date marks the beginning of an epoch in every branch of biology. Before
it, Biology had been almost entirely a descriptive science, but within a
few years after the publication of the _Origin_ its effects began to
colour all aspects of biological research. A co-ordinating and unifying
principle had been found, and the leading idea of biologists ceased to
be to describe living things as they are, and became transformed into
the attempt to discover how they are related to one another. The first
effect of this change of attitude was chiefly to turn biologists towards
the task of tracing phylogenetic or evolutionary relationships between
different groups of animals--the drawing up of probable or possible
genealogical trees and the explanation of natural classification on an
evolutionary basis. When once, however, the notion of cause and effect,
or more correctly of relationship, between the phenomena seen in living
beings had become familiar to biologists, it spread far beyond the
limits of tracing genealogical connexions between different animals and
plants. It made possible the conception of a true Science of Life, in
which every phenomenon seen in a living organism should fall into its
true place in relation to the rest, and in which also the phenomena of
life should be correlated with those discovered in the inorganic
sciences of Chemistry and Physics.
The history of the various branches of biological science in the past
sixty years reflects the general course of these tendencies. Until
shortly after 1859, the study of morphology, or the comparative
structure of animals (and of plants) was intimately related with that of
physiology, that is, with the study of function. In the years following
the appearance of the _Origin_, however, anatomists and morphologists
were seized with a new interest. For the time at least, the chief aim in
studying structure was no longer to explain function, but rather to
explain how that structure had come into being in the course of
evolution, and how it was related with homologous but different
structures in other forms. The result was a tendency to a divorce
between morphology and physiology, or at least between morphologists and
physiologists, which led to the division into two more or less distinct
sciences of what had hitherto been regarded as closely inter-related
branches of one. The greater men of the early part of the period, such
as Huxley, remained both morphologists and physiologists, but most of
their followers fell inevitably into one or the other group, and in
discussing the later phases of biological progress it will be necessary
to keep them separate.
Apart from its effect on the systematic and anatomical side of Biology,
the idea of Evolution, and especially of Darwin's theory of Natural
Selection, had important consequences on that side of the science which
may be described as Natural History. Before the appearance of Darwin's
work, Natural History consisted chiefly in the observation and
collection of facts about the habits and life-history of animals and
plants, which as a rule had no unifying principle unless they were used,
as in the Bridgewater Treatises, to illustrate 'the power, wisdom, and
goodness of God'. Now, however, a new motive was provided--that of
discovering the uses to the organism of its various colours,
structures, and habits, and the application of the principle of natural
selection to show how these characters conduced to the preservation and
further evolution of the species. And out of this interest in the theory
of natural selection grew in the last twenty years of the nineteenth
century the greatly increased attention to the facts and theories of
heredity, which was stimulated by Darwin's hypothesis of Pangenesis and
especially by Weismann's speculations about the nature and behaviour of
the 'germ-plasm'. Before the appearance of Weismann's work, the
germ-cells, which bear somehow or other the hereditary characters that
appear in the offspring, were supposed to be produced directly from the
body of the parent. Darwin provisionally suggested that every cell of
every organism gives off minute particles which become congregated in
the germ-cells, and that these cells thus contain representative
portions of all parts of the parent's body. Weismann, on the basis of
his work on the origin of the germ-cells in Medusae and Insects,
maintained that these cells are not derived from the body, but only from
pre-existing germ-cells stored within it--that, in fact, although an egg
gives rise to a hen, a hen does not give rise to an egg, but only keeps
inside her a store of embryonic eggs which mature and are laid as the
time comes round. The theory had to be modified to suit the facts of
regeneration and vegetative reproduction, but in essence it was accepted
by the biological world and is the orthodox opinion (if such a word may
be used in Science) at the present day. The difference between the two
views is not only of theoretical interest, for it involves the whole
question of whether characteristics acquired by an individual during its
life in response to external conditions can or cannot be transmitted to
offspring. If the germ-cells contain representatives of all parts of the
body, modifications impressed on the body during its life may at least
possibly be transmitted to offspring born after the modifications have
taken place. If, however, the germ-cells are independent of the rest of
the body, and only stored within it for safe-keeping like a deed-box in
the vaults of a bank, it would seem impossible for any environmental
influence, whether for good or ill, to take effect on the offspring.
This controversy on the heritability of 'acquired characters' was one of
the most important towards the end of last century, and although the
majority of biologists now follow Weismann in so far as they deny that
'acquired' characters are transmissible, the question is not yet
completely settled; all that can be said is that, in spite of many
attempts to prove the contrary, there is no satisfactory evidence of the
transmission to offspring of effects impressed on the body of the
parent, unless the germ-cells themselves have been affected by the same
cause--as for example in some cases of long-continued poisoning by
alcohol or similar drugs.
While the problem of the transmission of acquired characters, and of the
cause of variation and its relation to evolution, was occupying much of
the attention of biologists, the whole problem entered upon a new phase
in the year 1900 with the re-discovery of Mendel's work on heredity.
Mendel worked with plants, and published his results in 1865, but at
that time the biological world was too much occupied with the fierce
controversy which raged over _The Origin of Species_ to take much notice
of a paper the bearing of which upon it was not appreciated. Mendel's
discovery never came to the notice of Darwin, was buried in an obscure
periodical, and remained unknown until many years after the death of its
author. In 1900 it was unearthed, and, largely owing to the work of
Bateson, it rapidly became known as one of the most important
contributions to Biology made during the period under review.
This is not the place to describe in detail the nature of Mendel's
theory. Its essence is, firstly, that the various characteristics of an
organism are in general inherited quite independently of one another;
and, secondly, that the germ-cells of a hybrid are pure in respect of
any one character, that is to say, that any one germ-cell can only
transmit any unit character as it was received from one parent or the
other, and not a combination of the two. This leads to a conception of
the organism as something like a mosaic, in which each piece of the
pattern is transmitted in inheritance independently of the rest, and in
which any piece cannot be modified by association with a different but
corresponding piece derived from another ancestor. It is impossible to
say as yet whether this conception at all completely represents the
nature of the living organism, but it is one which is exercising
considerable influence in biological thought, and if established it will
mark a revolution in Biology hardly inferior to that brought about in
Physics and Chemistry by the discovery of radio-activity.
An important consequence of the advance in our knowledge of heredity
associated with the work of Mendel and his successors is a tendency to
doubt whether natural selection is of such fundamental importance in
shaping the course of evolution as was supposed in the years of the
first enthusiasm which followed the publication of the _Origin_.
Darwin based his theory of Natural Selection on the belief which he
derived from breeders of plants and animals, that the kind of variation
used by them to produce new breeds was the small and apparently
unimportant differences which distinguish a 'fine' from a 'poor'
specimen. He supposed that the skilled breeder picked out as parents of
his stock those individuals which were slightly superior in one feature
or another, and that by the accumulative effect of these successive
selections not only was the breed steadily improved, but also, by
divergent selection, new breeds were produced. Experience shows,
however, that although this method is used to keep breeds up to the
required standard, it is rarely, if ever, the means by which new breeds
arise. New breeds commonly come into existence either by a 'sport' or
mutation, or by crossing two already distinct races, and by selecting
from among the heterogeneous descendants of the cross those individuals
which show the required combination of characters. And it is further
found that most of the distinguishing features of various breeds of
domestic animals and plants are inherited according to Mendel's Law,
suggesting that each of these characters is a unit, like one piece of a
mosaic, independent of the rest. Now it is easy to see how the selection
of small, continuously varying characters could take place in Nature by
the destruction of all those individuals which failed to reach a certain
standard, but it is much more difficult to understand how natural
selection could act on comparatively large, sporadic, unco-ordinated
'sports'. There is thus a distinct tendency at present to regard natural
selection as less omnipotent in directing the course of evolution than
was formerly supposed, but it must be admitted that no very satisfactory
alternative hypothesis has been suggested. Some have supposed that there
is a kind of organic momentum which causes evolution to continue in
those directions in which it has already proceeded, while others have
postulated, like Bergson, an _élan vital_ as a kind of directive agency.
Others again have reverted towards the older belief in the inherited
effects of environment--a belief which, in spite of the arguments of
Weismann and his followers, has never been without its supporters. The
present condition of this part of biology, as of many others, is one of
open-mindedness approaching agnosticism. There is dissatisfaction with
the beliefs which satisfied the preceding generation, and which were
held up almost as dogmas, but there is no clear vision of the direction
in which a truer view may be sought.
Before leaving this side of the subject, reference must be made to one
important aspect of modern work on heredity--that of the inheritance of
'mental and moral' characteristics. As a result of the work of the
biometric school founded by Galton and Pearson, it has been shown that
the so-called mental and moral characteristics of man are inherited in
the same manner and to the same extent as his physical features. Of the
theoretical importance of this demonstration this is not the place to
speak; its practical value is unquestionable, and may in the future have
important effects on sociological problems.
Another notable line of advance, entirely belonging to the period under
review, and chiefly the product of the present century, is seen in the
science of Cytology--the investigation of the microscopic structure of
the cells of which the body is composed. The marvellous phenomena of
cell and nuclear division have revealed much of the formerly unsuspected
complexity of living things, while the universality of the processes
shows how fundamentally alike is life in all its forms. In recent years
great progress has been made in correlating the phenomena of heredity
and of the determination of sex with the visible structural features of
the germ-cells. Weismann attempted a beginning of this over thirty years
ago, but the detailed knowledge of the facts was then insufficient.
Since the discovery of Mendel's Law, a great amount of work has been
done, chiefly in America, by E.B. Wilson and T.H. Morgan and their
pupils, on tracing the actual physical basis of hereditary transmission.
Although the matter is far from being completely known, the results
obtained make it almost indubitable that inherited characters are in
some way borne by the _chromosomes_ in the nuclei of the germ-cells.
The work of Morgan and his school has shown that the actual order in
which these inherited 'factors' are arranged in the chromosomes can
almost certainly be demonstrated, and his results go far to support the
conception of the organism, referred to above, as a combination or
mosaic of independently inherited features.
It was said at the beginning of this sketch that most of the more
notable lines of advance in Biology could be traced back to the impetus
given by the acceptance of the theory of Evolution, and the desire to
test and prove that theory in every biological field. It is most
convenient, therefore, to take this root-idea as a starting-point, and
to see how the various branches of study have diverged from it and have
themselves branched out in various ways, and how these branches have
often again become intertwined and united in the later development of
the science.
Perhaps the most obvious method of testing the theory of evolution is by
the study of fossil forms, and our knowledge of these has progressed
enormously during the period under review. Not only have a number of new
and strange types of ancient life come to light, but in some cases, e.g.
in that of the horse and elephant, a very complete series of
evolutionary stages has been discovered. In this branch, however, as in
almost all others, the results have not exactly fulfilled the
expectations of the early enthusiasts. On the one hand, evolution has
been shown to be a much more complex thing than at first seemed
probable; and on the other, many of the gaps which it was most hoped to
fill still remain. A number of most remarkable 'missing links' have been
discovered, such as, for example, _Archaeopteryx_, the stepping-stone
between the Reptiles and the Birds, and the faith of the palaeontologist
in the truth of evolution is everywhere confirmed. But the hope of
finding all the stages, especially in the ancestry of Man, has not been
realized, and it has been found that what at one time were regarded as
direct ancestors are collaterals, and that the problem of human
evolution is much less simple than was once supposed.
A second important piece of evidence in favour of evolution is provided
by the study of the geographical distribution of animals, on which much
work was done in the earlier part of the period under review. And in
this connexion mention must be made of the science of Oceanography, for
our whole knowledge of life in the abysses of the ocean, and almost all
that we know of the conditions of life in the sea in general, has been
gained in the last fifty years.
Another of the chief lines of evidence for the truth of the evolution
theory is based on the study of embryology, and this also was followed
with great vigour by the zoologists of the last thirty years of the
nineteenth century. It is found that in many instances animals
recapitulate in their early development the stages through which their
ancestors passed in the course of evolution. Land Vertebrates, including
man, have in their early embryonic life gill-clefts, heart and
circulation, and in some respects skeleton and other organs of the type
found in fishes, and this can only be explained on the assumption that
they are descended from aquatic fish-like ancestors. On the basis of
such facts as these, the theory was formulated that every animal
recapitulates in ontogeny (development) the stages passed through in its
phylogeny (evolution), and great hopes were founded upon this principle
of discovering the systematic position and evolutionary history of
isolated and aberrant forms. In many cases the search has led to
brilliant results, but, as in the case of palaeontology, in many others
the light that was hoped for has not been forthcoming. For it soon
became evident that the majority of animals show adaptation to their
environment not only in their adult stages but also in their larval or
embryonic period, and these adaptations have led to modifications of the
course of development which are often so great as to mask, or obscure
altogether, the ancestral structure which may once have existed.
Although, therefore, the results of embryological research have provided
most convincing proof of the truth of the theory of evolution in
general, they have not completely justified the hopes of the early
embryologists that by this method all the outstanding phylogenetic
problems might be solved.
The detailed study of embryology, however, has led to most important
results apart from the particular purpose for which most of the earlier
investigations in this field were originally undertaken. For the study
of embryology, at first purely descriptive and comparative, was soon
found to involve fundamental problems concerning the factors which
control development. An egg consists of a single cell, and it develops
by the division of this cell into two, then into four, eight, and so
forth, until a mass of cells is produced. In some cases all these cells
are to all appearance alike, or nearly alike; in others the included
yolk is from the first segregated more or less completely into some
cells, leaving the other cells without it. But in any case, after this
process of cell-division has proceeded for a certain time,
differentiation begins to set in--some cells become modified in one way,
others in another, and from what was a relatively homogeneous mass an
organized embryo, with highly differentiated parts, appears. The problem
immediately propounds itself--what are the factors which control this
differentiation? This problem is essentially a physiological one, and
yet, since it arises most conspicuously in a field which has been worked
by professed zoologists rather than physiologists, it has been studied
more by those trained in zoology and botany than by those who have
specialized in physiology. In this way, as in many other directions,
such as in the study of heredity, of sex, and of the effects of the
environment on the colours and structure of animals, the trend of
zoology in recent years has returned towards the physiological side, and
the old division which separated the sciences (but which has never so
seriously affected students of plant life) is being obliterated.
Hence we are led back to consider the progress of Physiology as a
whole--a subject with which the present writer hesitates to deal except
in a very superficial manner. Physiology as an organized science has
inevitably been deeply influenced by its close relation with medicine,
with the result that through a large portion of the period under review
it has concerned itself chiefly with the functions of the human body in
particular, or at least chiefly with Vertebrates from which, by analogy,
the human functions may be inferred. In this field it has made enormous
progress, and a vast amount of knowledge has been gained with regard to
the function and mechanism of all the parts and organs of the body. It
may perhaps be suggested, however, that in the pursuit of this detailed
(and in practice absolutely necessary) knowledge, physiologists have to
some extent lost sight of the wood in their preoccupation with the
trees. That is to say, while they have advanced an immense distance in
their knowledge of organs, they have not yet got as far as might be
hoped in the understanding of the organism--which is to say no more than
that the great and fundamental problem of Biology, the nature and
meaning of Life, is apparently almost as far from solution as ever. To
this further reference will be made below.
The progress of Physiology has been so great in all its branches that it
is difficult to decide which most deserve mention; perhaps the most
important advances are those connected with the nervous system and with
internal secretions. Little or nothing was known fifty years ago of the
minute structure of the nervous system, nor of the special functions of
its different parts. Now the main functions of the various parts of the
brain, and the relation of these parts to the activities of the other
organs of the body, are well known, although much remains to be
discovered with regard to the more detailed localization of function.
The study of the microscopic structure of brain and nerve, and
experiment on the conduction of nervous impulse, have given us some
insight into the mechanism of the nervous system, but the fundamental
nature of nervous action still remains unsolved.
The nervous system is the chief co-ordinating link between the various
organs of the body, but in recent years it has been discovered that the
relations of the different parts to one another are greatly influenced
by substances known as internal secretions or 'hormones'. These
substances are produced by ductless glands (the thyroid, suprarenals,
&c.), from which they diffuse into the blood-stream and exercise a
remarkable influence either on particular organs or systems, or on the
body as a whole. Some of these secretions act specifically on the
involuntary muscles of the body, others control growth, others the
development of the secondary sexual characters, such as the distinctive
plumage of male birds, and also greatly influence the sexual instinct.
Much still remains to be discovered with regard to them, but it seems
clear that they are of immense importance in the economy of the body. It
has been suggested, without much experimental support, however, that if
a part of the body becomes modified by use or environment, it may
produce a modified hormone, and that so, by the action of this on the
germ-cells, the modification may be transmitted to subsequent
generations.
Before leaving the subject of physiology in the more special or
technical application of the term, reference must be made to another
science the growth of which has been largely under the influence of
medicine. This is bacteriology, one of the newest branches of biology,
and yet one which both from its practical importance and from the
theoretical interest of its discoveries is rapidly taking a foremost
place. Of its practical achievements in connexion with disease, and with
the part played by bacteria and other minute organisms in the life and
affairs of man, it is not necessary to speak. Every one knows the great
advances that have been made in recent years in identifying (and to a
less extent in controlling) disease-producing organisms, whether
bacteria, protozoa (such as the organisms causing malaria, dysentery,
etc.), or more highly organized parasites. The attempt, however, to
combat these pathogenic bacteria has led to discoveries of the highest
importance with regard to the production of immunity, not only against
specific germs, but against many organic poisons such as snake venom and
various vegetable toxins. That an attack of certain diseases leaves the
patient immune to that disease for a longer or shorter time has of
course been known for centuries, but it is a modern discovery that a
specific poison induces the body to produce a specific antidote which
neutralizes it, and the detailed working out of this principle and the
study of the means by which the immunity is brought about promise to
lead us a long way towards the central problem of the nature and
activities of life itself.