W. T. (William Thompson) Sedgwick.

An introduction to general biology online

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chemical elements, and it is probably too low an estimate to say
that at least six elements must unite in order that life may ex-
ist. Moreover, only a very few out of all the elements are able,
under any circumstances, to form this living partnership.

The most significant fact, however, is that there is no loss of
weight when living matter is killed. The total weight of the
lifeless products is exactly equal to the weight of t\iQ living sub-
stance analyzed, and if anything has escaped at death it is im-
ponderable, and, havhig no weight, is not material. It follows-
that living matter contains no material substance peculiar to it-
self, and that every element found in living matter may be found
also, under other circumstances, in lifeless matter.

Considerations like these lead ns to recognize a fundamental
fact, namely, that the terms living and lifeless designate two
different states or conditions of matter. "We do not know, at
present, what causes this difference of condition. But so far as.
the evidence shows, the li^ang state is never assumed except
under the influence of antecedent living matter, wliicli, so to
speak, infects lifeless matter and in some way causes it to as-
sume the living state.

Distinctive Properties of Living Matter. Those properties of
living matter which, taken together, distinguish it absolutely
from every form of lifeless matter, are :

1. Its chemical composition.

2. Its power of waste and rejDair, and of growth.

3. Its power of reproduction.

Living matter invariably contains substances known as pro-
teids, which are believed to constitute its essential material basis
(see p. 33). Proteids are complex compounds of Carbon, Oxy-
gen, Hydrogen, Xitrogen, Sulphur, and (in some cases at any
rate) Phosphorus.

It has been frequently pointed out that each of these six elements is
remarkable in some way : oxygen, for its vigorous combining powers ;
nitrogen, for its chemical inertia ; hydrogen, for its great molecular
mobility ; carbon, sulphur, and phosphorus, for their allotropic properties,
etc. All of these peculiarities may be shown to be of significance when
considered as attributes of living matter. (See Herbert Spencer, Principles
of Biology, vol. i.)


It is not, however, tlie mere presence of proteids which is
characteristic of Hving matter. White-of -egg (albumen) contains
an abundance of a typical proteid and jet is absolutely lifeless.
Living matter does not simply contain proteids, but has tbe
jpower to Tnamtfacticre tliem out of other substances ; and this is
a property of li^dng matter exclusively.

The waste and repair of living matter are equally character-
istic. The living substance continually wastes away by a kind
of internal combustion, but continually repairs the waste. More-
over, the growth of living things is of a characteristic kind, dif-
fering absolutely from the so-called growth of lifeless things.
Crystals and other lifeless bodies grow, if at all, by accretion^ or
the addition of new j)articles to the outside. Living matter
grows from within by intussusception., or the taking-in of new
particles, and fitting them into the interstices between those
already present, throughout the whole mass. And, lastly, liv-
ing matter not only thus repairs its own waste, but also gives
rise by reproduction to new masses of living matter which,
becoming detached from the parent mass, enter forthwith upon
an independent existence.

We may perceive how extraordinary these properties are by
supposing a locomotive engine to possess like powers : to carry
on a process of seK-repair in order to compensate for wear ; to
grow and increase in size, detaching from itself at intervals
pieces of brass or iron endowed with the power of grooving up
step by step into other locomotives capable of running them-
selves, and of reproducing new locomotives in their turn. Pre-
cisely these things are done by every li^dng thing, and nothing
like them takes place in the lifeless world.

Huxley has given the best statement extant of the distinctive properties
of living matter, as follows :

"1. Its chemical composition — containing, as it invariably does, one
or more forms of a complex compound of carbon, hydrogen, oxygen, and
nitrogen, the so-called protein (which has^ never yet been obtained except
as a product of living bodies), united with a large proportion of water,
and forming the chief constituent of a substance which, in its primary
unmodified state, is known as protoplasm.

" 2. Its universal disintegration and waste by oxidation^ and its con-
comitant reintegration by the intussuscejytion of new matter. A process
of waste resulting from the decomposition of the molecules of the proto-


plasm in virtue of which they break np into more highly oxidated products,
which cease to form any part of the living body, is a constant concomitant
of life. There is reason to believe that carbonic acid is always one of these
waste products, while the others contain the remainder of the carbon, the
nitrogen, the hydrogen, and the other elements which may enter into the
composition of the protoplasm.

" The new matter taken in to make good this constant loss is either a
ready-formed protoplasmic material, supplied by some other living being,
or it consists of the elements of protoplasm, united together in simpler
combinations, which constantly have to be built up into protoplasm by the
agency of the living matter itself. In either case, the addition of molecules
to those which already existed takes place, not at the surface of the living
mass, but by interposition between the existing molecules of the latter. If
the processes of disintegration and of reconstruction which characterize
life balance one another, the size of the mass of living matter remains sta-
tionary, while if the reconstructive process is the more rapid, the living
body grows. But the increase of size which constitutes growth is the
result of a process of molecular intussusception, and therefore differs alto-
gether from the process of growth by accretion, which may be observed in
crystals, and is elfected purely by the external addition of new matter ; so
that, in the well-known aphorism of Linnaeus, the word ' grow ' as applied
to stones signifies a totally different process from what is called ' growth *
in plants and animals.

" 3. Its tendency to undergo cyclical changes. In the ordinary course
of nature, all living matter proceeds from pre-existing living matter, a
portion of the latter being detached and acquiring an independent exist-
ence. The new form takes on the characters of that from which it arose ;
exhibits the same power of propagating itself by means cf an offshoot ;
and, sooner or later, like its predecessor, ceases to live, and is resolved
into more highly oxidated compounds of its elements.

" Thus an individual living body is not only constantly changing its
substance, but its size and form are undergoing continual modifications,
the end of which is the death and decay of that individual ; the continua-
tion of the kind being secured by the detachment of portions which tend
to run through the same cycle of forms as the parent. No forms of matter
which are either not living or have not been derived from living matter
exhibit these three properties, nor any approach to the remarkable phe-
nomena defined under the second and third heads." {Encyclopcedia Bri^
tan7iica, 9th ed., art. " Biology," vol. iii. p. 679.)

For the purposes of biological study life must be regarded as
a property of a certain kind of compounded matter. But we
are forced to regard the properties of compounds as the result-
ants of the properties of their constituent elements, even though
we cannot well imagine how such a relation exists ; and so in the


long-run we have to fall back upon tlie properties of carbon,
lijdi-ogen, nitrogen, oxygen, etc., for the properties of living

Scope of Biology. The Biological Sciences. It follows from
the broad definition given to Biology that this science includes
the study of whatever pertains to living matter or to living
things. It considers the forms, structures, and functions of living
things in health and m disease ; their habits, actions, modes of
nutrition ; their surroundings and distribution m space and time,
their relations to the lifeless world and to one another, their
sensations, mental processes, and social relations, their origin and
then- fate, and many other toj^ics. It includes both zoology and
botany, and deals with the phenomena of animal and vegetal life
not only separately, but in their relations to one another. It
includes the medical sciences and vegetal j^athology.

The field covered by biology as thus understood is so wide as
to necessitate a subdivision of the subject into a number of principal
branches which are usually assigned the rank of distinct sciences.
These are arranged in a tabular view on p. 7. The table shows
two dift'erent ways of regarding the main subject, according as
the table is read from left to right m* vice versa. Under the more
usual arrangement biology is primarily divided into zoology and
botany, according as animals or plants, respectively, form the
subject of study. Such a division has the great advantage of
practical convenience since, as a matter of fact, most biologists
devote their attention mainly either to plants alone or to animals
alone. From a scientific j)oint of view, however, a better sub-
division is into Morphology {jxofi(pii^ formj Xoyo?, a discourse)
and Physiology (0U(Tz?, nature^ Xoyos, a discourse). The
formel' is based upon the facts of form, structure, and arrange-
ment, and is essentially statical ; the latter upon those of action
or function, and is essentially dynamical. But morphology and
physiology are so intimately related that it is impossible to sepa-
rate either subject absolutely from the other.

Besides the sub-sciences given ui the table a distinct branch
called Etiology is often recognized, having for its object the in-
vestigation of the causes of biological 23lienomena. But the sci-
entific study of every phenomenon has for its ultimate object the
discovery of its cause. -Etiology is therefore inseparable from


' Morphology.

The science

of form,






science of
all living

things ;

i.e., of
matter in
the living



The science
of action or

The science of struc-
ture ; the term being
usually applied to the
coarser and more ob-
vious composition of
plants or animals.


Microscopic anatomy.
The ultimate optical
analysis of structure
by the aid of the
m i c r o s c op e ; sepa-
rated from anatomy
only as a matter of

Taxonomy or Classifi-

The classification of
living things. Based
chiefly on phenomena
of structure.


Considers the position
of living things in
space and time, their
distribution over the
present face of the
earth and their distri-
bution and succession
at former periods, as
displayed in fossil re-


The science of develop-
ment from the germ.
Includes many mixed
problems pertaining
both to morphology
and physiology. At
present largely mor-


The special science of
the functions of the
individual in health
and in disease ; hence
including Pathology.


The science of mental


The science of social
life, i.e., the life of
communities, wheth-
er of men or of lower

Botany, y

The science
of vegetal


matter or


\ Biology.


science of

all living

things ;

i.e., of

matter in

the living


1- Zoology.

The science
of animal

matter or


any of the .several branches of biology and need not be assigned
an independent place.

Psychology and Sociology are not yet generally admitted to
constitute branches of biology, and it is customary and con-
venient to set them apart from it. The establishment of the
theory of evolution has clearly shown, however, that the study
of these sciences is inseparable from that of biology in the ordi-
nary sense. The instincts and other mental actions of the lower
animals are as truly subjects of ^psychological as of physiological
inquiry ; the complex social life of such animal communities as
we hnd, for instance, among the bees and ants are no less truly
problems of Sociology.

It will be observed that in the scheme morphology and physi-
ology overlap ; that is, there are certain biological sciences in
which the study of structure and of action cannot be sej^arated.
This is especially true of embryology, which considers the suc-
cessive stages of embryonic structure and also the modes of
action by which they are produced. And Una! ly it must not be
forgotten that any particular arrangement of the biological sci-
ences must be in the main a matter of convenience only ; for it
is impossible to study any one order of phenomena in complete
isolation from all others.

The term General Biology does not designaie a particular
member of the group of biological sciences, but is only a con-
venient phrase, which has come into use for the general introduc-
tory study of biology. It bears precisely the same relation to
biology that general chemistry bears to chemistry or general
physics bears to physics. It includes an examination of the gen-
eral properties of living matter as revealed in the structures and
actions of particular living things, and may sei*ve as a basis for
subsequent study of more special branches of the science. It
deals with the broad characteristic phenomena and laws of life as
illustrated by the thorough comparative study of a series of
plants and animals taken as representative types; but in this
study the student should never lose sight of the fact that all the
varied phenomena which may come under his observation are in
the last analysis due to the properties of matter in the living
state^ and that this matter and these properties are the real goal
of the study.



Lifeless things occur in masses of the most various sizes
and forms, and may differ widely in structure and chemical com-
position. Living things, on the other hand, occur only in rela-
tively small masses, of which perhaps the largest are, among
plants, the great trees of California and, among animals, the
whales ; while the smallest are the micro-organisms or bacteria.
Moreover, the individual masses in which living things occur
possess a peculiar and characteristic structure and chemical com-
position which have caused them to be known as organisms^ and
their substance as organic. All organisms are built up to a

remarkable extent in the same way and of the same materials,

Fig, 1. (After Sachs.)— Longitudinal section through the growing apex of a young
pine-shoot. The dotted portion represents the protoplasm, the narrow lines be^
ing the partition-walls composed of cellulose (CsHjoOs). (Highly magnified.)

and we may conveniently begin a study of living things with the
larger and more complex forms, which exhibit most clearly
those structural peculiarities to which we have referred.

Organisms composed of Organs. Functions. It is character-
istic of any living body — for example, a rabbit or a geranium —
that it is composed of unlike parts, having a structure whicli
enables them to perform various operations essential or accessory
to the life of the w^hole. The plant has stem, roots, branches,

leaves, stamens, pistil, seeds, etc. ; the animal has externally




head, trunk, limbs, eyes, ears, etc., and internally stomach, in-
testines, liver, lungs, heart, brain, and many other parts of

Fig. 2.— Cross-section through part of the young leaf of a fern (Pteris cujuilina)^
showing thick- walled cells ; most of the walls are double. The granular sub-
stance is protoplasm. Most of the cells contain a large central cavity (vacuole)
filled with sap, the protoplasm having been reduced to a thin layer inside the
partitions. Nuclei are shown in some of the cells, and lifeless grains of starch
in others : ?i, nuclei ; s, starch ; r, vacuole ; u\ double partition-wall. ( X 500.)

the most diverse structure. These parts are known as oi^ganSy
and the living body, because it possesses them, is called an or-

The word organism, as here used, applies best to the higher animals
and plants. It will be seen in the sequel that there are forms of life so
simple that organs as here defined can scarcely be distinguished. Such
living things are nevertheless really organisms because they possess
parts analogous in function to the well-defined organs of higher form.
(See p. 157.)

Since organisms are composed of unlike parts, they are said
to be heterogeneous in structure. They are also heterogeneous
in action, the different organs performing different operations
Q,2XiQA functions. For instance, it is the function of the stomach
to digest food, of the heart to pump the blood into the vessels,
of the kidneys to excrete waste matters from the blood, and
of the brain to direct the functions of other organs. A similar
diversity of functions exists in plants. The roots hold the



Fig. 3. (After Sachs.)— Cross-section
through a group of dead, thick-
walled wood-cells from the stem of
maize. The cells contain only air or
water. (Highly magnified.)

plant fast and absorb various substances from tlie soil ; the stem
supports the leaves and flowers
and conducts the sap ; the leaves
absorb and elaborate portions of
the food; and the reproductive
organs of the flower serve to
form and bring to maturity seeds
destined to give rise to a new gen-

Heterogeneity of the kind
just indicated, accompanied by a
division of labor among the
parts, is one of the most char-
acteristic features of living things,
and is not known in any mass of
lifeless matter, however large and

Organs composed of Tissues. Differentiation. In the next
place, it is to be observed that the organs also, when fully
formed, are not homogeneous, but are in turn made up of
different parts. The human hand is an organ w^hich consists
of many parts, differing widely in structure and function. On
the outside are the skin, the hairs, the nails ; inside are bones,
muscles, tendons, ligaments, blood-vessels, and nerves. The leaf
of a plant is an organ consisting of a woody framework (the
'' veins I') which supports a green pulp, the whole being covered
on the outside by a delicate transparent skin. In like manner
every organ of the higher plants or animals may be resolved
into different parts, and these are known as tissues. The
tissues of fully formed organs are often very different from one
another, as in tlie cases just mentioned ; that is, they are well
differentiated; but frequently in adult organs, and always in those
which are sufficiently young, the tissues shade gradually into
one another, so that no definite line can be drawn between them.
In such cases they are said to be less differentiated. For ex-
ample, in the full-grown leaf of a plant the woody framework, the
green cells, and the skin exist as three plainly different tissues.
But in younger leaves these same tissues are less different, and
in very young leaves, still in the bud, there are no visible differ-



ences and the whole organ is very nearly homogeneous. In this
case the tissues are undifferentiated^ though potentially capal)le
of differentiation . In the same way, the tissues of the embry-

FiG. 4. — Cross-section through dead wood-like cells from the underground stem of a
fern (Ptei'is aquilina) . The walls are uncommonly thick and the protoplasm has
disappeared. The channels shown served in life to keep the cells in vital con-
nection, (x 450.)

onic human hand are imperfectly differentiated, and at a very
early stage are undifferentiated.

Tissues composed of Cells. Finally, microscopical examina-
tion shows every tissue to be composed of minute parts known
as cells, which are nearly or quite shnilar to one another through-
out the whole tissue, and form the ultimate units into which the
tissues and organs, and hence the whole organism, become more
or less perfectly divided, somewhat as a nation is divided into
states and these into counties and townships.


It will be sliown beyond that these ultimate units or cells
possess everywhere the same fundamental structure; but they
differ immensely in form, size, and mode of action, not only in
different animals and plants, but even in different parts of the
same individual. As a rule, the cells of any given tissue are
closely similar one to another and are devoted to the same func-
tion, but differ from those of other tissues in form., size, arrange-
ment, and especially in function. Indeed, the differences be-
tween tissues are merely the outcome of the differences between
the cells composing them. The skin of the hand differs in ap-
pearance and uses from the muscle which it covers, because skin-
cells differ from muscle-cells in form, size, color, function, etc.
Hence a tissue may be defined as a groiij) of similar cells hav-
ing a similar function.^ As a rule, each organ consists of
several such groups of cells or tissues, but, as stated above, young
organs are nearly or quite homogeneous ; that is, all of the cells
are nearly or quite alike. It is only when the organ grows
older that the cells become different and arrange themselves in
different groups, — a process known as the differentiation of the
tissices. In the case of some organs — for instance the leaf of a
moss — the cells remain permanently nearly alike, somewhat as
in the embryonic condition, and the whole organ consists of a
single tissue. .

What has been said thus far applies only to higher plants
and animals. But it is an interesting and suggestive fact that
there are also innumerable isolated cells, both vegetal and
animal, which are able to carry on an independent existence as
one-celled plants or animals. Physiologically these must cer-
tainly be regarded as individuals ; but it is no less certain that
they are equivalent, morphologically, to the constituent cells of
ordinary many-celled organisms. It will appear hereafter thai
the study of such unicellular organisms forms the logical ground-
work of all biological science. (See p. 157.)

Since organisms may be resolved successively into organs,
tissues and cells, it is evident that cells must contam living
matter. And a cell may be defined as a small mass of living
TTiatter either living apart or forming one of the ultimate units

* Tissues frequently contain matters deposited between cells ; but these
Lave usually been directly derived from tlie cells, and vary as the cells vary.




of an organism. The cell is an " organic individual of the first
order. "^"^ (Lang.)

Living and Lifeless Matter in the Living Organism. Since our
own bodies and those of lower animals and of plants are com-
posed of matter, it may be supposed, from what has been said
in the last chapter, that they are composed of li\dng
matter. This, however, is true only in part. It is
strictly true that every plant or animal contains living
matter, but a little reflection will show that it contains,
lifeless matter also. In the human body lifeless mat-
ter is found in the hairs, the ends of the nails, and
the outer layers of the skin, — structures which are
not simply devoid of feeling, as every one knows them
to be, but are really lifeless in every sense, although
forming part of a living body. Nor is lifeless mat-
ter confined to the exterior of the body. The mineral
matter of the bones is not alive; and this is true,
though less obviously, of many other parts, such as-
the liquid basis or plasma of the blood, the fat (which
is never wholly absent), and various other forms of mat-
ter occurring in many parts of the body.

In lower animals examples of this truth occur on
every hand. The calcareous shells of animals like the

snail and the oyster ; the skeletons of
corals and sponges ; the hard outer crust
'f^A of insects, lobsters, and related animals ;
the scales of fish and reptiles; the

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Online LibraryW. T. (William Thompson) SedgwickAn introduction to general biology → online text (page 2 of 20)