Chapter VII: The Meaning of Evolution (1)

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.

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.

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.