-joints, where pressure, with little friction and greater
warmth and moisture prevails? I need do no more than ask which is the
more reasonable of the two forthcoming explanations of such phenomena,
on the one hand that they are adapted _for_, and on the other adapted
_by_ this experience? I doubt if at any stage of the long process
this slow manufacture of differing fabrics ever conferred on man any
survival value or better matrimonial prospects. At any period or stage
which I have supposed it can only be claimed for the results on the
skin that they did _not_ cause the animal to pass through the meshes of
the sieve, and theoretically might be classed among the _indifferent_
modifications, even if they added a little to the comfort of their
possessor.
Skin of Palm and Sole.
One can examine in more detail the remarkable form of skin which is
found to cover the palmar and plantar surfaces in many mammals. It is
highly specialised and appears in many degrees of efficiency for the
purposes, or uses, of walking and climbing, grasping and discrimination
of objects. With two or three insignificant exceptions these are the
only regions even of man’s body where hairs do not grow in the normal
state, and in most other mammals hair is absent from the component
parts or pads, which correspond to our palms and soles. In the absence
of hairs and sebaceous glands and the presence of as many as 320
sweat-glands to the square centimetre, and especially the papillary
ridges, the mammalian hand and foot present a fruitful field for study.
They have been studied by none more earnestly and thoroughly than Dr.
H. Wilder Harris and Mrs. Wilder Harris (_née_ Inez Whipple). This
small area of skin as an organ for grasping and discrimination has
been studied by persons from different, but not conflicting points of
view. Time would fail me even to mention these, but I would recall
here one aspect of the matter, that is the name given to it by these
eminent authorities, Friction Skin. I think I do them no injustice, nay
even honour, when I claim them as allies for us “Old Contemptibles”
in the struggle, Lamarck _v._ Darwin in respect of these characters
of the “mammalian chiridium.” This is a term employed by them for the
hand and foot of all mammals, and is very convenient for descriptive
purposes. From this point of view this organ has been produced from
more generalised ancestral structures by reason of friction and
pressure, and not for the purpose of resisting them, at least in their
initial stages--again, adapted _by_ and not adapted _for_ meeting those
forces. There are other views of the matter held by Pan-Selectionists,
notably that of Dr. Hepburn, in regard to the papillary ridges. He
would, as I gather, treat them as primarily induced, by selection, for
the better grasping of objects cylindrical or more or less globular.
I have referred elsewhere[63] at some length to this in a book
describing the examination of the hands and feet of eighty-six species
of mammals. The varieties of epidermis were divided into the smooth,
corrugated, scaly, nodular, hairy, rod-like and ridge-covered forms,
also four mixed varieties, such as corrugated with coarse transverse
ridges on the digits, corrugated with papillary ridges, nodular with
papillary ridges, and hairy with coarse transverse ridges and smooth
pads. Of these the species with smooth epidermis and hair are few and
unimportant, and the largest group examined was that of the Primates,
thirty in all, in which papillary ridges were always present. It is
highly probable that the causes of these modifications of the epidermis
in diverse groups of animals could be traced to the habits and modes of
life of each, but I make no attempt here to do this. It is also matter
for inquiry, upon which no agreement has apparently been reached, how
it came to pass that man has virtually lost his hairy coat, and in
regard to the palms and soles of animals, what may be the reason that
so few have any hair on them, and why man has no sebaceous glands, but
has very numerous sweat-glands in these regions.
[63] _The Sense of Touch in Mammals and Birds._ A. & C. Black, 1907.
This is all of great interest, and possibly some day the Mendelians
will solve for us the mysteries thereof. But here I need only ask how
it would have been possible for hairs to grow, or, if growing, not
to be promptly worn away on a surface used by animals from monotreme
to man for walking-pads, and by most of them also for grasping and
discrimination between objects as well. We are so familiar with the
thickening of the skin on the hands of manual workers and on the feet
of those who walk much, to say nothing of what we call a “corn,” from
pressure of tight boots, that we are in danger of forgetting that the
protecting skin over the hands and feet of animals was of necessity
adjusted in a crude way to the measure and kind of walking in past ages
and in all levels of life, and that it is maintained in that adjusted
condition by the use, or disuse, of each life. Another familiar example
is that of knee-pads, as in the gnu and other ungulates. Some such
process it is legitimate to assume whether it be reckoned backwards
to monotremes or later levels of life-forms. We see then before our
eyes how this living tissue becomes adapted in varied ways by response
to the stimuli of friction and pressure, and the modifications thus
slowly effected must, one would suppose, be transmitted to offspring
ultimately from the original groups with which the process began,
when by frequent repetition small changes of structure have arisen at
last. I acknowledge the limited force of the answer, that this picture
involves the continuance in each succeeding generation of the stimuli
which initiated the changes, but the fact remains that _ex hypothesi_
the changes are there, written in tablets of animal tissue, and that
the making-up of an organism in course of many ages is not and cannot
be conceived as being governed alone by the “tyranny,” even in the
_good_ Greek sense of that word, of rigid unit-characters.
In the assumed process the correcting force of the Lamarckian
drill-sergeant is always at hand, as it superintends the construction
of tissues and parts, and I doubt if even Professor Thomson will here
interpose the difficulty of “correlation with useful characters,”
for the only important functions which are invoked as the invariable
antecedent of these structures are the elementary habits of walking,
climbing or grasping objects in certain different ways, and without
these habits or functions there would be neither lemur, monkey nor
man to interest the mind of a biologist from Mars. As I am desirous
of condensing such replies as I can make to certain opinions of
opponents and objections, I will remind the reader that Professor
Bateson in the _Jubilee Volume_ of 1909, pp. 100, 101, uses a metaphor
to illustrate his view that among the facts of nature we meet certain
definite structures and patterns in which we ought not, if desiring
rightly to interpret them, to expect to find _purposefulness_. He says:
“Such things are, as often as not, I suspect rather of the nature of
tool-marks, mere incidents of manufacture, benefiting their possessor
not more than the wire-marks in a sheet of paper, or the ribbing on the
bottom of an oriental plate renders these objects more attractive in
our eyes.” Metaphors are both indispensable and delightful, they are
the very salt of scientific and other sober writings, but they have a
rather “slim” way of betraying their employers. They express at times
the truth too well, and at others when vague and inaccurate lead the
reader right astray. Thinking of this metaphor of tool marks I was in a
modern church the other day and saw just before me a stone pillar the
pediment of which was marked with oblique parallel marks of a mason’s
tool. Here then there were marks left by a human hand at some date or
other and by means of some tool or other. I know one may not reason by
an analogy from inorganic to organic phenomena in which the push and
force of life is in full blast, and that inheritance in the former is
ruled out; but, taking the metaphor seriously, you have to account for
the appearance of the ribbing of paper and the mason’s marks on the
stone. To call them “by-products” or “tool marks” or _obiter facta_,
or by any suggestive name, does not advance the reply to the question,
“Whence came this great multitude?” If I were unwary enough to be here
trying to attack Selection and to respond to the invitation of the more
learned arachnida to walk into his parlour with a scheme of organic
evolution for him to demolish at his leisure, I should have to enter
upon the question of adaptation, specific difference and perhaps other
great disputed doctrines. But, knowing my own limits, and desiring to
keep to the self-imposed limits of the title of this book, I again
plead that I am here contending, as all through it, for the origin
of initial modifications by use and habit, and for nothing else. No
one who reads of the immense amount of research and learning that are
being carried on by the students of Mendelism and Mutationism can fail
to admire them. But, as I have remarked before, these are systems of
thought which in the main deal with characters by distribution or
“unpacking,” as it is called. Such a process of course leads to new
characters by amphimixis, and no one of whom I know denies it. Such
work is concerned with fresh views of the origin of species, but with
lamentable cowardice, or humility, I leave all that great sphere to
those who are incomparably more fit for it, and just seek to mind my
own business.
In subsequent chapters on modifications and their origin I shall not
need to repeat these observations.
Some Chosen Examples of Palms and Soles.
The facts then of a few selected examples of the palms and soles of
mammals are shortly these.
A heavy, burrowing animal, the _earth wolf_ of the Cape, has a very
smooth, hard epidermis covering its foot-pads and is thus a generalised
structure which I have found in no other animal.
The common _mole_ which uses its broad strong fore-feet like a pair
of spades, and depends chiefly for discrimination of its habitat on
the delicate sensory nerve-endings of its snout, has a hard nodular
skin which is much less developed on the hind feet than the fore feet,
the latter being less active tools. It has no papillary ridges, in
accordance with this fact, and is a very efficient miner that never
practises ca’ canny, as we know to our cost when we go out in the
morning and find great heaps of soft earth thrown up in the line of its
advance from its base or fortress. Such a mode of life lends itself
remarkably to the kind of skin on its feet, and this is _now_ at any
rate adapted _to_ its environment.
The _capybara_ is a large, heavily-built rodent, and has rather a
smooth epidermis not specially thick, with long and efficient papillæ
of the corium shown in microscopical sections. Being largely aquatic in
its habitat, and given to frequenting marshy ground and to enjoying as
much sleep as it can manage, it depends a good deal for discrimination
of objects on its sensitive corium, and its epidermis is not much
specialised for, or by friction and pressure in walking. It does not
acquire by reason of stimuli and response any unnecessary tools.
With this may be classed the _echidna_ or Australian ant-eater which
has sparse hairs set on a hard and slightly corrugated epidermis, and,
being mainly a nocturnal animal and living a secluded life, it does not
walk much or far in its stealthy pursuit of worms and insects, and the
stimuli of friction or pressure encountered by it are few.
A similar condition is found on the feet of many small carnivores.
Animals with scales on their feet, which are held to constitute the
earliest stage of the Primate modification of papillary ridges are such
as the _potoroo_, _wallaby_, _kangaroo_ and _giant ant-eater_. Such
scales register a long, long series of stimuli of friction and pressure
in these and their ancestors, in a level of life before any delicate
discrimination of surfaces came into operation.
The nodular form of skin is present in the Canadian _tree porcupine_,
where rough nodules cluster closely on the surface of both feet, and
it is a significant fact that it shares with the American _opossum_
the peculiarity of nodules on the ventral surface of the powerful
prehensile tail. This adaptation tends to efficiency in its arboreal
life, and may well have been produced by infinitely small degrees of
response in structure in the course of a long evolution.
The _rabbit_ alone have I found with rod-like projections of the
epidermic cells, among which are set in dense order the soft, long,
delicate hairs and which thus conduce to its wonderful power of
treading on sharp objects without injury. We thus see the inner meaning
of dear old Brer Rabbit’s jeer of triumph to Brer Fox, “Born and bred
in a brier bush.” This adaptation might be an unit-character segregated
from the ancestral stock of the Leporidæ, or it might not, but at any
rate the rabbit leads a life in which its walking or running is no
more prominent or frequent than is a good “run” on the part of a hunter
which pursues the hare with his beagles, and one may say at least
this--that its mode of life has _not_ produced a hard rough nodular
surface on its feet by stimuli of pressure and friction and response.
One may observe that there’s a divinity doth shape our ends, rough
hew them as we may, even if some objection be taken to the present
view of rough-hewing of parts of our organism on the ground of its
piecemeal character, rather than dealing with the organism as a whole.
To which it may be replied that the Mendelians give high support to the
piecemeal study of the profound subject of genetics, and further that
the business here is to look separately and simply at a few selected
attributes of parts of an organism, and see how they _began_ to grow
big enough to avoid passing through the meshes of the sieve.
The foregoing examples of animals in which papillary ridges are absent
have been given not in their zoological order, nor as representative
of a great many groups, but as taken from the eighty-six species I
examined myself. The following belong to the same series, but all
present papillary ridges in an ascending scale towards perfection in
man.
Examples of Ridge-covered Palms and Soles.
The common _hedgehog_ though a burrowing animal like the _mole_ is
not always underground as his distant relative is. He is not always
mining and though of ancient lineage he is a “slacker” compared with
the mole, hibernating for months, and spending also much time in his
nest and prowling slowly about above ground for insects. He has thus
acquired his somewhat indifferent epidermis that one finds, but with
the addition of sparse papillary ridges. It is the species among this
list with the fewest of these tactile structures, for there are but
three or four separate ridges on six of the ten digits, and radiating
groups on only three of all the palmar and plantar pads. So _quâ_ touch
it is ill-equipped, though it has adapted a higher form of tool than
the rabbit.
The common _squirrel_, that sits much and walks mainly on branches
of trees just as much as it needs to do, has an epidermis little
differentiated, and one which is corrugated with scanty papillary
ridges on the palmar and plantar pads, and none on the digits.
The _squirrel-like phalanger_ which flies always more or less downwards
by a kind of parachute-arrangement has most of its palmar and plantar
skin covered with papillary ridges encroaching upon its corrugated
areas, and a response to more delicate tactile experience has been thus
produced by its intermittent performance of ordinary progression.
_Azara’s opossum_ presents about as large a part of the surface covered
with nodules as with papillary ridges, the latter highly-developed for
an animal so low, zoologically-speaking, but one in which delicate
discrimination is much practised.
The _kinkajou_, another arboreal animal which walks about on trees more
than it uses its feet for prehension, trusting much to its prehensile
tail, shows its corrugated epidermis and papillary ridges developed in
about equal proportions.
These five mammals thus show that the stimuli of pressure and friction
and the response to them are being complicated by the addition of the
more delicate tactile organs known as papillary ridges, and these,
perhaps, in a secondary way are becoming useful in preventing friction.
But I must not omit to point out that, _quâ_ prevention of slipping,
the few sparse papillary ridges of the _hedgehog_, _squirrel_,
_kinkajou_ and _flying-phalanger_, especially those on the extreme tips
of the digits, could have no effect in this prevention and no survival
value. It is otherwise when they are developed in large areas as in the
succeeding groups.
Primates.
All the thirty species of Primates possessed papillary ridges to such
an extent that only small areas of the palmar and plantar skin of the
lemurs showed any other than these remarkable characters. It is so much
a property of the Primate hand and foot to possess these that it might
be almost made a matter of ordinal rank belonging to the Primates, were
it not that a few stray lower mammals also possess it.
The _black-headed lemur_ is the lowest Primate examined and it is
characterised by highly developed patterns of ridges on the palm
and sole, and these are interspersed with nodules on the regions
less exposed to pressure. The complexity of the patterns of another,
the _ring-tailed lemur_, is greater still. Now these nodules are
distinguished from the rough undifferentiated nodules of lower forms,
such as the Canadian _tree-porcupine_, and from the scales in others.
When examined with a lens the separate nodules show small groups of
papillary ridges two, three or four on each nodule, arranged in a
direction parallel to those of neighbouring nodules. They are in fact
papillary ridges in embryo, and shortly above this lemur-stage in
the ascent of animal life they are merged into papillary ridges in
patterns. All this is well told at length by Dr. and Mrs. H. Wilder
Harris. I refer to it here because the disappearance of the rough,
plain, nodular or corrugated epidermis in mammals is coincident with
increasing activity and intelligence in forms who employ or acquire
a more delicate sense of touch in their hands and feet. The cruder
response of structure to stimuli of friction and pressure, evident in
the lower forms, is abandoned in the higher, as tactile delicacy in
prehension comes more into play. Here, for example, may be a subtle
case of the co-operation of the mould and sieve in action.
From this lemur-level the degree of development in the Primate palm and
sole rises and falls, but always advances through the _lemuroidea_,
_monkeys_ and _anthropoid apes_ to man. No attempt at the tracing of
the lineage is made here, and from the present limited point of view
little remains to be said about different Primates. Only two of those
examined will be briefly referred to, the _slow loris_ and man.
The _slow loris_ shares with many monkeys and apes a very soft moist
skin of the palm and sole, and in this and other refinements of this
region it is much beyond many more intelligent, active and higher
Primates. I have never had social intercourse with a _loris_, but
I have shaken the friendly little hand of a _chimpanzee_ with a
combination of pleasure, mild shock and perhaps memories of my own
palms in the more nervous moments of early life. It is a strange,
cool, soft and damp surface, but the sensation conveyed by the skin
of a _loris_ lately dead show that in life it is a wonderfully
sensitive and tender structure. The whole of the palm and sole is
covered with well-developed patterns of papillary ridges especially
on the palmar and plantar pads. No trace of old-fashioned nodules,
scales or corrugation is to be found. The structures due to stimuli
of friction and pressure in its ancestors have disappeared for ever
from this specialised and small group, and we may fairly hold, in
accordance with the law of conservation of energy, that the past is
somehow enwrapped in the present in the strange hands and feet of
the _loris_. The adaptations of the hand and foot of the _loris_ are
most obviously now of value to it in its wary and dangerous life in
the branches of trees, but are equally unfitted for that higher life
which, in his case, consists in going lower down, on the ground. The
extraordinary deliberate life of the _loris_ has been often described.
As he moves from place to place on a branch, fixing one limb before he
moves another, much as we do in going up a ladder, he is subjected much
to the stimuli of pressure, but hardly at all to those of friction.
He sets us a good example of leaving nothing to chance. Thus his soft
sensitive skin suits well his mode of progression, but he would find
the harder, rougher skin of an African baboon very inferior for the
purpose. Here, indeed, I have ventured on the edge of Tom Tiddler’s
ground, and the Pan-Selectionist or Mendelian will make a grab at me
so that I escape with just the loss of a portion of clothing. After
escaping I have only to observe to him as to the adaptations of a
_loris’s_ hand and foot that in human life, of which we know a little,
one can in a measure forecast what a man will be like if we are told
on reliable authority what he and his ancestors have _not_ done in the
way of muscular or cerebral output, without information as to what he
has done. This is too obvious, but also too complex to prove here by
numerous illustrations and it may be left as a mere suggestion as to
the past life of the _loris_ and his ancestors for many generations. He
has _not_ walked in the ordinary method of terrestrial mammals, he has
always moved very slowly about the branches of trees, he sleeps most
of the day in a hollow of a tree, curled up like a ball, and his home
is in moist, tropical regions. No habits and conditions of life could
be better calculated to soften and moisten the skin over his palms and
soles or expose it less to stimuli of friction, while even those of
pressure in his tenacious grasp of boughs are decidedly intermittent.
Unless one may assume the appearance in the distant past of some
unit-character of soft, moist skin in this and other Primates, it seems
difficult to refuse the Lamarckian claim of long, long absence of
effectual stimuli of friction and equally long presence of enervating
“negative” conditions. Proof of such a view is, of course, wanting.
Palm and Sole of Man.
The palm of man’s hand is a miracle of adaptations for touch and
grasping, but has lost most of the coarse structure formed in response
to stimuli of pressure and friction which we saw were common in lower
mammals. This indeed he shares with most simian forms. The skin of
our hands is now very much what we make it and responds very soon to
fresh positive or passive conditions. The horny, cracked epidermis on
palm and digit of the old sailor may be contrasted with the soft and
flexible and pale surface of his twin-brother, the bank clerk, who is
of studious habits and has neither the vice of gardening nor golf. If
one compares the hand of the ordinary maid with that of her mistress
the difference is striking. But if one compares the hand of that
mistress with that of her spinster sister who has lain for twenty years
in bed or on a couch, the difference is equally significant. Indeed the
sofa-and-bed-ridden invalid, of whom I knew a few once, but who have
gone out of fashion, gives the observer some useful thoughts as to the
why and wherefore of the strange skin of the hands of the _slow loris_
previously referred to. And if he be disposed also to the pleasant
pursuit of moralizing at the expense of others he will feel led to
reflect over harshly on the invalid and compare her outlook on life
with that of the _loris_. Even in this concrete case of the hand of an
invalid there may be evidence of positive as well as negative response,
if one examines the right forefinger so much used in sewing, where the
skin becomes hard and thick.
The foot of man has a good deal of negative evidence in favour of my
contention as well as positive. As to the latter, in the thickening of
the skin over the heel and ball of the great toe in those who walk much
we find changes precisely similar to those on the hand. The negative
or degenerative changes visible on man’s foot consist chiefly in the
remarkable simplicity of pattern of the papillary ridges as well as
their flattening and blurring, through wasting of those which occupy
mainly the arch of the foot. These will be shown in the next chapter
in a drawing. When this portion of skin is compared with that of the
foot of any monkey or anthropoid ape it is clear that in this respect
the skin of man’s foot has undergone even more degeneration than his
hand has shown of higher development. This degeneration has coincided
with two facts, first that man’s terrestrial locomotion has advanced
far beyond that of any other Primate, and second, that he alone has a
plantar arch. This subject belongs to a later chapter and is referred
to here because the possession of an arch to his foot has caused man to
escape, on the under surface of it, a vast proportion of the stimuli of
pressure and friction involved in his mode of walking, and the extreme
simplicity of his plantar papillary ridges, and relatively thin, soft
skin under the plantar arches affords a fairly conclusive example of
change of structure from disuse _per se_.
I have thus only selected and used two striking types of the Primates,
the _loris_ and man, not wishing to burden this part of the subject
unduly with intervening and less characteristic forms of life. It
may be legitimate here to say in defence of this long chapter that
it illustrates what I desire to keep before me all through, the fact
that use, habit, environment and selection go ever hand in hand. In
all matters of science one has to descend to particulars, so it seemed
necessary to select a few scattered phenomena in the best known groups
of higher animals and endeavour to understand how certain “characters”
or better “modifications” _began_ to grow big enough to avoid passing
through the meshes of the sieve.