Evolution The important relations which the scientific subject has assumed to religious literature justifies, us. in a more copious and particular treatment than was appropriate under the general head of DEVELOPMENT SEE DEVELOPMENT (q.v.).*
*We present, unmodified, the facts and positions of our esteemed correspondent on this subject, who views it in it scientific aspect, although we dissent from some of his conclusions. — ED. SEE SCEPTICISM, in this volume.
I. Definition. — Evolution in its widest sense, and viewed from the scientific standpoint, is the continuous transformation and differentiation of an identical substance. More specifically, it is the continuous unfolding of a material existence according to such method that constituent parts which were germinal or potential become actual and functional, and according to such an order that the primitive existence is successively more differentiated, with parts progressively more and more specialized in structure and function. It is the passage from the homogeneous to the heterogeneous. It implies continuity and unity of existence. It also implies persistence of the fundamental conception embodied in the primitive substance, so that, however diversified, all its parts still conform to a changeless type.
It is a mode which reveals itself transcendentally as. the necessary product of mind; it reveals thought as all-pervading and all-enduring throughout the material realm in which the law of evolution finds its exemplification.
Whether the phenomena of the natural world come into existence under a method conformable to the above definition of evolution is a question of fact, to be decided by investigation of the phenomena.** This question of fact falls, therefore, strictly within the domain of natural science. Whatever verdict may be prouounced at this tribunal can never be invalidated by any a priori considerations, nor by any delineation of supposed consequences or implications of the verdict. Nor can it be set aside as proceeding from incompetent authority, since no authority in a question of fact can be conceived more competent than that of a body of witnesses who have surpassed all others in the study of that about which they testify. For our present purpose we must ascertain, therefore, what are the determinations of natural science in reference to the nature of the successions of phenomena in the natural world. Does science find a material continuity running through these successions; or does it find them marked by interruptions, discontinuity, and new beginnings?
** Not speculatively viewed, however, but in the light of all the evidence, both natural and revealed. — ED.
II. History of Opinion. — In searching for the best judgment of mankind in reference to the question of material continuity in the natural world we ought to cite first the opinions of thinkers antedating the epoch when scientific research had supplied material for a proper demonstration of the doctrine. As all philosophizing on the laws of nature must, of necessity, be grounded on an observation of nature more or less extensive and more or less exact, so the opinions of the ancient philosophers, however slender the basis of their inductions, must be regarded as essentially scientific. Science had not yet been distinguished from philosophy. Theories as to the origin of the world and of organic existence were in vogue some centuries before the Christian aera. The hylozoism of the Ionian physicists conceived a primordial matter endowed with generative or transmutative powers through which cosmic forms, successively differentiated, came into being. Trhe speculation presents analogies with the modern one of Buffon. Heraclitus, about 500 B.C., taught the doctrine of a perpetual flux of things, involving ceaseless conflicts between opposites, in the midst of which individual things survive, by superior fitness, the processes of destruction aid renovation. A developmental mode of cosmic origins was taught by Anaxagoras of Clazomenae (Aristotle, Physica, 8:1) about 500 B.C. He supposed the primitive condition of things to be a heterogeneous commixture of substances without order or motion. This continued an indefinite period, when the mind began to act upon it by instituting a revolving motion at a single point. This propagated itself into the surrounding realm, and led to the separation of the elementary contraries, fire and air, water and earth. The process was repeated in the resulting masses, and thus, by continuous differentiation of likes and unlikes, the actual constitution of the world resulted (Ueberweg, Hist of Philos. 1:66). The views of Leucippus and Democritus, about 430 B.C., contemplated.a gradual evolution of things. They held that immensity was eternally filled with atoms actuated by an eternal motion. These, in disposing themselves according to size, produced collisions which originated vortical motions. These, extending farther and farther, led to the formation of worlds. Such views were extended by Epicurus and the Roman Lucretius; and long afterwards, similar theories, but with more theistic leanings, were entertained by Torricelli. Galileo, and Gassendi. The Greek atomists attributed the lateral motions of the atoms to choice — a conception of the animated nature of atoms which was revived in the monads of Gassendi, Leibnitz, Rosmini, Campanela, Bruno, and Maupertuis; and reproduced in the conscious atoms and molecules of Hackel, Elsberg, and other moderns. The evolution of the cosmic system through the intervention of viortices was undertaken in the well-known theory of Descartes (Principia Philosophiae, 1644); and Kepler made use, also, of a.vortical movement in the matter of a primitive chaos, but invoked the Empedoclean conception of attractions and repulsions for the initiation of the primitive motions. The speculations of Swedenborg (Principia Rerum Naturalium, 1733-34) also posited vortical atomic motions, which expanded to cosmical movements and led to the differentiation of worlds. These various speculations (more fully set forth in Winchell's World Life, or Conmparative Geology, part 4), opened the way for the better defined and better-defended nebular cosmogonies of Kant and his successors. The evolution of the earth's physical features by means. of fire and water was first undertaken by Leibnitz (Protogea, etc., 1749, first, in abstract, in Acta Eruditorum, Leipzig, 1683). These eminent thinkers, whom, in this connection, we can only mention, all conceive the earth and the solar system to have originated through the progressive differentiations of a primitive chaotic matter. This is the conception of modern evolution.
Meantime the notion of a material continuity in the successions of the organic world was repeatedly shadowed forth. Empedocles taught the progressive origination of organic forms. Aristotle maintained that immanent divine mind determines in nature a tendencye towards improvement and perfection. Lucretius held that the races of men, however diverse, are derived from a common origin, and this through the continual survival of those best fitted for the environment. In later times, Sir Mathew Hale (Primitive Origination of Mankind, 1677, page 211), enumerates distinctly the results of the struggle for existence in the animal. De Maillet (Telliamed, Amsterdam, 1748), attempted to explain how animal forms undergo transmutation through the influence of changed environment; and Lamarck (Philosophic Zoologique, new ed. 1873) to this influence added the principle of use and disuse, and admitted also an underlying inherent conatus towards beneficial change. These very concise references to the history of opinion. may be supplemented by a perusal of the article on "Evolution" in the Encyclopcedia Britannica, and by a study of the later works to be mentioned in the progress of this article. Within our restricted limits it will be more profitable to proceed to an outline of the evidences of evolution as at present understood.
III. The Scientific Evidences. —
1. Inorganic Evolution. The processes of change in the topographical and hydrographical features of the earth's surface are so familiar that we almost fail to note the fact that these re cent transformations are but the last terms of a series of changes which have moulded the globe and imparted to it the features that complete its fitness for the reception of organic populations. But, in fact, the filling and drainage of a pond or lakelet in a human lifetime is the same kind of work as that which spread the deposits of the prairies of the Mississippi, the tchornosjom of southern Russia, the pampas of Buenos Ayres, and the steppes of southern Siberia. The alluvial sediment left by a Mississippi overflow of this year is only one of the succession of contributions which, in ages past, have formed the entire delta of the great river. The delta grows; ocean sediments accumulate; the hillsides waste; the mountains wear out; whole shore-lines rise or sink; and the integration of these minute annual changes between vast limits of time shows that all the grander features of our planet have grown into existence by progressive transformations of the original matter. All this is obvious.
So it is obvious that the observed and admitted tenor of events implies an ancient course of change, in times so remote that the conditions had not yet approximated to those revealed in the human period. The pages of geological science enumerate those changes. It is not necessary to assume that all or any of the conclusions of science are exact in reference to the particular events of the geological past; it cannot be doubted, however, that research has successfully shown that the present is the outcome of the past, and that the rocks and waters and gases which we observe are only a transformed portion of the material of the primeval world. The actual earth has passed, by material continuity, from a primitive state, in which all its physical conditions were extremely different from the present. Its mountains, rivers, islands, and seas have progressively come into existence. Its different portions have become more and more differentiated. It was once more homogeneous. It has undergone a real evolution. But the geognostic data which pass before our observation disclose the primitive world in a process of emergence from a molten state. The world's history has been a history of cooling; and there are numerous indications that the actual records of geology note only the last stages of the world's cooling history. We have not the space at command, nor is it necessary, to enter into an enumeration of the grounds on which science has traced terrestrial evolution backward to a nebular state, and even to a remoter one, in which the matter of the whole solar system is disclosed in a process of common evolution, under the action of the same forces as enter into the transformations of the earth's surface in these times, before human eyes. That our planetary system has had a nebular history is almost unanimously admitted by the science of the present. The chief divergences of opinion concern only some details of that history. This conclusion implies a material continuity through the totality of the changes. Rocks and ocean and atmosphere have grown out of fire-mist and nebula. World-life is a grand spectacle of evolution, and it illustrates continuity and unity of method on a scale of vastness which is deeply impressive. The details of the evolution must be sought in special works (see Winchell, World Life, 1883). The conception of modern nebular theory is itself an evolution. It was first shadowed forth by the Greek and mediaeval thinkers already quoted. It began to assume a consistent and modern aspect at the hands of Immanuel Kant (Allgemeine Naturgeschichte und Theorie des Himmels, 1755, and a prize essay, read in 1754 before the Berlin Academy of Science). Sir William Herschel's nebular researches disclosed the apparent existence of enormous patches of chaotic world-stuff, which seemed to undergo a process of differentiation into stars and planets (see sundry memoirs, read before the Royal Society of London between 1783 and 1818, but especially in 1784, 1785, 1791, 1795, 1811, and 1814; also Sir John Herschel, Observations of Nebulce and Clusters of Stars at Slough, 1825-33; Phil. Trans. November 21, 1833). Laplace, in apparent ignorance of Kant's remarkable speculation, brought the conception of nebular cosmogony to a rigorously scientific statement (Exposition du Systeme du Monde, 1796); and the general form of his theory enters into the most recent cosmological speculations, though the progress of discovery and of thought has necessitated slight modifications, and has greatly extended the scope of the grand generalization. That which for years was known as "the nebular hypothesis"* has strengthened into a nebular theory, accepted now with almost the same confidence as the Newtonian theory of universal gravitation. This is the verdict of science on a question in its own appropriate field. No dissent from the outside is deserving of consideration: though, of course, exceptions taken by a scientific minority must be honestly examined. For a discussion of alleged difficulties of nebular cosmogony, see Winchell's World Life, pages 153-198.
* The "nebular theory" here referred to is based upon the supposition that the universe originally existed in the form of gaseous vapor diffused by intense heat throughout space, and that all the heavenly bodies have resulted from this by rotation and gradual condensation through cooling off. Most or all the phenomena which they exhibit, such as sphericity, orbital and axial revolution, together with earthquakes and volcanoes (as showing the still liquid central mass), are thought to be best explained on this hypothesis, and the fact that nebulse are yet discovered in the starry spaces is held as confirmatory of it. On the other hand, some of these nehule have already been resolved by powerful telescopes into a mass of separate stars, and the presumption is therefore strong that such is the composition of all of them. Comets are too little known to be of much weight in the argument. Many astronomical facts, however, are decidedly antagonistic to the "nebular" view, such as the wait of ascertainable ratio between the magnitudes, distances from the sun and periods of revolution of our own planets and the obliquity of their orbits, some celestial bodies actually moving in the opposite direction. Experiments with the spectrum show that they are not all composed of the same elements. Moreover it is impossible to see, if space were at first filled with incandescent gas, where the excessive heat could have radiated to. For these and other reasons some of the ablest astronomers, Proctor for example, wholly discard the theory as insufficient and disproved. The question is a purely scientific one, of no especial interest to the theologian, so long as the origination of matter, motion, and life, with their laws and properties, be attributed to the divine flat. But the attempt to identify the processes of the nebular theory of cosnmoollny with any part of the narrative in the first chapter of Genesis is exegetically preposterous. Whatever therefore may become of that theory, Moses is not responsible for it, and revelation has nothing to do with it. — ED.
According to this conclusion, the cosmic realm is the grandest conceivable exemplification of the method of evolution pursued in nature. This evolution guides and determines all the ulterior details of inorganic history. The total inorganic universe, as we know it, is the final outcome of the method of efficient activity revealed in nature, and it has been exerted upon identical portions of matter from the dawn of cosmical history to the present. The question of fact, so far as concerns inorganic nature, can no longer be agitated.
2. Organic Evolution. — This is a greater and more serious question. Does a material continuity run through the succession of organic types which have appeared and disappeared in the history of the world? Are the higher species of the modern world descended from the lower species of the ancient world? Are the diversified types derived from a common ancestry? Is man's bodily organism the outcome of genealogical descent? That these queries must be answered affirmatively seems to be the inevitable conclusion from an enormous amount of modern research. The proofs are numerous and diverse; but we may range them along five lines of argumentation, converging towards the conclusion.
(1.) Ontogeny. — By this we mean the history of the individual. This, beyond all controversy, is an evolution. The succession of changes from the beginning of conscious life to maturity is great, but they are wrought in the same identical being. Still greater ontogenetic transformations may be traced back through embryonic life to the earliest changes wrought in the fertilized ovum. The unfertilized ovum is itself only a transformed epithelial cell, and consists of yolk, germinative vesicle, and germinative dot. The successive transformations of these elements bring into view, first, the faint outlines of the most fundamental structures, as vertebrae, spinal marrow and brain, heart and digestive structures, then the complete details, and finally the accessory structures belonging to the perfected form. The particulars of the history are too technical to be enumerated in this place. This succession of embryonic transformations in a higher vertebrate reveals a wonderful case of characteristic evolution, beginning in a cell and ending in a complicated animal structure. But the most impressive significance of the history will be mentioned in another connection. For details, see Balfour, A Treatise on Comparative Emnbryology (1880, volume 1); Ktolliker, Entwickelungsgeschichte des Menschen und der hoheren Thiere (1876); Foster and Balfour, Elements of Embryology (1874, volume 1, on the chick); Hackel, Anthropogenie (1874); Packard, Life Histories (1876); and, further, the important-works of Huckel, Owen, Bischoff, Parker, Remak, Agassiz, Clark, Reichert, von Baer, etc.
(2.) Morphology. — The forms of animals and plants are said to be similar in proportion to their affinities; but the implications of the statement are seldom appreciated. Among men, family resemblances are understood to signify blood relationship more or less remote. All men of the same race possess so many points of resemblance that every one admits their common descent from the same original parent. All mankind, according to the doctrine of evolution, however diverse in feature or endowment, must have descended from a common primitive human ancestry. But when we speak of two so-called species of the cat family, say the leopard of Africa and the panther of Asia, the popular opinion is that they are primordially distinct; though their resemblances are vastly closer than those of the Bushman and his neighbor, the Cape Englishman, the denial of whose kinship we resent. In fact, these two cats are so closely similar that some zoologists unite them in one species. If pronounced one species, popular opinion would assign them a common descent; if two species, it would hold them primordially distinct. Yet the animals, with all their characteristics, remain the same, whatever view may be taken of the systematic value of their slight distinctions. Now the question of consanguinity is one of fact, not depending on the opinion which may be entertained respecting differences.* Whatever that opinion may be, it continues manifest that we have a better reason for ascribing these cats to a common ancestry than for doing this with a Congo African and a blonde Scandinavian. But suppose we compare the leopard and the tiger — two distinct species by all admissions. The nature of their resemblances is precisely the same as in the other case, and only a little less in degree. To admit the common descent of the leopard and panther is to compel, at the risk of inconsistency, the admission of the common descent of the leopard and the tiger. When we assent to the consanguineous relation of two recognised species the whole proposition, in all' its breadth, is conceded, that not only all cats, but all mammals, are derived from some primitive stock; and the divergences existing have been acquired during the progress of the generations. But since mammals present so many graduations towards birds, in egg-laying ornithorhynchus and echidna, towards reptiles in the chelonians, and fishes in the cetaceans, we cannot refuse a common descent to mammals and all other vertebrates. This admission brings the whole animal kingdom with it, for some tunicates and cephalopods would be admitted close kinll to some of the lowest vertebrates. Indeed, if we compare any two representatives of the animal kingdom, however divergent, we shall find that they resemble each other in more points than the number of their differences; and the argument for their common descent is of the same nature as in the case of the negro and Scandinavian. This, then, is an indication of the nature of the argument from morphology — and we can only present the indication (for further details, see works on zoology and botany). Some striking animal portraits may be found in Johnson, Natural History (2 volumes, 8vo); Cassell, Natural History (1883, 6 volumes, 8vo); Knight, Animated Nature (2 volumes, 4to) ; Brehm, Thierleben (9 volumes, 8vo). Details of structure in Owen, Comparative Anatomy (3 volumes, 8vo); Hackel, Generelle Morphologie der Organismen (volume 1); Gegenbaur, Grundriss der vergleichenden Anatomie (8vo); Huxley, Manual of the Anatomy of Vertebrated Animals (8vo), etc.
* But on this question we have, in the book of Genesis, historical proof which cannot safely be neglected; and it is more definite than the scientific. — ED.
(3.) Palaeontology. — The doctrine of the descent of all living species from a common remote ancestry implies that in former times the divergences of organic types were less than at present. Such a retral convergence of genealogical lines is precisely what palaeontology shows. Within historic times this convergence is almost imperceptible; but as soon as we enter the aeons of geology no fact is more conspicuous. To take an example which has been much bruited, the domestic horse, now so widely differentiated from five-toed quadrupeds, we find that in the age immediately preceding the present true horses lived, in which the rudimentary second and fourth digits, or splint bones, of the modern horse were more developed. Further back were horses with the same bones terminated by dangling hooflets. Still further back were horses having these hooflets more developed, and reaching the ground. But these horses had other splint bones, the rudimentary condition of a first digit, and in remoter times these rudiments are found terminated by dangling hooflets, and in still remoter, by functional hoofs. So we trace the succession of equine types back to a four-toed quadruped which, when we consider the corresponding divergences in the teeth, tibiae, and other structures, we should hesitate to group with modern horses, if they were not connected by a gradation so gentle that we find no place to draw the dividing line.* The ancient four-toed horses are connected with a type of five-toed predecessors by a similar kind of relationship. The equine succession leads back, therefore, to a five-toed quadruped. If we take the modern ox or sheep or pig or camel or rhinoceros, we shall be able to trace back similarly a succession which leads towards a primitive fivetoed quadruped; and in every case such quadruped approximates the form which stands at the beginning of the equine succession. The details of facts establishing such a
-generalization are accessible to all readers in the writings of Leidy, Cope, Marsh, Gaudry, Owen, Huxley, and other palaeontologists. See Cope's memoirs in reports of surveys under Hayden and Wheeler, and briefer papers in American Naturalist; Marsh, in American Journal of Science (sermon 3); Leidy, U.S. Geol. Survey of the Territories (volume 1); Ancient Fauna of Nebraska (1853); "Extinct Mammalia of Dakota and Nebraska," in the Jour. Acad. Nat. Science (Phila. 1869, volume 7). In a manner precisely similar the two types of modern birds — "flying " and "running" — may be traced back along two successional lines, to Mesozoic Samirian reptiles. So, progress has been made in tracing lines of succession among invertebrate animals and plants. The facts show what the doctrine of descent requires, a gradual convergence backward of all the lines of organic succession.
*But there does not seem to be a particle of proof that these latter races were genetically or actually derived from the former ones. On the contrary, these very differences all the evidence we possess on the subject- go to show that they are not their offspring. — ED.
But, if these successions are genealogical,** there must have been uninterrupted continuity along each line.
** This genealogy is, in our view, a pure assumption. — ED.
The chain connecting the past and the present exhibited no missing links. It is the attempt of palaeontology to discover traces of all the links; but obviously the attempt is more difficult than to find all the fragments of a meteorite which exploded in the sky before the Christian era. The work of paleontology is necessarily incomplete; the relics of many types which once contributed to the continuity of the successions worked out remain undiscovered. There are, indeed, many missing links in our knowledge; but the tenor of discovery is such as to imply that no missing links interrupted .the continuity of the actual successions. Every year's acquisition of new facts narrows the great gaps, and closes up some of the smaller ones. Some successions are already reconstructed with marvellous completeness; beyond question much more is destined to be accomplished; and we may logically forecast the future state of the evidence and anticipate the conclusion. So we reason from palaeontology, and it seems entirely logical to conclude that in the actual life-history of our planet the successions of specific forms were nicely graduated from the rude and generalized types of the remote past to the large-brained and highly specialized types of the present. But this admission does not establish any genetic connections running through the several series. Each species may still have resulted from a special origination. Only the presumptions to be, drawn from embryology and morphology suggest genetic descent in palaeontology. The facts of palaeontology might be as they are, with every species a primordial and persistent form; butt the establishment of these graduated successions establishes what must have been the fact on the theory of common descent, and constitutes a link in the chain of argument.
(4.) Variability. — Is it within the economy of nature that organic types shall undergo indefinite secular variation, or maintain essential permanence? Within the historic period few undomesticated species are known to have varied to any marked extent; but all those domesticated have become differentiated, and sometimes to a striking extent. The different breeds of horses, cattle, dogs, fowls, and pigeons differ to such an extent that many of them, but for our knowledge of their common origin, would be set down by any naturalist as distinct species. They are distinct species in the same sense as the jaguar and the ounce and the panther are distinct. The elder Agassiz, though no evolutionist, used to proclaim the different races of men as widely distinct as the different families of monkeys. The suggestion that these divergences have not arisen in a state of nature seems to possess no relevancy, for it is still shown that the aptitude to vary is possessed by nature's organisms. Moreover, the influences brought to bear on these animals through man's treatment are the same in kind as those which sometimes arise from natural operations; they only differ in intensity, and thus accelerate changes for which nature fitted, and perhaps destined, the being. Finally, the changed forms result from the same kind of action of the same physiological forces as are in play in animals uninfluenced by domestication. Only powers like those of digestion, respiration, growth, and adaptation have been employed in the development of these varieties, and these are the functional activities of all animals. It would seem, therefore, that the results of domestication may be fairly appealed to as tests of the permanence of species. (See Darwin, Animals and Plants under Domestication.)
But it appears that great variations sometimes occur among animals and plants in a state of nature. Conflicts between individuals and conflicts with physical conditions are influences continually making their impressions on the organism. These are not causes, but only conditions, of organic change. By the law of adaptation the forces of the organism effect such changes as changed environment demands. The same species of birds, mammals, and molluscs, in their wide range across a continent from east to west, and from north to south, are found to vary according to the latitude, longitude, altitude, and other circumstances. A thorough knowledge of such variations in North America has led to the merging of large numbers of once accepted species (Allen, Proc. Bos. Soc. Nat. Hist. 15:156; 16:276; Bull. Mus. Comp. Zool. 2, No. 4, page 345, August 1876; Amer. Naturalist, October 1876, page 625; Baird, Mem. National Acad. January 1863; Amer. Jour. Sci. II, 41, January, March, and May 1863; Ridgeway, Amer. Jour. Sci. III, 4:454, 5:415). Similar extreme variability is observed in many invertebrate species, both recent and extinct. Hackel, in a remarkable work on calcareous sponges, has reached the conclusion that all the forms belong to one species, so gradual are the transitions between the several nominal species (Die Kalkschwamme, 1872, 2 volumes, 8vo). Many forms of fossil shells formerly regarded as distinct species have more recently been united, simply because series of intermediate forms became known. Hilgendorf has traced minutely the secular variations of a species of Planorbis (Ueber Planorbis multiformis in Steinheimner Susswasserkalk), and Hyatt has extended these studies (Proc. Amner. Assoc. 1880, and "Anniversary Mem." in Bost. Soc. Nat. Hist. 1880). Similar work has been done among Paleozoic brachiopods.
The influence of changed environment is sometimes accelerated by human intervention. The axolotl, permanently gill-bearing in its native elevated home, loses its gills when kept near the sea-level, and becomes a land salamander. In Japan certain leeches and planarians have become adapted to land life, and a fish, even (Periophthalmnus), frequents the land and seems in a transition state. Certain brine shrimps are reported by Schmaukevitch as undergoing important structural changes in the course of a few generations, when the brine is gradually freshened; and return to the original state as the salinity is again restored (Zeitsch. wiss. Zodlogie, 25: Suppl. 1, 1875, page 103, pl. 6; Annals and Mag. Nat. Hist. March 1876; ib. 29:429-494, 1877. See, also, Contributions on Knowledge of the Influence of External Conditions of Lfe upon the Organization of, Animals, transl. in Hayden's twelfth Ann. Rep. part 1, 473-514. But compare Verrill, Proc. Amer. Assoc. 1869, 230; Amer. Jour. Sci. II, 48, 244, 430; Packard, Amer. Jour. Sci. III, 2:108). The domestic cat on the Pribilov Islands becomes thickened, short, losing the tail, and undergoing great change of voice. Certain domestic pigs in Texas are well known to have become solid-hoofed.
Through hybridity, also, probably, result forms divergent from recognized species. Among cultivated plants hybrids are not uncommon. In the wild state the number of reputed hybrid forms may be judged from a glance through any manual of botany. (See also, Hooker, Flora of New Zealand; Candolle, "Etude sur l'Espece," in the Bibliothequle Univ. de Geneve, November 1862; Hooker and Thomson, Flora Indica, volume 1," Introductory Essay," London, 1855; Gray, Amer. Jour. Sci. II, 21:134; Naudin: Hybridity in the Vegetable Kingdom). Among animals, fertile hybridity, as well as infertile, is pretty well established.* From the hare and the rabbit has arisen a self-sustaining hybrid now extensively employed in Europe for food. Fertile hybrids of the common and Chinese geese are extensively reared in India, as also in England; while several generations of the hybrid from the mallard and muscovy ducks are reported living in Mt. Auburn cemetery (Brewer, Proc. Bos. Soc. Nat. Hist. 21 January 1874). Carl Vogt reports fertile hybrids of the wolf and dog, as also of the goat and sheep, and the latter is confirmed by Hackeli Von Tschudi and Vogt both report the same of the goat and steinbock, and of the fox and dog. The same is alleged of the buffalo and bison. Without relying on the intervention of hybridity, enough has been observed of the power of organic forms to adapt themselves permanently to the permanent changes of the environment to fully establish the conclusion that it is the economy of nature to permit structural variations without limits.* If a full survey of the facts to which we have too briefly alluded justifies the conclusion, as we think it does, then no bar exists to the conclusion that the successions of Palseontological types have arisen through the continued variation of primitive forms; and that the latter, also, may have arisen through variation and descent from one primordial, life-endowed being. This extreme conclusion, however, is not at all necessary to the proof of a method of evolution in the world, since the genealogical lines may have proceeded from any such number of beginnings as the state of the observed relationships may allow.
* But we believe this is true only to a very limited extent, and the fertility very rarely extends to successive generations. — ED.
(5.) Comprative Embryology. — A careful study of the aspects of the developing embryo of a higher vertebrate, as indicated above, under "Ontogeny," shows that it reaches, in ascending order, a succession of stages which may be enumerated and defined. Now the facts to which we wish to direct attention particularly, constitute a series of significant parallelisms.
(a) Ontogenetic parallelism. Research shows that every higher vertebrate passes through the same embryonic stages, and no divergences revealing the characteristics of class, genus, and. species make their appearance until the development is well advanced. To a certain stage the human embryo cannot be distinguished from that of a fish; at later stages, it diverges successively from the embryo of reptiles, birds, quadrupeds, and quadrumana. The embryo chick is absolutely undistinguishable from the embryo of man until about the sixth day of incubation. Even invertebrates pursue a course of development closely parallel with that of the earlier stages of the mammalian embryo. (Hackel, Naturliche Schopfungsgeschichte, 11 Vortrag; Anthropogenie, 13-19 Vortrage; Balfour, British Assoc. Address, 1880, Nature, 22:418).
(b) Taxonomic parallelism. The succession of aspects presented by the mammalian embryo is identical with that shown in the gradations of living animals. The disappearance of the nucleus of the egg results in a simple cytode, which is paralleled in the living world by Protamaeba, the lowest known animal. The new-formed nucleus gives the ovum the character of Amoeba. The "morula" mass resulting from the divisions of the yolk is paralleled by Labyrinthula. The spheroid formed of a single layer of cells corresponds to the larves of Planula. The invagination of this, forming a two-walled spheroid or urn ("gastrula") is paralleled by the larves of Protascus. The four-layered, elongated form answers to the worm Turbellarid. The fibrous, semi-tubular cranium and gelatinous spine are found adult in the lancelet. The gill-arches of the embryo are permanent in the dog-fish and other sharks. The tailed condition represents the maturity of the reptile. So, without further particulars, it may be broadly asserted that the gradations of living animals are pictured in the successive stages of the mammalian embryo. (See especially Hickel and Balfour, as cited; Baer, Nachrichten uber Leben und Schrifien 1865.) The principle has, indeed, found useful application in some cases, in determining the relative rank of animals.
(c) Palaeontological parallelism. It was amply shown by the elder Agassiz that the geological succession of organic types presents an order identical with that of the classificatory arrangement of animals. (See especially, Essay on Classification.) This has been more fully illustrated by Hackel (see citations above). Owing, however to the recognised imperfection of our knowledge of extinct life, this parallelism is less detailed than the others. We know specifically, however, that the primitive form, Eozoon, must have been akin to Amoeba and Labyrinthula; that the turbellarian grade was reached in Scolithus, of the Potsdam sandstone; that the shark type was attained in the Upper Silurian and Devonian; the transition from aquatic to terrestrial creatures, in the Amphibia of the Coal Measures, with some advance in the Trias; that reptiles succeeded in the Mesozoic, and birds appeared on their decline; that the lowest mammalian types existed in the Jurassic and higher types followed through. the Tertiary; that the lowest four-handed animals were of Lower Eocene age, and that tailed monkeys, anthropoid apes, and men followed in due order.
*We submit that these very limited variations do not prove a capacity for unlimited variation. — ED.
The established facts of comparative embryology show a prolonged and detailed succession of organic conceptions literally three times repeated. The doctrine of chances demonstrates that this must result from some mutual dependence and connection among them. The palaeontological succession must result from the order of succession under a law of development as primitively exemplified in the evolution of the individual. In the latter, each successive stage arises demonstrably by continuity with the preceding. The palaeontological series consists of the final terms of many genetically related embryonic series successive in the extinct world. The taxonomic series consists of the final terms of many genetically related embryonic series simultaneous in the actual world. All the terms in each series are therefore materially connected through the embryonic series of which they are several parts.*
*The force of this argument, however, seems to us to be wholly invalidated by two facts: 1. No instance of the propagation of one species of animal by parents of another, has been historically found; 2. The embryo in every instance stops at the precise point prescribed by its specific character; and becomes either an abortion or a monster if it fails to reach it. — ED.
IV. Evolution Theories. — While most evolutionists believe that the intellectual and moral elements of man are, equally with the material organism. the outcome of a long process of improvement, Mr. A.R. Wallace holds that both body and mind of man may have arisen in a different manner. (Wallace, Contributions to the Theory of Natural Selection, Am. ed. 1871; Address at Glasgow Meeting. Brit. Assoc. 1871, Amer. Jour. Sci. III, 13:377), while St. George Mivart limits the exception to man's psychic nature (Genesis of Species, 1871; Lessons from Nature, 1876). The majority of evolutionists maintain that man's body is so intimately identified in structure with that of lower animals that it is incredible that it has not participated in the common history. As to his psychic nature, it is held to be identical in many of its manifestations with the natures of brutes, and a strong presumption hence arises that even man's highest powers exist germinally in the lower animals.
The speculations of theorists concern chiefly the causes, conditions, and instrumentalities on which organic evolution depends. De Maillet, in a work whose title (Telliamed, 1748) was an anagram of the author's name, represents that organic beings possess an aptitude for structural changes, and that changes arise when, under changed conditions, the animal puts forth efforts to exercise changed functions. Lamarck (Philosophie Zoologique, 1809; new ed. by Martins, Paris, 1873) maintained that primitive rudiments of the great divisions of the organic kingdoms arose by spontaneous generation; that these were endowed with an inherent tendency to improvement, which becomes effective especially through use and disuse of organs, while the influence of external conditions determines use and disuse. The author of the Vestiges of Creation, 1844, suggested that life first appeared on our planet "in simple germinal vesicles," "produced by some chemico-electrical operation," and that successive steps of advance were effected "through the agency of the ordinary process of generation." The conditions under which this process resulted in an improved being were presented, he thought, in abnormally prolonged gestation. Next, the principle of natural selection was suggested simultaneously by Charles R. Darwin and A.R. Wallace (Jour. Linnaean Soc. London, August 1858; preceded by Wallace's paper in Ann. and Mag. Nat. Hist. September 1855), and this was most industriously and ably elaborated and illustrated by Darwin in a subsequent series of publications which have constituted an epoch in the history of scientific thought (Origin of Species, 1859; Variations of Animals and Plants, 1868; The Descent of Man, 1871; Expression of the Emotions, 1872; Insectivorous Plants, 1875; Effects of Cross- and Self-Fertilization, 1876, and; numerous other works and memoirs bearing more or less directly on the question of natural selection). This theory is not to be identified with the broad doctrine of evolution, as is commonly done. It assumes that a method of evolution exists in nature, and undertakes to explain by what means and agencies it is carried on. Recognising the fact that a perpetual struggle exists among individuals for existence, and for most favorable conditions of existence, and that the strongest always succeed the best, while the feeblest tend to perish, the obvious and necessary inference is drawn that the species is perpetuated by its best representatives, and thus undergoes continual improvement, precisely as when man intervenes to improve the breeds of domestic animals. Darwin inclined at first to consider this tendency a full explanation of organic progress, but later he admitted other influences, including, like Lamarck, an inherent nisus towards improvement, and the effects of use and disuse of organs. For an ampler exposition of the doctrine, see the article "Darwinism " in the Encyclopaedia Americana. That a process of natural selection goes on, and that its tendency is what Darwin claims, all must admit. But there is a growing belief that organic advances and relapses require an appeal also, to other conditions, instrumentalities, and causes. For instance, professor Parsons, of Harvard, inclined to regard specific variation as the result of extraordinary births (Amer. Jour. Science, July 1860, II, 30:1), and soon afterwards Richard Owen advanced an almost identical idea (Anat. of Vertebrates, chapter 40; Amer. Jour. Science, II, 47:33). Galton's theory seems to be the same (Hereditary Genius, 1869, pages 363-383). Killiker varied this conception by suggesting heterogeneous generation through agamic and parthenogenic reproduction-a profound misapprehension of proper generation (Ueber die Darwin'sche Schopfungsgeschichte, 1864). Huxley, while accepting Darwinism for what it is worth, has indicated some qualifications and additions (Lay Sermons, Addresses, and Reviews, 1862; On the Origin of Species, 1863; Critiques and Addresses, 1869, etc.). He holds particularly that nature sometimes makes considerable jumps; that the process of natural selection goes on among the molecules of the organism, and that there exists an inherent tendency of organization to vary. The latter point he emphasizes. Alpheus Hyatt, in 1868, pointed out that degradational metamorphoses in the old age of the individual, or the type, could not rationally be referred to natural selection, which acts in the contrary direction. An internal law fixes the duration of the species as of the individual. Specific advance he attributes to habitual acceleration of embryonic development. In the advanced age of species the reverse takes place, and thus the decline of a species reproduces, in inverted order, the succession of types which appeared during the rise of the species (Mem. Boston Soc. Nat. Hist. 1867, 1, part 2; Amer. Naturlalist, June 1870, 4:230; Fossil Cephalopods, Museum Comparatur Zool. Cambridge, 1872). Professor E.D. Cope varied this conception by attributing the recession of organic types to the influence of retarded development (Synopsis of Cyprinidae of.Penn. 1866; "Origin of Genera," in the Proc. Acad. Nat. Science, Phila. October 1868; "The Hypothesis of Evolution," in Lipp. Mag. 1870, and University Series, New Haven, 1873; "The Method of Creation of Organic Types," in the Proc. Acad. Nat. Science, Phila. 1871, and other papers). Probably the suggestions based on rate and duration of embryonic changes are all available. At the same time it is quite conceivable that the principle of natural selection obtains in embryonic life, both in conditions immediately present with the embryo and those external conditions which produce them — the circumstances surrounding the female parent, or even the male. This becomes intelligible on the basis of some such theory as Spencer's "Physiological Units," Darwin's "Pangenesis," Elsberg's "Plastidule Hypothesis" (Proc. Amer. Assoc. 1874, 1876), Hackel's "Perigenesis" (Die Perigenesis, 1876; Die heutige Entwickelungslehre, etc., 1879; Nature, October 4, 1877, and Pop. Scien. Monthly Suppl.) or Brooks' "Law of Heredity " (New York, 1883). Still, it must be admitted that in some cases widely variant forms, as in the Ancon breed of sheep, arise suddenly where, to all appearance, some other condition not yet known determines the divergence. We think also it must be finally admitted that the organism is affected by an implanted destination or law, which bends it constantly towards conformity to the environment, and employs the several agencies mentioned for the accomplishment of this result. In the history of the world the environment has undergone a progressive differentiation and improvement. Organization has advanced correspondingly. When the environment remains persistent, or deteriorates, organic forms persist or even deteriorate to a corresponding extent. If, however, no existing theory of organic evolution proves final, the fact of organic evolution remains highly probable.
V. Limitations of the Doctrine. — We have stated, preliminarily, that the question of evolution is simply one of fact. In ascertaining whether a method of evolution is a fact in the natural world, we are not concerned in anything outside of this simple inquiry. It is of no import whether the result is effectuated by necessity or free-will, by inherent forces, by implanted forces or external forces, by material forces or spiritual forces, by mediate action or immediate action. We are not even concerned in determining what conditions are favorable, what instrumentalities are employed, whether the action is prenatal or postnatal, whether through embryonic development, prolonged, accelerated, or retarded. All these questions are interesting — some of them may be important. The human mind cannot be restrained from investigating them. But it is important to understand clearly that a verdict on any one of these questions does not bear on the antecedent question of fact. If the fact exists, different persons may explain and interpret it differently. The explanation falls within the domain of science; the interpretation touches philosophy and theism. Scientific explanations are already various — each probably partial. Interpretations may be materialistic or spiritualistic — that will depend on the antecedent philosophy of the thinker. They may be theistic or atheistic — that depends on the predisposition of the interpreter. Philosophic and theological opinions must rest on other grounds. The fact of a method of evolution in the world is not responsible for them.
More categorically, we may state:
(1) The fact of evolution implies nothing in respect to causation. It throws no light on secondary cause or first cause. It does not imply the evolution of life from inorganic matter. It knows nothing of beginnings; it discovers only a method of continuance; the beginning may have been a creation by fiat. It knows nothing of the cause or causes of continuance; it may be by immanent divine agency.
(2) There is no assumption of inherent forces or necessary activities, or eternal matter. It is allowable to deny inherent forces and necessary actions, and hold to the creation of matter and force, and even to the identification of natural force with the divine volition.
(3) There is no implication concerning the nature or origin of mind. It may arise with each distinct organism; it may arise only in the human organism.
(4) Nothing is implied concerning the interpretation of the activities going forward in the organism. We are at liberty to affirm that they imply choice selection, intelligence. We are at full liberty to trace intelligence in the methods of the inorganic world, or to affirm that the all-embracing method of evolution is itself the highest possible manifestation of intelligence and unity.
(5) We may also, if we please, maintain that the method of the world and the collocations of the world imply determination and motive. Thus, in brief, the limitations of the essential doctrine of evolution are such that, in spite of the speculative views of some evolutionists, the full acceptance of the doctrine does not conflict with any fundamental conception of Christian theology.
VI. Literature. — Many of the most important original works have been cited in the progress of this article. Some other titles may be added: Spencer, First Principles of Philosophy; Principles of Biology; Gray, Darwiniana (1878); Romanes, The Scientific Evidences of Organic Evolution (1882), Chapman, The Evolution of Life (1873); Semper, Animal Life as Affected by the Natural Conditions of Existence; Die Verwandt schaftsbezie hungen der gegliederten Thiere (1875); Lankester, Degeneration, a Chapter in Darwinism (1880); Lindsay, Mind in the Lower Animals (1879); Seidlitz, Beitrage zur Descendenz-Theorie (1876); Fritz Muller, Fur Darwin (eod.); Zacharias, Zur Entwickelungstheorie (eod.); Jacoby, Etudes sur la Selection dans ses Rapports avec l'Heredite chez l'Homme; Canestrini, Teoria di Darwin Criticamente Exposta (Milan, 1880); Du Prel, Der Kampf ums Dasein am Himmel; Faivre, La Variabilite des Especes (1868); Weismann, Studien zur Descendenz- Theorie (1876); Ribot, Heredity; O. Schmidt, Descent and Darwinism (1875); H. Muller, Die Befruchtung der Blumen durch Insecten (1873; an Engl. translation, 1883); Alpenblumen und ihre Befruchtung durch Insecten (1881); Fechner, Einige Ideen zur Schopfungs- und Entwickelungsgeschichte der Organismen; Mivart, Man and Apes (1874); Bastian, Evolution and the Origin of Life; Roux, Der Kampnf der Theile im Organismus (1881); Gazelles, Outline of the Evolution Philosophy (1875). On the interpretation of evolution: Dreher, Der Darwinismus und seine Stellung in der Philosophie (1877); von Gizycki, Philosophische Consequenzen der Lamarck-Darwin'schen Entwickelungstheorie (1876); R. Schmidt, Die Darwin'schen Theorien und ihre Stellung zur Philosophie, Religion, und Moral (eod.; id. Engl. translation); Henslow, The Theory of the Evolution of Living Things, and the Application of the Principles of Evolution to Religion (1873); Leconte, Religion and Science; Simcox, Natural Law (1877); Wright, Philosophical Discussions, especially pages 97-266; Weismann, Ueber die letzten Ursachen der Transmutationen (1876); Spiller, Die Unrkraft des Weltalls nach ihrem Wesen und Wirken (eod.); Schneider, Der thierische Wille (1880);
Romanes, Animal Intelligence (1883); Mental Evolution in Animals (eod.); Savage, The Religion of Evolution (1877); Beale, Life Theories, their Influence upon Religious Thought (1871); Winchell, The Speculative Consequences of Evolution (1881); Sparks from a Geologist's Hammer, pages 301-385 (eod.), pages 301-385; Beckett, On the Origin of the Laws of Nature. Critical and adverse writings: von Hartmann, Wahrheit und Irrthum im Darwinismus (1875); Wigand, Der Darwinismus u. die Naturforschung Newtons u. Cuviers (1874-77, 3 volumes); Virchow, Die Freiheit der Wissenschaft im modernen Staat (1877; Engl. translation); Semper, Hackelismus in der Zoologie (1876); Michaelis, Anti- Darwinistische Beobachtungen (1877); Mivart, Lessons fron Nature, as Manifested in Mind and Matter (1876); Contemporary Evolution (eod.); Agassiz, Contributions to the Natural History of the U.S. volume 1, Introduction; Amer. Jour. Science, July 1860; Dawson, The Story of the Earth and Man (1873); Hodge, What is Darwinism?; Barrande,Trilobites (1871); Cephalopodes (1877); Brachiopodes (1879). A monthly journal of highest ability, devoted to evolution, is Kosmos, Stuttgart. (A.W.)