THE MEANING OF EVOLUTION
Apart from experimental investigation, the results of comparative
anatomy, even if they are amplified by those of comparative embryology,
and even if they include fossil as well as living organisms, do no more
than suggest the occurrence of an evolutionary process. It is in vain
that we attempt a demonstration of transmutation of forms of life by
showing that a similarity of structure is to be observed in all animals
belonging to the same group. We may show successfully that the skeleton
of the limbs and limb-girdles of vertebrate animals is anatomically
the same series of parts, whether it be the arms and legs of man,
or the wings and legs of birds, or the pectoral and pelvic fins of
fishes: such homologies as these were indeed suggested by the mediæval
comparative anatomists apart altogether from any notions as to an
evolutionary process. We may show that the simplicity of the skeleton
of the head of man is apparent only, and that in it are to be traced
most of the anatomical elements that enter into the skull and visceral
arches of the fish; and that fusions and losses and translocations
of parts have occurred and can be made to account for the observed
differences of form. All this might just as easily be explained by
assuming a process of special creation, or the gradual development of a
plan or design. Just as God made Eve from a superfluous rib taken from
the body of her husband, so He may have formed the auditory ossicles of
the higher vertebrate from those parts of the visceral arches of the
lower forms which had become superfluous in the construction of the
more highly organised creature. However much the language of evolution
may force itself on biology, it does no more than symbolise the results
of anatomy and embryology, and provide a convenient framework on which
these may be arranged.
But if, as all modern experimental work shows, the form of the
organism is, in the long run, the result of its interaction with the
environment; if, as indeed we see, this form is not an immutable one,
but a stage in a flux; and if deviations from it may occur with all
the appearance of spontaneity, then it would appear that the observed
facts of comparative anatomy and embryology are capable of only one
explanation. They represent the results of an evolutionary process, and
the relationships that morphological studies indicate are no longer
merely logical, but really material ones. We can now endeavour to
utilise these results in the attempt to trace the directions taken by
the process of evolution.
In so doing we set up the schemes of phylogeny. We divide all organisms
into plants and animals, and then we subdivide each of these kingdoms
of life into a small number of sub-kingdoms, in each of which we set up
classes, orders, families, genera, and species. But our classification
is no longer merely a formal arrangement whereby we introduce order
into the confusion of naturally occurring things. It is now a “family
tree,” and from it we attempt to deduce the descent of any one of the
members represented in it.
The sub-kingdoms, or phyla, of organisms are the primary groups in this
evolutionary classification. We divide all animals into about nine of
these phyla--the Protozoa or unicellular organisms; the Porifera or
sponges; the Cœlenterates, a group which includes all such organisms as
Zoophytes, Corals, Sea-Anemones, and “Jelly-fishes”; the Platyhelminth
worms, that is the Tapeworms, Trematodes, and some other structurally
similar animals which live freely in nature; the Annelids, a rather
heterogeneous assemblage of creatures which includes all those animals
commonly called worms; the Echinoderms, which are the Star-fishes,
Sea-Urchins, and Feather-Stars found in the sea; the Molluscs, that
is the animals of which the Oyster, the Periwinkle, the Garden-Slug
and the Octopus are good examples; the Arthropods, which include the
Crustacea, the Insects, and the Spiders; and lastly the Vertebrates.
Any such classification we naturally endeavour to make as complete a
one as possible, but round the bases of the larger groups there cling
small groups of organisms the precise relationships of which are
doubtful. Yet, on the whole, these sub-kingdoms of organisms represent
clearly the main directions along which the present complexity of
animal structure has been evolved.
There is an essential structure which we endeavour to assign to all
the animals of each phylum, and which is different from the structure
of the animals belonging to all other phyla. The Protozoa, which for
the present we regard as animals, are organisms the bodies of which
consist of single cells. These cells may become aggregated into
colonies, but they may as well exist apart from each other. They may
be enclosed in limy, siliceous, or cellulose skeletons or shells, or
they may possess limy or siliceous spicules in their tissues--these
parts are non-essential, and the schematic Protozoan is a cell
containing a single nucleus, and capable of independent existence.
The Porifera, and all the other phyla, include organisms the bodies of
which are made up of aggregates of cells. In the Porifera the cells,
which are specially modified in structure, are arranged to form the
internal walls of a “sponge-work” the cavities of which open to the
outside by series of pores through which water is circulated. The
bodies of the Cœlenterates are typically sacs formed by a double wall
of cells--endoderm and ectoderm. This sac opens to the exterior by a
single opening, or mouth, surrounded by a circlet of tentacles, and
its cavity is the only one contained in the body of the animal. The
Platyhelminth worms are animals the bodies of which are also composed
of ectodermal and endodermal tissues, between which is intercalated
another mesodermal tissue. They have a single digestive sac or
alimentary canal opening to the exterior by means of a mouth only; and
they all possess a complex, hermaphrodite, reproductive apparatus. In
all the other phyla there are also three principal layers or kinds of
tissue, but in addition to the cavity of the alimentary canal there
is also a body cavity, or cœlom, which is contained in the mesodermal
tissues. The Echinoderms are such cœlomate animals, but the alimentary
canal now opens to the exterior by means of both mouth and anus;
there are separate systems of vessels through which water and blood
circulate; the blood-vascular system of vessels is closed to the
exterior, the water-vascular system being open; and the integument is
armed by means of calcareous spines or plates. The Annelids are animals
with cylindrically shaped bodies, segmented so as to form numerous
joints. Each segment bears spines or hairs or appendages of some sort,
and also contains a separate nerve-centre. The alimentary canal opens
externally by a mouth and anus, and there is a spacious body cavity.
The Molluscs are unsegmented animals. The dorsal part of their bodies
contains the viscera, and is protected by a shell; while the ventral
part is modified for the purpose of locomotion. A fold of integument
hangs down all round the body and encloses a cavity in which the gills
are contained. The Arthropods are segmented animals. The body is armed
by a calcareous carapace or shell which forms the exo-skeleton. Each
bodily segment bears a pair of jointed appendages, and also contains
a separate nerve-centre. The whole series of ganglia are connected
together by means of a nerve-cord, and the nervous system lies ventral
to the alimentary canal. The Vertebrata are also segmented animals,
but the segmentation is not apparent externally. The skeleton is an
internal one, and is built up round an axial rod or notochord. The
nervous system is situated dorsally to the alimentary canal. There are
two pairs of limbs.
Thus we set up an essential or schematic structure characteristic
of each phylum. These schemata have no real existence: they are
morphological types from which the actual bodily structure of the
animals in each phylum may be deduced. They represent the minimum of
parts which must be present in order that an animal may be placed in
the phylum to which we assume that it may belong. But these anatomical
parts need not actually be present in the fully developed organism:
thus there are Crustacea in which the body is not segmented, and in
which neither calcareous exo-skeleton nor jointed appendages are
present; and there are Vertebrata in which the limbs may be absent.
But in such cases we require evidence that the essential anatomical
characters which are absent in the fully developed animal have appeared
at some stage in its ontogeny, and this evidence is usually available.
Or if embryological evidence cannot be obtained, we require proof
that the animal can be traced backwards in time, by means of other
characters, to some form in which the missing structures reappear.
The schemata are thus the generalised or conceptual morphology of the
phyla. They are not the morphology of an individual organism, but they
include the morphology of the race.
They are, Bergson says, themes on which innumerable variations have
been constructed. Structural elements may be suppressed, as when the
notochord disappears in the development of the individual Tunicate,
though it is present in the larva. Or elements may disappear and become
replaced by other structures, as when the true molluscan gills are lost
in the Nudibranchs and are replaced by the respiratory plumes. They may
be reduced to vestiges, as in the case of the “pen” of the Squids, or
the “cuttlebone” of the cuttlefish, remnants of the domed shell of the
primitive mollusc; or in the appendix vermiformis of the human being, a
remnant of the voluminous cæcum of the herbivorous animal. Structures
which were originally distinct may coalesce, as when the greater number
of the primitively distinct segments of the thorax of the crustacean
fuse to form the “body” of the crab; or when the segmental ganglia of
the same animal fuse together to form the great thoracic nerve-centre.
The form and situation of a structure may vary within wide limits: thus
the digestive cavity of some Cœlenterates may be a simple sac, as in
the Hydra, but it may be partially subdivided by numerous mesenteries
as in the zooid of the Corals; or the simple tubular alimentary
canal in some Platyhelminth worms may be bifurcated in others,
triple-branched in others again, or even provided with numerous lateral
branches, as in the more specialised species in the group. Organs
originally simple may undergo progressive modification: thus the eye of
a mollusc may be a simple integumentary cavity in the floor of which
there are some simple nerve-endings, and some black pigment; or this
cavity may close up so as to form a sac, and the anterior part of the
sac may become transparent so as to form a cornea. Behind the cornea a
lens may be formed, and the simple terminal twigs of the nerve-endings
may become a many-layered retina of great complexity of structure. In
the lowest Chordates the central part of the blood-vascular system is
a simple contractile vessel, but this becomes the two-chambered heart
of the fish, the three-chambered heart of the reptile, or the powerful
four-chambered heart of the warm-blooded animal. Anatomical elements
may change in function; thus parts of the visceral skeleton in the fish
may become the ossicles of the middle ear in the Reptiles and Mammals;
while its swim-bladder may possibly be represented in the higher
vertebrates by the lungs.
Thus there may be suppression of parts leading to entire disappearance
or to mere vestiges of the original morphology. A structure
degenerating through disuse may become removed from its typical
relations with other structures and may acquire altogether new ones. Or
its increasing importance may lead to its hypertrophy and to increased
complexity of structure, and perhaps to the inclusion of new anatomical
elements, or to the incorporation of other parts, the function of which
may originally have been quite different. In all sorts of ways organs
and organ-systems may become anatomically different as the result of
adaptive modifications, or indirectly as non-adaptive modifications
induced by the adaptive modifications of adjacent parts. It is the task
of comparative anatomy to trace these changes of morphology, aided
by the study of embryology and by the comparison of the structure of
the parts of fossil animals. Regarding the process of transformism as
proved by experiments and observations in breeding and heredity, the
naturalist endeavours to trace the lines along which evolution has
proceeded from the results of morphological investigations.
Such results cannot have more than a very limited value, and it is
often the case that several interpretations of morphological results
are equally probable. We may conclude that the existing Teleost and
Elasmobranch fishes are descended from a common stock which no longer
exists; we may similarly conclude that the Birds and Reptiles are
closely allied, more so than either group is to the Mammals; and we
may conclude that the Primates--the group of Mammals to which Man
belongs--is descended from some group allied to the existing Ungulates
or Insectivores, while the Mammals themselves may have come down
from some group of vertebrates related to both the Amphibia and the
Reptiles. But as to the nature of the animals which combined the
characters of the Birds and Reptiles, or of the Reptiles and Amphibia,
we know nothing. Palæontology, if its results were more numerous than
they are, would afford us the material for the discovery of these
“missing links,” and there can be no doubt that as the world becomes
better known our knowledge of palæontological stages in the history of
existing groups will become more complete, so that we may, in time,
possess an actual historical record of the phylogeny of the main groups
of animals. But it is remarkable that while the results of comparative
anatomy and embryology, aided by those of palæontology, enable us to
trace back short series of stages in the evolutionary process, they
still show us gaps at all the places where lines of descent ought to
converge. They show us, for instance, that the oldest Birds known were
decidedly reptilian in their morphology, but they do not show us an
animal which was neither Bird nor Reptile, but from which both groups
of Vertebrata have descended; and this is almost always the case in
our hypothetical schemes of phylogeny. Morphology has continually
to postulate the existence of “annectant” forms, “Archi-Mollusc,”
“Protosaurian,” “Protochordate,” etc.: hypothetical animals which
combine the characters of those which lie near the bases of diverging
lines of descent. There is nothing to guide us in the construction of
these annectant forms except the progressive simplicity of structure
indicated in the morphological and palæontological series. The earlier
Birds had teeth, for instance, and so have the Reptiles, therefore the
annectant form had teeth, and it was an animal combining the schematic
morphology of both Birds and Reptiles. But just according to the value
which we attach to one morphological character rather than another,
so will the structure of the annectant form differ. Is, for instance,
the alimentary canal of the Vertebrate the most fundamental and
conservative part of its morphology: that is, is it the structure which
has been most resistant to change in the course of the evolutionary
process? Then we may regard the Vertebrates as having descended from
some animal which was closely related to the Annelid worms. Or is the
nervous system the most conservative part of the Vertebrate anatomy? If
so, we may trace back the main Chordate stem to animals which included
among their characters those of the most primitive Arthropods. In the
one case the annectant form joins together the Vertebrate and Annelid
stems, but in the other case it would join together the Vertebrate and
Arthropod stems, a conclusion which a rigid application of the results
of morphology would seem to make the more probable one.
[Illustration: FIG. 25.]
But, however this may be, we must not fail to notice that annectant
forms--“Archi-Mollusc,” “Protosaurian,” “Protochordate,” and the like,
are only fictions which we base on the precise importance that we
attach to one part of the essential morphology of a group of animals
rather than another. These hypothetical animals, and the genealogical
schemes or phylogenies of which they form the roots, are conventional
summaries of the results of comparative anatomy, this term being used
to include the anatomy of the developing animal and that of extinct
forms. So long as we do not possess a representative series of the
fossil remains of the animals which have existed in the past, all
schemes of descent founded on the comparison of the parts or the organs
of living animals, or on the comparison of stages of development, must
possess doubtful value when they profess to indicate the direction
taken by evolution. Their true value lies rather in the way they
epitomise our knowledge of morphology, and in the incentive which they
give to sustained and minute investigation of the structure of animals.
[Illustration: FIG. 26.]
Why did Haeckel’s “Gastrea-Theorie” gain the acceptance that it did
during the latter part of the nineteenth century? It correlated a
great number of facts, in that it postulated a general uniformity
of structure in the early developmental stages of very many animals
belonging to widely separated groups. In all of these the ovum segments
into a mass of cells, which then become arranged as a hollow ball
(_A_). One side of this ball becomes pushed in so that the inner
part of the hollow sphere becomes opposed to the inner wall of the
upper part. Thus a little sac, consisting of two layers of cells,
ectoderm and endoderm, and opening to the outside by an aperture, the
blastopore, is formed (_B_). This is essentially the anatomy of the
schematic Cœlenterate animal--_Hydra_, for instance, strongly suggests
it. Suppose now that the lips of the blastopore fuse together at one
place so that there are two openings into the cavity of the gastrula
instead of one; and suppose that the spherical organism elongates so
as to form a cylinder, the elongation involving the fused part of the
blastoporic region. Then we obviously have a worm-like animal with
an alimentary canal, a mouth and an anus (_C_). Suppose further that
an additional layer of cells becomes formed between the endoderm and
ectoderm by proliferation from one of these tissues, and suppose that
this becomes double and that a cavity appears between the two sheets of
cells forming this middle layer: this cavity becomes the body cavity
or cœlom (_D_). Now such blastula and gastrula stages appear in the
ontogeny of animals belonging to widely different groups, and such a
formation of the middle layer, or mesoblast, and of the mesoblastic or
cœlomic cavities also actually occurs. Let us assume therefore that
all multicellular animals have descended from a primitive Gastrea-form
essentially similar in morphology to the gastrula larva; and let us
assume that all cœlomate animals have descended from a form in which
a third layer of cells, or mesoblast, became intercalated between
the other two. These two assumptions are the bases of the classic
phylogenies of the last century; all Cœlenterate animals have descended
from a Gastrea-form, and all animals higher than the Cœlenterates have
been evolved from a three-layered form. Implied in this hypothesis is
also a third one, that the Gastrea-stage of evolution possesses such
a degree of stability that it has persisted, though in an obscure
condition it may be, in the development of nearly all multicellular
animals. The triple germinal layers, endoderm, ectoderm, and mesoderm,
which first became distinct from each other in the primitive cœlomate
animal, also acquired a high degree of stability, and they have been
transmitted by heredity to all animals higher than Cœlenterates. The
Gastrea and the three germinal layers are therefore to be sought for
in the developmental stages of all the higher animals, and they have
usually been found. Let it be admitted that they may make a transient
appearance--that they may be obscured in many ways, still they ought to
be there.
The Gastrea-Theorie ceased to be useful, as a means of description, or
a working hypothesis of investigation, after the rise of experimental
embryology. It could not be proved that the process of development
by gastrulation and the cleavage of a mesodermal layer are so very
conservative that they have persisted throughout the greater part of
the evolution of the animal world, yet without this proof it could not
be contended that the veiled gastrula of the developing frog’s egg,
for instance, is related genetically to the gastrula of the Echinoderm
larva. What experimental embryology does indicate is that the formation
of gastrula and (in most groups) the three germinal layers are only
the _means_ of morphogenesis. In the division of the ovum, and the
arrangement of the cells to form the organ-rudiments, the formation
of the gastrula and the mesoderm are in general the line of least
resistance in the process of development. If they do not appear, or
are difficult to recognise in the ontogeny of a group of animals, it
is not a sound method to assume their presence in an abbreviated or
distorted form, postulating that they _ought_ to be present, having
been transmitted by heredity. Physical conditions undoubtedly influence
developmental processes and there is no reason for assuming that all
ontogenetic processes were originally the same.
If we do not strain the facts of our descriptions of organic nature,
and if we do not build on unprovable conjectures, all that morphology
certainly shows us is that the evolutionary process has led to the
establishment of some dozen or so great groups of organisms, each
with appended smaller groups more or less closely related to them.
Whether these greater lines of descent are to be represented, as they
usually are, as branches springing from a single stem, or whether they
are truly collateral, each evolved independently of all the others,
is a question which is not to be solved merely by the methods of
comparative anatomy or embryology. The widely different, and equally
probable, phylogenies of the past indicate that data for the solution
of such a problem do not exist, not just yet at all events. What we
may discuss with greater advantage is the question as to which of
the great subdivisions of life represents the main results of the
evolution of complex organic entities from the simple living substances
in which we suppose life first became materialised on our earth. What
activities and structural forms represent the main manifestations of
the evolutionary process?
That is to say, what great groups of organisms are the _dominant_ ones
on the earth? Greater or less degrees of dominance are indicated by
the extent to which a group of organisms is distributed on the earth,
by its abundance, and by the period of time during which it can be
recognised in the fossil condition. Ubiquitous distribution implies
a high degree of adaptability: a group of organisms inhabiting land
and sea and atmosphere is obviously one in which the morphological
structure has been elastic enough to admit of the development of
various modes of locomotion; and the limbs may be either the appendages
of a terrestrial animal, or the fins, or other swimming organs, of an
aquatic creature, or the wings of one adapted for flight. Dominance
in this respect implies mobility and activity, and a relatively
highly developed nervous system; it implies the development of organs
specialised for prehension, that is, for the capture of food; and
it also implies a high degree of adaptability to widely different
physical conditions, to temperature changes, for instance. Dominance
in geological time means also this great adaptability to changes in
climatic conditions, and the development of means of distribution
sufficient to overcome extensive physical changes on the surface of the
earth. A terrestrial species might become isolated by the formation
of a mountain range, or the submergence of the land adjacent to that
which it inhabited, and some widely distributed species of plants and
insects must have been able to traverse oceanic areas. The abundance
of a group obviously implies great powers of reproduction, the ability
to withstand physical changes, and the ability to resist competition
with other predatory creatures. Dominance, in short, means that the
organism possesses in high degree the inherent powers of reproduction;
and also those activities which enable it to respond by adaptations of
morphology, functioning, and behaviour, to environmental changes. These
environmental changes are those which must have been experienced during
lengthy geological periods, and also those experienced by the organism
in its attempt continually to enlarge its area of distribution.
If we make a broad survey of the animal world we shall find that
dominance in these respects has been acquired by three great groups
of organisms, (1) the Bacteria, (2) the chlorophyllian organisms, (3)
the Arthropods, and (4) the Vertebrates. In each case the threefold
condition of wide distribution over all the earth, both in fresh and
marine water areas, on the land and in the atmosphere; of existence
throughout the greater part of geological time; and of ability to
withstand environmental change, are satisfied. The bacteria are known
to have existed in the carboniferous period. At the present time
their distribution on the earth is universal: no part of the land
surface, and no water masses, either marine or lacustrine--no matter
how unsuitable they may be for the life of more highly organised
creatures--are untenanted by bacteria. They are able to withstand
extremes of temperature, or of salinity, which would be fatal to the
multicellular plant or animal. Parasitism is a mode of life which they
exhibit in a more manifold degree than do any other organisms. The
upper regions of the atmosphere are the only parts of the earth and
its envelopes which they do not inhabit.
The chlorophyllian organisms include those unicellular plants and
animals--the distinction becomes obscure with regard to these
organisms--which are pigmented blue, green, brown, or red owing to the
existence in the cells of chlorophyll, or of some substance allied to
this compound, and they include, of course, the green plants. Like the
Bacteria their distribution is world-wide, extending over land and sea
and fresh-water areas; and it is restricted mainly by the distribution
of sunlight, and by a lower limit of temperature. The Marine Algæ, the
Diatoms, the Peridinians, and other chlorophyll-containing organisms
appear to inhabit all parts of the world ocean, certainly within a
depth of about twenty to fifty fathoms from the surface of the sea.
Green plants inhabit the land everywhere except within polar areas,
the tops of high mountains, and over areas desert by reason of lack of
water, or by the presence of mineral substances.
These conditions--temperature, light, soil, etc.--do not appear to
limit the distribution of the Arthropods and Vertebrates. We find
both kinds of animals in the deepest oceanic abysses (deep-sea fishes
and Crustacea), in polar land and sea regions (Man, some Insects,
Crustacea, and Birds), as well as in desert areas and on the summits of
the loftiest mountains. The Ants share the subsoil with the Bacteria.
Birds and Insects conquer the atmosphere by their activity and not,
like the Bacteria, merely by being blown about. Crustaceans such as the
Copepoda have much the same distribution in the sea as the Insects have
in the atmosphere, while Isopods and Amphipods are a parallel, so far
as the sea bottom is concerned, to the Spiders, Millipedes, and Ants on
the land. Fishes are distributed throughout all depths, and in almost
all physical conditions in the sea. Some species of marine Mammalia
and Birds are quite cosmopolitan except that they are restricted to
the upper layers of the ocean. Land Mammals are subject to the same
restrictions as are the green plants, being unable to survive in desert
and polar areas. The only parts of the sea which are not inhabited by
Arthropods and Vertebrates are those limited deep strata of water (as
in the case of the deeper layers of the Black Sea) where there are
accumulations of poisonous chemical substances in solution. But the
Bacteria inhabit even these regions.
Green plants, Arthropods, and Vertebrates appear as fossils in almost
every part of the stratified rocks. The Trilobites represent the end of
a long evolutionary process, and the same is to be said of the first
fishes found in Silurian rocks, so that these groups of animals must
have existed in the geological periods represented by those remains
of rocks which are older than the earliest fossiliferous ones. Plant
remains are present in Silurian rocks, but there can be no doubt that
Ferns and other chlorophyllian organisms must have been in existence
long before this time. We can hardly suppose that the Bacteria found
in the Carboniferous rocks first appeared at this time in the earth’s
history: like the other great groups of life they probably had a
prolonged history prior to that date of the geological formations in
which they are first to be recognised. Our dominant groups of organisms
may therefore be traced back almost to the very beginnings of life on
the earth.
Dominance, such as we have defined it, cannot be said to have been
attained by any other of the sub-kingdoms of life. Cœlenterates and
sponges appear to have existed throughout the whole period during
which the remains of organisms are to be traced in the rocks, but
they have always been exclusively aquatic animals and they are very
sparsely distributed in fresh water regions. Echinoderms are also a
very old group, but they were more abundant in the past than they
are now, and they appear to have been an exclusively marine group of
animals. Molluscs have existed since the beginnings of stratified
deposits and they are both aquatic and terrestrial animals, but they
belong predominantly to the sea. They have always been relatively
sluggish and inactive animals, with the exceptions of the great Squids
and Cuttlefishes, but fortunately for the other inhabitants of the
sea these formidable creatures appear to possess restricted powers of
reproduction, and they have never been very abundant. All the smaller
groups of animals are restricted in their distribution: the flat-worms
occur sparingly both on the land and in the sea, and they attain their
highest development as parasites in the bodies of other animals.
Annelid worms, Gephyrea, Nemertine worms, Polyzoa, Rotifers, etc., are
all groups of animals occurring mainly in fresh and sea water and none
of them is abundant. Related to most of the great phyla are smaller
groups: the extinct Trilobites, Eurypterids, etc., in relation to the
Arthropoda; the group represented now by _Peripatus_ in relation to the
Arthropods and Annelids; the Enteropneusta and some other creatures
which appear to possess affinities with the Echinoderms and Chordates;
and the extinct Ostracoderms, which appear to have been related to
either the Arthropods or Vertebrates, or to both. All these smaller
groups of animals we must regard as representing sidepaths taken by
the evolutionary process--paths which have either ended blindly, as in
the case of those groups which have become extinct, or which we can
still trace in the existing remnants of groups which were formerly more
abundant than they are now.
Only among the existing Bacteria, chlorophyllian organisms, Arthropods,
and Vertebrates has the vital impetus found its most complete
manifestation, and we may even narrow down the main path that evolution
has taken to certain groups in each of these phyla. Some of the
Bacteria--those which are exclusively parasitic in the bodies of the
warm-blooded animals--have adopted a most specialised mode of life,
and may even be said to exist only with difficulty, since the healthy
animal is able to destroy them. Only those Bacteria living in the open
or upon the dead tissues of plants and animals have attained to real
dominance. Some green plants, like the Ferns, are far less abundant now
than they were in the past; while the Fungi and some other saprophytic
and parasitic plants have specialised in much the same way as have the
parasitic worms, and are restricted in their distribution. Marine Algæ
are confined to a relatively narrow selvedge of sea round the land
margin. The great trees, the grasses, and the microscopic green plants
such as the Diatoms and Peridinians, represent the truly dominant
organisms in the vegetable kingdom. On the side of the Arthropods and
Vertebrates there have been many unsuccessful lines of evolution: the
Trilobites, for instance, in the former group; and the armoured Ganoid
fishes, the armed Reptiles, the volant Reptiles, and the giant Saurians
and Mammals among the Vertebrates. Among the existing Arthropods and
Vertebrates there are some smaller groups which persist, so to speak,
only with difficulty. Such are the Spiders, Mites, and Scorpions among
the Arthropods; and the Tunicates, the Dipnoan fishes, the tailed
Amphibians, many Reptiles, and the volant Mammals among the Chordates:
such are, of course, only instances of the less successful lines of
evolution in these phyla. The dominant Arthropods and Vertebrates are
the Crustacea, the Hymenopterous Insects, the Teleost and Elasmobranch
fishes, and the terrestrial Mammals. The earth belongs to Man, to
the social and solitary Ants, Wasps and Bees, the marine Crustacea,
the Teleost fishes, the Trees, Grasses, and unicellular Diatoms and
Peridinians, and to the putrefactive and prototrophic Bacteria.
These are the organisms in which life has attained its fullest
manifestations, and has been most successful in its mastery over inert
matter.
In what kinds of activity and morphology, then, has the vital impetus
found most complete expression? We see at once that in relation to
energetic processes life has followed two divergent lines--animal
and vegetable. There is no absolute distinction between the
energy-transformations which proceed in the living plant and animal--we
return to this point later on--but we may trace an unmistakable
difference in tendency, that is, in the direction taken by evolution.
This difference we have already considered in an earlier chapter, but
we may illustrate it by considering a lifeless earth, and also one
tenanted only by plants, or animals, or by both.
In a lifeless earth all energetic processes would tend continually
toward a condition of stability. The crust of the earth, that is, the
part known to us by direct observation, is made up of rocks and the
remains of rocks; materials consisting of compounds of oxygen, silicon,
iron, aluminium, sodium, potassium, calcium, and so on. They are
substances which would be stable but for the eroding action of water,
the gases of the atmosphere, and volcanic activity. But as volcanic
activity tends always toward cessation, the oxygen of the atmosphere
would gradually disappear, first by its combination with oxidisable
substances, and second by its combination with the nitrogen of the
atmosphere under the influence of electric discharges. Carbon dioxide
would either combine with materials in the rocks, or would remain in
the atmosphere along with nitrogen and other inert gases in a stable
condition. Water, moved by the tides and winds, would gradually plane
down the surface of the land, unless along with other gases it would
gradually become dissipated into outer space. We see, then, that the
materials of the earth tend to fall into stable combinations, and that
they approximate toward conditions in which potential chemical energy
becomes reduced to a minimum, the whole energy possessed by matter
being that of the motions of the molecules, that is, kinetic energy
unavailable for transformations of any kind. It would be an earth
devoid of phenomena.
Vegetable life alone would be possible only for a time on an earth such
as we know it at present. The green plant depends for its existence on
the presence in the soil of mineral substances such as salts of nitric
acid and of ammonia, and on the presence of water and carbon dioxide in
the atmosphere. The chlorophyllian apparatus is essentially a mechanism
whereby these substances become built up into carbohydrates, like
starch and sugar; hydrocarbons, like resins and oils; and proteids. The
energy necessary for these syntheses is obtained from solar radiation
through the agency of the chlorophyll plastids. The green plant would
depend for its supply of nitrate or ammonia on the combination of the
nitrogen of the atmosphere with oxygen, or on the exhalations from
volcanoes, and these are irreversible processes which tend continually
toward cessation. The plant requires also carbon dioxide and the amount
of this substance in the atmosphere is very limited, while the only
inorganic source from which it can be renewed seems to be volcanic
activity: this substance also would tend to disappear. A time would
therefore come when plant life on the earth would cease to be possible
because of the disappearance of the materials on which it depends; but
while it did exist its result would be the accumulation of chemical
compounds of high potential energy. The result of the metabolism of
the plant is the formation of such compounds as cellulose from woody
tissues and shed leaves, of other plant carbohydrates, of oils and
resins, and of proteids. In the absence of bacteria such substances
would persist unchanged: even in an earth tenanted by bacteria such
products as oils, lignite, peat, coal, etc., have been able to
accumulate throughout geological time. The tendency of plant life
is therefore toward the accumulation of compounds of high potential
energy, and this process also is irreversible.
Bacterial activity would, of itself, make continued plant life possible
on the earth. The essential characters of these organisms are their
ability to bring about the most varied energy-transformations. From
our present point of view bacteria may be divided into paratrophic,
metatrophic, and prototrophic forms. Paratrophic bacteria are those
which live as parasites within the living tissues of plants and
animals: this mode of life is obligatory, and these organisms are
unable to live in the open. The result of their activity is the
breaking down of protoplasmic substance. Metatrophic bacteria are
those that produce putrefaction and fermentation of organic compounds.
They may be parasitic in their mode of life, but most of them live in
soil, in water, and in the cavities of the animal body--the mouth,
alimentary canal, nose, and vagina. Proteids are decomposed by them
into simple chemical compounds such as amido-acids, and then these
substances, along with carbohydrates, are fermented so as ultimately
to form water, carbonic acid, and salts of nitric acid. These bacteria
obtain their energy from the conversion of chemical compounds of high
potential energy into compounds of low potential energy. Prototrophic
bacteria are never parasites, nor do they live in the cavities of the
bodies of animals: they always live in the open. They carry on still
further the action of the putrefactive bacteria by converting ammonia
into nitrous acid, and nitrous acid into nitric acid. Others reverse
this series of changes by reducing nitric acid to nitrous acid, nitrous
acid to ammonia, and ammonia to free nitrogen. Others again oxidise
sulphuretted hydrogen to sulphuric acid, others ferrous hydrate to
ferric hydrate, while it has recently been shown that some bacteria
are apparently able to oxidise the carbon of coal to carbonic acid.
Some are able to oxidise the free nitrogen of the atmosphere into
nitrous and nitric acids. How precisely the energy necessary for these
transformations is obtained is not at all clearly understood, and it
may be possible that some of the prototrophic bacteria obtain their
energy by making use of the un-co-ordinated kinetic energy of the
medium in which they live. From our point of view the net result of the
activity of the predominant species of bacteria which inhabit the earth
is that they _reverse_ the processes which are the manifestations of
the metabolism of plants and animals. The result of the metabolism of
plants is the accumulation of stores of high potential compounds such
as carbohydrates, and the depletion of the terrestrial stores of carbon
dioxide and other materials necessary for the continued existence of
the plants themselves. The result of the metabolism of the bacteria is
the break-down of this accumulation of such compounds as carbohydrates,
and the replenishing of the stores of carbon dioxide and nitrogenous
mineral substance on which the plant depends. If bacteria are present,
the life process becomes a reversible one.
Plant life and bacterial life are thus complementary to each other,
for, on the whole, the energetic processes of the green plant proceed
in the opposite direction to those of the bacteria. An organic world
consisting of green plants and bacteria would therefore be one capable
of permanent existence. Now, so far, we need only consider these
various kinds of organisms as living protoplasmic substances in which
energy-transformations of different types proceed. The bacterium is
simply a cell containing a nucleus, and the green plant need only be
a nucleated cell containing a chlorophyll plastid: this is, indeed,
all that it is in the case of a Diatom or a Peridinian. The morphology
of the green plant is only accessory to the chlorophyllian apparatus.
Neglecting the reproductive apparatus, the higher green plant consists
essentially of the chlorophyllian cells in the parenchyma of the leaf,
for roots and stomata are only organs for the absorption of water and
mineral salts from the soil and carbon dioxide from the atmosphere;
while the tissues of the trunk, stems, and branches are, in the main,
apparatus for the conduction of these raw materials through the body
of the plant, and, of course, the nutritive substances into which they
are elaborated. All the innumerable variations of form in the plant
(apart from the structure of the flower or other reproductive organ)
are adaptations which provide for the absorption and distribution of
these substances; or for the mechanical support of the plant body; or
are non-adaptive variations, pure luxuries, so to speak.
More than this is represented by the structure of the animal body, but
we must first of all consider the points of difference between plant
and animal regarded merely as apparatus in which energy-transformations
occur. In the green plant energy is accumulated in the form of
high potential chemical compounds, but in the animal energy is
expended. Inorganic mineral substances are built up by the plant
into carbohydrate, proteid, and fat or oil, but in the animal body
carbohydrate, proteid, and fat are dissociated into water, carbonic
acid, and urea (or some other nitrogenous excretory substance); and
the urea or other analogous substance is broken down by bacteria into
nitrate, water, and carbon dioxide. The metabolic activities of the
animal are said to be “analytic” or destructive, while those of the
plant are said to be “synthetic” or constructive, but these contrasting
terms hardly describe accurately the essential nature of the activities
of the two kinds of organisms. What further constitutes “animality”? It
is _purposeful mobility_, and the energy-transformations that occur are
the means whereby this mobility is attained. The plant is essentially
immobile, for such movements as the turning of leaves toward the
light, the down-growth of roots, the up-growth of stems, the twining
of tendrils round supporting objects, and the opening and closing of
flowers are only the movements of parts of the plant organism. They are
constant, directed responses to external stimuli--real tropisms--and
the extension of this kind of response so as to describe in general the
movements of animals is only an instance of the insufficient analysis
of facts. The movements of the typical green plant are therefore
movements of its parts, they are few in number, they belong to a few
simple types, and they are evoked by simple external physical changes
in the medium. The movements of the typical animal are movements of the
organism as a whole; they are infinitely varied in their nature; they
are evoked by individualised stimuli and they are continually being
modified by the experience of the organism.
The bodily structure of the animal is the means whereby this purposeful
mobility is attained and the energy-transformations directed; and
the greater and more varied the movements of the animal, the more
complex is its structure. In respect of the manner in which the
energy-transformations are effected, that is, in respect of the
material means whereby energy falls from a state of high potential to a
state of low potential, the morphology of the animal is similar to that
of the plant, that is, the energy-transformations are the functions of
nucleated cells. But in the plant the kinetic energy of solar radiation
passes into the potential energy of chemical compounds which become
stored in the body of the plant; while in the animal the potential
energy of ingested chemical compounds passes into the kinetic energy
of the movements of the animal itself. How exactly it moves, how this
kinetic energy is employed is determined by the sensori-motor system.
It is the existence of the sensori-motor system that makes the animal
an animal. What, then, is the sensori-motor system? It is the skeleton
and muscles, that is, the organs of locomotion, aggression, prehension,
and mastication; the peripheral sensory and motor nerves; and the
central nervous system or brain. The skeleton of an animal, whether
it be the carapace or exoskeleton of a crustacean, or the vertebral
column, limb-girdles, and limb-bones of a vertebrate, is a rigid and
fixed series of supports to which the muscles are attached. Organs
of locomotion are, for instance, the appendages of a crustacean, the
wings of a bird or insect, the tail and fins of a fish, or the limbs
of a vertebrate. Organs of aggression are the mandibles of a spider
or blood-sucking fly, the chelate claws of a crab or lobster, the
jaws of a fish, or the claws and teeth of a terrestrial vertebrate.
Organs of prehension and mastication are in the main also those of
aggression. All these parts consist of modified skeletal structures,
teeth, claws, etc., attached to muscles which originate in the rigid
parts of the skeleton. When we speak of the movements of an animal we
speak of the motions of such parts as we have mentioned; other parts do
indeed move--the heart pulsates, the lungs dilate and contract, and the
blood and other fluids circulate through closed vessels; but these are
movements of the parts of the animal, and are comparable rather with
those movements of the plant organism that we have considered. They are
not to be regarded as examples of the mobility of the animal in the
sense of the exercise of its sensori-motor system.
A central and peripheral nervous system is, of course, bound up with
a motor system. Receptor organs, eyes, olfactory, auditory, tactile
organs of sense, and so on, are the means whereby the animal is
_affected by_ changes in its environment--it need not be cognisant
of, or become aware of, or perceive these impressions on its receptor
organs. These stimuli are transmitted along the sensory, or afferent,
nerves to the central nervous system: this is the way in. The effector
nervous organs are the motor plates, that is, the nervous structures in
the muscles in which the nerves terminate. The motor nerves are the
efferent paths, the way out from the central nervous system.
The central nervous system is essentially the organ for the integration
of the activities of the whole body. It is the “seat of multitudinous
synapses,” a description which better than any other applies to the
morphology of the brain of the vertebrate animal. We have already
considered what is meant by the term “reflex action,” it is the series
of processes which occur when a “reflex arc” becomes functionally
active. A reflex arc consists of (1) a receptor organ, say a tactile
corpuscle in the skin; (2) an afferent nerve fibre; (3) a nerve cell
in the brain or spinal cord; (4) an efferent nerve fibre; and (5) an
effector nerve organ, say a motor plate in a muscle fibre. The series
of processes involved in a reflex action consist of the stimulation
of the receptor organ, the passage of the afferent impulse into the
brain or cord, the passage of the impulse through a series of cells in
the nerve centre forming a synapse, the transmission of the impulse
through the efferent nerve fibre into the effector organ in the muscle
and the stimulation of the latter to an act of contraction. This is a
purely schematic description of the structures and processes forming
a reflex action and arc: in reality the path both into and out from
the central nervous system is interrupted again and again, and at each
place of interruption there are alternative paths. The interruptions
occur at the synapses. At a synapse the nervous impulse passes through
an arborescence of fine nervous twigs, into which the fibre breaks
up, into a similar arborescence, and these two arborescences are not
in actual physical contact: the impulse leaps over a gap. At numerous
places in both brain and cord there are alternative synapses and at
these places the impulse may travel in more than one direction.
The brain and cord are a switch-board of unimaginable complexity, so
that an efferent impulse entering it from, say, the eye, can be shunted
on to one nerve path after another, so that it may affect any muscle
in the whole body. This is no fiction: it may actually be the case.
In normal respiration a centre in the hind-brain is stimulated to
rhythmical activity by the presence of carbon dioxide in the blood, and
from it efferent impulses originate which stimulate the muscles of the
chest wall and diaphragm. But in the distress of asphyxia every muscle
of the body may be stimulated to activity in the effort to accelerate
the oxygenation of the blood, and these are not spasmodic movements
of the muscles of limbs, etc., but purposeful contractions having for
their object the increased intake of air into the lungs. The central
nervous system is, therefore, a switch-board--so mechanistic physiology
teaches, neglecting any idea of an _operator_. But the whole trend of
modern investigation is to show that every increase of specialisation
in the evolution of the higher animal adds to the complexity of this
nervous apparatus by increasing the number of alternative paths that an
impulse originating anywhere in the body may take before it issues from
the brain or spinal cord. Yet with all this increase of complexity it
is nevertheless the case that in the higher animal the various parts of
the central and peripheral nervous system are more and more integrated,
so that in the actions of the animal it becomes more and more the
organism _as a whole_ that acts.
All other organs in the animal body--excepting always the reproductive
apparatus--are accessory to the sensori-motor system. The alimentary
canal and its glands dissolve the food-stuffs ingested; the metabolic
organs, that is, the cells of the wall of the intestine, the liver,
etc., transform these ingested proteids, fats, and carbohydrates of the
food into the proteids, fats, and carbohydrates of the animal itself;
the heart, blood, and lymph vessels carry this food material to the
muscles and nervous organs; the respiratory organs absorb oxygen which
is distributed throughout the body in the blood stream; the excretory
organs, that is, the lungs, skin, and kidneys, remove noxious materials
like carbonic acid and urea, or its precursors; and purposeful changes
of functioning of all these organs are brought about by changes in
motor activity. Round the sensori-motor system all the rest of the
structure of the animal body is built up.
What we see clearly in the evolution of the animal body is the
progressive increase of activity of the sensori-motor system. The
_animal becomes more and more mobile_. It is in this way that dominance
has been attained and all the directions of structural evolution in the
past that have not tended in this direction have been unsuccessful,
irreversible, evolutionary processes. Great size has not succeeded in
the animal kingdom, and so the gigantic reptiles and mammals of the
secondary and tertiary periods have become extinct. Defence against
enemies by the development of dermal armour has not succeeded, and so
the Dinosaurs, and other armed animals of the Tertiary Age have also
become extinct. The transformation of the fore limbs of the reptile
into wings, or the legs of the mammal into flappers, did not succeed,
because all the rest of the structure of these animals had become
adapted to locomotion on dry land, and the change of structure had
become too profound to be modified: so the Pterodactyls passed away,
as the whales of our own period are also passing. Only in the lightly
boned, feathered bird, with the possibility of the development of
powerful pectoral muscles, did indefinite possibilities of flight
reside; and only in the fish, with the concomitant evolution of gills,
the reduction of a minimum of the mass of the alimentary canal and its
glands, and the conversion of most of the muscles of the body into
organs actuating the tail fin, was the completeness of adaptation
to aquatic life realised. Mobility, a bodily structure capable of
indefinitely varied movements, and a nervous system by the aid of which
any part of the body might become linked to any other part--these were
the structural adaptations that have been successful alike in Arthropod
and Vertebrate.
There were apparently two main types of structure by means of which
this mobility and elasticity could be attained, the Arthropod type
and the Vertebrate type. There seems little to choose between them
if we had to select one of them in order to obtain a highly mobile
organic mechanism. Arthropod and Vertebrate seem to be equally complex
if we take account of difference in size and the additional bodily
mechanism that great size must involve. Certainly the musculature of
the Vertebrate is more complex than in the Arthropod. But greater
weight must require larger and more powerful muscles if the same degree
of mobility relative to the size of the animal is to be attained,
and this more complex musculature must carry with it a more complex
brain. It must also be concomitant with a more massive skeleton, for
rigid supports for the muscles must be present in the mechanism. Why
are there no great insects or crustaceans? Mr Wells has suggested in
one of his novels the formidability of a wasp two feet long! Such a
creature would indeed be more dreadful than any predatory bird that we
know if its activity were also that of the wasps that we know, just as
a Copepod as large as a shark would be a more formidable animal than
the fish. It seems possible that the reason for the smaller size of
the Vertebrate is to be found in the nature of the skeleton. Powerful
muscles would require a very strong and thick carapace, and this would
attain a mass in a very large insect or crustacean which would require
too much energy for its rapid transport. A rigid exoskeleton like that
of an Arthropod also means that growth must take place by a process
of ecdysis, that is, the animal grows only during the periods when it
casts its shell; and the necessity of this process of ecdysis must
be a formidable disadvantage in the case of a very large animal, if
indeed it would be possible at all. Thus the Arthropod developing an
exoskeleton must remain small, and this smallness, fortunately for the
Vertebrate, has made it the less formidable animal. It was an accident
of evolution that the Arthropods developed an exoskeleton instead of an
endoskeleton.
Undoubtedly the internal skeleton of the Vertebrates, with its light,
hollow, cancellated bones, was mechanically the best means for the
attachment of muscles. It made possible a greater degree of freedom
of movement of the parts of the body, greater variety and plasticity
of action, and it removed, to some extent, the limit of size and the
embarrassing discontinuity of growth by ecdysis, with all the dangers
that this involves. Above all, it led to the increased complexity
of the central nervous system, since this became bound up with the
increasing variety of bodily movement.
In the evolution of the dominant groups of organisms we see, then, the
development of several tendencies. First, that tendency which seems
to offer the greatest contrast to the universal tendency displayed in
inorganic processes, the dissipation of energy. The plant organism is
essentially a system in which energy is accumulated in the potential
form. Then, in the animal kingdom we see that the main tendency of
evolution has been the development of systems in which energy becomes
expended in infinitely varied movements. It may seem, on superficial
examination, that in the animal mode of metabolism energy is dissipated
as it is in inorganic processes; and this is the conclusion that we
should reach if we considered the actions, and the results of the
actions, of the lower animals only, that is, animals lower than man. We
return to this point later on, but in the meantime it is to be noted
that the fundamental division of organisms is that founded upon their
activities as energy-transformers, that is, into plants and animals.
Within each of these kingdoms of organisms structural evolution has
occurred: the unicellular green plant has evolved along very numerous
lines, each of them characterised by a different type of morphological
structure. The unicellular animal has also evolved in a similar way
with the result that the present phyla have become established. Looking
at these great groups of animals, we see that two of them have attained
dominance by the development along different lines of a sensori-motor
system. Here we see another fundamental difference between the
plant and animal organism, but one which is a consequence of the
difference that exists between the two kingdoms in respect of the
energy-transformations carried out by them. The plant is characterised
by immobility, the animal by mobility.
Immobility implies unconsciousness, mobility consciousness, and this
physical difference is the third one which we can establish between the
plant and the animal. Now few physiologists are likely to accept this
distinction as one which has any real objective meaning. Consciousness
is not a concept to be dealt with in any process of reasoning, it is
not even something felt in the way in which we speak of the feelings of
pain, or light, or hunger: these are all states of our consciousness.
The difference in ourselves, says Ladd, when we are sunk in sound
dreamless sleep, and when we are in full waking activity, _that_ is
consciousness. If we reason about organisms and their activities
as we do about inorganic things we have no right to speak about
consciousness, for outside our own Ego it has no existence. The acting
animal is only a body, or a system of bodies, moving in nature, and all
its activities are to be described by a system of generalised force
and position co-ordinates with reference to some arbitrarily chosen
point of space. “This animal machine,” says a zoologist, writing about
instinct, “which I call my wife, exhibits certain facial contortions
and emits certain articulate sounds which correspond with those emitted
by myself when I have a headache, but I have no right to say that she
has a headache.” This kind of argument does not appear to be capable
of refutation except, perhaps, by the domestic conflicts which it
would usually evoke if applied in such cases as that quoted. In a
description of nature by the methods and symbolism of science we see
only systems of molecules in motion, and in those systems which we
describe as organisms the motions are only more complex than they are
in inorganic systems. Such is the method of science, as irrefutable
in the study of the organism as we know that it is false. Valid in
pure speculation according to the methods of the intellect it would
nevertheless be absurd in the everyday affairs of common civilised life
and the scientific man who applies it in his writing would nevertheless
hesitate to apply it in the affairs of his own household.
We must recognise that our knowledge that other beings like
ourselves, as well as animals lower in organisation than ourselves,
are consciously acting organisms is intuitive knowledge, attainable
because of community of organisation: our intuitive knowledge of the
behaviour and feelings of our own brothers and sisters is greater
than our knowledge of other men and women; and we can, by intuition,
place ourselves within the consciousness of an intelligent dog to a
greater extent than in the case of other animals. This knowledge of
the consciousness of other animals is not scientific knowledge and it
is unattainable and unprovable by reasoning or methods of scientific
observation. It is a conviction in itself incapable of analysis or
proof, but yet a conviction on which we confidently base most of our
dealings with our fellow-creatures, and which is justified by our
experience.
It is nevertheless a scientific hypothesis of much the same validity as
many other scientific hypotheses. We cannot bring ourselves to doubt
that other men and women are consciously acting organisms, however
impossible it may be to adduce scientific reason for the faith that
is in us. We cannot doubt that a compass needle which “responds” by
turning one or other of its poles towards us according as we push
forwards one or other of the poles of a magnet is an unconscious
piece of metal, though we find it impossible to say why this belief
possesses such conviction. From this to the movements of the typical
green plant is only a step. The turning of a green leaf towards the
source of light, or the downward movement of a root into the soil, are
responses to external stimuli which exhibit most of the inevitability
of response of the magnet. They are “tropisms”: the plant leaf is
obliged to turn towards the light so that the latter strikes against
its surface perpendicularly, and the root must grow downwards because
gravity acts along vertical lines. But suppose that reflex actions are
tropistic: suppose, for instance, that the moth is bound to fly into
the candle flame because the light stimulates both sides of its body
equally and this orientates it and guides it towards the direction from
which the stimulus proceeds. Complex actions, in the higher animal, on
this view are chains of reflexes, and the acting must be unconscious
and inevitable, just as the turning of the magnet or green leaf are
unconscious movements. Therefore the actions of our fellow-creatures
are unconscious and automatic, a conclusion toward which the whole
tendency of mechanistic physiology forces us. Yet we know that the
conclusion cannot be true.
Between the obligatory reaction of the compass needle to the magnet, or
the analogous heliotropism and geotropism of the plant organism, and
the infinitely variable responses of the higher animal toward changes
in its environment, consciousness must come into existence. It is
absent in the inorganic system and the typical green plant; it is dim
in the sedentary sea-anemone or mollusc; it becomes brighter in the
freely moving Arthropod or fish; and it is most intense in man. This,
it must be admitted, is only a belief, but accepting it as such we may
attempt to support it by showing a parallelism of stages of structural
complexity and actions. The sensori-motor system is absent in the
green plant; it is simple in the extreme in the sea-anemone; and it
is rudimentary or vestigial in the sedentary mollusc. It becomes more
complex in the Arthropod or fish, and it is developed to the greatest
degree in ourselves. If we now examine our own mental states, with
their corresponding conditions of bodily activity, we see as clearly as
possible that our consciousness waxes and wanes with our activities.
It is absent in normal sleep, when bodily activity in the real sense
ceases almost absolutely, when the cerebral cortex becomes inactive,
and when the only movements performed are those truly automatic ones
of parts of the body which are analogous to the movements of the plant
organism. Such movements are the rhythmic ones of the heart and lungs,
the movements of the blood, and so on, in general the movements leading
to constructive metabolism. Consciousness is most intense in difficult
unfamiliar actions: the lad learning to row; the child learning scales
on the piano, or the fingering of the violin; the engineer assembling
together the parts of a new machine; or the artist engaged on a
picture. In each of these cases the worker is acutely conscious, in a
deliberative manner, of his own bodily actions. But with the habitual
exercise of these movements, and with the ease and facility with which
they are performed, consciousness that they are being performed fades
towards nothingness.
What does this mean but that degrees of consciousness are parallel to
degrees of complexity of deliberated and purposeful bodily movements
or actions? Or degrees of consciousness are also parallel to the
attempt of the organism to perform these actions. What is pain, the
most acutely felt of all our mental states? It is, Bergson says, the
consciousness of the persistent and unsuccessful effort of the tissues
to respond purposefully to a persistently renewed stimulus. But complex
actions require for their performance systems of skeletal and muscular
parts capable of moving in the most varied ways, and a system of
afferent and efferent nerves with all their connections in the central
nervous system: that is, a sensori-motor system. Therefore just as
the sensori-motor system is more or less complex so, in general, is
consciousness more or less acute.
Yet in the same organism consciousness is the more or less acute as
the actions which it performs are more or less familiar. The pianist
who plays scales as a matter of exercise carries out most complex
movements of hands and wrists unconsciously and without effort, but
to play an unfamiliar composition for the first time without error
involves attention of the highest degree. A girl who counts the sheets
of paper coming from a machine seizes a handful in one hand, and drops
a separate sheet between every two fingers of the other hand, repeating
this most difficult operation with great rapidity, and counting the
handfuls of sheets accurately while thinking and talking deliberately
about some other matter. At the beginning of her work these actions
were clumsily performed and facility was only attained by sustained
attention to the movements of the hands, yet with experience they
become unconsciously performed. Complex movements of the body and limbs
and digits, involving the co-ordinated activity of numerous muscles,
nerves, and nerve centres, are performed at first only after a high
degree of conscious effort, but with each repetition of the series of
movements the animal ceases to be aware of them, or at least of their
difficulty. In the higher animals there are, therefore, two categories
of actions, (1) those unfamiliar actions which are _difficult_, and
in the performance of which the animal becomes conscious of complex
muscular activities; and (2) those habitual actions which have
become _easy_ by dint of repetition, and the performance of which is
unattended by conscious effort. Analysis of our own activities reveals
these two categories of actions, and we have no doubt whatever that the
higher animals have the same feelings of difficulty and effort in the
one case, and of lack of conscious effort in the other.
The difference is one of those which separate instinctive from
intelligent activities. Now we hesitate to attempt the discussion of
this much-controverted question of the distinction between instinct and
intelligence: after reading much that has been said as to the nature of
this difference, one rises with the uncomfortable impression that the
time is not yet ripe for its discussion, and that the problem is still
one far more for the naturalist than for the psychologist. Reliable
data are still urgently required. Yet it is a question which we cannot
fail to consider. The typical plant differs from the typical animal in
that a sensori-motor system has been evolved in the one but not in the
other; and among the animals in which this system is developed to a
high degree the activities which involve its exercise differ in their
form. Actions of a stereotyped pattern characterise the behaviour of
the higher Invertebrate, while in the higher Vertebrate all that we
see indicates that the behaviour is the result of deliberation, and
that the actions performed are not stereotyped but differ infinitely
in their patterns. Just as clearly as differences in morphology
differentiate Arthropod from Vertebrate, so also do differences in the
mode of activity of the sensori-motor system mark divergent lines of
evolution culminating in the Hymenopterous Insect on the one hand and
in Man on the other.
What is the essential difference between an action performed
instinctively and one performed intelligently? It is not that the
animal is unaware of its activity in the first case and not in the
second; however much we tend to “explain” organic activity in terms of
inorganic reactions, we do not really believe that the instinctively
acting wasp is a pure automaton, while admitting that the schoolgirl
is acutely conscious of her own multifarious activities. It is not
that the instinctive action displays a “finish,” or perfection of
technique, that the deliberative action lacks: the comb built by the
wasp is not more perfect in its way than is the doorway constructed
by a skilled mason, or the “buttonholes” stitched by a seamstress.
It is not that instinctive actions are so absolutely stereotyped, as
is sometimes assumed, while intelligent actions grow more perfect in
their result by repetition: the work of the insect or bird is often
faulty and it is improved by practice. The most obvious difference is
that the instinctive action is _effective_ the very first time it is
performed, while the intelligent action only becomes effective after it
has been attempted several times, or very many times, according to its
difficulty. The flight of the young swallow is effective inasmuch as it
sustains the bird in the air, but it is also an exceedingly difficult
series of muscular efforts which is at first clumsily performed
and which becomes more perfect by repetition. But the flight of an
aeroplane, even now after years of experiment, is not always effective,
and exhibits at its best all the imperfections of the flight of the
young swallow. Yet can we doubt that in time it will exhibit all the
ease and certainty and finish of the flight of the bird?
The typical intelligently performed action is the action of a _tool_,
or of a part of the body which is used for some other purpose than
that which is indicated by its immediate evolutionary history, or by
its previous use. The typical instinctively performed action is always
the action of a bodily organ, the structure and immediate evolutionary
history of which indicates that it originated as an adaptation for
the performance of these particular actions, or category of actions.
Here it seems to us that we find the distinction between the two kinds
of bodily activity; and the distinction is one which depends for its
validity on our notions as to what a tool is. An implement made by man
is a piece of inert matter fashioned in order that it may be used for
a definite preconceived purpose. It has an existence as a definite
specific object _apart from its use_; and its exercise by the man who
made it and its existence in nature are two different things. Its use
must be _learned_, and the results obtained by its employment become
more perfect with every repetition of its use. But the mandibles of an
insect are implements purposefully adapted for some action or series
of actions, just as the pincers of the blacksmith are so adapted.
They are, however, implements which are part of the organisation of
the animal using them--organised tools--and it does not seem as if we
ought to think of them, and of their shape and nature, as something
apart from their exercise. Must we think of an animal as having to
_learn_ how to use any part of its body? If so, then the problem of
instinct remains with us in all its historic obscurity. But if we think
of the existence of a bodily tool as something inseparable from the
functioning of the tool, the problem becomes less obscure, or at least
it can be stated in terms of some other problems which we have already
considered.
We do actually think of bodily parts or organs as material structures
quite apart from the consideration of their functions: it is the
distinction between morphology and physiology--an altogether artificial
one. An animal, for the morphologist, is a complex of skeleton,
muscles, nerves, glands, and so on; and it does not matter whether
it is contained in a jar of methylated spirit or is running about in
a cage. For the physiologist it is “something happening”; but is it
not really both things, and are not the structure and the functioning
only two convenient, but arbitrary, aspects from which we consider the
organism? We ought not to think of diaphragm and lungs apart from the
movements of these organs, and we do not say that the first breath
drawn by the newly-born mammal is an instinctive action, involving the
use of inborn bodily tools--the diaphragm, lungs, etc. We ought not
to think of the lips and mouth and pharynx of the young baby apart
from the actions of suckling the mammæ of its mother, but usually we
say that this action is an instinctive one. Where does the ordinary
functioning of an organ end and its instinctive functioning begin?
Are the muscular actions of the lobster when it frees its body and
appendages from the carapace during the act of ecdysis instinctive
ones? Most zoologists would say that they are not, any more than the
movements of the maxillipedes in respiration are instinctive ones, yet
they probably would not hesitate to say that the action of the “soft”
lobster in creeping into a rock crevice is instinctive. Does a young
child really “learn” to walk? It is more likely that the actions of
walking are potential in its limbs and that they become actual when all
the connections of nerve tracts and centres in its brain and spinal
cord become established. What is the difference between the acquirement
of the ability to walk and to write? The latter series of actions are
unfamiliar combinations of nervous and muscular activities which are
no part of the organisation of the young child; while the former are
simply the result of the complete functional development of certain
nervous and muscular apparatus.
It seems difficult, then, to express clearly what is the essential
difference between instinctive and intelligent behaviour; and it is
doubtless the case that reasoned experiments and observations are
still too few to enable us to make sound deductions. But it certainly
seems as if we ought to think of instinctive actions as having evolved
concomitantly with the structure of the organs which effect them:
they are those _inheritable adaptations of behaviour_ which are bound
up with--are indeed the same things as--inheritable adaptations
of structure. In performing them the instinctively acting animal
is doubtless aware of its own activity, but we must think of this
awareness as being of much the same nature as our consciousness of the
automatic activities of our own bodies--the rhythmic activities of the
heart and respiratory organs, or the actions of our arms and legs in
walking, for instance. It is knowledge of the inborn ability of the
organisms to use an inborn bodily tool.
In the intelligent action we certainly see something different from
this. The organ or organ-system which carries out such an action
functions in a manner which is different from that for which it was
evolved: the action is the conscious adaptation of the organ for some
form of activity new to it, and this acquirement of activity seems
to be non-inheritable--at least it is non-inheritable in the sense
in which we speak of acquired characters being non-inherited. It is
accompanied, while it is being acquired, by a consciousness which is
deliberative, and is different from that awareness of its own activity
which accompanies the acting of the instinctive animal--the knowledge
that it is acting in an effective manner. It does not seem as if the
animal in so acting is aware of the relation of the bodily tool to the
object on which it is acting. But intelligence seems to imply more than
this: it implies the knowledge of the organism that some parts of its
body bear certain relations to the parts of the environment on which
they are acting, and that these relations are variable ones and may be
the objects of conscious choice.