Several writers have misapprehended or objected to the term Natural

Selection. Some have even imagined that natural selection induces

variability, whereas it implies only the preservation of such

variations as arise and are beneficial to the being under its

conditions of life. No one objects to agriculturists speaking of the

potent effects of man’s selection; and in this case the individual

differences given by nature, which man for some object selects, must of

necessity first occur. Others have objected that the term selection

implies conscious choice in the animals which become modified; and it

has even been urged that, as plants have no volition, natural selection

is not applicable to them! In the literal sense of the word, no doubt,

natural selection is a false term; but who ever objected to chemists

speaking of the elective affinities of the various elements?--and yet

an acid cannot strictly be said to elect the base with which it in

preference combines. It has been said that I speak of natural selection

as an active power or Deity; but who objects to an author speaking

of the attraction of gravity as ruling the movements of the planets?

Everyone knows what is meant and is implied by such metaphorical

expressions; and they are almost necessary for brevity. So again it is

difficult to avoid personifying the word Nature; but I mean by Nature,

only the aggregate action and product of many natural laws, and by laws

the sequence of events as ascertained by us. With a little familiarity

such superficial objections will be forgotten.

We shall best understand the probable course of natural selection by

taking the case of a country undergoing some slight physical change,

for instance, of climate. The proportional numbers of its inhabitants

will almost immediately undergo a change, and some species will

probably become extinct. We may conclude, from what we have seen of the

intimate and complex manner in which the inhabitants of each country

are bound together, that any change in the numerical proportions of

the inhabitants, independently of the change of climate itself, would

seriously affect the others. If the country were open on its borders,

new forms would certainly immigrate, and this would likewise seriously

disturb the relations of some of the former inhabitants. Let it be

remembered how powerful the influence of a single introduced tree

or mammal has been shown to be. But in the case of an island, or of

a country partly surrounded by barriers, into which new and better

adapted forms could not freely enter, we should then have places in the

economy of nature which would assuredly be better filled up, if some

of the original inhabitants were in some manner modified; for, had the

area been open to immigration, these same places would have been seized

on by intruders. In such cases, slight modifications, which in any

way favoured the individuals of any species, by better adapting them

to their altered conditions, would tend to be preserved; and natural

selection would have free scope for the work of improvement.

We have good reason to believe, as shown in the first chapter, that

changes in the conditions of life give a tendency to increased

variability; and in the foregoing cases the conditions have changed,

and this would manifestly be favourable to natural selection, by

affording a better chance of the occurrence of profitable variations.

Unless such occur, natural selection can do nothing. Under the term

of “variations,” it must never be forgotten that mere individual

differences are included. As man can produce a great result with

his domestic animals and plants by adding up in any given direction

individual differences, so could natural selection, but far more easily

from having incomparably longer time for action. Nor do I believe

that any great physical change, as of climate, or any unusual degree

of isolation to check immigration, is necessary in order that new and

unoccupied places should be left for natural selection to fill up by

improving some of the varying inhabitants. For as all the inhabitants

of each country are struggling together with nicely balanced forces,

extremely slight modifications in the structure or habits of one

species would often give it an advantage over others; and still further

modifications of the same kind would often still further increase the

advantage, as long as the species continued under the same conditions

of life and profited by similar means of subsistence and defense. No

country can be named in which all the native inhabitants are now so

perfectly adapted to each other and to the physical conditions under

which they live, that none of them could be still better adapted or

improved; for in all countries, the natives have been so far conquered

by naturalized productions, that they have allowed some foreigners to

take firm possession of the land. And as foreigners have thus in every

country beaten some of the natives, we may safely conclude that the

natives might have been modified with advantage, so as to have better

resisted the intruders.

As man can produce, and certainly has produced, a great result by his

methodical and unconscious means of selection, what may not natural

selection effect? Man can act only on external and visible characters:

Nature, if I may be allowed to personify the natural preservation or

survival of the fittest, cares nothing for appearances, except in so

far as they are useful to any being. She can act on every internal

organ, on every shade of constitutional difference, on the whole

machinery of life. Man selects only for his own good: Nature only for

that of the being which she tends. Every selected character is fully

exercised by her, as is implied by the fact of their selection. Man

keeps the natives of many climates in the same country; he seldom

exercises each selected character in some peculiar and fitting manner;

he feeds a long and a short-beaked pigeon on the same food; he does

not exercise a long-backed or long-legged quadruped in any peculiar

manner; he exposes sheep with long and short wool to the same climate.

He does not allow the most vigorous males to struggle for the females.

He does not rigidly destroy all inferior animals, but protects during

each varying season, as far as lies in his power, all his productions.

He often begins his selection by some half-monstrous form; or at

least by some modification prominent enough to catch the eye or to

be plainly useful to him. Under nature, the slightest differences of

structure or constitution may well turn the nicely-balanced scale in

the struggle for life, and so be preserved. How fleeting are the wishes

and efforts of man! how short his time! and consequently how poor will

be his results, compared with those accumulated by Nature during whole

geological periods! Can we wonder, then, that Nature’s productions

should be far “truer” in character than man’s productions; that they

should be infinitely better adapted to the most complex conditions of

life, and should plainly bear the stamp of far higher workmanship?

It may metaphorically be said that natural selection is daily and

hourly scrutinizing, throughout the world, the slightest variations;

rejecting those that are bad, preserving and adding up all that are

good; silently and sensibly working, whenever and wherever opportunity

offers, at the improvement of each organic being in relation to its

organic and inorganic conditions of life. We see nothing of these slow

changes in progress, until the hand of time has marked the lapse of

ages, and then so imperfect is our view into long-past geological ages,

that we see only that the forms of life are now different from what

they formerly were.

In order that any great amount of modification should be effected in

a species, a variety when once formed must again, perhaps after a

long interval of time, vary or present individual differences of the

same favourable nature as before; and these must be again preserved,

and so onwards step by step. Seeing that individual differences of

the same kind perpetually recur, this can hardly be considered as an

unwarrantable assumption. But whether it is true, we can judge only by

seeing how far the hypothesis accords with and explains the general

phenomena of nature. On the other hand, the ordinary belief that the

amount of possible variation is a strictly limited quantity is likewise

a simple assumption.

Although natural selection can act only through and for the good of

each being, yet characters and structures, which we are apt to consider

as of very trifling importance, may thus be acted on. When we see

leaf-eating insects green, and bark-feeders mottled gray; the Alpine

ptarmigan white in winter, the red-grouse the colour of heather,

we must believe that these tints are of service to these birds and

insects in preserving them from danger. Grouse, if not destroyed at

some period of their lives, would increase in countless numbers;

they are known to suffer largely from birds of prey; and hawks are

guided by eyesight to their prey--so much so, that on parts of the

Continent persons are warned not to keep white pigeons, as being the

most liable to destruction. Hence natural selection might be effective

in giving the proper colour to each kind of grouse, and in keeping

that colour, when once acquired, true and constant. Nor ought we to

think that the occasional destruction of an animal of any particular

colour would produce little effect: we should remember how essential

it is in a flock of white sheep to destroy a lamb with the faintest

trace of black. We have seen how the colour of the hogs, which feed on

the “paint-root” in Virginia, determines whether they shall live or

die. In plants, the down on the fruit and the colour of the flesh are

considered by botanists as characters of the most trifling importance:

yet we hear from an excellent horticulturist, Downing, that in the

United States smooth-skinned fruits suffer far more from a beetle, a

Curculio, than those with down; that purple plums suffer far more from

a certain disease than yellow plums; whereas another disease attacks

yellow-fleshed peaches far more than those with other coloured flesh.

If, with all the aids of arts, these slight differences make a great

difference in cultivating the several varieties, assuredly, in a state

of nature, where the trees would have to struggle with other trees and

with a host of enemies, such differences would effectually settle which

variety, whether a smooth or downy, a yellow or purple-fleshed fruit,

should succeed.

In looking at many small points of difference between species, which,

as far as our ignorance permits us to judge, seem quite unimportant,

we must not forget that climate, food, etc., have no doubt produced

some direct effect. It is also necessary to bear in mind that, owing to

the law of correlation, when one part varies, and the variations are

accumulated through natural selection, other modifications, often of

the most unexpected nature, will ensue.

As we see that those variations which, under domestication, appear at

any particular period of life, tend to reappear in the offspring at the

same period; for instance, in the shape, size, and flavour of the seeds

of the many varieties of our culinary and agricultural plants; in the

caterpillar and cocoon stages of the varieties of the silkworm; in the

eggs of poultry, and in the colour of the down of their chickens; in

the horns of our sheep and cattle when nearly adult; so in a state of

nature natural selection will be enabled to act on and modify organic

beings at any age, by the accumulation of variations profitable at that

age, and by their inheritance at a corresponding age. If it profit

a plant to have its seeds more and more widely disseminated by the

wind, I can see no greater difficulty in this being effected through

natural selection, than in the cotton planter increasing and improving

by selection the down in the pods on his cotton trees. Natural

selection may modify and adapt the larva of an insect to a score of

contingencies, wholly different from those which concern the mature

insect; and these modifications may effect, through correlation, the

structure of the adult. So, conversely, modifications in the adult may

affect the structure of the larva; but in all cases natural selection

will insure that they shall not be injurious: for if they were so, the

species would become extinct.

Natural selection will modify the structure of the young in relation

to the parent, and of the parent in relation to the young. In social

animals it will adapt the structure of each individual for the benefit

of the whole community; if the community profits by the selected

change. What natural selection cannot do, is to modify the structure

of one species; without giving it any advantage, for the good of

another species; and though statements to this effect may be found

in works of natural history, I cannot find one case which will bear

investigation. A structure used only once in an animal’s life, if

of high importance to it, might be modified to any extent by natural

selection; for instance, the great jaws possessed by certain insects,

used exclusively for opening the cocoon--or the hard tip of the beak of

unhatched birds, used for breaking the egg. It has been asserted, that

of the best short-beaked tumbler-pigeons a greater number perish in the

egg than are able to get out of it; so that fanciers assist in the act

of hatching. Now if nature had to make the beak of a full-grown pigeon

very short for the bird’s own advantage, the process of modification

would be very slow, and there would be simultaneously the most rigorous

selection of all the young birds within the egg, which had the most

powerful and hardest beaks, for all with weak beaks would inevitably

perish; or, more delicate and more easily broken shells might be

selected, the thickness of the shell being known to vary like every

other structure.

It may be well here to remark that with all beings there must be much

fortuitous destruction, which can have little or no influence on

the course of natural selection. For instance a vast number of eggs

or seeds are annually devoured, and these could be modified through

natural selection only if they varied in some manner which protected

them from their enemies. Yet many of these eggs or seeds would perhaps,

if not destroyed, have yielded individuals better adapted to their

conditions of life than any of those which happened to survive. So

again a vast number of mature animals and plants, whether or not they

be the best adapted to their conditions, must be annually destroyed by

accidental causes, which would not be in the least degree mitigated

by certain changes of structure or constitution which would in other

ways be beneficial to the species. But let the destruction of the

adults be ever so heavy, if the number which can exist in any district

be not wholly kept down by such causes,--or again let the destruction

of eggs or seeds be so great that only a hundredth or a thousandth

part are developed,--yet of those which do survive, the best adapted

individuals, supposing that there is any variability in a favourable

direction, will tend to propagate their kind in larger numbers than the

less well adapted. If the numbers be wholly kept down by the causes

just indicated, as will often have been the case, natural selection

will be powerless in certain beneficial directions; but this is no

valid objection to its efficiency at other times and in other ways; for

we are far from having any reason to suppose that many species ever

undergo modification and improvement at the same time in the same area.

SEXUAL SELECTION

Inasmuch as peculiarities often appear under domestication in one sex

and become hereditarily attached to that sex, so no doubt it will be

under nature. Thus it is rendered possible for the two sexes to be

modified through natural selection in relation to different habits

of life, as is sometimes the case; or for one sex to be modified in

relation to the other sex, as commonly occurs. This leads me to say

a few words on what I have called Sexual Selection. This form of

selection depends, not on a struggle for existence in relation to other

organic beings or to external conditions, but on a struggle between the

individuals of one sex, generally the males, for the possession of the

other sex. The result is not death to the unsuccessful competitor, but

few or no offspring. Sexual selection is, therefore, less rigorous than

natural selection. Generally, the most vigorous males, those which are

best fitted for their places in nature, will leave most progeny. But in

many cases, victory depends not so much on general vigour, as on having

special weapons, confined to the male sex. A hornless stag or spurless

cock would have a poor chance of leaving numerous offspring. Sexual

selection, by always allowing the victor to breed, might surely give

indomitable courage, length to the spur, and strength to the wing to

strike in the spurred leg, in nearly the same manner as does the brutal

cockfighter by the careful selection of his best cocks. How low in the

scale of nature the law of battle descends, I know not; male alligators

have been described as fighting, bellowing, and whirling round, like

Indians in a war-dance, for the possession of the females; male

salmons have been observed fighting all day long; male stag-beetles

sometimes bear wounds from the huge mandibles of other males; the

males of certain hymenopterous insects have been frequently seen by

that inimitable observer, M. Fabre, fighting for a particular female

who sits by, an apparently unconcerned beholder of the struggle, and

then retires with the conquerer. The war is, perhaps, severest between

the males of polygamous animals, and these seem oftenest provided with

special weapons. The males of carnivorous animals are already well

armed; though to them and to others, special means of defence may be

given through means of sexual selection, as the mane of the lion, and

the hooked jaw to the male salmon; for the shield may be as important

for victory as the sword or spear.

Amongst birds, the contest is often of a more peaceful character.

All those who have attended to the subject believe that there is the

severest rivalry between the males of many species to attract, by

singing, the females. The rock-thrush of Guiana, birds of paradise,

and some others, congregate; and successive males display with the

most elaborate care, and show off in the best manner, their gorgeous

plumage; they likewise perform strange antics before the females,

which, standing by as spectators, at last choose the most attractive

partner. Those who have closely attended to birds in confinement well

know that they often take individual preferences and dislikes: thus

Sir R. Heron has described how a pied peacock was eminently attractive

to all his hen birds. I cannot here enter on the necessary details;

but if man can in a short time give beauty and an elegant carriage to

his bantams, according to his standard of beauty, I can see no good

reason to doubt that female birds, by selecting, during thousands

of generations, the most melodious or beautiful males, according

to their standard of beauty, might produce a marked effect. Some

well-known laws, with respect to the plumage of male and female birds,

in comparison with the plumage of the young, can partly be explained

through the action of sexual selection on variations occuring at

different ages, and transmitted to the males alone or to both sexes at

corresponding ages; but I have not space here to enter on this subject.

Thus it is, as I believe, that when the males and females of any

animal have the same general habits of life, but differ in structure,

colour, or ornament, such differences have been mainly caused by sexual

selection: that is, by individual males having had, in successive

generations, some slight advantage over other males, in their weapons,

means of defence, or charms, which they have transmitted to their

male offspring alone. Yet, I would not wish to attribute all sexual

differences to this agency: for we see in our domestic animals

peculiarities arising and becoming attached to the male sex, which

apparently have not been augmented through selection by man. The tuft

of hair on the breast of the wild turkey-cock cannot be of any use, and

it is doubtful whether it can be ornamental in the eyes of the female

bird;--indeed, had the tuft appeared under domestication, it would have

been called a monstrosity.

ON THE DEGREE TO WHICH ORGANISATION TENDS TO ADVANCE

Natural Selection acts exclusively by the preservation and accumulation

of variations, which are beneficial under the organic and inorganic

conditions to which each nature is exposed at all periods of life. The

ultimate result is that each creature tends to become more and more

improved in relation to its conditions. This improvement inevitably

leads to the gradual advancement of the organisation of the greater

number of living beings throughout the world. But here we enter on

a very intricate subject, for naturalists have not defined to each

other’s satisfaction what is meant by an advance in organisation.

Amongst the vertebrata the degree of intellect and an approach in

structure to man clearly come into play. It might be thought that

the amount of change which the various parts and organs pass through

in their development from the embryo to maturity would suffice as a

standard of comparison; but there are cases, as with certain parasitic

crustaceans, in which several parts of the structure become less

perfect, so that the mature animal cannot be called higher than its

larva. Von Bar’s standard seems the most widely applicable and the

best, namely, the amount of differentiation of the parts of the same

organic being, in the adult state as I should be inclined to add, and

their specialisation for different functions; or, as Milne Edwards

would express it, the completeness of the division of physiological

labour. But we shall see how obscure this subject is if we look,

for instance, to fishes, amongst which some naturalists rank those

as highest which, like the sharks, approach nearest to amphibians;

whilst other naturalists rank the common bony or teleostean fishes as

the highest, inasmuch as they are most strictly fishlike, and differ

most from the other vertebrate classes. We see still more plainly

the obscurity of the subject by turning to plants, amongst which the

standard of intellect is of course quite excluded; and here some

botanists rank those plants as highest which have every organ, as

sepals, petals, stamens, and pistils, fully developed in each flower;

whereas other botanists, probably with more truth, look at the plants

which have their several organs much modified and reduced in number as

the highest.

If we take as the standard of high organisation, the amount of

differentiation and specialisation of the several organs in each

being when adult (and this will include the advancement of the brain

for intellectual purposes), natural selection clearly leads towards

this standard; for all physiologists admit that the specialisation

of organs, inasmuch as in this state they perform their functions

better, is an advantage to each being; and hence the accumulation

of variations tending towards specialisation is within the scope of

natural selection. On the other hand, we can see, bearing in mind that

all organic beings are striving to increase at a high ratio and to

seize on every unoccupied or less well occupied place in the economy of

nature, that it is quite possible for natural selection gradually to

fit a being to a situation in which several organs would be superfluous

or useless: in such cases there would be retrogression in the scale of

organisation. Whether organisation on the whole has actually advanced

from the remotest geological periods to the present day will be more

conveniently discussed in our chapter on Geological Succession.

But it may be objected that if all organic beings thus tend to rise

in the scale, how is it that throughout the world a multitude of the

lowest forms still exist; and how is it that in each great class some

forms are far more highly developed than others? Why have not the

more highly developed forms everywhere supplanted and exterminated

the lower? Lamarck, who believed in an innate and inevitable tendency

towards perfection in all organic beings, seems to have felt this

difficulty so strongly, that he was led to suppose that new and simple

forms are continually being produced by spontaneous generation. Science

has not as yet proved the truth of this belief, whatever the future

may reveal. On our theory the continued existence of lowly organisms

offers no difficulty; for natural selection, or the survival of the

fittest, does not necessarily include progressive development--it only

takes advantage of such variations as arise and are beneficial to each

creature under its complex relations of life. And it may be asked

what advantage, as far as we can see, would it be to an infusorian

animalcule--to an intestinal worm--or even to an earth-worm, to be

highly organised. If it were no advantage, these forms would be left,

by natural selection, unimproved or but little improved, and might

remain for indefinite ages in their present lowly condition. And

geology tells us that some of the lowest forms, as the infusoria and

rhizopods, have remained for an enormous period in nearly their present

state. But to suppose that most of the many now existing low forms

have not in the least advanced since the first dawn of life would be

extremely rash; for every naturalist who has dissected some of the

beings now ranked as very low in the scale, must have been struck with

their really wondrous and beautiful organisation.

Nearly the same remarks are applicable if we look to the different

grades of organisation within the same great group; for instance,

in the vertebrata, to the co-existence of mammals and fish--amongst

mammalia, to the co-existence of man and the ornithorhynchus--amongst

fishes, to the co-existence of the shark and the lancelet

(_Amphioxus_), which latter fish in the extreme simplicity of

its structure approaches the invertebrate classes. But mammals and

fish hardly come into competition with each other; the advancement

of the whole class of mammals, or of certain members in this class,

to the highest grade would not lead to their taking the place of

fishes. Physiologists believe that the brain must be bathed by warm

blood to be highly active, and this requires aërial respiration;

so that warm-blooded mammals when inhabiting the water lie under a

disadvantage in having to come continually to the surface to breathe.

With fishes, members of the shark family would not tend to supplant the

lancelet; for the lancelet, as I hear from Fritz Müller, has as sole

companion and competitor on the barren, sandy shore of South Brazil,

an anomalous annelid. The three lowest orders of mammals, namely,

marsupials, edentata, and rodents, co-exist in South America in the

same region with numerous monkeys, and probably interfere little with

each other. Although organisation, on the whole, may have advanced and

be still advancing throughout the world, yet the scale will always

present many degrees of perfection; for the high advancement of certain

whole classes, or of certain members of each class, does not at all

necessarily lead to the extinction of those groups with which they do

not enter into close competition. In some cases, as we shall hereafter

see, lowly organised forms appear to have been preserved to the present

day, from inhabiting confined or peculiar stations, where they have

been subjected to less severe competition, and where their scanty

numbers have retarded the chance of favourable variations arising.

Finally, I believe that many lowly organised forms now exist

throughout the world, from various causes. In some cases variations or

individual differences of a favourable nature may never have arisen

for natural selection to act on and accumulate. In no case, probably,

has time sufficed for the utmost possible amount of development.

In some few cases there has been what we must call retrogression

of organisation. But the main cause lies in the fact that under

very simple conditions of life a high organisation would be of no

service,--possibly would be of actual disservice, as being of a more

delicate nature, and more liable to be put out of order and injured.

Looking to the first dawn of life, when all organic beings, as we may

believe, presented the simplest structure, how, it has been asked,

could the first steps in the advancement of differentiation of parts

have arisen? Mr. Herbert Spencer would probably answer that, as soon as

simple unicellular organism came by growth or division to be compounded

of several cells, or became attached to any supporting surface, his law

“that homologous units of any order become differentiated in proportion

as their relations to incident forces become different” would come into

action. But as we have no facts to guide us, speculation on the subject

is almost useless. It is, however, an error to suppose that there would

be no struggle for existence, and, consequently, no natural selection,

until many forms had been produced; variations in a single species

inhabiting an isolated station might be beneficial, and thus the whole

mass of individuals might be modified, or two distinct forms might

arise. But, as I remarked towards the close of the Introduction, no

one ought to feel surprise at much remaining as yet unexplained on the

origin of species, if we make due allowance for our profound ignorance

on the mutual relations of the inhabitants of the world at the present

time, and still more so during past ages.

CONVERGENCE OF CHARACTER

Mr. H. C. Watson thinks that I have overrated the importance of

divergence of character (in which, however, he apparently believes),

and that convergence, as it may be called, has likewise played a

part. If two species, belonging to two distinct though allied genera,

had both produced a large number of new and divergent forms, it is

conceivable that these might approach each other so closely that they

would have all to be classed under the same genus; and thus the

descendants of two distinct genera would converge into one. But it

would in most cases be extremely rash to attribute to convergence a

close and general similarity of structure in the modified descendants

of widely distinct forms. The shape of a crystal is determined solely

by the molecular forces, and it is not surprising that dissimilar

substances should sometimes assume the same form; but with organic

beings we should bear in mind that the form of each depends on an

infinitude of complex relations, namely, on the variations which have

arisen, those being due to causes far too intricate to be followed

out,--on the nature of the variations which have been preserved or

selected, and this depends on the surrounding physical conditions, and

in a still higher degree on the surrounding organisms with which each

being has come into competition,--and lastly, on inheritance (in itself

a fluctuating element) from innumerable progenitors, all of which have

had their forms determined through equally complex relations. It is

incredible that the descendants of two organisms, which had originally

differed in a marked manner, should ever afterwards converge so closely

as to lead to a near approach to identity throughout their whole

organisation. If this had occurred, we should meet with the same form,

independently of genetic connection, recurring in widely separated

geological formations; and the balance of evidence is opposed to any

such an admission.

Mr. Watson has also objected that the continued action of natural

selection, together with divergence of character, would tend to make

an indefinite number of specific forms. As far as mere inorganic

conditions are concerned, it seems probable that a sufficient number

of species would soon become adapted to all considerable diversities

of heat, moisture, &c.; but I fully admit that the mutual relations

of organic beings are more important; and as the number of species in

any country goes on increasing, the organic conditions of life must

become more and more complex. Consequently there seems at first sight

no limit to the amount of profitable diversification of structure, and

therefore no limit to the number of species which might be produced.

We do not know that even the most prolific area is fully stocked with

specific forms: at the Cape of Good Hope and in Australia, which

support such an astonishing number of species, many European plants

have become naturalised. But geology shows us, that from an early part

of the tertiary period the number of species of shells, and that from

the middle part of this same period the number of mammals, has not

greatly or at all increased. What then checks an indefinite increase

in the number of species? The amount of life (I do not mean the number

of specific forms) supported on an area must have a limit, depending

so largely as it does on physical conditions; therefore, if an area

be inhabited by very many species, each or nearly each species will

be represented by few individuals; and such species will be liable to

exterminate from accidental fluctuations in the nature of the seasons

or in the number of their enemies. The process of extermination in

such cases would be rapid, whereas the production of new species

must always be slow. Imagine the extreme case of as many species as

individuals in England, and the first severe winter or very dry summer

would exterminate thousands on thousands of species. Rare species, and

each species will become rare if the number of species in any country

becomes indefinitely increased, will, on the principle often explained,

present within a given period few favourable variations; consequently,

the process of giving birth to new specific forms would thus be

retarded. When any species becomes very rare, close interbreeding will

help to exterminate it; authors have thought that this comes into play

in accounting for the deterioration of the Aurochs in Lithuania, of Red

Deer in Scotland, and of Bears in Norway, &c. Lastly, and this I am

inclined to think is the most important element, a dominant species,

which has already beaten many competitors in its own home, will tend to

spread and supplant many others. Alph. de Candolle has shown that those

species which spread widely, tend generally to spread very widely;

consequently, they will tend to supplant and exterminate several

species in several areas, and thus check the inordinate increase of

specific forms throughout the world. Dr. Hooker has recently shown that

in the S. E. corner of Australia, where, apparently, there are many

invaders from different quarters of the globe, the endemic Australian

species have been greatly reduced in number. How much weight to

attribute to these several considerations I will not pretend to say;

but conjointly they must limit in each country the tendency to an

indefinite augmentation of specific forms.

SUMMARY OF CHAPTER

If under changing conditions of life organic beings present individual

differences in almost every part of their structure, and this cannot

be disputed; if there be, owing to their geometrical rate of increase,

a severe struggle for life at some age, season, or year, and this

certainly cannot be disputed; then, considering the infinite complexity

of the relations of all organic beings to each other and to their

conditions of life, causing an infinite diversity in structure,

constitution, and habits, to be advantageous to them, it would be a

most extraordinary fact if no variations had ever occurred useful to

each being’s own welfare, in the same manner as so many variations

have occurred useful to man. But if variations useful to any organic

being ever do occur, assuredly individuals thus characterised will

have the best chance of being preserved in the struggle for life; and

from the strong principle of inheritance, these will tend to produce

offspring similarly characterised. This principle of preservation,

or the survival of the fittest, I have called Natural Selection. It

leads to the improvement of each creature in relation to its organic

and inorganic conditions of life; and consequently, in most cases, to

what must be regarded as an advance in organisation. Nevertheless,

low and simple forms will long endure if well fitted for their simple

conditions of life.

Natural selection, on the principle of qualities being inherited at

corresponding ages, can modify the egg, seed, or young, as easily as

the adult. Amongst many animals, sexual selection will have given its

aid to ordinary selection, by assuring to the most vigorous and best

adapted males the greatest number of offspring. Sexual selection will

also give characters useful to the males alone, in their struggles or

rivalry with other males; and these characters will be transmitted to

one sex or to both sexes, according to the form of inheritance which

prevails.

Whether natural selection has really thus acted in adapting the

various forms of life to their several conditions and stations, must

be judged by the general tenor and balance of evidence given in the

following chapters. But we have already seen how it entails extinction;

and how largely extinction has acted in the world’s history, geology

plainly declares. Natural selection, also, leads to divergence of

character; for the more organic beings diverge in structure, habits,

and constitution, by so much the more can a large number be supported

on the area,--of which we see proof by looking to the inhabitants of

any small spot, and to the productions naturalised in foreign lands.

Therefore, during the modification of the descendants of any one

species, and during the incessant struggle of all species to increase

in numbers, the more diversified the descendants become, the better

will be their chance of success in the battle for life. Thus the small

differences distinguishing varieties of the same species, steadily tend

to increase, till they equal the greater differences between species of

the same genus, or even of distinct genera.

We have seen that it is the common, the widely diffused and widely

ranging species, belonging to the larger genera within each class,

which vary most; and these tend to transmit to their modified offspring

that superiority which now makes them dominant in their own countries.

Natural selection, as has just been remarked, leads to divergence of

character and to much extinction of the less improved and intermediate

forms of life. On these principles, the nature of the affinities, and

the generally well-defined distinctions between the innumerable organic

beings in each class throughout the world, may be explained. It is

a truly wonderful fact--the wonder of which we are apt to overlook

from familiarity--that all animals and all plants throughout all time

and space should be related to each other in groups, subordinate to

groups, in the manner which we everywhere behold--namely, varieties of

the same species most closely related, species of the same genus less

closely and unequally related, forming sections and sub-genera, species

of distinct genera much less closely related, and genera related in

different degrees, forming sub-families, families, orders, sub-classes

and classes. The several subordinate groups in any class cannot be

ranked in a single file, but seem clustered round points, and these

round other points, and so on in almost endless cycles. If species had

been independently created, no explanation would have been possible of

this kind of classification; but it is explained through inheritance

and the complex action of natural selection, entailing extinction and

divergence of character....

The affinities of all the beings of the same class have sometimes been

represented by a great tree. I believe this simile largely speaks the

truth. The green and budding twigs may represent existing species; and

those produced during former years may represent the long succession

of extinct species. At each period of growth all the growing twigs

have tried to branch out on all sides, and to overtop and kill the

surrounding twigs and branches, in the same manner as species and

groups of species have at all times overmastered other species in the

great battle for life. The limbs divided into great branches, and these

into lesser and lesser branches, were themselves once, when the tree

was young, budding twigs; and this connection of the former and present

buds by ramifying branches may well represent the classification of

all extinct and living species in groups subordinate to groups. Of the

many twigs which flourished when the tree was a mere bush, only two or

three, now grown into great branches, yet survive and bear the other

branches; so with the species which lived during long-past geological

periods, very few have left living and modified descendants. From

the first growth of the tree, many a limb and branch has decayed and

dropped off; and these fallen branches of various sizes may represent

those whole orders, families, and genera which have now no living

representatives, and which are known to us only in a fossil state. As

we here and there see a thin straggling branch springing from a fork

low down in a tree, and which by some chance has been favoured and is

still alive on its summit, so we occasionally see an animal like the

Ornithorhynchus or Lepidosiren, which in some small degree connects by

its affinities two large branches of life, and which has apparently

been saved from fatal competition by having inhabited a protected

station. As buds give rise by growth to fresh buds, and these, if

vigorous, branch out and overtop on all sides many a feebler branch, so

by generation I believe it has been with the great Tree of Life, which

fills with its dead and broken branches the crust of the earth, and

covers the surface with its ever-branching and beautiful ramifications.

FOOTNOTES:

[Footnote 32: From the _Origin of Species_. Ch. IV.]

XXX

THEODOR SCHWANN

1810-1882

_Theodor Schwann, the son of a Prussian printer, was born at Neuss,

Prussia, December 7, 1810. He first studied medicine, but was persuaded

to devote himself to science by Johannes Mueller, who appointed him

assistant in the anatomical museum. In 1838 he was called to the

Catholic University of Louvain, and later removed to Liège. One of

the first to suggest the chemical explanation of life, he discovered

the presence and function of pepsin as a ferment in digestion. In

1839 he established his great theory that all life is composed of

inter-connected cellular units--a conception which revolutionized

biology. He died at Liège on January 11, 1882._

CELL THEORY[33]

The various opinions entertained with respect to the fundamental powers

of an organized body may be reduced to two, which are essentially

different from one another. The first is, that every organism

originates with an inherent power, which models it into conformity

with a predominant idea, arranging the molecules in the relation

necessary for accomplishing certain purposes held forth by this idea.

Here, therefore, that which arranges and combines the molecules is a

power acting with a definite purpose. A power of this kind would be

essentially different from all the powers of inorganic nature, because

action goes on in the latter quite blindly. A certain impression is

followed of necessity by a certain change of quality and quantity,

without regard to any purpose. In this view, however, the fundamental

power of the organism (or the soul, in the sense employed by Stahl)

would, inasmuch as it works with a definite individual purpose, be

much more nearly allied to the immaterial principle, endued with

consciousness which we must admit operates in man.

The other view is, that the fundamental powers of organized bodies

agree essentially with those of inorganic nature, that they work

altogether blindly according to laws of necessity and irrespective

of any purpose, that they are powers which are as much established

with the existence of matter as the physical powers are. It might be

assumed that the powers which form organized bodies do not appear at

all in inorganic nature, because this or that particular combination

of molecules, by which the powers are elicited, does not occur in

inorganic nature, and yet they might not be essentially distinct

from physical and chemical powers. It cannot, indeed, be denied that

adaptation to a particular purpose, in some individuals even in a

high degree, is characteristic of every organism; but, according to

this view, the source of this adaptation does not depend upon each

organism being developed by the operation of its own power in obedience

to that purpose, but it originates as in inorganic nature, in the

creation of the matter with its blind powers by a rational Being. We

know, for instance, the powers which operate in our planetary system.

They operate, like all physical powers, in accordance with blind laws

of necessity, and yet is the planetary system remarkable for its

adaptation to a purpose. The ground of this adaptation does not lie in

the powers, but in Him, who has so constituted matter with its powers,

that in blindly obeying its laws it produces a whole suited to fulfil

an intended purpose. We may even assume that the planetary system

has an individual adaptation to a purpose. Some external influence,

such as a comet, may occasion disturbances of motion, without thereby

bringing the whole into collision; derangements may occur on single

planets, such as a high tide, &c., which are yet balanced entirely by

physical laws. As respects their adaptation to a purpose, organized

bodies differ from these in degree only; and by this second view we are

just as little compelled to conclude that the fundamental powers of

organization operate according to laws of adaptation to a purpose, as

we are in inorganic nature.

The first view of the fundamental powers of organized bodies may be

called the teleological, the second the physical view. An example will

show at once, how important for physiology is the solution of the

question as to which is to be followed. If, for instance, we define

inflammation and suppuration to be the effort of the organism to remove

a foreign body that has been introduced into it; or fever to be the

effort of the organism to eliminate diseased matter, and both as the

result of the “autocracy of the organism,” then these explanations

accord with the teleological view. For, since by these processes the

obnoxious matter is actually removed, the process which effects them

is one adapted to an end; and as the fundamental power of the organism

operates in accordance with definite purposes, it may either set these

processes in action primarily, or may also summon further powers of

matter to its aid, always, however, remaining itself the “primum

movens.” On the other hand, according to the physical view, this is

just as little an explanation as it would be to say, that the motion of

the earth around the sun is an effort of the fundamental power of the

planetary system to produce a change of seasons on the planets, or to

say, that ebb and flood are the reaction of the organism of the earth

upon the moon.

In physics, all those explanations which were suggested by a

teleological view of nature, as “horror vacui,” and the like, have

long been discarded. But in animated nature, adaptation--individual

adaptation--to a purpose is so prominently marked, that it is

difficult to reject all teleological explanations. Meanwhile it must

be remembered that those explanations, which explain at once all

and nothing, can be but the last resources, when no other view can

possibly be adopted; and there is no such necessity for admitting the

teleological view in the case of organized bodies. The adaptation of

a purpose which is characteristic of organized bodies differs only in

degree from what is apparent also in the inorganic part of nature;

and the explanation that organized bodies are developed, like all the

phenomena of inorganic nature, by the operation of blind laws framed

with the matter, cannot be rejected as impossible. Reason certainly

requires some ground for such adaptation, but for her it is sufficient

to assume that matter with the powers inherent in it owes its existence

to a rational Being. Once established and preserved in their integrity,

these powers may, in accordance with their immutable laws of blind

necessity, very well produce combinations, which manifest, even in

a high degree, individual adaptation to a purpose. If, however,

rational power interpose after creation merely to sustain, and not

as an immediately active agent, it may, so far as natural science is

concerned, be entirely excluded from the consideration of the creation.

But the teleological view leads to further difficulties in the

explanation, and especially with respect to generation. If we assume

each organism to be formed by a power which acts according to a certain

predominant idea, a portion of this power may certainly reside in the

ovum during generation; but then we must ascribe to this subdivision

of the original power, at the separation of the ovum from the body of

the mother, the capability of producing an organism similar to that

which the power, of which it is but a portion, produced: that is, we

must assume that this power is infinitely divisible, and yet that each

part may perform the same actions as the whole power. If, on the other

hand, the power of organized bodies reside, like the physical powers,

in matter as such, and be set free only by a certain combination of the

molecules, as, for instance, electricity is set free by the combination

of a zinc and copper plate, then also by the conjunction of molecules

to form an ovum the power may be set free, by which the ovum is capable

of appropriating to itself fresh molecules, and these newly-conjoined

molecules again by this very mode of combination acquire the same

power to assimilate fresh molecules. The first development of the

many forms of organized bodies--the progressive formation of organic

nature indicated by geology--is also much more difficult to understand

according to the teleological than the physical view.

Another objection to the teleological view may be drawn from the

foregoing investigation. The molecules, as we have seen, are not

immediately combined in various ways, as the purpose of the organism

requires, but the formation of the elementary parts of organic

bodies is regulated by laws which are essentially the same for all

elementary parts. One can see no reason why this should be the case,

if each organism be endued with a special power to frame the parts

according to the purpose which they have to fulfil: it might much

rather be expected that the formative principle, although identical

for organs physiologically the same, would yet in different tissues

be correspondingly varied. This resemblance of the elementary parts

has, in the instance of plants, already led to the conjecture that

the cells are really the organisms, and that the whole plant is an

aggregrate of these organisms arranged according to certain laws.

But since the elementary parts of animals bear exactly similar

relations, the individuality of an entire animal would thus be lost;

and yet precisely upon the individuality of the whole animal does the

assumption rest, that it possesses a single fundamental power operating

in accordance with a definite idea.

Meanwhile, we cannot altogether lay aside teleological views if all

phenomena are not clearly explicable by the physical view. It is,

however, unnecessary to do so, because an explanation, according to

the teleological view, is only admissible when the physical can be

shown to be impossible. In any case it conduces much more to the object

of science to strive, at least, to adopt the physical explanation.

And I would repeat that, when speaking of a physical explanation of

organic phenomena, it is not necessary to understand an explanation by

known physical powers, such, for instance, as that universal refuge

electricity, and the like; but an explanation by means of powers which

operate like the physical powers, in accordance with strict laws of

blind necessity, whether they be also to be found in inorganic nature

or not.

We set out, therefore, with the supposition that an organized body

is not produced by a fundamental power which is guided in its

operation by a definite idea, but is developed, according to blind

laws of necessity, by powers which, like those of inorganic nature,

are established by the very existence of matter. As the elementary

materials of organic nature are not different from those of the

inorganic kingdom, the source of the organic phenomena can only

reside in another combination of these materials, whether it be in a

peculiar mode of union of the elementary atoms to form atoms of the

second order, or in the arrangement of these conglomerate molecules

when forming either the separate morphological elementary parts of

organisms, or an entire organism. We have here to do with the latter

question solely, whether the cause of organic phenomena lies in the

whole organism, or in its separate elementary parts. If this question

can be answered, a further inquiry still remains as to whether the

organism or its elementary parts possess this power through the

peculiar mode of combination of the conglomerate molecules, or through

the mode in which the elementary atoms are united into conglomerate

molecules.

We may, then, form the two following ideas of the cause of organic

phenomena, such as growth, &c. First, that the cause resides in the

totality of the organism. By the combination of the molecules into

a systematic whole, such as the organism is in every stage of its

development, a power is engendered, which enables such an organism to

take up fresh material from without, and appropriate it either to the

formation of new elementary parts, or to the growth of those already

present. Here, therefore, the cause of the growth of the elementary

parts resides in the totality of the organism. The other mode of

explanation is, that growth does not ensue from a power resident in the

entire organism, but that each separate elementary part is possessed of

an independent power, an independent life, so to speak; in other words,

the molecules in each separate elementary part are so combined as to

set free a power by which it is capable of attracting new molecules,

and so increasing, and the whole organism subsists only by means of

the reciprocal action of the single elementary parts. So that here the

single elementary parts only exert an active influence on nutrition,

and totality of the organism may indeed be a condition, but is not in

this view a cause.

In order to determine which of these two views is the correct one,

we must summon to our aid the results of the previous investigation.

We have seen that all organized bodies are composed of essentially

similar parts, namely, of cells; that these cells are formed and grow

in accordance with essentially similar laws; and, therefore, that these

processes must, in every instance, be produced by the same powers. Now,

if we find that some of these elementary parts, not differing from the

others, are capable of separating themselves from the organism, and

pursuing an independent growth, we may thence conclude that each of

the other elementary parts, each cell, is already possessed of power

to take up fresh molecules and growth; and that, therefore, every

elementary part possesses a power of its own, an independent life, by

means of which it would be enabled to develop itself independently,

if the relations which it bore to external parts were but similar to

those in which it stands in the organism. The ova of animals afford us

example of such independent cells, growing apart from the organism.

It may, indeed, be said of the ova of higher animals, that after

impregnation the ovum is essentially different from the other cells of

the organism; that by impregnation there is a something conveyed to the

ovum, which is more to it than an external condition for vitality, more

than nutrient matter; and that it might thereby have first received

its peculiar vitality, and therefore that nothing can be inferred from

it with respect to the other cells. But this fails in application to

those classes which consist only of female individuals, as well as

with the spores of the lower plants; and, besides, in the inferior

plants any given cell may be separated from the plant, and then grow

alone. So that here are whole plants consisting of cells, which can

be positively proved to have independent vitality. Now, as all cells

grow according to the same laws, and consequently the cause of growth

cannot in one case lie in the cell, and in another in the whole

organism; and since it may be further proved that some cells, which

do not differ from the rest in their mode of growth, are developed

independently, we must ascribe to all cells an independent vitality,

that is, such combinations of molecules as occur in any single cell,

are capable of setting free the power by which it is enabled to take

up fresh molecules. The cause of nutrition and growth resides not in

the organism as a whole, but in the separate elementary parts--the

cells. The failure of growth in the case of any particular cell, when

separated from an organized body, is as slight an objection to this

theory as it is an objection against the independent vitality of a bee,

that it cannot continue long in existence after being separated from

its swarm. The manifestation of the power which resides in the cell

depends upon conditions to which it is subject only when in connexion

with the whole (organism).

The question, then, as to the fundamental power of organized bodies

resolves itself into that of the fundamental powers of the individual

cells. We must now consider the general phenomena attending the

formation of cells, in order to discover what powers may be presumed

to exist in the cells to explain them. These phenomena may be arranged

in two natural groups: first, those which relate to the combination of

the molecules to form a cell, and which may be denominated the plastic

phenomena of the cells; secondly, those which result from chemical

changes either in the component particles of the cell itself, or in the

surrounding cytoblastema, and which may be called metabolic phenomena

(_to metabolikon_, implying that which is liable to occasion or to

suffer change).

The general plastic appearances in the cells are, as we have seen,

the following: at first a minute corpuscle is formed (the nucleolus);

a layer of substance (the nucleus) is then precipitated around it,

which becomes more thickened and expanded by the continual deposition

of fresh molecules between those already present. Deposition goes on

more vigorously at the outer part of this layer than at the inner.

Frequently the entire layer, or in other instances the outer part of

it only, becomes condensed to a membrane, which may continue to take

up new molecules in such a manner that it increases more rapidly in

superficial extent than in thickness, and thus an intervening cavity is

necessarily formed between it and the nucleolus. A second layer (cell)

is next precipitated around this first, in which precisely the same

phenomena are repeated, with merely the difference that in this case

the processes, especially the growth of the layer and the formation of

the space intervening between it and the first layer (the cell-cavity),

go on more rapidly and more completely. Such were the phenomena in

the formation of most cells; in some, however, there appeared to be

only a single layer formed, while in others (those especially in which

the nucleolus was hollow) there were three. The other varieties in

the development of the elementary parts were (as we saw) reduced to

these--that if two neighbouring cells commence their formation so near

to one another that the boundaries of the layers forming around each

of them meet at any spot, a common layer may be formed enclosing the

two incipient cells. So at least the origin of nuclei, with two or

more nucleoli, seemed explicable, by a coalescence of the first layers

(corresponding to the nucleus), and the union of many primary cells

into one secondary cell by a similar coalescence of the second layers

(which correspond to the cell). But the further development of these

common layers proceeds as though they were only an ordinary single

layer. Lastly, there were some varieties in the progressive development

of the cells, which were referable to an unequal deposition of the new

molecules between those already present in the separate layers. In this

way modifications of form and division of the cells were explained.

And among the number of the plastic phenomena in the cells we may

mention, lastly, the formation of secondary deposits; for instances

occur in which one or more new layers, each on the inner surface of

the previous one, are deposited on the inner surface of a simple or of

a secondary cell.

These are the most important phenomena observed in the formation and

development of cells. The unknown cause, presumed to be capable of

explaining these processes in the cells, may be called the plastic

power of the cells. We will, in the next place, proceed to determine

how far a more accurate definition of this power may be deduced from

these phenomena.

In the first place, there is a power of attraction exerted in the

very commencement of the cell, in the nucleolus, which occasions the

addition of new molecules to those already present. We may imagine

the nucleolus itself to be first formed by a sort of crystallization

from out of a concentrated fluid. For if a fluid be so concentrated

that the molecules of the substance in solution exert a more powerful

mutual attraction than is exerted between them and the molecules of

the fluid in which they are dissolved, a part of the solid substance

must be precipitated. One can readily understand that the fluid must be

more concentrated when new cells are being formed in it than when those

already present have merely to grow. For if the cell is already partly

formed, it exerts an attractive force upon the substance still in

solution. There is then a cause for the deposition of this substance,

which does not co-operate when no part of the cell is yet formed.

Therefore, the greater the attractive force of the cell is, the less

concentration of the fluid is required; while, at the commencement of

the formation of a cell, the fluid must be more than concentrated. But

the conclusion which may be thus directly drawn, as to the attractive

power of the cell, may also be verified by observation. Wherever the

nutrient fluid is not equally distributed in a tissue, the new cells

are formed in that part into which the fluid penetrates first, and

where, consequently, it is most concentrated. Upon this fact, as we

have seen, depended the difference between the growth of organized and

unorganized tissues. And this confirmation of the foregoing conclusion

by experience speaks also for the correctness of the reasoning itself.

The attractive power of the cells operates so as to effect the addition

of new molecules in two ways,--first, in layers, and secondly, in such

a manner in each layer that the new molecules are deposited between

those already present. This is only an expression of the fact; the

more simple law, by which several layers are formed and the molecules

are not all deposited between those already present, cannot yet be

explained. The formation of layers may be repeated once, twice, or

thrice. The growth of the separate layers is regulated by a law,

that the deposition of new molecules should be greatest at the part

where the nutrient fluid is most concentrated. Hence the outer part

particularly becomes condensed into a membrance both in the layer

corresponding to the nucleus and in that answering to the cell, because

the nutrient fluid penetrates from without, and consequently is more

concentrated at the outer than at the inner part of each layer. For

the same reason the nucleus grows rapidly, so long as the layer of the

cell is not formed around it, but it either stops growing altogether,

or at least grows much more slowly as soon as the cell-layer has

surrounded it; because then the latter receives the nutrient matter

first, and, therefore, in a more concentrated form. And hence the cell

becomes, in a general sense, much more completely developed, while

the nucleus-layer usually remains at a stage of development, in which

the cell-layer had been in its earlier period. The addition of new

molecules is so arranged that the layers increase more considerably in

superficial extent than in thickness; and thus an intervening space

is formed between each layer and the one preceding it, by which cells

and nuclei are formed into actual hollow vesicles. From this it may be

inferred that the deposition of new molecules is more active between

those which lie side by side along the surface of the membrane, than

between those which lie one upon the other in its thickness. Were it

otherwise, each layer would increase in thickness, but there would be

no intervening cavity between it and the previous one, there would be

no vesicles, but a solid body composed of layers.

Attractive power is exerted in all the solid parts of the cell. This

follows, not only from the fact that new molecules may be deposited

everywhere between those already present, but also from the formation

of secondary deposits. When the cavity of a cell is once formed,

material may be also attracted from its contents and deposited in

layers; and as this deposition takes place upon the inner surface

of the membrane of the cell, it is probably that which exerts the

attractive influence. This formation of layers on the inner surface of

the cell-membrane is, perhaps, merely a repetition of the same process

by which, at an earlier period, nucleus and cell were precipitated as

layers around the nucleolus. It must, however, be remarked that the

identity of these two processes cannot be so clearly proved as that of

the processes by which nucleus and cell are formed; more especially

as there is a variety in the phenomena, for the secondary deposits in

plants occur in spiral forms, while this has at least not yet been

demonstrated in the formation of the cell-membrane and the nucleus,

although by some botanical writers the cell-membrane itself is supposed

to consist of spirals.

The power of attraction may be uniform throughout the whole cell,

but it may also be confined to single spots; the deposition of new

molecules is then more vigorous at these spots, and the consequence of

this uneven growth of the cell-membrane is a change in the form of the

cell.

The attractive power of the cells manifest a certain form of election

in its operation. It does not take up all the substances contained in

the surrounding cytoblastema, but only particular ones, either those

which are analogous with the substance already present in the cell

(assimilation), or such as differ from it in chemical properties. The

several layers grow by assimilation, but when a new layer is being

formed, different material from that of the previously-formed layer

is attracted: for the nucleolus, the nucleus and cell-membrane are

composed of materials which differ in their chemical properties.

Such are the peculiarities of the plastic power of the cells, so far as

they can as yet be drawn from observation. But the manifestations of

this power presuppose another faculty of the cells. The cytoblastema,

in which the cells are formed, contains the elements of the materials

of which the cell is composed, but in other combinations; it is

not a mere solution of cell-material, but it contains only certain

organic substances in solution. The cells, therefore, not only attract

materials from out of the cytoblastema, but they must have the faculty

of producing chemical changes in its constituent particles. Besides

which, all the parts of the cell itself may be chemically altered

during the process of its vegetation. The unknown cause of all these

phenomena, which we comprise under the term metabolic phenomena of the

cells, we will denominate the metabolic power.

The next point which can be proved is, that this power is an attribute

of the cells themselves, and that the cytoblastema is passive under

it. We may mention vinous fermentation as an instance of this. A

decoction of malt will remain for a long time unchanged; but as soon as

some yeast is added to it, which consists partly of entire fungi and

partly of a number of single cells, the chemical change immediately

ensues. Here the decoction of malt is the cytoblastema; the cells

clearly exhibit activity, the cytoblastema, in this instance even a

boiled fluid, being quite passive during the change. The same occurs

when any simple cells, as the spores of the lower plants, are sown in

boiled substances.

In the cells themselves again, it appears to be the solid parts, the

cell-membrane and the nucleus, which produce the change. The contents

of the cell undergo similar and even more various changes than the

external the cytoblastema, and it is at least probable that these

changes originate with the solid parts composing the cells, especially

the cell-membrane, because the secondary deposits are formed on

the inner surface of the cell-membrane, and other precipitates are

generally formed in the first instance around the nucleus. It may

therefore, on the whole, be said that the solid component particles of

the cells possess the power of chemically altering the substances in

contact with them.

The substances which result from the transformation of the contents

of the cell are different from those which are produced by change

in the external cytoblastema. What is the cause of this difference,

if the metamorphosing power of the cell-membrane be limited to its

immediate neighbourhood merely? Might we not much rather expect that

converted substance would be found without distinction on the inner

as on the outer surface of the cell-membrane? It might be said that

the cell-membrane converts the substance in contact with it without

distinction, and that the variety in the products of this conversion

depends only upon a difference between the convertible substance

contained in the cell and the external cytoblastema. But the question

then arises, as to how it happens that the contents of the cell differ

from the external cytoblastema. If it be true that the cell-membrane,

which at first closely surrounds the nucleus, expands in the course of

its growth, so as to leave an interspace between it and the cell, and

that the contents of the cell consist of fluid which has entered this

space merely by imbibition, they cannot differ essentially from the

external cytoblastema. I think therefore that, in order to explain the

distinction between the cell-contents and the external cytoblastema,

we must ascribe to the cell-membrane not only the power in general of

chemically altering the substances which it is either in contact with,

or has imbibed, but also of so separating them that certain substances

appear on its inner, and others on its outer surface. The secretion of

substances already present in the blood, as, for instance, of urea, by

the cells with which the urinary tubes are lined, cannot be explained

without such a faculty of the cells. There is, however, nothing so

very hazardous in it, since it is a fact that different substances are

separated in the decompositions produced by the galvanic pile. It might

perhaps be conjectured from this peculiarity of the metabolic phenomena

in the cells, that a particular position of the axes of the atoms

composing the cell-membrane is essential for the production of these

appearances.

Chemical changes occur, however, not only in the cytoblastema and the

cell-contents, but also in the solid parts of which the cells are

composed, particularly the cell-membrane. Without wishing to assert

that there is any intimate connexion between the metabolic power

of the cells and galvanism, I may yet, for the sake of making the

representation of the process more clear, remark that the chemical

changes produced by a galvanic pile are accompanied by corresponding

changes in the pile itself.

The more obscure the cause of the metabolic phenomena in the cells

is, the more accurately we must mark the circumstances and phenomena

under which they occur. One condition to them is a certain temperature,

which has a maximum and a minimum. The phenomena are not produced in

a temperature below 0° or above 80° R.; boiling heat destroys this

faculty of the cells permanently; but the most favorable temperature is

one between 10° and 32° R. Heat is evolved by the process itself.

Oxygen, or carbonic acid, in a gaseous form or lightly confined, is

essentially necessary to the metabolic phenomena of the cells. The

oxygen disappears and carbonic acid is formed, or _vice versa_,

carbonic acid disappears, and oxygen is formed. The universality of

respiration is based entirely upon this fundamental condition to the

metabolic phenomena of the cells. It is so important that, as we shall

see further on, even the principal varieties of form in organized

bodies are occasioned by this peculiarity of the metabolic process in

the cells.

Each cell is not capable of producing chemical changes in every organic

substance contained in solution, but only in particular ones. The fungi

of fermentation, for instance, effect no changes in any other solutions

than sugar; and the spores of certain plants do not become developed in

all substances. In the same manner it is probable that each cell in the

animal body converts only particular constituents of the blood.

The metabolic power of the cells is arrested not only by powerful

chemical actions, such as destroy organic substances in general, but

also by matters which chemically are less uncongenial; for instance,

concentrated solutions of neutral salts. Other substances, as arsenic,

do so in less quantity. The metabolic phenomena may be altered in

quality by other substances, both organic and inorganic, and a change

of this kind may result even from mechanical impressions on the cells.

Such are the most essential characteristics of the fundamental powers

of the cell, so far as they can as yet be deduced from the phenomena.

And now, in order to comprehend distinctly in what the peculiarity of

the formative process of a cell, and therefore in what the peculiarity

of the essential phenomenon in the formation of organized bodies

consist, we will compare this process with a phenomenon of inorganic

nature as nearly as possible similar to it. Disregarding all that

is specially peculiar to the formation of cells, in order to find a

more general definition in which it may be included with a process

occurring in inorganic nature, we may view it as a process in which a

solid body of definite and regular shape is formed in a fluid at the

expense of a substance held in solution by that fluid. The process of

crystallization in inorganic nature comes also within this definition,

and is, therefore, the nearest analogue to the formation of cells.

Let us now compare the two processes, that the difference of the

organic process may be clearly manifest. First, with reference to the

plastic phenomena, the forms of cells and crystals are very different.

The primary forms of crystals are simple, always angular, and bounded

by plane surfaces; they are regular, or at least symmetrical, and

even the very varied secondary forms of crystals are almost, without

exception, bounded by plane surfaces. But manifold as is the form of

cells, they have very little resemblance to crystals; round surfaces

predominate, and where angles occur, they are never quite sharp, and

the polyhedral crystal-like form of many cells results only from

mechanical causes. The structure too of cells and of crystals is

different. Crystals are solid bodies, composed merely of layers placed

one upon another; cells are hollow vesicles, either single, or several

inclosed one within another. And if we regard the membranes of these

vesicles as layers, there will still remain marks of difference between

them and crystals; these layers are not in contact, but contain fluid

between them, which is not the case with crystals; the layers in the

cells are few, from one to three only; and they differ from each

other in chemical properties, while those of crystals consist of the

same chemical substance. Lastly, there is also a great difference

between crystals and cells in their mode of growth. Crystals grow by

apposition, the new molecules are set only upon the surface of those

already deposited, but cells increase also by intussusception, that

is to say, the new molecules are deposited also between those already

present.

But greatly as these plastic phenomena differ in cells and in crystals,

the metabolic are yet more different, or rather they are quite peculiar

to cells. For a crystal to grow, it must be already present as such in

the solution, and some extraneous cause must interpose to diminish its

solubility. Cells, on the contrary, are capable of producing a chemical

change in the surrounding fluid, of generating matters which had not

previously existed in it as such, but of which only the elements were

present in another combination. They therefore require no extraneous

influence to effect a change of solubility; for if they can produce

chemical changes in the surrounding fluid, they may also produce

such substances as could not be held in solution under the existing

circumstances, and therefore need no external cause of growth. If a

crystal be laid in a pretty strong solution, of a substance similar

even to itself, nothing ensues without our interference, or the crystal

dissolves completely: the fluid must be evaporated for the crystal

to increase. If a cell be laid in a solution of a substance, even

different from itself, it grows and converts this substance without

our aid. And this it is from which the process going on in the cells

(so long as we do not separate it into its several acts) obtains that

magical character, to which attaches the idea of Life.

From this we perceive how very different are the phenomena in the

formation of cells and of crystals. Meanwhile, however, the points

of resemblance between them should not be overlooked. They agree in

this important point, that solid bodies of a certain regular shape are

formed in obedience to definite laws at the expense of a substance

contained in solution in a fluid; and the crystal, like the cell, is

so far an active and positive agent as to cause the substances which

are precipitated to be deposited on itself, and nowhere else. We

must, therefore, attribute to it as well as to the cell a power to

attract the substance held in solution in the surrounding fluid. It

does not indeed follow that these two attractive powers, the power of

crystallization--to give it a brief title--and the plastic power of the

cells, are essentially the same. This could only be admitted, if it

were proved that both powers acted according to the same laws. But this

is seen at the first glance to be by no means the case: the phenomena

in the formation of cells and crystals, are, as we have observed, very

different, even if we regard merely the plastic phenomena of the cells,

and leave their metabolic power (which may possibly arise from some

other peculiarity of organic substance) for a time entirely out of the

question.

Is it, however, possible that these distinctions are only secondary,

that the power of crystallization and the plastic power of the cells

are identical, and that an original difference can be demonstrated

between the substance of cells and that of crystals, by which we

may perceive that the substance of cells must crystallize as cells

according to the laws by which crystals are formed, rather than in the

shape of the ordinary crystals? It may be worth while to institute such

an inquiry.

In seeking such a distinction between the substance of cells and that

of crystals, we may say at once that it cannot consist in anything

which the substance of cells has in common with those organic

substances which crystallize in the ordinary form. Accordingly, the

more complicated arrangement of the atoms of the second order in

organic bodies cannot give rise to this difference; for we see in

sugar, for instance, that the mode of crystallization is not altered by

this chemical composition.

Another point of difference by which inorganic bodies are distinguished

from at least some of the organic bodies, is the faculty of imbibition.

Most organic bodies are capable of being infiltrated by water, and

in such a manner that it penetrates not so much into the interspaces

between the elementary tissues of the body, as into the simple

structureless tissues, such as areolar tissue, &c.; so that they form

an homogeneous mixture, and we can neither distinguish particles

of organic matter, nor interspaces filled with water. The water

occupies the infiltrated organic substances, just as it is present in

a solution, and there is as much difference between the capacity for

imbibition and capillary permeation, as there is between a solution and

the phenomena of capillary permeation. When water soaks through a layer

of glue, we do not imagine it to pass through pores, in the common

sense of the term; and this is just the condition of all substances

capable of imbibition. They possess, therefore, a double nature,

they have a definite form like solid bodies; but like fluids, on the

other hand, they are also permeable by anything held in solution. As

a specifically lighter fluid poured on one specifically heavier so

carefully as not to mix with it, yet gradually penetrates it, so also,

every solution, when brought into contact with a membrane already

infiltrated with water, bears the same relations to the membrane, as

though it were a solution. And crystallization being the transition

from the fluid to the solid state, we may conceive it possible, or

even probable, that if bodies, capable of existing in an intermediate

state between solid and fluid could be made to crystallize, a

considerable difference would be exhibited from the ordinary mode of

crystallization. In fact, there is nothing, which we call a crystal,

composed of substance capable of imbibition; and even among organized

substances, crystallization takes place only in those which are capable

of imbibition, as fat, sugar, tartaric acid, &c. The bodies capable of

imbibition, therefore, either do not crystallize at all, or they do so

under a form so different from the crystal that they are not recognized

as such.

Let us inquire what would most probably ensue if material capable of

imbibition crystallized according to the ordinary laws, what varieties

from the common crystals would be most likely to show themselves,

assuming only that the solution has permeated through the parts of

the crystal already formed, and that new molecules can therefore

be deposited between them. The ordinary crystals increase only by

apposition; but there may be an important difference in the mode of

this apposition. If the molecules were all deposited symmetrically

one upon another, we might indeed have a body of a certain external

form like a crystal; but it would not have the structure of one,

it would not consist of layers. The existence of this laminated

structure in crystals presupposes a double kind of apposition of their

molecules; for in each layer the newly-deposited molecules coalesce,

and become continuous with those of the same layer already present;

but those molecules which form the adjacent surfaces of two layers

do not coalesce. This is a remarkable peculiarity in the formation

of crystals, and we are quite ignorant of its cause. We cannot yet

perceive why the new molecules, which are being deposited on the

surface of a crystal (already formed up to a certain point), do not

coalesce and become continuous with those already deposited, like the

molecules in each separate layer, instead of forming, as they do, a

new layer; and why this new layer does not constantly increase in

thickness, instead of producing a second layer around the crystal, and

so on. In the meantime we can do no more than express the fact in the

form of a law, that the coalescing molecules are deposited rather along

the surface beside each other, than in the thickness upon one another,

and thus, as the breadth of the layer depends upon the size of the

crystal, so also the layer can attain only a certain thickness, and

beyond this, the molecules which are being deposited cannot coalesce

with it, but must form a new layer.

If we now assume that bodies capable of imbibition could also

crystallize, the two modes of junction of the molecules should be

shown also by them. Their structure should also be laminated, at least

there is no perceptible reason for a difference in this particular,

as the very fact of layers being formed in common crystals shows that

the molecules need not be all joined together in the most exact manner

possible. The closest possible conjunction of the molecules takes place

only in the separate layers. In the common crystals this occurs by

apposition of the new molecules on the surface of those present and

coalescence with them. In bodies capable of imbibition, a much closer

union is possible, because in them the new molecules may be deposited

by intussusception between those already present. It is scarcely,

therefore, too bold an hypothesis to assume, that when bodies capable

of imbibition crystallize, their separate layers would increase by

intussusception; and that this does not happen in ordinary crystals,

simply because it is impossible.

Let us then imagine a portion of the crystal to be formed: new

molecules continue to be deposited, but do not coalesce with the

portion of the crystal already formed; they unite with one another

only, and form a new layer, which, according to analogy with the common

crystals, may invest either the whole or a part of the crystal. We

will assume that it invests the entire crystal. Now, although this

layer be formed by the deposition of new molecules between those

already present instead of by apposition, yet this does not involve

any change in the law, in obedience to which the deposition of the

coalescing molecules goes on more vigorously in two directions,

that is, along the surface, than it does in the third direction

corresponding to the thickness of the layer; that is to say, the

molecules which are deposited by intussusception between those already

present, must be deposited much more vigorously between those lying

together along the surface of the layer than between those which lie

over one another in its thickness. This deposition of molecules side

by side is limited in common crystals by the size of the crystal, or

by that of the surface on which the layer is formed; the coalescence

of molecules therefore ceases as regards that layer, and a new one

begins. But if the layers grow by intussusception in crystals capable

of imbibition, there is nothing to prevent the deposition of more

molecules between those which lie side by side upon the surface, even

after the lamina has invested the whole crystal; it may continue to

grow without the law by which the new molecules coalesce requiring to

be altered. But the consequence is, that the layer becomes, in the

first instance more condensed, that is, more solid substance is taken

into the same space; and afterwards it will expand and separate from

the completed part of the crystal so as to leave a hollow space between

itself and the crystal; this space fills with fluid by imbibition,

and the first-formed portion of the crystal adheres to a spot on its

inner surface. Thus, in bodies capable of imbibition, instead of a new

layer attached to the part of the crystal already formed, we obtain a

hollow vesicle. At first this must have the shape of the body of the

crystal around which it is formed, and must, therefore, be angular,

if the crystal is angular. If, however, we imagine this layer to be

composed of soft substance capable of imbibition, we may readily

comprehend how such a vesicle must very soon become round or oval. But

the first-formed part of the crystal also consists of substance capable

of imbibition, so that it is very doubtful whether it must have an

angular form at all. In common crystals atoms of some one particular

substance are deposited together, and we can understand how a certain

angular form of the crystal may result if these atoms have a certain

form, or if in certain axes they attract each other differently. But in

bodies capable of imbibition, an atom of one substance is not set upon

another atom of the same substance, but atoms of water come between;

atoms of water, which are not united with an atom of solid substance,

so as to form a compound atom, as in the water of crystallization, but

which exist in some other unknown manner between the atoms of solid

substance. It is not possible, therefore, to determine whether that

part of the crystal which is first formed must have an angular figure

or not.

An ordinary crystal consists of a number of laminæ; when so small as

to be but just discernible, it has the form which the whole crystal

afterwards exhibits, at least as far as regards the angles; we must

therefore suppose that the first layer is formed around a very small

corpuscle, which is of the same shape as the subsequent crystal. We

will call this the primitive corpuscle. It is doubtful what may be

the shape of this corpuscle in the crystals which are capable of

imbibition. The first layer, then, is formed around the corpuscle

in the way mentioned; it grows by intussusception, and thus forms

a hollow, round or oval vesicle, to the inner surface of which the

primitive corpuscle adheres. As all the new molecules that are being

deposited may be placed in this layer without any alteration being

required in the law which regulates the coalescence of the molecules

during crystallization, we must conclude that it remains the only

layer, and becomes greatly expanded, so as to represent all the

layers of an ordinary crystal. It is, however, a question whether

there may not exist some reasons why several layers can be formed.

We can certainly conceive such to be the case. The quantity of the

solid substance that must crystallize in a given time, depends upon

the concentration of the fluid; the number of molecules that may,

in accordance with the law already mentioned, be deposited in the

layer in a given time depends upon the quantity of the solution

which can penetrate the membrane by imbibition during that time. If

in consequence of the concentration of the fluid there must be more

precipitated in the time than can penetrate the membrane, it can only

be deposited as a new layer on the outer surface of the vesicle. When

this second layer is formed, the new molecules are deposited in it, and

it rapidly becomes expanded into a vesicle, on the inner surface of

which the first vesicle lies with its primitive corpuscle. The first

vesicle now either does not grow at all, or at any rate much more

slowly, and then only when the endosmosis into the cavity of the second

vesicle proceeds so rapidly that all that might be precipitated while

passing through it, is not deposited. The second vesicle, when it is

developed at all, must needs be developed relatively with more rapidity

than the first; for as the solution is in the most concentrated state

at the beginning, the necessity for the formation of a second layer

then occurs sooner; but when it is formed, the concentration of the

fluid is diminished, and this necessity occurs either later or not at

all. It is possible, however, that even a third, or fourth, and more,

may be formed; but the outermost layer must always be relatively the

most vigorously developed; for when the concentration of the solution

is only so strong, that all that must be deposited in a certain time,

can be deposited in the outermost layer, it is all applied to the

increase of this layer.

Such, then, would be the phenomena under which substances capable of

imbibition would probably crystallize, if they did so at all. I say

probably, for our incomplete knowledge of crystallization and the

faculty of imbibition, does not as yet admit of our saying anything

positively _a priori_. It is, however, obvious that these are the

principal phenomena attending the formation of cells. They consist

always of substance capable of imbibition; the first part formed is

a small corpuscle, not angular (nucleolus), around this a lamina is

deposited (nucleus), which advances rapidly in its growth, until a

second lamina (cell) is formed around it. This second now grows more

quickly and expands into a vesicle, as indeed often happens with

the first layer. In some rarer instances only one layer is formed;

in others, again, there are three. The only other difference in the

formation of cells is, that the separate layers do not consist of the

same chemical substance, while a common crystal is always composed

of one material. In instituting a comparison, therefore, between the

formation of cells and crystallization, the above-mentioned differences

in form, structure, and mode of growth fall altogether to the ground.

If crystals were formed from the same substance as cells, they would

probably, in these respects, be subject to the same conditions as the

cells. Meanwhile the metabolic phenomena, which are entirely absent in

crystals, still indicate essential distinctions.

Should this important difference between the mode of formation of

cells and crystals lead us to deny all intimate connexion of the two

processes, the comparison of the two may serve at least to give a clear

representation of the cell-life. The following may be conceived to be

the state of the matter: the material of which the cells are composed

is capable of producing chemical changes in the substance with which it

is in contact, just as the well-known preparation of platinum converts

alcohol into acetic acid. This power is possessed by every part of the

cell. Now, if the cytoblastema be so changed by a cell already formed,

that a substance is produced which cannot become attached to that cell,

it immediately crystallizes as the central nucleolus of a new cell. And

then this converts the cytoblastema in the same manner. A portion of

that which is converted may remain in the cytoblastema in solution,

or may crystallize as the commencement of new cells; another portion,

the cell-substance, crystallizes around the central corpuscle. The

cell-substance is either soluble in the cytoblastema, and crystallizes

from it, so soon as the latter becomes saturated with it; or else it is

insoluble, and crystallizes at the time of its formation, according to

the laws of crystallization of bodies capable of imbibition mentioned

above, forming in this manner one or more layers around the central

corpuscle, and so on. If we conceive the above to represent the mode

of formation of cells, we regard the plastic power of the cells as

identical with the power by which crystals grow. According to the

foregoing description of the crystallization of bodies capable of

imbibition, the most important plastic phenomena of the cells are

certainly satisfactorily explained. But let us see if this comparison

agrees with all the characteristics of the plastic power of the cells.

The attractive power of the cells does not always operate

symmetrically; the deposition of new molecules may be more vigorous in

particular spots, and thus produce a change in the form of the cell.

This is quite analogous to what happens in crystals; for although

in them an angle is never altered, there may be much more material

deposited on some surfaces than on others; and thus, for instance,

a quadrilateral prism may be formed out of a cube. In this case new

layers are deposited on one, or on two opposite sides of a cube. Now,

if one layer in cells represent a number of layers in a common crystal,

it may be easily perceived that instead of several new layers being

formed on two opposite surfaces of a cell, the one layer would grow

more at those spots, and thus a round cell would be elongated into a

fibre; and so with the other changes of form. Division of the cells

can have no analogue in common crystals, because that which is once

deposited is incapable of any further change. But this phenomenon

may be made to accord with the representation of crystals capable

of imbibition.... And if we ascribe to a layer of a crystal capable

of imbibition the power of producing chemical changes in organic

substances, we can very well understand also the origin of secondary

deposits on its inner surface as they occur in cells. For if, in

accordance with the laws of crystallization, the lamina has become

expanded into a vesicle, and its cavity has become filled by imbibition

with a solution of organic substance, there may be materials formed

by means of the converting influence of the lamina, which cannot any

longer be held in solution. These may, then, either crystallize within

the vesicle, as new crystals capable of imbibition under the form of

cells; or if they are allied to the substance of the vesicle, they may

so crystallize as to form part of the system of the vesicle itself:

the latter may occur in two ways, the new matters may be applied to

the increase of the vesicle, or they may form new layers on its inner

surface from the same cause which led to the first formation of the

vesicle itself as a layer. In the cells of plants these secondary

deposits have a spiral arrangement. This is a very important fact,

though the laws of crystallization do not seem to account for the

absolute necessity of it. If, however, it could be mathematically

proved from the laws of the crystallization of inorganic bodies, that

under the altered circumstances in which bodies capable of imbibition

are placed, these deposits must be arranged in spiral forms, it might

be asserted without hesitation that the plastic power of cells and the

fundamental powers of crystals are identical.

We come now, however, to some peculiarities in the plastic power of

cells, to which we might, at first sight, scarcely expect to find

anything analogous in crystals. The attractive power of the cells

manifests a certain degree of election in its operation; it does

not attract every substance present in the cytoblastema, but only

particular ones; and here a muscle-cell, there a fat-cell, is generated

from the same fluid, the blood. Yet crystals afford us an example

of a precisely similar phenomenon, and one which has already been

frequently adduced as analogous to assimilation. If a crystal of nitre

be placed in a solution of nitre and sulphate of soda, only the nitre

crystallizes; when a crystal of sulphate of soda is put in, only the

sulphate of soda crystallizes. Here, therefore, there occurs just the

same selection of the substance to be attracted.

We observed another law attending the development of the plastic

phenomena in the cells, viz. that a more concentrated solution is

requisite for the first formation of a cell than for its growth when

already formed, a law upon which the difference between organized and

unorganized tissues is based. In ordinary crystallization the solution

must be more than saturated for the process to begin. But when it is

over, there remains a mother lye, according to Thénard, which is no

longer saturated at the same temperature. This phenomenon accords

precisely with the cells; it shows that a more concentrated solution is

requisite for the commencement of crystallization than for the increase

of a crystal already formed. The fact has indeed been disputed by

Thomson; but if, in the undisputed experiment quoted above, the crystal

of sulphate of soda attracts the dissolved sulphate of soda rather

than the dissolved nitre, and _vice versa_, the crystal of nitre

attracts the dissolved nitre more than the dissolved sulphate of soda,

it follows that a crystal does attract a salt held in solution, because

the experiment proves that there are degrees of this attraction. But if

there be such an attraction exerted by a crystal, then the introduction

of a crystal into a solution of a salt, affords an efficient cause for

the deposition of this salt, which does not exist when no crystal is

introduced. The solution must therefore be more concentrated in the

latter case than in the former, though the difference be so slight

as not to be demonstrable by experiment. It would not, however, be

superfluous to repeat the experiments. In the instance of crystals

capable of imbibition, this difference may be considerably augmented,

since the attraction of molecules may increase perhaps considerably by

the penetrating of the solution between those already deposited.

We see then how all the plastic phenomena in the cells may be compared

with phenomena which, in accordance with the ordinary laws of

crystallization, would probably appear if bodies capable of imbibition

could be brought to crystallize. So long as the object of such a

comparison were merely to render the representation of the process

by which cells are formed more clear, there could not be much urged

against it; it involves nothing hypothetical, since it contains no

explanation; no assertion is made that the fundamental power of the

cells really has something in common with the power by which crystals

are formed. We have, indeed, compared the growth of organisms with

crystallization, in so far as in both cases solid substances are

deposited from a fluid, but we have not therefore asserted the

identity of the fundamental powers. So far we have not advanced beyond

the data, beyond a certain simple mode of representing the facts.

The question is, however, whether the exact accordance of the phenomena

would not authorize us to go further. If the formation and growth of

the elementary particles of organisms have nothing more in common with

crystallization than merely the deposition of solid substances from out

of a fluid, there is certainly no reason for assuming any more intimate

connexion of the two processes. But we have seen, first, that the laws

which regulate the deposition of the molecules forming the elementary

particles of organisms are the same for all elementary parts; that

there is a common principle in the development of all elementary parts,

namely, that of the formation of cells; it was then shown that the

power which induced the attachment of the new molecules did not reside

in the entire organism, but in the separated elementary particles (this

we called the plastic power of the cells); lastly, it was shown that

the laws, according to which the new molecules combine to form cells,

are (so far as our incomplete knowledge of the laws of crystallization

admits of our anticipating their probability) the same as those by

which substances capable of imbibition would crystallize. Now the

cells do, in fact, consist only of material capable of imbibition;

should we not then be justified in putting forth the proposition, that

the formation of the elementary parts of organisms is nothing but a

crystallization of substance, capable of imbibition, and the organism

nothing but an aggregate of such crystals capable of imbibition?

To advance so important a point as absolutely true, would certainly

need the clearest proof; but it cannot be said that even the premises

which have been set forth have in all points the requisite force. For

too little is still known of the cause of crystallization to predict

with safety (as was attempted above) what would follow if a substance

capable of imbibition were to crystallize. And if these premises were

allowed, there are two other points which must be proved in order to

establish the proposition in question: 1. That the metabolic phenomena

of the cells, which have not been referred to in the foregoing

argument, are as much the necessary consequence of the faculty of

imbibition, or of some other peculiarity of the substance of cells, as

the plastic phenomena are. 2. That if a number of crystals capable of

imbibition are formed, they must combine according to certain laws

so as to form a systematic whole, similar to an organism. Both these

points must be clearly proved, in order to establish the truth of the

foregoing view. But it is otherwise if this view be adduced merely as

an hypothesis, which may serve as a guide for new investigations. In

such case the inferences are sufficiently probable to justify such

an hypothesis, if only the two points just mentioned can be shown to

accord with it.

With reference to the first of these points, it would certainly be

impossible, in our ignorance as to the cause of chemical phenomena in

general, to prove that a crystal capable of imbibition must produce

chemical changes in substances surrounding it; but then we could not

infer, from the manner in which spongy platinum is formed, that it

would act so peculiarly upon oxygen and hydrogen. But in order to

render this view tenable as a possible hypothesis, it is only necessary

to see that it may be a consequence. It cannot be denied that it may:

there are several reasons for it, though they certainly are but weak.

For instance, since all cells possess this metabolic power, it is more

likely to depend on a certain position of the molecules, which in all

probability is essentially the same in all cells, than on the chemical

combination of the molecules, which is very different in different

cells. The presence, too, of different substances on the inner and

outer surface of the cell-membrane in some measure implies that a

certain direction of the axes of the atoms may be essential to the

metabolic phenomena of the cells. I think, therefore, that the cause of

the metabolic phenomena resides in that definite mode of arrangement

of the molecules which occurs in crystals, combined with the capacity

which the solution has to penetrate between these regularly deposited

molecules (by means of which, presuming the molecules to possess

polarity, a sort of galvanic pile will be formed), and that the same

phenomena would be observed in an ordinary crystal, if it could be

rendered capable of imbibition. And then perhaps the differences

of quality in the metabolic phenomena depend upon their chemical

composition.

In order to render tenable the hypothesis contained in the second

point, it is merely necessary to show that crystals capable of

imbibition can unite with one another according to certain laws. If

at their first formation all crystals were isolated, if they held

no relation whatever to each other, the view would leave entirely

unexplained how the elementary parts of organisms, that is, the

crystals in question, become united to form a whole. It is therefore

necessary to show that crystals do unite with each other according

to certain laws, in order to perceive, at least, the possibility

of their uniting also to form an organism, without the need of any

further combining power. But there are many crystals in which a union

of this kind, according to certain laws, is indisputable; indeed they

often form a whole, so like an organism in its entire form, that

groups of crystals are known in common life by the names of flowers,

trees, etc. I need only refer to the ice-flowers on the windows, or

to the lead-tree, etc. In such instances a number of crystals arrange

themselves in groups around others, which form an axis. If we consider

the contact of each crystal with the surrounding fluid to be an

indispensable condition to the growth of crystals which are not capable

of imbibition, but that those which are capable of imbibition, in which

the solution can penetrate whole layers of crystals, do not require

this condition, we perceive that the similarity between organisms and

these aggregations of crystals is as great as could be expected with

such difference of substance. As most cells require for the production

of their metabolic phenomena, not only their peculiar nutrient fluid,

but also the access of oxygen and the power of exhaling carbonic acid,

or _vice versa_; so, on the other hand, organisms in which there

is no circulation of respiratory fluid, or in which at least it is not

sufficient, must be developed in such a way as to present as extensive

a surface as possible to the atmospheric air. This is the condition of

plants, which require for their growth that the individual cells should

come into contact with the surrounding medium in a similar manner,

if not in the same degree as occurs in a crystal tree, and in them

indeed the cells unite into a whole organism in a form much resembling

a crystal tree. But in animals the circulation renders the contact of

the individual cells with the surrounding medium superfluous, and they

may have more compact forms, even though the laws by which the cells

arrange themselves are essentially the same.

The view then that organisms are nothing but the form under which

substances capable of imbibition crystallize, appears to be compatible

with the most important phenomena of organic life, and may be so

far admitted, that it is a possible hypothesis; or attempt towards

an explanation of these phenomena. It involves very much that is

uncertain and paradoxical, but I have developed it in detail, because

it may serve as a guide for new investigations. For even if no relation

between crystallization and the growth of organisms be admitted

in principle, this view has the advantage of affording a distinct

representation of the organic processes; an indispensable requisite for

the institution of new inquiries in a systematic manner, or for testing

by the discovery of new facts a mode of explanation which harmonizes

with phenomena already known.

FOOTNOTES:

[Footnote 33: Translated from _Mikroskopische Untersuchungen über die

Wachstum der Tiere und der Pflanzen_ (Berlin, 1839) by Henry Smith

in the _Publications of the Sydenham Society_ (1847).]

XXXI

HERMANN VON HELMHOLTZ

1821-1894

_Hermann von Helmholtz, born at Potsdam, Prussia, August 31, 1821,

studied medicine at the University of Berlin, from which he received

his degree in 1842. He then entered the German Army as surgeon and

in 1847 published his paper on “The Conservation of Energy,” which

summarized historically the development of the idea. In 1849 he was

appointed professor of physiology and general pathology at Königsberg.

In 1855 he was called to Bonn, and in 1858 was elected to the chair of

physiology at Heidelberg._

_In 1851 he invented the ophthalmoscope and later at Heidelberg he

continued his researches in the subject of sight, and also cleared up

the problem of the mechanical causes of sound. In 1871 he was appointed

professor of physics at the University of Berlin, where he remained

until his death, September 8, 1894._

THE CONSERVATION OF ENERGY[34]

A new conquest of very general interest has been recently made by

natural philosophy. In the following pages I will endeavour to give a

notion of the nature of this conquest. It has reference to a new and

universal natural law, which rules the action of natural forces in

their mutual relations towards each other, and is as influential on

our theoretic views of natural processes as it is important in their

technical applications.

Among the practical arts which owe their progress to the development of

the natural sciences, from the conclusion of the middle ages downwards,

practical mechanics, aided by the mathematical science which bears the

same name, was one of the most prominent. The character of the art

was, at the time referred to, naturally very different from its present

one. Surprised and stimulated by its own success, it thought no problem

beyond its power, and immediately attacked some of the most difficult

and complicated. Thus it was attempted to build automaton figures which

should perform the functions of men and animals. The wonder of the last

century was Vaucanson’s duck, which fed and digested its food; the

flute player of the same artist, which moved all its fingers correctly;

the writing boy of the older, and the pianoforte player of the younger

Droz: which latter, when performing, followed its hands with its eyes,

and at the conclusion of the piece bowed courteously to the audience.

That men like those mentioned, whose talent might bear comparison with

the most inventive heads of the present age, should spend so much

time in the construction of these figures, which we at present regard

as the merest trifles, would be incomprehensible, if they had not

hoped in solemn earnest to solve a great problem. The writing boy of

the elder Droz was publicly exhibited in Germany some years ago. Its

wheel-work is so complicated, that no ordinary head would be sufficient

to decipher its manner of action. When, however, we are informed that

this boy and its constructor, being suspected of the black art, lay

for a time in the Spanish Inquisition, and with difficulty obtained

their freedom, we may infer that in those days even such a toy appeared

great enough to excite doubts as to its natural origin. And though

these artists may not have hoped to breathe into the creature of

their ingenuity a soul gifted with moral completeness, still there

were many who would be willing to dispense with the moral qualities

of their servants if, at the same time, their immoral qualities could

also be got rid of; and accept, instead of the mutability of flesh

and bones, services which should combine the regularity of a machine

with the durability of brass and steel. The object, therefore, which

the inventive genius of the past century placed before it with the

fullest earnestness, and not as a piece of amusement merely, was boldly

chosen, and was followed up with an expenditure of sagacity which has

contributed not a little to enrich the mechanical experience which a

later time knew how to take advantage of. We no longer seek to build

machines which shall fulfil the thousand services required of one man,

but desire, on the contrary, that a machine shall perform one service,

but shall occupy in doing it the place of a thousand men.

From these efforts to imitate living creatures, another idea, also by

a misunderstanding, seems to have developed itself, which, as it were,

formed the new philosopher’s stone of the seventeenth and eighteenth

centuries. It was now the endeavour to construct a perpetual motion

machine. Under this term was understood a machine which, without being

wound up, without consuming in the working of it, falling water, wind

or any other natural force, should still continue in motion, the motive

power being perpetually supplied by the machine itself. Beasts and

human beings seemed to correspond to the idea of such an apparatus, for

they moved themselves energetically and incessantly as long as they

lived, were never wound up, and nobody set them in motion. A connection

between the taking in of nourishment and the development of force did

not make itself apparent. The nourishment seemed only necessary to

grease, as it were, the wheel-work of the animal machine, to replace

what was used up, and to renew the old. The development of force out of

itself seemed to be the essential peculiarity, the real quintessence of

organic life. If, therefore, men were to be constructed, a perpetual

motion must first be found.

Another hope also seemed to take up incidentally the second place,

which, in our wiser age, would certainly have claimed the first rank

in the thoughts of men. The perpetual motion was to produce work

inexhaustibly without corresponding consumption, that is to say, out

of nothing. Work, however, is money. Here, therefore, the practical

problem which the cunning heads of all centuries have followed in the

most diverse ways, namely, to fabricate money out of nothing, invited

solution. The similarity with the philosopher’s stone sought by the

ancient chemists was complete. That also was thought to contain the

quintessence of organic life, and to be capable of producing gold.

The spur which drove men to inquiry was sharp, and the talent of some

of the seekers must not be estimated as small. The nature of the

problem was quite calculated to entice poring brains, to lead them

round a circle for years, deceiving ever with new expectations, which

vanished upon nearer approach, and finally reducing these dupes of

hope to open insanity. The phantom could not be grasped. It would be

impossible to give a history of these efforts, as the clearer heads,

among whom the elder Droz must be ranked, convinced themselves of the

futility of their experiments, and were naturally not inclined to

speak much about them. Bewildered intellects, however, proclaimed

often enough that they had discovered the grand secret; and as the

incorrectness of their proceedings was always speedily manifest, the

matter fell into bad repute, and the opinion strengthened itself more

and more that the problem was not capable of solution; one difficulty

after another was brought under the dominion of mathematical mechanics,

and finally a point was reached where it could be proved that, at least

by the use of pure mechanical forces, no perpetual motion could be

generated.

We have here arrived at the idea of the driving force or power of

a machine, and shall have much to do with it in future. I must,

therefore, give an explanation of it. The idea of work is evidently

transferred to machines by comparing their arrangements with those of

men and animals to replace which they were applied. We still reckon

the work of steam engines according to horse-power. The value of

manual labor is determined partly by the force which is expended in

it (a strong laborer is valued more highly than a weak one), partly,

however, by the skill which is brought into action. A machine, on the

contrary, which executes work skilfully, can always be multiplied to

any extent; hence its skill has not the high value of human skill in

domains where the latter cannot be supplied by machines. Thus the idea

of the quantity of work in the case of machines has been limited to the

consideration of the expenditure of force; this was the more important,

as indeed most machines are constructed for the express purpose of

exceeding, by the magnitude of their effects, the powers of men and

animals. Hence, in a mechanical sense, the idea of work is become

identical with that of the expenditure of force, and in this way I will

apply it.

How, then, can we measure this expenditure, and compare it in the case

of different machines?

I must here conduct you a portion of the way--as short a portion

as possible--over the uninviting field of mathematico-mechanical

ideas, in order to bring you to a point of view from which a more

rewarding prospect will open. And though the example which I shall

here choose, namely, that of a water-mill with iron hammer, appears

to be tolerably romantic, still, alas, I must leave the dark forest

valley, the spark-emitting anvil, and the black Cyclops wholly out of

sight, and beg a moment’s attention to the less poetic side of the

question, namely, the machinery. This is driven by a water-wheel, which

in its turn is set in motion by the falling water. The axle of the

water-wheel has at certain places small projections, thumbs, which,

during the rotation, lift the heavy hammer and permit it to fall again.

The falling hammer belabors the mass of metal, which is introduced

beneath it. The work therefore done by the machine consists, in this

case, in the lifting of the hammer, to do which the gravity of the

latter must be overcome. The expenditure of force will, in the first

place, other circumstances being equal, be proportioned to the weight

of the hammer; it will, for example, be double when the weight of the

hammer is doubled. But the action of the hammer depends not upon its

weight alone, but also upon the height from which it falls. If it falls

through two feet, it will produce a greater effect than if it falls

through only one foot. It is, however, clear that if the machine, with

a certain expenditure of force, lifts the hammer a foot in height, the

same amount of force must be expended to raise it a second foot in

height. The work is therefore not only doubled when the weight of the

hammer is increased twofold, but also when the space through which it

falls is doubled. From this it is easy to see that the work must be

measured by the product of the weight into the space through which it

ascends. And in this way, indeed, do we measure in mechanics.

The unit of work is a foot-pound, that is, a pound weight, raised to

the height of one foot.

While the work in this case consists in the raising of the heavy

hammer-head, the driving force which sets the latter in motion is

generated by falling water. It is not necessary that the water should

fall vertically, it can also flow in a moderately inclined bed; but

it must always, where it has water-mills to set in motion, move from

a higher to a lower position. Experiment and theory coincided in

teaching, that when a hammer of a hundred weight is to be raised one

foot, to accomplish this at least a hundred weight of water must fall

through the space of one foot; or what is equivalent to this, two

hundred weight must fall full half a foot, or four hundred weight a

quarter of a foot, etc. In short, if we multiply the weight of the

falling water by the height through which it falls, and regard, as

before, the product as the measure of the work, then the work performed

by the machine in raising the hammer can, in the most favourable case,

be only equal to the number of foot-pounds of water which have fallen

in the same time. In practice, indeed, this ratio is by no means

attained; a great portion of the work of the falling water escapes

unused, inasmuch as part of the force is unwillingly sacrificed for the

sake of obtaining greater speed.

I will further remark, that this relation remains unchanged whether

the hammer is driven immediately by the axle of the wheel, or

whether--by the intervention of wheel-work, endless screws, pulleys,

ropes--the motion is transferred to the hammer. We may, indeed, by

such arrangements, succeed in raising a hammer of ten hundred weight,

when by the first simple arrangement, the elevation of a hammer of one

hundred weight might alone be possible; but either this heavier hammer

is raised to only one-tenth of the height, or tenfold the time is

required to raise it to the same height; so that, however we may alter,

by the interposition of machinery, the intensity of the acting force,

still in a certain time, during which the mill-stream furnishes us with

a definite quantity of water, a certain definite quantity of work, and

no more, can be performed.

Our machinery, therefore, has, in the first place, done nothing more

than make use of the gravity of the falling water in order to overpower

the gravity of the hammer, and to raise the latter. When it has lifted

the hammer to the necessary height, it again liberates it, and the

hammer falls upon the metal mass which is pushed beneath it. But why

does the falling hammer here exercise a greater force than when it is

permitted simply to press with its own weight on the mass of metal? Why

is its power greater as the height from which it falls is increased?

We find, in fact, that the work performed by the hammer is determined

by its velocity. In other cases, also, the velocity of moving masses

is a means of producing great effects. I only remind you of the

destructive effects of musket-bullets, which, in a state of rest, are

the most harmless things in the world. I remind you of the windmill,

which derives its force from the moving air. It may appear surprising

that motion, which we are accustomed to regard as a non-essential and

transitory endowment of bodies, can produce such great effects. But

the fact is, that motion appears to us, under ordinary circumstances,

transitory, because the movement of all terrestrial bodies is resisted

perpetually by other forces, friction, resistance of the air, etc.,

so that motion is incessantly weakened and finally neutralized. A

body, however, which is opposed by no resisting force, when once set

in motion, moves onward eternally with undiminished velocity. Thus

we know that the planetary bodies have moved without change, through

space, for thousands of years. Only by resisting forces can motion

be diminished or destroyed. A moving body, such as the hammer or the

musket-ball, when it strikes against another, presses the latter

together, or penetrates it, until the sum of the resisting forces which

the body struck presents to its pressure, or to the separation of its

particles, is sufficiently great to destroy the motion of the hammer

or of the bullet. The motion of a mass regarded as taking the place of

working force is called the living force (_vis viva_) of the mass.

The word “living” has of course here no reference whatever to living

beings, but is intended to represent solely the force of the motion as

distinguished from the state of unchanged rest--from the gravity of

a motionless body, for example, which produces an incessant pressure

against the surface which supports it, but does not produce any motion.

In the case before us, therefore, we had first power in the form of

a falling mass of water, then in the form of a lifted hammer, and,

thirdly, in the form of the living force of the fallen hammer. We

should transform the third form into the second, if we, for example,

permitted the hammer to fall upon a highly elastic steel beam strong

enough to resist the shock. The hammer would rebound, and in the most

favourable case would reach a height equal to that from which it

fell, but would never rise higher. In this way its mass would ascend:

and at the moment when its highest point has been attained, it would

represent the same number of raised foot-pounds as before it fell,

never a greater number; that is to say, living force can generate the

same amount of work as that expended in its production. It is therefore

equivalent to this quantity of work.

Our clocks are driven by means of sinking weights, and our watches by

means of the tension of springs. A weight which lies on the ground, an

elastic spring which is without tension, can produce no effects; to

obtain such we must first raise the weight or impart tension to the

spring, which is accomplished when we wind up our clocks and watches.

The man who winds the clock or watch communicates to the weight or

to the spring a certain amount of power, and exactly so much as is

thus communicated is gradually given out again during the following

twenty-four hours, the original force being thus slowly consumed

to overcome the friction of the wheels and the resistance which the

pendulum encounters from the air. The wheel-work of the clock therefore

exhibits no working force which was not previously communicated to it,

but simply distributes the force given to it uniformly over a longer

time.

Into the chamber of an air-gun we squeeze, by means of a condensing

air-pump, a great quantity of air. When we afterwards open the cock of

a gun and admit the compressed air into the barrel, the ball is driven

out of the latter with a force similar to that exerted by ignited

powder. Now we may determine the work consumed in the pumping-in of the

air, and the living force which, upon firing, is communicated to the

ball, but we shall never find the latter greater than the former. The

compressed air has generated no working force, but simply gives to the

bullet that which has been previously communicated to it. And while we

have pumped for perhaps a quarter of an hour to charge the gun, the

force is expended in a few seconds when the bullet is discharged; but

because the action is compressed into so short a time, a much greater

velocity is imparted to the ball than would be possible to communicate

to it by the unaided effort of the arm in throwing it.

From these examples you observe, and the mathematical theory has

corroborated this for all purely mechanical, that is to say, for

moving forces, that all our machinery and apparatus generate no

force, but simply yield up the power communicated to them by

natural forces--falling water, moving wind, or by the muscles of

men and animals. After this law had been established by the great

mathematicians of the last century, a perpetual motion, which should

make only use of pure mechanical forces, such as gravity, elasticity,

pressure of liquids and gases, could only be sought after by bewildered

and ill-instructed people. But there are still other natural forces

which are not reckoned among the purely moving forces--heat,

electricity, magnetism, light, chemical forces, all of which

nevertheless stand in manifold relation to mechanical processes. There

is hardly a natural process to be found which is not accompanied by

mechanical actions, or from which mechanical work may not be derived.

Here the question of a perpetual motion remained open; the decision of

this question marks the progress of modern physics.