OTHER FACTORS IN EVOLUTION addition to the three main factors of evolution so far dealt with, variation, heredity and competition, there are, as Darwin pointed out, subsidiary or auxiliary aids to natural selection. Of these isolation is perhaps the most important. As explained above, species are, generally speaking, not composed of individuals of perfectly uniform inheritance, but are assemblages of intercrossing races, differing from each other by some out of the vast number of genes making up their hereditary equipment. Now, sexual reproduction no doubt is the means of affording ever fresh combinations of genes, but it also works towards pre serving an average type by continually mixing varying strains. Selection without isolation may lead to evolution in a straight line, but if divergence is to take place, some sort of isolation seems necessary, and the fitting of organisms to new environ ments, the development of new functions and structures along ever diverging lines of adaptation, is one of the most character istic and significant features of organic evolution. Isolation is of several kinds : geographical, functional, structural, psychological. The means of dispersal are various; some organisms, such as microscopic animals and plants, spores, and seeds, are dis tributed passively by the wind , others by water currents, and still others move from place to place by their own efforts. Geographical isolation is due to physical barriers, mountains, deserts, rivers and oceans, separating groups of land animals. Marine organisms may be divided by the uprising of dry land, and the inhabitants of fresh water by the separation of river basins. Further subdivisions are due to organisms adopting dif ferent habitats to which they are adapted. Some live in dry, others in damp, places, some in the shade while others prefer the light, and so on. Aquatic forms may be restricted to certain depths, mountain forms to certain heights. Whatever the reason, isolation, sooner or later, leads to divergence from the parent form. Speaking generally, individuals sufficiently alike to be placed in the same species usually have a continuous distribution, closely allied species are usually found in neighbouring regions, especially if they have few means of dispersal, and they are more unlike the further they have strayed from the original centre of distribution.
Each large land area tends to acquire a characteristic flora and fauna, made up partly of indigenous forms evolved within it, and partly of forms which have migrated into it from neigh bouring areas. The more efficient the barriers enclosing it and the longer it has been isolated, the more different will be its flora and fauna from those of other areas. The organisms on an island resemble, on the whole, those of the continent from which it has been separated; many of them, however, are generally found to differ somewhat from the continental types. Particularly signifi cant is the fact that in an archipelago, a species may become split up into as many races as there are islands inhabited by it ; as has happened, for instance, with the golden oriole (Icterus) in the West Indies. In volcanic oceanic islands which have never been connected with a continent, and whose population is derived from rare strays capable of swimming, flying or being conveyed over seas from neighbouring lands, the number of peculiar species is strikingly large, and here again each island of a group may have its own form. A well known instance is that of the local races or species of birds and reptiles in the Galapagos islands. Such facts, of which many examples might be given, afford unmistak able evidence of the part played by geographical isolation in evolution. Somewhat similar isolation and divergence seem to result from parasitism ; for the tendency of the parasite to restrict itself to one host, to which it becomes closely adapted, may lead a parasitic form to split into as many different races as there are kinds of host. Similarly with plants fertilized by insects, the flowers tend to become specialized in various ways to attract certain kinds, while the insects undergo corresponding specializa tion to obtain nourishment from the flowers.
Many organisms, though closely allied, living in the same locality and coming into close contact, do not interbreed. Their crossing is effectually prevented by variations in structure, habit or temperament. The most important "physiological" barrier is sterility. of which there are several kinds due to different causes. Mere difference of size may prevent interbreeding, or differences in the structure of the copulatory organs, as in many insects; also the attainment of sexual maturity at different times of the year. Variations of the germ-cells themselves lead to what may be called true sterility, of which there are various degrees. Ferti lization may be imperfect or not take place at all, probably owing to incompatible differences in the protoplasm, in the num ber and character of chromosomes, or in the hereditary factors. Even if zygotes are formed they may fail to develop normally, or the offspring, if they reach maturity, may themselves be sterile, as in the mule. Whatever their origin, these various barriers to free and fertile interbreeding favour the preservation of divergent characters.
It may here be pointed out that other results follow on ferti lization besides the activation of the zygote and the mingling of genes already mentioned. It is popularly held that close and con tinued inbreeding leads to harmful results, to loss of fecundity and a weakening of the race ; while outbreeding or intercrossing is supposed to have beneficial results, to increase fecundity and general vigour. The almost universal forbidding of the marriage of near-of-kin in human societies is consonant with this belief ; and the many devices evolved in nature to facilitate cross-breeding and prevent self-fertilization among hermaphrodite plants and animals point to cross-breeding being advantageous. The ques tion, however, is by no means simple and cannot here be treated in detail (see INBREEDING) . It may be pointed out that continued close inbreeding due to self-fertilization does occur in many plants and in some animals (especially parasites) in nature, and may be carried on artificially without harmful consequences, pro vided vigorous individuals are selected for the purpose. One of the results of inbreeding is to reduce heterozygosity and secure homozygosity among the progeny; the opposite follows from cross-breeding. Now, since unfavourable characters are often determined by recessive genes, they are apt to become manifest in the individuals homozygous for them. Hence inbreeding of tainted stocks tends to bring out albinism, deaf-mutism and feeble-mindedness in man ; while no such evil results will follow if these particular genes are absent from the inheritance. On the other hand, there is ample evidence that the crossing of individuals or races differing slightly from each other in their inheritance or factorial make up, generally leads to increased vigour in the off spring. This beneficial result, apparently due to some degree of heterozygosity, is secured in nature by the separation of the sexes, and has long been known to breeders.
Great has been the influence of sex on the evolution of organ isms. To secure the nourishment, distribution, fertilization and survival of the reproductive cells is the chief function of all creatures ; to these ends have been developed that wonderful diversity and elaboration of bodily and mental structure found in living nature.
Palaeontology, the study of extinct fossil forms, has done much to strengthen this conclusion ; likewise embryology, the study of the ontogeny or development of individuals, which has thrown much light on obscure relationships and familiarized naturalists with the notion that small and simple beginnings may give rise to large and complex final products. Classification, then, is an attempt to group organisms according to their natural affinities and to trace out their pedigrees. Similar individuals are grouped into species, species into genera, these into families, orders, classes, phyla, divisions of increasing size and importance. The history of the conception of "species" has been mentioned at the beginning of this article. All sorts of attempts have been made to define species in accordance with modern theories of evolution ; but no stricter definition seems practicable than this, that a species is composed of closely allied individuals descended from a common ancestor, which normally interbreed, and are sufficiently alike and sufficiently distinguishable from related forms to be conveniently called by the same name.
Much stress has been laid on sterility between two forms as a test of their specific rank; but although interspecific sterility commonly occurs, perhaps favoured by natural selection to prevent intercrossing, it is by no means universal. Every gradation occurs between absolute sterility and perfect fertility in crosses between well-recognized species, especially among plants. Indeed many botanists claim that inter-specific hybridization is a frequent source of new species.
De Vries tried to define "elementary species", as strains dis tinguished by a single mutation. But mutations may be of any size from "sports" to scarcely perceptible differences, and most, if not all groups distinguished as species seem to be composed of interbreeding races differing from each other by small factorial differences, which races have sometimes been separated out by careful selection and breeding.
Species living at the present day represent the closely allied individuals at the extremities of the surviving twigs of the phylogenetic tree. If they have become marked off from allied species, it is because the intermediate connecting individuals have died out. Along the branches leading to them and along the main stem, there are no points at which a species can be said to begin or to end. It is only for convenience in description that the name species can be applied to ancestral forms along these lines of gradual evolution. The only "fixed points" in a phylogenetic system of classification are the points of bifurcation, where one branch diverges from another, and it is here that generic, family and other divisions should be made.
The living species are those which have succeeded in the struggle for existence. The greater the extinction of allied forms the more widely do the survivors become separated from each other. Hence as the phylogenetic tree grows in the course of geological ages it tends to give off ever spreading branches. Rarely, however, is the geological record well enough known to enable us to demonstrate the exact point of origin of a group, and most classifications are but approximations to a correct phylogeny.
Degeneration.—Mere resemblance is not always a sure guide to affinity. Forms which differ widely in the adult state often resemble each other much more closely in young or embryonic stages. By observing their development, affinities can sometimes be discovered which would scarcely be otherwise suspected. For instance, the sea-squirts (Tunicata, q.v.) are essentially sedentary animals which have become highly specialized and in some respects degenerate, owing to their peculiar mode of life. The adult lives fixed to the sea-bottom, and is of simple sac-like structure, little resembling an ordinary vertebrate; yet the free-swimming larva has the dorsal central nervous system, gill-slits and notochord characteristic of the vertebrates. The barnacles (Cirripedia) have also taken to an adult fixed life and lost many of the features of the Crustacea (q.v.) to which their larval stages show they belong.
Other striking instances of specialization accompanied by degeneration commonly occur among parasites. Since they often absorb their food directly from their hosts, they tend to lose organs of locomotion, special sense organs and even the alimentary canal, and devote themselves to producing enormous numbers of eggs and young to infest new hosts. Frequently they become simplified beyond recognition, and their true affinities are betrayed only in their development.
Degeneration, or the loss of special structures no longer required in the new environment to which an organism may become adapted, is a frequent phenomenon. One of the great merits of the doctrine of evolution by natural selection is that it accounts for this simplification. For both progressive and retrogressive mutations occur; which will be selected depends on the needs of the organism at the time.
Vestigial Organs.—One of the results of change of life-habit or environment is to leave behind, in the course of evolution, parts and organs, once perhaps of vital consequence, but now of little or no use. Numberless such vestigial organs are known amongst plants and animals, and their presence has been considered as affording the strongest evidence for evolution. Unless they become adapted to fulfil some new function, they are apt to diminish and finally disappear, particularly if their presence is in any way harm ful. Such vestiges are the splints representing the last trace of the side-digits on the foot of the horse, the small bones embedded in the body-wall of whales and representing the vanishing hind-limb, the vermiform appendix of the intestinal coecum and the hidden little bony tail of man. Flightless birds, like the kiwi (Apteryx) of New Zealand, preserve vestigial wings concealed below the feathers.
In the course of evolutionary progress, however, through varia tion and selection organs are usually transformed and converted to new uses. It is doubtful whether any really useless parts are ever preserved for long unless they are insignificant, and many of the so-called vestigial organs are now known to fulfil important functions. The small pineal organ in our brain is, in a sense, the degenerate remnant of a dorsal eye still fairly well developed in certain living lizards and the lamprey, and there is evidence that it was possessed by most of the primitive fossil fish and land vertebrates; but in ourselves and other mammals, though it has lost its primitive function, it secretes essential substances into the circulation.
Homology, Analogy and Convergence.—Organs converted to new uses become modified and adapted in different ways, along divergent lines of evolution. However different they may become, if they can be traced back to the same origin in a common ances tor they are called homologous. The tracing of homologies is one of the means of determining affinities. For instance, the creeping foot of a mollusc, as seen in the snail, may become an expanded flapping swimming organ in marine pteropods, a jumping organ in the cockle or be converted into prehensile arms in the squid. Our own five-fingered forelimb is homologous with the forelimb of a horse, the swimming paddle of a whale, the wing of a bat or of a bird. In spite of their modifications homologous organs gen erally show the same essential parts, the same fundamental plan of structure. Intermediate steps between the extremes of spe cialization may often be found, especially among fossil forms. However much the resemblances may be obscured or lost in the adult, they are generally clearer in the earlier stage of develop ment, in which the common origin of the most diverse organs may be manifest.
But all similar parts are not necessarily homologous, and just as adaptation to different uses leads to divergence, so adaptation to the same function may lead to resemblance. Organs of quite different origin may become so alike by convergence as to deceive the practised eye of the expert. Such specialized parts which cannot be traced back to a common ancestor are called analogous. As examples of analogy may be mentioned the wing of a bird and that of a butterfly, our own jaws and those of an insect. Organs may be both analogous and homologous, for example, the wings of the bat, the bird and reptilian pterodactyl are homologous in so far as they are all forelimbs, but analogous in so far as they are organs of flight independently derived from a walking limb. All such cases are powerful evidence in favour of evolution ; indeed it is difficult to describe them without assuming their evolutionary origin.
What has been said above of organs is equally true of organ isms as a whole. While evolution as a rule leads to divergence, it may by adaptation to similar environments and modes of life, lead, on the contrary, to convergence in colour, shape, structure and behaviour. Unrelated forms may thus become deceptively alike. The amphibian Siphonops, the lizard Amp/iisbaena, the snake Typhlops, all creeping and burrowing forms, have lost their limbs and come to resemble an earthworm. Striking cases of convergence are known among plants ; for instance, the inde pendent acquisition of a cactus-like structure by representatives of the Euphorbiaceae, Asclepiadaceae and Compositae in adapta tion to dry desert-like conditions.
Recapitulation Theory.—The study of embryology affords not only valuable help in the tracing out of obscure affinities, but also strong evidence in favour of the doctrine of evolution. As pointed out long ago by K. E. von Baer, the embryos of forms belonging to different groups are generally more alike than are the adults. Among vertebrates, for instance, the embryo reptile, bird and mammal possess gill arches like those of a fish at a corresponding stage of development. From such evidence it was rashly concluded that in the course of its development an organ ism passes through stages approximately representing the series of ancestors which preceded it, and E. Hackel enunciated his famous "Biogenetic Law" that ontogeny recapitulates phylogeny. This was a gross exaggeration, and all that can be maintained is that the ontogeny of an individual may more or less recapitulate the ontogeny of its ancestor. Owing to new conditions and adap tations the course of ontogeny may come to deviate from that of the ancestor at any period from egg to adult ; stages may be omitted, larval specializations may be intercalated, new structures developed. The fish, the reptile, the mammal, do not really start from the same point and pass through the same stages ; they are fish, reptile and mammal respectively from their beginning. Their ontogenies are similar only in so far as their zygotes contain the same factors and develop under the same conditions.
Apart from differences of environment, the basis of the evolu tionary changes seen in a phyletic series lies in the alteration of the factors contained in the continuous stream of germ-plasm, not in the adult organisms derived from it. And here it may be mentioned that this progressive or retrogressive change involves not only the gross structure, but also the intimate chemical compo sition of the germ-plasm, of the very molecules of which it is made up. There has been a phylogeny of the genes and of their ingredients, as there has been a phylogeny of the organisms themselves. Consequently protoplasm and its products come to differ along diverging phyletic lines. For instance the red colouring matter of the blood (haemoglobin) differs slightly in different vertebrates ; so also starch in various plants and many of the proteins in all organisms. The degree of resemblance may afford some measure of affinity.
Help in tracing relationships has recently been derived from the study of the physicochemical reactions of the blood and other body-fluids of vertebrates. The "precipitin test" applied to the blood affords evidence, for instance, that man is more nearly allied to the anthropoid apes than to the lower monkeys, and to the latter than to the other Mammalia. Some of the phe nomena of interspecific sterility mentioned above are probably related to such divergences in chemical composition.
So we often find scattered over the globe isolated and special ized remnants of a once extensive group betraying their origin by their archaic structure. For example, the arthropod Peripatus and a few closely allied genera, in central and south Africa, central and south America, the Malayan and Australian regions; or the lung-fishes (Dipnoi), of world-wide distribution in the fresh waters of Palaeozoic times, now surviving only as Ceratodus in Australia, Protopterus in central Africa and Lepidosiren in south America.
Most remarkable are the number of abortive branches of the phylogenetic tree. Over and over again large and flourishing groups are seen to dwindle quickly to insignificance, or die out altogether. These results of selective elimination are obvious, but the causes which bring it about are by no means so clear. Cli matic changes, catastrophes, epidemic diseases may account for a great deal, but overspecialization seems to be the most usual contributory cause. Natural selection can provide only for the needs of the moment; it cannot be prophetic. Once committed to some narrow path of specialization, it may be difficult, if not impossible, for an organism to turn aside. Once an organ has been lost, it can hardly be regained, though it may sometimes be replaced by an analogous structure.
Other mammalian orders have had a very similar history, and almost always increase in size has been a conspicuous feature of their evolutionary transformation. This has also occurred among plants, and many groups of invertebrates, such as ammonites and arachnids. The large stegocephalian Amphibia, the gigantic dino saurs among Reptilia, and Titanotheria, Dinocerata and Probos cides among Mammalia are examples of the same development. This great increase in size doubtless secured temporary success and world-wide expansion; but usually the success has been short lived and was quickly followed by decline and extinction. From these and similar phenomena it has sometimes been concluded that "trends" may be discerned in evolution leading organisms along some definite line of specialization. If by this is meant that behind ordinary evolutionary processes there is some mysterious force, something other than natural selection of variations, guid ing organisms inevitably along some preordained path and guiding them often to destruction, such a view cannot be accepted as a scientific explanation. The dangers of specialization have already been insisted upon; increase in size is a form of specialization, often clearly advantageous in overcoming enemies and other destructive influences.
One of the most efficient ways of responding advantageously to various needs is by correspondingly modifying "behaviour." Behaviour is response expressed in movements; it is a special kind of accommodation and has developed in association with the power of storing up the impressions of past responses ; it enables the organism to "learn" and benefit by "experience." These new powers "emerge" with the elaboration of the sense-organs and of the nervous system. In the long run it is adaptability, and more especially "brain power" which wins in the struggle.
Natural science is not directly concerned with metaphysical explanations or philosophical systems ; it does not seek to explain the ultimate cause or purpose of existence, the ultimate nature of things or of knowledge. Nevertheless, when dealing with the scientific description of evolutionary processes, we should en deavour to realize what are the limitations of scientific method, and what is the scope of natural science within which its conclu sions may be accepted.
What are commonly distinguished as mind and body are two one-sided incomplete abstractions, made in our own conscious minds and subject to its limitations. These two abstractions represent perhaps two aspects of some whole, more fundamental, reality. Like the two sides of a plane surface, they cannot both be seen at the same time. The mental and physical series of events are not two independent parallel series ; so far as we know, one cannot exist without the other; they are indissolubly con nected. Neither can one be said to be produced by the other, or be described in the same terms. The mental series cannot be held to interfere with, guide or break into the interconnected chain of physico-chemical events. The two series may perhaps be dis tinguished as respectively subjective and objective,—the one is felt, the other observed. The question, "What has been the influence of the mind in evolution?" has no scientific meaning, and should rather be asked in some such form as this : "What part has been played in evolution by that complex system of sense organs, nervous system, etc., to which correspond higher mental processes such as those we know in ourselves?" With such a ques tion natural science can legitimately deal, even though a com plete answer cannot yet be given, owing to our very incomplete knowledge of the metabolic processes involved.
The adoption of this position as, at all events, the only prac tical scientific working hypothesis, entails the conclusion that to every metabolic process in a living organism there corresponds some sort of mental process, that every organism may be said to have some sort of mind be it ever so "low" or rudimentary. And this conclusion is amply justified by the observation of behaviour, whether "instinctive" or "intelligent." Since the two series are indissolubly correlated, the "laws" of variation and inheritance hold good in mental as in material evo lution. This explains how man, by breeding and selecting the requisite mechanism, has altered the behaviour and with it the mental equipment of races of domestic animals.
Another important element besides simple tropisms in the be haviour of organisms is known as "differential sensibility," or the reaction to sudden and marked changes in the strength of a stimu lus. Leaves may move or flowers open in strong light; a shadow passing across may cause a worm to contract, a fly to escape.
The usual behaviour of lower organisms ; their movements when seeking food ; their habit of gathering together in dark, warm, or damp places ; their sexual instincts ; their instincts connected with reproduction, such as the laying of eggs on appropriate substances for the young to feed on, and so forth, can be interpreted as com posed of such simple responses. Instinctive behaviour is a chain of interlocked simple responses each one of which acts as a stimu lus to the next. An environmental stimulus releases the first and the others inevitably follow to a definite and predictable end, provided the necessary conditions are present.
Tropistic response is, of course, based on the fundamental irritability of protoplasm. It varies in extent and direction, and, like other variations, may be useful, harmful or indifferent. Use ful instinctive behaviour may be built up by the natural selection of tropisms of "survival value" into the wonderful instincts dis played, for instance, among social insects (q.v.). Such complex chains of reactions will be inherited, for they depend for their appearance on the interaction of certain transmitted factors of inheritance with the conditions present in the normal environ ment. Among animals, simple responses or tropisms become "reflex actions" owing to the great differentiation of the tissues, and elaboration of the mechanism consisting of sense-organs to receive special stimuli and a nervous system to convey impulses set up by them to muscles which carry out the movements. Se quences of interlocking and co-ordinated reflexes form the basis of the most complicated behaviour; yet another element, "asso ciative memory," enters more especially into its more elaborately differentiated forms. It depends on the fact that the effects of a response may persist as a lasting impression on the organism, in the form of some physico-chemical change which serves as an internal stimulus to modify subsequent responses. The cumulative effects of past responses may thus greatly influence behaviour.
No hard-and-fast line can be drawn between instinctive and intelligent behaviour, and they are usually combined ; the rudi ments of both seem to occur in the lowest organisms, but it is in the higher animals and more especially in man that intelligence has reached its highest development.
As the sense-organs and nervous system advance in complexity and perfection, new qualities emerge marking new levels in mental development; consciousness is one of these. We cannot tell at which stage in phylogeny, consciousness arose. In the case of our own species we can determine approximately at what stage con sciousness begins to manifest itself, as the tissues become dif ferentiated in ontogeny ; but, with regard to its evolution, it can only be said that it probably appeared, as we know it in ourselves, when the cerebral hemispheres reached the high state of develop ment seen in man, though the lower organisms may possess it in some simpler stage of development. Very striking in a survey of the evolution of the vertebrates, is the steady increase in the relative size and complexity of the brain, the centre of co-ordina tion and storage-house of past impressions, as we pass from fish to amphibian, reptile and mammal. This is particularly conspicu ous in the Primate series leading up to man. Most of the structural peculiarities of the human body are related to the great develop ment of the brain, in other respects it differs but little from that of our nearest relatives, the anthropoid apes. Man's great capacity for retaining the impressions of past responses, and for bringing them to bear on responses to new stimulations, has led to the wonderful growth of his powers of adaptability to new and vary ing conditions. The acquisition of stereoscopic vision and th, power of fashioning and using implements were important steps in the evolution of modern man. Brain power, not brute force, has secured him his mastery of the world. It is a grave mistake to represent this success in evolution as due merely to individual strength, skill and foresight. The triumph of the human race over the lower organisms, and again of the higher races over the lower, has been brought about through mutual help, co-operation, self sacrifice and the subordination of the individual.
Patriotism and religion, art, science and literature, have all their survival value and play a useful part in evolution. Morality ap pears not as an unrelated external force working against a ruth less and unmoral cosmic process, but as a product of evolution and an important influence moulding its development. We may hope and believe that in the long run those civilizations which are founded on justice and liberty, on law and order, on sound moral ity, will succeed best and last longest.