Mendel’s work published in an obscure horticultural journal

remained neglected for forty years, till in 1900 his principle

was independently rediscovered by three continental workers--de

Vries, Tschermak, and Correns. During that period the study of the

reproductive process had progressed rapidly. The way was being paved

for new and spectacular developments of the atomistic standpoint

in heredity. To appreciate the subsequent elaboration of Mendel’s

hypothesis in its historical perspective a brief digression into the

anatomy of the cell is necessary.

Mendel’s researches were confined to plants. When he started his work

the nature of fertilization in animals was still obscure. The bodies of

animals like plants were known to be built up of microscopic bricks, or

_cells_ as Robert Hooke had called them. With the use of more powerful

microscopes this had gained general recognition during the thirties

and forties. Two centuries had elapsed, since Leeuwenhoek with the

first microscope had seen seminal fluid teeming with minute vibratile

bodies, the _spermatozoa_. At the end of the eighteenth century

that inquisitive ecclesiastic Spallanzani had shown that the sperm

is the essential constituent of the seminal fluid. By 1841 Kolliker

had traced the development of the spermatozoa from single cells of

the testis. It was not until 1875-9 that Hertwig and Fol working on

sea urchins independently observed for the first time in history the

penetration of the egg by the sperm, and established the universal

rule that fertilization involves the union of a single sperm with a

single egg cell. All modern discussion of genetic differences takes its

starting point from the fact that anything which is implied by the word

inheritance has its material basis in the microscopic sperm contributed

by the father or in the egg cell with which it unites.

In all animals the sperm is a microscopic entity. In all animals

from the jellyfish to Man with very few exceptions its appearance is

extraordinarily similar. It consists of a thicker portion to which is

attached a long vibratile process, or flagellum. The eggs of different

animals are of very different dimensions. Sometimes they contain

immense stores of food material (yolk). Sometimes as they pass to the

exterior by the female generative tract they are invested with an

additional slimy coat and a leathery or calcareous shell, secreted by

special glands. The immature egg of all animals is essentially similar.

In the living condition it is a spherical or ellipsoidal body in

which a clear spherical vesicle is seen; this vesicle present in all

cells is called the _nucleus_. The thicker part or body of the sperm

consists mainly of the nucleus of the cell from which it is derived. At

fertilization it swells up and unites with the nucleus of the egg. The

fertilized egg then divides into two separate segments or cells, and

the process of dividing is repeated an indefinite number of times. The

cells or segments into which the fertilized egg divides each contain a

nucleus, and the process of segmentation which involves the division of

cells into two is accompanied by the division of the nucleus of each

dividing cell. Like the testis or ovary the substance of all the organs

of the animal body is built up of the microscopic bricks which we

have called cells. In some tissues like bone and cartilage the bricks

are separated by a good deal of mortar. Others, such as the lining

membranes of the body, consist simply of cells packed tightly together.

At the beginning of embryonic existence all the cells are very much

alike. In the course of development the cells of different tissues

are considerably differentiated. Throughout all stages the process of

cell division always involves the partition of the nucleus in a highly

characteristic manner.

The details of this peculiar process, first elucidated by Flemming

and others during the seventies, has proved to be of astonishing

significance for the further understanding of Mendel’s hypothesis.

When a cell is about to divide, the nucleus looks like a tangle of

fine threads; and this tangle of fine threads resolves itself into a

number of readily distinguishable filaments which become progressively

shorter, assuming the appearance of stout rods staining deeply

with basic dyes. These rods, visible only with high powers of the

microscope, are the _chromosomes_, whose behaviour has provided us

with a tangible basis for Mendel’s conception of inheritance, and have

thereby permitted an extensive clarification and amplification of the

original hypothesis. From one point of view they might be said to have

done as much for the Mendelian conception of heredity as the discovery

of alpha particles has done for our belief in the atomic structure

of matter. As the dividing cell begins to constrict, the chromosomes

arrange themselves at its equator, and split longitudinally into

halves, each half travelling to opposite poles, where they spin out

again into fine threads from which the nuclei of the daughter cells are

built up. Thus each of the chromosomes in the nucleus of any cell in

the body is structurally equivalent to a corresponding chromosome in

the preceding or succeeding cell generation. About the year 1875 it was

recognized that this numerical constancy extends beyond the life of a

single individual. In every species of animal or plant the number of

chromosomes which can be counted in dividing nuclei is a constant for

the species.

With the discovery of this fact a new problem arose so soon as the

essential features of fertilization were appreciated. How is this

constancy maintained from generation to generation of new individuals?

Two investigators, Van Beneden and Boveri (1881-3), who worked on

the horse threadworm, a form which has only four chromosomes in the

dividing cells of the segmenting egg, showed that the egg and sperm

each contain only half the number of chromosomes characteristic of

the cells of the embryo. This conclusion turned out to be a perfectly

general one. Attention was immediately directed to the nuclear changes

which happen in the formation of the gametes. Innumerable cell

divisions occur in the testis or ovary of an animal. These are at first

similar in all respects to those which occur in the segmentation of the

developing embryo; but cell division goes on in the testis or ovary

throughout life. If we trace backwards the history of an individual

sperm or egg in the testis or ovary in which it originates, we find

a reduction of the number of chromosomes effected during the last

division but one, leading up to the formation of a sperm or ripe egg.

This penultimate division of the germ nuclei is preceded by the fusion

of the chromosomes lengthwise in pairs. When the division actually

takes place, each pair behaves like a single chromosome, splitting in

such a way that one member of each pair goes to form each daughter

nucleus. The succeeding division being normal, each gamete receives

half the number of chromosomes present in ordinary cell division. At

fertilization the normal number is restored. Thus each ordinary cell of

the body has a chromosome set of which half the components are paternal

and half maternal in origin.

In many animals and plants the chromosomes are very distinctly of

different sizes and shapes, and can be sorted out into corresponding

pairs. Such arrangements are constant for the species, and could only

be maintained constant, if each gamete contains one representative of

each pair. This means that the maternal and paternal constituents of

a pair are distributed in the reduction division to different cells.

The chromosomes therefore exist in pairs of which one element is of

maternal origin and one of paternal origin. Each gamete receives

one element of each pair, just as Mendel supposed that each gamete

contained either the paternal or maternal element of his paired

“factors.” By a curious coincidence this far-reaching conclusion was

first established in the very year which witnessed the application of

Mendel’s principles to animals by Bateson in England and Cuenot in

France (1902). Its recognition accompanied the elucidation of another

peculiarity of nuclear division, also destined to have important

theoretical consequences. In many animals there is found to be an

unequally mated pair of chromosomes, the XY pair. When this occurs, it

occurs in one sex only. In the alternate sex there is a corresponding

equal pair (XX). In birds and moths the female is the XY, the male

the XX individual. In other animals the male is usually found with

sufficiently careful measurement to have an unequal (XY) pair which

is equally mated in the female (XX). During the nineties it was found

that some animals had in one sex an odd number of chromosomes, a fact

which at first sight seemed to conflict with the numerical constancy

of the chromosomes. In the early years of the present century American

zoologists provided the key to an understanding of the discrepancy.

In all such cases the alternate sex has one more chromosome. The case

of the large cockroach will serve as an illustration. The male of

_Periplaneta americana_ (its technical name) has 33, the female 34

chromosomes. The eggs will all have 17 chromosomes. One-half of the

sperm will have 17, the other half 16 chromosomes. If a sperm of the

former class fertilizes an egg, the individual produced will be a

female (17 + 17 = 34); and if a sperm of the second type fertilizes

an egg, the individual produced will be a male (17 + 16 = 33). In an

animal with an unequally mated (XY) pair of chromosomes in the male

reduction will result in one-half of the sperm carrying the X and

one-half the Y chromosome. The eggs will all have the X, since this

chromosome is equally paired in the female. Thus an egg fertilized by

a Y-bearing sperm will become a male, while an egg fertilized by an

X-bearing sperm will develop into a female.

§3

By statistical reasoning Mendel had deduced from his experimental

data the existence of entities which behave just as the chromosomes

do. He had no direct evidence that his factors had any material basis

in the architecture of the germ cells. The new cell anatomy provided

independent confirmation of his predictions from an unexpected quarter;

but it was not immediately recognized that this was so. Antagonism to

the belief that the chromosomes fulfilled the requirements of Mendel’s

hypothesis is easily explicable. To Mendel’s first disciples his second

and first laws were equally sacrosanct. Mendel’s second law implies

that different pairs of hereditary factors behave quite independently

of one another. On such an assumption one of two deductions is

inevitable. Either the applicability of Mendel’s first law is extremely

restricted; or the number of factors is too large to permit of their

localization in the chromosomes. The sweet-pea, for instance, has only

seven pairs of chromosomes. If Mendel’s second law were as general as

the first, only seven pairs of factors could be accounted for by the

behaviour of the chromosomes. From this dilemma further development

of the atomistic view of heredity was rescued, when it was discovered

that Mendel’s second law is only a particular case of the possibilities

inherent in the first.

In 1910 Bateson and Punnet first discovered in the sweet-pea what they

then called “coupling and repulsion,” or as we now say, _linkage_.

Without going into the experimental data, we may define the phenomenon

of linkage in the following way. Suppose that these are two varieties

A and B which obey Mendel’s first law and two other varieties C and

D which likewise conform to its requirements, when crossed with one

another. Mendel’s second law stated that in a cross between AC and BD

the second generation will consist of the types AC, AD, BC and BD in

numerical proportions agreeable to the assumption that it is equally

likely that the factor determining A will be present in the same gamete

as the factor determining C or the factor determining D. Bateson

and Punnet found that this does not always happen. There is another

category of cases in which the factor which determines A sticks more

or less completely to the factor for C in preference to the factor for

D. The detailed analysis of these cases was at first made difficult

by Mendel’s literal symbolism, and his way of thinking of factors in

_pairs_. But the discovery of linkage at once led Lock to formulate the

fruitful suggestion that factors located on the same chromosome pair

would satisfy the requirements of linkage, while factors located on

different pairs of chromosomes would fit in with Mendel’s second law.

From this point onwards the most spectacular development came from

the study of inheritance in animals, and the significance of the

chromosomes was immensely reinforced by newly gained knowledge of sex

determination. Almost contemporaneously with the discovery of the sex

chromosomes or XY mechanism, as we now say, Leonard Doncaster had

elucidated in moths the phenomenon of sex-linked inheritance. This was

soon found to be of common occurrence in animals. Till this discovery,

which was made in 1905, the same results had always been obtained

in crosses of pure-bred varieties, whether the male or the female

parent displayed one or the other characteristic distinguishing them.

Doncaster’s work on the currant moth showed that there is a category of

cases which at first sight obey Mendel’s first law in its simplest form

when the cross is made in one way, but yield a different type of result

when the cross is carried out reciprocally with respect to the sex of

the parents. In such cases one sex is only able to transmit certain

characters to its offspring of the opposite sex. It was already known

that the XY sex (male in Man and most animals) can only transmit its X

chromosome to the XX type. The facts did not dovetail at first sight,

because sex-linked inheritance was originally elucidated in birds and

moths of which the female is the XY type. There was still an attitude

of hesitancy towards accepting Lock’s hypothesis, strengthened by the

persistence of an incorrect interpretation of the process of reduction

which had been made the basis of Weismann’s metaphysical speculations

concerning “germinal selection.”

When in 1914 Doncaster summed up the case for regarding the chromosomes

as the material basis of Mendel’s first law, a new era had already

dawned. Thomas Hunt Morgan, the central figure of a group of ardent

investigators at Columbia, had initiated a body of enquiries which

within half a decade eclipsed all other achievements that had

succeeded Mendel’s pioneer labours. About the time when Bateson first

encountered the phenomenon of linked inheritance Morgan began to rear

the fruit-fly Drosophila for breeding experiments. Till then genetic

experiment had been held in check by the slow rate at which most

convenient animals and plants reproduce and the expense entailed in

breeding them in sufficiently large numbers to permit statistical

inference. The fruit-fly completes its life cycle, if kept in warm

laboratory conditions, in a period of ten days. It is prolific. It

feeds on rotten banana skins. It therefore costs little to breed. To

these immense advantages it adds two others of supreme importance.

It has only four pairs of chromosomes readily distinguishable from

one another by size and shape; and it has produced in the laboratory

a crop of several hundreds of sports or _mutants_. Each mutant type

differs from the wild parent stock in some well-defined characteristic

inherited in crosses with the wild type in accordance with Mendel’s

first law. The mutant characters are extremely varied. One is

distinguished from the red-eyed parent by having white eyes, another

by having purple eyes, another by having no eyes at all. One is

distinguished by having wings that are practically vestiges, another

by wings that turn up at the tips, another by wings that are truncated

at their extremities. From the wild type which has a greyish body, one

mutant is distinguished by a deep black, another by yellow coloration.

The mutant characters are thus in general clear-cut differences

lending themselves to easy identification. With an animal that breeds

so rapidly and prolifically information accumulated with astonishing

rapidity. From data based on the study of a large assemblage of mutant

characters there soon emerged the precise requirements of Lock’s

hypothesis. All the mutant characters of Drosophila fall into four

groups. Members of the same group always tend to stick together in

hereditary transmission. Members of different groups like Mendel’s

dihybrids behave independently of one another. Of several hundred

mutant characters in Drosophila every one belongs to one of these four

linkage groups; and the number of chromosome pairs in Drosophila is

four.

This discovery was only the beginning of what might well be called

one of the faery tales of modern scientific research. In the way of

accepting Lock’s hypothesis there were still difficulties. It was in

evading the principal difficulty that Morgan’s school extended the

atomistic concept of heredity much further than his predecessors had

done. Till then the main outcome of experiments on breeding had been

to show that Mendel’s principle was of vastly wider applicability

than was at first supposed, and to engender the suspicion that the

patient unravelling of difficult and elusive cases would establish its

universal validity. As yet the world of Mendel’s atoms was without

form. Morgan and his colleagues gave it a map. Not content with

showing that Mendel’s atoms of heredity have their material basis in

the chromosomes, nor with actually identifying which chromosome is

significantly associated with a particular mutant character, Morgan

went further and localized the region of an individual chromosome in

which a particular Mendelian factor resides. He thus gave to Mendel’s

factors spatial co-ordinates in the living cell.

At the outset the study of linkage upon which the chromosome map is

based was facilitated in the case of Drosophila, because the varieties

dealt with were all known to be mutants from a fixed wild type. Thus

it was possible to break away from Mendel’s conception of “pairs”

of hybridizing characters. The Mendelian factor was replaced by the

mutant _gene_, by saying which is implied that a mutant arises because

at some point on a particular chromosome a physical change has taken

place. The gene is the Mendelian factor for the mutant condition, but

no assumptions are made about what determines the wild-type condition.

The inter-relationship of different characters is greatly simplified

by thinking only of the relation of one _mutant_ gene to another. The

discovery that all the genes fall into four groups corresponding to the

four groups of chromosomes presented one stumbling-block. Members of

the same group in general do not invariably stick together. When two

mutants are crossed the numerical proportions of the various types of

offspring give a definite value for the probability that the gene A and

the gene B will stick together or separate apart. This is a constant

for A and B. The constant used in practice is the tendency for A and B

to separate. Expressed as a percentage, it is called the _cross-over

value_. Some additional information was necessary to explain why A

and B do not always stick together, if they are associated with the

same chromosome. It was from the solution of this problem that the

chromosome map took shape.

Here the sex chromosomes came to the rescue. One very interesting

type of sport which has turned up in breeding the fruit-fly is not

recognizable by any discrete bodily peculiarity but merely by an

abnormality in the number of chromosomes. Of these the first to be

discovered was a type of female which has in addition to its usual

four pairs of chromosomes an additional Y chromosome. The XXY females

yield very extraordinary numerical results both as regards the sex

ratio and other characteristics, when used in making crosses involving

mutant characters. There is a class of mutant characters in Drosophila,

more than a hundred in all, distinguished by the fact that they are

not inherited symmetrically with regard to sex. They display linkage

_inter se_. They behave as “sex-linked” characters. The introduction of

XXY females into crosses involving such mutant characters results in

numerical ratios which are inexplicable on any assumption other than

the view that the sex-linked gene is referable to the X chromosome

alone. Yet, although the sex-linked genes are all borne on the same

chromosome, they do not invariably stick together in crossing. The

holistic chromosome clearly would not do. An atomistic chromosome had

to be put in its place.

The clue to this was provided by studying more closely the extent

to which the different genes stick together. Taking all the genes

located on the X chromosome this remarkable generalization emerged

from Morgan’s researches. If A, B, and C are three sex-linked genes;

if the probability that A and B will not stick together is _x_, and

the probability that B and C will not stick together is _y_, the

probability that A and C will not stick together is either the sum

or the difference of _x_ and _y_. The correspondence here stated is,

of course, subject to the margin of error permitted by the theory of

probability. To interpret this new _law of the linear alignment of the

genes_ Morgan made use of a structural peculiarity of the reduction

process. When the chromosomes pair in the reduction division, they

appear to become twisted. The appearance suggests that in the ensuing

split corresponding lengths of the original pair are interchanged. It

is very natural to assume that the likelihood that two points will be

separated from one another in such a manner is proportional to their

distance apart. So if the sex-linked genes are arranged in a series

along the length of the chromosome, the probability that A and C will

not stick together must be the sum of the probabilities that A and B

and B and C will be separated. This is just what experiment had shown

to be true. Thus all the genes on the X chromosome can be arranged in a

linear series. The intervals between consecutive genes in such a series

represents a space dimension.

The law of the linear alignment of the genes was soon found to apply

to the other groups of linked characters. Abnormalities in the

number of chromosomes have made it possible to identify each of the

remaining three linkage groups of the fruit-fly with its corresponding

pair of chromosomes. The first chromosome map of Drosophila was

constructed in 1916. It revealed the suggestive coincidence that the

number of ascertained points on each pair of chromosomes is roughly

proportional to its size. There is now very little doubt that the

work of the Columbia school has revealed an aspect of inheritance

which is of general significance. After years of patient work with the

relatively slow breeding sweet-pea, Punnet has at length elucidated

seven linkage groups corresponding to its seven pairs of chromosomes.

He has constructed a chromosome map of a seed plant on the basis of

the principle first established for the fruit-fly. A law which holds

good for two organisms so far apart in the evolutionary scale can

hardly be supposed to be lacking in universal validity. The chromosome

hypothesis may now take its place as one of the major generalizations

of biological science. The law of linear alignment has transformed

Mendel’s original conception of inheritance in a way which might

be compared with the elaboration of Dalton’s hypothesis after the

discovery of the law of combination of gases by volume. Mendel’s

atoms of heredity are now units spatially localized in larger units

of microscopically visible dimensions. These supermolecules are the

chromosomes.

Being a portion of living matter the chromosome is constantly

undergoing chemical change. Some critics of the chromosome hypothesis

have based objections upon this circumstance. The difficulty is more

apparent than real. Like the individuality of the modern atom the

individuality of the chromosome must be conceived in statistical terms.

For the discussion of the more familiar chemical reactions the statical

atom of traditional chemistry is adequate. For the interpretation of

hybridization experiments the diagrammatic chromosome of the text-book

suffices. In the field of radioactivity the statical atom makes way for

a dynamical model. So also in the domain of cell physiology we conceive

the chromosome as an ever-changing entity. The logical situation is

analogous in the two cases. Those who hold with Dr. Haldane that the

biologist must interpret his data in a manner different from that in

which the chemist or physicist interpret theirs have now to fall back

on the contention that the Mendelian view is only a partial picture of

heredity transmission. Anything which might have been said in favour

of this contention ten years ago has been weakened by recent work on

the inheritance of size. The pioneers of Mendelism selected clear-cut

hereditary differences which ordinarily manifest themselves in any

environment in which the animal or plant can live. They succeeded

in showing that a vast number of hereditary differences involving

a great variety of anatomical and physiological features conform to

the requirements of Mendel’s hypothesis. There was one category of

phenomena which remained obscure till quite recently. Differences in

size, height, body weight and the like vary greatly with environmental

conditions. Two stocks may be distinguished from one another by the

fact that the average member of one is measurably different from the

average member of another; but any given individual of one stock may

be indistinguishable from another individual of the other, because,

even when the environment is standardized as much as is practicable,

the range of variability of the two stocks overlaps. The analysis of

such cases cannot be undertaken by the ordinary technique of Mendelian

experiments; but certain statistical requirements of Mendel’s laws may

nevertheless be verified. By elementary statistical reasoning we can

deduce that the coefficient of variability of the progeny of a cross

between two inbred stocks must be a minimum in the first generation and

a maximum in the second. This has been shown to be true in a number of

crosses in which it is impossible to distinguish individual genetic

types by direct observation.

There is no longer any adequate reason to support the contention that

Mendel’s atomistic concept leads us to an incomplete understanding of

biparental inheritance in animals and plants. Those who assert that it

is so are now forced to fall back upon the last resort of obscurantism

by appealing to the magnitude of our ignorance. The modern theory of

the gene is a statistical construction consistently developed by a

logical interpretation similar to that adopted in elaborating the great

generalizations of physical science. The mechanist is often accused of

attributing vital processes to “chance” combinations of phenomena.

If the word chance is used to imply that we do not know the precise

conditions which determine such combinations, the statement is hardly

exceptionable. It might also imply that the phenomena which biologists

study can be successfully interpreted in terms of the mathematical

laws of chance. The history of Mendelism shows that these laws provide

a fruitful basis for predicting the behaviour of living systems, even

where physico-chemical hypotheses at present fail to throw light on

the phenomena which the biologist studies. The biologist is able to

progress to greater certainty of prediction only when he interprets his

data with the same logical method employed by the chemist and physicist

to deduce physical “laws.” Whatever the future holds in store for

further interpretation of heredity and variation on physico-chemical

lines, the progress already achieved has at every stage involved

elimination of holistic concepts by the ruthless application of

mechanistic logic. To the application of physico-chemical hypotheses no

branch of physiology has proved more recalcitrant than the physiology

of inheritance. No branch of physiology might more suitably be chosen

to cast doubt on Dr. Haldane’s recent statement that “anything which

can properly be called scientific physiology is impossible apart from

the assumption of _holism_.”

III. THE NATURE OF LIFE

“I am sorry then, I have pretended to be a philosopher: for I find

your questions very perplexing; and am in danger, if my answer be

too rigid and severe, of passing for a pedant and scholastic: if

it be too easy and free, of being taken for a preacher of vice and

immorality. However, to satisfy you, I shall deliver my opinion

upon the matter, and shall only desire you to esteem it of as

little consequence as I do myself. By that means you will neither

think it worthy of your ridicule nor your anger.”--David Hume, _The

Sceptic_

§1

Since a man must needs live before he can be a philosopher, no problem

of philosophy is more fundamental than the nature of life. There

is also no issue which provides more scope for vague, barren and

undisciplined discussion. A Regius Professor of Moral Philosophy,

whether he accepts the fact with resignation or refuses to do so, is a

piece of living matter. Perhaps this is why physicists are more vocal

than biologists in promoting a pacific solution of the territorial

dispute between science and traditional philosophy. At the present

moment it is the fashion among those who are writing on scientific

philosophy either to neglect the contribution of the biologist to

the world symposium, or to assume that the biologist in dealing

with living matter operates with different methods and different

concepts from those employed in physics. No phase in the history of

biology is more fitted than the present to illustrate the fundamental

unity of scientific method. In no branch of science is the limit of

applicability of scientific method a more significant issue.

Since a scientific concept is only a way of describing a class of

properties, the nature of life cannot refer to anything but the nature

of the properties of living things. Having arrived at some general

classification of the characteristic properties of living things, a

discussion of the nature of life in the light of modern biological

science presents two issues of pre-eminent interest. One is how far

the _methods_ employed in physical science have been successful

and are likely to continue to prove successful in dealing with the

properties of living matter. The other is how the increasing measure of

success which attends the utilization of purely physical _concepts_ to

interpret the properties of living matter is calculated to influence

our evaluation of the place of science in human thought. Whatever

differences of interpretation may exist among biologists on matters of

detail, it should at least be possible to infer from a survey of the

progress of biology whether the study of living matter is progressing

satisfactorily along the lines of quantitative analysis of experimental

data towards greater certainty of prediction, and whether there is good

reason to believe that the preservation of the teleological standpoint

in dealing with living matter is likely to ensure conspicuous success

in the same direction.

It may be admitted that there exists among biologists more unanimity

with reference to the first than towards the second issue. Every infant

science makes use of notions peculiar to its own province. Chemistry

has but lately passed beyond the stage when the concept of _affinity_

first became amenable to interpretation in thermodynamical quantities.

There are still many biologists who would assert that the concept of

_adaptation_ demarcates the province of biology from that of physics

and chemistry by an impassable gulf. There are others, fewer in

number, who, surveying the teleological growing pains of the more exact

sciences and bearing in mind that only 300 years have passed since the

properties of familiar chemical compounds were literally personified as

spirits of wood, spirits of salt and the like, do not feel compelled to

regard the concept of adaptation as final. They are able to entertain

the possibility that those properties which enable an organism to

maintain its continued existence as an organism are not permanently

more incapable of physical interpretation than the polarization of a

voltaic battery, a phenomenon which the consistent teleologist would

presumably regard as an attempt on the part of the latter to save its

own life. Clearly the onus of defining what precisely is implied in the

concept of adaptation lies on those who assert its uniqueness. Until

the vitalist is more definite on this issue, the mechanist is under no

obligation to refrain from classifying the properties of living matter

in the light of his own experience. The mechanist denies that anything

is to be gained by clinging to the teleological standpoint with its

implication of some extra- or intra-mundane purpose which has been

abandoned in all branches of science that lay claim to exactitude. He

refuses to deal with living matter except in as far as it is considered

as a series of “events” whose characteristics must be interpreted with

rigid economy of hypothesis.

In approaching any lump of living matter, let us say the author of

these essays, as an object of the external world, the maintenance of

economy of hypothesis compels the enquirer to seek as far as possible a

common basis for the characteristic properties of living and non-living

systems. This necessitates a clear definition of the distinction

between the two. Taking a comparatively complex organism, as, for

instance, the common frog, a distinction might be attempted along the

following lines. In the first place, its possibilities of behaviour

are more varied than those of any machine which can be manufactured

by man; yet, while possessing a greater range of reversible response

than any non-living system, it would be difficult to specify in a

living system any single activity which could not be reproduced by a

mechanical system. Apart from this diversity, which we may refer to

under the generic term _reactivity_, living matter is characterized

in general by the wide range of external influences which are

significant in determining its characteristic reversible responses.

This peculiarity, in view of the subjective preconceptions implicit

in the older terms irritability, sensation, etc., may be denoted by

the term _receptivity_. Here again it is impossible to isolate any

single agency (or “_stimulus_”) capable of evoking reversible change

in any living system and incapable of evoking reversible change in

any non-living system. Finally--and at first sight--a more diagnostic

difference between living and non-living matter is seen in the property

of _reproduction_ (taken in the broader sense of the term, to include

growth). A given piece of living matter comes into being in our

experience only through the agency of other pieces of living matter

closely resembling itself.

§2

Were the more obscure process of sexual reproduction universally

characteristic of living matter, this distinction would appear

especially fundamental. Experimental biology is far from the

achievement of a complete physico-chemical analysis of asexual

reproduction in any type of organism. On the other hand, in the life

cycles of those multitudes of micro-organisms which multiply by simple

fission after attaining a certain limit of growth, there is nothing

which compels an unprejudiced investigator to regard the process as

more intrinsically incapable of physical interpretation than the

splitting into two of a liquid drop. Although our knowledge of the

nature of sexual reproduction is fragmentary, in this very field some

particularly spectacular advances have been registered in substituting

physical agencies as effective instruments for initiating processes

which at one time were only amenable to the influences of living matter

itself. Thirty years have now passed since Loeb’s discovery that

changes in the osmotic pressure of the external medium or alteration

of the permeability of the egg itself, leading to changes in its own

internal osmotic pressure, can initiate without any assistance from the

sperm the development of the ovum into a new and complete organism.

That discovery was the starting-point of a body of investigations whose

influence has radiated into many other fields of biological enquiry.

Especially noteworthy in this connexion is the work of Warburg during

the last decade. Warburg was able to show that sea-urchin eggs, and

later animal cells in general, if rapidly dehydrated and ground to

a powder, will, like the intact cell, absorb oxygen for some time

when moistened. He showed also that this property, like respiration

in the intact cell, can be abolished by the action of cyanides and

other classes of tissue poisons. By doing so, Warburg has taken a

characteristic and highly complex property of living matter out of

the realm of vitalism into that of physical chemistry. His analysis

went further. Experiment showed that three classes of poisons which

inhibit tissue respiration can be distinguished by their quantitative

relations. Of these, the efficacy of one class, the cyanides, was

shown by Warburg to be correlated with the iron content of the

cell. On the hypothesis that iron catalysis is the main factor in

the oxidation of organic material in the cell, Warburg manufactured

suspensions of charcoal with a high iron content capable of catalysing

the auto-oxidation of sugars, fats, etc. The catalytic activity of

these suspensions was found to be related quantitatively to the three

categories of respiratory poisons in a manner closely parallel to the

action of the latter on tissue respiration.

Though reproduction is, in some respects, to the biologist at least,

the most fundamental of all the three features which I have defined

above, the ever-changing reactivity and manifold receptivity to

external influences so characteristic of living matter pre-eminently

engage our attention in connexion with the more intimate and subtle

issues of a field of enquiry which biology may yet claim. I refer to

the analysis of human behaviour. In this connexion I shall mention

progress in three directions as illustrating the transition from

teleological to quantitative treatment during the last half century;

namely, the physical analysis of the events which constitute an

isolated unit of response or reflex, the integration of reflexes in the

normal behaviour of animals, and the determination of new behaviour

patterns along the lines laid down by Pavlov’s school. With regard to

the first, we will consider the effect of flashing a bright light upon

an animal that has been previously kept in the dark. The characteristic

response, let us say, blinking of the eyelids, and the intervening

events involved are, first, a physical change in a receptive area,

namely, the retina; secondly, the propagation of the disturbance

there set up along a certain path, the nervous system; thirdly, the

liberation of a considerable quantity of energy at the seat of response

or effector organ, that is to say, the muscles of the eyelid.

Our knowledge of the nature of receptivity is least complete. That it

is a measurable physical event is beyond dispute. When light impinges

upon a given area of the retina there follows a characteristic series

of changes in electrical potential of the excited area with reference

to a non-excited area. Through the work of Jolly, Adrian and others

the sequence, the time relations and the magnitude of these changes

are being related to the intensity and duration of the stimulus

within predictable limits for a given species. These events initiate

the propagation of the disturbance known as the nervous impulse. The

nervous impulse is a physical event whose space-time relations can be

defined as concretely as the passage of an electric current through a

wire. Three-quarters of a century ago Helmholtz showed that the time

which elapses between the application of a stimulus to a nerve and

contraction of its attached muscle is a linear function of the distance

between the latter and the point of application of the stimulus. The

conception of the nervous impulse as a physical event had been, till

this discovery, entirely repugnant to scientific thought. We now know

not only, as Helmholtz showed, that the nervous impulse has definite

space-time co-ordinates, but that it has the dimensions of energy. Its

passage corresponds to the rate of propagation of an electrical change

of an analogous character to the electrical response of the excited

retina. The total energy of its propagation has been recently measured

by Gerrard and A. V. Hill from determinations of heat production during

its passage. Its mass relations are attested by a measurable increase

in the carbon dioxide production of stimulated nerve. The rate of

propagation of the nervous impulse varies like all chemical reactions

in a characteristic way with increase in temperature. The goal of the

nervous impulse after it has traversed one or more synapses in the

central nervous system is the effecter organ itself--in the case of

blinking of the eyelids, a muscle fibre. During the past two decades

a series of brilliant researches based on calorimetric methods have

revolutionized our knowledge of the final component of the reflex.

A. V. Hill and Meyerhof have correlated the chemical and energetic

changes accompanying muscular contraction with a precision of the order

expected in purely physico-chemical determinations. They have shown

that the total energy of muscular contraction can be quantitatively

related to the energy liberated _in vitro_ by the breakdown into lactic

acid of an amount of glycogen equivalent to that which is converted

into lactic acid in the actual contractile process.

Passing from the analysis of the constituent events of the reflex to

the integration of reflexes in normal behaviour, we are faced with a

striking change in the attitude of enquiry adopted in the study of

those aspects of behaviour determined by generalized stimuli such as

light and gravity and denoted by the term _tropisms_. Three-quarters

of a century ago, after Helmholtz had dispelled the belief that

identified the nervous impulse with an imponderable psychical

principle, biologists like Lubbock were content for the most part with

the statement that the moth flies towards the candle because it likes

the light. The work of Loeb and others has shown that the state of

contraction of particular groups of muscles is reflexly determined by

the stimulation of particular areas of the retina. It is a mechanical

necessity that when different areas are unequally stimulated,

differences in tension of different groups of muscles will bring the

body into such a position that symmetrically opposite areas will be

equally illuminated. The animal must move, as in fact it does, along

the path of the incident beam, whether by so doing it brings itself

into a brighter, or, as can easily be arranged experimentally, a darker

situation. Whereas the older and purely teleological attitude permits

us to predict nothing of consequence, the objective interpretation of

tropisms by experimental methods permits us to make many verifiable

predictions, as, for instance, the fact that the moth will move in

circles, if one eye is blackened, owing to the fact that the muscles on

that side will be more relaxed.

By the end of the nineteenth century, experimental biologists were

generally disposed to the belief that the analysis of the reflex and

the integration of reflex systems were problems not of apologetics but

of energetics. Investigation had been confined to those aspects of

behaviour which are for practical purposes invariable responses to a

particular situation. From the human standpoint the most fascinating

feature of the behaviour of an organism is, after all, the extent

to which its behaviour is conditioned not by the immediate but by

the antecedent situation. In the opening years of this century the

researches of Sherrington were elucidating the integration of reflexes

in normal behaviour. Restricted as they were to the decerebrate

animal, the traditional distinction between reflex and voluntary

activity remained as a defeatist formula in biological nomenclature.

The distinction was not a gratuitous olive branch to introspective

philosophy. It had its objective basis in the domain of behaviour

which is not uniquely determined by the immediate stimulus, when all

synchronous conditions have been standardized. That distinction has

been superseded to-day by the work of Pavlov’s school, which has shown,

first, that in the higher animals with the cerebrum intact, new reflex

systems can be built up experimentally under perfectly definable and

reproducible conditions; that the relations between such conditioned

reflexes can be defined in the language of space and time; and that the

concept of sensation can be externalized by reference to the ability of

a given stimulus to become a specific agent in the building up of a new

reflex system. In short, it is legitimate to anticipate the possibility

of giving a complete specification of how such an animal as a dog

will behave in a given situation without recourse to the traditional

nomenclature of memory, consciousness, sensation, etc.

Pavlov’s work is now accessible to the English reader through two

translations of the Russian physiologist’s own writings and several

excellent résumés, such as the one given in Lovatt Evans’ _Recent

Advances in Physiology_. How far-reaching are its consequences has not

been widely recognized even by biologists themselves. Experimental

biology, during its brief career, has attempted to accommodate itself

to the introspective temper of traditional philosophy by a compromise

explicitly formulated in the writings of Descartes, who bequeathed

to physiology the dualism of mind and matter. In conformity with the

Chaldæan mythos, many philosophers, Descartes among them, have endowed

Man alone with soulfulness. The coming of the Evolutionary hypothesis

has broken down so inflexible a distinction between Man and other

forms of living matter. Evolutionists in the nineteenth century, like

Haeckel, were prepared to equip the Amœba with a soul. In our time

the Cartesian compromise has again shifted its boundaries. By the

beginning of this century the moth once more had gone to join the

candle. Still Man stood with a little family of mammals around him,

each with one leg on either side of the frontier that separates the

universe of space and time from the Platonic world of universals.

Pavlov has taken those aspects of behaviour which would have been

referred twenty years ago to exclusively introspective concepts, and

has treated them successfully as predictable configurations in a

space-time framework. The little family of mammals has been let through

the tollgate of the Cartesian compromise. A new school of psychologists

has come into being with the express object of making psychology a

physical science, relieving Man, the celestial pilgrim, of his burden

of soul.

Philosophers have always had a legitimate cause for complaint that

biologists were unable to deal with those aspects of human life

which interest people most. The distinction between reflex and

voluntary activity provided the fullest absolution for that amiable

libertarianism which we all entertain under the influence of alcohol

and love. Because that distinction was implicit in the outlook of the

most radical mechanists of the last generation, Loeb among them, Dr.

Haldane finds it so easy to point out the inadequacy of the mechanistic

outlook. In the light of Pavlov’s work we can now envisage the

possibility that the methods of physical science will one day claim the

whole field of what can be properly called knowledge. If I am right in

cherishing such an opinion, it would thus appear that the investigation

of the conditioned reflex initiates a new epoch in biology, pregnant

with more far-reaching philosophical implications than the evolutionary

speculations of the nineteenth century. The fact that no reference to

the conditioned reflex is contained in Dr. Haldane’s Gifford Lectures

may in part account for the fact that he can so easily dispose of the

mechanistic position. The modern mechanist does not say that thought

and love and heroism do not exist. He says, show me behaviour to

which you apply the adjectives thoughtful or loving or heroic, and we

will, one fine day, endeavour to arrive at predictable conclusions

with reference to it by following the only method of enquiry which

we have learned by experience to trust. When Dr. Haldane goes out of

his way to dispose of the puerile formula that thought is a secretion

of the brain, as bile is a secretion of the liver, and does so, I

gather, under the impression that mechanists either believe it to mean

something or alternatively shut their eyes to the major problems of

existence, I can only respectfully suggest that he is flogging a dead

horse, while the living ones are getting out of the vitalistic stables.

I have endeavoured so far to indicate the increasing measure of

success that has crowned the application of physical methods and the

use of physical concepts in modern biological investigation. I have

attempted to illustrate the continuous retreat from teleological

concepts that has accompanied this advance. In asking what progress

may be anticipated from encouraging the teleological attitude to the

nature of life, I wish now to urge that the important advances of

biological science during the last hundred years have not only involved

continual abandonment of teleological concepts, but have consistently

been made in the teeth of opposition from the vitalists, organicists

and holists of their time. A century ago, in the same year that

witnessed Wöhler’s announcement of the successful synthesis of Urea,

the great chemist Henry wrote (1827) concerning organic compounds:

“It is not probable that we shall ever attain the power of imitating

Nature in these operations. For in the functions of a living plant a

directing principle appears to be concerned peculiar to animated bodies

and superior to and differing from the cause which has been termed

chemical affinity.” Only six years before Helmholtz’s determination

of the velocity of the nervous impulse in 1851, Johannes Müller had

declared that to measure the propagation of that imponderable psychical

principle was a theoretical absurdity.

It is not unlikely that before another celebration of the centenary

of Wöhler’s achievement, Fischer’s synthesis of an octadecapeptide

will have been surpassed by the manufacture of complex proteins in the

laboratory. Looking forward a little in the light of what success has

crowned the construction of physical models of vital processes, it

is, as Sir Edward Sharpey Schafer scandalously suggested at a meeting

of the British Association some years ago, perfectly legitimate to

entertain the possibility, even the likelihood, that scientists will

one day construct from artificially synthesized organic materials,

systems with so wide a range of reversible reactivity and receptivity

to external influences that they would be called organisms, if met with

in Nature. While taking a more hopeful view in this matter than some

biologists, I would remark that the validity of the mechanistic outlook

is quite independent of this possibility. The security of any dynamical

system of treating the motions of the heavenly bodies is independent of

the possibility that human effort could manufacture a new satellite for

Jupiter.

§3

If we can assert that the present phase of biological enquiry is

a peculiarly fruitful one, and that there is no reason to see any

immediate cessation of progress in the use of physical concepts as

the basis of our analysis of the properties of living matter, can we

not go further and state that we have absolutely no encouragement

for entertaining the hope that any deeper knowledge will accrue from

apostrophizing under the sobriquets of entelechy, life force, élan

vital that elusive entity to which, perhaps, the poet William Blake

referred as Old Nobodaddy? It is doubtful whether we shall see a

recrudescence of such frankly animistic devices as these. As biology

becomes more technical and more exact, an aptitude for rehabilitating

oriental mysticism in somewhat unusual verbiage will be regarded

as an insufficient equipment for entering the field of biological

controversy. The investigator who abandons physics for the pursuit

of biology will contribute new ideas. Fruitful contributions need no

longer be expected from those who combine the pursuit of literature

with an amiable interest in natural history. The days of Butler and

Bergson are passed.

Dr. Haldane, the most vigorous contemporary critic of the mechanistic

standpoint, is very anxious to avoid any suspicion of being tainted

with the cruder forms of vitalism. He disowns any allegiance to the

life force, élan vital et hoc genus omne, except in so far as he,

somewhat mysteriously, contrives to introduce an adventitious deity

into the latter portion of his Gifford Lectures. This does not make

its appearance until his major thesis is complete. Anxious as is Dr.

Haldane to disclaim adherence to the tenets of vitalism, he is very

definite in denying the possibility that atomistic concepts will

ever successfully deal with the problem of what he calls “conscious

behaviour.” In the light of Pavlov’s work we see that the problem of

what is usually called conscious behaviour, or as we should rather say

_conditioned behaviour_, can now be approached as a problem in the

study of those conditions which determine whether a new reflex will,

or will not, be brought into being. We may state this in other words

by enquiring how the passage of impulses along particular tracts in

the central nervous system _influences the facility with which the

nervous impulse will pass across a particular type of synapse_. Since

the problem of the conductivity of the synapse is, as we have seen,

an essentially physical problem, it is not overstating the case to

say that the work of Pavlov’s school has brought the study of what

Dr. Haldane calls “conscious behaviour” within the realm of physical

enquiry. Once this is fully grasped it no longer seems incredible

that the interpretation of conditioned behaviour will eventually come

within the scope of physico-chemical analysis. Contrary to the holistic

standpoint, we are thus led to an atomistic concept of individuality.

This I shall venture to formulate as _the statistical probability

that in an immensely elaborate system of reversible reactions a

certain number of states characteristic of any given moment will be

reproducible at another moment_.

In his Lowell lectures Professor A. V. Hill lays down two general

conclusions derived from the extension of modern biological enquiry.

First, as I have endeavoured to show, there is no limit to the extent

to which the mechanisms of life can be elucidated with the aid of

physical methods and concepts. Second, that, however far we get, we

shall still find function, adaptation, organization and purpose in

the processes we explore. I would venture to suggest that, however

alluring such a compromise between vitalism and mechanism may appear,

these two conclusions, though formally in nowise inconsistent, are,

nevertheless, in practice incompatible. As Henderson points out in his

_Fitness of the Environment_, if we wish to indulge in teleological

phantasies, we can find as much scope in physics and chemistry as in

biology. We do not dismiss the hypothesis that thunderstorms occur when

a blue unicorn sneezes on Uranus, because it is actually possible to

disprove so engaging a fancy, but simply because other ways of treating

thunderstorms lead to more useful conclusions.

Hence it seems to me that as we come to understand more and more about

the mechanics of living systems by using methods of which Professor

Hill is so brilliant an exponent, we shall inevitably find ourselves

talking less and less about purpose and function. In consequence many

of the problems which now engage the attention of philosophers will be

relegated to the same status as the philosopher’s stone. No doubt such

a change will come very gradually, so gradually that we shall hardly

notice it. Nevertheless, one may venture to predict that philosophers,

already forced by the developments of modern physics to divert their

attention from the pretentious crossword puzzles of the Hegelian

tradition, will sooner or later be driven to take account of the

post-evolutionary developments in biology, and more especially those

which have their starting-point in Pavlov’s researches.

By undertaking the analysis of the characteristics of conscious

behaviour without departing from the methods of the traditional

physiology of reflex action, biological science in our generation has

shown that there is no nicely defined boundary at which physiology

ends and moral philosophy begins. Hitherto physiology and academic

philosophy have developed independently, because physiologists

themselves have accepted the common sense dualism of mind and matter.

Moral philosophy can no longer claim that there is any distinctive

aspect of the Nature of Life, which lies beyond the province of

physiological enquiry. If any fundamental distinction between mind

and matter remains, that distinction henceforth defines the antinomy

of a _public world_ of common beliefs which all can share, the

conceptual world of science in which ethical neutrality and economy

of hypothesis reign supreme, and, in contradistinction to that public

world, many _private_ worlds which for the present remain impenetrable

through the medium of discourse. Biological science is continually

socializing our beliefs. What seems irrevocably part of the private

worlds of one generation becomes irrevocably part of the public world

of its grandchildren. Thus the new pluralism will not be, like the

Cartesian system, static, but dynamic. It is ever tending towards a

monistic outlook as a limiting case. Such a monism, unlike traditional

materialism and traditional idealism will be regarded not as a formula

but as an asymptote. It is evidently immaterial to _public_ discourse

whether we _privately_ entertain the view that _the_ public world is

more or less _real_ than _our_ private worlds. It would thus seem that

as biological science invades the province of human behaviour the

concept of _publicity_, as I venture to call the communicability of

beliefs, will come to occupy the status of importance which _reality_

has held in the systems of egocentric philosophers.

The public world, as I have conceived it, is a construction based on

the continuous extension of the principle of mechanism. The principle

of mechanism, that a complex system is interpretable only by reference

to the properties of its constituent parts, is not urged in the spirit

of dogmatic assertion, but because it has served us well in the past.

We still await any single verifiable conclusion that is uniquely

developed from any alternative principle. Holism, the newest form

of Vitalism, claims to have found an alternative or supplementary

principle that is essentially teleological. The holist does not specify

by reference to any single concrete situation how he proposes to use

his principle. It is admitted even by the mechanist that we are not

in a position to construct a symbolic relation which will completely

describe the vagaries of a Ford car in terms of the field equations

of the proton and electron. Does the holist wish us to believe that

we can help anyone to drive a car by assuring him that at every level

of complexity between the internal structure of the atom and the

newly licensed automobile there emerges an ever-increasing urge to a

wholeness or unity which is somehow indefinably different from the

interaction of the parts? Verily the mechanist of all people knows that

we know in part and we prophesy in part. For this very reason, because

he is prepared to await with patience the slow advance of science, he

refuses to subscribe to high-sounding pseudonyms for ignorance and

principles that are never seriously intended to be put into practice.

In the recent symposium on _The Nature of Life_ before the British

Association both General Smuts in his exposition of the holistic

standpoint and Dr. Haldane who supported him dwelt upon the supposed

collapse of mechanistic principles in physics itself. The former

cited in support of his point a somewhat rhetorical remark by Dr.

Whitehead in this sense. It is of course evident that if our mechanical

principles undergo modification our biological interpretations must

share in the general change of outlook. It is, therefore, beside the

point to criticize the mechanistic standpoint on the ground that our

mechanical principles are undergoing revision. Let us examine this

objection a little more closely. Experimental biology, we are told,

has been directed towards the attempt to describe the properties

of living matter in terms of the traditional physical concepts of

mass, length, time, energy, etc. Since these concepts now appear to

be less fundamental than we once believed, the hope that a complete

mathematical description of the universe is realizable, has, as

Mr. Sullivan asserts with triumphant _naïveté_, “no longer any

plausibility.” Surely it is evident that a signal advance towards

a more monistic interpretation of nature has been made, when the

analysis of any biological phenomenon has been achieved with the aid

of traditional physical concepts, and when concepts once peculiar

to biology, as affinity was once peculiar to chemistry, have been

translated into the traditional language of physics. Physics to-day

is seeking a new synthesis to take into one system of equations all

the old data, and many new ones which have lately accumulated. This

is not a new situation. The old mechanics remains as valid as ever

for the realm in which it was developed to operate. To effect a more

comprehensive scheme it has been necessary to examine many of the

old postulates. In the meantime we have to recognize that we are not

so near to a single unifying hypothesis as the rise of energetics

led the physicists of Kelvin’s generation to hope. What does this

signify? Certainly not that mechanics has abandoned the principles

of mechanism. Is it not rather a fact that the modern physicist is

complaining that the inadequacy of Newtonian principles is in part

attributable to teleological implications insufficiently recognized

till now? The very hope of finality which Kelvin’s generation

entertained seems from the new mechanistic standpoint, as I have stated

it, to savour of scholasticism.

In taking this line General Smuts and Dr. Haldane seem to me to have

laid bare the source of a misunderstanding that lies at the root of

most of the criticism which vitalists old or new direct against the

new or the old mechanistic standpoint. Those who have the scholastic

predilection for finality and the scholastic predilection for the

abstract noun, do not seem to be able to believe in the existence of

people who are not like themselves. They cannot, it appears, understand

that unless one starts off with the obsession that the universe can

be summed up in a monosyllable, one is under no imperative necessity

on the one hand to be resentful towards or disappointed with science

because it lays no claim to the finality of religious dogma, nor

on the other to make the assumption that such finality ought to be

obtainable. The mechanist does not claim that his system is, or ever

will be, complete in the sense that science will one day find an answer

for all the conundrums which the scholastic temperament dictates. On

the contrary, it is the essence of the mechanistic position that there

is a technique of asking questions profitably as well as a way of

answering them satisfactorily. All the mechanist claims is that as far

as we can see at present his way of dealing with things leads to the

most complete unanimity which it is possible to attain. Against the

old vitalism, that of Dr. Haldane, who denies that the principle of

mechanism can ever deal with conscious behaviour the older mechanistic

outlook was secure in the assurance that, if the principle of

mechanism failed at such a level, no other principle led to verifiable

predictions in the same field. Against the new vitalism or holistic

standpoint of General Smuts which no longer asserts dogmatically that

the principle of mechanism is inapplicable at any specific level of

existence, but contends that it does not anywhere give a complete

account, the new mechanistic or _publicist_ standpoint which I have

outlined contends that if the principle of mechanism fails to give a

complete account at any level no alternative or supplementary principle

has been discovered. The reply of the mechanist old or new to the

vitalist old or new is that of Mr. W. B. Yeats’ faeries:

“Is anything better, anything better

Tell us it then...”

It follows that, in any discussion between the two, the combatants

are generally at cross-purposes. The mechanist is primarily concerned

with an epistemological issue. His critic has always an ontological

axe to grind. The mechanist is concerned with how to proceed to a

construction which will represent as much about the universe as human

beings with their limited range of receptor organs can agree to accept.

The vitalist or holist has an incorrigible urge to get behind the

limitations of our receptor organs and discover what the universe is

_really_ like. What we mean by _really_ in this connexion evidently

depends upon whether we view the question socially or individually. In

our relation to other human beings the nearest approach to what the

universe is really like is found in the schematization of our common

experiences. If there is any other reality its sanction is non-social.

Thus in contradistinction to the _reality_ of traditional philosophy

which is an individualistic concept, the concept of _publicity_, which

it is proposed to substitute as the goal of synthetic philosophy, is an

essentially social one.

IV. THE CONCEPT OF ADAPTATION

“No philosopher who is rational and modest has ever pretended to

assign the ultimate cause of any natural operation, or to show

distinctly the action of that power which produces single effect

in the universe. It is confessed that the utmost effort of human

reason is to reduce the principles productive of natural phenomena

to a greater simplicity.... The most perfect philosophy of the

natural kind only staves off our ignorance a little longer, as

perhaps the most perfect philosophy of the moral or metaphysical

kind serves only to discover larger portions of it.”--David Hume,

_Sceptical Doubts_

§1

By those who hesitate to commit themselves to an explicit advocacy of

either the vitalistic or mechanistic views about the Nature of Life it

has often been urged that the concept of adaptation is fundamental to

biological science. Professor A. V. Hill is perhaps the most brilliant

physiologist now living. He adopts a hopeful attitude to the progress

which awaits further analysis of the properties of living matter in

physico-chemical terms. He also thinks that, however far mechanistic

principles are extended, the biologist will always encounter

“adaptation” in the phenomena which he studies. Another distinguished

physiologist, Professor Lovatt Evans has expressed himself in rather

more emphatic terms.

“Physiologists,” he states, “in attempting to know what life is,

have in my opinion attempted too much, and I think that a new point

of view is essential.... The idea of adaptation, urged by Claude

Bernard, should be adopted by physiology as its basal principle, as

the chemist accepts the conservation of matter or the physicist

the conservation of energy. We need not seek to know why it is so,

that is the province of the philosopher.... It is not a definition

of what life is, but a brief statement of its way.... Life is

conserved by adaptation.”

When I first read these words I was not sure that I agreed with them.

I was not quite certain that I knew what they meant. I had already

come to the conclusion that the word adaptation is frequently used

by biologists without a very clear agreement as to its content. I

cannot subscribe to the view that there is a sort of trade union of

philosophers to which physiologists are ineligible, unless they can

show their articles of apprenticeship. Nor can I conceive what is meant

by a concept of life except such as is implied in a statement of _its

way_. A scientific concept defines a class of properties. A scientific

concept of life or adaptation must conform to this requirement. In

this essay my object is not to criticize Professor Lovatt Evans for

whose breadth of view I entertain a very sincere respect. I have

quoted his words, because they focus attention on some significant and

controversial issues. They serve to reveal how imperative it has become

that biologists should agree about the sense in which they intend to

use the word _adaptation_.

The quotation given above might be interpreted to mean two very

different things. If the term adaptation is used to define certain

very general characteristics of living systems, it becomes almost

co-extensive with a scientific concept of life itself. If we use

_principle_ in a somewhat archaic sense to indicate a field for

investigation, like the principle of affinity or the active principle

of the thyroid gland, there can be no question that the idea of

adaptation is the basic principle of physiology. The comparison of the

biologist with the chemist or physicist seems to go beyond this, and

imply that adaptation is not something to be explored and interpreted,

but part of the logical procedure of biology, something by the aid of

which we can predict conclusions of universal validity in the field

of biological enquiry. I do not think that Professor Lovatt Evans

really means this. I do urge that biologists continually confuse within

the compass of the concept of adaptation the notion of a problem for

solution and of a _vera causa_. This in everything but verbiage is

precisely what the cruder type of vitalist does, when he invokes the

vital principle. He first introduces a term to describe a large number

of things about which we are ignorant and wish that we knew more. He

then falls into the trap of imagining that the invention of a new term

has solved the problem.

Quite apart from this difference which, if it is to define the scope

of our scientific enquiries, cannot be dismissed as metaphysics,

biologists differ a good deal concerning the extent of the phenomena

and the kinds of phenomena they are dealing with, when they speak

of adaptation. The physiologist--in the restricted sense of the

term--is usually referring to something which might be called

the self-regulating characteristic of the body. The evolutionary

biologist--who to-day is a physiologist in the broader sense of the

term--is usually thinking of “a change in the structure, and by

implication also in the habits of an animal which render it better

fitted” for life. I here quote Professor D. M. S. Watson’s suggestive

address on adaptation from the evolutionary standpoint.[3] Sometimes

the word adaptation has a more comprehensive significance and includes

both definitions which I have distinguished. It then amounts to saying

that living systems are self-regulating and self-propagating, which

is one way of defining the nature of life as a scientific concept.

None of these technical uses of the word adaptation imply anything

that the most dogmatic mechanist could decry. If we define adaptation

as the self-regulating processes by which living matter retains its

recognizable characteristics, it is a truism to say that life is

preserved by adaptation. In that case, if adaptation is to be made

the paramount issue for biological enquiry, we can hardly upbraid our

predecessors for presumptuously seeking to know what life is. If we are

to reach any agreement about the use of the word adaptation we must

therefore retrace our steps, and examine more closely what are the

characteristics of a living system. It is useless to define the goal

of biological enquiry in terms of a concept which is as vague as life

itself. I suggest that when, in its various uses, the term adaptation

has any objective utility, it refers to these two more or less

distinct categories of characteristics which living beings display,

i.e. self-regulating and self-propagating. They are separable issues

inasmuch as a worker bee and a Dominican friar are self-regulating but

not self-propagating systems. There is no particular reason to object

to the use of the prefix, so long as no personalistic implications of

the word self are imposed upon it without further discussion.

§2

Of the two ways in which the word adaptation is used in biological

discussion, that which implies the notion of self-regulation is

most fundamental. A living organism is an extremely complex system

in dynamic equilibrium with its environment. The idea of dynamical

equilibrium is not peculiar to biology. The atom, which for traditional

chemistry was a statical concept, is no longer regarded in that way

by the modern physicist. What is more peculiar about living matter

is its amazing complexity, and the idea of adaptation in the sense

of self-regulation calls attention to the fact that a system of such

extreme complexity, a system with so many characteristics, continues

to maintain its individuality, i.e., its manifold characteristics,

in spite of all the changes that are taking place within it and

without. The recognition of this complexity is common ground. If the

mechanist underrates the difficulty of the problem, he is certainly

to be discouraged, except in so far as the scientist in attacking any

problem must always focus his attention on a limited range of data

and rule out certain things as insignificant for his present purpose.

It may be true, as Professor Lovatt Evans opines, that mechanistic

interpretations tend to become arrogant and superficial. Is he on surer

ground in holding that “it is unthinkable that a chance encounter

of physico-chemical phenomena can be the explanation”? Might we not

reflect with David Hume that “our own mind being narrow and contracted,

we cannot extend our conception to the variety and extent of nature,

but imagine that she is as much bounded in her operations, as we are in

our speculation”?

Scientific hypotheses are not always thinkable, if by that we mean

pleasant, easy or conformable to common sense. Our grandfathers

thought it “inconceivable” that her Gracious Majesty, Queen of Great

Britain and Ireland, Empress of India and Defender of the Faith,

could be descended from an ape. The atomic structure of matter was

unthinkable to many people little more than a century ago. To-day the

quantum atomic model is unthinkable; but we think it is the best way

of interpreting the data. Given this amazingly complex system in

dynamically stable equilibrium with its environment, we have to decide

consistently with the fullest requirements of the problems what is the

most economical way in which we can envisage its existence. Seeing that

a mechanistic interpretation is evidently the most economical one, the

real issue is to decide whether there are any characteristics of the

complex which are inconsistent with such an attitude.

From the modern standpoint the individuality of the atom is a

statistical concept. The atom is in dynamically stable equilibrium

with its surroundings. It might, therefore, be argued that a Ford car

is an example of a complex mechanism which is in dynamical equilibrium

with its environment. This would be a superficial analogy for the

order of complexity which we encounter in living matter. The molecular

constitution of the parts of a Ford car is comparatively static. In

the minutest parts of an organism new molecules are being built up and

replacing others that have been broken down. Nevertheless, in all this

astonishing panorama of microscopic revolutions which underlie the

microscopic continuity of the organism we know of no events which are

in conflict with the great generalizations of physical science.

On this point Professor Hill speaks with special authority, when he

declares:

“Fortunately for physiology several of the generalizations of

science appear to be fairly strictly true, even when applied

to the living organism. Although such exact experiments are

not possible on man, or animals, or plants, as may be made on

non-living objects, there is little evidence--indeed, I would be

bold and say there is no evidence--that such living creatures can,

in any manner or degree, evade the ordinary laws of mechanics,

chemistry and physics, the principles of the Conservation of Energy

and Mass.... There really is _no_ evidence that momentum and

kinetic energy, that chemical transformations, that electrical and

magnetic phenomena, occur in the living body in any manner, or to

any extent, which differs from that obtaining in the more readily

investigated non-living world.”

In the same lecture Professor Hill replies to a statement which has

been frequently reiterated by vitalistic writers including General

Smuts and Professor Julian Huxley. Referring to the Second Law of

Thermodynamics, he says:

“Philosophically speaking, the Second Law of Thermodynamics,

dealing with the limitations of the availability of Energy, is

more liable to doubt. It is known to rest on a statistical basis,

and when we are dealing with units, complete, self-producing,

yet as invisible and intangible as the filter-passing or other

micro-organisms, it is, theoretically speaking, possible that some

means may be available of evading the statistical relations which

govern the behaviour of larger systems. But here again we must ask

for evidence--and there is none of a precise or definite character

which suggests, in the least degree, that the living cell can

escape the jurisdiction of the Second Law.”

We are thus forced to consider the order of complexity of the living

system maintained in dynamical equilibrium with its surroundings as

the essential feature which distinguishes it from non-living things.

This complexity can be arbitrarily divided into many levels; but for

convenience we may confine ourselves to two, the macroscopic and the

microscopic. Let us be explicit about the meaning of this distinction.

In the more familiar animals, we are accustomed to recognize a variety

of responses to a variety of external agencies. Generally speaking

in the more complex animals each kind of reactivity and each kind of

receptivity is spatially localized. For instance, light impinging upon

the retina evokes contraction of the pigment cells in the toes of a

frog. From this macroscopic complexity of the gross architecture of

the animal body arise two types of problems: first, the problems of

co-ordination dealing with the way in which a disturbance recurring in

some receptive area is propagated to an effector organ (gland, muscle,

etc.) in some other region; and second, the problems of metabolic

exchange, dealing with how the supply and distribution of sources

of energy for all this display of activity is maintained. The first

involves the study of the nervous impulse along the peripheral nerve

fibres and through the central nervous system; it also involves the

study of the internal secretions. The second involves the study of

digestion and assimilation of foodstuffs, the intake of oxygen to burn

up the waste products of chemical activity, and the removal of carbon

dioxide, water and other products of oxidation. In contradistinction

to the gross complexity of organs or populations of cells, we have to

take into account the microscopic complexity of the cell itself. This

presents a more general issue, because there exist many organisms whose

complexity is of the same order as that of the separate cells which

make up the bodies of familiar animals of visible dimensions. Two of

the major problems of cell physiology concern the way in which the cell

maintains its semi-permeability, and the way in which it maintains

a constant renewal of chemical materials by utilizing the energy

liberated in certain organic oxidations.

If we remove the magneto from a car, we can keep it intact for an

indefinite period: it is fundamental to our idea of a mechanism that it

can be taken to pieces and put together again. We are so accustomed to

think of a leg or an arm as dependent for their activity on the rest

of the body, that the conception of a living mechanism is repugnant to

common sense. In the laboratory it is possible to study properties

of nerve, muscle, the cell membrane, absorption of food in the gut,

oxidation of nitrogenous materials in the liver, etc., as isolated

events. A person who is not a biologist almost invariably expresses

bewilderment when he sees the isolated heart of an animal beating

regularly in a perfusion apparatus. There exists the idea that the

living organism differs from a mechanical system in that the parts

cannot persist without the whole.[4] Behind this illusion of common

sense the holistic concept of adaptation stands securely entrenched.

The holistic conception implies that for living systems the part must

be interpreted in relation to the whole, and not the whole from the

interaction of parts. We have seen that the ultimate non-biological

constituents of living matter, molecules, atoms, etc., do not behave

differently when united to form a living system. In transcending this

level of organization we are faced with an equally striking conclusion.

The contraction of an isolated muscle preparation is essentially the

same as the contraction of a muscle considered as an isolated aspect

of the behaviour of the intact organism. The passage of the nervous

impulse along an isolated nerve is not fundamentally different from the

passage of the nervous impulse in the normal animal. The conversion

of sugar into alcohol by the isolated enzyme zymase obtained from

crushed yeast cells is a process like that of the conversion of sugar

into alcohol by the living yeast fungus. The whole development of

physiology, from the time when Haller first made an isolated muscle

preparation and Spallanzani produced animal light by moistening a

dessicated powder of luminescent jelly fishes, bears witness to the

conclusion that the separated constituents of a living whole do not

at any level of complexity behave differently from the way in which

they behave as parts of a more complex order. Thus, when the fullest

recognition is given to the extreme complexity of living systems, the

problem of self-regulation submitted to experimental analysis does not

bring forward any confirmation for the holistic view of adaptation as

the creative interpolation of new irreducible properties at different

levels of complexity. The holist may reply that it is one thing to take

the living machine to pieces, and another thing to put it together

again. Even here the analogy with the machine holds good. To graft the

eye of one salamander tadpole on to the head of another individual

is now a commonplace of experimental embryology. Five-legged and

two-headed newts are now manufactured in the laboratory.

Self-regulation, the way in which an organism maintains a seeming

continuity of arrangement in spite of the uninterrupted and ubiquitous

flux of macroscopic and microscopic changes which its existence

implies, defines the sense in which the term adaptation is ordinarily

used by the physiologist. In contradistinction to this physiological

and individual use, adaptation is employed in biological discussion

in a morphological and specific sense, when we consider how one

animal comes to be distinguished from another by some architectural

arrangement appropriate to a particular kind of environment. In this

sense the problem of adaptation has played a prominent part in the

evolutionary speculations of the past century. Given the fact that

organisms are not only self-regulating but self-propagating, the

evolutionary theory sets out to explain how living systems come to

exist in so many specific forms, and how it is that these specific

forms are on the whole _fitted_ or _adapted_ to their respective

surroundings. The qualification _on the whole_ is highly significant.

Organisms display many peculiarities of architecture which by no

stretch of imagination can be regarded as necessarily fitting them

better for their conditions of life. To assume that every peculiarity

of structure in an animal is useful to it in the struggle for existence

is a pure assumption unfounded on anything but teleological prejudice.

Adaptation in the morphological sense really includes two ideas which

to some extent coalesce, and are therefore all the more readily

confused. At times the word implies nothing more than _viability_.

In this sense adaptation is the whole problem of evolution. Up to

a certain point an organism must be “suited” to its environment in

order to live at all. At other times adaptation is extended to mean an

essential utility in every detail of the structure of an organism. This

is a mischievous implication which, as will be seen later, has hindered

the formation of a clear conception of the evolutionary process.

Even if we could justify the belief that female peafowl are as much

impressed by peacocks as are some male biologists, we have still failed

to supply a criterion of survival value which has any satisfactory

significance. The enthusiast who describes an adaptation is often like

the advertizing manager who tells us how many customers we shall get,

if we advertize with him, but is inclined to be reticent about whether

the profit derived from more customers is commensurate with the fees

he proposes to exact for his services. Bateson, who first applied

Mendel’s principles to animals, wrote five years before the Mendelian

Renaissance in terms which may still be commended to the thoughtful

examination of every student of the evolutionary problems:

“Whereas the only possible test of the utility of a structure is

a quantitative one, such a quantitative method of assessment is

entirely beyond our powers. To know that the presence of a certain

organ may lead to the preservation of the race is useless, if we

cannot tell how much preservation it can effect... unless we know

also the degree to which its presence is harmful, unless, in fact,

we know how its presence affects the profit and loss account of the

organism.” (_Materials for the Study of Variation_).

That animals do in fact display many structural characteristics which

are in no sense useful to them is generally admitted to-day. It

thus becomes as much the function of any theory of the evolutionary

process to explain the origin of useless as to explain the origin

of useful devices. There is a practical limit to the use of the

concept of adaptation in morphology. There is a no less obvious limit

to the use of the concept of adaptation in physiology. An animal

is a self-regulating system up to a point; but it cannot in every

contingency take arms against a sea of troubles and by opposing end

them. If we could define in some general terms where this limit lies,

we should be justified in speaking of a principle of adaptation in

the sense that we speak of a principle of conservation of matter.

The ideally self-regulating unit of living matter endowed with the

secret of perpetual youth is as imaginary as the Economic Man. At

present the fact that organisms cease to regulate themselves and die

is as fundamental a problem of biology as the converse fact that they

regulate themselves and thereby continue to live. The fact that the

organism has a good deal of useless anatomical equipment seems to be

as true as the fact that on the whole its anatomy is suited to the

requirements of its surroundings. In whichever way we employ the term

adaptation we are forced to the conclusion that it is only legitimate

to speak of a principle of adaptation in the sense in which we speak

of the active principle of the thyroid gland. Adaptation defines a

field of problems which await solution. In that sense the concept of

adaptation is as fundamental to mechanistic as to any other theories of

the organism.

§3

This is not what is generally meant when it is said that adaptation

is a fundamental principle of biological enquiry. I believe that it

is the only legitimate sense in which it can be said that there is

a biological principle of adaptation. It seems to me that, when we

go further and put more than this into our concept of adaptation, we

are driven to formulating the problems of biology in a wrong way. By

inventing hypotheses to explain facts which do not exist, we then

proceed to give false interpretations of the significance of facts that

do exist.

When the principle of adaptation is treated as a principle which

enables us to predict conclusions, it constantly leads us to fantastic

distortions of what really happens. If I wished to illustrate this in

connexion with the self-regulating aspect of the concept of adaptation,

I could not do better than refer to current speculations about the rôle

of the ductless glands in the economy of the organism. The physiologist

who interprets his field of observation in a manner analogous to

that of the physicist and chemist realizes that we have no reason

to believe that every chemical entity found in the animal body is

necessary or even useful to its owner. He will not therefore draw any

conclusions of a far-reaching nature from the discovery that a certain

tissue extract has highly specific physiological properties, unless

he can show that the removal of the tissue itself produces effects

of an opposite nature to those which ensue on injecting its active

constituent. The student of ductless glands who regards adaptation as

a principle to be applied rather than as field to be explored will

not be held back by such restraint. We must thank the “principle” of

adaptation in endocrinology for the romantic guess-work of that school

which undertakes to interpret the whole of human history in terms of a

glandular explanation of temperament. Most speculations on which the

glandular theory of temperament are based have their only experimental

basis in the presence of supposedly specific active substances in one

or other tissue extract. A “principle of adaptation” does not assist

us to understand why the pituitary gland of a fish should contain one

specific constituent which produces expansion of the black pigment

cells in the skin of a frog, another specific constituent which causes

the uterus of the mammal to contract, and yet a third which produces

a specific fall of blood pressure in the bird and a specific rise of

blood pressure in the mammal.

It is especially in the field of evolutionary biology that we must look

for most guidance, because the concept of adaptation has occupied such

a prominent part in the evolutionary controversy. As an example of how

“the principle of adaptation” leads to incorrect conclusions I need

cite only one example from an exceedingly able and provocative address

of Professor D. M. S. Watson.

“It is not unusual for a student of fossils to discuss the habits

of an extinct animal on the basis of a structural resemblance of

its ‘adaptive features’ with those of a living animal and then to

pass on to make use of his conclusions as if they were facts in

the discussion of an evolutionary history or of the mode of origin

of a series of sediments. In extreme cases such evidence may be

absolutely reliable: no man faced with an ichthyosaur so perfectly

preserved that the outlines of its fins are visible can possibly

doubt that it is an aquatic animal, and such a conclusion based on

structure is supported by the entire absence of ichthyosaurs in

continental deposits of appropriate ages and their abundance in

marine beds. But if extremes give good evidence, ordinary cases are

always disputable. For example, there is, so far as I know, not the

least evidence in the post-cranial skeleton that the hippopotamus

is aquatic; its limbs show no swimming modification whatsoever, and

the dorsal position of the eyes would be a small point on which to

base assumptions. Most palæontologists believe that the dentition

of a mammal, and by inference also that of a reptile or fish, is

highly adaptive, that its character will be closely correlated with

the animal’s food, and that from it the habits of an extinct animal

can be inferred with safety. Here again the extreme cases are

justified, the flesh-eating teeth of a cat and the grinding battery

of the horse are clearly related to diet. Crushing dentitions, with

the modification of skull and jaw shape and of musculature which

go with them, seem equally characteristic. I had always believed

that the horny plates and the jaws of Platypus were adapted to

hard food, and that that animal possessed them, whilst the closely

allied Echidna was toothless, because it was aquatic and lived in

rivers which might be expected to have a rich molluscan fauna which

could serve as food. But the half-dozen specimens whose stomachs I

have opened contained no molluscs whatsoever, and seem to have fed

on insect larvæ, the ordinary soft bottom fauna of a stream.” (_Op.

cit._)

We are now beginning to see that the evolutionists of the nineteenth

century focused their attention far too exclusively on adaptation. In

other words, they regarded adaptation as a principle like the principle

of conservation of matter, one of universal validity within the field

of biology. Any theory of evolution has to explain why non-adaptive,

as well as adaptive, features arise. In that sense the fundamental

problem of evolution is not the origin of adaptation but the origin of

species. Both the theories of Lamarck and Darwin implicitly assumed

that the differences between species, in the traditional, i.e. Linnæan

sense, are mainly utilitarian. Having started with an incorrect

apprehension of the facts they proceeded to elaborate hypotheses to

account for them. Thence inevitably they drew from these hypotheses an

unsatisfactory account of the way in which new species do arise. From

the modern standpoint analysis of the species problem does not demand a

recognition that species differences are even in the main utilitarian,

though such a statement would probably be true of differences between

larger units such as genera. Nor from the modern standpoint do the

hypotheses of either Lamarck or Darwin give us any clue to the way in

which the species barrier, i.e. inability to breed with other species

successfully, can have arisen. Anything which remains of the Lamarckian

principle in the light of modern research has no special relevance

to the origin of adaptations. Whatever remains of the theory of

natural selection has been completely divested of the implication that

non-adaptive characters were necessarily adaptive at their inception.

A discussion of the fate of the Lamarckian and Darwinian theories must

be undertaken elsewhere. Here it is sufficient to point out that both,

more particularly the latter, had a peculiarly sterilizing influence

on the growth of experimental biology. Obsessed with the principle of

universal adaptation which Natural Selection had secularized, zoology,

from the publication of the _Origin of Species_ to the rediscovery of

Mendel’s Laws, wandered for forty years in a wilderness of phylogenetic

speculation. Biological research in the words of Professor Punnett

became

“devoted to the construction of hypothetical pedigrees suggesting

the various tracks of evolution.... The result of such work may be

said to have shown that the diverse forms under which living things

exist to-day, and have existed in the past, so far as palæontology

can tell us, are consistent with the view that they are all

related by the community of descent.... It is obvious that all this

work has little or nothing to do with the manner in which species

are formed.”

According to the Selectionist doctrine in its original form, characters

originated and persisted in virtue of their utility and what Darwin

somewhat vaguely called “the strong principle of inheritance.” To

explain any peculiarity of structure or habit, it became necessary

only to show one of two things, either it was useful to its owner or

was once useful to an ancestor of its owner. Everything was or had

been an adaptation. This resulted in a complete divorce of comparative

anatomy from comparative physiology. The morphologist and systematic

zoologist regarded it as an impertinence of the physiologist to seek

for experimental evidence, where a perfectly good case of adaptation

was evident to anyone who would accept their premisses.

I will illustrate this from a field in which I have myself carried out

experimental investigations for twelve years. Writing of colour change

in frogs Dr. Hans Gadow makes the following remarks in the _Cambridge

Natural History_ (_Amphibia and Reptiles_, p. 36):

“Biedermann concludes that the chromatic function of frogs in

general depends chiefly upon the sensory impressions received by

the skin, while that of fishes depends upon the eye. All this

sounds very well, but the observations and experiments are such

as are usual in physiological laboratories, and the frogs, when

absorbed in their native haunts, or even when kept under proper

conditions, do not always behave as the physiologist thinks they

should. There is no doubt that in many cases the changes of colour

are not voluntary but reflex actions. It is quite conceivable that

the sensation of sitting on a rough surface starts a whole train of

processes: roughness means bark, bark is brown, change into brown;

but one and the same tree frog does not always assume the colour of

the bark, when it rests or when it sleeps upon such a piece. He

will if it suits him remain grass green on a yellow stone or on a

white window frame.... The sensory impression received through the

skin of the belly is the same, no matter if the board be painted

white, black or green, and how does it then come to pass that the

frog adjusts its colour to a nicety to the general hue or tone of

its surroundings.”

It is safe to say that no one, unless at the outset prejudiced by the

principle of adaptation, could be led to entertain the view that frogs

as a rule are able to adjust themselves “to a nicety” to the general

hue and tone of their surroundings. The state of the pigment cells in

the skin is influenced independently by a number of diverse factors,

including moisture, temperature, diffuse light acting on the skin

and reflected light acting on the retina of the eye in the opposite

sense. Individual frogs differ in basic pattern, but the range of hue

between the dark and pale condition for any frog is fixed, as is also

true of the proverbial chameleon. When the conditions affecting colour

change in a frog are defined, it is possible to predict the pigmentary

response of a frog and its time relations with as much confidence as

any other physical event in nature. It is, on the other hand, quite

impossible to draw any far-reaching conclusions about colour change

from uncontrolled observation of the frog in its native haunts, because

the number of significant variables is far too numerous to handle

in this way. I have quoted this passage to show the attitude which

zoologists under the influence of the post-Darwinian tradition adopted

towards experimental enquiry of any description. Dr. Gadow applies

the “principle of adaptation,” as it was then used in morphology, to

the self-regulating aspect of the organism with results which show

what might well happen to physiology if the physiologist employed

the principle of adaptation as the chemist employs the principle of

conservation of matter.

Ecology, or the study of the relation of species to particular types

of environment, provides a clear illustration of the progress that

has been achieved by detailed study of physiological mechanisms in

place of the speculative application of the principle of “adaptation.”

Krogh and his pupils have made a special study of the physico-chemical

properties of the blood pigments of the lower organisms, and have

thereby thrown a good deal of light on the conditions which determine

their ecological distribution. Let us take the case of two common bony

fishes, the carp and the trout. It is a matter of common experience

that in nature the trout will only live in running streams. It can

be kept with great difficulty in aquaria, if special precautions for

aerating the water are taken. The carp will live in still water, where

the oxygen content is low, and like its ally the goldfish accommodates

itself to the aquarium with great ease. The difference between the two

types is at once understood, when we know that the hæmoglobin of the

carp has a much higher affinity for oxygen than the hæmoglobin of the

trout. In consequence the blood of the carp is completely saturated

with oxygen when the oxygen content of the water in which it swims is

far below that which is in equilibrium with the oxygen pressure of the

atmosphere. The blood of the trout on the other hand is only fully

saturated with oxygen when the water is itself nearly saturated.

The concentration of salts in the blood of fishes like the trout

and carp is kept constant at a level below that of sea water. The

concentration of dissolved substances in the blood of sharks and

dogfishes which are all marine is in equilibrium with the osmotic

pressure of the sea. The respiratory centre of the wrasse is

paralysed at 60° C. {sic}, while the heart of the English dogfish

shows irreversible changes above 18° C. Taking these facts together

we can deduce a good deal about the viability of a species in a given

locality. A fish like the skate placed near the estuary of a large

river is forced to remain where the salt concentration is above a

certain level. A salmon is not subject to this restraint. Assuming that

the fish can pass the estuarine boundary and proceed upstream, two

alternatives present themselves. He can remain in the swiftly moving

main stream or take to backwaters and stagnant lakes connected with

it. If he has the hæmoglobin of a trout, he is committed irretrievably

to the former alternative. Being compelled to remain in the swiftly

running part of the river bed, he might stay in the lowlands or make

for the source, which in general will be much colder. In the case of

a fish like the wrasse, whose respiratory centre is paralysed at a

temperature of 6° C., the latter course is impossible, if the river

rises in a high range. Thus in place of vaguely speculating about how

an organism is specially “adapted” to live in some particular place,

experimental biology is gathering clearly defined ideas about why an

organism cannot live in any place other than that in which it does live.

The idea that a problem can be solved by invoking the principle of

adaptation assumes its most grotesque form in Haeckel’s discussion of

Recapitulation. The classical example of what is called recapitulation

is provided by the gill clefts of vertebrates. All vertebrate

embryos have pits or clefts at the sides of the throat, supplied by

a characteristic arrangement of blood-vessels. In fishes the clefts

acquire filaments richly supplied with blood-vessels, and act as

gills. Both the gill clefts and the characteristic arrangement of

blood-vessels associated with them persist throughout life. In frogs

and salamanders gill filaments are developed in the tadpole stage, but

the clefts disappear in adult life and the characteristic arrangement

of their blood supply becomes profoundly changed. In Man and most land

vertebrates the gill clefts are never used as respiratory organs, and

with their blood-vessels disappear at an early stage in development.

During the first half of the last century Van Baer, the pioneer

embryologist, propounded a generalization which may be stated thus:

embryos of different species of animals of the same group are more

alike than the adults, and the younger the embryo the greater are the

resemblances. This generalization, well illustrated by the gill clefts,

was later extended by Haeckel with the sonorous title “Biogenetische

Grundgesetz.” It is thus defined by its author: “The history of the

fœtus is a recapitulation of the history of the race, or in other

words, ontogeny is a recapitulation of phylogeny.”

The way in which the modern geneticist handles the problem of

development offers a striking contrast to the attitude of Haeckel and

a generation of zoologists unduly preoccupied with the concept of

adaptation. A recent investigation from the laboratory of Professor

Julian Huxley will illustrate the difference. In the little crustacean

_Gammarus_ there are a number of varieties distinguished by the colour

of their eyes. All coloured eyes are at an early stage of development

colourless. They then become scarlet owing to the formation of a red

pigment. They may subsequently darken owing to the deposition of the

black substance known as melanin. Varieties with eye colour from a

dark red through various grades of chocolate to dark brown and black

are distinguished by the time at which the deposition of melanin

begins and the rate at which it occurs. Here there is no difficulty in

seeing what conditions must be fulfilled in order that a new variety

should or should not recapitulate the characteristic of the ancestral

stock from which it arises. If a red-eyed variety of Gammarus arose

from a white-eyed stock, it would necessarily exhibit the ancestral

condition at the beginning of development, because all eyes are at

first colourless. If a black-eyed form arose as a sport from a red-eyed

stock, it would also recapitulate the ancestral characteristic, because

all black eyes are at first red. If a white-eyed form arose as a sport

in a red-eyed stock, or a red-eyed form emerged from a black-eyed

stock, in neither case would the ancestral condition be manifest at any

stage of development. There is no question of the intrinsic usefulness

of a new character involved in this. Whether recapitulation does or

does not occur here depends upon whether the Mendelian factor which

distinguishes a new variety hastens or retards some feature of the

developmental process.

Now Haeckel’s “Grundgesetz” implies an additional statement to that

contained in Van Baer’s Law. It signifies that the embryonic stages of

one form are to be compared with adult rather than embryonic stages

of another. This in fact is not correct, as the classical cases of

recapitulatory phenomena demonstrate most clearly. The mammalian

embryo never possesses true gills. It goes through a stage at which it

has the characteristic clefts and arterial arches which in the _fish

embryo_ precede the development of functional gills. This is also

true of crustacean larvæ. _Sacculina_, the crab gall, passes through

the two characteristic larval forms of the true _barnacles_. It has

no resemblance to an adult barnacle in any stage. A more serious

objection to Haeckel’s way of stating the idea of recapitulation in

development is the vagueness it assumes when brought face to face with

the exceptions that are as numerous as the applications of the rule. An

illustration of the exceptions is provided by eye colour in the human

species. It is fairly certain that the blue-eyed condition has arisen

as a mutant in a brown-eyed stock; yet the eyes of brown-eyed adults

are often blue in the newly born.

It is not difficult to discover in Haeckel’s own writings the train of

reasoning which led him to distort the facts of development in stating

the law which is often associated with his name.

“The evolution of the fœtus (or ontogenesis),” states Haeckel, “is

a condensed and abbreviated recapitulation of the evolution of the

stem (or phylogenesis); is preserved by a constant heredity; on the

other hand, it becomes less complete in proportion as a varying

adaptation to new conditions increases the disturbing factors in

the development (or cenogenesis). The cenogenetic alterations or

distortions of the original paligenetic course of development take

the form, as a rule, of a gradual displacement of the phenomena,

which is slowly effected by adaptation to the changed conditions

of embryonic existence during the course of thousands of years.

This displacement may take place as regards either the locality or

the time of the phenomenon. The first is called heterotopism, the

second heterochronism.”

So naïve a combination of garrulous teleology and self-contradiction

is characteristic of the hopeless confusion of thought which existed

in evolutionary biology, while it remained dominated by the principle

of adaptation. The larval “adaptations” should on the face of it

recapitulate their ancestral story--and so on in endless regression.

There is no intelligible meaning in Haeckel’s explanation of the

admittedly ubiquitous exceptions to his rule.

Haeckel’s so-called Biogenetische Grundgesetz exerted a profound

influence on biology during the second half of the nineteenth

century, and perhaps did more than anything else to divert zoologists

from the study of activity to the pursuit of insignificant details

of no conceivable physiological interest. Instead of furthering

the development of zoology as an exact science, it substituted

the construction of architectural mnemonics for the search after

quantitative laws. With Haeckel’s law is associated an interesting

logical fallacy in the development of the argument for evolution.

Huxley made a good debating point when he disclosed the embarrassing

information that a bishop at one stage of the episcopal life cycle

has gill structures like those of a fish. From the standpoint of

formal logic the point is worthless. Only the atmosphere of religious

propaganda which surrounds the birth of the evolutionary doctrine

can explain the perennial reappearance of the contention that

recapitulation constitutes an argument _sui generis_ in favour of the

doctrine of descent. If experimental breeding taught us that mutant

forms recapitulate the characteristics of the stock from which they

originate, the resemblance of developmental stages of present-day

forms to adult organisms which existed in the geological past would

constitute a special consideration in favour of regarding fossil

remains as ancestral to contemporary animals. As yet experimental

breeding teaches us no such thing. We do not find that a white-eyed fly

originating as a sport in a red-eyed stock invariably has red eyes at

any prior stage of development. Recapitulatory phenomena are difficult

to explain on a theological basis, but they do not constitute a special

argument in favour of the evolutionary alternative. To-day biologists

are beginning to realize that evolution must furnish an explanation

of specific differences which are not adaptive as much as specific

differences which are adaptive. With this change of outlook it is

becoming possible to discuss the logical status of the evolutionary

hypothesis without recourse to arguments which belong more properly to

propaganda than to science.