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EVOLUTION, the term now generally applied to the doctrine that the existing universe has been gradually unfolded by the action of natural causes in the immeasurable course of past time. The question how the present order of things originated seems natural to the human mind, and has been put by all the races of men. The answer given in their cosmogonies, that it was created as we now see it by supernatural power, has been generally accepted as a matter of religious faith. The early Greek philosophers first brought the question into the field of speculation, and taught that all natural things have sprung from certain primal elements, such as air, water, or fire. As regards the origin of life, Anaximander is said to have held that animals were begotten from earth by means of moisture and heat, and that man did not originate in a perfectly developed state, but was engendered from beings of a different form. Empedocles taught that the various parts of animals, arms, feet, eyes, &c., existed separately at first; that they combined gradually, and that these combinations, capable of subsisting, survived and propagated themselves. Anaxagoras believed that plants and animals owe their origin to the fecundation of the earth whence they sprung by germs contained in the air. Aristotle, the father of natural history, entertained much more rational views upon the subject, and it is maintained that he held opinions as to the causes of diversity in living beings similar to those that are entertained by the latest zoölogists. It has been asserted that some of the early theologians, including St. Augustine and St. Thomas Aquinas, announced doctrines that harmonize apparently with the modern views of evolution. We however find no development of the ideas thus shadowed forth. Linnæus and Buffon seem to have been the first among modern naturalists who formed definite conceptions of a progressive organic development, but they did little to elucidate the idea. Immanuel Kant announced in 1755 his theory of the mechanical origin of the universe, and supposed that the different classes of organisms are related to each other through generation from a common original germ. Dr. Erasmus Darwin, grandfather of Charles Darwin, in his Zoonomia (1794), maintained the natural genesis of organic beings. But the first to frame a distinct hypothesis of development was Lamarck, who published his Philosophie zoologique in 1809, and developed his views still further in 1815 in his Histoire naturale des animaux sans vertèbres. He held that all organic forms, from the lowest to the highest, have been developed progressively from living microscopic particles. Similar conclusions were arrived at by Goethe in Germany, and by Geoffroy Saint-Hilaire in France in his work Sur le principe de l'unité de composition organique, published in 1828. The views thus far were of a general and highly speculative nature, and without firm scientific ground-work. It was only when the question was narrowed down to that of the mutability or immutability of species, and to the causes and extent of variation as determined by observation and experiment, that the real difficulties of the case were grappled with, and the inquiry assumed a strictly scientific character. In 1813 Dr. W. C. Wells read a paper before the London royal society, in which he recognized distinctly the principle of natural selection as applied to certain races of mankind, In 1822 the Rev. William Herbert, afterward dean of Manchester, declared his conviction that “botanical species are only a higher and more permanent class of varieties;” and he extended this opinion to animals. Leopold von Buch, in his Physikalische Beschreibung der Canarischen Inseln (1825), expresses the opinion that varieties change gradually into permanent species, which are no longer capable of intercrossing. In 1826 Prof. Grant of Edinburgh published a paper on the spongilla in the “Philosophical Journal,” in which he held that species are descended from other species, and that they become improved in the course of modification. Karl Ernst von Baer, in his Ueber Entwickelungsgeschichte der Thiere (1828), maintains similar views as to animals. Oken, in his Naturphilosophie (1843), published his belief in the development of species; and in 1846 J. d'Omalius d'Halloy of Brussels expressed his opinion that probability favors this theory rather than that of separate creations Isidore GeoiFroy Saint-Hilaire, in his lectures published in 1850, gives reasons for his belief in the modification of species by circumstances, and in the transmission of differences thus produced. In 1852 Herbert Spencer argued that species have undergone modification through change of circumstances. M. Naudin in the same year published a paper on the origin of species in the Revue horticole, in which he averred his belief that botanical species are formed in a manner analogous to varieties under cultivation; and Franz Unger also in 1852, expressed similar opinions in his Versuch einer Geschichte der Pflanzenwelt. In 1853 Dr. Schaffhausen, in a paper published in the Verhandlungen des Naturhistorischer Vereins des preussischen Rheinlands, &c., maintained the doctrine of progressive development of organic forms. On July 1, 1858, two essays were read before the Linnæan society, one by Charles Robert Darwin, entitled “0n the Tendency of Species to form Varieties, and on the Perpetuation of Species and Varieties by means of Natural Selection;” the other by Alfred Russel Wallace, entitled “On the Tendency of Varieties to depart indefinitely from the Original Type.” These papers showed that these two naturalists had arrived at almost exactly the same general conclusions; but the priority may safely be assigned to Darwin, who, although he had not previously made public his views, had submitted a sketch of them as early as 1844 to Sir Charles Lyell, Dr. Hooker, and others. In 1859 he published the treatise entitled “On the Origin of Species by means of Natural Selection,” which was the means of diffusing so widely the theory, elaborated by him through years of patient and careful investigation, that it is commonly designated by his name. In this work he did not apply the doctrine of evolution to the human race, although he had long held the opinion that man must be included with other organic beings; and it was not until after Huxley, Spencer, Lyell, Lubbock, Gegenbaur, Vogt, Rolle, Haeckel, Canestrini, Francesco, and others, had accepted the extreme conclusion, that he published “The Descent of Man, and Selection in Relation to Sex” (1871). In 1872 Haeckel, who previously had discussed the genealogy of man in Naturliche Schöpfungsgeschichte (1868), published his Monographie der Kalkschwämme, in which he claims to give an analytical demonstration of the problem of the development of species.—The theory as now generally held is thus stated by Prof. Huxley: “Those who hold the theory of evolution (and I am one of them) conceive that there are grounds for believing that the world, with all that is in it, did not come into existence in the condition in which we now see it, nor in anything approaching that condition. On the contrary, they hold that the present conformation and composition of the earth's crust, the distribution of land and water, and the infinitely diversified forms of animals and plants which constitute its present population, are merely the final terms in an immense series of changes which have been brought about, in the course of immeasurable time, by the operation of causes more or less similar to those which are at work at the present day.” The idea expressed by the term development involves the same principle, but it is usually restricted to the evolution of organic beings. We will first consider the doctrine as applied to the development of the various forms of life, and then in its broader aspects as a theory of universal evolution.—It has been proved by geology that the earth and its life, instead of being called suddenly into existence a few thousand years ago, have existed for millions of years; and as the mountains and continents are known to have attained their present form by the action of natural agencies, it is thought probable that other objects of nature have been produced in a similar way. The earth has teemed with living beings through incalculable periods of time, and fossil remains of them are found distributed through the rocky layers that have been successively formed, until they are several miles in thickness. But not all kinds of animals and plants existed from the beginning, leaving their mingled remains in the lowest strata; the lowest types of life, vegetable and animal, appeared first. The successive phases of life are so definite that they have been held as marking off the earth's history into a series of ages. The invertebrates (radiates, mollusks, and articulates) are found in the Silurian or oldest stratified rocks; and from the predominance of the mollusks the period has been called the age of mollusks. Fishes, which are higher in the scale, begin to appear in the Silurian, but become so abundant in the later Devonian period that it is called the age of fishes. Amphibious animals, as an advance on the fishes, appear in the carboniferous age, which again is followed by the age of reptiles. To this succeeds the age of mammals, and lastly comes the age of man, the series, which began with the lowest forms of life, terminating with the highest. That the order has been progressive, and that its lower terms have been more general in character, while the later terms have been more specialized and perfect, is admitted by all naturalists. Prof. Owen says: “In regard to animal life and its assigned work on this planet, there has plainly been an ascent and a progress in the main;” and he has “never omitted a proper opportunity for impressing the results of observation showing the more generalized structure of extinct as compared with the more specialized forms of recent animals.” Prof. Agassiz holds that “the more ancient animals resemble the embryonic forms of existing species;” that is, are lower in the scale of development than the later forms. Mr. Wallace remarks: “As we go back into past time and meet with the fossil remains of more and more ancient races of extinct animals, we find that many of them are actually intermediate between distinct groups of existing animals;” the ancient fishes, for example, present unmistakable reptilian traits, while the early reptilians combined also the characters of birds which had not yet appeared. As regards the continuity of the course of life, Prof. Dana remarks: “Geological history is like human history in this respect; time is one in its course, and all progress one in plan. . . . The germ of the period was long working onward in preceding time, before it finally came to its full development and stood forth as a characteristic of a new era of progress. . . . The beginning of an age will be in the midst of a preceding age; and the marks of the future, coming out to view, are to be regarded as prophetic of that future. The age of mammals was foreshadowed by the appearance of mammals long before in the course of the reptilian age, and the age of reptiles was prophesied in the types that lived in the earlier carboniferous age.” The animal kingdom displays a unity of plan or a correlation of parts by which common principles are traced through the most disguising diversities of form, so that in aspect, structure, and functions the various tribes of animals pass into each other by slight and gradual transitions. The arm of a man, the fore limb of a quadruped, the wing of a bird, and the fin of a fish are homologous; that is, they contain the same essential parts modified in correspondence with the different circumstances of the animal; and so with the other organs. Prof. Cope says: “Every individual of every species of a given branch of the animal kingdom is composed of elements common to all, and the differences which are so radical in the higher grades are but the modifications of the same elemental parts.” There are many cases of rudimentary and useless organs in animals and plants. During the development of embryos organs often develop to certain points, and are then reabsorbed without performing any function, although generally the partially developed organs are retained through life. Certain snakes have rudimentary hind legs hidden beneath the skin; the paddle of the seal has toes that still bear external nails; some of the smooth-skinned amphibia have scales buried under the dermal surface; rudimental teeth have been traced even in birds; and there are rudimentary eyes in cave fishes and rudimentary mammæ in men. Classification is an arrangement of living beings by related characters. In the earliest attempts the organic tribes were arranged in a serial order or a chain from the bottom to the top of the scale; but this has been abandoned, as also have those symmetrical systems which assumed that the characters of different groups are equivalents of each other. The endeavor to thrust animals and plants into these artificial partitions is of the same nature as the endeavor to arrange them in a linear series; and it assumes a regularity which does not exist in nature. Classification now represents the animal kingdom as consisting of certain great sub-kingdoms very widely divergent, each made up of classes much less widely divergent, severally containing orders still less divergent, and so on with genera and species, like the branches of a growing tree; and the old method of classification, as Mr. Spencer remarks, involves exactly the difficulty “which would meet the endeavor to classify the branches of a tree as branches of the first, second, third, fourth, and fifth orders; the difficulty, namely, that branches of intermediate degrees of composition exist.” There is a remarkable analogy between the present distribution of animals in space over the earth and their past distribution in time as we trace their fossils in the successive geological formations. The larger groups, such as classes and orders, are generally spread over the whole earth, while smaller groups, such as families and genera, are commonly confined to limited districts; but when a group is restricted to one region, and is rich in the minor groups called species, it is almost invariably the case that the most closely allied species are found in the same locality or in closely adjoining localities. The same fact is seen in geological distribution. Mr. Wallace observes: “Most of the larger and some smaller groups extend through several geological periods. In each period, however, there are peculiar groups, found nowhere else, and extending through one or several formations. As generally in geography no species or genus occurs in two very distant localities without being also found in intermediate places, so in geology the life of a species or genus has not been interrupted. In other words, no group or species has come into existence twice.” From these facts Mr. Wallace deduces the following important law: “Every species has come into existence coincident both in space and time with a preëxisting closely allied species.” The adherents of development maintain that these facts, and many others of kindred significance, are only to be explained by the continuous operation of a great natural law of descent and divergence by which the present life of the earth has been derived from its preëxisting life. That the numberless forms of life should have been held as independently created, so long as the earth was regarded as having been recently and suddenly called into existence, was inevitable; but now, when it is known that the order of nature is extended backward into immeasurable time, the supposition that species were called into existence by hundreds of thousands of separate and special creations, running through the geological ages, and as we approach our own epoch suddenly and unaccountably ceasing, is held to be an unwarranted assumption which science can no longer accept. As remarked by the Rev. Baden Powell: “The introduction of a new species is part of a series. But a series indicates a principle of regularity and law, as much in organic as in inorganic changes. The event is part of a regularly ordained mechanism of the evolution of the existing world out of former conditions, and as much subject to regular laws as any changes now taking place. If the series be regular, its subordinate links must each be so; the part cannot be less subject to law than the whole. That species should be subject to exactly the same general laws of structure, growth, nutrition, and all other functions of organic life, and yet in the single instance of their mode of birth or origin should constitute exceptions to all physical law, is an incongruity so preposterous that no inductive mind can for a moment entertain it.” This is the ground taken by the great majority of contemporary naturalists. They believe in evolution in some form as a great fact of nature; but many think that we know nothing as to how it has been brought about, while others hold that the problem of the modes and causes of evolution, although obscure, is no more barred from successful investigation than are the other phenomena of nature.—The following facts have been offered as throwing light upon the way in which the diversities of life have originated. Organic beings differ from inorganic in their modifiability. They are capable in various degrees of adaptation to new conditions. Plants taken from their native situations and cultivated in gardens undergo changes so great as often to render them no longer recognizable as the same plants. The muscles are strengthened by exercise and the skin thickened and hardened by pressure, while the bones of men who put forth great physical exertion are more massive than the bones of those who do not labor. In the words of Mr. Spencer: “There is in living organisms a margin of functional oscillations on all sides of a mean state, and a consequent margin of structural variation.” These variations may become fixed through the law of hereditary descent. It is the law of transmission of characters which preserves species and varieties from generation to generation, oaks being always derived from oaks and dogs from ancestral dogs. It is not only the normal qualities that are perpetuated, but malformations, diseases, and individual peculiarities are also transmitted. While offspring tend to grow in the likeness of parents, they also tend to grow in unlikeness; while moulded upon the parental type, the resemblance is usually imperfect. Nor are variations confined to any particular organs or characters, but they may be manifested by every part, quality, or instinct of the creature. These divergences may be selected and fixed by breeding so as to give rise to new kinds or varieties. Nature begins the variation, art secures its perpetuation and increase. How profound are the modifications that may be thus produced is shown in the numerous breeds of dogs, all of which belong to the same species. Not only have they reached extreme diversities in size (the largest being, according to Cuvier, 100 times larger than the smallest), but in muscular, bony, and nervous development, in form, strength, fleetness, and variety of instinct and intelligence, their divergences are almost equally remarkable. Domestic pigeons afford another example of the great plasticity of the living organism, by which it can be moulded into the extremest diversities. Naturalists believe that from a single species, the wild rock pigeon, there have arisen no fewer than 150 kinds that breed true or hold to the variety; and how deep have become the differences among them is thus stated by Prof. Huxley: “In the first place, the back of the skull may differ a good deal, and the development of the bones of the face may vary a good deal; the beak varies a good deal; the shape of the lower jaw varies; the tongue varies very greatly, not only in correlation to the length and size of the beak, but it seems also to have a kind of independent variation of its own. Then the amount of naked skin round the eyes and at the base of the beak may vary enormously; so may the length of the eyelids, the shape of the nostrils, and the length of the neck. I have already noticed the habit of blowing out the gullet, so remarkable in the pouter, and comparatively so in the others. There are great differences, too, in the size of the female and the male, the shape of the body, the number and width of the processes of the ribs, the development of the ribs, and the size, shape, and development of the breast bone. We may notice, too (and I mention the fact because it has been disputed by what is assumed to be high authority), the variation in number of the sacral vertebræ. The number of these varies from 11 to 14, and that without any diminution in the number of the vertebræ of the back or of the tail. Then the number and position of the tail feathers may vary enormously, and so may the number of the primary and secondary feathers of the wings. Again, the length of the feet and of the beak, although they have no relation to each other, yet appear to go together; that is, you have a long beak wherever you have long feet. There are differences, also, in the periods of the acquirement of the perfect plumage, the size and shape of the eggs, the nature of flight, and the powers of flight, so-called ‘homing’ birds having enormous flying powers; while on the other hand, the little tumbler is so called because of its extraordinary faculty of turning head-over-heels in the air, instead of pursuing a distinct course. And lastly, the dispositions and voices of the birds may vary. Thus the case of the pigeons shows you that there is hardly a single particular, whether of instinct or habit, or bony structure, or of plumage, of either the internal economy or the external shape, in which some variation or change may not take place, which by selective breeding may become perpetuated and form the foundation of and give rise to a new race.” Nor is this variation confined to domestic animals. Wild species both of plants and animals vary, become diversified, and give rise to new varieties. As many as 28 varieties of oak have been made out within the limits of a single species. The wolf species exhibits some 15 varieties, and lions, tigers, bears, hyænas, foxes, birds, reptiles, and fishes all exhibit marked varieties, which show that wild species undergo modification in a state of nature. What was needed to make out the analogy of variation between wild and domesticated animals was to discover some process in nature which is the equivalent of human agency in breeding. Mr. Darwin believes that he has discovered this process, and calls it the principle of “natural selection.” He says that living beings in a state of nature are subject to certain external conditions, such as climate, situation, character of soil, and exposure to enemies, by which they are surrounded and limited. They are endowed with enormous powers of increase, so that any one of the hundreds of thousands of species of plants or animals, if all its progeny were preserved, would go on multiplying until it covered the earth or filled the sea. Space is fixed and food limited, and the consequence is a universal conflict, the war of races; and in the “struggle for existence” multitudes perish and comparatively few survive. This survival is not a matter of chance. Mr. Darwin maintains that it is regulated by law, and that those only survive which are in some way best adapted to the conditions of life. The strongest, the fleetest, the most cunning, and the best adapted to the conditions will live and multiply, while the less fit will disappear. The introduction of European plants and animals into New Zealand affords an instructive example of how races encroach on each other's areas, the weaker being extirpated by the stronger in the competition for existence. Dr. Hooker says: “The cow grass has taken possession of the roadsides; dock and water cress choke the rivers; the sow thistle is spread over all the country, growing luxuriantly up to 6,000 feet; white clover in the mountain districts displaces the native grasses; and the native (Maori) saying is: ‘As the white man's rat has driven away the native rat, as the European fly drives away our own, and the clover kills our fern, so will the Maoris disappear before the white man himself.’ ” Mr. Darwin in his works gives a great number of facts showing how apparently trifling variations give advantages to their possessors, which determine their survival and become perpetuated in the race. The principle of natural selection, or, as it is termed by Herbert Spencer, the “survival of the fittest,” is now generally recognized as a genuine agency or vera causa, and the opponents of development admit that it may give rise to varieties, although they deny that it is competent to produce the deeper diversities of species. The extent of its operation remains yet to be determined, but many naturalists agree with Prof. Helmholtz that Mr. Darwin has contributed to science an “essentially new creative idea.” Mr. Darwin, however, does not assume to be the discoverer of the principle of natural selection, and he points out that others before him have recognized the action of the process, though without seeing its full significance. What he claims is to have first shown the efficacy of the principle in producing divergency of types under the laws of variation and heredity. But having discovered a new factor in organic development, and published his work on the “Origin of Species” at the fortunate moment when naturalists had become widely dissatisfied with the old views, he became prominently identified with the development doctrine, and this has led many into the error of regarding Darwinism as the equivalent of evolution, of which, as we are now to see, it is but a minor part.—The advance of civilization in the historical period gave rise to the modern idea of progress, which was strengthened by the discoveries made early in the present century concerning the past course of terrestrial life. The process was crudely conceived, in the one case as the successive development of all living creatures in a graded and linear series, and in the other case as the continuous movement of humanity toward a state of final perfection. About the year 1850 Mr. Herbert Spencer entered upon the systematic study of the subject. The problem was strictly a scientific one, and he had a wide and accurate preparation for it by a mastery of scientific knowledge which Mr. Mill has pronounced “encyclopædic.” Mr. Spencer was also remarkable for his power of analysis, his grasp of wide-reaching principles, and his independence of opinion. The essence of progress is change. Mr. Spencer asked what, then, are the laws of change by which it is effected? Complying with the Newtonian canon that the fewest causes possible are to be assumed in the explanation of phenomena, he took up the question as resolvable in terms of matter, motion, and force. Progress being a theory of the successive changes by which things are produced, his task was to ascertain the dynamical conditions or laws under which the forms of nature rise, continue, and disappear. The objects of nature coexist and are maintained in a certain order in space. Newton discovered that this is effected by the operation of a simple and universal law. The objects of nature undergo changes in time, emerging and vanishing, some quickly and others slowly: is there a universal law by which these changes also are governed? This was the aim of the research. Mr. Spencer early found that the conception of progress which implies movement in one direction only is erroneous. There is no unbroken march of events; breaks and regressions alternate with advancement, and descending as well as ascending changes have to be accounted for. He therefore rejected the term progress as having erroneous implications, and adopted the term evolution, as more fully indicating the scope of the inquiry and better expressing the strictly scientific nature of his theory. The naturalist Von Baer had already attempted to define and generalize the changes of organic growth, and had formulated them as from the homogeneous germ state to the heterogeneous adult state by a process of differentiation. Mr. Spencer soon found that this formula gave but a very partial account of what takes place in organic development. The change was shown to be not only from uniformity to unlikeness, or a differencing of parts, but from the indefinite to the definite, from the incoherent to the coherent, producing the integration of parts, or increasing unity with increasing complexity. The conditions and course of changes in which organic evolution consists being ascertained, the question arose as to their extent, and Mr. Spencer became convinced that the law of organic movement is not an isolated fact in nature, but “that the process of change gone through by each evolving organism is a process gone through by all things.” Science had shown that the universe, past and present, is subject to orderly changes; he discovered that fundamentally this order is one. The nebular hypothesis proposed by Kant, confirmed by Herschel and Laplace, and accepted by astronomers explained the origin and motions of suns and planets by slow condensation from a nebulous mist diffused through space. The geological history of our earth shows that it has undergone a vast series of progressive changes, and, as Prof. Dana says, “was first a featureless globe of fire, then had its oceans and dry land, in course of time received mountains and rivers, and finally all those diversities of surface which now characterize it.” The course of organic life, as we have seen, was a progressive unfolding into greater diversity and specialty. Mind is developed with the body, and therefore mental phenomena obey a law of unfolding. As human society is made up of units that are capable of these changes, it presents in the past a gradual development of intelligence, arts, and institutions, as now embodied in our diverse and complex civilization. By a careful analysis of the phenomena in these widely separated cases, Mr. Spencer showed that they all conform to a great general law, of which individual life is but a special case. Equally in the inorganic, the organic, and the super-organic spheres, the progressive changes are from the homogeneous to the heterogeneous by differentiation. But with increasing divergences there is also increasing definiteness, coherence, complexity, and integration. Evolution is thus a universal law, while the development of the individual and the career of the race, so far from being exceptional phenomena, are but parts of the great system of change to which the whole cosmos conforms. Evolution being thus disclosed as a universal dynamical law, the question next arises, how is it to be interpreted? Is it an ultimate law like gravitation, or is it a derivative principle deducible as a necessity from the established laws of matter, motion, and force? Mr. Spencer proves that evolution is a resultant of dynamical agencies, and that, given matter as a vehicle of change, motion as the result of change, and force as the cause of change, such are their established laws of interaction that evolution follows as an inevitable consequence. We can here only touch upon the leading elements of the elucidation, and must refer the reader to Mr. Spencer's “System of Philosophy” for the full elaboration of the subject. Modern science has established the great principles of the indestructibility of matter and the conservation of force. (See Correlation of Forces.) Mr. Spencer maintains that these resolve themselves into the single law of the persistence of force, and that this is the fundamental postulate of evolution. Whatever interpretation is given to the principle, it certainly becomes a fundamental condition of the changes taking place in nature. If matter and force throughout the universe are neither created nor destroyed, all changes must be changes of transformation. The stock of material and energy being limited, each new effect must be at the expense of something preëxisting; and hence in the ongoings of nature one thing is necessarily derived from another, while the problem of advance becomes one of transmutation. Mr. Spencer traces out the several causes of transformation or factors of evolution, and shows that they are all corollaries from the supreme law of the persistence of force. Briefly indicated, these are as follows: 1. The principle of the rhythm of motion. Under the law of the persistence of forces and the diversity of their forms, there arise constant conflicts of effect, so that motions are not uniform but varying. Action is met by counteraction, and the result is that movements take a rhythmical form. Boughs, for example, sway in the wind, water is thrown into waves, sound arises in vibrations, earthquakes are propagated in shocks, planets swing through eccentric orbits, breathing is recurrent, the heart beats, scarcity alternates with abundance, and prices rise and fall. From the minutest organism throughout the whole frame of things to the most distant systems, from momentary pulses to geological cycles, the agitations of things take the form of thrills and surges, which produce incessant and universal redistributions of matter and force. How are these redistributions directed? 2. They are controlled first by the law of the instability of the homogeneous. The relatively homogeneous is the commencing stage of all evolution, and Mr. Spencer has shown that this is an unstable condition, and under rhythmic disturbance tends constantly to rearrangement and greater complexity. No object can exist without being acted upon and altered by forces, and no mass can be thus acted upon in all parts alike; unequal action therefore tends to destroy homogeneity and produce ever increasing diversity. For this cause the nebulous condition could not continue; the homogeneous germ divides into unlike parts; a class of animals or plants distributed over a geographical area, being unequally acted upon by environing conditions, would fall into diversity; and for the same reason a uniform social condition would be resolved into heterogeneous societies. 3. The transformations of evolution are further explained by the dynamical principle of the multiplication of effects. Throughout all nature simple agencies produce diverse consequences, every impulse of force yielding a multiplicity of results. A simple mechanical collision of two bodies may produce effects of sound, heat, light, electricity, and various chemical and structural changes; an accident to the foot may entail a train of consequences affecting the whole constitution; the upheaval of a continent may produce the most extensive alterations in the life of races; while an invention like that of the steam engine works its multiform effects throughout civilization. By this law the principle of the instability of the homogeneous is powerfully reënforced, and the cause of universal movement toward greater diversity is rationally explained. But these modes of action alone could only result in a vague chaotic heterogeneity, and could not account for that orderly heterogeneity in which evolution essentially consists. 4. This finds explanation in the principle of segregation. When a mass is acted upon by forces which promote the redistribution of its parts, its units are not only differentiated and regrouped, but there is a segregation of like units which become separated from the neighboring groups. A familiar example of this is seen in the winnowing process, by which a force applied to a mixed mass brings all the grain together in one place and the chaff in another. The same thing is seen when several salts are dissolved in a liquid, and each crystallizes out by the combination of like chemical molecules. The organism conforms to this principle from its earliest stage of growth, the special elements of the bony, muscular, and nervous systems being withdrawn from the nourishing fluids and segregated in the distinctive parts. We have already seen that natural selection is a winnowing process, by which the unfit are excluded, and the better adapted are separated and preserved. In social development the same thing is seen. Not only are there continual differentiations of groups and classes by which society becomes heterogeneous, but these groups are unified by similarity of occupation, character, taste, and race. Stock brokers cluster in Wall street, and the Mormons segregate in Utah. Thus in all the spheres of change redistribution leads to unification. 5. This end is farther promoted by the important dynamical law that motion takes place along lines of least resistance. The operation of this principle in inorganic nature is self-evident. Water forms its channels in the direction of least obstacles. Mr. James Hinton has shown that organic growth takes place in obedience to this law, and Mr. Spencer proves that it governs both mental and social changes. This law, in connection with the principle that movement set up in any direction is a cause of further movement in that direction, by which lines of connection become established, goes far to account for that integration of structures and functions which is disclosed in all phases of evolution. But can evolution go on for ever, or is it limited? This brings us to the process by which it is constantly antagonized and always finally terminated, the counter-agency of dissolution. All redistributions of matter and motion are either evolution or dissolution, but neither of these processes ever goes on absolutely unqualified by the other, and the change in either direction is but a differential result of the conflict. Mr. Spencer's formula, to be complete, must embrace both sets of correlative changes, and its determination led him to the following universal law: 6. Every change wrought in an object must be either a transposition of its mass, or a variation of its internal or molecular motion. As it loses this contained or insensible motion, there follows a concentration of the parts and increasing integration; if it acquires insensible motion, there is dispersion of the particles, or disintegration; that is, with concentration of matter there is dispersion of motion, and with absorption of motion there is diffusion of matter. These are the two aspects of the universal metamorphosis, and when approximately balanced there is equilibration. Evolution is integration; dissolution is disintegration. We have here confined ourselves to the most abstract statement of Mr. Spencer's theory; its concrete applications will be found extensively worked out in his “First Principles” and in the biological, psychological, and sociological divisions of his “Philosophical System.” As a method of philosophy it aims only to explain phenomena; all phenomena being regarded as manifestations of the unknown power which transcends the reach of thought. Philosophy is regarded as the highest explanation of things, and as each science is unified by its largest inductions, the family of sciences is brought into a completer unity by a law that comprehends them all.—Whatever ultimate form the theory of evolution may take, its influence must be powerfully felt in the direction of future inquiries; for many who withhold their assent from it as an established truth of nature nevertheless recognize it as an invaluable working hypothesis. As remarked by Prof. Grove: “The first question is, does the newly proposed view remove more difficulties, require fewer assumptions, and present more consistency with observed facts than that which it seeks to supersede? If so, the philosopher will adopt it, and the world will follow the philosopher, after many days.” Mr. Spencer's theory has been clearly summed up by himself in the following propositions: “1. Throughout the universe, in general and in detail, there is an unceasing redistribution of matter and motion. 2. This redistribution constitutes evolution where there is a predominant integration of matter and dissipation of motion, and constistutes dissolution where there is a predominant absorption of motion and disintegration of matter. 3. Evolution is simple when the process of integration, or the formation of a coherent aggregate, proceeds uncomplicated by other processes. 4. Evolution is compound when, along with this primary change from an incoherent to a coherent state, there go on secondary changes due to differences in the circumstances of the different parts of the aggregate. 5. These secondary changes constitute a transformation of the homogeneous into the heterogeneous—a transformation which, like the first, is exhibited in the universe as a whole and in all (or nearly all) its details: in the aggregate of stars and nebulæ; in the planetary system; in the earth as an inorganic mass; in each organism, vegetal or animal (Von Baer's law); in the aggregate of organisms throughout geologic time; in the mind; in society; in all products of social activity. 6. The process of integration, acting locally as well as generally, combines with that of differentiation to render this change not simply from homogeneity to heterogeneity, but from an indefinite homogeneity to a definite heterogeneity; and this trait of increasing definiteness, which accompanies the trait of increasing heterogeneity, is like it exhibited in the totality of things, and in all its divisions and subdivisions down to the minutest. 7. Along with this redistribution of the matter composing any evolving aggregate, there goes on a redistribution of the retained motion of its components in relation to one another; this also becomes step by step more definitely heterogeneous. 8. In the absence of a homogeneity that is infinite and absolute, this redistribution of which evolution is one phase is inevitable. The causes which necessitate it are: 9. The instability of the homogeneous; which is consequent upon the different exposures of the different parts of any limited aggregate to incident forces. 10. The transformations hence resulting are complicated by the multiplication of effects: every mass and part of a mass on which a force falls subdivides and differentiates that force, which thereupon proceeds to work a variety of changes, and each of these becomes the parent of similarly multiplying changes; the multiplication of these becoming greater in proportion as the aggregate becomes more heterogeneous. 11. These two causes of increasing differentiations are furthered by segregation, which is a process tending ever to separate unlike units and to bring together like units; so serving continually to sharpen, or make definite, differentiations otherwise caused. 12. Equilibration is the final result of these transformations which an evolving aggregate undergoes. The changes go on until there is reached an equilibrium between the forces which all parts of the aggregate are exposed to, and the forces these parts oppose to them. Equilibration may pass through a transition stage of balanced motions (as in a planetary system) or of balanced functions (as in a living body) on to the ultimate equilibrium; but the state of rest in inorganic bodies, or death in organic bodies, is the necessary limit of the changes constituting evolution. 13. Dissolution is the counter change which sooner or later every evolved aggregate undergoes. Remaining exposed to surrounding forces that are unequilibrated, each aggregate is ever liable to be dissipated by the increase, gradual or sudden, of its contained motions; and its dissipation, quickly undergone by bodies lately animate and slowly undergone by inanimate masses, remains to be undergone at an indefinitely remote period by each planetary and stellar mass, which since an indefinitely remote period in the past has been slowly evolving; the cycle of its transformations being thus completed. 14. This rhythm of evolution and dissolution, completing itself during short periods in small aggregates, and in the vast aggregates distributed throughout space, completing itself in periods which are immeasurable by human thought, is as far as we can see universal and eternal; each alternating phase of the process predominating now in this region of space and now in that, as local conditions determine. 15. All these phenomena, from their great features down to their minutest details, are necessary results of the persistence of force, under its forms of matter and motion. Given these in their known distributions through space, and their quantities being unchangeable either by increase or decrease, there inevitably result the continuous redistributions distinguishable as evolution and dissolution, as well as all those special traits above enumerated. 16. That which persists unchanging in quantity but ever-changing in form, under these sensible appearances which the universe presents to us, transcends human knowledge and conception is an unknown and unknowable power, which we are obliged to recognize as without limit in space and without beginning or end in time.”—Besides the works already mentioned, the following are important: Spencer's “First Principles,” “Principles of Biology,” “Principles of Psychology,” “ Principles of Sociology,” and “Descriptive Sociology”(1860-'73); Darwin's “Variation of Animals and Plants under Domestication” (1868); St. George Mivart's “The Genesis of Species” (1871); Huxley's “Man's Place in Nature” (1864), “Lay Sermons” (1870), and “Critiques and Addresses” (1873). The relation of the doctrine of evolution to Christianity is discussed in “The Bible and the Doctrine of Evolution,” by W. W. Smyth (1873); “The Theory of Evolution,” by the Rev. E. Henslow (1873); “What is Darwinism?” by Charles Hodge, D. D. (1874); and “The Doctrine of Evolution,” by Alexander Winchell, LL. D. (1874).