INDEX 277
ILLUSTRATIONS.
FIG. PAGE
1. Diagram of a Cell. (_Drawn by Mr. E. P. Copeland from a
sketch by the Author._) 5
2. Stylonychia. (_Drawn by Mr. E. P. Copeland._) 9
3. Amœba. (_Drawn by Mr. E. P. Copeland._) 11
4. Rotalia. (_Drawn by Dr. A. L. Lawrence._) 12
5. Difflugia. (_Drawn by Dr. A. L. Lawrence._) 17
6. Noctiluca. (_Drawn by Dr. A. L. Lawrence._) 18
7. Gromia. (_Drawn by Dr. A. L. Lawrence._) 21
8. Diagram Illustrating Mitosis. (_Drawn by Mr. E. P. Copeland
from a sketch by the Author._) 26
9. Diagram Illustrating the Maturation and Fertilization of
the Ovum. (_Drawn by Mr. E. P. Copeland from a sketch
by the Author._) 30
10. Diagram Illustrating Segmentation and Gastrulation. (_Drawn
by Mr. E. P. Copeland from a sketch by the Author._) 34
11. Archæan North America. (_From a drawing in Shaler’s “First
Book in Geology.”_) 84
12. Cretaceous North America. (_From a drawing in Shaler’s
“First Book in Geology.”_) 86
13. Tertiary North America. (_From a drawing in Shaler’s “First
Book in Geology.”_) 89
14. Genesis of Horse’s Feet. (_Drawn by Mr. E. P. Copeland._) 112
15. Bipes, Cheirotes, and Snake. (_From a drawing in Shaler’s
“First Book in Geology.”_) 114
16. Domesticated Pig and Wild Boar. (_From Romanes’ “Darwin
and After Darwin.”_) 121
17. Tadpoles and Frog. (_Drawn by Dr. A. L. Lawrence._) 175
18. Diagram of Development. (_Drawn by Mr. B. H. Esterly from
a sketch by the Author._) 183
19. Brain of Fish. (_Drawn by Mr. B. H. Esterly from specimens
of the Author._) 198
20. Brain of Reptile. (_Drawn by Mr. B. H. Esterly from
specimens of the Author._) 199
21. Brain of a Marsupial. (_Drawn by Mr. B. H. Esterly from
specimens of the Author._) 200
22. Brain of a Lemur. (_Modified by Mr. B. H. Esterly from
Flower._) 201
23. Ideal Sections of Brains. (_Modified from Le Conte. Drawn
by Dr. A. L. Lawrence from a sketch by the Author._) 202
24. Brain of Human Fœtus—Fish Phase. (_Drawn by Mr. B. H.
Esterly from specimens of the Author._) 204
25. Brain of Human Fœtus—Reptile Phase. (_Drawn by Mr. B. H.
Esterly from specimens of the Author._) 204
26. Brain of Human Fœtus—Marsupial Phase. Side View. (_Drawn
by Mr. B. H. Esterly from specimens of the Author._) 204
27. Brain of Human Fœtus—Marsupial Phase. Dorsal View. (_Drawn
by Mr. B. H. Esterly from specimens of the Author._) 205
COLORED PLATES
PLATE FACING PAGE
I. Wild Rock Pigeon and Pouter. (_Drawn by Mr. J. L. Ridgway
from specimens in the Smithsonian Institution._) Frontispiece
II. Butterflies. (_Drawn by Miss L. Sullivan from specimens
in the United States Bureau of Entomology._) 138
III. Caterpillar of Geometer Moth. (_Drawn by Miss L. Sullivan
from specimens in the United States Bureau of Entomology._) 142
V. Spider on Flower. (_Drawn by Miss L. Sullivan from a plate
in McCook’s “American Spiders.”_) 144
VI. Elaps and Erythrolamprus. (_Modified by Mr. J. L. Ridgway
from a figure in Romanes’ “Darwin and After Darwin.”_) 146
VII. Salamandra maculosa. (_Drawn by Mr. J. L. Ridgway from
specimens in the United States National Museum._) 148
VIII. Paradise Birds. (_Drawn by Mr. J. L. Ridgway from specimens
in the Smithsonian Institution._) 152
IX. Primrose Flowers. (_Drawn by Miss L. Sullivan from
specimens._) 162
X. Bee Fertilizing Flower. (_Drawn by Miss L. Sullivan from
specimens in the United States Bureau of Entomology._) 164
PLATES ENGRAVED IN BLACK AND WHITE
IV. Leaf-hoppers. (_Drawn by Miss L. Sullivan from drawings
collected by Dr. L. O. Howard._) 140
XI. Babies’ Grasping Power. (_From a Photograph taken by Dr.
Louis Robinson._) 192
XII. Brain of Man. (_From Carus’ “The Soul of Man.”_) 202
INTRODUCTION.
It is extremely difficult to realize the variety, wealth, and grandeur
of animal and plant life as they exist on the globe to-day. Even if we
endeavor to recall, in imagination, all that we have seen in streams and
woodland, in ponds and rivers, in meadows, and in the air; even when
we call to mind the multifarious specimens we have beheld in all the
museums of natural history we have visited, and remember the most vivid
descriptions that we have ever read of the wealth of tropical life;—even
then we have only the faintest conception of the multitude and variety of
_living forms_, not to mention the _vanished hosts of bygone ages_.
It has always been the endeavor of students of nature to reduce this
great host of living creatures to order by some system of classification.
First one system of classification was adopted, and then another, with
the progress of Botany and Zoölogy, until eventually, before the theory
of evolution was entertained by scientific men, that system was adopted
which naturalists likened to a tree. In this system, those lowest
organisms which cannot properly be called either animals or plants, may
be represented by a short trunk. Soon this short trunk divides into two
large trunks, one of which represents the animal kingdom and the other
the vegetable kingdom. Each of these trunks then sends off large branches
representing classes, from which smaller and more numerous branches,
representing orders, are given off. From these, other branches and
sub-branches are separated, representing families and genera, and finally
the terminal twigs or leaves represent species.
In this tree, while there is a general advance in organization from
below upwards, there are many deviations in this respect. Some leaves
may be growing on different branches at the same level, which means
that species belonging to widely divergent classes or orders may still
possess an equal grade of organization. On the same branch there may be
growing leaves at different levels. This means that one species may be
more highly organized than another belonging to the same class. Not only
may all living species be classified in the form of a family tree, but
all the extinct hosts of species that lived in the ages of the past may
be similarly classified. _If all the animals that have ever lived on the
globe should be represented by a tree, those existing on the earth to-day
would be indicated by the topmost twigs and leaves, while the extinct
forms would be represented by the trunk and main branches._ (Vid. Diagram
of Development.)
The detecting of this tree-like arrangement of species in nature is
the progressive work of naturalists for centuries past. At about the
commencement of the present century, when it was finally detected,
naturalists were unable to understand the significance of it. They did
not perceive the underlying principle that accounts for the fact that
groups of living forms have such natural affinities that they are
arranged like a family tree.
When Darwin came upon the scene and found that his predecessors had
already empirically worked out the tree-like system of classification, he
convinced naturalists that the great underlying principle of this system
was =Heredity=; and that, therefore, the grouping of living and extinct
organisms in a family tree according to their natural affinities is a
grouping based upon genetic affinities; and further that the ultimate
meaning of classification is the tracing of lines of pedigree. This
signifies that all the creatures living on the globe to-day, and all
the hosts that lived in the ages of the past, are blood relations in
greater or less degree, and that they have all been evolved from simple
microscopic creatures that appeared on the globe at the dawn of life. All
organisms, then, have undergone an evolution.
The factors of fundamental importance in the study of the theory of
=Evolution=, or the doctrine of the Transmutations of Organisms, are
=Cells=, =Heredity= and =Variation=, =Environment=, =Natural Selection=,
and =Isolation=.
_The evolution of an organism means its descent from preceding organisms
with continuous adaptation to its Environment. Its adaptation occurs
chiefly, if not entirely, through the Natural Selection of its useful
Variations, and the tending of these variations to be transmitted to the
offspring by the forces of Heredity. The summation of the variations, by
Heredity and Isolation, leads ultimately to specific, generic, ordinal,
and other differences in the descendants; leads, in other words, to the
transmutations of organisms._
An organism consists of two great groups of _structural units_ called
_cells_,—=Germ-Cells= and =Body-Cells=,—harmoniously associated together.
As far as Evolution is concerned, the most fundamental parts of an
organism are the Germ-Cells.
In order that the reader may have a working knowledge of the wonderful
powers of Germ-Cells and their progeny, it is vitally important that some
concrete illustrations of the activities of different kinds of cells
should be given.
SECTION I.
ORGANIC CELLS: THE VISIBLE UNITS OF LIFE.
ORGANIC CELLS: THE VISIBLE UNITS OF LIFE.
In the sanctuary of S. Vitale at Ravenna, in Italy, is a very interesting
representation, in mosaic pictures, and over life-size, of the Emperor
Justinian and his Empress Theodora, attended by a numerous suite of
ladies and courtiers. The mosaics are small bits of glass, of varying
pattern and color, cemented together so nicely as to form beautiful
delineations of the Emperor and his attendants.
The bits of glass or mosaics that form the figures may very appropriately
be called _structural units_.
The body of man, a bird, a lizard, an oak tree, and many other animals
and plants, may usefully be compared to such mosaic figures; for, just
as the mosaic figure has its structural unit, the little bit of glass
or stone, called the mosaic; so the bodies of men, birds, and other
creatures may be looked upon as infinitely complex figures formed of
minute mosaics called _cells_.
The groups of minute mosaics or _cells_ that make up the bodies of
animals and plants differ profoundly from the mosaics that form the
figures of the dead Emperor and his companions, inasmuch as each
mosaic or cell of the animal or plant body is so small as to require
the microscope to reveal it; also each mosaic of the living animal or
plant is a living mosaic or cell, in that it can absorb food, digest
it, assimilate it, grow, and multiply in numbers. Cells, then, are the
_morphological or structural units_ which compose the bodies of all
living creatures.
All animals and plants begin life as _single_ cells, which, in the vast
majority of cases, are microscopic in size. Those that remain single
cells, and dissociated throughout life, are called _Unicellular_ Animals
(Protozoa) and Plants (Protophyta), and are to be seen mostly by the
microscope alone; but those which, by multiplication and growth, form
large numbers of cells that remain associated together as in the body of
a bird or lizard, are called _Multicellular_ Animals (Metazoa) or Plants
(Metaphyta).
A cell (Fig. 1) is a _nucleated lump of protoplasm_, or cytoplasm, and
most often of microscopic size and more or less covered on its exterior
by, and holding in its interior, various products and formations
resulting from its activity, which are called _metaplasm_. Since the
protoplasm of a cell, under the microscope, presents a superficial
resemblance to a minute speck of that jelly-like substance (albumen)
which forms the white of an egg, it is often called an albumenoid
substance. But it is very misleading to use such an expression, for
protoplasm is not a single chemical substance of great complexity; but it
is rather composed of a large number of different chemical substances of
great complexity. Many of these substances, it is true, are albumenoid
in character. The same is true as to the chemical complexity of the
nucleus, which is a physically and chemically differentiated part of the
protoplasm.
The protoplasm contains certain globulins, and also albumins and
peptones; it also contains large quantities of nucleo-albumins, with
other substances. The nucleus not only contains these same substances,
but also nuclein and nucleo-proteids. It is important to state that
nuclein consists of an albumin and nucleic acid.
[Illustration: FIG. 1.—Diagram of a Cell, highly magnified.]
The protoplasm, structurally, is made up of threads forming a complex,
sponge-like substance, or reticulum, called _spongioplasm_; and in the
meshes of the spongioplasm is a more or less fluid-like substance known
as _hyaloplasm_: suspended in the hyaloplasm are various kinds of living
bodies known as _plastids_, besides various products resulting from the
activity of the protoplasm and which are designated _metaplasm_.
In many cells the protoplasm has formed on its periphery a layer of
metaplasm which is frequently called a cell-wall. This cell-wall prevents
amœboid movements of the protoplasm, and a cell possessing it is said to
be _encysted_.
In many cells, especially vegetable ones, will also be observed clear
spaces termed _vacuoles_. These vacuoles contain water with various
chemical substances held in solution, which serve the purpose chiefly of
food-reservoirs.
The nucleus also is formed of threads called _nuclear or chromatin
threads_ (_chromosomes_), the interstices of which are filled with
hyaloplasm or _achromatin_. In the nucleus can also be observed the
nucleolus.
The protoplasmic and nuclear threads show various structural
modifications in different regions and under different physiological
states of the cell.
As will be observed later on, the nuclear threads are of special interest
to the student of heredity. They may in one phase of cell-activity look
like one thread forming an inextricable network, while in other phases
they may look like thick, short, distinct rods.
The _centrosome_ (Fig. 1), with its enveloping attraction-sphere,
constitutes another fundamentally important part of the cell. It
is especially concerned with the phenomena of cell division and
multiplication.
Just as the living body consists of an infinitely complex figure of
living mosaics termed cells, so the cell itself consists of an infinitely
complex figure of still smaller living mosaics called, by Spencer,
_Physiological Units_. These units have been given different names by
various writers, viz.: by Darwin, _gemmæ_ (gemmules); by de Vries,
_pangennæ_; by Hertwig, _idioblasts_; by Weismann, _biophors_, etc., etc.
Like the atom of the chemist and the molecule of the physicist, the
physiological unit of the biologist is merely at present an intellectual
conception, yet it is, at the same time, an intellectual necessity and
plays a very important part as the theoretical component of many vital
questions. Just as the cells are the _visible_ units of life, so the
physiological units are the _invisible_ units.
The physiological activities of cells are those that pertain to their
_nutrition_ and _reproduction_.
The nutrition of cells includes all processes that are subservient
to their life and well-being, such as irritability, contractility,
absorption of food, its digestion and assimilation, secretion, etc.
In consequence of the wonderful _nutritive_ activities of cells, we may
well speak of them as marvelous magicians. Hertwig, following Haeckel,
speaks of many cells as being _builders_. In the same spirit, we can
say that multitudes of cells are expert chemists, artists, sculptors,
mathematicians, and so on, in that they make all the myriad chemical
products of organic nature, such as spices, pigments, sugars, starches,
acids, perfumes, and numerous other substances; they paint in colors
that rival the hues of the rainbow; they construct all of the beautiful
forms in the animal and plant worlds; and they draw lines as straight and
curves as graceful as the most expert mathematician.
One of the most important _reproductive_ activities of a cell is
_mitosis_ (see below). Mitosis essentially consists of a series of
processes by which each _nuclear thread_ of the nucleus _splits
longitudinally into two equal parts_, and then these equivalent parts
separate from each other, so that from the one nucleus we get two smaller
nuclei. Then each of these smaller nuclei appropriates its share of the
enveloping protoplasm, finally splitting it into two parts. Thus from the
larger cell (nucleated piece of protoplasm) we get two smaller cells
(two smaller nucleated pieces of protoplasm). In technical language,
we say that the larger cell is the _mother cell_, and the two smaller
cells that it has divided into are the _daughter cells_. In consequence
of the method of mitosis, the two daughter cells very frequently are
exactly like the mother cell, except in size. But by the absorption of
nutriment, and through digestion and assimilation, they grow and finally
become exactly like the mother cell. This is the simplest illustration of
_heredity_. The reproductive process may be repeated very many times, so
that from one cell we may get millions of cells.[1]
It is necessary to assume that the nutritive and reproductive activities
of _cells_ are based upon and controlled by the nutritive and
reproductive activities of the _physiological units_, inasmuch as these
are the ultimate living units.
In the activities of a cell the nucleus and protoplasm are intimately
correlated with one another.
The nucleus is looked upon by the majority of cytologists as the
_formative center_ of the cell in a chemical, and also, consequently, in
a morphological, sense. Active exchanges of material take place between
the nucleus and the protoplasm during the nutritive processes of the
cell. Possibly this may be altogether a chemical process, or possibly it
may be due, as Hertwig suggests, to the migrations of the _physiological
units_ as carriers and elaborators.
In these exchanges, and in the upbuilding chemical activities (anabolism)
of the cell, the nucleic acid plays a leading part. Here the nucleic
acid in the physiological units of the nuclear threads, combines with
albumins from the protoplasm, forming nuclein. Much of this nuclein,
undergoing further elaboration, is passed into the protoplasm as one of
its finished products (metaplasm). The more purely nutritive the activity
of a cell, the more nuclein its nuclear threads contain; on the other
hand, when the cell is in the phase of reproductive activity, the nucleus
contains little nuclein, and is almost entirely composed of pure nucleic
acid.
[Illustration: FIG. 2.—Stylonychia: c, an entire animal, showing planes
of section; the middle piece of c contains two nuclei and can regenerate
a perfect animal; a, and b, contain no nuclei,—they live and swim about
for a while and then die.]
That the nucleus is the formative center of the cell is indicated
by the following, among many facts: If a unicellular animal, such
as =Stylonychia= (Fig. 2), for instance, be broken up into several
fragments, it will be observed that some of the fragments are nucleated
and others non-nucleated. The nucleated fragments have the power of
quickly healing the wounds on them, regenerating the missing parts,
and thus restoring the mutilated fragments to perfect individuals.
These nucleated fragments have the power to perform all the activities
of the perfect animal. The non-nucleated portions, on the other hand,
cannot undergo regeneration. They cannot digest food, or grow or secrete
substances as the nucleated fragments can. They can simply live for
awhile, responding to stimuli and moving about. They finally perish.
Having mentioned in a general way some of the wonderful powers of cells,
it will be well now to describe briefly a few of the _unicellular plants
and animals_ that can be so easily obtained and studied in warm weather,
and which may thus serve as illustrations of the powers of nucleated
pieces of protoplasm or cells. Many unicellular plants and animals
can be obtained in summer from the superficial ooze on the bottom of
slow-running streams and also on the under surfaces of the leaves of
water plants, a study of which will be of the greatest value and interest.
=Amœba proteus= (Fig. 3). This little unicellular animal, which belongs
to the Rhizopod type, is very common in ponds and streams in warm
weather. In the resting state it is spherical in form, but when active
its form is as changeable as the fabled =Proteus=, hence its name, _Amœba
proteus_. This little creature is a naked piece of protoplasm, with its
outer layer differentiated into a firmer and pellucid part called the
_ectoplasm_ (Fig. 3, ec); its interior, the _endoplasm_ (en), is quite
granular and much more fluid, the granular particles moving quite freely
upon one another when the animal changes its shape. The superficial
portion of the endoplasm is firmer than its more central parts, and
graduates insensibly into the more consistent ectoplasm.
[Illustration: FIG. 3.—Amœba proteus: n, nucleus; cv, contractile
vesicle; ec, ectoplasm; en, endoplasm; p, pseudopodia.]
In the periphery of the granular endoplasm, and adherent to the inner
surface of the ectoplasm, is a clearly defined _nucleus_ (n). When
most distinctly seen, it presents the appearance of a clear vesicle
surrounding a solid and more or less spherical nucleolus. A _contractile
vacuole_ (cv) is also uniformly present, located in the endoplasm. The
creature has the power of putting out projections (p) from the surface
called false feet (_pseudopodia_). Sometimes the protrusion consists
of ectoplasm alone, but more commonly endoplasm extends into it, when
a current of granules will be observed moving from the more central
portions of the Amœba into its protrusion, whilst from some other
protrusion that is being withdrawn a similar current may set towards the
center of the body, and thus the animal moves, in a creeping manner,
from place to place. While moving about in this way the little animal
comes across other one-celled creatures, such as _Desmids_ and _Diatoms_,
seizing them and forcing them through its ectoplasm into the endoplasm,
where the nutritious parts are digested and assimilated. After the animal
has taken its prey through its ectoplasm, no break in the continuity
of the ectoplasm remains, but the parts immediately come together in a
perfect manner. After it has abstracted all the nutriment from its prey,
the Amœba casts away from it the parts that are indigestible.
[Illustration: FIG. 4.—Rotalia Freyeri: a many-shelled Foraminifer, or a
colony of many single-shelled Foraminifera, with pseudopodia extended.]
=Foraminifera.= These are little protoplasmic unicellular animals
that have the power of secreting for themselves more or less complex
envelopes composed of _limestone_. They may be single, as in _Lagena_,
or composed of a number of individuals with the shells cemented together
as in Globigerina or _Rotalia_ (Fig. 4). They have played a part of vast
importance in the geological development of the world. Their myriads of
shells remaining at the bottom of seas millions of years after the little
protoplasmic bodies have perished, they have been consolidated into vast
expanses of limestone rocks, and finally uplifted into such formations as
the huge chalk cliffs of England.
=Osteoblasts and Osteoclasts.= These cells are naked pieces of
protoplasm, the latter much the larger and having many nuclei. They are
concerned in some of the most interesting phenomena of many growing
animals. Just as the Foraminifera have the power of forming complex
aggregations of limestone shells, so the Osteoblasts have the power to
construct the bones of animals. And when a bone is broken as the result
of accident, these little cells do the mending. While the Osteoblasts are
_bone-formers_, the Osteoclasts are _bone-destroyers_. It is very curious
that little specks of living jelly, like these Osteoclasts, should have
the power of destroying hard tissue like bone, but such is the fact.
These Osteoclasts can, by their wonderful chemical processes, liquefy
and absorb, and by these means destroy, ivory pegs that are driven into
living bone. They are the agents by which the roots of children’s milk
teeth are destroyed, so that the crowns of the teeth are shed and the way
paved for the appearance of the permanent teeth. The wonderful activity
of these little Osteoblasts and Osteoclasts is well exemplified in the
growth and shedding of the antlers of deer. While these antlers are
growing in the spring, they are covered with a delicate skin, technically
called “velvet.” This velvet is very sensitive and quite warm from the
nutrient blood circulating through it. In it are hundreds and thousands
of busy, living Osteoblasts that work together under some mysterious,
directing or coördinating agency, to build up the splendid beams, tynes
and snags that constitute the antlers, which in many deer of the Rocky
Mountains reach such a size that a man may walk under the archway made by
setting the shed antlers up on their points. No hive of bees is busier
or more replete with active life than the antler of a stag as it grows
beneath the warm, soft velvet, through the agency of the Osteoblasts.
The building of the antlers by these little agents continues through the
spring and summer. In the autumn the Osteoblasts cease their activity
and die; the delicate, sensitive velvet dries and peels off, leaving the
dead, hard, bony substance exposed, and they now become weapons adapted
for fighting. This is the season when the stags challenge one another to
single combat, the hinds standing timidly by to be taken by the victor
as his mates. When the loves and battles of the autumn are over and the
mating is completed, the antlers no longer serve a useful purpose, and
they are shed. The shedding is accomplished through the agency of the
bone-destroyers, the little jelly-like cells called Osteoclasts.
=Bacteria= are exceedingly minute specks of naked protoplasm. They are
unicellular plants. Some of them are harmless to mankind; some are very
useful to him, and others are his deadly enemies. Many of them are
concerned in the production of the infectious diseases. They do so by
elaborating various chemical products that are virulent poisons, hence
these products are called toxines; when taken up by the blood, they are
carried to various parts of the body. In this manner they cause the
particular symptoms that are characteristic of a special infectious
disease. Why is it that some persons, on exposure to an infectious
disease, contract the malady while others similarly exposed do not? In
other words, what gives immunity to disease? The explanation is probably
as follows: Just as the invading bacteria have the power of secreting
_toxines_, so the cells of the body, normally, have the power of
elaborating chemical products that are antidotes to the toxines, and are
appropriately called _antitoxines_. Infectious diseases and immunity from
them, are the result of a contest between the invading bacteria and the
protecting cells of the body. If the bacteria secrete toxines in greater
quantities than can be neutralized by the cells of the body, we have
disease; if the reverse occur, we have immunity.
The =white blood-corpuscles= (cells) also take part in this warfare. They
have the power of traveling, in virtue of their amœboid movements, from
the blood to the part invaded by the bacteria. Here a contest takes place
between them, the corpuscle takes the bacteria into its interior, and
either kills them or is itself killed. The result of this contest helps
to produce either immunity or infection.
=Tetanus bacillus= is a cell shaped like a slender rod. It has the power
of secreting a poison which, when introduced into the body, produces
convulsions and other symptoms of lockjaw. These much resemble those
induced by strychnine poisoning.
=Bacillus diphtheriæ= is an exceedingly small unicellular plant, and has
the power of producing a poison called _toxalbumin_, which is analogous
to the poison of certain venomous serpents. It is the speck of protoplasm
through whose activity diphtheria is caused.
Many useful bacteria have the power of so acting on dead organic bodies
as to decompose them, the three most conspicuous end-products of this
decomposition being water, carbonic oxide and ammonia. When the dead
bodies are decomposed in the soil there are other bacteria, in addition,
that have the power of further acting on the ammonia, causing its
oxidation and producing nitrous and nitric acids and their salts. The
unicellular plants that bring about these changes are the _nitrifying
bacteria_. Conspicuous illustrations of the functional activity of these
little naked pieces of protoplasm are seen in the immense saltpeter
beds of Peru and Chili, where, from the enormous fecal accumulations of
sea-fowls, the immense quantities of nitrates are produced that supply
the commercial world.
[Illustration: FIG. 5.—Difflugia Pyriformis.]
=Arcella=, of which there are many species, is a unicellular animal whose
protoplasmic body has secreted from its surface an enclosing “test” that
is composed of a _horny membrane_, resembling very much in constitution
the chitin which gives firmness to the integuments of insects. This
creature is commonly discoidal in shape, with one face arched and the
other flat, an aperture being situated in the center of the flat side
through which the creature may thrust its pseudopodia or withdraw them.
The surface of the testaceous covering is often marked with a regular
but minute and attractive pattern. In =Difflugia= (Fig. 5), the test
is somewhat pitcher-shaped, and is mostly made up (by the constructive
activity of the protoplasm) of exceedingly small particles of shell and
gravel cemented together. Many testaceous amœbans form tests of singular
beauty and remarkable regularity. In some of the animals the minute
plates of which the tests are formed have been picked up from the surface
over which the animals crawl, and are cemented into various charming
patterns; and in other cases they are formed by secretion from their own
bodies. In =Quadrula symmetrica= the protoplasmic body has constructed
a pear-shaped testaceous covering, of complete transparence-like glass,
composed of a great number of square plates touching each other by
their edges. The protoplasmic body of the animal does not entirely
fill the test, the intervening space being occupied by a clear liquid
and traversed by bands of protoplasm. A clear, large spherical nucleus
is seen in the part farthest from the pseudopodia. It contains a dark
and well-defined nucleolus. In front of the nucleus two contractile
vesicles are to be observed. The pseudopodia in these creatures, it
must be remembered, are not appendages, but lobate protrusions of the
protoplasmic body, are few in number, rounded, short and broad.
=Diatoms= are unicellular plants, isolated or aggregated together, that
have the power of constructing flint coverings, often of great complexity
and charming pattern. The tracings on many of these flint coverings are
so constant and small, that they are frequently employed for the purpose
of testing the power of modern compound microscopes. In various parts
of the world vast deposits of Diatoms have been discovered. The most
remarkable of these for extent, as well as for the beauty and number of
the species contained in it, is that on which the city of Richmond, in
Virginia, is built, which is over thirty feet deep and extends for many
miles.
[Illustration: FIG. 6.—Noctiluca miliaris. A, dorsal view; B, side view;
n, nucleus; f, flagellum; a, entrance to atrium; b, atrium; o, œsophagus;
r, superficial ridge.]
=Noctiluca miliaris= (Fig. 6) is a very large unicellular, flagellate
animal. It is spheroidal in form, and has an average diameter of not
quite one-half a millimeter. It is just large enough to be observed
by the unaided eye when the water in which the animal may be swimming
is contained in a glass jar held up to the light. It has a tail-like
appendage (flagellum) by which the animal moves about. Along one side
of the cell is a meridional groove resembling that of a peach, and
leading into a deep depression of the surface termed the atrium (Fig.
6, B, b). It is from the shallow commencement of this depression that
the flagellum (Fig. 6, f) originates. At the base of the flagellum the
depression sinks down to the mouth (o). A slightly elevated ridge (r)
extends along the opposite meridian and commences with a bifurcation at
that end of the atrium farthest from the flagellum. The mouth opens into
a short œsophagus, which leads down directly to the central protoplasmic
mass. The central protoplasmic mass sends off branching prolongations
of its substance in all directions, the ramifications of which freely
inosculate. The farther these ramifications extend out to the periphery,
the thinner they become, until finally a protoplasmic network of extreme
tenuity is formed immediately under the enveloping membrane of the cell.
In addition to these ramifying prolongations, the central protoplasmic
mass sends off a thin, broad, irregular extension to the superficial
ridge and coalesces with it. Near the central protoplasmic mass is seen
the nucleus (n).
The flagellum is a flattened, whip-like filament, having a striated
appearance, and gradually tapers from the base to its extremity. It
slowly bends over five or six times a minute to the mouth, and then,
more slowly still, bends away again. It is through the movements of the
flagellum that particles of food are driven into the mouth and down
the œsophagus into the central protoplasmic mass. In this mass and its
extensions the food is digested and assimilated.
This little one-celled animal has the power, through its special chemical
activities, of manufacturing and emitting light. It is through the agency
of myriads of these little creatures that the diffused luminosity of some
seas is produced and can be observed at night. The Noctiluca is very
transparent, and for this reason it is a particularly favorable subject
for the study of its luminosity or phosphorescence. They can be obtained
by the tow-net in unlimited quantities from the sea and transferred into
a jar of sea water. Here they soon rise to the surface, forming a thick
layer. If the jar be placed in the dark and agitated in the slightest
degree, there is an instantaneous display of light, which is of a
beautiful greenish tint. The light emitted by the Noctiluca is so vivid
that it can even be observed in ordinary lamp-light. This phosphorescence
is only of an instant’s duration, and a short rest is necessary for its
renewal. The special locality for the formation of the phosphorescence
is in the very fine protoplasmic network, which lines the external
structureless membrane or cell wall. These wonderful little Noctilucæ may
well be figuratively called the fire-flies of the ocean.
[Illustration: FIG. 7.—Gromia oviformis with protoplasmic threads
(pseudopodia) extended and forming an elaborate network in which a
captured unicellular organism is seen; d, diatom captured; p, protoplasm
containing captured diatoms; s, shell.]
=Gromia oviformis= (Fig. 7) is often found in fresh water adhering to
confervæ and other plants of running streams. The protoplasmic body of
this animal is enveloped in a chitinous covering that is egg-shaped and
of a brownish-yellow color. It is about two millimeters in diameter.
When the animal is quiet, no one would suspect its real nature, so much
does it look like the seed of an aquatic plant. The testaceous envelope
has a single round orifice at its more pointed end. The animal, when in
an active state, pushes out the protoplasmic substance, which speedily
gives off ramifying extensions, and these by further ramification and
inosculation form a complicated network. The protoplasm of the animal
also extends itself in such a way as to form a continuous layer on the
external surface of the test. From this layer numerous protoplasmic
threads may extend out, forming more or less complicated networks. By
the alternate contraction and extension of its protoplasmic threads and
networks, minute one-celled plants and animals are entrapped like flies
in a spider’s web (d). When caught they are carried, by retraction of the
protoplasmic thread-like pseudopodia, into the endoplasm in the test.
Here the nutritious parts of the entrapped creatures are abstracted and
assimilated. In transparent species, the indigestible parts, such as the
silicious valves of diatoms, may be distinguished in the midst of the
endoplasm, from which they are ultimately extruded.
When gromia oviformis reproduces by mitosis it gives off a bud (small
cell), which finally separates from the parent form and constitutes a
distinct individual. This process may be repeated many times, so that a
great number of separate individuals may be formed, all of which lead
detached and independent lives.
Most of the protozoa, which are produced by fission (cell division by
mitosis), separate entirely from each other, as in Gromia; but in many of
these unicellular animals, the new creatures produced by fission do not
separate from one another, but remain more or less closely connected, and
thus form _colonies_ of Protozoans. These colonies are of the greatest
interest, for they represent a lower stage of the _cell colonies_ of the
Metazoa (multicellular animals). They reproduce, in many cases, in a way
which is strongly suggestive of reproduction in the Metazoa.
=Microgromia socialis= is a little unicellular animal, having a thin,
nearly globular, calcareous shell that it secretes upon its surface. It
multiplies by fission, and forms a number of distinct individuals which
have the curious habit of fusing their pseudopodia and uniting into
a more or less closely associated colony. The individuals sometimes
remain at a distance from one another, but sometimes associate themselves
together into a compact colony. These individuals are all alike,
performing the same functions. There is no division of labor among the
units, but they live practically an independent life. If the individual
animals were detached from one another, they would live and build new
colonies.
=Codosiga umbellata= is another unicellular animal—a flagellate
Protozoan. It has a collar-like extension of its ectoplasm from the
anterior extremity of its body, forming a sort of funnel from the bottom
of which the thread-like structure (flagellum) arises. The vibrations of
this flagellum cause a current of the surrounding fluid to set into the
funnel so that particles of food reach the soft protoplasmic substance
which serves as a mouth. The nucleus is seen near the base of the collar.
Near the posterior extremity of the body two contractile vesicles are to
be observed. This posterior extremity of the animal has a cylindrical
extension of its ectoplasm by which it attaches itself to an object. This
protozoan multiplies by longitudinal fission. In some species the animals
separate completely from one another and lead entirely independent lives.
But in _Codosiga umbellata_ the fission does not extend through the
cylindrical extension, so that a group of animals are associated in a
colony.
=Rotalia or Globigerina.= The shelled amœba (_Lagena_) gives off a bud
(unseparated cell) which grows to the full size, secretes its calcareous
shell and remains connected with the parent form. This process may be
repeated a number of times until a colony of shelled amœbæ, of varying
pattern in different species, may be formed and permanently associated
together (Fig. 4). The individual amœbæ are all alike and perform the
same functions. There is no division of labor, no specialization, among
them. If the individual animals could be separated from one another they
would live and build new colonies.
=Pandorina morum= forms a small colony of sixteen cells (solid sphere) of
mulberry-like shape and enclosed in a common gelatinous envelope. Each
cell in the mulberry mass bears two flagella on its peripheral end. These
project out beyond the surface of the gelatinous envelope, and are agents
for locomotion of the colony. The cells in the colony are all alike.
There is no division of labor among them. They all act alike. The cells
(flagellate protozoans) of the colony may reproduce in two ways. Each
animal in the colony may subdivide into sixteen smaller units, each of
which by growth and multiplication may form a new mulberry mass, a new
colony, each unit of which acquires two flagella. Or two of the small
units may amalgamate (conjugate), and then develop (by fission) into a
new colony. The conjugating units are nearly of the same size and look
very much alike.
=Volvox globator= is a spheroidal shaped colony (hollow sphere) of
unicellular flagellate animals, about one-half a millimeter in size.
It was formerly supposed to be a fresh-water _Alga_. It is now known
to be a colony of _Protozoans_. All the animals in this colony are not
alike. There is a division of labor among the cells, for some are merely
vegetative, serving purposes of nutrition, and having no reproductive
powers; while other members of the colony are purely reproductive
animals. Furthermore, there is quite a marked specialization of the
reproductive cells. Those reproductive cells that may be spoken of as
the female cells are large and non-motile encysted cells. The male cells
are small and actively motile, in that they have two flagella developed
on them. A small flagellate male cell penetrates the large encysted
female cell, and as the result of this conjugation, fission takes place
repeatedly, and a new colony of flagellate protozoans (volvox) is formed.
Volvox approaches very suggestively towards the type of animals known as
Metazoan.
In order to comprehend, in some measure, the transition from _Colonial
Protozoa_ to _Metazoa_, it will be well for the reader to study a
typical sponge. For a long time the Porifera (Sponges) were looked upon
as compound Protozoa (colonial Protozoa), but while they are nearer the
Protozoa than any of the other types of Metazoa, their position in the
animal series is unquestionably among the Metazoa. The Sponge, like the
rest of the Metazoa, develops from a fertilized egg by a process of cell
multiplication, differentiation, gastrulation, etc.
CELL REPRODUCTION BY MITOSIS.
The multiplication of cells plays a part of such fundamental importance
in Evolution, and therefore in Embryology and studies in Heredity,
that it is necessary to study the subject somewhat in detail. It is a
wonderful process, and is worthy of very careful attention.
[Illustration: FIG. 8.—Diagram illustrating Mitosis. A, the cell
commencing activity; B, C, D, phases in the formation of the spindle and
the chromatin loops or V’s, also showing that the mother V’s have split
into daughter V’s; D, the chromatin loops forming the equatorial plate,
_chr_; E, F, G, separation of the daughter loops (daughter chromosomes)
and their passage towards the poles of the spindle, thus forming daughter
nuclei; H, I, division of the protoplasm so as to form two daughter
cells; at, attraction sphere enclosing a centrosome; n m, nuclear
membrane; chr., chromatin threads; p, protoplasm; c w, cell wall; sp,
spindle.]
The process by which one cell (a mother cell) divides into two
cells (daughter cells) is called _mitosis_, and is inaugurated by the
_centrosome_ (Fig. 8, A, _at_). The centrosome divides into _two_
centrosomes, which at first remain close together (Fig. 8, B, _sp_),
and then gradually separate from one another. Each centrosome becomes
the center of a system of fine achromatin fibers arranged round it in
a radiating manner and forming what is called the _attraction sphere_;
also, at the same time, a spindle-shaped bundle of achromatin fibers,
called the _spindle_ (Fig. 8, B, sp), extends between the centrosomes.
In the meantime, important changes have been taking place in the
chromosomes (hereditary threads) of the nucleus. The chromosomes, which
at first are arranged in an apparently inextricable tangle or network,
frequently assume U-shaped or V-shaped forms (Fig. 8, C, _chr_), and
the nuclear membrane disappears. Sooner or later each chromosome splits
longitudinally into two daughter chromosomes, with which the achromatin
fibers of the spindle become connected (Fig. 8, D). In this phase of
mitosis the split V-shaped chromosomes form a single group called the
_equatorial plate_ (chr), and extend across the axis of the spindle.
It is to be observed from the diagrams in the figure, that one of the
centrosomes has traveled to the opposite pole of the nucleus, thus
causing the achromatin fibers of the spindle to extend across the
original site of the nucleus. The equatorial plate of split V-shaped
mother chromosomes (hereditary threads) thus divides the fibers of the
spindle into two parts, one half extending from one centrosome to one
group of daughter chromosomes, while the remaining half extend from the
other centrosome to the other group of daughter chromosomes. Soon the
achromatin fibers of the spindle contract, and in this way separate the
two groups of daughter chromosomes, so that one group is drawn towards
one centrosome, and the other group to the other centrosome (Fig. 8, E,
F, G, H). After the two groups of daughter chromosomes have been drawn to
their respective centrosomes, each group assumes the tangle or network
phase like the nucleus of the mother cell, and an investing nuclear
membrane reappears for each (Fig. 8, I). Thus from the mother nucleus of
the mother cell we get two daughter nuclei (I). In a further phase of
the mitotic process, a furrow appears on the surface of the protoplasm
and surrounds it in the form of a ring. This furrow is in a plane at
right angles to the long axis of the spindle, and gradually deepens until
the protoplasm is divided into two parts, each segment of protoplasm
containing its own nucleus and centrosome; in short, the mother cell has
divided into two daughter cells (I).
It will thus be observed that the centrosomes and their achromatin fibers
are a beautiful mechanism by which the heredity threads (chromosomes) are
exactly divided into two equivalent halves.
There are some cells (_Amœba proteus_, for instance,) which divide in a
much simpler manner than by mitosis; in these there is no complicated
rearrangement of the chromosomes and no disappearance of the nuclear
membrane, the nucleus simply becoming separated into two parts
(_Amitosis_).
=The Human Ovum.= The human ovum is a typical cell about one-fifth
of a millimeter in size and spherical in shape. It is a nucleated
piece of protoplasm possessing an enveloping cell-wall (metaplasm).
The protoplasm contains nutrient material or yolk (metaplasm). The
_maturation_ of the ovum essentially consists in throwing out half of its
chromosomes. In doing this the nucleus (mother) approaches the surface of
the protoplasm (Fig. 9, A), and divides, by mitosis, into two daughter
nuclei; then the unripe ovum divides into two cells, but of very unequal
size. This process is repeated a second time. Thus two small cells are
formed which are known as the _polar bodies_ (Fig. 9, A, B, pol. b). The
large cell remaining after the formation of the two polar bodies is the
_mature ovum_ (B). Its nucleus, which recedes towards the center of the
protoplasm, is called the female _pronucleus_ (Fig. 9, B, f. pr.). The
pronuclei of mature ova differ from the nuclei of all the other cells of
the body in that they only contain half as many chromosomes (hereditary
threads).
The ripe _spermatozoid_ (a flagellate sexual cell of the male)
corresponds to the ovum of the female. It also contains a pronucleus
having only half the number of chromosomes that the other cells of the
adult body possess.
=Fertilization.= The male sexual cell (_spermatozoid_) is vastly smaller
than the female sexual cell (ovum). Having a _flagellum_ (Fig. 9, B,
s), it moves about, like a tadpole in water, and seeks the ovum. When
it comes in contact with the ovum it penetrates into its interior
(usually only one doing so), as indicated at (Fig. 9, B, s). The tail or
_flagellum_ of the spermatozoid fuses with the protoplasm of the ovum,
and disappears from view. Its _pronucleus_ (C, m. pr.), accompanied
by its _centrosome_ (C, m. c.), approaches the _female pronucleus_
(f. pr.) of the ovum (Fig. 9, C, D). Finally the male and female
pronuclei coalesce to form a single nucleus (Fig. 9, E, n. os.). The
centrosome of the ovum persists for awhile and then disappears; that of
the spermatozoid remains in the ovum, and is the agency by which cell
multiplication, through _mitosis_, takes place.
[Illustration: FIG. 9.—Diagram illustrating the maturation and
fertilization of the human ovum. A, one polar body is formed and a
second is in process of formation; B, both polar bodies are formed and
a spermatozoid is penetrating the ovum; C and D represent the approach
of the male pronucleus towards the female pronucleus; E indicates the
amalgamation of the two pronuclei to form the nucleus of the oösperm
(segmentation nucleus); pol. b, polar bodies; pol. c, centrosome of the
polar body; chr. p, chromatin of the polar body; f pr, female pronucleus;
p, protoplasm; p p, peripheral protoplasm (but _not_ cell wall); f c,
female centrosome; m c, male centrosome; m pr, male pronucleus; n, os,
nucleus of the oösperm (first stage of a human being).]
The cell resulting from the coalescence of the male pronucleus with the
pronucleus of the ovum, and which is only one fifth of a millimeter in
size, is the first stage in the existence of a human being. Man thus
starts his career as a Protozoan-like creature,—as a unicellular animal.
The fertilized ovum is called the sperm-egg (_oösperm_), and contains now
the normal number of _hereditary threads_ (chromosomes); for those of the
male have been added to those of the female.
=Segmentation of the Oösperm.= The fertilization of the ovum imparts
to it a wonderful stimulus, so that the oösperm divides, by mitosis,
into two cells, these two into four, the four into eight, the eight
into sixteen, these into thirty-two, and so on repeatedly, until a
large number of comparatively small cells are formed (Fig. 10). This
mass of cells is spherical in shape, and the little round cells towards
the surface project in such a way as to give to the mass an appearance
somewhat similar to the fruit of the mulberry, whence it is termed the
mulberry body or _morula_ (Fig. 10, 4). In the morula stage of his
existence man resembles the solid colony of protozoans represented by
_Pandorina_. The cells of the morula next become arranged regularly in a
single layer at the circumference, by which the embryo assumes the form
of a hollow sphere, and is known as the _blastula_ (Fig. 10, 5). This
phase of man’s existence is quite suggestive of _Volvox_.
Soon one side of the blastula is invaginated or pushed in, as one
would push in one side of a hollow india-rubber ball. The result of
this invagination (called _gastrulation_ technically) is the formation
of a sort of cup. This is the _gastrula_ (Fig. 10, 6) phase of man’s
existence. It is a higher and fundamentally different phase of existence
than either the morula or the blastula. It corresponds, not to a
Protozoan, but to the higher Metazoan. It possesses the fundamental
anatomical qualities of a low Cœlenterate (Polyp).
The mode of gastrulation is different in man from that just described,
and varies in different animals, but the essential point common to all
is the formation of a double cellular-membrane; the outer membrane being
called _epiblast_ or ectoderm, and the inner one _hypoblast_ or endoderm,
the enclosed cavity being the primitive digestive cavity. These two
layers are the primary germinal layers. A third layer is subsequently
formed between them, by the agency of one or both of them, and is called
the _mesoblast_. From these three simple membranes, which are composed
exclusively of cells, are formed all the complex tissues and organs of
the adult man. The _epiblast_ develops into the nervous system (brain,
spinal cord, and nerves), and the cuticle, hair, etc. The _hypoblast_
develops into the cellular parts of the digestive canal, the liver,
lungs, etc. The _mesoblast_ develops into the muscles, bones, ligaments,
blood-vessels, etc. To trace the details of this evolution of a human
being from the microscopic _oösperm_ is a fascinating and instructive
study, but is beyond the limits and purpose of this little book. It will
well repay further careful study by the reader.
A careful study of the life-histories of the few unicellular animals and
plants mentioned in the preceding pages will help to let one realize the
wonderful powers of nucleated pieces of protoplasm (cells). If isolated
pieces of protoplasm can accomplish so much, one will not be astonished
to learn that many diverse cells associated together in intimate
correlations, as occurs in the higher animals, may accomplish results
that are profoundly interesting and marvelous. As far as anatomy and
physiology alone can reveal, it is the result of millions of cells acting
together that makes possible the existence of such a living, sentient,
thinking creature as man or the highly intelligent elephant or any other
multicellular animal. It is due to the mysterious powers of protoplasm
that one little microscopic cell, like the fertilized ovum of a woman, is
able to hold all the heritages of the race, and gradually unfold them as
it builds up the body into myriads of diverse cells intimately associated
together.
All of the wonderful results of Embryology are accomplished through
cell-multiplications, cell-differentiations, cell-associations,
invagination and evaginations of cell-groups (tissues or organs), and
unequal growth of parts (cells or groups of cells).
[Illustration: FIG. 10.—Segmentation of the fertilized ovum and
Gastrulation: 4, morula; 5, section through blastula showing hollow
sphere; 6, gastrula showing outer layer of cells (epiblast) and inner
layer (hypoblast); the 6 is at the mouth of the cavity (enteron) of the
gastrula.]
In concluding this brief but, we hope, useful study of a few selected
cells, we may say that an eminent English physiologist has made the
statement that a student who has not looked through the microscope and
observed the circulation of the blood in the web of a frog’s foot is not
fit to study medicine. However beautiful, fascinating and instructive the
sight of this circulation may be, we are tempted to make the assertion
that the student who has not looked through the microscope at some of
the superficial ooze from the bottom of any slow-running stream, in
summer, and observed the structure and actions of that wonderful little
unicellular animal, the Amœba proteus, is still less prepared to study
medicine. In this little speck of protoplasm the problems of life are
reduced to their simplest forms, for all higher plants and animals may be
regarded as groups of more or less modified amœbæ peculiarly associated
together. In our studies of the amœba we will be forcibly reminded
of a very clever trick which is practiced in India and is called the
mango-trick. In this trick a seed is put into the ground and covered up,
and after divers incantations a full-blown mango-bush appears within five
minutes. We have never met any one who knew how this thing was done, nor
have we ever seen a person who believed it to be anything else than a
conjuring trick. So it is with the _amœba_, a beautiful and fascinating
trickster of nature. We understand some of its activities, interesting
and exceedingly instructive, but there are many others beyond our ken. We
see the commencement and ending of many of its chemical activities, but
there are numerous other intermediary processes that take place in the
hidden recesses of the protoplasm, and concerning which we know nothing.
It may fall to the lot of some reader of this little book, as a patient
and keen observer, to unravel some of these mango-like tricks of the
amœba or other unicellular creature.
SECTION II.
HEREDITY WITH VARIATION.
HEREDITY WITH VARIATION.
That an offspring always inherits from its parents many of their
characteristics is well known; that it always varies, more or less, from
them is also equally well known. Heredity and variation are twin forces
that play upon every creature, holding it rigidly true to the parental
type or compelling more or less divergence therefrom, according to
the strength of the one or other power; so that every creature is the
resultant of the activities of these two great parallel forces. Variation
is coextensive with heredity, and every living creature gives evidence of
the existence of variations.
=Examples of Variations.= No two leaves on a plant are exactly alike;
no two children of the same parents give a perfect resemblance; no two
individuals of the same species are molded in precisely the same pattern;
of the thousands and thousands of faces that we observe in a city in the
course of a year, each has some distinctive peculiarity.
The trained eye of the gardener recognizes each hyacinth among hundreds
of bulbs; of the shepherd, each sheep in his flock; of the Laplander,
each reindeer crowded in his herd like ants on the anthill. In a flock of
1,000 sheep each mother can even recognize a variation in the _voice_ of
her own lamb, all alike to _us_.
Every part of an animal is subject to variations, not only in bodily
structure, but also in habits and instincts, and these variations are
large in amount, numerous and diverse in character. Many observations,
experiments and measurements that have been made at various times attest
the truth of this assertion. Not only do variations take place in animals
and plants under domestication, but also in the wild state.
=Illustrations of Heredity.= _Mental heredity_ can be illustrated by
studying the genealogies of such persons as Aristotle, Goethe, Darwin,
Coleridge, Milton, etc. Probably the Bach family, of Germany, supply one
of the best illustrations of the inheritance of intellectual character
that we know of. The record of this family begins in 1550, lasting
through eight generations to 1800. For about two centuries it gave to the
world musicians and singers of high rank. The founder was Weit Bach, a
baker of Presburg, who sought recreation from his routine work in song
and music. For nearly two hundred years his descendants, who were very
numerous in Franconia, Thuringia, and Saxony, retained a musical talent,
being all church singers and organists.
When the members of the family had become very numerous and widely
separated from one another, they decided to meet at a stated place once a
year. Often more than a hundred persons—men, women and children—bearing
the name of Bach were thus brought together. This family reunion
continued until nearly the middle of the eighteenth century. In this
family of musicians twenty-nine became eminent.
_Inheritance of moral character_ is well established. Heredity, in its
relation to crime and pauperism, has been thoroughly investigated by Mr.
Dugdale in his most instructive little work entitled _The Jukes_. In
this work the descendants of _one_ vicious and neglected girl are traced
through a large number of generations. It reveals that a large proportion
of the descendants of this woman became licentious, for, in the course of
six generations, fifty-two per cent. of the females became harlots and
twenty-three per cent. of the children were illegitimate. It shows also
that there were seven times more paupers among the women than among the
average women of the State, and nine times more paupers among the male
descendants than among the average men of the State.
The inheritance of _physical peculiarities_ is so obvious as to need
no illustration. Among the ancients the Romans stereotyped its truth
by the use of such expressions as the _labiones_, or thick-lipped;
the _nasones_, or big-nosed; the _capitones_, or big-headed; and the
_buccones_, or swollen-cheeked, etc. In more recent times we read of the
Austrian lip and the Bourbon nose.
Questions of heredity and variation are cytological ones—that is,
questions of the anatomy, physiology, physiological chemistry, and
pathology of cells. The most important part of a cell, as far as these
questions are concerned, is the =nucleus=. The nucleus is the physical
basis of all the heritages of an organism, from the simplest to the most
complex. The nuclear threads may, therefore, very appropriately be termed
the _hereditary threads_, or, collectively, the hereditary mass; and
the physiological units in them the _hereditary units_. The nucleus is
of fundamental importance in the reproduction or multiplication of both
unicellular and multicellular animals and plants.
In unicellular creatures multiplication may take place by fission and by
conjugation. Both of these processes can be studied by observation of
the infusorians. Maupas’s beautiful investigations on these unicellular
animals have demonstrated that multiplication by fission may proceed to
a prodigious extent for many generations, but that a time comes when
the process fails, and the species will become exhausted and die out
unless there is a rejuvenation of it by conjugation of individuals. In
conjugation two individual infusoria come in apposition with each other,
the nucleus in each undergoes subdivision. They reciprocally exchange
part of their nuclear contents so that each infusorian comes to contain
hereditary threads of two distinct individuals. From these rejuvenated
(or fertilized) individuals multitudes of others may be derived by
fission until exhaustion again takes place.
Multiplication in multicellular creatures may be accomplished by budding
(which is allied to fission), and is exemplified in the plant, hydra,
the queen bee (parthenogenesis), etc., and by fertilization (which is
allied to conjugation). A knowledge of the phenomena of fertilization of
the ovum by the spermatozoid is essential to any understanding of the
problems of heredity and variation in mankind. The nuclear threads of
the ovum are its hereditary threads—the _groups of maternal hereditary
units_; likewise, the nucleus of the spermatozoid contains the _paternal
groups of hereditary units_.
=Fertilization.= In fertilization, the spermatozoid (a nucleated
flagellate cell) penetrates the ovum (a nucleated, encysted cell), its
protoplasm mixes with that of the ovum, and its nuclear threads come into
relation with the nuclear threads of the ovum; so that the fertilized
ovum (a new creature, a veritable microcosm) is still a nucleated cell,
but one in which the nucleus is compound, is _hermaphroditic_, in that
it contains maternal and paternal threads—that is, maternal and paternal
hereditary units which constitute its hereditary mass.
It will be convenient to speak of the maternal and paternal hereditary
units in the fertilized ovum as _ancestral hereditary units_.
This hermaphroditic cell passes through complex phases, illustrated by
embryology, to the adult. In doing so this hermaphroditic cell (mother
cell) first divides into two smaller cells (daughter cells). The mother
cell divides in such a way (by mitosis) that one-half of its nucleus
and part of its protoplasm goes to one daughter cell and the other half
of the nucleus, with the remainder of the protoplasm, goes to the other
daughter cell. It is an interesting fact that although the amount of
protoplasm which goes from the mother cell to the two daughter cells may
be unequal at times, yet the amount of the nucleus in one daughter cell
is always exactly equal[2] to that in the other; _so that each daughter
cell contains maternal and paternal hereditary masses of equal quantity
and quality_, being in fact one-half that of the fertilized ovum. In
consequence of the fact that each hereditary unit in the nucleus of the
daughter cell can absorb nutriment and grow, it comes about that the
nucleus of each daughter cell attains to the size of that of the mother
cell. The enveloping protoplasm of the nuclei also grows to a greater or
less extent, so that the cells as a whole grow. These two daughter cells
go through the same process and form other daughter cells, and so on
through all the mitoses of development, until all the myriads of cells
of a living organism are produced, _each of which contains maternal and
paternal hereditary masses of equal quality and quantity, and also of
the same character as that of the fertilized ovum whence they are all
derived_.
Apart from their activity in absorbing nutriment and growing, the great
majority of the hereditary units in the nuclei of the forming cells
remain latent. But some of the hereditary units in each cell produced are
active. They multiply, grow and migrate out of the nucleus, and get among
the units of the enveloping protoplasm. During this activity they undergo
physical and chemical changes and effect corresponding differentiations
in the protoplasm of the cell. Thus, through many mitoses and many
differentiations of the protoplasm of cells, we finally derive from the
fertilized ovum all the cells that constitute the adult body, such as
muscle cells, glandular cells (as liver cells, kidney cells, etc.), nerve
cells, skeletal cells (as bone, cartilage, and connective-tissue cells),
and the ova and spermatozoids. According to this theory, the _nucleus_ of
each cell in the adult animal or plant is _pure hereditary mass_, exactly
like that in the nucleus of the fertilized ovum; but the _protoplasm_
of each adult cell that envelopes the nucleus may differ greatly in
different cell groups, as in muscle, nerve, cartilage, and the like. Of
course, the protoplasm of those cells that develop into the ova and the
spermatozoids has differentiated along such lines as to become like that
of the ovum and spermatozoid, the junction of which formed the fertilized
ovum. These statements hold true for plants and the lower animal
organisms, although they cannot be verified for the higher animals. More
than likely the pure hereditary masses are present in the body (somatic)
cells of the higher organism in latent conditions, but are unable ever to
be developed owing to the greater specialization of these cells. It is
thus seen that all the cells of many animals and plants can perform their
own special functions and at the same time contain all of the hereditary
units of the complex organism in a state ready to develop under favoring
conditions.
Since the ova and spermatozoids are cells specially differentiated for
the purpose of propagating the species by sexual generation, and since
their conjugation produces the germ of a new creature, they may very
appropriately be spoken of as =germ cells=. Since all the other cells of
the adult form the great bulk of the body that envelopes and protects the
germ cells, they may be termed the =body cells=, or =somatic cells=.
Suppose that ova, containing maternal and paternal groups of hereditary
units, are fertilized by similarly complex spermatozoids, and the process
is repeated generation after generation. There will come a time when the
fertilized ovum will have a highly complex nucleus composed of _many
different ancestral groups of hereditary units_.
One often hears the expression that a child is a chip of the old block;
but this is only a very partial truth, for a child is preëminently a
composite chip of many old blocks.
The complex nucleus of the fertilized ovum may be compared to a modern
Italian building which has been constructed of material—a column here, a
cornice there, a lintel yonder—gathered from different classic buildings
of varying antiquity. In view of the increasing number of ancestral
groups of hereditary units that must have accumulated in the nuclei
of ova in the course of time, there must necessarily, for mechanical
reasons, have arrived a period when these nuclei could receive no more
of them by fertilization, unless natural selection should develop some
saving device; hence we have, possibly, an explanation of the phenomena
of maturation in ova (the _reducing process_ of Weismann and Hertwig).
Here the ovum, prior to fertilization, undergoes mitosis twice in
succession, by which the polar bodies are formed and the hereditary mass
is diminished by one-half (the mature germ cells having only one-half
the number of nuclear threads that the body cells possess). A homologous
process takes place in the maturation of spermatozoids. Fertilization
increases the amount of the hereditary mass in the ovum to the original
quantity, and thus restores the number of nuclear threads to the specific
number. All the body cells derived from the fertilized ovum possess,
also, the specific number of threads.
This union of two distinct hereditary masses is called _amphimixis_ as
well as _fertilization_.
Maturation and amphimixis or fertilization are _the source of many
variations in the body_, good and bad, beautiful and ugly, geniuses and
monstrosities; because, in the commingling of distinct hereditary masses,
there is a struggle for existence between the hereditary units and a
survival of the fittest.
In this struggle some of the hereditary units are strengthened so that
heritages may be augmented; some mix so that there may be a blending
of characteristics; some are mutually exclusive; some are prepotent;
some are neutralized; some are destroyed; some lie dormant (latent) for
varying lengths of time, and some are so altered as to produce much
modified forms; and thus the possibilities of combination, reactions
and modifications of the hereditary units, and therefore heritages, are
almost endless.
The _augmentation of heritages_ in the fertilized ovum is well displayed,
for instance, where fleet horses are bred with fleet ones, until, by
careful selection, generation after generation, a progeny may be secured
much more swift than the original stock from whence they were derived. In
the same way good milch cows have been produced.
The mixing of hereditary units and _blending of heritages_ is shown in
the color of the skin, as where a mulatto child is born to a negress by
a white father; _mutually exclusive heritages_ are well illustrated in
the color of the eyes, as where a child has either the blue eyes of one
parent or the black of the other, but never any blending of the colors;
this may also be illustrated where the white game bird and black one
are crossed, the young being either white or black, but never blended.
_Prepotency_ is illustrated where the silky variety of fan-tailed pigeon
is mated with any other small-sized variety of pigeon, for the silkiness
is invariably transmitted. A most interesting case of prepotency in
mankind, mentioned by Ribot, is that of Lislet-Geoffrey, an engineer in
Mauritius. He was the son of a very stupid negress and an educated white
man. In physical constitution he was as much a negro as his mother; he
had the woolly hair, the features, the complexion, and the peculiar odor
of his race. He was so thoroughly a white man as regards intellectual
development that he succeeded in vanquishing the prejudices of race,
so strong in the French colonies, and in being admitted into the most
aristocratic houses. At the time of his death he was Corresponding Member
of the Academy of Sciences.
In this, it will be observed, we have prepotency in the mother’s physical
constitution, and in the father’s intellectual characteristics.
The struggle of heritages in the impregnated ovum may lead to such
structural changes of the nucleus, and therefore of the cell, as to
develop the most marked variations—such variations as the biologists call
_sports_.
In the latter part of the eighteenth century the farmers of Massachusetts
had flocks of ordinary sheep on their farms. These sheep were continually
jumping fences and getting on neighboring farms. They were the source of
many disputes and much irritation between neighboring farmers. Finally,
one of the sheep had a lamb which, when grown, displayed well-marked
peculiarities (a sport). It had a longer body than the ordinary sheep
and shorter legs, which were bowed. It was noticed that this sheep could
not get over the fences. The cute Yankee farmer, noticing this valuable
peculiarity, carefully preserved this peculiar sheep, and from it was
ultimately derived, by careful selective breeding, a special variety
known as the Ancon sheep.
The germinal variations resulting from the mixing of two separate
hereditary masses by impregnation find their expression in the most
varied qualities of the minds and bodies of developing children. If the
variations are not especially marked, they are looked upon as normal and
attract no special attention.
But if the variations are so pronounced as to compel attention, and at
the same time it is known that they are useful, they are spoken of as
talents, or, on the other hand, if they are harmful or useless, they are
designated as pathological or monstrosities.
These are truly what the biologists call sports; and to those classes of
sports that occur as specially gifted in human culture, in the varied
fields of science, art, or literature, we assign such a person as
Shakespeare, and call the remarkable variations embodied in him genius.
On the other hand, such variations as lead to certain forms of pigmentary
degeneration of the retina, and to Daltonism, to dyschromatopsia and
achromatopsia, to certain supernumerary glands, polydactylism, and such
like, which are either useless or harmful, we designate as pathological
cases or monstrosities.
Hereditary units (the carriers of heritages) may be _latent_—that is,
they may appear late in life, or in the offspring, or, still again, in
remote descendants; in the latter cases the heritages are spoken of as
reversional or _atavistic_. Latent hereditary units may very usefully be
compared to dormant seeds buried in the ground. It is stated that buried
seeds may lie dormant for many years, so that when a plot of ground is
plowed deeply and upturned, plants that have not been seen there within
the memory of one will often make their appearance and flourish. The
hereditary units are veritable _living seeds_, that, under certain and
often unknown stimuli, grow and unfold their heritages as do the buried
seeds.
Latent heritages are well illustrated by a study of secondary sexual
characters as developed at puberty.
Among our barnyard fowls, the hens often, when they have atrophy
or degeneration of the ovaries, although up to this time they have
laid eggs for years, stop this function, put aside the plumage and
appearance proper to their sex, and don more or less completely the
garments of the rooster. Thus females have latent in them many secondary
sexual characters of the male. For similar reasons the male develops,
occasionally, female characters.
This latency is illustrated again in deer. In most species of the
deer tribe the males alone possess antlers, yet it is a well-known
circumstance that in females with degenerations of the ovaries
rudimentary horns that are never shed appear. A study of congenital
color-blindness illustrates beautifully latent heritages, showing how the
females of one generation may be free from the malady and the males of
the next afflicted.
A study of the _regeneration of lost parts_ in various animals and plants
illustrates well the latency of many hereditary units. A cutting made
from the willow and planted sends out roots and finally reaches the
dimensions of the adult tree. Here the body cells of the stem evidently
contained, in a latent condition, many hereditary units in their nuclei,
which became active through the special stimulus of being planted as
a cutting. Adult plants can be raised from the cuttings of many other
plants.
If the garden worm is cut in two, the head-part will reproduce the
tail-portion. If the little fresh-water polyp, Hydra, be cut into a
number of pieces, each segment will reproduce a perfect animal. Many
lizards, after losing their tails by violence, manufacture new tails
through the agency of the latent hereditary units contained in the body
cells of the stump left. If the tentacle or “horn” of a snail, which
contains an eye with a perfect lens and retina, be cut off, the animal
can reproduce another one with a perfect eye, and this can be repeated a
number of times. Often newts, when fighting with one another, or lobsters
when fighting, lose a leg or a claw. These highly organized animals
have the power of creating new limbs, making bones, ligaments, muscles,
nerves, cuticle, and so on. All of this is done _through the hereditary
masses in the nuclei of the body cells_ at the site of the injury.
A study of the phenomena of polymorphism in hydroids and insects will
beautifully and most interestingly illustrate latent hereditary units.
Much that is speculative and fanciful is included under the subject
of atavism, and the safest plan for pathologists and biologists, in
considering any abnormality, is to remember a golden rule of Gegenbaur,
that only those structures are reversional which are taxonomically not
far distant or phylogenetically not very old. Embryology is also a very
important check in considering such subjects.
In mankind supernumerary limbs and digits, microcephalia and
micrencephalia, have been looked upon as reversions to the simian type.
Lombroso, in contrasting the criminal with normal man, looks upon his
_homo delinquens_ as an illustration of atavism, contrasting with _homo
sapiens_. But, as Ziegler, in his _Pathology_, well observes, many
writers have gone too far in this respect, and have characterized as
atavistic formations various acquired pathological formations and fresh
variations of germ cells.
I think one can safely say that supernumerary ribs and those
supernumerary nipples and mammary glands along the line of the deep
epigastric and internal mammary arteries are truly atavistic structures;
also certain muscles normally belonging to those mammalia which come near
to man in the scale of relationship, and which appear in man as muscular
variations, are reversional.
Children are often born with pigmented hairy patches on their bodies
known as moles; sometimes these hairy moles are only of the size of a
split pea, in other cases they are several square inches in area, while
in rare cases almost all of the trunk may be thus covered. Although many
similar pathological cases are often but marked variations called sports,
yet the illustrations mentioned are undoubtedly reversional. Of the
multitudinous illustrations of atavism that could be mentioned I wish to
refer to but one more case.
The conjunctiva is a modification of skin, and frequently proclaims its
ancestry by reverting to its original form. It is by no means a very
rare event to see a patient having a patch of hair-covered skin growing
upon the ocular conjunctiva. While a clinical assistant at the Royal
Ophthalmic Hospital in London, we saw one such case, and Dr. Treacher
Collins, the pathologist of that eye hospital, has stated that about
twelve cases are seen there annually of this pathological condition,
which is atavistic, according to Sutton, although it seemingly violates
Gegenbaur’s rule about phylogenetic remoteness, and may be looked upon by
some as a pathological illustration of a sport.
In speaking of inheritance, we should carefully discriminate between
_heredity_ and _pseudo-heredity_. Physicians constantly write of
tuberculosis, lepra, smallpox, and syphilis as hereditary; but it is
incorrect and misleading to do so. When a person has syphilis, say, from
the earliest existence—that is, from the fertilized ovum by transmission
of a syphilitic microbe through the germ cells of the parents—this should
be designated by its proper name as _congenital bacterial infection_.
This is totally different from the hereditary qualities that flow from
the structural equilibrium following the commingling and struggle for
existence of multitudes of hereditary units.
The one set of hereditary qualities is _purely germinal_, while the
other is germinal _profoundly modified by the presence of an infecting
microbe_. Of course, to the extent that any toxines that are secreted by
the bacteria may cause permanent structural changes in the germ-cells, to
that extent may the germinal characteristics be transmitted and become
hereditary.
Many instances of infection of the child _in utero_ have been reported
in cases of endocarditis, scarlet fever, and smallpox; and there can
no longer be any doubt, from experimental investigation and recent
observation, that pneumococci, typhoid bacilli, anthrax bacilli, and
pus cocci are able to pass to the fœtus through the placenta. But the
diseases that develop in this way can be called hereditary with even less
semblance of correctness than in the case of the fertilized ovum that is
invaded with a microbe.
All of these cases are illustrations of pseudo-hereditary transmission,
and should, for the sake of clearness and accuracy, be spoken of as
_prenatal_ infections.
So far as the problems of heredity and variation are concerned, we may
say that the life cycle begins and ends with the germ cell. Insects lay
their eggs in old age; among plants the annuals flower but to die; in
higher creatures the cessation of the procreative power often marks the
beginning of bodily decline.
Bearing in mind that the human body consists of two great classes of
cells, germ cells and somatic cells, the following scheme will be found
very useful in discussing heredity with variation—viz.:
{ Stable
{ (Heredity) { Temperature. }
{ { Chemical }
{ { substances in }
{ { solution in the }
Germ-Cell { { Blastogenetic { fluids that }
{ { { bathe the }
{ { { germ-cells, as }
{ { { food, drugs, }
{ Unstable { { poisons, etc. }
{ (Variation) { } Environment
{ { Habitat }
{ { Temperature }
{ { Climate }
{ { Air }
{ Somatogenetic { Food }
{ Soil }
{ Water }
{ “Use” }
{ “Disuse” }
Just in proportion as fertilized germ cells during the mitoses of
ontogeny give origin, among the somatic cells, to other germ cells that
are structurally, and therefore physiologically, like themselves, just to
that extent do we have heredity; on the other hand, just to the degree
that the new germ cells which are produced are unstable, to that degree
also do we meet with variations.
ENVIRONMENT.
In zoölogy the environment of an organism means the sum-total of the
conditions of life that surround and affect it, such as food, air, water,
climate, etc.
We have already stated that as far as evolution is concerned the
structures of fundamental importance in an organism are the Germ-Cells;
therefore, for our purposes, we will define _environment to be the
sum-total of the conditions that directly or indirectly influence in any
way the germ-cells_, by which variations in them may be produced, or
through which stability may be maintained.
There are two great classes of environmental factors that bring about
variations in the germ-cells. One of these classes acts directly on
the germ-cells, and is therefore called _blastogenetic_; the other
acts indirectly through the body cells, and is therefore designated
_somatogenetic_. Many of the blastogenetic factors bringing about
structural changes in the delicate mechanism of germ-cells are entirely
unknown, and are therefore designated as fortuitous. Many other causes,
such as poisons in solution in the fluids that bathe them, can readily
enough be appreciated.
=Blastogenetic Factors.= It has been demonstrated that various _chemical
substances_, such as chloroform, morphia, chloral, etc., _have a
pronounced influence upon the vital activities of cells_. It is well
known that microscopic unicellular plants constitute the essential part
of yeast. These little cells have the power of causing fermentation in
solutions of grape sugar by which alcohol and carbon dioxide are formed,
the latter being a gas and escaping as bubbles. If chloroform or ether be
added to the solution of sugar, before adding the yeast, no fermentation
takes place, for the yeast-cells are paralyzed. But when the yeast is
separated from the chloroform solution and rinsed with distilled water,
it soon regains the power of causing fermentation in pure solutions of
sugar.
_Ova and Spermatozoids_ are subject to the action of drugs in a similar
manner. If actively motile spermatozoids of a sea-urchin be placed
in a one-half of one per cent. solution of chloral in sea water, it
will be found that after five minutes their action will be completely
arrested. These motions can soon be restored if the chloral solution be
sufficiently diluted with pure sea water. These temporarily paralyzed
spermatozoids, when completely recovered, will unite as quickly with ova
as fresh spermatozoids. When spermatozoids are kept for half an hour in
the chloral solution, a more decided paralysis will be observed, which
persists for some time after the removal of the poisonous agent. A few
minutes elapse before some of the spermatozoids exhibit feeble movements
which finally become active. Even when placed near ova, it is some time
before they fertilize them, although several may attach themselves to
the egg’s surface. But, finally, fertilization does take place by the
penetration of one spermatozoid, and the egg normally develops.
In like manner, if ova are subjected to chloral solution of varying
strength, they also are influenced in a marked degree; for, when
fertilized, they develop in an abnormal manner. Ordinarily, normal ova
are fertilized by one spermatozoid. If fertilized by two or more, they
become diseased, and develop pathologically. The chloral solution favors
this fertilization by several spermatozoids. The stronger the solution
of chloral, the larger the number of spermatozoids that fertilize the
ovum. Experiment and observation show that the behavior of the nuclear
hereditary mass is modified, during mitosis, by the chloral and other
solutions. It is thus seen that _the germ-cells of the lower animals can
be profoundly modified by various substances_.
Equally true is it that the man or woman who makes use of such drugs
as alcohol, opium, chloral, and such like, in an intemperate manner,
contains these poisons in solution in the blood, circulating to every
part of the body, and thus bathing and profoundly influencing the
germ-cells. In consequence of this fact an acquired and habitual
intemperance will seldom fail to leave its impress upon one or more of
the offspring, either like the original vice or one very closely allied
to it. Intemperate people not only profoundly impair the health, the
intelligence, and the morals of their offspring, by poisoning these
delicate germ-cells, but they also transmit the fatal tendency to crave
for the very substances that have acted as poisons on these germ-cells
before and after fertilization. And one of the saddest features of this
great medical truth is that the hereditary units which are concerned
in transmitting these grave abnormal tendencies may lie dormant in the
germs of one generation, to become active in those of the next; so that
children of intemperate parents may lead honorable and temperate lives,
and take every pains to rear, in turn, their own children in a wholesome
and refining atmosphere, and yet these children of good environment may
become intemperate through heredity, so that the sins of the grandparents
may be visited, not on the children, but on the grandchildren.
These profound truths should lead all, and especially law-makers, to
remember that “the man who inherits from his parents an impulsive or
easily tempted nature and an inert will and judgment, and commits a crime
under the influence of strong emotion, can no more be placed in the same
category of responsibility with a man of more favorable constitution
and temperament than can a man who steals a loaf under the pangs of
starvation with a merchant who commits a forgery to afford him the means
of prolonging a guilty career.”[3]
Not only do certain known poisons circulating in the blood, or other
fluid that may bathe the germ-cells of living creatures, profoundly
affect the germ-cells, but many other substances probably have great
influence upon them. Certainly, the _amount and character of the food_
have a very decided influence on them, as will be understood from the
following facts.
According to Yung, who has experimented very extensively upon tadpoles,
all tadpoles pass through a bisexual (hermaphroditic) stage, as is the
case probably with most animals. During this tadpole phase external
influences, and, more particularly, food, determine their fate as regards
sex. In Yung’s experiments it was found that when tadpoles were left to
themselves, the percentage of females was in the majority, the average
being probably about 57 per cent. females and 43 per cent. males. In
experimenting with three broods, those fed on beef gave 78 per cent.
females; those fed on fish gave 80 per cent. females; and those fed on
the highly nutritious flesh of frogs gave 92 per cent. females.
In Mrs. Treat’s interesting experiments on moths and butterflies, it
was observed that if caterpillars were confined and starved before they
entered the chrysalis state, the resultant moths or butterflies were
males, but others of the same brood that had been highly nourished came
out females.
The study of bees illustrates the same conclusions. It is well known that
in a beehive there are three kinds of inmates, as the queen, the drones,
and the workers,—the last-mentioned being females whose reproductive
organs are imperfectly developed. It is believed that the eggs that
give rise to queens and workers are fertilized and developed normally.
But it is a very curious fact that the eggs which develop into drones
do so without fertilization (_parthenogenesis_). What factor or factors
decide the destiny of the two former, determining whether a given ovum
will develop into a queen, and thus be the possible mother of a new
generation, or stay at the lower grade of a working, non-fertile female?
These factors are the quality and quantity of the food. An abundance of
what is called royal food causes the development of the larva in such a
way that the queen with her reproductive organs is formed. If a larva
on the road to develop into a worker (non-fertile female) “receive by
chance some crumbs from the royal superfluity,” it is found that the
reproductive organs may develop to such an extent that workers partially
fertile may be formed. A worker larva may, by this royal food, be
intentionally reared into a queen bee.
It is thus seen how profoundly the germ-cells, in their growth, may be
affected and made to vary by such a blastogenetic factor as food.
=Somatogenetic Factors.= As to somatogenetic factors—granting that
structural changes in the body (body-cells) of an animal or plant can
profoundly influence in some way the germ-cells, and that, therefore,
acquired characters can be transmitted—they are many and well defined.
Some of them are the habitat of an animal or plant, the temperature,
climate, air, food, soil, water, _structures in use or disuse_ (so-called
“Use” and “Disuse”), etc.
The following brief descriptions will enable the reader to understand
that change in the surroundings (environment) of a living creature may
cause its body (body-cells) to vary.
A certain species of snail was introduced into Lexington, Virginia, a
few years ago from Europe. In its new _habitat_ it varied very much. One
hundred and twenty-five varieties have been discovered there, sixty-seven
of which are new and unknown in Europe, the native home of the species.
The common ringed snake, when living in its natural habitat, deposits
eggs in the sand, which are hatched by the heat of the sun; but when
this snake is confined in a cage in which no sand is strewn, it gives
birth to little living snakes.
In experimenting on moths it has been found that the _variations of
temperature_ to which the pupæ, and probably also the larvæ, are
subjected, tend to bring about very pronounced differences in the moths.
Cold has a tendency to develop a darker hue in the perfect insect.
English dogs when taken to hot climates, like that of India, are known to
degenerate in a few generations. It is well known how climate affects the
hairiness of animals. When greyhounds are taken to the uplands of Mexico
they are unable to course on account of the rarity of the air.
In 1870 a number of pupæ of a certain species of moth (Saturnia) were
taken from Texas to Switzerland. After passing the winter there, the pupæ
emerged from their cocoons as moths, and resembled the Texan species
entirely. The young of these moths were fed on the leaves of a plant
different from that the moths in Texas feed on, and they developed into
moths so different in form and color-pattern from their parents that
entomologists classified them as a distinct species.
We have seen how certain foods affect the germ-cells and act as
blastogenetic factors; the preceding case and the following show how
_certain foods act as somatogenetic factors_ and modify the body-cells.
If the bullfinch be fed on hemp seed, its color is changed to black; if
the canary be fed on cayenne, its plumage becomes darker; if the common
green Amazonian parrot be fed on the fat of siluroid fishes, it assumes a
beautiful variegation of red and yellow.
The _character of the soil_ has a marked influence in inducing
somatogenetic variations. In France an experimenter collected seed from
the wild radish and sowed one lot in heavy soil in the country, while
another lot was sown by him in the dry, light soil near the Museum of
Natural History in Paris. The radish “roots” grown in these two places
presented marked differences in color and form. Those grown in Paris
were either of a rose or white color and elongated; while those from
the country were violet, dark-brown or nearly black in color, and more
rounded than the former.
In the summer of 1847 Professor Buckman gathered seed from wild parsnips,
and sowed them in the spring of 1848 under changed conditions of life.
Most of the plants grown from these seeds were like the wild parsnips,
but some of them developed the light-green color and hairless, smooth
appearance characteristic of the cultivated plant. The roots also were
found to be more fleshy than those of the wild variety.
Peas and squashes, when grown in different soils, often show remarkable
variations.
There is one species of shrimp that inhabits brackish water, and another
that lives in water which is much more salt. These crustaceans differ
from one another in the character of the spines they bear and in the form
of the tail-lobes. They have been regarded as distinct species, and yet
either of them can be transformed into the other in the course of a few
generations, by gradually altering the saline conditions of the water.
For a long while the siredon and amblystoma were regarded as being
distinct genera of amphibians. Siredon was looked upon as a permanent
gill-breather, while amblystoma passes through a metamorphosis and
becomes a permanent lung-breather. It is now known that the former can
change into the latter. If there is plenty of water the siredon remains
indefinitely a gill-breather and reproduces freely; but when the water
dries up it changes into the lung-breathing amblystoma. These two cases
illustrate very well the power of environment to modify the development
of organic forms.
As to “_use_” and “_disuse_”: It can readily be observed that exercise
increases the size of muscles; that by steady application the capacity
for thinking can be developed; that the oarsman’s constant use of his
hands leads to the hardening and thickening of the cuticle; that the arm
of the blacksmith and the legs of the mountaineer are much enlarged, etc.
When an organ is exercised properly, there is an increased blood supply
to it, and, consequently, stimulated nutrition and growth in various
parts, such as in the muscular, nervous or other tissues.
When an organ is disused there is diminished blood supply, and,
consequently, diminished growth and functional capacity. In man it is
known that certain activities, such as coal-heaving, shoemaking, etc.,
produce recognizable effects upon the muscular system, the skeleton, and
other parts of the body.
The peculiar habits of a tribe, such as tree-climbing among those natives
of the interior of New Guinea, who build their houses in the upper limbs
of lofty trees, modify the body in ways that are readily recognizable.
After considering many facts in connection with the brains of rabbits,
Darwin announced that this most complicated and important organ in an
animal is subject to the law of decrease in size from disuse. We have
very interesting illustrations of the effects of “use” and “disuse” in
causing somatogenetic variations, in the differences between domestic
ducks and the wild ones from which they have been undoubtedly derived.
The wild duck, which must constantly be on the alert for enemies, and
uses its wings so much more extensively and its legs comparatively less
than the domestic duck, is a much more intelligent fowl than the stupid,
well-protected domestic one. The wings of the wild duck are stronger and
its legs shorter than those of the barnyard duck. It has been shown that
in the wild duck the brain is nearly twice as heavy in proportion to the
body as it is in the comparatively imbecile domestic duck.
Many other useful illustrations of disuse, such as the cattle and goats
in India, that have dependent ears; also cats in China, and horses in
parts of Russia, whose ears are dependent, could be referred to. Use
and disuse are included among the factors of environment, because by
those terms we mean certain _groups of body-cells that are functionally
active or inactive_; for body-cells on any theory of modified pangenesis
constitute an exceedingly important environment of the germ-cells.
The surrounding conditions (environment) of an animal or plant having
the power to cause variations in the living creature by affecting its
germ-cells or its body-cells, the _environment_ may be spoken of as
_blastogenetic_ and _somatogenetic_.
Whether it is a fact or not that somatic variations can induce
corresponding variations in the germ-cells, and thus be transmitted by
heredity, it is certainly true that all heritages must come through the
germ-cells. For this reason, it is clearly seen that so far as evolution
is concerned the germ-cells are the factors of fundamental importance in
organisms. Therefore, we may repeat that environment is the sum-total of
the conditions of life that affect the germ-cells directly or indirectly.
ACQUIRED CHARACTERS.
All heritages, then, are derived directly through the germ cells.
Can there be any heritages indirectly from the somatic cells through
the germ-cells, as has hitherto been assumed? In other words, can
acquired characteristics be transmitted to the offspring? This question
has given origin to the battle royal that is still going on between
opposing schools of biology. The contending parties have appealed to
such biological evidence as is furnished by a study of use-inheritance,
reflex and instinctive actions in animals, etc., and to such experimental
evidence as the induction of traumatic epilepsy in guinea pigs, a change
in the shape of the ear by cutting the cervical sympathetic nerve,
protrusion of the eyeball through injury to the restiform body of the
brain, and such like, noting the effects on the offspring, and have drawn
very different conclusions.
As to the transmission or non-transmission of acquired characters, some
have maintained that only germinal variations are transmitted (because
they believe the germ cells are _insulated_ from the body cells, and
therefore from somatic influences). For instance, Ziegler, in his work
on _General Pathology_, says: “If a disease, such as nearsightedness,
is the product of a special inherited predisposition, _plus_ the effect
of harmful influences which have acted upon the body during life, only
that part can be transmitted which was received by inheritance, but not
that part which was derived from external influences.” In other words,
there is no transmission of acquired character. In this belief it will be
observed that he follows Weismann.
On the contrary, other investigators, like Darwin and Spencer, teach
that somatic variations—the plus element in Ziegler’s illustration of
nearsightedness—do influence the germ-cells (through some such agency as
Darwin’s theory of pangenesis suggests), and that, therefore, acquired
characters can be transmitted. The question is one of fundamental
importance, and yet no crucial experiment has been devised or fact
observed which can compel the correct answer. The evidence seems to favor
the view that acquired characters can be transmitted.
The theories as to the transmission or non-transmission of acquired
characters may be better understood by reference to schemes No. 1, 2 and
3. Scheme No. 1 represents the theory of =Pangenesis=, which teaches
that reproductive cells are not formed from pre-existing reproductive
cells, but by the body cells themselves. Darwin taught that all the
cells of the body, such as skeletal-cells, muscle-cells, nerve-cells,
and so on, are continually giving off infinitely small cell germs or
gemmules, which have the power of growing and forming cells exactly
like themselves. These gemmules have a great affinity for one another,
and, circulating in the blood in countless numbers, they finally come
together in the reproductive glands and form the reproductive cells.
On this theory the fact of the transmission of acquired characters can
readily be appreciated, and it can easily be understood how the parent
molds the child. Suppose, for instance, that the parent, by exercise, has
become a skillful athlete. In him certain muscles have become greatly
developed and strengthened. During all the time of the exercise of these
muscles, the modifying muscle cells have been continually giving off to
the blood modified gemmules, which collect in the reproductive cells and
make it possible for the offspring to develop into an athlete because the
modified gemmules develop into modified muscles like those of the athlete.
Scheme 1 shows the absence of any arrow like those shown in schemes 2
and 3, directly connecting germ-cell with germ-cell; this means that in
this theory there is no continuity of the germ-cells. But arrows are seen
extending from the various body-cells (skeletal, glandular, etc.) to the
germ-cell; this means that the germ-cells are formed by influences or
gemmules emanating from the various body cells.
Scheme 2 teaches that a germ-cell (when fertilized, of course) can
produce many cells, some of which differentiate, finally, into skeletal
cells, some into glandular, some into muscle and nerve cells, and some
into new germ-cells; so that an animal or plant, I, is formed. In like
manner a germ-cell of animal, I, can give rise to the germ and body cells
of animal, II, and so on indefinitely. This scheme shows that there is
a direct =continuity of the germ-cells=; and it also shows that the
germ-cells are entirely _insulated_, as it were, from the body-cells
(skeletal, glandular, etc.), inasmuch as no influences (arrows) extend
from the body-cells to the germ-cells. This means that the transmission
of acquired characters, bodily, mental, moral, etc., is impossible. It
means, in other words, that none of the advantages gained by a parent in
the course of his life can be handed on to his offspring by heredity.
There are many biologists and pathologists who teach this theory as the
correct one.
The majority of biologists accept the theory illustrated in Scheme 3.
This is the theory of =modified Pangenesis=, which teaches that there
is a direct continuity of the germ-cells and that these germ-cells are
not insulated from the body-cells, but that the latter, when modified as
the result of experience, can send off influences that correspondingly
modify the germ-cells; so that the latter, when developing into a new
individual, may cause the same body variations that exist in the parent.
In short, this scheme illustrates not only that there is a _germinal
inheritance_, but also an _inheritance of acquired characters_. In this
Scheme 3, the oblique arrows show that germ-cells produce other germ
cells; the perpendicular arrows show that the germ-cells are modified by
influences that proceed from the body cells.
_Germinal characteristics are transmitted with vastly greater amplitude
and swiftness than merely body (acquired) characteristics._ If, for
instance, a man were born with that physical constitution that makes
with ease a first-class pianist out of him, his sons may easily, through
heredity, be first-class pianists. But if a man be born without such a
congenital tendency and has by constant labor and practice so developed
the muscles of his forearm, his nerves, his brain, etc., that he becomes
a very good pianist (acquired characters); and, further, if his male
descendants for thousands of generations, in succession, have become very
good pianists by constant practice, we may expect that the sons of these
last generations may obtain a congenital tendency to become first-class
pianists quite easily. The constant improvement, by practice, of groups
of body-cells (muscle-cells, nerve-cells, etc.) for generations, has,
in each generation, tended to so correspondingly modify the germ-cells
that they have acquired the power to develop into men who may become very
good pianists with very little practice. This illustrates that there may
be a continuous summation of feeble germ-cell variations that have been
induced by prolonged influences emanating from somatic variations, so
that, in the course of many generations, robust acquired characters may
ultimately be translated into strong congenital characters (Scheme 3).
Scheme 1. =Illustrating the theory of Pangenesis.= Here the germ-cell (a)
develops into the body-cells, e, e, e, e, of animal I, as indicated by
the oblique arrows, but not into any germ-cells, as indicated by absence
of arrow between germ-cell (a) and germ-cells (b). The germ-cells (b) in
animal I are formed by the aggregation of infinite numbers of gemmules
from the various groups of body-cells, e, e, e, e, as indicated by the
perpendicular arrows. The germ-cell (a) transmits germinal heritages to
the body-cells e, e, e, e; these body-cells transmit the heritages to the
germ-cells (b) by means of the gemmules. If the body-cells are modified
in any way, correspondingly modified gemmules are sent to the germ-cells
(b), and these germ-cells are modified and thus transmit acquired
characters to animal II, and so on.
[Illustration:
I
—> Skeletal-cells (e)
(a) Germ-cell —> Glandular-cells (e)
—> Muscle-cells (e)
—> Nerve-cells (e)
| | | |
| | | | II
| | | |
| | | | —> Skeletal-cells (f)
v v v v —> Glandular-cells (f)
(b) Germ-cell —> Muscle-cells (f)
—> Nerve-cells (f)
| | | |
| | | | III
| | | | —> Skeletal-cells (g)
v v v v —> Glandular-cells (g)
(c) Germ-cell —> Muscle-cells (g)
—> Nerve-cells (g)
| | | |
| | | |
v v v v
(d) Germ-cell.
]
Scheme 2. =Illustrating the theory of Continuity of the Germ-Cells;
pure germinal inheritance; and the non-transmissibility of acquired
characters. The germ-cells are insulated from the body cells.= The
germ-cell (a) develops into the body-cells, e, e, e, e, and the
germ-cells (b), in animal I. The body-cells, e, e, e, e, do not
influence in any way the germ-cells (b), as indicated by the absence of
perpendicular arrows. The germ-cells (b) get all their heritages from the
antecedent germ-cell (a), as indicated by the oblique arrow from (a) to
(b). All heritages are purely through the germ-cells. The same with the
animals II and III. Germ-cells (a), (b), (c), (d), are connected together
by obliquely placed arrows, indicating the continuity of the germ-cells.
[Illustration:
I
—> Skeletal-cells (e)
(a) Germ-cell —> Glandular-cells (e)
\ —> Muscle-cells (e)
\ —> Nerve-cells (e)
\ | | | |
\ | | | | II
\ | | | |
\ v v v v —> Skeletal-cells (f)
(b) Germ-cell —> Glandular-cells (f)
\ —> Muscle-cells (f)
\ —> Nerve-cells (f)
\ | | | |
\ | | | | III
\ | | | |
\ v v v v —> Skeletal-cells (g)
(c) Germ-cell —> Glandular-cells (g)
\ —> Muscle-cells (g)
\ —> Nerve-cells (g)
\ | | | |
\ | | | |
\ v v v v
(d) Germ-cell.
]
Scheme 3. =Illustrating the theory of Continuity with Modified
Pangenesis.= A germ-cell (a) develops into the body-cells, e, e, e,
e, and the germ-cells (b) of animal I. The germ-cells (b) get their
heritages directly from the germ-cell (a), as indicated by the long,
obliquely-situated arrow (continuity of the germ-cells). The germ-cells
(b) are, moreover, modified by influences extending from the body-cells,
e, e, e, e, as indicated by the perpendicular arrows. A modified
germ-cell (b) can develop into a modified animal II, and the body-cells
of this animal can influence and modify the germ-cells (c); and so on,
indefinitely. The perpendicular arrows indicate that acquired characters
are transmitted, and that, too, through the germ-cells.
[Illustration:
I
—> Skeletal-cells (e)
(a) Germ-cell —> Glandular-cells (e)
\ —> Muscle-cells (e)
\ —> Nerve-cells (e)
\ | | | |
\ | | | | II
\ | | | |
\ | | | | —> Skeletal-cells (f)
\ v v v v —> Glandular-cells (f)
(b) Germ-cell —> Muscle-cells (f)
\ —> Nerve-cells (f)
\ | | | |
\ | | | | III
\ | | | |
\ | | | | —> Skeletal-cells (g)
\ v v v v —> Glandular-cells (g)
(c) Germ-cell —> Muscle-cells (g)
\ —> Nerve-cells (g)
\ | | | |
\ | | | |
\ v v v v
(d) Germ-cell.
]
Professor Morgan, of England, has advanced the ingenious theory, which
may reconcile the above-mentioned antagonistic views, that somatic
variations, in the direction of adaptation, pave the way for germinal
variations, so that, while somatic modifications _as such_ are not
inherited, they are yet the _favoring conditions_ under which germinal
variations are preserved by the great principle of natural selection.
If this is true, as we think it is, then we can safely state that each
man in his totality is the resultant of two great factors—_heredity_
and _environment_, the latter including not only food, water, climate,
occupation, etc., but also the character of the civilization, the state
of morals in society, the ideals and examples most frequently seen, etc.,
etc.
Heredity brings down to him the streams of tendency of former
generations, often of a healthy and beneficent character, but also often
surcharged with lust and passion, and reeking with disease.
Environment is the coöperating and, to us, vitally important factor,
inasmuch as it may supplement and thus reënforce the hereditary
tendencies, whether good or bad; or it may even tend to turn them into
new channels, correcting the evil or vitiating the good.
Man is not simply a creature of the present, but profoundly a product of
the past. Bodily structure, moral and intellectual tendencies, disease,
vices, and virtues are all in the marvelous stream of heritage that comes
to him from the past. “Diseases that no facts in the individual life can
account for point gaunt fingers of blame from one generation to another.
Not a murderer is hung, not a daughter starts on the downward way, but
a great company, like those who were present at the stoning of Stephen,
stand by inaugurating and consenting to the ruin.”[4]
Truly has it been said that the past is at work in the present, its
powers reaching down through the ages, to all the race, largely molding
every human life, touching and influencing every individual’s thought and
will, and, more than any other force, coloring history.
Studies in heredity illustrate most luridly that the continuity of the
human race is a terrible but remorseless reality.
If the ignorance and the perverted pleasures of one generation may
produce the vices and the crimes and the diseases of another, a question
of tremendous import arises: Is heredity as potent in the direction of
virtue and health as of vice and disease? At the first look one is almost
tempted to answer Nay! for the most striking examples of heredity seem
to be in the direction of evil. But this is perfectly natural. Decay is
always more rapid than growth. A cherry rots much more quickly than it
ripens. Vice and disease spread much more quickly and widely than virtue
and health. But all history and all social and medical science teach that
vice and disease carry within themselves the seeds of decay, and virtue
and health the seeds of endurance and growth.
Through the great Darwinian principle of natural selection, or survival
of the fittest, vice and disease will become less and less predominant,
and virtue and hygienic constitutions more and more disseminated.
As influencing a man’s life and character,[5] which is the stronger
factor, heredity or environment? Fatalism or choice? In our opinion, as
the result of long study and reading, where we have an average man of
“_mens sana in corpore sano_,” environment will be the stronger factor
whether for good or for evil—that is, in men in general, who have no
organic defect, such as insanity or idiocy, and allied affections, the
stronger force is environment; but in those having such defect, heredity
is the controlling power, and, we may add, the destroying power.
It must be recalled, though, that the average man with a “sound mind
in a sound body,” in his development to his present estate, has become
possessed of a vast aggregate of diverse heritages, of varying age,
strength and dignity. Some of them are so old and strong that they seem
to be cast in unyielding molds, while others are so weak and recent that
they fluctuate with every passing circumstance. The most dignified and
important of all his heritages is that of rational volition. It is the
play of this volition upon many of his other heritages that gives him the
power of selecting, to a limited extent, his environment.
Every man is born into the world with a certain physical constitution,
and, therefore, with a given temperament; with certain passions; with
the power of judgment; and with a certain strength of will. If the power
of his will be not equal to the strength of his passions, the latter
will surely predominate and will display him as the slave of heredity.
If he has such an organization of his nervous system that his volition
is superior to his passions, he will be none the less the servant of
heredity, though a being now possessed of the power of Free-Will.
Man is, to a far greater degree than is ordinarily realized, the servant
of heredity. It seems to us an incontrovertible fact that every living
creature, at any given moment, is swayed infinitely more by the _totality
of its heritages_ than by its environment. No one can possibly deny this
so far as plants and most animals are concerned. Nor, if one look below
the surface, can it be denied of the higher animals and of man. Happily,
the average man, with his present constitution, has his diverse heritages
so proportioned that we may repeat that his life and character (in
customs, morals, and religion) are vastly more influenced by environment
than by heredity.
The standards for estimating the life and character of men, namely,
human customs, morals, and religions, are such recent acquisitions,
_geologically_ speaking, that they have, as yet, very slightly if at all
influenced the germ-cells. They are acquired (somatic) characteristics,
and not congenital (germinal) qualities. They are preëminently the
creations of environment. If the infants of a Catholic family which is
descended from a long line of Catholic ancestors were to be placed and
retained in a purely Mohammedan environment, heredity would carry no
Christian customs, morals or religion into that environment, but, on
the contrary, the Mohammedan surroundings would instill new customs,
different ethical ideas, and a different religion. This illustrates
how very feebly indeed are germ-cells correspondingly impressed by
pure acquired characters. _It is almost certain that the translation
of somatic changes into germinal changes is appallingly slow._ As far,
then, as social customs, morals, and religion are concerned, the average
man is, in our opinion, infinitely more the creature of nurture than
of nature. But, as far as his temperament, his emotional nature, his
judgment, his strength of will, in short, his physical and therefore
his mental constitution, are concerned, he is _almost_ absolutely the
creature of heredity. The equilibrium of qualities or heritages in
the average man, resident in a given, stable community, is in harmony
with the average customs, ethical ideas, and religious beliefs of that
community. But in all stable communities there are men whose resultant of
heritages, some in one direction and some in another, places them out of
harmony with the average of their social environment, and they are looked
upon, some as idiots, some as geniuses, some as criminals, and others as
saints, and so on. So that again we may say that a man’s character in a
community is the resultant of an hereditary physical constitution, and
his environment. Some men may inherit such a physical constitution that
in spite of the best environment they are much debased below the average
man; others may possess such heritages that, notwithstanding adverse
circumstances, they reach a level of character much above the average
man. And there are all gradations between the two extremes.
SECTION III.
UNSTABLE ENVIRONMENT.
UNSTABLE ENVIRONMENT.
Where living creatures are in harmony with their surroundings,—where, in
other words, they are adapted to their environment,—and where, further,
this environment is apparently in a state of equilibrium; there we find
the fewest and least marked variations in the living creatures. To the
casual observer the face of nature maintains the same guise from year
to year. The earth seems solid and unyielding; the mountains appear to
be everlasting; the restless waters of rivers and brooks seemingly move
and throb in the same channel; the tides ebb and flow in apparently
unchanging ocean beds; the birds and flowers and woodlands look alike
from year to year; and all the varied phenomena of nature appear
completed and permanent, as if the present world were constructed in an
unyielding mold.
But nothing is fixed and rigid in nature. The earth itself travels
rapidly through space and brings in due season spring, summer, autumn
and winter; revolves upon its axis and alternates the starry night with
sunshine; and periodically changes its orbit so that at one time the
northern pole has a temperate climate where water lilies may grow, and
at another period presents an arctic climate with impassable barriers
of ice. Ice and frost and other forces are breaking up the rocks of
mountains, making larger and smaller fragments and even powder; the
rains descend and the mountain brooks are swollen to resistless torrents
which carry the fragments and mud to the rivers, and these latter take
the mud on to the ocean.
Thus, by degrees, the mountains, hills and all the earth are being
eroded and the great bulk of the detritus carried by the rivers to the
sea and deposited along the sea margins. Thus sedimentation goes along
with erosion, and gradually marginal sea bottoms of immense thickness
are formed, which will in time be consolidated into rocks and uplifted
as dry land. The ceaseless grinding of waves and tides erodes the coast
line and adds _débris_ to the marginal sea bottom. The finest sediment is
carried out by the tides so far as to reach the ocean currents, and thus
is strewn broadcast over portions of deep-sea bottom, and will also in
due time be consolidated into rock.
Myriads of animals that form calcareous shells live and die in the ocean.
The shells of the dead animals are falling like a perpetual shower on
certain ocean bottoms, year after year, so that immense accumulations
of calcareous substances occur there, which will also in time be
consolidated into limestone rocks, and uplifted as dry land.
Deltas are forming at present at the mouths of certain rivers, and
estuaries at others. Lands are now gradually emerging from the sea in
some places and at others sinking into the sea. The entire coast of
Scandinavia, both on the Baltic and Atlantic sides, is rising out of the
sea, and has been doing so for a long time. It is rising at the rate of
more than two feet in a hundred years. During an immense period of time
there has been a gradual elevation of all the southern part of the South
American continent. Sometimes a large elevation takes place rapidly. In
1822, and again in 1835, the southwest coast of South America, after
severe earthquakes, was elevated several feet along a distance of several
hundred miles. It is known that the coast of Greenland, for five hundred
miles, is subsiding. From a study of coral barriers and atolls it is
believed that an area of the mid-Pacific sea bottom covering over ten
million square miles is sinking and has been doing so for a long time.
The foregoing facts tend to illustrate the truth that nothing is
permanent in the environment of living creatures at present. All
surroundings are perpetually changing, though apparently ever so slowly.
The changes that are now going on were also taking place yesterday, last
week, last century, last æon, and so on throughout geologic time. Gradual
oscillations of the earth’s crust on a grand scale and affecting whole
continents, but usually so slowly as to escape popular observation,
have been taking place ceaselessly through inconceivable ages. These
oscillations have produced all the great inequalities of the earth’s
surface, such as ocean basins, continents and mountain chains. The
oscillations are probably due to the slow cooling and unequal shrinking
of the whole earth which has been progressive through all geologic time.
The state of the contest between the eroding and the uplifting agencies
of the world at any time determines the height of mountains and
continents, the depths of seas, the distribution of land and water, for
that period.
[Illustration: FIG. 11.—Archæan North America. The white part of the
drawing indicates the emerged land; the dark shading indicates the
submerged land covered by a shallow sea; the light shading indicates the
deep sea.
From Shaler’s First Book in Geology. By courtesy of the publishers, D. C.
Heath & Co.]
Knowing that the present physical agencies at work on the globe have been
acting through long ages,[6] it can readily be appreciated how small
effects have been accumulated and low elevations, for instance, have
become immense, high mountain ranges. The growing mountain ranges alter
the climate and the meteorological conditions. The rainfall on one side
differs from that on the other. The temperature varies with the altitude,
and so on.
Although continents have gradually and steadily grown from the earliest
times, there have been many local alterations of land and sea. Marginal
sea bottoms have become great mountain ranges. Islands have appeared and
sunk from view. Lakes have been gradually converted into solid land or
into peat bogs. Fresh water bodies have become brackish. Dry lands have
become marshes, and forests have been buried beneath the waves. Geologic
changes have caused great alterations in climate at given times and in
given areas.
These statements may be illustrated and emphasized by a brief reference
to the =development of the Continent of North America=. This Continent
has grown from comparatively a small beginning to its present great
proportions. In doing so it has passed through eras of stupendous
duration. These eras in the order of their occurrence are as follows:
(1) =Archæan era=; (2) =Palæozoic era= (subdivided into =Cambrian=,
=Silurian=, =Devonian= and =Carboniferous= periods); (3) =Mesozoic era=
(subdivided into =Triassic=, =Jurassic= and =Cretaceous= periods); (4)
=Cenozoic era= (subdivided into =Tertiary= and =Quaternary= periods); the
Tertiary is subdivided into =Eocene=, =Miocene= and =Pliocene= epochs;
the Quaternary is subdivided into =Glacial=, =Champlain= and =Terrace=
epochs; (5) =Psychozoic era=, or recent epoch.
The physical geography of the continent at the close of that early
geologic era known as the =Archæan= is shown on the map (Fig. 11). At
this time the vast portion of the continent, whose outlines nevertheless
existed, was submerged under a shallow sea, as indicated by the dark
shading on the figure. The white V-shaped mass, starting just above the
site of the great lakes and extending on the one hand in a northeasterly
direction to Labrador, and on the other in a northwesterly direction to
the Arctic Ocean, is the emerged land of this _Archæan_ time. Smaller
masses of Archæan land are also seen at the site of the Blue Ridge
Mountains in the east and at that of the Rocky Mountains in the west.
Around these lands as a nucleus the North American Continent has been
built. Therefore, at the close of the Archæan or beginning of the next,
or =Palæozoic era=, the whole interior portion of the continent was
covered by a shallow sea which beat against the Canadian Archæan land
on the north, the Blue Ridge Archæan land on the east, and the Rocky
Mountain Archæan land on the west. This shallow sea is known as the
_Palæozoic Sea_. Throughout the vast ages of the Palæozoic era, immense
sediments were being deposited along the marginal sea bottoms. The
deposition of these sediments was simultaneous with a further sinking
of the submerged continent, so that the shallowness of the Palæozoic
Sea was maintained; finally, the uplifting forces predominated, and the
submerged land along the margins of the Canadian Archæan appeared as dry
land, and thus increased the area of the infant continent. During all
this period there was a steady and slow growth of the land southward from
the Canadian Archæan, so that towards its close the visible continent had
increased nearly, though not exactly, to the proportions attained in a
still later (=Cretaceous=) period (Fig. 12).
[Illustration: FIG. 12.—Cretaceous North America. The white portion of
the figure indicates emerged land—the growing continent.
From Shaler’s First Book in Geology. By courtesy of the publishers, D. C.
Heath & Co.]
At the close of the Palæozoic era the slow, steady changes that had been
going on were replaced by more rapid and comparatively revolutionary
changes, which caused great alterations in the physical geography and
climate. Hitherto the continent had been comparatively low. Now the vast
sedimentary accumulations constituting the marginal sea bottom of the
eastern portion of the Palæozoic Sea, which had been accumulating through
all Palæozoic time, were uplifted into the great Appalachian chain of
mountains.
During the earlier ages (=Silurian= and =Devonian=) of this Palæozoic
era, the place of the Appalachian chain of mountains was marginal sea
bottom; but during the later ages (=Carboniferous=) it was, through
repeated oscillations, in an uncertain state, being sometimes swamp land,
sometimes covered with river sediment, and sometimes covered by the sea.
It was during this _Carboniferous age_ that the great coal measures were
formed; at this time also the climate was probably very uniform, warm
and moist, loaded with carbonic acid gas and deficient in oxygen. This
period was undoubtedly a paradise for the great coal-forming plants, but
was very unsuitable for the hot-blooded air-breathing animals, such as
mammals and birds, none of which existed at that period. Throughout all
geological time the excessive amount of moisture in the air has been
gradually removed by the growth of continents in size and height; also
the superabundant carbon dioxide in the atmosphere has been removed in
many ways, especially by the plants in the coal period appropriating
the carbon. Many ages later, at the close of the =Jurassic= period, the
Sierra Nevada range of mountains was uplifted. Up to this time the site
of these mountains was a marginal sea bottom receiving vast amounts
of sediment, and the Pacific coast-line was east of the site of the
Sierra range. Naturally vast changes in physical geography and climate
occurred in consequence. During these and the following =Cretaceous=
ages that the continent was growing, the great interior Palæozoic sea
and what may be called the Gulf of Mexico were more and more restricted,
as shown in the map of North America in the Cretaceous period of its
growth (Fig. 12). This great inland sea, separating the continent into
an eastern and western portion, is now called _Cretaceous Sea_ instead
of Palæozoic. This Cretaceous Sea covered the whole plains and plateau
region of the continent, and extended from the Gulf of Mexico to the
Arctic Ocean. At the end of the Cretaceous period of the continent
this sea was obliterated by the gradual upheaval of this region and
replaced by great lakes. At the same time the western marginal bottom
of the sea was uplifted into the Wahsatch range of mountains; also at
this time a line of islands in the Cretaceous Sea was uplifted into the
Colorado mountains. All these events were entailing tremendous changes in
physical geography and climate. Fig. 13 is a representation of the map of
North America in the early =Tertiary= period, the time succeeding the
Cretaceous period. In this period, the continent continuing to uplift,
the lakes that occupied the site of the Cretaceous Sea are obliterated;
the Coast Range mountains of California and Oregon are uplifted from
marginal sea bottom (Fig. 13, dark shading); the Atlantic and Gulf
borders are extended (dark shading), so that at the close of the Tertiary
period the North American continent had attained its present form, except
the southern portion of Florida and its keys. Since then the latter have
grown and are still growing.
[Illustration: FIG. 13.—Early Tertiary North America.
From Shaler’s First Book in Geology. By courtesy of the publishers, D. C.
Heath & Co.]
A later epoch still in the history of the globe is known as the
=Quaternary= period, the period that immediately preceded our present
epoch. The great features of this period, which is divided into
=Glacial=, =Champlain= and =Terrace= epochs, are the wide-spread
oscillations of the earth’s crust in high latitudes towards the north and
south poles, attended with great changes of climate from temperateness
to extreme cold.
The Glacial epoch was characterized by upward crust movements, the land
becoming over one thousand feet higher than at present. The land was
covered with ice, and an arctic severity of climate extended almost to
the Gulf of Mexico. The Champlain epoch was characterized by a downward
movement of the coast until it became five hundred feet or more below
the present level, so that many lower portions of the continent became
covered with sea. At this time there was a moderation of the temperature,
a melting of the vast sheets of ice, and consequently a flooding of
rivers and lakes, with many icebergs floating in them. The last or
Terrace epoch of the Quaternary period was characterized by a crust
movement up to the present condition of things.
What is true of the instability of the North American Continent is true
of _all_ the continents of the globe. They have all grown from small
beginnings to their present huge proportions, and are _now_ undergoing
slow but irresistible changes. When these facts are held in mind, one may
form a faint conception of the colossal changes that have taken place
throughout the sweep of bygone ages. _Environment means a complexity of
conditions almost infinite in their number and character, and almost
infinite in their variations._
SECTION IV.
TRANSMUTATION OF LIVING FORMS.
TRANSMUTATION OF LIVING FORMS.