Thus the electrostatic effects would be due to the elasticity of the

ether and the electro-dynamical phenomena to the living force. Now,

ought this elasticity itself to be explained, as Lord Kelvin thinks, by

rotations of small parts of the fluid? Different reasons may render

this hypothesis attractive; but it plays no essential part in the

theory of Maxwell, which is quite independent of it.

In the same way, I have made comparisons with divers mechanisms. But

they are only similes, and pretty rough ones. A complete mechanical

explanation of electrical phenomena is not to be sought in the volumes

of Maxwell, but only a statement of the conditions which any such

explanation has to satisfy. Precisely what will confer long life on the

work of Maxwell is its being unentangled with any special mechanical

hypothesis.

FOOTNOTES:

[Footnote 36: Translated from a paper by M. Henri Poincaré.]

XXXIV

AUGUST WEISMANN

1834-1914

_August Weismann was born at Frankfort-on-Main, January 17, 1834,

and studied medicine at Göttingen, 1852-1856. He was physician to the

Austrian Archduke for two years (1860-62), but was compelled to retire

because of his poor eyesight. He was called to the chair of zoology

at Freiburg University. After a close study of Darwin’s theory, he

published in 1876 his “Studies in the Theories of Descent,” a book

which at once attracted much attention among scientists, for it

proposed the theory of the germ-plasm as the basis of heredity, and

denied the theory of the transmissibility of acquired characteristics.

He died at Freiburg-in-Baden, November 6, 1914._

THE CONTINUITY OF THE GERM-PLASM AS THE FOUNDATION OF A THEORY OF

HEREDITY[37]

INTRODUCTION

When we see that, in the higher organisms, the smallest structural

details, and the most minute peculiarities of bodily and mental

disposition, are transmitted from one generation to another; when we

find in all species of plants and animals a thousand characteristic

peculiarities of structure continued unchanged through long series of

generations; when we even see them in many cases unchanged throughout

whole geological periods; we very naturally ask for the causes of

such a striking phenomenon: and inquire how it is that such facts

become possible, how it is that the individual is able to transmit its

structural features to its offspring with such precision. And the

immediate answer to such a question must be given in the following

terms:--“A single cell out of the millions of diversely differentiated

cells which compose the body, becomes specialized as a sexual cell; it

is thrown off from the organism and is capable of reproducing all the

peculiarities of the parent body, in the new individual which springs

from it by cell-division and the complex process of differentiation.”

Then the more precise question follows: “How is it that such a single

cell can reproduce the _tout ensemble_ of the parent with all the

faithfulness of a portrait?”

The answer is extremely difficult; and no one of the many attempts

to solve the problem can be looked upon as satisfactory; no one of

them can be regarded as even the beginning of a solution or as a

secure foundation from which a complete solution may be expected in

the future. Neither Häeckel’s “Perigenesis of the Plastidule,” nor

Darwin’s “Pangenesis,” can be regarded as such a beginning. The former

hypothesis does not really treat of that part of the problem which

is here placed in the foreground, viz., the explanation of the fact

that the tendencies of heredity are present in single cells, but it

is rather concerned with the question as to the manner in which it

is possible to conceive the transmission of a certain tendency of

development into the sexual cell, and ultimately into the organism

arising from it. The same may be said of the hypothesis of His, who,

like Häeckel regards heredity as the transmission of certain kinds of

motion. On the other hand, it must be conceded that Darwin’s hypothesis

goes to the very root of the question, but he is content to give, as

it were, a provisional or purely formal solution, which, as he himself

says, does not claim to afford insight into the real phenomena, but

only to give us the opportunity of looking at all the facts of heredity

from a common standpoint. It has achieved this end, and I believe it

has unconsciously done more, in that the thoroughly logical application

of its principles has shown that the real causes of heredity cannot

lie in the formation of gemmules or in any allied phenomena. The

improbabilities to which any such theory would lead are so great that

we can affirm with certainty that its details cannot accord with

existing facts. Furthermore, Brooks’ well-considered and brilliant

attempt to modify the theory of Pangenesis cannot escape the reproach

that it is based upon possibilities, which one might certainly describe

as improbabilities. But although I am of the opinion that the whole

foundation of the theory of Pangenesis, however it may be modified,

must be abandoned, I think, nevertheless, its author deserves great

credit, and that its production has been one of those indirect roads

along which science has been compelled to travel in order to arrive

at the truth. Pangenesis is a modern revival of the oldest theory of

heredity, that of Democritus, according to which the sperm is secreted

from all parts of the body of both sexes during copulation, and is

animated by a bodily force; according to this theory also, the sperm

from each part of the body reproduces the same part.

If, according to the received physiological and morphological ideas

of the day, it is impossible to imagine that gemmules produced by

each cell of the organism are at all times to be found in all parts

of the body, and furthermore that these gemmules are collected in the

sexual cells, which are then able to reproduce again in a certain

order each separate cell of the organism, so that each sexual cell is

capable of developing into the likeness of the parent body; if all

this is inconceivable, we must inquire for some other way in which we

can arrive at a foundation for the true understanding of heredity. My

present task is not to deal with the whole question of heredity, but

only with the single although fundamental question--“How is it that a

single cell of the body can contain within itself all the hereditary

tendencies of the whole organism?” I am here leaving out of account

the further question as to the forces and the mechanism by which these

tendencies are developed in the building-up of the organism. On this

account I abstain from considering at present the views of Nägeli, for

as will be shown later on, they only slightly touch this fundamental

question, although they may certainly claim to be of the highest

importance with respect to the further question alluded to above.

Now if it is impossible for the germ-cell to be, as it were, an extract

of the whole body, and for all the cells of the organism to dispatch

small particles to the germ-cells, from which the latter derive their

power of heredity; then there remain, as it seems to me, only two other

possible, physiologically conceivable, theories as to the origin of

germ-cells, manifesting such powers as we know they possess. Either

the substance of the parent germ-cell is capable of undergoing a

series of changes which, after the building-up of a new individual

leads back again to identical germ-cells; or the germ-cells are not

derived at all, as far as their essential and characteristic substance

is concerned, from the body of the individual, but they are derived

directly from the parent germ-cell.

I believe that the latter view is the true one: I have expounded it

for a number of years, and have attempted to defend it, and to work

out its further details in various publications. I propose to call it

the theory of “The Continuity of the Germ-plasm,” for it is founded

upon the idea that heredity is brought about by the transference from

one generation to another of a substance with a definite chemical,

and above all, molecular constitution. I have called this substance

“germ-plasm,” and have assumed that it possesses a highly complex

structure, conferring upon it the power of developing into a complex

organism. I have attempted to explain heredity by supposing that in

each ontogeny a part of the specific germ-plasm contained in the

parent egg-cell is not used up in the construction of the body of

the offspring, but is reserved unchanged for the formation of the

germ-cells of the following generation.

It is clear that this view of the origin of germ-cells explains the

phenomena of heredity very simply, inasmuch as heredity becomes thus

a question of growth and of assimilation,--the most fundamental of

all vital phenomena. If the germ-cells of successive generations are

directly continuous, and thus only form, as it were, different parts

of the same substance, it follows that these cells must, or at any

rate may, possess the same molecular constitution, and that they

would therefore pass through exactly the same stages under certain

conditions of development, and would form the same final product. The

hypothesis of the continuity of the germ-plasm gives an identical

starting point to each successive generation, and thus explains how it

is that an identical product arises from all of them. In other words,

the hypothesis explains heredity as part of the underlying problems

of assimilation and of the causes which act directly during ontogeny;

it therefore builds a foundation from which the explanation of these

phenomena can be attempted.

It is true that this theory also meets with difficulties, for it seems

to be unable to do justice to a certain class of phenomena, viz.,

the transmission of so-called acquired characters. I therefore gave

immediate and special attention to this point in my first publication

on heredity, and I believe that I have shown that the hypothesis of

the transmission of acquired characters--up to that time generally

accepted--is, to say the least, very far from being proved, and

that entire classes of facts which have been interpreted under this

hypothesis may be quite as well interpreted otherwise, while in many

cases they must be explained differently. I have shown that there is

no ascertained fact which, at least up to the present time, remains

in irrevocable conflict with the hypothesis of the continuity of

the germ-plasm; and I do not know any reason why I should modify

this opinion to-day, for I have not heard of any objection which

appears to be feasible. E. Roth has objected that in pathology we

everywhere meet with the fact that acquired local disease may be

transmitted to the offspring as a predisposition; but all such cases

are exposed to the serious criticism that the very point that first

needs to be placed on a secure footing is incapable of proof, viz.,

the hypothesis that the causes which in each particular case led to

the predisposition were really acquired. It is not my intention, on

the present occasion, to enter fully into the question of acquired

characters; I hope to be able to consider the subject in greater detail

at a future date. But in the meantime I should wish to point out that

we ought, above all, to be clear as to what we really mean by the

expression “acquired character.” An organism cannot acquire anything

unless it already possesses the predisposition to acquire it: acquired

characters are therefore no more than local or sometimes general

variations which arise under the stimulus provided by certain external

influences. If by the long-continued handling of a rifle, the so-called

“_Exercierknochen_” (a bony growth caused by the pressure of

the weapon in drilling) is developed, such a result depends upon

the fact that the bone in question, like every other bone, contains

within itself a predisposition to react upon certain mechanical

stimuli, by growth in a certain direction and to a certain extent. The

predisposition towards an “_Exercierknochen_” is therefore already

present, or else the growth could not be formed; and the same reasoning

applies to all other “acquired characters.”

Nothing can arise in an organism unless the predisposition to it is

pre-existent, for every acquired character is simply the reaction

of the organism upon a certain stimulus. Hence I should never have

thought of asserting that predispositions cannot be transmitted, as

E. Roth appears to believe. For instance, I freely admit that the

predisposition to an “_Exercierknochen_” varies, and that a

strongly marked predisposition may be transmitted from father to son,

in the form of bony tissue with a more susceptible constitution. But

I should deny that the son could develop an “_Exercierknochen_”

without having drilled, or that, after having drilled, he could

develop it more easily than his father, on account of the drilling

through which the latter first acquired it. I believe that this is as

impossible as that the leaf of an oak should produce a gall without

having been pierced by a gall-producing insect, as a result of the

thousands of antecedent generations of oaks which have been pierced by

such insects, and have thus “acquired” the power of producing galls. I

am also far from asserting that the germ-plasm--which, as I hold, is

transmitted as the basis of heredity from one generation to another--is

absolutely unchangeable or totally uninfluenced by forces residing in

the organism within which it is transformed into germ-cells. I am also

compelled to admit that it is conceivable that organisms may exert a

modifying influence upon their germ-cells, and even that such a process

is to a certain extent inevitable. The nutrition and growth of the

individual must exercise some influence upon its germ-cells; but in the

first place this influence must be extremely slight, and in the second

place it cannot act in the manner in which it is usually assumed that

it takes place. A change of growth at the periphery of an organism,

as in the case of an “_Exercierknochen_,” can never cause such a

change in the molecular structure of the germ-plasm as would augment

the predisposition to an “_Exercierknochen_,” so that the son

would inherit an increased susceptibility of the bony tissue or even of

the particular bone in question. But any change produced will result

from the reaction of the germ-cell upon changes of nutrition caused by

alteration in growth at the periphery, leading to some change in the

size, number, or arrangement of its molecular units. In the present

state of our knowledge there is reason for doubting whether such

reaction can occur at all; but, if it can take place, at all events

the quality of the change in the germ-plasm can have nothing to do

with the quality of the acquired character, but only with the way in

which the general nutrition is influenced by the latter. In the case of

the “_Exercierknochen_” there would be practically no change in

the general nutrition, but if such a bony growth could reach the size

of a carcinoma, it is conceivable that a disturbance of the general

nutrition of the body might ensue. Certain experiments on plants--on

which Nägeli showed that they can be submitted to strongly varied

conditions of nutrition for several generations, without the production

of any visible hereditary change--show that the influence of nutrition

upon the germ-cells must be very slight, and that it may possibly leave

the molecular structure of the germ-plasm altogether untouched. This

conclusion is also supported by comparing the uncertainty of these

results with the remarkable precision with which heredity acts in the

case of those characters which are known to be transmitted. In fact,

up to the present time, it has never been proved that any changes in

general nutrition can modify the molecular structure of the germ-plasm,

and far less has it been rendered by any means probable that the

germ-cells can be affected by acquired changes which have no influence

on general nutrition. If we consider that each so-called predisposition

(that is, a power of reacting upon a certain stimulus in a certain way,

possessed by any organism or by one of its parts) must be innate, and

further that each acquired character is only the predisposed reaction

of some part of an organism upon some external influence; then we must

admit that only one of the causes which produce any acquired character

can be transmitted, the one which was present before the character

itself appeared, viz., the predisposition; and we must further

admit that the latter arises from the germ, and that it is quite

immaterial to the following generation whether such predisposition

comes into operation or not. The continuity of the germ-plasm is amply

sufficient to account for such a phenomenon, and I do not believe that

any objection to my hypothesis, founded upon the actually observed

phenomena of heredity, will be found to hold. If it be accepted, many

facts will appear in a light different from that which has been cast

upon them by the hypothesis which has been hitherto received,--a

hypothesis which assumes that the organism produces germ-cells afresh,

again and again, and that it produces them entirely from its own

substance. Under the former theory the germ-cells are no longer looked

upon as the product of the parent’s body, at least as far as their

essential part--the specific germ-plasm--is concerned: they are rather

considered as something which is to be placed in contrast with the

_tout ensemble_ of the cells which make up the parent’s body, and

the germ-cells of succeeding generations stand in a similar relation

to one another as a series of generations of unicellular organisms,

arising by a continued process of cell-division. It is true that in

most cases the generations of germ-cells do not arise immediately

from one another as complete cells, but only as minute particles of

germ-plasm. This latter substance, however forms the foundation of the

germ-cells of the next generation, and stamps them with their specific

character. Previous to the publication of my theory, C. Jäger, and

later M. Nussbaum, have expressed ideas upon heredity which come very

near to my own. Both of these writers started with the hypothesis that

there must be a direct connection between the germ-cells of succeeding

generations, and they tried to establish such a continuity by supposing

that the germ-cells of the offspring are separated from the parent

germ-cell before the beginning of embryonic development, or at least

before any histological differentiation has taken place. In this form

their suggestion cannot be maintained, for it is in conflict with

numerous facts. A continuity of the germ-cells does not now take place,

except in very rare instances; but this fact does not prevent us from

adopting a theory of the continuity of the germ-plasm, in favour of

which much weighty evidence can be brought forward. In the following

pages I shall attempt to develop further the theory of which I have

just given a short account, to defend it against any objections which

have been brought forward, and to draw from it new conclusions which

may perhaps enable us more thoroughly to appreciate facts which are

known, but imperfectly understood. It seems to me that this theory of

continuity of the germ-plasm deserves at least to be examined in all

its details, for it is the simplest theory upon the subject, and the

one which is most obviously suggested by the facts of the case, and we

shall not be justified in forsaking it for a more complex theory until

proof that it can be no longer maintained is forthcoming. It does not

presuppose anything except facts which can be observed at any moment,

although they may not be understood,--such as assimilation, or the

development of like organisms from like germs; while every other theory

of heredity is founded on hypotheses which cannot be proved. It is

nevertheless possible that continuity of the germ-plasm does not exist

in the manner in which I imagine that it takes place, for no one can at

present decide whether all the ascertained facts agree with and can be

explained by it. Moreover, the ceaseless activity of research brings to

light new facts every day, and I am far from maintaining that my theory

may not be disproved by some of these. But even if it should have to

be abandoned at a later period, it seems to me that, at the present

time, it is a necessary stage in the advancement of our knowledge, and

one which must be brought forward and passed through, whether it prove

right or wrong, in the future. In this spirit I offer the following

considerations, and it is in this spirit that I should wish them to be

received.

THE GERM-PLASM

I entirely agree with Strasburger when he says, “The specific qualities

of organisms are based upon nuclei”; and I further agree with him in

many of his ideas as to the relation between the nucleus and cell-body:

“Molecular stimuli proceed from the nucleus into the surrounding

cytoplasm; stimuli which, on the one hand, control the phenomena of

assimilation in the cell, and, on the other hand, give to the growth

of the cytoplasm, which depends upon nutrition, a certain character

peculiar to the species.” “The nutritive cytoplasm assimilates, while

the nucleus controls the assimilation, and hence the substances

assimilated possess a certain constitution and nourish in a certain

manner the cyto-idioplasm and the nuclear idioplasm. In this way the

cytoplasm takes part in the phenomena of construction, upon which the

specific form of the organism depends. This constructive activity

of the cyto-idioplasm depends upon the regulative influence of the

nuclei.” The nuclei therefore “determine the specific direction in

which an organism develops.”

The opinion--derived from the recent study of the phenomena of

fertilization--that the nucleus impresses its specific character

upon the cell, has received conclusive and important confirmation

in the experiments upon the regeneration of Infusoria, conducted

simultaneously by M. Nussbaum at Bonn, and by A. Gruber at Freiburg.

Nussbaum’s statement that an artificially separated portion of a

_Paramaecium_, which does not contain any nuclear substance,

immediately dies, must not be accepted as of general application, for

Gruber has kept similar fragments of other Infusoria alive for several

days. Moreover, Gruber had previously shown that individual Protozoa

occur, which live in a normal manner, and are yet without a nucleus,

although this structure is present in other individuals of the same

species. But the meaning of the nucleus is made clear by the fact,

published by Gruber, that such artificially separated fragments of

Infusoria are incapable of regeneration, while on the other hand those

fragments which contain nuclei always regenerate. It is therefore only

under the influence of the nucleus that the cell substance re-develops

into the full type of the species. In adopting the view that the

nucleus is the factor which determines the specific nature of the cell,

we stand on a firm foundation upon which we can build with security.

If therefore the first segmentation nucleus contains, in its molecular

structure, the whole of the inherited tendencies of development, it

must follow that during segmentation and subsequent cell-division, the

nucleoplasm will enter upon definite and varied changes which must

cause the differences appearing in the cells which are produced; for

identical cell-bodies depend, _ceteris paribus_, upon identical

nucleoplasm, and conversely different cells depend upon differences

in the nucleoplasm. The fact that the embryo grows more strongly in

one direction than in another, that its cell-layers are of different

nature and are ultimately differentiated into various organs and

tissues,--forces us to accept the conclusion that the nuclear substance

has also been changed in nature, and that such changes take place

during ontogenetic development in a regular and definite manner.

This view is also held by Strasburger, and it must be the opinion of

all who seek to derive the development of inherited tendencies from

the molecular structure of the germ-plasm, instead of from preformed

gemmules.

We are thus led to the important question as to the forces by which the

determining substance or nucleoplasm is changed, and as to the manner

in which it changes during the course of ontogeny, and on the answer

to this question our further conclusions must depend. The simplest

hypothesis would be to suppose that, at each division of the nucleus,

its specific substance divides into two halves of unequal quality, so

that the cell-bodies would also be transformed; for we have seen that

the character of a cell is determined by that of its nucleus. Thus in

any Metazoon the first two segmentation spheres would be transformed in

such a manner that one only contained the hereditary tendencies of the

endoderm and the other those of the ectoderm, and therefore, at a later

stage, the cells of the endoderm would arise from the one and those of

the ectoderm from the other; and this is actually known to occur. In

the course of further division the nucleoplasm of the first ectoderm

cell would again divide unequally, _e. g._, into the nucleoplasm

containing the hereditary tendencies of the nervous system, and into

that containing the tendencies of the external skin. But even then,

the end of the unequal division of nuclei would not have been nearly

reached; for, in the formation of the nervous system, the nuclear

substance which contains the hereditary tendencies of the sense-organs

would, in the course of further cell-division, be separated from that

which contains the tendencies of the central organs, and the same

process would continue in the formation of all single organs, and in

the final development of the most minute histological elements. This

process would take place in a definitely ordered course, exactly as

it has taken place throughout a very long series of ancestors; and

the determining and directing factor is simply and solely the nuclear

substance, the nucleoplasm, which possesses such a molecular structure

in the germ-cell that all such succeeding stages of its molecular

structure in future nuclei must necessarily arise from it, as soon as

the requisite external conditions are present. This is almost the same

conception of ontogenetic development as that which has been held by

embryologists who have not accepted the doctrine of evolution: for we

have only to transfer the primary cause of development, from an unknown

source within the organism, into the nuclear substance, in order to

make the views identical.

I believe I have shown that theoretically hardly any objection can be

raised against the view that the nuclear substance of somatic cells

may contain unchanged germ-plasm, or that this germ-plasm may be

transmitted along certain lines. It is true that we might imagine _a

priori_ that all somatic nuclei contain a small amount of unchanged

germ-plasm. In Hydroids such an assumption cannot be made, because only

certain cells in a certain succession possess the power of developing

into germ-cells; but it might well be imagined that in some organisms

it would be a great advantage if every part possessed the power of

growing up into the whole organism and of producing sexual cells under

appropriate circumstances. Such cases might exist if it were possible

for all somatic nuclei to contain a minute fraction of unchanged

germ-plasm. For this reason, Strasburger’s other objection against my

theory also fails to hold; viz., that certain plants can be propagated

by pieces of rhizomes, roots, or even by means of leaves, and that

plants produced in this manner may finally give rise to flowers, fruit

and seeds, from which new plants arise. “It is easy to grow new plants

from the leaves of begonia which have been cut off and merely laid upon

moist sand, and yet in the normal course of ontogeny the molecules of

germ-plasm would not have been compelled to pass through the leaf; and

they ought therefore to be absent from its tissue. Since it is possible

to raise from the leaf a plant which produces flower and fruit, it is

perfectly certain that special cells containing the germ-substance

cannot exist in the plant.” But I think that this fact only proves

that in begonia and similar plants all the cells of the leaves or

perhaps only certain cells contain a small amount of germ-plasm, and

that consequently these plants are specially adapted for propagation

by leaves. How is it then that all plants cannot be reproduced in this

way? No one has ever grown a tree from the leaf of the lime or oak,

or a flowering plant from the leaf of the tulip or convolvulus. It

is insufficient to reply that in the last mentioned cases the leaves

are more strongly specialized, and have thus become unable to produce

germ-substance; for the leaf-cells in these different plants have

hardly undergone histological differentiation in different degrees.

If, notwithstanding, the one can produce a flowering plant, while the

others have not this power, it is of course clear that reasons other

than the degree of histological differentiation must exist; and,

according to my opinion, such a reason is to be found in the admixture

of a minute quantity of unchanged germ-plasm with some of their nuclei.

In Sach’s excellent lectures on the physiology of plants, we read on

page 723--“In the true mosses almost any cell of the roots, leaves and

shoot-axes, and even of the immature sporogonium, may grow out under

favourable conditions, become rooted, form new shoots, and give rise to

an independent living plant.” Since such plants produce germ-cells at

a later period, we have here a case which requires the assumption that

all or nearly all cells must contain germ-plasm.

The theory of the continuity of the germ-plasm seems to me to be

still less disproved or even rendered improbable by the facts of the

alternation of generations. If the germ-plasm may pass on from the egg

into certain somatic cells of an individual, and if it can be further

transmitted along certain lines, there is no difficulty in supposing

that it may be transmitted through a second, third, or through any

number of individuals produced from the former by budding. In fact, in

the Hydroids, on which my theory of the continuity of the germ-plasm

has been chiefly based, alternation of generations is the most

important means of propagation.

THE SIGNIFICANCE OF THE POLAR BODIES

We have already seen that the specific nature of a cell depends upon

the molecular structure of its nucleus; and it follows from this

conclusion that my theory is further, and as I believe strongly,

supported, by the phenomenon of the expulsion of polar bodies, which

has remained inexplicable for so long a time.

For if the specific molecular structure of a cell-body is caused

and determined by the structure of the nucleoplasm, every kind of

cell which is histologically differentiated must have a specific

nucleoplasm. But the egg-cell of most animals, at any rate during

the period of growth, is by no means an indifferent cell of the most

primitive type. At such a period its cell-body has to perform quite

peculiar and specific functions; it has to secrete nutritive substances

of a certain chemical nature and physical constitution, and to store

up this food material in such a manner that it may be at the disposal

of the embryo during its development. In most cases the egg-cell

also forms membranes which are often characteristic of particular

species of animals. The growing egg-cell is therefore histologically

differentiated: and in this respect resembles a somatic cell. It

may perhaps be compared to a gland-cell, which does not expel its

secretion, but deposits it within its own substance. To perform such

specific functions it requires a specific cell-body, and the latter

depends upon a specific nucleus. It therefore follows that the growing

egg-cell must possess nucleoplasm of specific molecular structure,

which directs the above mentioned secretory functions of the cell.

The nucleoplasm of histologically differentiated cells may be called

histogenetic nucleoplasm, and the growing egg-cell must contain such

a substance, and even a certain specific modification of it. This

nucleoplasm cannot possibly be the same as that which, at a later

period, causes embryonic development. Such development can only be

produced by the true germ-plasm of immensely complex constitution, such

as I have previously attempted to describe. It therefore follows that

the nucleus of the egg-cell contains two kinds of nucleoplasm:--germ

and a peculiar modification of histogenetic nucleoplasm, which

may be called ovogenetic nucleoplasm. This substance must greatly

preponderate in the young egg-cell, for, as we have already seen, it

controls the growth of the latter. The germ-plasm, on the other hand,

can only be present in minute quantity at first, but it must undergo

considerable increase during the growth of the cell. But in order

that the germ-plasm may control the cell-body, or, in other words, in

order that embryonic development may begin, the still preponderating

ovogenetic nucleoplasm must be removed from the cell. This removal

takes place in the same manner as that in which differing nuclear

substances are separated during the ontogeny of the embryo: viz., by

nuclear division, leading to cell-division. The expulsion of the polar

bodies is nothing more than the removal of ovogenetic nucleoplasm from

the egg-cell. That the ovogenetic nucleoplasm continues greatly to

preponderate in the nucleus up to the very last, may be concluded from

the fact that two successive divisions of the latter and the expulsion

of two polar bodies appear to be the rule. If in this way a small part

of the cell-body is expelled from the egg, the extrusion must in all

probability be considered as an inevitable loss, without which the

removal of the ovogenetic nucleoplasm cannot be effected.

ON THE NATURE OF PARTHENOGENESIS

It is well known that the formation of polar bodies has been repeatedly

connected with the sexuality of germ-cells, and that it has been

employed to explain the phenomena of parthenogenesis. I may now perhaps

be allowed to develop the views as to the nature of parthenogenesis at

which I have arrived under the influence of my explanation of polar

bodies.

The theory of parthenogenesis adopted by Minot and Balfour is

distinguished by its simplicity and clearness, among all other

interpretations which had been hitherto offered. Indeed, their

explanation follows naturally and almost as a matter of course, if the

assumption made by these observers be correct, that the polar body is

the male part of the hermaphrodite egg-cell. An egg which has lost its

male part cannot develop into an embryo until it has received a new

male part in fertilization. On the other hand, an egg which does not

expel its male part may develop without fertilization, and thus we are

led to the obvious conclusion that parthenogenesis is based upon the

non-expulsion of polar bodies. Balfour distinctly states “that the

function of forming polar cells has been acquired by the ovum for the

express purpose of preventing parthenogenesis.”

It is obvious that I cannot share this opinion, for I regard the

expulsion of polar bodies as merely the removal of the ovogenetic

nucleoplasm, on which depended the development of the specific

histological structure of the egg-cell. I must assume that the

phenomena of maturation in the parthenogenetic egg and in the sexual

egg are precisely identical, and that in both, the ovogenetic

nucleoplasm must in some way be removed before embryonic development

can begin.

Unfortunately the actual proof of this assumption is not so complete

as might be desired. In the first place, we are as yet uncertain

whether polar bodies are or are not expelled by parthenogenetic eggs;

for in no single instance has such expulsion been established beyond

doubt. It is true that this deficiency does not afford any support

to the explanation of Minot and Balfour, for in all cases in which

polar bodies have not been found in parthenogenetic eggs, these

structures are also absent from the eggs which require fertilization

in the same species. But although the expulsion of polar bodies in

parthenogenesis has not yet been proved to occur, we must assume it to

be nearly certain that the phenomena of maturation, whether connected

or unconnected with the expulsion of polar bodies, are the same in the

eggs which develop parthenogenetically and in those which are capable

of fertilization, in one and the same species. This conclusion depends,

above all, upon the phenomena of reproduction in bees, in which,

as a matter of fact, the same egg may be fertilized or may develop

parthenogenetically, as I shall have occasion to describe in greater

detail at a later period.

Hence when we see that the eggs of many animals are capable of

developing without fertilization, while in other animals such

development is impossible, the difference between the two kinds of eggs

must rest upon something more than the mode of transformation of the

nucleus of the germ-cell into the first segmentation nucleus. There

are, indeed, facts which distinctly point to the conclusion that the

difference is based upon quantitative and not qualitative relations.

A large number of insects are exceptionally reproduced by the

parthenogenetic method, _e. g._, in Lepidoptera. Such development

does not take place in all the eggs laid by an unfertilized female,

but only in part, and generally a small fraction of the whole, while

the rest die. But among the latter there are some which enter upon

embryonic development without being able to complete it, and the

stage at which development may cease also varies. It is also known

that the eggs of higher animals may pass through the first stages of

segmentation without having been fertilized. This was shown to be

the case in the egg of the frog by Leuckart, in that of the fowl by

Oellacher, and even in the egg of mammals by Hensen.

Hence in such cases it is not the impulse to development, but the power

to complete it, which is absent. We know that force is always bound up

with matter, and it seems to me that such instances are best explained

by the supposition that too small an amount of that form of matter

is present, which, by its controlling agency, effects the building

up of the embryo by the transformation of mere nutritive material.

This substance is the germ-plasm of the segmentation nucleus, and I

have assumed above that it is altered in the course of ontogeny by

changes which arise from within, so that when sufficient nourishment

is afforded by the cell-body, each succeeding stage necessarily

results from the preceding one. I believe that changes arise in the

constitution of the nucleoplasm at each cell-division which takes place

during the building up of the embryo, changes which either correspond

or differ in the two halves of each nucleus. If, for the present, we

neglect the minute amount of unchanged germ-plasm which is reserved

for the formation of the germ-cells, it is clear that a great many

different stages in the development of somatic nucleoplasm are thus

formed, which may be denominated as stages 1, 2, 3, 4, etc., up to

_n_. In each of these stages the cells differ more as development

proceeds, and as the number by which the stage is denominated becomes

higher. Thus, for instance, the two first segmentation spheres would

represent the first stage of somatic nucleoplasm, a stage which may

be considered as but slightly different in its molecular structure

from the nucleoplasm of the segmentation nucleus; the first four

segmentation spheres would represent the second stage; the succeeding

eight spheres the third, and so on. It is clear that at each successive

stage the molecular structure of the nucleoplasm must be further

removed from that of the germ-plasm, and that, at the same time, the

cells of each successive stage must also diverge more widely among

themselves in the molecular structure of their nucleoplasm. Early in

development each cell must possess its own peculiar nucleoplasm, for

the further course of development is peculiar to each cell. It is

only in the later stages that equivalent or nearly equivalent cells

are formed in large numbers, cells in which we must also suppose the

existence of equivalent nucleoplasm.

If we may assume that a certain amount of germ-plasm must be contained

in the segmentation nucleus in order to complete the whole process of

the ontogenetic differentiation of this substance; if we may further

assume that the quantity of germ-plasm in the segmentation nucleus

varies in different cases; then we should be able to understand why

one egg can only develop after fertilization, while another can

begin its development without fertilization, but cannot finish it,

and why a third is even able to complete its development. We should

also understand why one egg only passes through the first stages of

segmentation and is then arrested, while another reaches a few more

stages in advance, and a third develops so far that the embryo is

nearly completely formed. These differences would depend upon the

extent to which the germ-plasm, originally present in the egg, was

sufficient for the development of the latter; development will be

arrested as soon as the nucleoplasm is no longer capable of producing

the succeeding stage, and is thus unable to enter upon the following

nuclear division.

From a general point of view such a theory would explain many

difficulties, and it would render possible an explanation of the

phyletic origin of parthenogenesis, and an adequate understanding

of the strange and often apparently abrupt and arbitrary manner

of its occurrence. In my works on Daphnidae I have already laid

especial stress upon the proposition that parthenogenesis in insects

and Crustacea certainly cannot be an ancestral condition which has

been transmitted by heredity, but that it has been derived from a

sexual condition. In what other way can we explain the fact that

parthenogenesis is present in certain species or genera, but absent

in others closely allied to them; or the fact that males are entirely

wanting in species of which the females possess a complete apparatus

for fertilization? I will not repeat all the arguments with which I

attempted to support this conclusion. Such a conclusion may be almost

certainly accepted for the Daphnidae, because parthenogenesis does not

occur in their still living ancestors, the Phyllopods, and especially

the Estheridae. In Daphnidae the cause and object of the phyletic

development of parthenogenesis may be traced more clearly than in any

other group of animals. In Daphnidae we can accept the conclusion with

greater certainty than in all other groups, except perhaps the Aphidae,

that parthenogenesis is extremely advantageous to species in certain

conditions of life; and that it has only been adopted when, and as far

as, it has been beneficial; and further, that at least in this group

parthenogenesis became possible and was adopted in each species as soon

as it became useful. Such a result can be easily understood if it is

only the presence of more or less germ-plasm which decides whether an

egg is or is not capable of development without fertilization.

If we now examine the foundations of this hypothesis we shall find that

we may at once accept one of its assumptions, viz., that fluctuations

occur in the quantity of germ-plasm in the segmentation nucleus; for

there can never be absolute equality in any single part of different

individuals. As soon therefore as these fluctuations become so great

that parthenogenesis is produced, it may become, by the operation of

natural selection, the chief mode of reproduction of the species or

of certain generations of the species. In order to place this theory

upon a firm basis, we have simply to decide whether the quantity of

germ-plasm contained in the segmentation nucleus is the factor which

determines development; although for the present it will be sufficient

if we can render this view to some extent probable, and show that it is

not a contradiction of established facts.

At first sight this hypothesis seems to encounter serious difficulties.

It will be objected that neither the beginning nor the end of embryonic

development can possibly depend upon the quantity of nucleoplasm in the

segmentation nucleus, since the amount may be continually increased

by growth; for it is well known that during embryonic development

the nuclear substance increases with astonishing rapidity. By an

approximate calculation I found that in the egg of a Cynips the

quantity of nuclear substance present at the time when the blastoderm

was about to be formed, and when there were twenty-six nuclei, was even

then seven times as great as the quantity which had been contained

in the segmentation nucleus. How then can we imagine that embryonic

development would ever be arrested from want of nuclear substance, and

if such deficiency really acted as an arresting force, how then could

development begin at all? We might suppose that when germ-plasm is

present in sufficient quantity to start segmentation, it must also be

sufficient to complete the development; for it grows continuously, and

must presumably always possess a power equal to that which it possessed

at the beginning, and which was just sufficient to start the process of

segmentation. If at each ontogenetic stage the quantity of nucleoplasm

is just sufficient to produce the following stage, we might well

imagine that the whole ontogeny would necessarily be completed.

The flaw in this argument lies in the erroneous assumption that the

growth of nuclear substance is, when the quality of the nucleus and

the conditions of nutrition are equal, unlimited and uncontrolled. The

intensity of growth must depend upon the quantity of nuclear substance

with which growth and the phenomena of segmentation commenced. There

must be an optimum quantity of nucleoplasm with which the growth of

the nucleus proceeds most favourably and rapidly, and this optimum

will be represented in the normal size of the segmentation nucleus.

Such a size is just sufficient to produce, in a certain time and

under certain external conditions, the nuclear substance necessary

for the construction of the embryo, and to start the long series

of cell-divisions. When the segmentation nucleus is smaller, but

large enough to enter upon segmentation, the nuclei of the two first

embryonic cells will fall rather more below the normal size, because

the growth of the segmentation nucleus, during and after division will

be less rapid on account of its unusually small size. The succeeding

generations of nuclei will depart more and more from the normal size in

each respective stage, because they do not pass into a resting stage

during embryonic development, but divide again immediately after their

formation. Hence nuclear growth would become less vigorous as the

nuclei fell more and more below the optimum size, and at last a moment

would arrive when they would be unable to divide, or would be at least

unable to control the cell-body in such a manner as to lead to its

division.

The first event of importance for embryonic development is the

maturation of the egg, _i. e._, the transformation of the

nucleus of the germ-cell into a nuclear spindle and the removal of

the ovogenetic nucleoplasm by the separation of polar bodies, or by

some analogous process. There must be some cause for this separation,

and I have already tried to show that it may lie in the quantitative

relations which obtain between the two kinds of nucleoplasm contained

in the nucleus of the egg. I have suggested that the germ-plasm, at

first small in quantity, undergoes a gradual increase, so that it

can finally oppose the ovogenetic nucleoplasm. I will not further

elaborate this suggestion, for the ascertained facts are insufficient

for the purpose. But the appearances witnessed in nuclear division

indicate that there are opposing forces, and that such a contest is

the motive cause of division; and Roux may be right in referring the

opposition to electrical forces. However this may be, it is perfectly

certain that the development of this opposition is based upon internal

conditions arising during growth in the nucleus itself. The quantity

of nuclear thread cannot by itself determine whether the nucleus can

or cannot enter upon division; if so, it would be impossible for two

divisions to follow each other in rapid succession, as is actually

the case in the separation of the two polar bodies, and also in their

subsequent division. In addition to the effects of quantity, the

internal conditions of the nucleus must also play an important part in

these phenomena. Quantity alone does not necessarily produce nuclear

division, or the nucleus of the egg would divide long before maturation

is complete, for it contains much more nucleoplasm than the female

pronucleus, which remains in the egg after the expulsion of the polar

bodies, and which is in most cases capable of further division. But

the fact that segmentation begins immediately after the conjugation of

male and female pronuclei, also shows that quantity is an essential

requisite. The effect of fertilization has been represented as

analogous to that of the spark which kindles the gunpowder. In the

latter case an explosion ensues, in the former segmentation begins.

Even now many authorities are inclined to refer the polar repulsion

manifested in the nuclear division which immediately follows

fertilization, to the antagonism between male and female elements. But,

according to the important discoveries of Flemming and van Beneden, the

polar repulsion in each nuclear division is not based on the antagonism

between male and female loops, but depends upon the antagonism and

mutual repulsion between the two halves of the same loop. The loops of

the father and those of the mother remain together and divide together

throughout the whole ontogeny.

What can be the explanation of the fact that nuclear division follows

immediately after fertilization, but that without fertilization it

does not occur in most cases? There is only one possible explanation,

viz., the fact that the quantity of the nucleus has been suddenly

doubled, as the result of conjugation. The difference between the male

and female pronuclei cannot serve as an explanation, even though the

nature of this difference is entirely unknown, because polar repulsion

is not developed between the male and female halves of the nucleus, but

within each male and each female half. We are thus forced to conclude

that increase in the quantity of the nucleus affords an impulse for

division, the disposition towards it being already present. It seems

to me that this view does not encounter any theoretical difficulties,

and that it is an entirely feasible hypothesis to suppose that, besides

the internal conditions of the nucleus, its quantitative relation to

the cell-body must be taken into especial account. It is imaginable, or

perhaps even probable, that the nucleus enters upon division as soon

as its idioplasm has attained a certain strength, quite apart from the

supposition that certain internal conditions are necessary for this

end. As above stated, such conditions may be present, but division may

not occur because the right quantitative relation between nucleus and

cell-body, or between the different kinds of nuclear idioplasm has not

been established. I imagine that such a quantitative deficiency exists

in an egg which, after the expulsion of the ovogenetic nucleoplasm

in the polar bodies, requires fertilization in order to begin

segmentation. The fact that the polar bodies were expelled proves that

the quantity of the nucleus was sufficient to cause division, while

afterwards it was no longer sufficient to produce such a result.

This suggestion will be made still clearer by an example. In _Ascaris

megalocephala_ the nuclear substance of the female pronucleus

forms two loops, and the male pronucleus does the same; hence the

segmentation nucleus contains four loops, and this is also the case

with the first segmentation spheres. If we suppose that in embryonic

development the first nuclear division requires such an amount of

nuclear substance as is necessary for the formation of four loops,--it

follows that an egg, which can only form two or three loops from its

nuclear reticulum, would not be able to develop parthenogenetically,

and that not even the first division would take place. If we further

suppose that, while four loops are sufficient to start nuclear

division, these loops must be of a certain size and quantity in order

to complete the whole ontogeny (in a certain species), it follows

that eggs possessing a reticulum which contains barely enough nuclear

substance to divide into four segments, would be able to produce

the first division and perhaps also the second and third, or some

later division, but that at a certain point during ontogeny, the

nuclear substance would become insufficient, and development would be

arrested. This will occur in eggs which enter upon development without

fertilization, but are arrested before its completion. One might

compare this retardation leading to the final arrest of development,

to a railway train which is intended to meet a number of other trains

at various junctions, and which can only travel slowly because of some

defect in the engine. It will be a little behind time at the first

junction, but it may just catch the train, and it may also catch the

second or even the third; but it will be later at each successive

junction, and will finally arrive too late for a certain train; and

after that it will miss all the trains at the remaining junctions. The

nuclear substance grows continuously during development, but the rate

at which it increases depends upon the nutritive conditions together

with its initial quantity. The nutritive changes during the development

of an egg depend upon the quantity of the cell-body which was present

at the outset, and which cannot be increased. If the quantity of

the nuclear substance is rather too small at the beginning, it will

become more and more insufficient in succeeding stages, as its growth

becomes less vigorous, and differs more from the standard it would

have reached if the original quantity had been normal. Consequently it

will gradually fall more and more short of the normal quantity, like

the train which arrives later and later at each successive junction,

because its engine, although with the full pressure of steam, is unable

to attain the normal speed.

It will be objected that four loops cannot be necessary for nuclear

division in _Ascaris_, since such division takes place in the

formation of the polar bodies, resulting in the appearance of the

female pronucleus with only two loops. But this fact only shows that

the quantity of nuclear substance necessary for the formation of four

loops is not necessary for all nuclear divisions; it does not disprove

the assumption that such a quantity is required for the division of

the segmentation nucleus. In addition to these considerations we must

not leave the substance of the cell-body altogether out of account,

for, although it is not the bearer of the tendencies of heredity, it

must be necessary for every change undergone by the nucleus, and it

surely also possesses the power of influencing changes to a large

extent. There must be some reason for the fact that in all animal eggs

with which we are acquainted, the nucleus moves to the surface of the

egg at the time of maturation, and there passes through its well known

transformation. It is obvious that it is there subjected to different

influences from those which would have acted upon it in the center of

the cell-body, and it is clear that such an unequal cell-division as

takes place in the separation of the polar bodies could not occur if

the nucleus remained in the center of the egg.

This explanation of the necessity for fertilization does not exclude

the possibility that, under certain circumstances, the substance of the

egg-nucleus may be larger, so that it is capable of forming four loops.

Eggs which thus possess sufficient nucleoplasm, viz., germ-plasm, for

the formation of the requisite four loops of normal size (namely, of

the size which would have been produced by fertilization), can and must

develop by the parthenogenetic method.

Of course the assumption that four loops must be formed has only

been made for the sake of illustration. We do not yet know whether

there are always exactly four loops in the segmentation nucleus. I

may add that, although the details by which these considerations are

illustrated are based on arbitrary assumptions, the fundamental view

that the development of the egg depends, _ceteris paribus_, upon

the quantity of nuclear substance, is certainly right, and follows as

a necessary conclusion from the ascertained facts. It is not unlikely

that such a view may receive direct proof in the results of future

investigations. Such proof might, for instance, be forthcoming if we

were to ascertain, in the same species, the number of loops present

in the segmentation nucleus of fertilization, as compared with those

present in the segmentation nucleus of parthenogenesis.

The reproductive process in bees will perhaps be used as an argument

against my theory. In these insects the same egg will develop into a

female or male individual, according as fertilization has or has not

taken place, respectively. Hence one and the same egg is capable of

fertilization, and also of parthenogenetic development, if it does

not receive a spermatozoon. It is in the power of the queen-bee to

produce male or female individuals: by an act of will she decides

whether the egg she is laying is to be fertilized or unfertilized.

She “knows beforehand” whether an egg will develop into a male or a

female animal, and deposits the latter kind in the cells of queens and

workers, the former in the cells of drones. It has been shown by the

discoveries of Leuckart and von Siebold that all the eggs are capable

of developing into male individuals, and that they are only transformed

into “female eggs” by fertilization. This fact seems to be incompatible

with my theory as to the cause of parthenogenesis, for if the same

egg, possessing exactly the same contents, and above all the same

segmentation nucleus, may develop sexually or parthenogenetically, it

appears that the power of parthenogenetic development must depend on

some factor other than the quantity of germ-plasm.

Although this appears to be the case, I believe that my theory

encounters no real difficulty. I have no doubt whatever that the same

egg may develop with or without fertilization. From a careful study of

the numerous excellent investigations upon this point which have been

conducted in a particularly striking manner by Bessels (in addition

to the observers quoted above), I have come to the conclusion that

the fact is absolutely certain. It must be candidly admitted that

the same egg will develop into a drone when not fertilized, or into

a worker or queen when fertilized. One of Bessels’ experiments is

sufficient to prove this assertion. He cut off the wings of a young

queen and thus rendered her incapable of taking “the nuptial flight.”

He then observed that all the eggs which she laid developed into

male individuals. This experiment was made in order to prove that

drones are produced by unfertilized eggs; but it also proves that the

assertion mentioned above is correct, for the eggs which ripen first

and are therefore first laid, would have been fertilized had the queen

been impregnated. The supposition that, at certain times, the queen

produces eggs requiring fertilization, while at other times her eggs

develop parthenogenetically, is quite excluded by this experiment; for

it follows from it that the eggs must all be of precisely the same

kind, and that there is no difference between the eggs which require

fertilization and those which do not.

But does it therefore follow that the quantity of germ-plasm in

the segmentation nucleus is not the factor which determines the

beginning of embryonic development? I believe not. It can be very

well imagined that the nucleus of the egg, having expelled the

ovogenetic nucleoplasm, may be increased to the size requisite for the

segmentation nucleus in one of two ways: either by conjugation with

a sperm-nucleus, or by simply growing to double its size. There is

nothing improbable in this latter assumption, and one is even inclined

to inquire why such growth does not take place in all unfertilized

eggs. The true answer to this question must be that nature pursues

the sexual method of reproduction, and that the only way in which the

general occurrence of parthenogenesis could be prevented was by the

production of eggs which remained sterile unless they were fertilized.

This was effected by a loss of the capability of growth on the part of

the egg-nucleus after it had expelled the ovogenetic nucleoplasm.

The case of the bee proves in a very striking manner that the

difference between eggs which require fertilization, and those which

do not, is not produced until after the maturation of the egg and the

removal of the ovogenetic nucleoplasm. The increase in the quantity of

the germ-plasm cannot have taken place at any earlier period, or else

the nucleus of the egg would always start embryonic development by

itself, and the egg would probably be incapable of fertilization. For

the relation between egg-nucleus and sperm-nucleus is obviously based

upon the fact that each of them is insufficient by itself, and requires

completion. If such completion had taken place at an early stage the

egg-nucleus would either cease to exercise any attractive force upon

the sperm-nucleus, or else conjugation would be effected, as in Fol’s

interesting experiments upon fertilization by many spermatozoa; and,

as in these experiments, malformation of the embryo would result. In

Daphnidae I believe I have shown that the summer eggs are not only

developed parthenogenetically, but also that they are never fertilized;

and the explanation of this incapacity for fertilization may perhaps be

found in the fact that their segmentation nucleus is already formed.

We may therefore conclude that, in bees, the nucleus of the egg, formed

during maturation, may either conjugate with the sperm-nucleus, or

else if no spermatozoon reaches it the egg may, under the stimulus of

internal causes, grow to double its size, thus attaining the dimensions

of the segmentation nucleus. For our present purpose we may leave

out of consideration the fact that in the latter case the individual

produced is a male, and in the former case a female.

FOOTNOTES:

[Footnote 37: From _Essays upon Heredity and Kindred Biological

Problems_, Vol. I (1889).]

XXXV

SIR NORMAN LOCKYER

1836-1920

_Sir Joseph Norman Lockyer, born at Rugby, England, May 17, 1836,

entered the War Office in 1857. Through his own exertions he educated

himself in science and was one of the first to suggest the hypothesis

that the earth and other spheres were the result of the aggregation

of meteorites. He was also the first to apply the spectroscope to

the corona of the sun, revealing the chemical composition of solar

prominences as chiefly hydrogen, calcium, and helium. He died at

Sidmouth, Devonshire, August 16, 1920._

THE CHEMISTRY OF THE STARS[38]

The importance of practical work, the educational value of the seeking

after truth by experiment and observation on the part of even young

students, are now generally recognized. That battle has been fought

and won. But there is a tendency in the official direction of seats of

learning to consider what is known to be useful, because it is used,

in the first place. The fact that the unknown, that is, the unstudied,

is the mine from which all scientific knowledge with its million

applications has been won is too often forgotten.

Bacon, who was the first to point out the importance of experiment in

the physical sciences, and who predicted the applications to which I

have referred, warns us that “_lucifera experimenta non fructifera

quaerenda_”; and surely we should highly prize those results which

enlarge the domain of human thought and help us to understand the

mechanism of the wonderful universe in which our lot is cast, as well

as those which add to the comfort and the convenience of our lives.

It would be also easy to show by many instances how researches,

considered ideally useless at the time they were made, have been the

origin of the most tremendous applications. One instance suffices.

Faraday’s trifling with wires and magnets has already landed us in one

of the greatest revolutions which civilization has witnessed; and where

the triumphs of electrical science will stop no man can say.

This is a case in which the useless has been rapidly sublimed into

utility so far as our material wants are concerned.

I propose to bring to your notice another “useless” observation

suggesting a line of inquiry which I believe sooner or later is

destined profoundly to influence human thought along many lines.

Fraunhofer at the beginning of this century examined sunlight and

starlight through a prism. He found that the light received from the

sun differed from that of the stars. So useless did his work appear

that we had to wait for half a century till any considerable advance

was made. It was found at last that the strange “lines” seen and named

by Fraunhofer were precious indications of the chemical substances

present in worlds immeasurably remote. We had, after half a century’s

neglect, the foundation of solar and stellar chemistry, an advance in

knowledge equaling any other in its importance.

In dealing with my subject I shall first refer to the work which

has been done in more recent years with regard to this chemical

conditioning of the atmospheres of stars, and afterwards very briefly

show how this work carries us into still other new and wider fields of

thought.

The first important matter which lies on the surface of such a general

inquiry as this is that if we deal with the chemical elements as judged

by the lines in their spectra we know for certain of the existence of

oxygen, of nitrogen, of argon, representing one class of gases, in no

celestial body whatever; whereas, representing other gases, we have a

tremendous demonstration of the existence of all the known lines of

hydrogen and helium.

We see, then, that the celestial sorting out of gases is quite

different from the terrestrial one.

Taking the substances classed by the chemist as non-metals, we find

carbon and silicium--I prefer, on account of its stellar behavior, to

call it silicium, though it is old fashioned--present in celestial

phenomena. We have evidence of this in the fact that we have a

considerable development of carbon in some stars and an indication

of silicium in others. But these are the only non-metals observed.

Now, with regard to the metallic substances which we find, we deal

chiefly with calcium, strontium, iron, and magnesium. Others are not

absolutely absent, but their percentage quantity is so small that they

are negligible in a general statement.

Now do these chemical elements exist indiscriminately in all the

celestial bodies, so that practically, from a chemical point of view,

the bodies appear to us of similar chemical constitution? No; it is not

so.

From the spectra of those stars which resemble the sun, in that they

consist of an interior nucleus surrounded by an atmosphere which

absorbs the light of the nucleus, and which, therefore, we study by

means of this absorption, it is to be gathered that the atmospheres

of some stars are chiefly gaseous--i. e., consisting of elements we

recognize as gases here--of others chiefly metallic, of others again

mainly composed of carbon or compounds of carbon.

Here, then, we have spectroscopically revealed the fact that there is

considerable variation in the chemical constituents which build up the

stellar atmospheres.

This, though a general, is still an isolated statement. Can we connect

it with another? One of the laws formulated by Kirchhoff in the infancy

of spectroscopic inquiry has to do with the kind of radiation given

out by bodies at different temperatures. A poker placed in a fire

first becomes red, and, as it gets hotter, white hot. Examined in a

spectroscope, we find that the red condition comes from the absence of

blue light; that the white condition comes from the gradual addition of

blue as the temperature increases.

The law affirms that the hotter a mass of matter is the farther its

spectrum extends into the ultraviolet.

Hence the hotter a star is the farther does its complete or continuous

spectrum lengthen out toward the ultraviolet and the less it is

absorbed by cooler vapors in its atmosphere.

Now, to deal with three of the main groups of stars, we find the

following very general result:

Gaseous stars Longest spectrum.

Metallic stars Medium spectrum.

Carbon stars Shortest spectrum.

We have now associated two different series of phenomena, and we are

enabled to make the following statement:

Gaseous stars Highest temperature.

Metallic stars Medium temperature.

Carbon stars Lowest temperature.

Hence the differences in apparent chemical constitutions are associated

with differences of temperature.

Can we associate with the two to which I have already called attention

still a third class of facts? Laboratory work enables us to do this.

When I began my inquiries the idea was, one gas or vapor, one spectrum.

We now know that this is not true; the systems of bright lines given

out by radiating substances change with the temperature.

We can get the spectrum of a well known compound substance--say

carbonic oxide; it is one special to the compound; we increase the

temperature so as to break up the compound, and we then get the spectra

of its constituents, carbon and oxygen.

But the important thing in the present connection is that the spectra

of the chemical elements behave exactly in the same way as the spectra

of known compounds do when we employ temperatures far higher than those

which break up the compounds; and indeed in some cases the changes

are more marked. For brevity I will take for purposes of illustration

three substances, and deal with one increase of temperature only, a

considerable one and obtainable by rendering a substance incandescent,

first by a direct current of electricity, as happens in the so-called

“arc lamps” employed in electric lighting, and next by the employment

of a powerful induction coil and battery of Leyden jars. In laboratory

parlance we pass thus from the arc to the jar-spark. In the case of

magnesium, iron, and calcium, the changes observed on passing from

the temperature of the arc to that of the spark have been minutely

observed. In each, new lines are added or old ones are intensified at

the higher temperature. Such lines have been termed “enhanced lines.”

These enhanced lines are not seen alone; outside the region of high

temperature in which they are produced, the cooling vapors give us the

cool lines. Still we can conceive the enhanced lines to be seen alone

at the highest temperature in a space sufficiently shielded from the

action of all lower temperatures, but such a shielding is beyond our

laboratory expedients.

In watching the appearance of these special enhanced lines in stellar

spectra we have a third series of phenomena available, and we find that

the results are absolutely in harmony with what has gone before. Thus:

Gaseous stars Highest temperature Strong helium and faint

enhanced lines.

Metallic stars Feeble helium and strong

Medium temperature enhanced lines.

Carbon stars No helium and strong arc lines.

Lowest temperature Faint arc lines.

It is clear now, not only that the spectral changes in stars are

associated with, or produced by, changes of temperature, but that

the study of the enhanced spark and the arc lines lands us in the

possibility of a rigorous stellar thermometry, such lines being more

easy to observe than the relative lengths of spectrum.

Accepting this, we can take a long stride forward and, by carefully

studying the chemical revelations of the spectrum, classify the stars

along a line of temperature. But which line? Were all the stars in

popular phraseology created hot? If so, we should simply deal with

the running down of temperature, and because all the hottest stars

are chemically alike, all cooler stars would be alike. But there are

two very distinct groups of coolest stars; and since there are two

different kinds of coolest stars, and only one kind of hottest stars,

it cannot be merely a question either of a running up or a running down

of temperature.

Many years of very detailed inquiry have convinced me that all stars

save the hottest must be sorted out into two series--those getting

hotter and those, like our sun, getting cooler, and that the hottest

stage in the history of a star is reached near the middle of its life.

The method of inquiry adopted has been to compare large-scale

photographs of the spectra of the different stars taken by my

assistants at South Kensington; the complete harmony of the results

obtained along various lines of other work carries conviction with it.

We find ourselves here in the presence of minute details exhibiting the

workings of a chemical law, associated distinctly with temperature;

and more than this, we are also in the presence of high temperature

furnaces, entirely shielded by their vastness from the presence of

those distracting phenomena which we are never free from in the most

perfect conditions of experiment we can get here.

What, then, is the chemical law? It is this: In the very hottest

stars we deal with the gases hydrogen, helium, and doubtless others

still unknown, almost exclusively. At the next lowest temperatures we

find these gases being replaced by metals in the state in which they

are observed in our laboratories when the most powerful jar-spark is

employed. At a lower temperature still the gases almost disappear

entirely, and the metals exist in the state produced by the electric

arc. Certain typical stars showing these chemical changes may be

arranged as follows:

This, then, is the result of our first inquiry into the existence of

the various chemical elements in the atmospheres of stars generally.

We get a great diversity, and we know that this diversity accompanies

changes of temperature. We have also found that the sun, which we

independently know to be a cooling star, and Arcturus are identical

chemically.

We have now dealt with the presence of the various chemical elements

generally in the atmospheres of stars. The next point we have to

consider is, whether the absorption which the spectrum indicates for

us takes place from top to bottom of the atmosphere or only in certain

levels.

In many of these stars the atmosphere may be millions of miles high. In

each the chemical substances in the hottest and coldest portions may be

vastly different. The region, therefore, in which this absorption takes

place, which spectroscopically enables us to discriminate star from

star, must be accurately known before we can obtain the greatest amount

of information from our inquiries.

Our next duty then, clearly, is to study the sun--a star so near us

that we can examine the different parts of its atmosphere, which we

cannot do in the case of the more distant stars. By doing this we

may secure facts which will enable us to ascertain in what parts of

the atmosphere the absorption takes place which produces the various

phenomena on which the chemical classification has been based.

It is obvious that the general spectrum of the sun, like that of stars

generally, is built up of all the absorptions which can make themselves

felt in every layer of its atmosphere from bottom to top; that is, from

the photosphere to the outermost part of the corona. Let me remind you

that this spectrum is changeless from year to year.

Now, sun-spots are disturbances produced in the photosphere; and the

chromosphere, with its disturbances, called prominences, lies directly

above it. Here, then, we are dealing with the lowest part of the sun’s

atmosphere. We find first of all that, in opposition to the changeless

general spectrum, great changes occur with the sun-spot period, both in

the spots and chromosphere.

The spot spectrum is indicated, as was found in 1866, by the widening

of certain lines; the chromospheric spectrum, as was found in 1868, by

the appearance at the sun’s limb of certain bright lines. In both cases

the lines affected, seen at any one time, are relatively few in number.

In the spot spectrum, at a sun-spot minimum, we find iron lines chiefly

affected; at a maximum they are chiefly of unknown or unfamiliar

origin. At the present moment the affected lines are those recorded

in the spectra of vanadium and scandium, with others never seen in

a laboratory. That we are here far away from terrestrial chemical

conditions is evidenced by the fact that there is not a gram of

scandium available for laboratory use in the world at the present time.

Then we have the spectrum of the prominences and the chromosphere. That

spectrum we are enabled to observe every day when the sun shines, as

conveniently as we can observe that of sun spots. The chromosphere is

full of marvels. At first, when our knowledge of spectra was very much

more restricted than now, almost all the lines observed were unknown.

In 1868 I saw a line in the yellow, which I found behaved very much

like hydrogen, though I could prove that it was not due to hydrogen;

for laboratory use the substance which gave rise to it I called helium.

Next year I saw a line in the green at 1474 of Kirchhoff’s scale. That

was an unknown line, but in some subsequent researches I traced it to

iron. From that day to this we have observed a large number of lines.

They have gradually been dragged out from the region of the unknown,

and many are now recognized as enhanced lines, to which I have already

called attention as appearing in the spectra of metals at a very high

temperature.

But useful as the method of observing the chromosphere without an

eclipse, which enables us

“... to feel from world to world,”

as Tennyson has put it, has proved, we want an eclipse to see it face

to face.

A tremendous flood of light has been thrown upon it by the use of large

instruments constructed on a plan devised by Respighi and myself in

1871. These give us an image of the chromosphere painted in each one

of its radiations, so that the exact locus of each chemical layer is

revealed. One of the instruments employed during the Indian eclipse of

this year is that used in photographing the spectra of stars, so that

it is now easy to place photographs of the spectra of the chromosphere

obtained during a total eclipse and of the various stars side by side.

I have already pointed out that the chemical classification indicated

that the stars next above the sun in temperature are represented by γ

Cygni and Procyon, one on the ascending, the other on the descending

branch of the temperature curve.

Studying the spectra photographed during the eclipse of this year we

see that practically the lower part of the sun’s atmosphere, if present

by itself, would give us the lines which specialize the spectra of γ

Cygni or Procyon.

I recognize in this result a veritable Rosetta stone, which will enable

us to read the celestial hieroglyphics presented to us in stellar

spectra, and help us to study the spectra and to get at results much

more distinctly and certainly than ever before.

One of the most important conclusions we draw from the Indian eclipse

is that, for some reason or other, the lowest, hottest part of the

sun’s atmosphere does not write its record among the lines which build

up the general spectrum so effectively as does a higher one.

There was another point especially important on which we hoped for

information, and that was this: Up to the employment of the prismatic

camera insufficient attention had been directed to the fact that in

observations made by an ordinary spectroscope no true measure of the

height to which the vapors or gases extended above the sun could be

obtained; early observations, in fact, showed the existence of glare

between the observer and the dark moon; hence it must exist between us

and the sun’s surroundings.

The prismatic camera gets rid of the effects of this glare, and its

results indicate that the effective absorbing layer--that, namely,

which gives rise to the Fraunhofer lines--is much more restricted in

thickness than was to be gathered from the early observations.

We are justified in extending these general conclusions to all the

stars that shine in the heavens.

So much, then, in brief, for solar teachings in relation to the record

of the absorption of the lower parts of stellar atmospheres.

Let us next turn to the higher portions of the solar surroundings, to

see if we can get any effective help from them.

In this matter we are dependent absolutely upon eclipses, and I shall

fulfill my task very badly if I do not show you that the phenomena

then observable when the so-called corona is visible, full of awe and

grandeur to all, are also full of precious teaching to the student

of science. This also varies like the spots and prominences with the

sun-spot period.

It happened that I was the only person that saw both the eclipse of

1871 at the maximum of the sun-spot period and that of 1878 at minimum;

the corona of 1871 was as distinct from the corona of 1878 as anything

could be. In 1871 we got nothing but bright lines, indicating the

presence of gases; namely, hydrogen and another, since provisionally

called coronium. In 1878 we got no bright lines at all, so I stated

that probably the changes in the chemistry and appearance of the corona

would be found to be dependent upon the sun-spot period, and recent

work has borne out that suggestion.

I have now specially to refer to the corona as observed and

photographed this year in India by means of the prismatic camera,

remarking that an important point in the use of the prismatic camera is

that it enables us to separate the spectrum of the corona from that of

the prominences.

One of the chief results obtained is the determination of the position

of several lines of probably more than one new gas, which, so far, have

not been recognized as existing on the earth.

Like the lowest hottest layer, for some reason or other, this upper

layer does not write its record among the lines which build up the

general spectrum.

GENERAL RESULTS REGARDING THE LOCUS OF ABSORPTION IN STELLAR ATMOSPHERES

We learn from the sun, then, that the absorption which defines the

spectrum of a star is the absorption of a middle region, one shielded

both from the highest temperature of the lowest reaches of the

atmosphere, where most tremendous changes are continually going on and

the external region where the temperature must be low, and where the

metallic vapors must condense.

If this is true for the sun it must be equally true for Arcturus,

which exactly resembles it. I go further than this, and say that in

the presence of such definite results as those I have brought before

you it is not philosophical to assume that the absorption may take

place at the bottom of the atmosphere of one star or at the top of the

atmosphere of another. The _onus probandi_ rests upon those who

hold such views.

So far I have only dealt in detail with the hotter stars, but I have

pointed out that we have two distinct kinds of coolest ones, the

evidence of their much lower temperature being the shortness of their

spectra. In one of these groups we deal with absorption alone, as in

those already considered; we find an important break in the phenomena

observed; helium, hydrogen, and metals have practically disappeared,

and we deal with carbon absorption alone.

But the other group of coolest stars presents us with quite new

phenomena. We no longer deal with absorption alone, but accompanying

it we have radiation, so that the spectra contain both dark lines and

bright ones. Now, since such spectra are visible in the case of new

stars, the ephemera of the skies, which may be said to exist only for

an instant relatively, and when the disturbance which gives rise to

their sudden appearance has ceased, we find their places occupied by

nebulæ, we cannot be dealing here with stars like the sun, which has

already taken some millions of years to slowly cool, and requires more

millions to complete the process into invisibility.

The bright lines seen in the large number of permanent stars which

resemble these fleeting ones--new stars, as they are called--are those

discerned in the once mysterious nebulæ which, so far from being stars,

were supposed not many years ago to represent a special order of

created things.

Now the nebulæ differ from stars generally in the fact that in their

spectra we have practically to deal with radiation alone; we study them

by their bright lines; the conditions which produce the absorption by

which we study the chemistry of the hottest stars are absent.

A NEW VIEW OF STARS

Here, then, we are driven to the perfectly new idea that some of the

cooler bodies in the heavens, the temperature of which is increasing

and which appear to us as stars, are really disturbed nebulæ.

What, then, is the chemistry of the nebulæ? It is mainly gaseous;

the lines of helium and hydrogen and the flutings of carbon, already

studied by their absorption in the groups of stars to which I have

already referred, are present as bright ones.

The presence of the lines of the metals iron, calcium, and probably

magnesium, shows us that we are not dealing with gases merely.

Of the enhanced metallic lines there are none; only the low temperature

lines are present, so far as we yet know. The temperature, then, is

low, and lowest of all in those nebulæ where carbon flutings are seen

almost alone.

A NEW VIEW OF NEBULÆ

Passing over the old views, among them one that the nebulæ were holes

in something dark which enabled us to see something bright beyond, and

another that they were composed of a fiery fluid, I may say that not

long ago, they were supposed to be masses of gases only, existing at a

very high temperature.

Now, since gases may glow at a low temperature as well as at a high

one, the temperature evidence must depend upon the presence of cool

metallic lines and the absence of the enhanced ones.

The nebulæ, then, are relatively cool collections of some of the

permanent gases and of some cool metallic vapors, and both gases and

metals are precisely those I have referred to as writing their records

most visibly in stellar atmosphere.

Now, can we get more information concerning this association of certain

gases and metals? In laboratory work it is abundantly recognized that

all meteorites (and many minerals) when slightly heated give out

permanent gases, and under certain conditions the spectrum of the

nebulæ may in this way be closely approximated to. I have not time to

labor this point, but I may say that a discussion of all the available

observations to my mind demonstrates the truth of the suggestion, made

many years ago by Professor Tait before any spectroscopic facts were

available, that the nebulæ are masses of meteorites rendered hot by

collisions.

Surely human knowledge is all the richer for this indication of the

connection between the nebulæ, hitherto the most mysterious bodies in

the skies, and the “stones that fall from heaven.”

CELESTIAL EVOLUTION

But this is, after all, only a stepping stone, important though it be.

It leads us to a vast generalization. If the nebulæ are thus composed,

they are bound to condense to centers, however vast their initial

proportions, however irregular the first distribution of the cosmic

clouds which compose them. Each pair of meteorites in collision puts us

in mental possession of what the final stage must be. We begin with a

feeble absorption of metallic vapors round each meteorite in collision;

the space between the meteorites is filled with the permanent gases

driven out farther afield and having no power to condense. Hence

dark metallic and bright gas lines. As time goes on the former must

predominate, for the whole swarm of meteorites will then form a gaseous

sphere with a strongly heated center, the light of which will be

absorbed by the exterior vapor.

The temperature order of the group of stars with bright lines as well

as dark ones in their spectra has been traced, and typical stars

indicating the chemical changes have been as carefully studied as those

in which absorption phenomena are visible alone, so that now there are

no breaks in the line connecting the nebulæ with the stars on the verge

of extinction.

Here we are brought to another tremendous outcome--that of the

evolution of all cosmical bodies from meteorites, the various stages

recorded by the spectra being brought about by the various conditions

which follow from the conditions.

These are, shortly, that at first collisions produce luminosity among

the colliding particles of the swarm, and the permanent gases are given

off and fill the interspaces. As condensation goes on, the temperature

at the center of condensation always increasing, all the meteorites

in time are driven into a state of gas. The meteoritic bombardment

practically now ceases for lack of material, and the future history

of the mass of gas is that of a cooling body, the violent motions in

the atmosphere while condensation was going on now being replaced by a

relative calm.

The absorption phenomena in stellar spectra are not identical at

the same mean temperature on the ascending and descending sides of

the curve, on account of the tremendous difference in the physical

conditions.

In a condensing swarm, the center of which is undergoing meteoritic

bombardment from all sides, there cannot be the equivalent of the

solar chromosphere; the whole mass is made up of heterogeneous vapor

at different temperatures and moving with different velocities in

different regions.

In a condensed swarm, of which we can take the sun as a type, all

action produced from without has practically ceased; we get relatively

a quiet atmosphere and an orderly assortment of the vapors from top to

bottom, disturbed only by the fall of condensed metallic vapors. But

still, on the view that the differences in the spectra of the heavenly

bodies chiefly represent differences in degree of condensation and

temperature, there can be _au fond_, no great chemical difference

between bodies of increasing and bodies of decreasing temperature.

Hence we find at equal mean temperatures on opposite sides of the

temperature curve this chemical similarity of the absorbing vapors

proved by many points of resemblance in the spectra, especially the

identical behavior of the enhanced metallic and cleveite lines.

CELESTIAL DISSOCIATION

The time you were good enough to put at my disposal is now exhausted,

but I cannot conclude without stating that I have not yet exhausted

all the conceptions of a high order to which Fraunhofer’s apparently

useless observation has led us.

The work which to my mind has demonstrated the evolution of the cosmos

as we know it from swarms of meteorites, has also suggested a chemical

evolution equally majestic in its simplicity.

A quarter of a century ago I pointed out that all the facts then

available suggested the hypothesis that in the atmospheres of the sun

and stars various degrees of “celestial dissociation” were at work,

a “dissociation” which prevented the coming together of the finest

particles of matter which at the temperature of the earth and at all

artificial temperature yet attained here compose the metals, the

metalloids and compounds.

On this hypothesis the so-called atoms of the chemist represent not the

origins of things, but only early stages of the evolutionary process.

At the present time we have tens of thousands of facts which were not

available twenty-five years ago. All these go to the support of the

hypothesis, and among them I must indicate the results obtained at the

last eclipse, dealing with the atmosphere of the sun in relation to

that of the various stars of higher temperature to which I called your

attention. In this way we can easily explain the enhanced lines of iron

existing practically alone in Alpha Cygni. I have yet to learn any

other explanation.

I have nothing to take back, either from what I then said or what I

have said since on this subject, and although the view is not yet

accepted, I am glad to know that many other lines of work which are now

being prosecuted tend to favor it.

I have no hesitation in expressing my conviction that in a not distant

future the inorganic evolution to which we have been finally led by

following up Fraunhofer’s useless experiment will take its natural

place side by side with that organic evolution, the demonstration of

which has been one of the glories of the nineteenth century.

And finally now comes the moral of my address. If I have helped to show

that observations having no immediate practical bearing may yet help

on the thought of mankind, and that this is a thing worth the doing,

let me express a hope that such work shall find no small place in the

future University of Birmingham.

FOOTNOTES:

[Footnote 38: From an address delivered at the University of Birmingham

(1900).]

XXXVI

ROBERT KOCH

1843-1910

_Robert Koch, born at Klausthal, Hanover, Germany, December 11,

1843, graduated from Göttingen in 1866. After a short period as

assistant surgeon in the General Hospital in Hamburg, he practised

medicine at Langenhagen, Kackwitz, and Wollstein from 1872 to 1880,

during which time he began his researches in bacteriology. By 1878 he

had placed bacteriology on a scientific basis. In 1880 he was called

to Berlin as chief of the Sanitary Institute, where he continued his

studies of tuberculosis and cholera. After inventing new microscopical

appliances and a new technique, in 1882 he stated his discovery of

the tubercle bacillus. In 1883, after publishing a method for the

prevention of anthrax by inoculation, he was sent by his government

to Egypt and India to investigate cholera. During that work he

discovered the cholera bacillus. In 1884 he returned to Germany and

in the following year went to France as cholera commissioner. In 1888

he published a paper on the prophylaxis of infectious diseases in the

army. In later years he investigated the bubonic plague, malaria, and

sleeping-sickness. He died at Baden-Baden, May 28, 1910._

THEORY OF BACTERIA[39]

I am well aware that the investigations above described are very

imperfect. It was necessary, in order to have time for those parts

of the investigation which seemed the most important and essential,

to omit the examination of many organs, such as the brain, heart,

retina, etc., which ought not to pass unnoticed in researches on

infective diseases. For the same reason no record was kept of the

temperature, although this would undoubtedly have yielded most

interesting results. I have intentionally refrained from entering into

details of morbid anatomy, as only the etiology interested me, and as

I did not feel qualified to undertake a study of the morbid anatomy of

traumatic infective diseases. I must therefore leave this part of the

investigation to those who are better able to undertake it.

Nevertheless I consider that the results of my researches are

sufficiently definite to enable me to deduce from them some well

founded conclusions.

In this summary I shall, however, confine myself to the most obvious

conclusions. It has indeed of late become too common to draw the most

sweeping conclusions as to infective diseases in general from the

most unimportant observations on bacteria. I shall not follow this

custom, although the material at my command would furnish rich food

for meditation. For the longer I study infective diseases the more am

I convinced that generalisations of new facts are here a mistake, and

that every individual infective disease or group of closely allied

diseases must be investigated for itself.

As regards the artificial traumatic infective diseases observed by me,

the conditions which must be established before their parasitic nature

can be proved, we completely fulfilled in the case of the first five,

but only partially in that of the sixth. For the infection was produced

by such small quantities of fluid (blood, serum, pus, etc.) that the

result cannot be attributed to a merely chemical poison.

In the materials used for inoculation bacteria were without exception

present, and in each disease a different and well marked form of

organism could be demonstrated.

At the same time, the bodies of those animals which died of the

artificial traumatic infective diseases contained bacteria in

such numbers that the symptoms and the death of the animals were

sufficiently explained. Further, the bacteria found were identical

with those which were present in the fluid used for inoculation, and a

definite form of organisms corresponded in every instance to a distinct

disease.

These artificial traumatic infective diseases bear the greatest

resemblance to human traumatic infective diseases, both as regards

their origin from putrid substances, their course, and the result of

post-mortem examination. Further, in the first case, just as in the

last, the parasitic organisms could be only imperfectly demonstrated

by the earlier methods of investigation; not till an improved method of

procedure was introduced was it possible to obtain complete proof that

they were parasitic diseases. We are therefore justified in assuming

that human traumatic infective diseases will in all probability be

proved to be parasitic when investigated by these improved methods.

On the other hand, it follows from the fact that a definite pathogenic

bacterium, e. g., the septicæmic bacillus, cannot be inoculated on

every variety of animal (a similar fact is also true with regard to the

bacillus anthracis); that the septicæmia of mice, rabbits, and man are

not under all circumstances produced by the same bacterial form. It is

of course possible that one or other of the bacteric forms found in

animals also play a part in such diseases in the human subject. That,

however, must be especially demonstrated for each case; _a priori_

one need only expect that bacteria are present; as regards form, size

and conditions of growth, they may be similar, but not always the same,

even in what appear to be similar diseases in different animals.

Besides the pathogenic bacteria already found in animals there are no

doubt many others. My experiments refer only to those diseases which

ended fatally. Even these are in all probability not exhausted in the

six forms mentioned. Further experiments on many different species

of animals, with the most putrid substances and with every possible

modification in the method of application, will doubtless bring to

light a number of other infective diseases, which will lead to further

conclusions regarding infective diseases and pathogenic bacteria.

But even in the small series of experiments which I was able to carry

out, one fact was so prominent that I must regard it as constant,

and, as it helps to remove most of the obstacles to the admission of

the existence of a centagium vivum for traumatic infective diseases,

I look on it as the most important result of my work. I refer to

the differences which exist between pathogenic bacteria and to the

constancy of their characters. A distinct bacteric form corresponds, as

we have seen, to each disease, and this form always remains the same,

however often the disease is transmitted from one animal to another.

Further, when we succeed in reproducing the same disease _de novo_

by the injection of putrid substances, only the same bacteric form

occurs which was before found to be specific for that disease.

Further, the differences between these bacteria are as great as could

be expected between particles which border on the invisible. With

regard to these differences, I refer not only to the size and form

of the bacteria, but also to the conditions of their growth, which

can be best recognized by observing their situation and grouping. I

therefore study not only the individual alone, but the whole group of

bacteria, and would, for example, consider a micrococcus which in one

species of animal occurred only in masses (i. e., in a zooglæa form),

as different from another which in the same variety of animal, under

the same conditions of life, was only met with as isolated individuals.

Attention must also be paid to the physiological effect, of which I

scarcely know a more striking example than the case of the bacillus

and the chain-like micrococcus growing together in the cellular tissue

of the ear; the one passing into the blood and penetrating into the

white blood corpuscles, the other spreading out slowly into the tissues

in its vicinity and destroying everything around about; or again, the

case of the septicæmic and pyæmic micrococci of the rabbit in their

different relations to the blood; or lastly, the bacilli only extending

over the surface of the aural cartilage in the erysipetalous disease,

as contrasted with the bacillus anthracis, likewise inoculated on the

rabbit’s ear, but quickly passing into the blood.

As, however, there corresponds to each of the diseases investigated a

form of bacterium distinctly characterized by its physiological action,

by its conditions of growth, size, and form, which, however often the

disease be transmitted from one animal to another, always remains the

same and never passes over into another form, e. g., from the spherical

to the rod shaped, we must in the meantime regard these different forms

of pathogenic bacteria as distinct and constant species.

This is, however, an assertion that will be much disputed by botanists,

to whose special province this subject really belongs.

Amongst those botanists who have written against the subdivision of

bacteria into species, is Nägeli, who says, “I have for ten years

examined thousands of different forms of bacteria, and I have not yet

seen any absolute necessity for dividing them even into two distinct

species.”

Brefeld also states that he can only admit the existence of specific

forms justifying the formation of distinct species when the whole

history of development has been traced by cultivation from spore to

spore in the most nutritive fluids.

Although Brefeld’s demand is undoubtedly theoretically correct it

cannot be made a _sine qua non_ in every investigation on

pathogenic bacteria. We should otherwise be compelled to cease our

investigations into the etiology of infective diseases till botanists

have succeeded in finding out the different species of bacteria by

cultivation and development from spore to spore. It might then very

easily happen that the endless trouble of pure cultivation would be

expended on some form of bacterium which would finally turn out to be

scarcely worthy of attention. In practice only the opposite method can

work. In the first place certain peculiarities of a particular form of

bacterium different from those of other forms, and in the second place

its constancy, compel us to separate it from other less known and less

interesting, and provisionally to regard it as a species. And now, to

verify this provisional supposition, the cultivation from spore to

spore may be undertaken. If this succeeds under conditions which cut

out all sources of fallacy, and if it furnishes a result corresponding

to that obtained by the previous observations, then the conclusions

which were drawn from these observations and which led to its being

ranked as a distinct species must be regarded as valid.

On this, which as it seems to me is the only correct practical method,

I take my stand, and, till the cultivation of bacteria from spore to

spore shows that I am wrong, I shall look on pathogenic bacteria as

consisting of different species.

In order, however, to show that I do not stand alone in this view, I

shall here mention the opinion of some botanists who have already come

to a similar conclusion.

Cohn states that, in spite of the fact that many dispute the necessity

of separating bacteria into genera or species, he must nevertheless

adhere to the method as yet followed by him, and separate bacteria

of a different form and fermenting power from each other, so long as

complete proof of their identity is not given.

From his investigations on the effects of different temperatures and

of desiccation on the development of bacterium termo, Eidam came to

the conclusion that different forms of bacteria require different

conditions of nutriment, and that they behave differently towards

physical and chemical influences. He regards these facts as a further

proof of the necessity of dividing organisms into distinct species.

I shall bring forward another reason to show the necessity of looking

on the pathogenic bacteria which I have described as distinct species.

The greatest stress, in investigations on bacteria, is justly laid on

the so-called pure cultivations, in which only one definite form of

bacterium is present. This evidently arises from the view that if, in a

series of cultivations, the same form of bacterium is always obtained,

a special significance must attach to this form: it must indeed be

accepted as a constant form, or in a word as a species. Can, then,

a series of pure cultivations be carried out without admixture of

other bacteria? It can in truth be done, but only under very limited

conditions. Only such bacteria can be cultivated pure, with the aids

at present at command, which can always be known to be pure, either by

their size and easily recognizable form, as the bacillus anthracis, or

by the production of a characteristic coloring matter as the pigment

bacteria. When, during a series of cultivations, a strange species of

bacteria has by chance got in, as may occasionally happen under any

circumstances, it will in these cases be at once observed, and the

unsuccessful experiment will be thrown out of the series without the

progress of investigation being thereby necessarily interfered with.

But the case is quite different when attempts are made to carry

out cultivations of very small bacteria, which, perhaps, cannot be

distinguished at all without staining; how are we then to discover the

occurrence of contamination? It is impossible to do so, and therefore

all attempts at pure cultivation in apparatus, however skilfully

planned and executed, must, as soon as small bacteria with but little

characteristic appearances are dealt with, be considered as subject to

unavoidable sources of fallacy, and in themselves inconclusive.

But nevertheless a pure cultivation is possible, even in the case

of the bacteria which are smallest and most difficult to recognise.

This, however, is not conducted in cultivation apparatus, but in

the animal body. My experiments demonstrate this. In all the cases

of a distinct disease, e. g., of septicæmia of mice, only the small

bacilli were present, and no other form of bacterium was ever found

with it, unless in the case where that causing the tissue gangrene was

intentionally inoculated at the same time. In fact, there exists no

better cultivation apparatus for pathogenic bacteria than the animal

body itself. Only a very limited number of bacteria can grow in the

body, and the penetration of organisms into it is so difficult that

the uninjured living body may be regarded as completely isolated

with respect to other forms of bacteria than those intentionally

introduced. It is quite evident, from a careful consideration of

the two diseases produced in mice--septicæmia and gangrene of the

tissue--that I have succeeded in my experiments in obtaining a pure

cultivation. In the putrefying blood, which was the cause of these two

diseases, the most different forms of bacteria were present, and yet

only two of these found in the living mouse the conditions necessary

for their existence. All the others died, and these two alone, a small

bacillus and a chain-like micrococcus, remained and grew. These could

be transferred from one animal to another as often as was desired,

without suffering any alteration in their characteristic form, in

their specific physiological action and without any other variety of

bacteria at any time appearing. And further, as I have demonstrated, it

is quite in the power of the experimenter to separate these two forms

of bacteria from each other. When the blood in which only the bacilli

are present is used, these alone are transmitted, and thenceforth are

obtained quite pure; while on the other hand, when a field mouse is

inoculated with both forms of bacteria, the bacilli disappear, and

the micrococcus can be then cultivated pure. Doubtless an attempt to

unite these two forms again in the same animal by inoculation would

have been successful. In short, one has it completely in one’s power

to cultivate several varieties of bacteria together, to separate them

from each other, and eventually to combine them again. Greater demands

can hardly be made on a pure cultivation, and I must therefore regard

the successive transmission of artificial infective diseases as the

best and surest method of pure cultivation. And it can further claim

the same power of demonstrating the existence of specific forms of

bacteria, as must be conceded to any faultless cultivation experiments.

From the fact that the animal body is such an excellent apparatus for

pure cultivation, and that, as we have seen, when the experiments are

properly arranged and sufficient optical aids used, only one specific

form of bacterium can be found in each distinct case of artificial

traumatic infective disease, we may now further conclude that when, in

examining a traumatic infective disease, several different varieties

of bacteria are found, as e. g., chains of small granules, rods, and

long, oscillating threads--such as were seen together by Coze and Feltz

in the artificial septicæmia of rabbits--we have to do either with a

combined infective disease,--that is, not a pure one,--or, what in the

case cited is more probable, an inexact and inaccurate observation.

When, therefore, several species of bacteria occur together in any

morbid process, before definite conclusions are drawn as to the

relations of the disease in question to the organisms, either proof

must be furnished that they are all concerned in the morbid process,

or an attempt must be made to isolate them and to obtain a true

pure cultivation. Otherwise we cannot avoid the objection that the

cultivation was not pure, and therefore not conclusive. I shall only

briefly refer to a further necessary consequence of the admission of

the existence of different species of pathogenic bacteria. The number

of the species of these bacteria is limited; for, of the numerous

diverse forms present in putrid fluids, one or but few can in the most

favorable cases develop in the animal body. Those which disappear

are, for that species of animal at least, not pathogenic bacteria.

If, however, as follows from the foregoing, there exist hurtful and

harmless bacteria, experiments performed on animals with the latter,

e. g., with bacterium termo, prove absolutely nothing for or against

the behavior of the former--the pathogenic--forms. But almost all the

experiments of this nature have been carried out with the first mixture

of different species of bacteria which came to hand without there being

any certainty that pathogenic bacteria were in reality present in the

mixture. It is therefore evident that none of these experiments can

be regarded as furnishing evidence of any value for or against the

parasitic nature of infective diseases.

In all my experiments, not only have the form and size of the bacteria

been constant, but the greatest uniformity in their actions on the

animal organisms has been observed, though no increase of virulence, as

described by Coze and Feltz, Davaine, and others. This leads me to make

some remarks on the supposed law of the increasing virulence of blood

when transmitted through successive animals, discovered or confirmed by

the investigators just named.

The discovery of this law has, as is well known, been received with

great enthusiasm, and it has excited no little interest owing to its

intimate bearing on the doctrine of natural selection (Anpassung and

Vererbung). Some investigators, who are in other things very exact,

have allowed themselves to be blinded by the seductive theory that

the insignificant action of a single putrefactive bacterium may, by

continued natural selection in passing from animal to animal, be

increased in virulence till it becomes deadly though a drop of the

infective liquid be diluted in a quadrillion times. They have founded

thereon the most beautiful practical applications, not suspecting that

the bacteria in question have never been certainly demonstrated.

The original works of Coze and Feltz, as also that of Davaine, are

not at my disposal for reference; and I cannot therefore enter into

a complete criticism of them. So far, however, as I can gather from

the references accessible to me, especially from the detailed notices

in Virchow and Hirch’s “Jahnesbericht,” no complete proof that the

virulence of septicæmic blood increases from generation to generation

seems to have been furnished. Apparently blood more and more diluted

was injected, and astonishment was felt when this always acted, the

effect being then ascribed to its increasing virulence. But controlling

experiments to ascertain whether the septicæmic blood were not already

as virulent in the second and third generations as in the twenty-fifth,

do not seem to have been made. My experiments so far support and are in

accordance with those of Coze, Feltz, and Davaine in that for the first

infection of an animal relatively large quantities of putrid fluid are

necessary; but in the second generation, or at the latest in the third,

the full virulence was attained, and afterwards remained constant.

Of my artificial infective diseases the septicæmia of the mouse has

the greatest correspondence with the artificial septicæmia described

by Davaine. If we were to experiment with this disease in the same

manner as Davaine experimented, we would, if no controlling experiments

were employed, find the same increase in virulence of the disease. It

would only be necessary to use blood in slowly decreasing quantities in

order to obtain in this way any progressive increase of the virulence

that might be desired. I, however, took from the second or third

animal the smallest possible quantity of material for inoculation, and

thus arrived more quickly at the greatest degree of virulence. Till,

therefore, I am assured that, in the septicæmia observed by Davaine,

such controlling experiments were made, I can only look on an increase

in virulence as holding good for the earlier generations. In order

to explain this we do not, however, require to have recourse to the

magical wand of natural selection; a feasible explanation can be very

naturally furnished. Let us take again the septicæmia of mice, as being

the most suitable example.

If two drops of putrefying blood be injected into such an animal

there is introduced not only a number of totally distinct species

of bacteria, but also a certain amount of dissolved putrid poison

(sepsin), not sufficient to produce a fatal effect, but yet certainly

not without influence on the health of the animal. Different factors

must therefore be considered as affecting the health of the animal. On

the one hand there is the dissolved poison, on the other the different

species of bacteria, of which, however, perhaps only two, as in the

example before us, can multiply in the body of the mouse and there

exert a continuous noxious influence. Only one of these two species can

penetrate into the blood, and if the blood alone be used for further

inoculations, only this one variety will come victorious out of the

battle for existence. The further development of the experiment depends

entirely on the quantity of the putrid poison, and on the relation

of the two forms of bacteria to each other in point of numbers. If

one injects a large amount of septic poison and a large number of

that variety of bacteria which increase locally (in this case the

chain-like micrococci causing the gangrene of the tissue), but only a

very small number of the bacteria which pass into the blood (here the

bacilli), the first animal experimented on will die, as a result of the

preponderation influence of the first two factors before many bacilli

can have got into the blood and multiplied there. Of the blood of this

first animal, containing, as it does, proportionately very few bacilli,

one-fifth to one-tenth of a drop must be inoculated in order to convey

the disease with certainty. In the second animal, however, only the

bacilli are introduced, and these develop undisturbed in the blood. For

the infection of the third animal the smallest quantity of this blood

which can produce an effect is then sufficient, and after this third

generation the virulence of the blood remains uniform.

We may also imagine another case in which the increase of the virulence

may go on through more than two generations without any modification

resulting from natural selection and transmission from animal to

animal. This would take place if several species of bacteria capable

of passing into the blood were introduced into the animal at the first

injection. Let us suppose, for example, that in the same putrefying

blood which served for the foregoing experiment, the bacilli of

anthrax were also present, there would then be contained in the blood

of the first animal not only the septicæmic bacillus, but also

bacillus anthracis, and of each only a small number; of the anthrax

bacilli there would be even fewer than of the other, because in mice

they are deposited chiefly in the spleen, lungs, etc.; while in the

blood of the heart they are, even in the most favorable cases, only

sparsely distributed. On the other hand, the anthrax bacilli have

this advantage, that, provided they be inoculated in considerable

numbers, they kill even within twenty hours, while the septicæmic

bacilli only destroy life after fifty hours. In the blood of the second

animal, therefore, both species of bacilli would be present in larger

numbers than in the first, although not yet so numerous as if either

organism had been inoculated singly. Hence a larger quantity of blood

is necessary to ensure transmission to a third animal. Perhaps this

might be the case even in the fourth generation, till finally one or

other variety of bacillus would alone be present in the blood injected.

Probably this would be the septicæmic bacillus.

In this way the experiments of Coze, Feltz, and Davaine may admit of

simple explanation and be brought into harmony with my results.

FOOTNOTES:

[Footnote 39: From the English translation (1880) of _Untersuchungen

über die Aetiologie der Wundinfectionskrankheiten_ (1878).]

=TRANSCRIBER’S NOTES=

Simple typographical errors have been silently corrected; unbalanced

quotation marks were remedied when the change was obvious, and

otherwise left unbalanced.

Punctuation, hyphenation, and spelling were made consistent when a

predominant preference was found in the original book; otherwise they

were not changed.