These five theses of the modern cell-dogma are by no means sound; they

are incompatible with the theory of evolution. I have, therefore,

consistently resisted them for thirty-eight years, and consider them

to be so dangerous that I will briefly give my reasons. First, let

us clearly understand the modern definition of the cell. It is now

generally defined (in accordance with the second thesis) as being

composed of two essentially different parts, the nucleus and the

cell-body, and it is added that these organella differ constantly

both in respect of chemistry, morphology, and physiology. If that

is really so, the cell cannot possibly be the primitive organism; if

it were, we should have a miracle at the beginning of organic life

on the earth. The theory of natural evolution clearly and distinctly

demands that the cell (in this sense) is a secondary development from

a simpler, primary, elementary organism, a homogeneous cytode. There

are still living to-day very simple protists which do not tally with

this definition, and which I designated _monera_ in 1866. As they must

necessarily have preceded the real cells, they may also be called

"precellular organisms."

The earliest organisms to live on the earth, with which the wonderful

drama of life began, can, in the present condition of biological

science, only be conceived as homogeneous particles of plasm--biogens

or groups of biogens, in which there was not yet the division of

nucleus and cell-body which characterizes the real cell. I gave the

name "cytodes" to these unnucleated cells in 1866, and joined them

with the real nucleated cells under the general head of "plastids."

I also endeavored to prove that such cytodes still exist in the form

of independent monera, and in 1870 I described in my _Monograph on

the Monera_ a number of protists which do not tally with the above

definition.

Fifty years ago I made the first careful observations of living monera

(_protamœba_ and _protogenes_), and described them in my _General

Morphology_ (vol. i., pp. 133-5; vol. ii., p. xxii.) as structureless

organisms without organs and the real beginnings of organic life.

Soon afterwards, during a stay in the Canary Islands, I succeeded

in following the continuous life-history of a related organism of

the rhizopod type, which behaved like a very simple mycetozoon, but

differed in having no nucleus; I have reproduced the picture of it in

the first plate of my _History of Creation_. The description of this

orange-red globule of plasm (_protomyxa aurantiaca_) appeared first in

my _Monograph on the Monera_. Most of the organisms which I comprised

under this name exhibited the same movements as true rhizopods (or

sarcodina). It was afterwards proved of some of them that there was

a nucleus hidden within the homogeneous particle of plasm, and that,

therefore, they must be regarded as real cells. But this discovery

was wrongly extended to the whole of the monera, and the existence of

unnucleated organisms was denied altogether. Nevertheless, there are

living to-day several kinds of these organisms without organs, some

of them being very widely distributed. The chief examples are the

chromacea and the bacteria, the former with vegetal and the latter with

animal metabolism (or the former plasmodomous = plasma-forming, and the

latter plasmophagous = plasma-feeding). On the ground of this important

chemical difference, I distinguished two principal groups of the monera

in my _Systematic Phylogeny_ twenty years ago--the phytomonera and

the zoomonera, the former being unnucleated protophyta and the latter

unnucleated protozoa.

Among living organisms the chromacea are certainly the most primitive

and the nearest to the oldest inhabitants of the earth. Their simplest

forms, the chroococcacea, are nothing but small structureless particles

of plasm, growing by plasmodomism (formation of plasm) and multiplying

by simple cleavage as soon as their growth passes a certain limit

of individual size. Many of them are surrounded by a thin membrane

or somewhat thicker gelatinous covering, and this circumstance had

prevented me for some time from counting the chromacea as monera.

However, I became convinced afterwards that the formation of a

protective cover of this kind around the homogeneous particle of

plasm may indeed be regarded from the physiological stand-point as a

"purposive" structure, but at the same time may be looked upon, from

the purely physical stand-point, as a result of superficial strain.

On the other hand, the physiological character of these plasmodomous

monera is especially important, as it gives us the simple key to the

solution of the great question of spontaneous generation (or archigony,

_cf._ chapter xv.).

The chromacea are to-day found in every part of the earth, living

sometimes in fresh water and sometimes in the sea. Many species form

blue-green, violet, or reddish deposits on rocks, stones, wood, and

other objects. In these thin gelatinous plates millions of small

homogeneous cytodes are packed close together. Their tint is due to a

peculiar coloring matter (phycocyan), which is chemically connected

with the substance of the plasma-particle. The shade of this color

differs a good deal in the various species of chromacea (of which more

than eight hundred have been distinguished); in the native species it

is generally blue-green or sage-green, sometimes blue, cyanine blue,

or violet. Hence the common name cyanophyceæ (_i.e._, blue algæ).

It is incorrect, for two reasons; firstly, because only a part of

these protophyta are blue, and, secondly, because they (as simple,

primitive plants without tissue) must be distinguished from the real

algæ (phyceæ), which are multicellular, tissue-forming plants. Other

chromacea are red, orange, or yellow in color, as the interesting

_trichodesmium erythræum_, for instance, the flaky masses of which,

gathering in enormous quantities, cause at certain times the yellow

or red coloring of the sea-water in the tropics; it is these that are

responsible for the name "Red Sea" on the Arabian and "Yellow Sea" on

the Chinese coast. When I passed the equator in the Sunda Straits on

March 10, 1901, the boat sailed through colossal accumulations, several

miles in width, of this trichodesmium. The yellow or reddish surface of

the water looked as if it were strewn with sawdust. In the same way,

the surface of the Arctic Ocean is often colored brown or reddish-brown

by masses of the brown _procytella primordialis_ (formerly described as

_protococcus marinus_).

It is clearly quite illogical to regard the chromacea as a class

or family of the algæ, as is still done in most manuals of botany.

The real algæ--excluding the unicellular diatomes and paulotomes,

which belong to the protophyta--are multicellular plants that form

a _thallus_ or bed of a certain form and characteristic tissue. The

chromacea, which have not advanced as far as the real nucleated cell,

are unnucleated cytodes of a lower and earlier stage of plant-life.

If one would compare the chromacea with algæ or other plants at all,

the comparison cannot be with their constituent cells, but merely

with the chromatophora or chromatella, which are found in all green

plant-cells, and form _part_ of their contents. To be more precise,

these green granules of chlorophyll must be regarded as organella of

the plant-cell, or separated plasma-formations which arise beside the

nucleus in the cytoplasm. In the embryonic cells of the germs of plants

and in their vegetation points the chromatophora are as yet colorless,

and are developed, as solid, very refractive, globular, or roundish

granules, from the firm layer of plasm which immediately surrounds the

nucleus. Afterwards they are converted, by a chemical process, into

the green chlorophyll granules or chloroplasts, which have the most

important function in the plasmodomism or carbon-assimilation of the

plant.

The fact that the green chlorophyll granules grow independently

within the living plant-cell and multiply by segmentation is very

important and interesting. The globular chloroplasts are constricted

in the middle, and split into two equal daughter-globules. These

daughter-plastids grow, and multiply in turn in the same way. Hence

they behave within the plant-cell just like the free-living chromacea

in the water. On the strength of this significant comparison, one of

our ablest and most open-minded scientists, Fritz Müller-Desterro, of

Brazil, pointed out in 1893 that we may see in every green vegetal cell

a symbiosis between plasmodomous green and plasmophagous not-green

companions (_cf._ my _Anthropogeny_, figs. 277 and 278, and in the

text).

Many species of the simplest chromacea live as monobia (individually).

When the tiny plasma globules have split into two equal halves by

simple segmentation, they separate, and live their lives apart. This

is the case with the common, ubiquitous chroococcus. However, most

species live in common, the plasma granules forming more or less thick

cœnobia, or communities or colonies of cells. In the simplest case

(_aphanocapsa_) the social cytodes secrete a structureless gelatinous

mass, in which numbers of blue-green plasma globules are irregularly

distributed. In the _glœocapsa_, which forms a thin blue-green

gelatinous deposit on damp walls and rocks, the constituent cytodes

cover themselves immediately after cleavage with a fresh gelatinous

envelope, and these run together into large masses. But the majority

of the chromacea form firm, threadlike cell communities or chains

of plastids (catenal cœnobia.) As the transverse cleavage of the

rapidly multiplying cytodes always follows the same direction, and

the new daughter-cytodes remain joined at the cleavage surfaces, and

are flattened into discoid shape, we get stringlike formations or

articulated threads of considerable length, as in the oscillaria and

nostochina. When a number of these threads are joined together in

gelatinous masses, we often get large, irregular, jelly-like bodies, as

in the common "shooting-star jellies" (_nostoc communis_). They attain

the size of a plum.

In view of the extreme importance which I attach to the chromacea as

the earliest and simplest of all organisms, it is necessary to put

clearly the following facts with regard to their anatomic structure and

physiological activity:

1. The organism of the simplest chromacea is _not_ composed of

different organella or organs; and it shows no trace of purposive

construction or definite architecture.

2. The homogeneous tinted plasma granule which makes up the entire

organism in the simplest case (_chroococcus_) exhibits no plasma

structure (honeycomb, threads, etc.) whatever.

3. The original globular form of the plasma particle is the simplest of

all fundamental types, and is also that assumed by the inorganic body

(such as a drop of rain) in a condition of stable equilibrium.

4. The formation of a thin membrane at the surface of the structureless

plasma granule may be explained as a purely physical process--that of

surface strain.

5. The gelatinous envelope which is secreted by many of the chromacea

is also formed by a simple physical (or chemical) process.

6. The sole essential vital function that is common to all the

chromacea is self-maintenance, and growth by means of their vegetal

metabolism, or plasmodomism (=carbon assimilation); this purely

chemical process is on a level with the catalysis of inorganic

compounds (chapter x.).

7. The growth of the cytodes, in virtue of their continuous

plasmodomism, is on a level with the physical process of crystal growth.

8. The reproduction of the chromacea by simple cleavage is merely the

continuation of this simple growth process, when it passes the limit of

individual size.

9. All the other vital phenomena which are to be seen in some of the

chromacea can also be explained by physical or chemical causes on

mechanical principles. Not a single fact compels us to assume a "vital

force."

Especially noteworthy in regard to the physiological character of these

lowest organisms are their bionomic peculiarities, especially the

indifference to external influences, higher and lower temperatures,

etc. Many of the chromacea live in hot springs, with a temperature

of fifty to eighty degrees centigrade, in which no other organism is

found. Other species may remain for a long time frozen in ice, and

resume their vital activity as soon as it thaws. Many chromacea may be

completely dried up, and then resume their life if put in water after

several years.

Next in order to the chromacea we have the bacteria, the remarkable

little organisms which have been well known in the last few decades

as the causes of fatal diseases, and the agents of fermentation,

putrefaction, etc. The special science which is concerned with

them--modern bacteriology--has attained so important a position in

a short period--especially as regards practical and theoretical

medicine--that it is now represented by separate chairs at most of

the universities. We may admire the penetration and the perseverance

with which scientists have succeeded, with the aid of the best modern

microscopes and methods of preparation and coloring, in making so

close a study of the organism of the bacteria, determining their

physiological properties, and explaining their great importance for

organic life by careful experiments and methods of culture. The

bionomic or economic position of the bacteria in nature's household

has thus secured for these tiny organisms the greatest scientific and

practical interest.

However, we find that certain general views have been maintained by

specialists in bacteriology up to our own time which are in curious

contrast with these brilliant results. The biologist who studies the

systematic relations of the bacteria from the modern point of view of

the theory of descent is bewildered at the extraordinary views as to

the place of the bacteria in the plant-world (as segmentation-fungi),

their relations to other classes of plants, and the formation of their

species. When we carefully consider the morphological properties that

are common to all true bacteria and compare them with other organisms,

we are forced to the conclusion that I urged years ago in various

writings: the bacteria are not real (nucleated) cells, but unnucleated

cytodes of the rank of the monera; they are not real (tissue-forming)

fungi, but simple protists; their nearest relatives are the chromacea.

The individual organisms of the simplest kind, which bacteriologists

call "bacteria-cells," are not real nucleated cells. That is the clear

negative result of a number of most careful investigations which

have been made up to date with the object of finding a nucleus in

the plasma-body of the bacteria. Among recent exact investigations

we must especially note those of the botanist Reinke, of Kiel, who

sought in vain to detect a nucleus in one of the largest and most

easily studied genera of the bacteria, the _beggiatoa_, using every

modern technical aid. His conviction that this important cell-structure

is really lacking is the more valuable, as it is very prejudicial

to his own theory of "dominants." Other scientists (especially

Schaudinn) have recently claimed, as equivalent to a nucleus in some

of the larger bacteria, a number of very small granules, which are

irregularly distributed in the plasm, and are strongly tinted under

certain coloring processes. But even if the chemical identity of these

substances which take the same color were proved--which is certainly

not the case--and even if the appearance of scattered nuclein-granules

in the plasm could be regarded as a preliminary to, or a beginning

of, the differentiation of an individual, morphologically distinct

nucleus, we should not yet have shown its independence as an organellum

of the cell.

Nor is this any more proved from the circumstance that in some bacteria

(not all) we find a severance of the plasm into an inner and outer

layer, or a frothy structure with vacuole-formation, or a special

sharply outlined membrane on the plastid. Many bacteria (but not all)

have such a membrane, like the nearly related chromacea, and also the

secretion of a gelatine envelope. Both classes have also in common

an exclusively monogenetic reproduction. The bacteria multiply, like

the chromacea, by simple segmentation; as soon as the structureless

plasma-granule has reached a certain size by simple growth, it is

constricted and splits into two halves. In the long-bodied bacteria

(the rod-shaped bacilli) the constriction always goes through the

middle of the long axis, and is, therefore, simple transverse cleavage.

Many bacteria have also been said to multiply by the formation of

spores. But these so-called "spores" are really permanent quiescent

forms (without any multiplication of individuals); the central part

of the plastid (endoplasm) condenses, separates from the peripheral

part (exoplasm), and undergoes a chemical change which makes it very

indifferent to external influences (such as a high temperature).

The great majority of the bacteria differ so little morphologically

from the chromacea that we can only distinguish these two classes

of monera by the difference in their metabolism. The chromacea,

as protophyta, are plasmodomous. They form new plasm by synthesis

and reduction from simple inorganic compounds--water, carbonic

acid, ammonia, nitric acid, etc. But the bacteria, as protozoa, are

plasmophagous. They cannot, as a rule, form new plasm, but have to

take it from other organisms (as parasites, saprophytes, etc.);

they decompose it by analysis and oxydation. Hence the colorless

bacteria are without the important green, blue, or red coloring matter

(phycocyan) which tints the plastids of the chromacea, and is the real

instrument of the carbon-assimilation. However, there are exceptions

in this respect: _bacillus virens_ is tinted green with chlorophyll,

_micrococcus prodigiosus_ is blood-red, other bacteria purple, and

so on. Certain earth-dwelling bacteria (_nitro-bacteria_) have the

vegetal property of plasmodomism; they convert ammonia by oxydation

into nitrous acid, and this into nitric acid, using as their source of

carbon the carbonic acid gas in the atmosphere. They are thus quite

independent of organic substances, and feed, like the chromacea, on

simple inorganic compounds.

Hence the affinity between the plasmodomous chromacea and plasmophagous

bacteria is so close that it is impossible to give a single safe

criterion that will effectually separate the two classes. Many

botanists accordingly combine both groups in a single class with the

name of _schizophyta_, and within this distinguish as "orders" the

blue-green chromacea as _schizophycæ_ (cleavage-algæ) and the colorless

bacteria as _schizomycetes_ (cleavage-fungi). However, we must not

take this division too rigidly; and the absolute lack of a nucleus

and tissue-formation separates the chromacea just as widely from the

multicellular tissue-forming algæ as the bacteria from the fungi. The

simple multiplication by the halving of the cell, which is expressed in

the name "cleavage-plants" (_schizophyta_), is also found in many other

protists.

The number of forms that can be distinguished as species in the

technical sense is very great in the case of the bacteria, in

spite of the extreme simplicity of their outward appearance; many

biologists speak of several hundred, and even of more than a thousand,

species. But when we look solely to the outer form of the living

plasma-granule, we can only distinguish three fundamental types: (1)

Micrococci, or spherobacteria (briefly, cocci), globular or ellipsoid;

(2) bacilli, or rhabdo-bacteria (also called eubacteria, or bacteria in

the narrower sense), rod-shaped, cylindrical, and often twisted like

worms (comma-bacilli); (3) spirilla, or spirobacteria, screw-shaped

rods (vibriones when the screw is slight, and spirochæta when it

has many coils). Besides this threefold difference in the forms of

the cytodes, we have a ground of distinction in many bacilli and

spirilla in the possession of one or more very thin lashes (flagella),

which proceed from one of both poles of the lengthened plastid. The

construction and vibration of these serves for locomotion in the

swimming bacteria; but they are only found for a time in many species,

and in many others are altogether wanting.

Since, then, neither the simple outer form of the bacterium-cytodes

nor their homogeneous internal structure provides a satisfactory

ground for the systematic distinction of the numerous species,

their physiological properties are generally used for the purpose,

especially their different behavior towards organic foods (albumin,

gelatine, etc.), their chemical actions, and the various effects of

poisoning and decomposition which they produce in the living organism.

No bacteriologist now doubts that all the vital activities of the

bacteria are of a chemical nature, and precisely on this account

these microbes are of extreme importance. When we bear in mind how

complicated are the relations of the various species of bacteria to

the tissues of the human body, in which they cause the diseases of

typhus, hypochondriasis, cholera, and tuberculosis, we are bound to

admit that the real cause of these maladies must be sought in the

peculiar molecular structure of the bacterium-plasm, or the particular

arrangement of its molecules and the innumerable atoms (more than a

thousand) which are, in a very loose way, made up into special groups

of molecules. The chemical products of their mutual action are what we

call ptomaines, which are partly very virulent poisons (toxins). We

have succeeded in producing several of these poisonous matters in large

quantities by artificial culture, and isolating them and experimentally

ascertaining their nature; as, for instance, tetanin, which causes

tetanus, typhotoxin, the poison of typhus, etc.

In thus declaring the action of bacteria to be purely chemical and

analogous to that of well-known inorganic poisons, I would particularly

point out that this very justifiable statement is a pure hypothesis;

it is an excellent illustration of the fact that we cannot get on

in the explanation of the most important natural phenomena without

hypotheses. We can see nothing whatever of the chemical molecular

structure of the plasm, even under the highest power of the microscope;

it lies far below the limit of microscopic perception. Nevertheless,

no expert scientist has the slightest doubt of its existence, or that

the complicated movements of the sensitive atoms and the molecules and

groups of molecules they make up are the causes of the vast changes

which these tiny organisms effect in the tissues of the human and the

higher animal body.

Moreover, the distinction of the many species of bacteria is of

interest in connection with the general question of the nature and

constancy of a species. Whereas formerly in biological classification

only definite morphological characters, or definable differences in

outer form or inner structure, were regarded as of any moment in the

distinction of species, here, in view of the vagueness or total lack

of these characters, we have to look mainly to the physiological

properties, and these are based on the chemical differences in their

hypothetical molecular structure. But even these are not absolutely

constant; on the contrary, many bacteria lose their specific qualities

by progressive culture under changed food-conditions. By a change in

the temperature and the nutritive field in which a number of poisonous

bacteria have been reared, or by the action of certain chemicals, not

only the growth and multiplication are altered, but also the injurious

effect they have on other organisms by the generation of poisons.

This poisonous effect is weakened, and--what is most important--the

weakening is transmitted by heredity to the following generations. On

this is based the familiar process of inoculation, an admirable example

of the inheritance of acquired characteristics.

As the bacteria are still often described as "cleavage-fungi" and

classified along with the real fungi, we must particularly point

out the wide gulf that separates the two groups. The real fungi (or

_mycetes_) are metaphyta, their multicellular body (_thallus_) forming

a very characteristic sort of tissue, the mycelium; this is composed

of a number of interlaced and interwoven threads (or hyphens). Each

fungus-thread consists of a row of lengthened cells, which have a thin

membrane and enclose a number of small nuclei in the colorless plasm.

Moreover, the two sub-classes of the real fungi, the ascomycetes and

basimycetes, form peculiar fruit-bodies which generate spores (ascodia

and basidia). There is no trace whatever of these real characteristics

of the true fungus in the bacteria. Nor is it less incorrect to class

them with the fungilli, the so-called unicellular fungi or phycomycetes

(ovomycetes and zygomycetes); these form a special class of protists

which has the closest affinity to the gregarinæ.

Like the closely related chromacea, many of the bacteria show a marked

tendency to form communities or cell-colonies. These cell-communities

arise, as elsewhere, from the fact that the individuals, which

multiply rapidly by continuous cleavage, remain joined together.

This may happen in two ways. When the social bacteria secrete large

quantities of gelatine, and remain distributed in this, we have the

_zooglœa_ (as in the case of the _aphanocapsa_ and _glœocapsa_

among the chromacea). If, on the other hand, the long-bodied

bacilli remain fastened together in rows, we get the knotted

threads of _leptothrix_ and _beggiatoa_ (which may be compared with

the oscillaria). And, if these threads go into branches, we have

_cladothrix_. Other cœnobia of bacteria have the appearance of

disks, the cytodes dividing in a plane, usually in groups of four (as

in _merismopedia_), or of cube-shaped packets when they are in all

three directions of space (_sarcina_).

The two classes of bacteria and chromacea seem, in the present

condition of our knowledge, on account of their simple organization,

to be the simplest of all living things, real monera, or organisms

without organs. Hence we have to put them at the lowest stage of

the protist kingdom, and must regard the difference between them

and the most highly differentiated unicellular beings (such as the

radiolaria, ciliated infusoria, diatomes, or siphonea) as no smaller

than the difference (in the realm of the histona) between a lower

polyp (_hydra_) and a vertebrate, or between a simple alga (_ulva_)

and a palm. But if the kingdom of the protists is badly divided, on

the older rule, into a plant kingdom and an animal kingdom, the only

discriminating mark we have left is the difference in metabolism; in

that case we have to include the plasmophagous bacteria in the animal

kingdom (as Ehrenberg did in 1838) and the plasmodomous chromacea

in the plant kingdom. The remarkable class of the flagellata, which

includes ciliated unicellulars of both groups, contains several

forms which are only distinguished from the typical bacterium by the

possession of a nucleus. If it is true that in some of the protists

which were counted as bacteria a real nucleus has been detected, these

must be separated from the others (unnucleated) and included in the

nucleated flagellata.

The monera which I described in 1866, and on which I based the theory

of the monera in my monograph, belong to a different division of the

protists from the classes of bacteria and chromacea. These are the

forms which I described as _protamœba_, _protogenes_, _protomyxa_,

etc. Their naked mobile plasma-bodies thrust out pseudopodia, or

variable "false feet," from their surface, like the (nucleated) real

rhizopods (=sarcodinæ); but they differ essentially from the latter in

the absence of a nucleus. Afterwards (in my _Systematic Phytogeny_)

I proposed to separate these unnucleated rhizopods from the others,

giving the name of _lobomonera_ (_protamœba_) to the amœba-like

monera with flap-shaped feet, and the name of _rhizomonera_

(_protomyxa_, _pontomyxa_, _biomyxa_, _arachnula_, etc.) to the

gromia-like, root-feet forming monera. However, of late years, real

nuclei have been detected in each of these large monera, and so they

have been proved to be true cells. This discovery was made possible by

the improved modern methods of coloring the nucleus which I had not the

use of thirty years ago in my first observations. On the strength of

these recent discoveries many scientists claim that all the monera I

described are true cells, and must have nuclei. This baseless assertion

is much employed by the opponents of the theory of evolution in order

to deny the existence of the monera altogether.

Of the genus of monera which we call protamœba I have given an

illustration in my _History of Creation_ (tenth edition), which has

been frequently reproduced. Several species (at least two or three)

of this genus still exist, and are distinguished by the shape of

their flap-formation and their method of motion. They resemble

ordinary simple amœbæ, and only differ from these to any extent

in the absence of a nucleus. The _protamœba primitiva_ seems

to be pretty widely distributed; it has been found repeatedly by

observers (Gruber, Cienkowski, Leidy, etc.) in inland waters. In the

zoological demonstrations which I have given at the University of Jena

for forty years, and in the course of which the lowly inhabitants

of our fresh water are regularly examined with the microscope, the

_protamœba primitiva_ has been found four or five times. It

always had the same form, as I described it, moved about by the slow

formation of flaps at its surface, multiplied by simple cleavage,

and showed no trace of a nucleus in its homogeneous plasma-body even

with the most careful application of the modern methods of tinting

the nucleus. A larger number of very fine granules (microsoma) that

were irregularly distributed in the plasm, and were more or less

colored by nucleus-reagents, cannot be reckoned as clear equivalents

of the nucleus in this or in similar cases; they are probably products

of metabolism. The same may be said of the larger marine form of

rhizomoneron, which A. Gruber has recently called _pelomyxa pallida_.

The large marine form of rhizomoneron to which Huxley gave the name

of _bathybius Haeckelii_ in 1868, and as to the real nature of which

many opinions have been expressed, seems, according to the latest

investigation, not to have the significance ascribed to it. However,

the much-discussed question of the bathybius is superfluous as far as

our monera theory and the associated hypothesis of archigony (chapter

xv.) are concerned, since we have now a better knowledge of the much

more important monera-forms of the chromacea and bacteria.

In the case of some of the protists I described in my _Monograph on the

Monera_, it is at present doubtful whether their plasma-body contains

a nucleus or not, and, therefore, whether they are to be classed as

true cells or cytodes. This applies especially to the forms which

only happened to come under observation once, such as _protomyxa_

and _myxastrum_. In these obscure cases we must wait for fresh

investigations and the application of the modern methods of tinting

the nucleus. I may, however, point out, in passing, that these famous

methods of nucleus-coloring give by no means the absolute certainty

which is ascribed to them; there are other substances which take color

in the same way as chromatin. As far as my monera theory is concerned,

or the great general importance which I attach to these unnucleated

living granules of plasm, it does not matter whether a nucleus is

detected in these problematic monera or not. The chromacea alone--the

most important of all monera--completely suffice to provide a base for

the far-reaching theoretical conclusions which I draw from it.

At the close of these observations on the monera I will briefly

recapitulate the weighty inferences which we can deduce from their

simple organization. They serve as a solid foundation for the chief

theses of our monistic biology; and they are inconsistent with the

dualistic views of modern vitalists. In the first place, I emphasize

the fact that the structureless plasm-body of the simple monera has

no sort of organization and no composition from dissimilar parts

co-operating for definite vital aims. Reinke's conscious "dominant"--as

well as Weismann's mechanical "determinants"--have nothing to do here.

The whole vital activity of the simplest monera, especially of the

chromacea, is confined to their metabolism, and is therefore a purely

chemical process, that may be compared to the catalysis of inorganic

compounds. The simple formation of individuals in this primitive living

matter is merely a question of the cleavage of plasma globules of a

certain size (_chroococcus_); and their primitive multiplication (by

simple self-division) is only a continued growth (analogous to that

of the crystal). When this simple growth passes a certain limit, that

is fixed by the chemical constitution, it leads to the independent

existence of the redundant growth-products.

X

NUTRITION

Functions of nutrition--Assimilation and disassimilation--Plasmodoma

and plasmophaga--Phytoplasm and zooplasm--Plasmodomism of

plants--Chlorophyll granules and nitro-bacteria--Plasmophagism of

fungi and animals--Metasitism (conversion of metabolism)--Nutrition

of the monera (chromacea, bacteria, rhizomonera)--Nutrition of the

protophyta and metaphyta (cell-plants and tissue-plants)--Nutrition

of the metazoa--- Gastræa theory--Gastro-canal system of the

cœlenteria (gastræads, sponges, cnidaria, platodes)--Nutrition

of the cœlomaria (digestion, circulation, respiration,

evacuation)--Saprositism--Parasitism--Symbiosis.

The wonder of life which we call, in the widest sense of the word,

"nutrition" is the chief factor in the self-maintenance of the organic

individual. It is always bound up with a chemical modification of the

living matter, an organic metabolism (circulation of matter), and a

corresponding circulation of force. In this chemical process plasm is

used up, built up afresh, and once more disintegrated. The metabolism

which lies at the root of this chemistry of food is the essential

feature in the manifold processes of nutrition. A large part of the

several nutritive processes are explained without further trouble

by the known physical and chemical properties of inorganic bodies;

for another part of them we have not yet succeeded in doing this.

Nevertheless, all impartial physiologists now agree that it is possible

in principle, and that we have no reason to introduce a special vital

principle. All the trophic (nutritive) processes, without exception,

are subject to the law of substance.

In all the higher plants and animals the chemical process of

metabolism, with the stream of energy that accompanies it, is a very

complex vital activity, in which many different functions and organs

co-operate with the common aim of self-maintenance. As a rule, they are

distributed in four groups--namely: (1) Intussusception of food and

digestion: (2) distribution of the food in the body, or circulation;

(3) respiration, or exchange of gases; and (4) excretion of unusable

matter. In most of the histona, either tissue-plants or tissue-animals,

a number of organs are differentiated for the accomplishment of these

tasks. At the lower stages of life this division of labor is not found,

the entire process of nutrition being accomplished by a single layer of

cells (lower algæ, gastræads, sponges, lower polyps). In the protists,

again, it is the single cell that does all these things itself; in the

simplest cases, the monera, a homogeneous plasma-globule. As a long

gradation uninterruptedly unites these lowest forms of nutrition with

the more complicated forms, we must regard the latter no less than the

former as physico-chemical processes.

When we take the whole of the metabolic functions in organisms

together, we may look upon them as the outcome of two opposite chemical

processes--on the one hand the building-up of living matter by taking

in food (assimilation), and on the other the breaking-down of it in

consequence of its vital activity (disassimilation). As in every case

the plasm is the active living matter, we may say: _Assimilation_ (or

plasma-production) consists in the conversion within the organism into

the special plasm of the particular species of food that has been

received from without; _disassimilation_ (or plasma-destruction)

is the result of the work done by the plasm, which is the cause of

its partial decomposition or breakdown. In both respects there is a

striking difference between the two great kingdoms of organic nature.

The plant kingdom is, on the whole, the agent of assimilation, forming

new plasm by synthesis and reduction from inorganic matter. In the

animal world, on the contrary, disassimilation preponderates, the plasm

received being resolved by oxydation, and the actual energy taken out

of it by analysis being converted into heat and motion. Plants are

plasmodomous; animals, plasmophagous.

Of all the chemical processes the most important, because the most

indispensable, for the origin and maintenance of organic life is the

constant reconstruction of plasm. We give it the name of plasmodomism

(_domeo_=to build up), or carbon-assimilation. Botanists have the

habit of late of calling it briefly assimilation, and have thus

caused a good deal of misunderstanding. The more common and older

meaning of assimilation in animal physiology is, in the widest sense,

the intussusception and preparation of the food received. But the

carbon-assimilation in plants--what I call plasmodomism--is only the

first and original form of plasma-production. It means that the plant

is able, under the influence of sunlight, to form carbohydrates,

and from these new plasm, out of simple inorganic compounds (water,

carbonic acid, nitric acid, and ammonia) by synthesis and reduction.

The animal is unable to do this. It has to take its plasm in its

food from other organisms--plant-eaters directly, and animal-eaters

indirectly. We therefore give the title of _plasmophagous_ to these

animal "plasma-eaters." In working up the foreign plasm it has eaten,

and converting it into its own specific form of plasm, the animal

also accomplishes assimilation; but this animal albumin-assimilation

is totally different from the vegetal carbon-assimilation. The

fresh-formed animal plasm is then broken up by oxydation, and by this

analysis the energy needed for the vital movements is obtained.

The physiological contrast which we thus find between the two principal

forms of living matter, the synthetic plasm of the plant and the

analytic plasm of the animal, is of great importance for the lasting

maintenance of the whole organic world. It depends on a reversal of

the molecular movement in the plasm, the intimate nature of which is

just as little known to us as the chemical constitution of the albumins

in general, and that of living albumin, the plasm, in particular. As

I mentioned in chapter v., modern physiological chemistry has good

reason to believe that the invisible albumin-molecule is, comparatively

speaking, gigantic, and is composed of more than a thousand atoms.

These are in such an unstable equilibrium, so complicated and

impermanent an arrangement, that the slightest push or stimulus

suffices to alter them and form a new kind of plasm. As a fact, the

number and variety of kinds of plasm are immense. This is seen at

once from the ontogenetic fact that the ovum and sperm-cell of each

species (and each variety) have a specific chemical constitution.

In reproduction this is transmitted to the offspring. But, setting

aside these countless finer modifications, we may distinguish two

chief groups of kinds of plasm: the phytoplasm of the plant, with the

synthetic property of plasmodomism, and the zooplasm of the animal,

which is destitute of this property, and so confined to plasmophagy.

The remarkable synthetic process of building up the plasm, to which

we give the name of plasmodomism, or carbon-assimilation, usually

needs as its first condition the radiant energy of sunlight. Every

green plant-cell contains in its chlorophyll-granules so many tiny

laboratories, their green plasm being able to form new plasm out of

inorganic compounds under the influence of light. The water that is

needed for this, besides nitrogenous compounds (nitric acid, ammonia),

is drawn from the earth by the roots; the carbonic acid is taken from

the atmosphere by the green leaves. The immediate products of the

synthesis, due to the separation of the carbonic acid, is, as a rule, a

non-nitrogenous starch-flour (_amylum_). This is further used for the

composition of the nitrogenous albumin by an as yet unknown synthetic

process, with the aid of nitrogenous mineral compounds. In this process

of reduction the separated free oxygen is returned to the atmosphere.

The carbohydrates that chiefly co-operate in this are glucoses and

maltoses: the mineral substances, especially salts of potassium and

magnesium, and compounds of these elements with nitric acid, sulphuric

acid, and phosphoric acid. Iron is also found to be an important

element in the process, though in a very small quantity. As a rule,

the ferruginous chlorophyll can only form new plasm with the help of

light-waves. The most important part of the spectrum for this purpose

is that containing the red, orange, and yellow waves.

The chief factor in plasma-formation in the organic world is the

photosynthesis, or ordinary carbon-assimilation by chlorophyll, the

wonderful green matter that amounts to only a very small percentage

(about one-tenth) of the weight of the chlorophyll-granules, and can

be separated from their plasmatic substance by certain methods. Even

when the plant has some other color than green the chlorophyll is still

the real plasmodomous substance. Its green color is then masked by

some other color--diatomin in the yellow diatomes, phycorhodin in the

red rhodophyceæ, phycophæin in the brown phæophyceæ, and phyocyan in

the blue-green chromacea or cyanophyceæ. The latter have an especial

interest for us, because in the simplest specimens the entire organism

is merely a globular bluish-green granule of plasm. Moreover, in the

simplest forms of nucleated primitive plants (_algariæ_)--many of the

so-called unicellular algæ--the metabolism is effected by a single

grain of chlorophyll. There is usually a large number of them in the

plasm of the plant-cells.

Another kind of plasm-synthesis, quite different from the ordinary

plasmodomism by chlorophyll and sunlight has lately been discovered in

some of the lowest organisms (by Heraeus, Winogradsky, and others). The

nitro-bacteria (or nitromonades) are tiny monera (unnucleated cells)

that live in complete darkness underground. Their globular colorless

plasma-bodies contain neither chlorophyll nor nucleus. They have the

remarkable capacity of forming carbohydrates, and from these plasm,

by a peculiar synthesis out of purely inorganic compounds--water,

carbonic acid, ammonia, and nitric acid. Pfeffer has called this

carbon-assimilation, on account of its purely chemical nature,

"chemosynthesis," in opposition to the ordinary photosynthesis by

means of sunlight. There are also other bacteria (sulphur-bacteria,

purple-bacteria, etc.) that show various peculiarities of metabolism.

The nitro-bacteria must belong to the oldest monera, and represent a

transition from the vegetal chromacea to the animal bacteria.

The extensive class of the fungi (or _mycetes_) resembles a part of

the bacteria in regard to metabolism. These organisms are, it is true,

generally regarded as plants, but they have not the capacity of the

green, chlorophyll-bearing plants to supply themselves with carbon

from the carbonic acid in the atmosphere. They have to take it from

organic substances, such as albumin, carbohydrates, etc., like the

animals. But while the animals have to derive their nitrogen from the

latter, the fungi can obtain it from inorganic matter in the earth.

Fungi cannot support life without the addition of organic compounds;

but we can make them grow in a food solution consisting of sugar and

purely inorganic nitrogenous salts. Thus they are on the border that

separates the plasmodomous plants from the plasmophagous animals. Like

the latter, the fungi have evolved from the plants through changed food

conditions. We find this process even among the unicellular protists in

the phycomycetes, which descend from the siphonea. In the same way the

real multicellular fungi (ascomycetes and basimycetes) may be traced to

the tissue-forming algæ.

All true animals have to derive their food from the plant kingdom, the

vegetal feeders directly, and the flesh feeders indirectly, when they

consume vegetal feeders. Hence the animals are, in a certain sense, as

the older natural philosophy put it four hundred years ago, "parasites

of the plant world." From the point of view of phylogeny, the animal

kingdom is, therefore, clearly much younger than the plant kingdom. The

development of the animals from the plants was determined originally by

a change in the method of nutrition which we call metasitism.

The chemical modification of the living matter which is connected

with the loss of plasmodomism--in other words, the conversion of the

reducing phytoplasm into oxidizing zooplasm--must be regarded as

one of the most important changes in the history of organic life.

This "reversal of metabolism" is polyphyletic; it has been repeated

many times in the course of biological history, and has taken place

independently in very different groups of the organic world--whenever

a plasmodomous cell or group of cells (=tissue) had occasion to feed

directly on ready-made plasm, instead of giving itself the trouble of

building it up out of inorganic compounds. We see this particularly

among the unicellular protists in the independent ciliated cells.

The longer plasmophagous flagellata, which are colorless, and have

no chlorophyll (monodina, conoflagellata), closely resemble in

form and movement the older plasmadomous and chlorophyll-bearing

mastigota, from which they are descended (volvocina, peridinia); they

only differ in the manner of nutrition. The colorless flagellata

feed on ready-formed plasm, which they obtain either by means of

their lashes or by a special cell mouth in their cell body. On the

other hand, their ancestors, the green or yellow mastigota, form

new plasm by photosynthesis like true cells. But there are also

complete intermediate forms between the two groups--for instance, the

chrysomonades and the gymnodinia; these may behave alternately as

protozoa or protophyta. In the same way we can derive the phycomycetes

by metasitism from the siphonea, the fungi from the algæ; and, finally,

the process is also found in many of the higher parasitic plants

(orchids, orobanches, etc.). (See under "Parasitism.")

As is the case with every other vital function, so for the function

of metabolism we find a starting-point in the lowest and simplest

group of the protophyta, the chromacea. In their oldest forms, the

chroococcacea, the whole body is merely a blue-green, structureless,

globular plasma particle, growing by means of its plasmodomous power,

and splitting up as soon as it reaches a certain stage of growth.

There the miracle of life consists merely of the chemical process of

plasmodomism by photosynthesis. The sunlight enables the blue-green

phytoplasm to form new plasm of the same kind out of inorganic

compounds (water, carbonic acid, ammonia, and nitric acid). We may look

upon this process as a special kind of catalysis. In this case there

is absolutely nothing to be done by Reinke's "dominants," or conscious

and purposive vital forces. There are, as yet, no differentiated

physiological functions in these organisms without organs, and no

anatomically distinct members; and so their one vital activity, growth,

may very well be compared to the simple growth of inorganic crystals.

It has been pointed out repeatedly that the remarkable monera which

now play so important a part in biology as bacteria stand, in many

respects, quite apart from the ordinary vital phenomena of the

higher organisms. This is especially true of their metabolism, which

has the most striking peculiarities. Morphologically, many of the

bacteria cannot be distinguished from their nearest relatives and

direct ancestors, the chromacea, differing from them only in the

absence of coloring matter in the plasm. Many of them are simple,

globular, ellipsoid, or rod-shaped plasma particles, without any

visible organization or movement. Others move about by means of one or

more very fine lashes (like the flagellata). No real nucleus can be

discovered in the structureless plasma body. The very fine granules

which are found in some species, and the vacuole-formation that we see

in others, may be regarded as products of metabolism; and the same

may be said of the thin membrane or the thicker gelatinous envelope

which many of the bacteria secrete. This makes all the more remarkable

the peculiarity of their chemical constitution and the metabolism

determined thereby. The nitro-bacteria we have mentioned previously are

plasmodomous; the anaërobe bacteria (of butyric acid and tetanus) only

flourish where oxygen is excluded; the sulphur bacteria (_beggiatoa_)

secrete--by the oxydation of sulphuretted hydrogen--pure regulation

sulphur in the form of round granules. The ferruginous bacteria

(_leptothrix ochrocea_) store up oxyhydrate of iron (by the oxydation

of carbonic protoxide of iron). The saprogenetic bacteria cause

putrefaction, and the zymogenetic fermentation. Finally, we have the

very interesting pathogenetic bacteria which cause the most dangerous

diseases by the secretion of special poisons--toxins--festering,

small-pox, tetanus, diphtheria, typhus, tuberculosis, cholera, etc.

On account of their great practical importance, these bacteria have

of late been taken over by a special branch of biology, bacteriology.

But only a few of the many experts in this department have pointed out

the extreme theoretical significance which these zoomonera have for

the important questions of general biology. These structureless plasma

bodies show unmistakably that their vital activity is a purely chemical

phenomenon. Their great variety proves how manifold and complicated

must be the molecular composition of the plasm, even in these simplest

organisms.

The unicellular protophyta exhibit the same form of metabolism and

plasmodomism as the familiar green cells of the tissue-plants; but

in most of the protozoa we find special features of nutrition and

plasmophagy. The great class of the rhizopods is distinguished by the

fact that their naked plasma body can take in ready-formed solid food

at any point of its surface. On the other hand, most of the infusoria

have a definite mouth-opening in the outer wall of their unicellular

body, and sometimes a gullet-tube as well. Besides this cell-mouth

(_cytostoma_) we usually find also a second opening for the discharge

of indigestible matter, a cell-anus (_cytopyge_).

Metabolism in the tissue plants (metaphyta) forms a long gradation from

very simple to very complicated arrangements. The lowest and oldest

thallophyta, especially the simplest algæ, are not far removed from

the communities of protophyta, and, like these, are merely definitely

grouped colonies of cells. The social cells which form their most

rudimentary tissue are quite homogeneous, with no differentiation

beyond that of sex. The thallus or bed-formation consists in the

simplest specimens of plain or branched fine threads, consisting of

rows or chains of homogeneous cells (so _conferva_ among the green,

_ectocarpus_ among the brown, and _callithamnion_ among the red

algæ). Other algæ (such as the ulva) form thin leaf-shaped forms of

the thallus, a number of homogeneous cells lying side by side along a

level. In the larger algæ compact tissue-bodies are formed, in which

frequently firmer rows of cells exhibit the rudiments of fibres; and

the thallus divides, as in the cormophyta, into root, stalk, and

leaves. There is also a trophic differentiation, the fibres undertaking

special functions of nutrition (the conduction of the sap). The

same must be said of the mosses (_bryophyta_). Their lowest forms

(_ricciadinæ_) are close akin to the algæ; the highest mosses (the

_mnium_ and _polytrichum_, for instance) approach the cormophyta. Many

botanists comprise these lower plants--algæ, fungi, and mosses--under

the title of "cell-plants" (_cytophyta_), and oppose the higher

plants--ferns and flowering-plants--to them as "vascular plants"

(_angiophyta_), because they have complex fibres or sap vessels. This

distinction has a phylogenetic significance similar to the division

between cœlenteria and cœlomaria in the animal kingdom.

While most of the cell-plants either live in the water (algæ) or are

very simply organized on account of their saprophytic or parasitic

habits (fungi), the vascular plants mostly live on land, and have to

adapt themselves to much more complicated conditions. Their nutrition

is accordingly distributed among different functions, and special

organs have been evolved to discharge them. This is equally true of

the crytogam ferns (_pteridophyta_) and the phanerogam flowering

plants (_anthophyta_). The most important later acquisition which

distinguishes both groups from the lower cell-plants is the possession

of vascular or conducting fibres. These organs for conducting water

pass through the entire body of the vascular plant in the shape of

long tubes, formed by the combination of rows of cells; the cells

themselves die off, and their plasma content disappears. The stream of

water that rises constantly in these tubes is taken up by the roots,

conducted by the fibres to all parts, and given off (transpiration) by

the pores of the leaves. But these pores also serve for the breathing

of plants, being connected with the air-containing intercellular

passages; through these air-spaces, which serve for the aëration of

the higher plant-body, air and moisture can enter, and oxygen be given

off in respiration. Finally, many of the vascular plants have special

glands that serve for secretion (of oil, resin, etc.). In the higher

flowering plants this division of work among the various digestive

organs gives rise to a very complicated apparatus for nutrition. Among

the many remarkable structures that have been developed in this way by

adaptation to special conditions we may particularly note the organs

for catching and digesting insects in the insect-eating plants, the

European _drosera_ and _utricalaria_, and the tropical _nepenthas_ and

_dionæa_.

The long scale of evolutionary forms which we find in the tissue

animals (metazoa) leads up uninterruptedly from the simplest to

the most elaborate physiological functions and a corresponding

morphological complexity of organs. The two principal divisions of

the metazoa are chiefly distinguished by the circumstance that in the

cœlenteria one single system of organs, the gastro-canal system,

discharges the whole (or most part) of the partial functions of

nutrition; while in the cœlomaria they are usually distributed among

four different systems of organs, each of which is made up of a number

of organs. To an extent, we find once more in each great division

characteristic types of organization. However, comparative ontogeny

teaches us that all these various structures have been developed

from one simple fundamental form, as I have shown in my theory of the

gastræa (1872).

The older research into the origin of the nutritive apparatus in

the metazoa--especially its chief part, the alimentary or gastric

canal--had led to the erroneous belief that in several groups of the

metazoa it owed its origin to very different growth-processes, and

that particularly in the higher vertebrates (the amniotes) it was

a comparatively late product of evolution. On the other hand, the

comparative study of the embryology of the lower and higher animals

led me thirty-four years ago to the opposite conclusion, that a simple

gastric sac was the first and oldest organ of all the metazoa, and that

all the different forms of it had been developed from this primitive

type. I gave this view in my _Biology of the Sponges_ in 1872; and I

developed and established it in my _Studies of the Gastræa Theory_ in

1873. In the latter book I also worked out the important conclusions

that follow from this monistic reform of the theory of germinal layers

for the phylogenetic natural classification of the animal kingdom.

I began with the consideration of the simplest sponges (_olynthus_)

and cnidaria (_hydra_). The whole body of these lowest and oldest

of the cœlenteria is in essence nothing but a round, oval, or

cylindrical gastric vesicle, a digestive sac, the thin wall of which

consists of two simple layers of cells. The outer layer (the ectoderm

or skin-layer) is the covering layer of the external skin (epidermis);

it is the instrument of sensation and movement. The inner layer of

cells (entoderm or gastric layer) serves for nutrition; it clothes

the simple cavity of the sac, which admits the food by its opening

and digests it. This opening is the primitive mouth (_prostoma_ or

_blastoporus_), the inner cavity itself the primitive gut (_progaster_

or _archenteron_). I proved that there was the same composition in

the young embryos or larvæ of many of the lower animals, and showed

that the manifold and apparently very different embryonic form of all

the higher animals may be reduced to the same common type. To this I

gave the name of the "cup-embryo" or gastric larvæ (_gastrula_), and

concluded, in virtue of the biogenetic law, that it is the palingenetic

reproduction of a corresponding ancestral form (the _gastræa_)

maintained until the present by heredity. It was not until much later

(1895) that Monticelli discovered a modern gastræad (_pemmatodiscus_)

which corresponds completely to this hypothetical ancestor (see the

last edition of my _Anthropogeny_, fig. 287). The simplest living forms

of the sponges (_olynthus_) and the cnidaria (_hydra_) only differ from

this hypothetical primitive form of the gastræa by a few secondary and

subsequently acquired features.

The classes of the lower animals which we comprise under the name

cœlenteria (or cœlenterata in the widest sense) generally agree

in having all the functions of nutrition accomplished exclusively (or

for the most part) by a single system of organs, the gastro-canal or

gastro-vascular system. From their common stem-group, the gastræads,

three different stems have been evolved--the sponges, cnidaria, and

platodes. All these cœlenteria have three features in common: (1)

The gastric canal or tube has only one opening--the primitive mouth,

which serves at once for admitting food and ejecting indigestible

matter; there is no anus; (2) there is no special body-cavity

(_cœloma_) distinct from the gastric tube; (3) there is also no

trace of a vascular system. All cavities that are found in these lower

animals besides the digestive gut-cavity are direct processes from it

(with the exception of the nephridia in the platodes).

While the simple digestive gut is the sole organ of nutrition in the

stem-group of the gastræads, we find other structures co-operating in

the rest of the cœlenteria. The characteristic stem of the sponges

is distinguished by the piercing of the wall of the gastric vesicle

with several holes. Through these water pours into the body, bringing

with it the small particles of food which are received and digested

by the ciliated cells of the entoderm; the water emerges again by the

mouth-opening (_osculum_). The best-known of the sponges is the common

bath-sponge (_euspongia officinalis_), the horny skeleton of which

we use daily in washing. In these and most other sponges the large,

unshapely body is traversed by a number of branching canals, on which

there are thousands of tiny vesicles, produced by the multiplication

of a simple gastric vesicle of the primitive sponge (_olynthus_). Each

of these ciliated chambers is really a tiny gastræa, a "person" of the

simplest character (_cf._ chapter vii.). Hence we may regard the whole

sponge-body as a gastræad-stock (_cormus_).

The large group of the cnidaria offers a long series of evolutionary

stages, from very small and simple to very large and elaborate forms.

Some of them remain at a very low stage, as does our common green

fresh-water polyp (_hydra viridis_), which only differs from the

gastræa by a few variations in tissue and the formation of a crown of

feelers about the mouth. Most of the polyps form stocks (_cormi_),

the individuals shooting out buds which remain joined to the mother

animal. In these and all the other stock-forming animals the nutrition

is communistic; all the food that the individuals get and digest is

conducted by tubes to the common fund and equally distributed. In all

the larger cnidaria the body-wall becomes thicker, and is traversed by

branching gastro-canals; these convey the nutritive fluid to all parts

of the body.

While the fundamental type in the cnidaria is radial (determined

by the crown of radiating feelers or tentacles that surrounds the

mouth), it is bilateral-symmetrical in the platodes or "flat-worms"

(_plathelminthes_). In this animal-stem, moreover, the lowest forms,

the platodaria (also called _cryptocœla_ and _acæla_) come very

close to the gastræa. But most of the platodes are distinguished from

the rest of the cœlenteria by the formation of a pair of nephridia

(renal canals or water-vessels), thin tubes which, as excretory organs,

remove from the body the unusable products of metabolism, the urine.

Here we have a second organ of nutrition, the gut tube, added to the

first. In the lower platodes this remains very simple. As a rule, a

gullet tube (pharynx) is formed by the hollowing out of the mouth, as

in the corals; and as in the case of the latter branched canals, which

conduct the nutritive sap from the stomach to distant parts of the

body, grow out of the stomach, in the larger coil-worms (_turbellaria_)

and suction-worms (_trematodes_). On the other hand, the gut atrophies

in the tape-worms (_cestodes_); as these parasites live in the

intestines or other organs of animals, they can obtain their nutritive

sap directly from them through the surface of the skin.

The more highly organized cœlomaria differ from the simpler

cœlenteria chiefly by the greater complexity in the structure

and functions of their apparatus of nutrition. As a rule, these

functions are divided between four groups of organs, which are not yet

differentiated in the cœlenteria--namely: 1, organs of digestion

(gastric system); 2, organs of circulation (vascular system); 3, organs

of breathing (respiratory system); and 4, organs of excretion (renal

system). Moreover, in the cœlomaria the gastric canal has usually

two openings, the mouth and the anus. Finally, they all have a special

body-cavity (_cœloma_); this is quite separate from the gastric

canal, which is suspended in it, and serves for the formation of the

sexual cells. It is formed in the embryo by the hollowing out and

cutting off of a pair of sacs (cœlom-pouches) from the gut near the

mouth; the pouches touch, and then coalesce, as their division-walls

break down. If a part of the dividing wall remains, it serves as

mesentery to fasten the gut to the body-wall. The action of the four

groups of alimentary organs remains very simple in the lowest and

oldest cœlomaria, the worms (_vermalia_); but in the other higher

animals, which have been evolved from these, they have very varied and

often complicated features.

In the great majority of the cœlomaria the gastric system forms a

highly differentiated apparatus, composed, as in man, of a number of

different organs. The food is usually taken in by the mouth, ground

up by the jaws or the teeth, and softened with saliva, which the

salivary glands pour into the cavity of the mouth. From the mouth

the pulpy food passes in swallowing into the gullet, which often has

glandular appendages, and from this through the narrow esophagus into

the stomach. This most important part of the alimentary apparatus is

often divided into several sections, one of which (the masticating

stomach) is armed with teeth and prepared for a further triturition

of solid pieces, while the other (the glandular stomach) produces the

dissolving gastric juice. The liquefied food (_chylus_) then passes

into the small intestine (_ileum_), which has to absorb it, and is as a

rule the longest section of the alimentary canal. A number of different

digestive glands open into this intestine, the most important of them

being the liver. The small intestine is often sharply distinguished

from the large intestine (_colon_), the last large section of the

alimentary canal; into this also a number of glands and blind

intestines open. The last portion of it is called the _rectum_, and

this removes the indigestible remnants of the food (_fæces_) through

the anus.

This general plan of the alimentary system, which is common to most

of the cœlomaria in its chief features, is very much modified in

the various groups of these animals and adapted to their several

conditions of nutrition. The simplest structures are found in many of

the vermalia; the lowest forms of these, the rotifers, and especially

the gastrotricha, still closely resemble their platode ancestors, the

turbellaria. The higher type of animal-stems which have been evolved

from them are partly distinguished by special structures. Thus the

mollusks have a characteristic masticating apparatus; on their tongue

there is a hard plate (_radula_) armed with a number of teeth, which

grinds against a hard upper jaw, and so breaks up the food. In most of

the articulates this work is done by side-jaws, which consist of hard

rods and represent modified bones. The vertebrates and the closely

related tunicates are distinguished by the conversion of the first

sections of the alimentary canal into a characteristic respiratory

apparatus (gills). But the construction of the various sections of

the gastro-canal also varies a good deal in the small groups of

the cœlomaria, as it depends to a great extent on the nature of

the food and the conditions in which it is got and prepared. The

largest expenditure of mechanical and chemical energy is needed for

a voluminous solid vegetal diet. Hence the alimentary canal and its

many appendages are longest and most complicated in the plant-eating

snails, leaf-eating insects, and grass-eating ruminants. On the other

hand, they are shortest and simplest in parasitic cœlomaria, which

derive their fluid food already prepared from the contents of another

animal's intestines. In these cases the gut may altogether atrophy; as

in the _acanthocephala_ among the vermalia, the _entoconcha_ among the

mollusks, and the _sacculina_ among the crustacea.

The greater the extent of the body, and the more complex the

organization of the higher animals, the more necessary it is to have

an orderly and regular distribution of the nutritive fluid to all

parts. In the cœlenteria this work is accomplished by the gastric

canals (side branches from the gut, opening into its cavity) but in

the cœlomaria it is done much better by means of blood-vessels

(_vasa sanguifera_). These canals do not communicate directly with the

gastro-canal, but are formed independently of it in the surrounding

parenchyma of the mesoderm. They take up the filtered and chemically

improved food-fluid, which transudes through the intestinal walls, and

conduct it in the form of blood to all parts of the body. This blood

generally contains millions of cells, which are of great importance

in metabolism. The blood-cells of the lower cœlomaria are usually

colorless (leucocytes), while those of the vertebrates are mostly red

(rhodocytes).

The circulation of the blood in most of the cœlomaria is effected

by a heart, a contractile tube, formed by the local thickening of

a skin-vessel, which contracts and beats regularly by means of its

muscular bands. Originally two of these skin-vessels were developed in

the abdominal wall--a dorsal vessel in the upper and ventral vessel

in the lower wall (as in many of the vermalia). The heart is formed

from the dorsal vessel in the mollusks and articulates, but from the

ventral in the tunicates and vertebrates. The arteries are the vessels

which conduct the blood from the heart; those which conduct it from the

body to the heart are the veins. The finest branchlets of both kinds

of vessels, which form the connecting link between them, are called

capillaries; these immediately effect the interchange of matter in the

tissues by osmosis. The blood-vessels co-operate very closely with the

respiratory organs.

The interchange of gases in the organism, which we call breathing or

respiration--the taking in of oxygen and giving out of carbonic-acid

gas--does not require special organs in the lower animals. In these

it is accomplished by epithelial cells, which clothe the surface of

the body--the ectoderm of the outer skin layer and the entoderm of

the inner gut-covering. As nearly all these cœlenteria live in the

water, or (as parasites) in some fluid that contains air, and as these

fluids are constantly pouring in and out of the body, the exchange of

gases is accomplished at the same time. But in the higher animals this

is rarely found, only in the small animals of simple construction (such

as the rotifers and other vermalia, and the smallest specimens of the

mollusca and articulata). The majority of these cœlomaria attain a

considerable size, and so require special organs; these afford a larger

surface for the exchange of gases in the limited space, and accomplish

a very peculiar chemical work as the localized organs of respiration.

They fall into two groups according to the nature of the environment;

gills for breathing in water and lungs for breathing on land. The

latter take the oxygen directly from the atmosphere, and the former

from the water, in which atmosphere air is contained in solution.

The instruments of water-respiration which we call gills (_branchiæ_)

are generally attenuated parts or processes of the outer skin or the

inner gastric skin; hence we distinguish the two chief forms, external

and internal gills. Both are richly provided with blood-vessels which

bring the blood from the body for the purpose of aëration. Cutaneous

or external gills are especially found in the vertebrates, in the form

of threads, combs, leaves, pencils, tufts of feathers, etc., which are

drawn out from the entoderm as local processes of the outer skin, and

afford a wide surface for the interchange of gases between the body

and the water. In the mollusca there are usually a pair of comb-shaped

gills near the heart; in the articulates there are several pairs,

repeated in the different segments of the body. Gastric or internal

gills are peculiar to the vertebrates and the next-related tunicates,

with a small group of the vermalia, the enteropneusta. In these the

fore-gut or head-gut is converted into a gill-organ, the wall of which

is pierced with gill-fissures; the water that has been taken in by the

mouth passes away through the outer openings of these fissures. In the

lower aquatic vertebrates (acrania, cyclostoma, and fishes) the gills

are the sole organs of breathing; in the higher animals, that live in

the air, they fall into disuse, and their place is taken by lungs.

Nevertheless, heredity is so tenacious that we find from three to five

pairs of rudimentary gill-clefts in the embryo right up to man, though

they have long since ceased to have any function. This is one of the

most interesting of the palingenetic facts that prove the descent of

the amniotes (including man) from the fishes.

The group of the aquatic echinoderms has some very peculiar features of

respiration. Their body possesses an extensive water-duct, which takes

in the sea-water and returns it by special openings (skin-pores or

madreporites). The many branches of these water-vessels or ambulacral

vessels fill with water, especially the tiny feelers or feet, which

extend from the skin in thousands; they serve at once for movement,

feeling, and breathing. But many of the echinoderms have also special

gills--the star-fish have small finger-shaped cutaneous gills on

the back, the sea-urchins special leaf-shaped ambulacral gills, the

sea-cucumbers internal gastric gills (tree-shaped branching internal

folds of the rectum).

The organs of air-breathing are called, in general, lungs (_pulmones_).

Like the organs of water-breathing, they are formed sometimes from

the external and sometimes from the internal covering of the body.

Cutaneous or external lungs are found in several groups of the

vertebrates. Among the mollusks the land-dwelling lung-snails have

acquired a lung-sac by change in the work of the gill cavity: among

the articulata the lung-spiders and scorpions have two or more

trachea-lungs; that is to say, cutaneous sacs, in which are enclosed

fanwise a number of trachea-leaves. In the other air-breathing

articulates (tracheata) we find, instead of these simple or branched,

and often bushlike, air-tubes (_tracheæ_), which spread through the

whole body and conduct the air direct to the tissues. They take the

air from without by special air-holes in the skin (_stigmata_ and

_spiracula_). The myriapods and insects generally have numbers of

air-holes; the spiders only one or two, more rarely four, pairs. When

these air-tube animals return to an aquatic life (as happens with the

larvæ of various groups of insects), the outer air-holes close up, and

new thread-shaped or leaf-shaped trachea-gills are formed, which take

the air from the surrounding water by osmosis. The oldest and lowest

tracheata are the primitive air-tube animals, or protracheata, and form

the link between the older annelids and the myriapods. They have a

number of clusters of short air-tubes distributed over the whole skin,

and it is clear that these have been evolved from simple skin-glands by

change of function.

Gastric or internal lungs are only found in the higher animals, to

which we give the name of quadrupeds (or _tetrapoda_), the amphibia and

amniotes, and their fishlike ancestors, the dipneusta. These internal

lungs are sac-shaped folds of the fore-gut, formed originally from the

swimming-bladder (_nectocystis_) of the fishes by change of function.

This air-filled bladder, a sac-shaped appendage of the gullet, merely

serves the purpose of a hydrostatic organ, by varying the specific

weight, in the fishes. When the fish wishes to descend it contracts

the bladder and becomes heavier; it rises to the top by inflating it

again. The lungs were formed by the adaptation of the blood-vessels

in the wall of the swimming-bladder to the interchange of gases. In

the oldest living lung-fishes (_ceratodus_) it is still a simple sac

(_monopneumones_=one-lunged); in the others the simple gullet-cavity

divides early into a pair of sacs (_dipneumones_, two-lunged). The

wind-pipe (_trachea_--not to be confused with the organ of the same

name in the tracheata) is formed by the lengthening of their stalk

and strengthening of it with cartilaginous rings. At the anterior end

of the trachea we find already formed in the amphibia the larynx, the

important organ of voice and speech.

The function of removing unusable matter is not less important to the

organism than breathing. Just as breathing gets rid of the poisonous

carbonic acid, so the kidneys remove fluid and solid excreta in the

shape of urine; these are partly acid (uric acid, hippuric acid, etc.),

partly alkaline (urea, guanine, etc.). In most of the cœlomaria

special organs for removing these would be superfluous, as this is

accomplished (like breathing) by the stream of water that is constantly

passing through the whole body. But with the platodes we begin to

find important excretory organs in the nephridia, a pair of simple

and ramified canals which lie on either side of the gut, and open

outward. These primitive renal canals are transmitted by the platodes

to the vermalia, and by these to the higher stems of the cœlomaria.

In the latter they generally open by special funnels into the inner

body-cavity, which serves as first receptacle for the urine. Their

outer opening sometimes (primarily) goes through the outer skin at the

back (excretory pores), sometimes (secondarily) to the rectum, and

so out through the anus. The oldest articulates, the annelids, have

a pair of nephridia in each segment of the body; each renal canal,

or segmental canal, consists of three sections, an inner funnel

which opens into the body-cavity, a middle glandular section, and an

external bladder that ejects the urine by contraction. The disposition

of the renal system in the internally articulated vertebrates is very

similar to this; but now complicated structures begin to appear, a

pair of compact kidneys (_renes_), which are made up of a number of

branching nephridia. Three generations of kidneys succeed each other,

as phylogenetic stages of evolution--first the primary fore-kidneys

(_protonephros_), in the middle the secondary primitive kidneys

(_mesonephros_), and last the tertiary after-kidneys (_metanephros_).

The latter are only reached in the three highest classes of

vertebrates, reptiles, birds, and mammals. Mollusks also have a couple

of compact kidneys. They are developed from a pair of nephridia, the

funnels of which open internally into the heart-pouch (the remainder of

the reduced body-cavity); at the back they open outward. The crustacea

also have generally a pair of renal canals. On the other hand, the

protracheata (the stem-forms of the air-tube animals) have segmental

nephridia, a pair to each joint inherited from their annelid ancestors.

The rest of the tracheata, the myriapods, spiders, and insects, have,

instead of these, Malpighi tubes, funnel-shaped glands that arise from

the entodermal rectum, sometimes one pair or less, sometimes a number

in a cluster.

While most plants are purely plasmodomous, and most animals

plasmophagous, there are nevertheless in both organic kingdoms a

number of species (especially the lower) whose metabolism has assumed

peculiar forms by their relations to other organisms. To this class

belong especially the saprosites and parasites. By saprosites are

understood those plants and animals which feed entirely or mostly

on the corpses of other animals, or the decomposed matter which is

unfit for the food of higher animals. Among the unicellular protists

many of the bacteria, especially, belong to this class, and also many

fungilla (_phycomycetes_); among the metaphyta the fungi (mycetes),

and among the metazoa the sponges. I have already spoken of the many

peculiarities of metabolism in the ubiquitous bacteria; while many of

them cause putrefaction, they at the same time feed on the parts of

other organisms which have died. The fungi feed for the most part on

the decayed remains of plants and the products of putrefaction which

accumulate on the ground. In this character of scavengers they play the

same important part on land as the sponges do at the bottom of the sea.

But a number of small groups of the higher plants and animals have, as

a secondary habit, turned to saprositism. Among the metaphyta we have

especially the monotropea (to which our native asparagus, _monotropa

hypopitys_, belongs) and many orchids (_neottia_, _corallorhiza_). As

they find their plasm directly in the decayed matter in the woods,

they have lost their chlorophyll and green leaves. Among the metazoa

many of the vermalia, and some of the higher animals, such as the

rain-worm and many tube-dwelling annelids (the mud-eaters, _limicolæ_),

etc., live on putrid matter. The organs which their nearest relatives

use for obtaining, breaking up, and digesting food (eyes, jaws,

teeth, digestive glands) have been entirely or mostly lost by these

saprosites. Many of them form a transitional type to the parasites.

By parasites, in the narrower sense, we understand, in modern biology,

only those organisms which live on others and derive their nourishment

from them. They are numerous in all the chief divisions of the plant

and animal kingdoms, and their modifications are of great interest

in connection with evolution. No other circumstance has so profound

an influence on the organism as adaptation to a parasitic existence.

Moreover, there is no other section in which we can follow, step

by step, the course of the degeneration which is caused, and show

clearly the mechanical nature of the process. Hence the science of

parasites--parasitology--is one of the soundest supports of the theory

of descent, and provides an abundance of the most striking proofs of

the much-contested inheritance of acquired characteristics.

Among the unicellular organisms, the bacteria are the most conspicuous

instances of manifold adaptation to parasitic habits. As we count

these unnucleated protozoa among the oldest and simplest organisms,

and trace them directly by metasitism to the plasmodomous chromacea,

it is very probable that they turned to parasitism very early in the

history of life. Even a part of the monera (in which group we must

place the bacteria on account of their lack of a nucleus) found it

convenient and advantageous to prey on other protists and assimilate

their plasm directly, instead of going through the laborious process

of carbon assimilation themselves in the hereditary fashion. This is

also true of the large class of the sporozoa or fungilla (_gregarinæ_,

_coccidia_, etc.), real nucleated cells, which have adapted themselves

in various ways to parasitic habits. Many of them live in the

rectum, the cœlum, or other organs of the higher animals (the

gregarinæ, especially in the articulates); others in the tissues (for

instance, the sarcosporidia in the muscles of mammals, the coccidia

and myxosporidia in the liver of vertebrates). A good many of them

are "cell-parasites," and live inside the cells of other animals,

which they destroy; such are the hœmosporidia, which destroy the

blood-cells in man, and so cause intermittent fever.

Among the multicellular metaphyta it is particularly the fungi that

have taken to parasitism in various ways. Many of them are, as is

known, the most dangerous enemies of the higher animals and plants. The

various species of fungi cause certain diseases by their poisonous

(chemical) action on the tissues of their host. It is well known how

our most important cultivated plants, the vine, potato, corn, coffee,

etc., are threatened by fungoid diseases; and this is also true of many

of the lower and higher animals. It is probable that the fungi have

been evolved polyphyletically by metasitism from the algæ.

Among the higher metaphyta we find parasitism in many different

families, especially orchids, rhinanthacea (_orobranche_,

_lathraca_), convolvulacea (_cuscuta_), aristolochiacea, loranthacea

(_viscum_, _loranthus_), rafflesiacea, etc. These various kinds of

flowering-plants often grow to resemble each other by convergence

(that is to say, by their common adaptation to parasitic life); they

lose their green leaves, the plasmodomous chlorophyll of which is of

no further use to them. Frequently rudimentary leaves are left on them

in the form of colorless scales. For the purpose of clinging to the

plants they live on, and penetrating into their tissues, they evolve

special clinging apparatus (haustoria, suctorial cups, creepers). Their

stalks and roots are also modified in a characteristic way. The whole

productive force of these parasites is expended on their sexual organs;

_rafflesia_ has the largest flowers there are, more than a yard in

diameter.

Parasitism in the metazoa (in all groups) is even more frequent and

interesting than in the metaphyta. The mollusks and echinoderms

show the least disposition for it, and the platodes, vermalia, and

articulates the most. Even among the gastræada, the common ancestral

group of the metaphyta, we find parasites (kyemaria and gastremaria).

The protection they find inside their hosts is probably the reason

why these oldest of the metazoa have remained unchanged to the

present day. Real parasites are not numerous among the sponges and

cnidaria. But they are very numerous among the platodes. The suctorial

worms (_trematodes_) live partly externally (as ectoparasites)

on other animals and partly inside them (as endoparasites), and

produce serious diseases in them. They have lost the vibratory

coat of their free-living ancestors, the turbellaria, and acquired

clinging apparatus instead. The tape-worms (_cestodes_), which live

entirely in the interior of other animals, and are descended from the

suctorial worms, have lost their gastro-canal; they are nourished by

imbibition through the skin. The same degeneration is found in the

itchworms (_acanthocephala_) among the vermalia, the parasitic snails

(_entoconcha_) among the mollusks, and the root-crabs (_rhizocephala_)

among the crustacea.

The class of crustacea affords the most numerous and most instructive

examples of degeneration through parasitism, because in this class it

is found polyphyletically in very different orders and families, and

because their highly organized body shows every stage of degeneration

together in the different organs. The free-living crustacea generally

move about very rapidly and ingeniously; their numerous bones are

well jointed and excellently adapted for the most varied methods of

locomotion (running, swimming, climbing, digging, etc.); their organs

of sense are highly developed. As these are no longer used when

they take to parasitism, they atrophy and gradually disappear. The

younger crustacea all proceed from the same characteristic form of

the _nauplius_, and swim freely about; later, when they settle down

to parasitic habits, their organs of sense and locomotion atrophy. As

Fritz Müller-Desterro showed in his famous little work, _For Darwin_

(1864), forty years ago, the crustacea afford most luminous proofs of

the theory of descent and selection, and of progressive heredity and

the biogenetic law. These facts are the more important as the crab

undergoes the same degeneration by parasitic habits in a number of

different orders and families.

From parasitism we must entirely distinguish that intimate life-union

of two different organisms which we called symbiosis or mutualism.

Here we have an association of two living things for their mutual

benefit, while the parasite lives entirely at the expense of his

host. Symbiosis is found among the protista, being very wide-spread

among the radiolaria. In the gelatinous envelope (_calymma_) which

encloses the central capsule of their unicellular bodies we find a

number of motionless yellow cells (_zooxanthella_) scattered. These

are protophyta or (as it is said) "unicellular algæ" of the class of

paulotomea (_palmellacea_). They receive protection and a home from the

radiolaria, grow plasmodomously, and multiply by rapid segmentation.

A large part of the starch-flour and the plasm which they form by

carbon-assimilation goes as food directly to the radiolarium-host; the

other part of the xanthella goes on growing and multiplying. Similar

yellow zooxanthella or green zoochlorella are found as symbionta in the

tissues of many animals. Our common fresh-water polyp (_hydra viridis_)

owes its green color to the zoochlorella which live in great numbers

on the ciliated cells of its entoderm (the digestive gut-epithelium).

In general, however symbiosis is rarer in the metazoa than in the

metaphyta. In the latter case it is the fundamental feature of a whole

class of plants, the lichens. Each lichen consists of a plasmodomous

plant (sometimes a protophyte, sometimes an alga) and a plasmophagous

fungus. The latter affords a home, protection, and water to the green

alga, which repays the service by providing food.

XI

REPRODUCTION

Reproduction and generation--Sexual

and asexual reproduction--Superfluous

growth--Monogony--Self-cleavage--Budding--Formation of

spores--Amphigony--Ovum and sperm-cell--Hermaphrodite formation

and separation of the sexes--Hermaphrodism and gonochorism

of the cells--Monoclinism and diclinism--Monœcism and

diœcism--Alternation of sex-division--Sexual glands of

the histona--Hermaphroditic glands--Sexual ducts--Generative

organs--Parthenogenesis--Pædogenesis--Metagenesis--Heterogenesis--

Strophogenesis--Hypogenesis--Hybridism--Generation of hybrids and

the species--Graduation of forms of reproduction.

While nutrition secures the maintenance of the organic individual,

reproduction insures that of the organic species, or the group of

definite forms which we distinguish from others by the name "species."

All individuals are more or less restricted in the duration of their

lives, and die off after the lapse of a certain time. The succession

of individuals, connected by reproduction and belonging to a species,

makes it possible for the specific form itself to last for ages. In

the end, however, the species is temporary; it has no "eternal life."

After existing for a certain period, it either dies or is converted by

modification into other forms.

The rise of new individuals by reproduction from parent organisms

is a natural phenomenon with definite time-restriction. It cannot

have continued from eternity on our planet, as the earth itself is

not eternal, and even long after its formation was incapable of

supporting organic life on its surface. This only became possible when

the surface of the glowing planet had sufficiently cooled for liquid

water to settle on it. Until this stage carbon could not enter into

those combinations with other elements (oxygen, hydrogen, nitrogen, and

sulphur) which led to the formation of plasm. As I intend to deal with

this process of _archigony_, or spontaneous generation, in a special

chapter, I leave it for the present, and confine myself to the study of

_tocogony_, or parental generation.

* * * * *

The various forms of tocogony, or the reproduction of living things,

are generally divided into two large groups; on the one hand there is

the simple form of asexual generation (monogony), and on the other the

complex form of sexual generation (amphigony). In asexual generation

the action of one individual only is needed, this providing a product

of transgressive (redundant) growth which develops into a new organism.

In sexual generation it is necessary for two different individuals to

unite in order to produce a new being from themselves. This amphigony

(or _generatio digenea_) is the sole form of reproduction in man and

most of the higher animals. But in many of the lower animals and most

of the plants we find also asexual multiplication, or monogony, by

cleavage or budding. In the lowest organisms, the monera and many of

the protists, fungi, etc., the latter is the only form of propagation.

Strictly speaking, monogony is a universal life-process; even the

ordinary cell-cleavage, on which depends the growth of the histona, is

a cellular monogony. Hence historical biology must say that monogony

is the older and more primitive form of parental generation, and

that amphigony was secondarily developed from it. It is important to

emphasize this because, not only some of the older writers, but even

some recent ones, regard sexual generation as a universal function of

organisms, and declare that it dates from the very beginning of organic

life.

The complex and frequently very intricate phenomena of sexual

generation, as we find them in the higher organisms, become

intelligible to us when we compare them with the simpler forms of

asexual generation at the lowest stages of life. We then learn that

they are by no means unintelligible and supernatural marvels, but

natural physiological processes, which, like all others, may be traced

to the action of simple physical forces. The form of energy which

lies at the root of all tocogony is _growth_ (_crescentia_). And as

this phenomenon is also the cause, in the form of gravitation, of the

formation of crystals and other inorganic individuals, we do away with

another of the boundaries which people would establish between organic

and inorganic nature. Reproduction is a kind of nutrition and growth of

the organism beyond the individual standard, building up a part of it

into a whole. This _limit_ of individual size is determined for each

species by two factors--the inner constitution of the plasm, which is

inherited, and the dependence on the outer environment, which controls

adaptation. When this limit has been passed, the transgressive growth

takes the form of reproduction. Every species of crystal has also a

definite limit of growth; when this is passed, new crystal-individuals

are formed in the mother-water on the old individual, which grows no

further.

Asexual or monogenetic tocogony (also called "vegetative

multiplication") is always effected by a single organic individual,

and so must be traced to its transgressive growth. When this affects

the entire body as a total growth, the whole dividing into two or

more equal parts, we call the monogenetic process division (or

segmentation). But when the growth is partial, and affects only a

part of the individual, or when this special part separates from

the generating organism in the form of a bud (_gemma_), the process

is called budding or gemmation (_gemmatio_). Hence the essential

difference between the two forms of generation is that in division the

parent disappears in its partial products (children); these are of

the same age and form. But in budding the generating parent retains

its individuality; it is larger and older than the young bud. This

important difference between division and gemmation, which is often

overlooked, holds good both for protists (unicellulars) and histona

(multicellulars). The fact that in division the individual as such

is destroyed is a sufficient refutation of Weismann's theory of the

immortality of the unicellulars. (See above, and also the _Riddle_,

chapter xi.)

Reproduction by division is by far the most common of all forms of

propagation. It is the normal form of monogony, not only in many of the

protists, but also in the tissue-cells which compose the tissues of

the histona. It is, moreover, the sole method of propagation for most

of the monera, both chromacea and bacteria, which are in consequence

often comprised under the title of "cleavage-plants" (_schizophyta_).

Self-cleavage is also found among the higher multicellular

organisms--namely, the cnidaria (polyps, medusæ). It usually takes the

form of division into two parts (_dimidiatio_ or hemitomy), the body

splitting into two equal halves. The plane of division is sometimes

indefinite (fragmentary-cleavage), sometimes coincident with the long

axis (length-cleavage), sometimes with the transverse axis, vertical

to the long axis (transverse-cleavage), and less frequently with a

diagonal axis (oblique-cleavage). When the segmentation of a cell

proceeds so rapidly that the transverse-cleavage follows immediately

on the length-cleavage, and the two are at length made to coincide,

twofold division is changed into fourfold division. And when the

process is repeated in quick succession, and the body falls at last

into a number of small and equal pieces, we have manifold division

(polytomy); as in the spore-formation of the sporozoa and rhizopoda,

and in the embryonic sac of the phanerogams.

Asexual propagation by budding is chiefly distinguished from

segmentation by the fact that the determining transgressive growth is

only partial in the one and total in the other. The bud produced is,

therefore, younger and smaller than the parent from which it issues;

the latter may replace the lost part by regeneration and produce a

number of buds simultaneously or successively without losing its

individuality (whereas this is destroyed in division). Propagation

by budding is rare among the protists, and more common among the

histona--that is, with most of the tissue-plants and the lower,

stock-forming, tissue-animals (cœlenteria and vermalia). Most stocks

(cormi) are formed by a sprout or person shooting out buds which remain

united to it. The layer and shoots of tissue-plants are detached

buds. The two chief kinds of gemmation are terminal and lateral.

Terminal budding takes place at the end of the long axis, and is not

far removed from transverse division (for instance, the strobilation

of the acraspedæ medusæ and the chain tape-worms). Lateral budding is

much more common; it determines the branching of trees and generally of

complex plants, and also of the tree-shaped stocks of sponges, cnidaria

(polyps, corals), bryozoa, etc.

A third form of asexual reproduction is the formation of spores or

"germ-cells," which are usually produced in great numbers inside the

organism, then detached from it, and developed into new organisms

without needing fertilization. The spores are sometimes motionless

(rest-spores or paulospores); sometimes they have one or more lashes

which enable them to swim about (rambling-spores or planospores).

This monogenetic propagation is very common among the protists, both

protophyta and protozoa. Among the latter the sporozoa (gregarinæ,

coccidia, etc.) are remarkable for the passing away of the whole

unicellular organism in the formation of spores; in this case and

in many of the rhizopods (_mycetozoa_) the process coincides with

manifold cell-division. In other cases (radiolaria, thalamophora)

only a portion of the parental cells is used for the production of

spores. Spore-formation is very common among the cryptogams; here it

usually alternates with sexual propagation. The spores are generally

formed in special spore-capsules (sporangia). In the flowering plants

(anthophyta) sporogony has disappeared. It is found at times in the

tissue-animals (in the fresh-water sponges); in this case the sporangia

are called _gemmulæ_.

The essential feature of sexual generation is the coalescence of two

different cells, a female ovum (egg-cell) and a male sperm-cell.

The simple new cell which arises from the blending of these is the

stem-cell (_cytula_), the stem-mother of all the cells that make up

the tissues of the histon. But even among the unicellular protists

we find in many places the beginnings of sexual differentiation; it

is foreshadowed in the blending or copulation of two homogeneous

cells, the gameta. We may conceive this process, or zygosis, as a

peculiar and very favorable kind of growth, that is connected with a

rejuvenescence of the plasm; the latter is enabled to propagate by

repeated cleavage through the mixing of the two different plasma-bodies

on either side (_amphimixis_). When these two gameta become unequal

and differ in size and shape, the larger female body is called the

macrogameton or macrogonidion, and the smaller, male part, the

microgameton or microgonidion. Among the histona the first is called

the egg-cell (_ovulum_), and the latter the sperm-cell (_spermium_, or

_spermatozoon_). As a rule the latter is a very mobile ciliated cell,

the former an inert or amœboid cell. The vibratory movements of the

sperm-cells serve for approaching the ovulum in order to fertilize it.

The qualitative difference between the two copulating sexual cells

(_gonocyta_), or the chemical difference between the ovoplasm of the

female and the sperm-plasm of the male cell, is the first (and often

the only) condition of amphigony; subsequently we find in addition (in

the higher histona) a very elaborate apparatus of secondary structures.

With this chemical difference is associated a peculiar double form of

sensitive perception and an attraction based thereon, which is called

sexual chemotaxis or erotic chemotropism. This "sex-sense" of the two

gonocyta, or elective affinity of the male androplasm and the female

gynoplasm, is the cause of mutual attraction and union. It is very

probable that this sexual sense-function, akin to smell or taste, and

the movements it stimulates, are located in the cytoplasm of the two

sex-cells, while heredity is the function of the caryoplasm of the

nucleus. (_Cf._ the _Anthropogeny_, chapters vi. and vii.)

The sexual difference between the two forms of gonoplasm, the ovoplasm

of the female and spermoplasm of the male cell, is noticeable at the

very beginning of sexual differentiation in the different sizes of

the copulating gameta, and later in their increasing divergence as to

shape, composition, movement, etc. It leads further to the distribution

of the germinal regions (in which the sex-cells are formed) into two

different individuals. When the ovum and the sperm-cell are produced in

one and the same individual, we call this an hermaphrodite; and when

they are formed in two different individuals (male and female), we

call them monosexual, or gonochorists. In accordance with the various

stages of individuality which we distinguished above (chapter vii.), we

may indicate the following stages of hermaphrodism and gonochorism.

Some groups of protists, especially the highly organized ciliated

infusoria (_ciliata_), are distinguished by having a separation of

male and female plasm within the unicellular organism. The ciliata

propagate, as a rule, in large numbers by repeated division (by

indirect cell-cleavage). But this monogony has its limits, and has to

be interrupted from time to time by amphigony, a rejuvenation of the

plasm, which is effected by the conjugation of two different cells and

the partial destruction of their nuclear matter. By conjugation is

meant the partial and momentary union of two different unicellulars,

while copulation is a total and permanent coalescence. When two

ciliated infusoria conjugate they place themselves side by side, and

connect for a time by means of a bridge of plasm. A part of the nucleus

of each has already divided into two portions, one of which functions

as the female standing-nucleus (_paulocaryon_) and the other as the

male travelling-nucleus (_planocaryon_). The two mobile nuclei enter

the plasm-bridge, and move through it, pushing against each other,

into the body of the opposite cell; they then coalesce with the deeper

lying standing-nucleus. When a fresh nucleus has been thus formed (by

_amphimixis_) in each of the copulating cells, they again separate. The

two rejuvenated cells have once more acquired the power to propagate

for a long time by division.

This peculiar hermaphroditic formation of the cells, which

distinguishes the ciliated infusoria and some other protists, and

which we now know in its smallest details through the investigations

of Richard Hertwig, Maupas, and others, is especially interesting

because it proves that the chemical difference between the female

gynoplasm and the male androplasm can be found within a single cell.

This erotic division of labor is so important that formerly it was

universally ascribed to two different cells. Recent accurate research,

penetrating into the smallest visible processes of fertilization, has

shown that the essential feature in the formation of a fresh individual

(the stem-cell) is the blending of equal portions (hereditary parts)

of the male and female nuclei; the caryoplasm of the two copulating

cells is the vehicle of heredity from the parents. The cytoplasm of the

cell-body, on the other hand, serves the purposes of adaptation and

nutrition. As a rule the cell-body of the ovulum is very large, and

is, as a food-store, very richly provided with albumin, fat, and other

nutritive matter (food-yolk); while the cytoplasm of the sperm-cell is

very small, and generally forms a vibrating lash, with which it moves

along and seeks the ovum.

In most of the plants the female and male cells are produced by the

same sprout, and in many of the lower animals by one and the same

person. This kind of hermaphrodism in "individuals of the second order"

is called monoclinism ("one-beddedness"). In many of the higher plants

(monœcic stocks) and most of the higher animals we have diclinism

("two-beddedness")--in other words, the one sprout or person has

only male, and the other sprout or person only female, organs--this

is gonochorism of individuals of the second order. Monoclinism is

generally associated with sedentary life (and often necessary for it),

and diclinism with free movement. Adaptation to parasitic habits also

favors monoclinism; thus, the crabs, for instance, are for the most

part gonochoristic individuals, but the creeping crabs (_cirripedia_),

which have adopted sedentary (and to an extent parasitic) habits, have

become hermaphrodites in consequence. Many intestinal parasites among

the lower animals (such as tape-worms, suctorial worms, wonder-snails),

which live isolated lives inside other animals, have to be

hermaphroditic and able to fertilise themselves if the species is to be

maintained. On the other hand, many hermaphroditic flowers, although

they have both sorts of sex-organs, are incapable of fertilizing

themselves and have to receive this from insect visitors which carry

the pollen from one flower to another.

Individuals of the third order, which we call stocks (_cormi_) in both

the plant and animal worlds, also exhibit varying features in the

sex-persons which compose them. When male and female diclinic sprouts

or persons are found side by side on the same stock, we call this

hermaphrodism of the cormi _monœcia_ ("one-housedness"); this is the

case with most of the siphonophora and some of the corals. _Diœcia_

("two-housedness") is less common: in this one stock has only male and

the other only female sprouts or persons, as in poplars and osiers,

most of the corals, and some of the siphonophora. The physiological

advantages of crossing--the union of sex-cells of different

individuals--favor progressive sex-division in the higher organisms.

A comparative study of the features of hermaphrodism and sex-division

in the plant and animal worlds teaches us that both forms of

sex-activity are often found in closely related organisms of one and

the same group, sometimes even in different individuals of the same

species. Thus, for instance, the oyster is usually gonochoristic, but

sometimes hermaphroditic; and so with many other mollusks, vermalia,

and articulata. Hence, the question often raised, which of the two

forms of sex-division is original, is hardly susceptible of a general

answer, or without relation to the stage of individuality and the place

in classification of the group under discussion. It is certain that in

many cases hermaphrodism represents the original feature; for instance,

in most of the lower plants and many of the stationary animals

(sponges, polyps, platodes, tunicates, etc.). Where we find exceptions

in these groups, they are of secondary origin. It is equally certain,

on the other hand, that in other cases the separation of the sexes is

the primitive arrangement; as in siphonophoræ, ctenophoræ, bryozoa,

cirripedia, and mollusks. In these cases the hermaphrodism is clearly

secondary in the sense that the hermaphrodites descend originally from

gonochorists.

It is only in a few sections of the lowest histona that the two kinds

of sex-cells arise without a definite location in different parts of

the simple tissue, as in a few groups of the lower algæ and in the

sponges. As a rule they are formed only at definite positions and in a

special layer of the tissue-body, and mostly in groups, in the shape

of sexual glands (_gonades_). These bear special names in different

groups of the histona. The female glands are called archegonia in

the cryptogams, _nucellus_ (formed from the macrosporangia of the

pteridophyta) in the phanerogams, and ovaries in the metazoa. The male

glands are called antheridia in the cryptogams, pollen-sacs (formed

from the microsporangia of the ferns) in the phanerogams, and testicles

(_as spermaria_) in the metazoa. In many cases, especially in aquatic

lower animals, the ovula (as products of the ovaries) are discharged

directly outward. But, in most of the higher organisms, special sexual

ducts (_gonoductus_) have been formed to conduct both kinds of the

gonocyta out of the organism.

While the two kinds of sexual glands are usually located in different

parts of the generating organism, there are, nevertheless, a few cases

in which the sex-cells are formed directly and together from one and

the same gland. These glands are called hermaphroditic glands. Such

structures are very notable in several highly differentiated groups

of the metazoa, and have clearly been developed from gonochoristic

structures in lower forms. The class of crested medusæ, or ribbed

medusæ (ctenophoræ), contains glasslike, sea-dwelling cnidaria of a

peculiar and complicated build, which probably descend from hydromedusæ

(or craspedota). But whereas the latter have very simple gonochoristic

structures (four or eight monosexual glands in the course of the

radial canals or in the gastric wall), in the ctenophoræ the eight

hermaphroditic canals run in a meridian arch from one pole of the

cucumber-shaped body to the other. Each canal corresponds to a ciliary

streamer, and forms ovaries at one border and testicles at the other;

and these are so arranged that the eight intercostal fields (the spaces

between the eight streamers) are alternately male and female. Still

more curious are the hermaphroditic glands of the highly organized,

land-dwelling, and air-breathing lung-snails (_pulmonata_), to which

our common garden snail (_arion_) and vineyard snail (_helix_) belong.

Here we have a hermaphroditic gland with a number of tubes, each of

which forms ovaries in its outer part and sperma in the inner. Still

the two kinds of sex-cells lead separately outward.

In most of the lower and aquatic histona both kinds of sex-cells,

when they are ripe, fall directly into the water, and come together

there. But in most of the higher, and especially the terrestrial,

organisms special exits or conducting canals have been formed for the

sex-products, the sexual ducts (_gonoductus_); in the metazoa the

female have the general name of oviducts and the male spermaducts (or

_vasa deferentia_). In the viviparous histona special canals serve

for the conveyance of the sperm to the ovum, which remains inside the

mother's body; such are the neck of the archegonium in the cryptogams,

the pistil in the phanerogams, and the vagina in the metazoa. At the

outer opening of these conducting canals special copulative organs are

developed, as a rule.

When the ejected sex-cells do not directly encounter each other (as in

many aquatic organisms), special structures have to be formed to convey

the fertilizing sperm from the male to the female body. This process of

copulation becomes important, as it is associated with characteristic

feelings of pleasure, which may cause extreme psychic excitement; as

sexual love it becomes, in man and the higher animals, one of the most

powerful springs of vital activity. In many of the higher animals

(namely, vertebrates, articulates, and mollusks) there are also formed

a number of glands and other auxiliary organs which co-operate in the

copulation.

The manifold and intimate relations which exist, in man and the higher

animals (especially vertebrates and articulates), between their sexual

life and their higher psychic activity, have given rise to plenty of

"wonders of life." Wilhelm Bölsche has so ably described them in his

famous and popular work, _The Life of Love in Nature_, that I need only

refer the reader to it. I will only mention the great significance of

what are called "secondary sexual characters." These characteristics

of one sex that are wanting in the other, and that are not directly

connected with the sexual organs--such as the man's beard, the woman's

breasts, the lion's mane, or the goat's horns--have also an æsthetic

interest; they have, as Darwin showed, been acquired by sexual

selection, as weapons of the male in the struggle for the female,

and vice versa. The feeling of beauty plays a great part in this,

especially in birds and insects; the beautiful colors and forms which

we admire in the male bird of paradise, the humming-bird, the pheasant,

the butterfly, etc., have been formed by sexual selection (_cf._ the

_History of Creation_).

In various groups of the histona the male sex has become superfluous

in the course of time; the ovula develop without the need of

fertilization. That is particularly the case in many of the platodes

(trematodes) and articulates (crustacea and insects). In the bees we

have the remarkable feature that it is only decided at the moment

of laying the egg whether it is to be fertilized or not; in the one

event a female and in the other a male bee is formed from it. When

Siebold proved at Munich these facts of miraculous conception in

various insects, he was visited by the Catholic archbishop of the

city, who expressed his gratification that there was now a scientific

explanation possible of the conception of the Virgin Mary. Siebold

had, unfortunately, to point out to him that the inference from the

parthenogenesis of the articulate to that of the vertebrate was not

valid, and that all mammals, like all other vertebrates, reproduce

exclusively from impregnated ova. We also find parthenogenesis

among the metaphyta, as in the _chara crinita_ among the algæ, the

_antennaria alpina_ and the _alchemilla vulgaris_ among the flowering

plants. We are, as yet, ignorant for the most part of the causes

of this lapse of fertilization. Some light has been thrown on it,

however, by recent chemical experiments (the effect of sugar and

other water-absorbing solutions), in which we have succeeded in

parthenogenetically developing unfertilized ova.

In the higher animals the complete maturity and development of

the specific form are requisite for reproduction, but in many of

the lower animals it has been observed recently that ovula and

sperm-cells are even formed by the younger specimens in the larva

stage. If impregnation takes place under these conditions, larvæ of

the same form are born. And when these larvæ have afterwards reached

maturity and reproduced in this form, we call the process _dissogony_

("double-generation"). It is found in many of the cnidaria, especially

the medusæ. But if larvæ propagate by unfertilized ova, and so

reproduce their kind parthenogenetically, the process is known as

_pædogenesis_ ("young-generation"). It is found particularly in the

platodes (trematodes) and some of the insects (larvæ of _cecidomyca_

and other flies).

In a large number of lower animals and plants sexual and asexual

generation regularly alternate. Among the protists we find this

alternation of generation in the sporozoa; among the metaphyta in the

mosses and ferns; and among the metazoa in the cnidaria, platodes,

tunicates, etc. Often the two generations differ considerably in shape

and degree of organization. Thus, in the mosses the asexual generation

is the spore-forming moss capsule (_sporogonium_), while the sexual is

the moss plant with stalk and leaves (_culmus_). In the case of the

ferns, on the other hand, the latter is spore-forming and monogenetic,

while the thallus-formed, simple, and small fore-germ (_prothallium_)

is sexually differentiated. In most of the cnidaria a small stationary

polyp is developed out of the ovum of the free-swimming medusa, and

this polyp, in turn, generates by budding medusæ, which reach sexual

maturity. In the tunicates (salpa) a sexual social form alternates with

an asexual solitary form; the chain-salpa of the former are smaller and

differently shaped than the large individual salpa of the latter, which

again generate chains by budding. This special form of metagenesis was

the first to be observed, as it was in 1819 by the poet Chamisso, when

he sailed round the world. In other cases (for instance, in the closely

related _doliolum_) a sexual generation alternates with two (or more)

neutral ones. The explanation of these various forms of alternating

generations is given in the laws of latent heredity (atavism), division

of labor, and metamorphosis, and especially by the biogenetic law.

While in real metagenesis (alternation of generations in the strict

sense) the asexual generation propagates by budding or spore-formation,

this is done parthenogenetically in the cognate process of

heterogenesis. This it is which, especially in many of the articulates,

causes an immense increase of the species in a short time. Among the

insects we have the leaf-lice (aphides), and among the crustacea the

water-fleas (daphnides), that propagate in great numbers during warm

weather by unfertilized "summer-ova." It is not until the autumn that

males appear and fertilize the large "winter-ova"; in the following

spring the first parthenogenetic generation issues from the winter

eggs. The two heterogenetic generations are very different in the

parasitic suctorial worms (trematodes). From the fertilized ovum of the

hermaphrodite distoma we get simply constructed nurses (pædogenetic

larvæ), inside which cercaria are generated from unfertilized ova;

these travel, and are afterwards converted (inside another animal) into

distoma once more.

I have given (_General Morphology_, chap, ii., p. 104) the name of

strophogenesis to the complicated process of cell-reproduction,

which we find in the ontogeny of most of the higher histona, both

phanerogams and cœlomaria. In these there is not a real alternation

of generations, as the multicellular tissue-forming organism develops

directly from the impregnated ovum. But the process resembles

metagenesis in so far as the ontogenetic construction consists itself

in a repeated division of the cells. Many generations of cells proceed

by cleavage from the one stem-cell (the impregnated ovum) before two

of these cells become sexually differentiated, and form a generation

of sexual cells. However, the essential difference consists in the

fact that all these generations of cells--in the body of both the

higher animals and the flowering plants--remain joined together as

parts of a single bion (a unified physiological individual); but in

the alternation of generations each group produced is made up of a

number of bionta, which live as independent forms--often so different

from each other that they were formerly thought to be animals of

separate classes, such as the polyps and medusæ. Hence we must not

describe the reproductive circle of the phanerogams as an alternation

of generations, although it has started from the fern (by abbreviated

heredity).

All simple forms of sexual reproduction without alternation of

generations are comprised under the title of _hypogenesis_. The

generative cycle proceeds from ovum to ovum in one and the same

bion or physiological individual. This form of development is usual

with most of the higher animals and plants; it may proceed with or

without metamorphosis. The younger forms which arise temporarily in

the latter case, and are distinguished from the sexually ripe form

by the possession of the provisional (and subsequently disappearing)

organs--larva organs (for instance, the tadpole or the pupa), are

comprised under the general head of larvæ.

As a rule, only organisms of the same species seem to have sexual

union and generate fertile progeny. This was formerly a rigid dogma,

and served the purpose of defining the loose idea of the species.

It was said: "When two animals or plants can have fertile offspring

they belong to the same real species." This principle, which once

afforded support to the dogma of the constancy of species, has long

been discarded. We now know by numbers of sound experiments that not

only two closely related species, but even two species of different

genera, may have sexual intercourse in certain circumstances, and that

the hybrids thus generated can have fertile offspring, either by union

among themselves or with one of the parents. However, the disposition

to hybridism varies considerably, and depends on the unknown laws of

sexual affinity. This sexual affinity must be based on the chemical

properties of the plasm of the copulating cells, but it seems to show

a good deal of vagueness in its effect. As a rule, hybrids exhibit a

combination of the features of both parents.

It has been proved by many recent experiments that hybrids have a more

powerful build and can reproduce more strongly than pure offspring,

whereas pure selection has generally in time an injurious effect. A

freshening by the introduction of new blood seems to be good from time

to time. Hence, it is just the reverse of what the former dogma of the

constancy of species affirmed. The question of hybridism has, generally

speaking, no value in defining the species. Probably many so-called

"true species," which have relatively constant features, are really

only permanent hybrids. This applies especially to lower sea-dwelling

animals, the sexual products of which are poured into the water and

swarm together in millions. As we know of various species of fishes,

crabs, sea-urchins, and vermalia, that their hybrids are very easily

produced and maintained by artificial impregnation, there is nothing to

prevent us from believing that such hybrids are also maintained in the

natural state.

The short survey we have made of the manifold varieties of reproduction

is sufficient to give an idea of the extraordinary wealth of this world

of wonders. When we go more closely into details we find hundreds of

other remarkable variations of the process on which the maintenance

of the species depends. But the most important point of all is the

fact that all the different forms of tocogony may be regarded as

connected links of a chain. The steps of this long ladder extend

uninterruptedly from the simple cell-division of the protists to the

monogony of the histona, and from this to the complicated amphigony

of the higher organisms. In the simplest case, the cell-cleavage of

the monera, propagation (by simple transverse division) is clearly

nothing more than transgressive growth. But even the preliminary

stage of sexual differentiation, the copulation of two equal cells

(_gameta_), is really nothing but a special form of growth. Then, when

the two gameta become unequal in the division of labor, when the larger

inert macrogameton stores up food in itself, and the smaller, mobile

microgameton swims in search of it, we have already expressed the

difference between the female ovum and the male sperm-cell. And in this

we have the most essential feature of sexual reproduction.

The reproduction of the organism is often regarded as a perfect mystery

of life, and as the vital function which most strikingly separates

the living from the lifeless. The error of this dualistic notion is

clear the moment one impartially considers the whole gradation of

forms of reproduction, from the simplest cell-division to the most

elaborate form of sexual generation, in phylogenetic connection. It is

obvious all through that transgressive growth is the starting-point

in the formation of new individuals. But the same must be said of

the multiplication of inorganic bodies--the cosmic bodies on the

larger scale, crystals on the smaller scale. When a rotating sun

passes a certain limit of growth by the constant accession of falling

meteorites, nebulous rings are detached at its equator by centrifugal

force, and form into new planets. Every inorganic crystal, too, has

a certain limit of individual growth (determined by its chemical and

molecular constitution). However much mother-water you add, this

is never passed, but new crystals (daughter-crystals) form on the

mother-crystal. In other words, growing crystals propagate.

XII

MOVEMENT

Mechanics as the science of motion (kinematics and

phoronomism)--Chemistry of vital movement--Active and passive

movements--Undulatory movement--Mechanism of imbibition--Autonomous

and reflex movements--Will and willing--Mixed movements--Movements

of growth--Direction of the vital movement--Direction of the

crystallizing force--Direction of cosmic motion--Movements of

protists--Amœboid, myophenous, hydrostatic, secretory, vibratory

movements: cilia and lashes--Movements of histona, metaphyta, and

metazoa--Locomotion of tissue animals: ciliary motion and muscular

movements--Muscles of the skin--Active and passive organs of

movement--Radiata, articulata, vertebrata, mammalia--Human movements.