Part 21

EV'OLUTE. The evolute of a curve is the curve which is the envelope of all its normals or the locus of all its centres of curvature. The first curve is called the involute of the second. These names are given to the curves because the end of a stretched thread unwound from the evolute will describe the involute.

EVOLUTION, a term introduced into biological writings in the early part of the eighteenth century to denote the mode of generation of living things. At first it was used in the same sense as we now apply the word development, more especially with reference to the process whereby the germ of an animal or plant becomes transformed into the adult organism; but it is now used in biology for the process by which more complex plants or animals have been derived from a series of less specialized ancestors by transformation. In accordance with the teaching of modern biology, all living creatures are the progeny of one original group of microscopic unicellular organisms, different branches of which during many millions of years have become diversely modified in structure and function to form the vast multitudes of diverse species of plants and animals with which we are acquainted.

The idea of a transformation of one type of being into another is extremely ancient, and its origin was in all probability genetically related to the primitive conceptions which have survived to the present day in totemism (q.v.) and such myths as the story of the were wolf. For once it was believed that a totem-animal, like a cow or a pig, could give birth to human beings, or that the Great Mother could at will assume a great variety of living forms, ranging from a mollusc or a grain of barley to a higher mammal, it was a comparatively simple step to arrange these potential 'ancestors' in a series, and provide mankind with a mythical genealogy. It is possible that such beliefs may have suggested to the Greek philosophers, such as Aristotle, speculations as to the process by which custom and change of habit might modify the structure of animals. But it was not until the end of the seventeenth century that the trend of philosophical speculation, associated with the growing understanding of natural processes, started lines of investigation which, after many failures, eventually brought forth Charles Darwin's _Origin of Species_ in 1859, and established once for all the fact that different species of animals and plants have been produced by the differentiation of the progeny of the same ultimate ancestors. The history of these events was admirably summarized by Huxley in 1878 in an article _Evolution in Biology_, republished in _Darwiniana_ (1893, p. 187); in this account due credit is given to the pioneers of the eighteenth and early nineteenth centuries, such as Lamarck, and to Darwin's co-discoverer of the hypothesis of natural selection, Alfred Russell Wallace.

At the present time there is a general concensus of opinion among serious biologists, and, in fact, most educated men, as to the reality of evolution; but there is wide divergence of opinion as to the exact pedigrees of the various groups of plants and animals, and especially as to the mechanisms whereby the processes of transformation have been effected.

The evidence that establishes the proof of evolution is of manifold kinds. "The gradations of structure, from extreme simplicity to very great complexity, presented by living things, and of the relation of these graduated forms to one another. The existence of an analogy between the series of gradations presented by the species which compose any great group of animals or plants, and the species of embryonic conditions of the highest members of that group. Large groups of species of widely different habits present the same fundamental plan of structure; and parts of the same animal or plant, the functions of which are very different, likewise exhibit modifications of a common plan. Structures are found in a rudimentary or apparently useless condition in one species of a group which are fully developed and have definite functions in other species of the same group. These considerations, when studied in conjunction with the facts of the geological succession of the forms of life, of geographical distribution, and the effects of varying conditions upon living organisms, establish the truth of evolution" (Huxley).

The full meaning of these statements will be better understood if a concrete example is studied, and perhaps the case of man and his ancestry is most instructive for this purpose. The fact that man has a vertebral column, a brain and nervous system, a heart and blood-vessels, digestive and other systems of organs, built up in accordance with the arbitrary plan which is shared also by all mammals, birds, reptiles, amphibians, and fishes, proclaims that man belongs to the vertebrate group of animals, and that all such vertebrata must originally have sprung from the same common ancestors. The possession of four limbs with five fingers or toes on each, and a host of identical arrangements of bones, muscles, nerves, &c., in these limbs, reveals the fact that all the four-limbed creatures or Tetrapoda represent one group which developed from some fish-like ancestor to become an amphibian. The discovery of fossilized remains of extinct animals reveals that the fishes are much older than the amphibians, and that a number of intermediate stages demonstrate the process of gradual transformation which converted one group of fishes into four-limbed, semi-terrestrial amphibians. Geological evidence also proves that the reptiles came definitely later than the amphibians, and that only one small group of very primitive amphibians shared in the progressive modifications of brain, limbs, and organs of circulation, &c., to become reptiles--creatures able to live wholly on the dry land, and capable of a wider range of activities than the Amphibia. From the primitive reptiles were derived not only the highly specialized forms that have survived to the present day as lizards, tortoises, snakes, &c., creatures that differ profoundly from their earliest reptilian ancestors, but also the ancestors of birds and the ancestors of mammals. One particular group of primitive reptiles is known, from fossilized remains found in South Africa, that reveals many of the distinctive peculiarities of mammals not shared by other reptiles; and it is now certain that these cynodonts--so called from their dog-like teeth--include the parents of the mammals. The fact that all the Mammalia are provided with glands which in the female supply milk for the nutrition of their young, that they have a hairy coat, that they have a highly developed brain more fully adapted for learning by experience than is the case in other vertebrates, that they have limbs capable of a much more varied and active range of skilled movements, and a host of identical transformations of viscera, muscles, nerves, and vessels prove the common ancestry of mammals from some very primitive cynodont reptile. Different mammals have been specialized in structure for amazingly varied modes of life, on land, under the ground, in trees, in the air, or in rivers or the sea. Of the terrestrial animals some have been modified for fleetness, like the horse and the antelope; others for strength, like the elephant; others again, like the lion and the tiger, to prey upon their weaker relatives. At the dawn of the age of mammals one particular group was able to survive without any of the profound alterations of the structure of the limbs which such creatures as the horse and the ox, the elephant and the whale, the tiger and the bat had to adopt to avoid extinction, and retained the primitive type of limbs with their fingers and toes which became the most useful and plastic instruments for performing skilled movements and acquiring experience and knowledge as soon as the brain was sufficiently advanced in structure and capability to put these instruments to their full use. The group of mammals which delayed the time of specialization until it was able to profit by its greater adaptability was the Prosimiae, the ancestors of the apes and man. These small creatures for a long time lived a life of obscurity in trees without submitting to those extreme adaptations of structure which are found in most arboreal and flying mammals. But the cultivation of their powers of vision, and the acquisition of skill in the use of their primitive but plastic hands, guided by vision, eventually conferred upon some of these Prosimiae vastly enhanced powers of skilled action and of learning by experience and of acquiring knowledge, which culminated in the attainment of the supreme power of discrimination distinctive of human intelligence.

The fact that man belongs to the same order (Primates) as the apes is proved not merely by the possession of a body which in most respects is identical in structure with such of them as the gorilla and the chimpanzee, of a similar process of development characterized by identical stages up to a certain stage, but also by the fact that the blood of man and the apes react towards one another as those of relatives, and in a way not shared by the blood-reactions of other mammals. The apes, also, are subject to certain human diseases from which other mammals are immune. Man shares with the anthropoid apes (gorillas, chimpanzees, orangs, and gibbons) so many peculiarities which differentiate all of them from the tailed monkeys that there can be no doubt that the human family was derived from a primitive anthropoid ape, possibly a species that lived in the foothills of the Himalayas in Miocene times, as is suggested by fossils recently discovered by Dr. Pilgrim, director of the Geological Survey of India.

The vestigial remains of the muscles, blood-vessels, &c., of the tail reveal the fact that man's Primate ancestry began with a tailed form. In fact, the human embryo actually possesses a tail for some weeks of its existence. If India reveals the fossilized remains of a variety of Miocene anthropoid apes closely akin to the ancestors of man, the gorilla, the chimpanzee, and the orang, the Egyptian Fayum has provided the evidence of the origin of the anthropoid apes in Oligocene times as very diminutive creatures distantly akin to the gibbons, but bearing very obvious indications in the form of their teeth of an affinity with the Prosimian sub-order Tarsioidea, a very interesting group of Eocene Primates found in a fossilized condition in North America and France, one of the members of which has survived in the peculiar Spectral Tarsier still found living in the forests of Borneo, Java, and the Philippines. The detailed study of the structure and development of Tarsius, and comparisons with other Primates and mammals of other orders, provide the information necessary to fill in the gaps left in the geological record, and enable us to sketch out the general scheme of man's ancestry, and to appreciate the nature of the factors which determined the evolution of such an intelligent mammal as man.

Within recent years the increasing knowledge of embryology and comparative anatomy, and the recovery of fossilized remains of vast numbers of hitherto unknown animals, has established the truth of evolution and the exact line of ancestry of many animals. Professor Osborn's work on the evolution of the horse, and Dr. C. W. Andrews's revelation of the ancestry of the elephant, are striking recent illustrations of the exactness of the demonstration palaeontology can give of the past history of mammals. Dr. Robert Brown and Professor D. M. S. Watson have given conclusive proofs of the origin of mammals and birds from primitive reptiles, and the latter zoologist has pushed back the ancestry of these higher vertebrates still further, and shown how the reptiles were derived from primitive Amphibia, and the changes that occurred in vertebrate anatomy when certain fishes crawled out of the water and developed into four-footed Amphibia. All of these conclusions are matters of fact and not of theory, even if we are still in the dark as to the exact mechanism whereby the variations which the forces of evolution use in effecting transformations were themselves brought about. Within recent years there has been a revival of interest in the problem whether characters acquired by parents as the result of their individual experience can be transmitted to their offspring. For the last thirty years biologists have been influenced by the teaching of Weismann that nothing happening to the parents can affect the morphological capabilities of the germ plasm from which the next generation is derived; but recent research suggests that this negative doctrine is too rigid, and makes it probable that certain influences brought to bear upon the parents may be transmitted also to the offspring. If this is so, it opens one avenue of explanation of how structural modifications are effected and the possibility of evolution is created. But at the present moment the whole question is being actively investigated and discussed.

Just as every complex animal can be shown to be derived during development from a simple microscopic cell or egg, so the study of evolution reveals the fact that all animals were originally derived from microscopic unicellular animals known as protozoa, which are with difficulty distinguishable from unicellular plants, from which all the varied forms of complex vegetable life were derived. It is equally certain that these unicellular plants and animals are themselves only the specialized descendants of common ancestors--unspecialized unicellular organisms which are neither strictly vegetable nor animal. But we are quite in the dark as to the processes whereby these most primitive living organisms were evolved from inorganic matter, and how they acquired these peculiar properties of growth and differentiation and their powers of reproduction, commonly called vital, which are their distinctive characteristics. Bibliographical references to most of the matters mentioned in this article will be found in the Presidential Address to Section H of the British Association for the Advancement of Science, Dundee meeting (see Report of British Association, 1912).

EV'ORA, a town in Portugal, capital of the province of Alemtejo, 75 miles east of Lisbon. It is an ancient place, poorly built, and its walls, citadel, and forts are all in a state of ruin. It has a Roman aqueduct still serviceable, a Gothic cathedral, and an ecclesiastical seminary. Pop. 17,900. The district of Evora has a pop. of 144,300.

EVREMOND, or EVREMONT. See _St. Evremond_.

EVREUX (ev-r_eu_), a town of N.W. France, capital of the department of Eure, in a fertile valley on the Iton. It is an ancient town with narrow streets and has many fine buildings, including an ancient Gothic cathedral. The town was frequently occupied by the English in the fifteenth century. Pop. 18,950.

EWALD ([=a]'v[.a]lt), Georg Heinrich August von, a German Orientalist and Biblical critic, born at Goettingen 16th Nov., 1803, died there 5th May, 1875. After studying at the university of his native town, in 1827 he became extraordinary, in 1831 ordinary professor of theology, and in 1835 professor of Oriental languages. In 1837 he lost his chair at Goettingen on account of his protest against the king's abrogation of the liberal constitution, became professor of theology at Tuebingen, but in 1848 returned to his old chair at Goettingen. When Hanover was annexed by Prussia in 1866 he became a zealous defender of the rights of the ex-king. Among his chief works are the following: _Complete Course of the Hebrew Language_, _The Poetical Books of the Old Testament_, _History of the People of Israel_, _Antiquities of the People of Israel_. The _History_ is considered his greatest work.

EWALD ([=a]'v[.a]lt), Johannes, Danish poet, born at Copenhagen in 1743, died in 1781. After studying theology at Copenhagen University he ran away and enlisted in the Prussian service, which he soon deserted for the Austrian. On his return to Copenhagen an elegy which he wrote on the death of Frederick V of Denmark was received with general admiration, and awoke in himself the consciousness of poetic talent. His reputation rapidly increased with the publication of his tragedies, _The Death of Balder_ (English translation by George Borrow), _Adam and Eve_, and _Rolf Krage_; and his odes and songs, notable amongst which are: _King Christian_ and _Liden Gunver_. Ewald, who had dissipated habits, died in utter poverty. His collected works were published in 1914.

EWART, James Cossar, zoologist, was born at Penicuik, Midlothian, in 1851, studied medicine at Edinburgh University, graduated in 1874, and was soon after appointed demonstrator of anatomy in the university. From 1875 to 1878 he was Conservator of the museums of University College, London, in the latter year took his M.D. degree, and from 1878 to 1882 was professor of natural history in Aberdeen University, being then appointed to the natural history chair at Edinburgh. Since then (having been also connected with the Scottish Fishery Board for about ten years) he has devoted much attention to the question of fish-culture and preservation, and has visited North America, Denmark, and Norway for purposes of investigation. He has also carried out experiments in the hybridization of zebras and horses. His publications include: _The Locomotor System of the Echinoderms_ (with G. J. Romanes, 1881); _The Natural and Artificial Fertilization of Herring Ova_ (1884); _On Whitebait_ (1886); _On the Preservation of Fish_ (1887); _The Electric Organ of the Skate_ (1888-9); _The Cranial Nerves and Lateral Sense-organs of the Elasmobranchs_ (1889-91); _The Development of the Limbs of the Horse_ (1894); _The Penicuik Experiments_ (1899); _Guide to Zebras, Hybrids, &c._ (1900); _Multiple Origin of Horses and Ponies_ (1904).

EXALBUMINOUS SEEDS, those which, when ripe, contain no endosperm, this having been entirely absorbed into itself by the developing embryo. Opposed to albuminous seeds. See _Cotyledons_.

EXAMINER OF PLAYS, a British official and censor of plays, who acts for the Lord Chamberlain, under whose jurisdiction the theatres are placed. No play can be produced without the sanction of the examiner, to whom a copy of every new play intended for production must be sent seven clear days before the first performance. The examiner either grants or refuses his licence, and frequently insists upon an alteration of the text. The abolition of this censorship of plays is a subject which in recent years has given rise to much discussion in the theatre-loving world.

EXANTHE'MATA (eruption of the skin), a term applied to infectious diseases with skin eruptions, accompanied by general disturbances. The term includes scarlet fever, measles, German measles, smallpox, chicken-pox, and others.

EXARCHATE (egz-aer'k[=a]t), a name of a province or territory under an _exarch_, or viceroy. In the sixth century after Christ Justinian formed the middle part of Italy into a province of the Eastern Empire, and gave the government of it to an _exarch_. (See _Ravenna_.) Exarch was also the title of an ecclesiastical grade in the Greek Church, inferior to the patriarchs but superior to the metropolitans. Among the modern Greeks an _exarch_ is a deputy of the patriarch, who travels about in the provinces and visits the bishops and churches.

EXCAM'BION, in Scots law, the name given to the contract by which one piece of land is exchanged for another.

[Illustration: Excavating the Culebra Cut, Panama Canal]

EXCAVATION, the process of removing soil or rock in engineering or exploration works, such as for docks, retaining-walls, railway cuttings, canals, foundations, &c. On a small scale, or in situations unsuitable for machinery, it is performed by hand, the soil being first loosened by the pick, and then shovelled into barrows. In rocky soils, drilling and explosives may be employed. In large works power shovels or 'steam-navvies' are employed, which are essentially cranes carrying a large shovel, or a system of steel buckets of the dredger type. These carry their own means of propulsion, and run on temporary rails laid down as the work proceeds. They work against the face of the excavation, and load directly into bogies or wagons.

EXCAVATIONS. The forgotten history of the remote past has been reconstructed by those scientists who have explored the sites of ancient seats of civilization. Egyptian and Babylonio-Assyrian investigations date from the middle of the eighteenth century, but it was not until the middle of the nineteenth century, when modern scholars first penetrated the secrets of the lost languages (see _Cuneiform Writing_ and _Hieroglyphics_) that the ancient civilizations were rendered more or less articulate. As the texts were being deciphered with increasing ease and accuracy, a basis was provided for archaeology, and it became possible to frame chronological systems. During the latter years of the nineteenth and the early years of the present century, excavators, philologists, and ethnologists have provided a fairly continuous and detailed history of man from 3000 B.C. till classical times, thus bridging a gulf which used to be misty with doubtful legends and traditions. The most dramatic excavations were those begun by Heinrich Schliemann in 1871 at Hissarlik, the site of ancient Troy. On the hillock he dug through nine successive settlements. He afterwards excavated at Mycenae and Tiryns in Greece and found evidence of a high pre-Hellenic culture. Following up the clues thus afforded, Sir Arthur Evans and others, excavating in Crete during the opening years of the present century, discovered abundant relics, including palaces and towns, of the earliest Aegean civilization now referred to as Minoan (see _Crete_). Of late years Central and Western Europe have yielded evidence of the 'drift' of Minoan culture to outlying parts. Excavations in Russian Turkestan and Chinese Turkestan have revealed traces of ancient culture centres dating back beyond 2000 B.C. In the Americas the excavators have thrown considerable light on the pre-Columbian civilizations of Peru, Central America (Maya), and Mexico, but the picture-writings have not been deciphered. See _Babylonia_; _Crete_; _Egypt_; _Troy_.

EX'CELLENCY, a title given to ambassadors and plenipotentiaries, governors of colonies, the President of the United States, of France, &c.