Origin Of Species Of Plants And Animals
( Originally Published 1909 )
THERE are in the museums of the world at the present time representatives of several hundred thousand probably more than a million kinds or species of plants and animals, and thousands of new species are being discovered and named each year. A single group of insects has classified under it more species than there are stars to be seen in the heavens with the unaided eye on a clear night. Aristotle knew about five hundred kinds of animals, but a single new botanical or zoological work may now describe more than that number of species new to the records of Science. Linnaeus in 1758 published the tenth edition of his 'Systema Naturae' and named about four thousand animals, and every year since 1864 the 'Zoological Record' has listed three or four times this number of species previously undescribed, yet now, as in Linnaeus's time, it is certain that not half of the number of species of animal organisms is yet known. The six hundred thousand, more or less, on the registers of Science today are certainly far less than half of the millions which actually exist.
In botany the same conditions are to be found. There are fewer known species of plants than animals by half, and they are more easily preserved and handled, while the work of collection and investigation proceeds on a scale even more extensive, yet it would be a bold statement to say that today half the species of plants that exist are known.
All this refers to the forms now living, without reference to the host which composes their long ancestry, extending backward toward the dawn of creation. The species have come down through the geological ages, changing in form and function to meet the varying needs of changing environment. This enumeration takes no ac-count of the still vaster myriads of forms almost endlessly varied, which have perished utterly in the pressure of environment, leaving no trace in the line of descent.
It is evident that variety in life is a factor in the history of the globe, that it may be expressed in terms of number of species, but that the actual range of variation is far greater than the number of species, and that if causes are to be judged by range of effects, in the origin of species must be found the operation of world-wide forces, the cooperation of great influences, far-reaching in time and space, as broad as the surface of the globe and as enduring as its life.
The cause of this amazing variety in life, and the manner of accounting for origin of species are questions necessarily asked when attention is called to the existence of such vast numbers of species of plants and animals. In earlier chapters the theories of the origin of life have been described and proof that present living matter has descended from preexisting living matter has been outlined. It is, in fact, accepted in modern science that there has been a continuity of descent from the first living matter to that of the present time. But the form or species in which life first originated is another subject, and almost as great. Such a subject must deal with the questions as to whether life first appeared in each species of animal or plant separately, or whether it began in simple protoplasm from which have been evolved the almost numberless species known today.
It is quite clear that there are only two hypotheses in the field whereby it is possible so much as to suggest an explanation of the origin of species. Either all the species of plants and animals must have been supernaturally created, or else they must have been naturally evolved. There is no third hypothesis possible; for no one can rationally suggest that species have been eternal.
It should be noticed that whichever of the two rival theories is entertained, the concern is not with any question touching the origin of life, but only with the origin of particular forms of life that is to say, with the origin of species. The theory of descent starts from life as a 'datum' already granted. How life itself came to be, the theory of descent, as such, is not concerned to show. Therefore, in the present discussion, the existence of life is taken as a fact which does not fall within its range of debate.
The history of biology in the nineteenth century will be famous because of the discussion of these two hypotheses which attempt to account for the existence of the innumerable species of living things which inhabit the earth : the theory of creation and the theory of evolution. According to the theory of creation, all the individuals of every species existing at the present day the tens of thousands of dogs, oak trees, amoebae, and what not are derived by a natural process of descent from a single individual, or from a pair of individuals in each case precisely resembling, in all essential respects, their existing descendants which came into existence by a process outside the ordinary course of nature, and known as Creation.
According to the rival theory that of Descent or Organic Evolution every species existing at the present day is derived by a natural process of descent from some other species which lived in a former period of the world's history. If from generation to generation the individuals of any existing species could be traced back, on this hypothesis, their characters would be found gradually to change, until finally a period was reached at which the differences were so considerable as to necessitate the placing of the ancestral forms in a different species from their descendants at the present day. And in the same way if the species of any one genus could be traced back they would gradually approach one another in structure until they finally converged in a single species, differing from those now existing but standing to all in a true parental relation.
In regard to the present standing of these two theories it should be stated that the theory of descent is now generally accepted by men of science and the theory of special creation rejected. In fact no great naturalist since Agassiz has attacked the general theory, tho some have debated many of its minor details. As David Starr Jordan has said : "There is to-day no doubt in our minds of the truth, the actuality, of descent. It is not the theory of descent : it is the fact, the law, of descent, of which we talk and write. Organisms are blood-related; they are transformed, descended from one another."
At this point it will clarify some later considerations if it is emphasized that there is a great distinction to be drawn between the fact of evolution and the manner of it, or between the evidence of evolution as having taken place somehow, and the evidence of the causes which have been concerned in the process. This most important distinction is frequently disregarded by popular writers on evolution, and, therefore, in order to mark it as strongly as possible, it will be necessary to effect a complete separation between the evidence of evolution as a fact, and the evidence as to its method. In other words, not until the evidence of organic evolution as a process, which somehow or another has taken place, has been fully considered, is it advisable to consider how it has taken place, or the causes which Darwin and others have suggested as having probably been concerned in this process.
First there is to be considered, therefore, the evidences pointing to the fact of organic descent (evolution) of species of animals and plants and later there will be outlined the trend of the enormous amount of investigation which has been and now more actively than ever is being done toward the solution of the problems concerned with the causes or the factors of evolution. In passing, it must be noted that while Darwin wrote both concerning the evidences of evolution and the manner or causes of evolution, it is not correct, as many authors assume, to regard the 'Darwinian Theory' or 'Darwinism' as synonymous with the theory of descent or evolution. Rather should the terms 'Darwinism' and Darwinian Theory' be applied to the theory of natural selection, Darwin's great explanation of the cause of the origin of species by evolution or descent.
The late Professor Cope, of Philadelphia, defines evolution in the broadest sense, including both organic and inorganic evolution, as follows: "The doctrine of evolution may be defined as the teaching which holds that creation has been and is accomplished by the agency of the energies which are intrinsic in the evolving matter, and without the interference of agencies which are external to it. It holds this to be true of the combinations and forms of inorganic nature, and of those of organic nature as well. Whether the intrinsic energies which accomplish evolution be forms of radiant or other energy only, acting inversely as the square of the distance, and without consciousness, or whether they be energies whose direction is affected by the presence of consciousness, the energy is a property of the physical basis of tridimensional matter, and is not outside of it."
But a distinction must be made between organic and inorganic evolution. Professors Jordan and Kellogg have stated this most clearly in 'Evolution and Animal Life': "Biological evolution and cosmic evolution are not the same. From the biological side a certain objection must be made to this philosophical theory of universal or cosmic evolution. In organic and inorganic evolution there is much in common, so far as conditions and results are concerned; but these likenesses belong to the realm of analogy, not of homology. They are not true identities because not arising from like causes. The evolution of the face of the earth forces parallel changes in organic life, but the causes of change in the two cases are in no respect the same. The forces or processes by which mountains are built or continents established have no homology with the forces or processes which transformed the progeny of reptiles into mammals or birds.
"Tendencies in organic development are not mystic purposes, but actual functions of actual organs. Tendencies in inorganic nature are due to the interrelations of mass and force, whatever may be the final meanings attached to these terms or to the terms matter and energy. It is not clear that science has been really, advanced through the conception of the essential unity of organic evolution and cosmic evolution. The relatively little the two groups of processes have in common has been overemphasized as compared with their fundamental differences.
"The laws which govern living matter are in a large extent peculiar to the process of living. Processes which are functions of organs cannot exist where there are no organs. . The traits of protoplasm are shown only in the presence of protoplasm. or this reason we may well separate the evolution of astronomy, the evolution of dynamic geology and of physical geography, as well as the purely hypothetical evolution of chemistry, from the observed phenomena of the evolution of life.
"To regard cosmic evolution and organic evolution as identical or as phases of one process is to obscure facts by verbiage. There are essential elements in each not shared by the other or which are at least not identical when measured in terms of human experience. It is not clear that any force whatever or any sequence of events in the evolution of life is homologous with any force or sequence in the evolution of stars and planets. The unity of forces may be a philosophical necessity. A philosophical necessity in logic is unknown to science. We can recognise no logical necessity until we are in possession of all the facts. No ultimate fact is yet known to science.
"For reasons indicated above the term 'evolution' is not wholly acceptable as the name of a 'branch of science. The term 'bionomics' is a better designation of the changing of organisms influenced through unchanging laws. It is a name broader and more definite than the term organic evolution, it is more euphonious than any phrase meaning life adaptation, it involves and suggests no theory as to the origin of the phenomena it describes."
The theory of descent of plants and animals is defined by the same authors as the "belief that organs and species as we know them are derived from other and often simpler forms by processes of divergence and adaptation. According to this theory all forms of life now existing, or that have existed on the earth, have risen from other forms of life which have previously lived in turn. All characters and attributes of species and groups have developed with changing conditions of life. The homologies among animals are the results of common descent. The differences are due to various influences, one of the leading forces among these being competition in the struggle for existence between individuals and between species, where-by those best adapted to their surroundings live and pro-duce their kind."
This theory is now the central axis of all biological investigation in all its branches, from ethics to histology, from anthropology to bacteriology. In the light of this theory every peculiarity of structure, every character or quality of individual or species, has a meaning and a cause. It is the work of the investigator to find this meaning as well as to record the fact. "One of the noblest lessons left to the world by Darwin," says Frank Cramer, "is this, which to him amounted to a profound, almost religious conviction, that every fact in nature, no matter how insignificant, every stripe of color, every tint of flowers, the length of an orchid's nectary, unusual height in a plant, all the infinite variety of apparently insignificant things, is full of significance. For him it was an historical record, the revelation of a cause, the lurking place of a principle." It is therefore a fundamental principle of the science of bionomics that every structure and every function of today finds its meaning in some condition or in some event of the past.
Darwin's own view of the doctrine of descent is clearly set forth in the following passages from the 'Origin of Species' : "Authors of the highest eminence seem to be fully satisfied with the view that each species has been independently created. To my mind it accords better with what we know of the laws impressed on matter by the Creator, that the production and extinction of the past and present inhabitants of the world should have been due to secondary causes, like those determining the birth and death of the individual.
"When I view all beings not as special creations, but as the lineal descendants of some few beings which lived long before the first bed of the Cambrian system was deposited, they seem to me to become ennobled. Judging from the past, we may safely infer that not one living species will transmit its unaltered likeness to a distant futurity. And of the species now living very few will transmit progeny of any kind to a far distant futurity ; for the manner in which all organic beings are grouped, shows that the greater number of species in each genus, and all the species in many genera, have left no descendants, but have become utterly extinct. We can so far take a prophetic glance into futurity as to foretell that it will be the common and widely spread species, belonging to the larger and dominant groups within each class, which will ultimately prevail and procreate new and dominant species. As all the living forms of life are the lineal descendants of those which lived long before the Cambrian epoch, we may feel certain that the ordinary succession by generation has never once been broken, and that no cataclysm has desolated the whole world.
"It is interesting to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent upon each other in so complex a manner, have all been produced by laws acting around us. There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, while this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been and are being evolved."
What organic evolution does not mean is a topic which deserves some attention before discussing the evidences for evolution. President Jordan has written in 'Footnotes to Evolution' a strong statement concerning this, wherein he says : "Evolution is not a theory that 'man is a developed monkey.' The question of the immediate origin of man is not the central or overshadowing question of evolution. This question offers no special difficulties in theory, altho the materials for exact knowledge are in many directions incomplete. Homologies more perfect than those connecting man with the great group of monkeys could not exist. These imply the blood relation-ship of the human race with the great host of apes and monkeys. As to this there can be no shadow of a doubt, and, as similar homologies connect man with all members of the group of mammals, similar blood relationship must exist; and homologies less close but equally unmistakable connect all backboned animals one with another, and the lowest backboned types are closely joined to wormlike forms not usually classed as vertebrates.
"It is perfectly true that in the higher or anthropoid apes the relations with man are extremely intimate ; but man is not simply 'a developed ape.' Apes and men have diverged from the same primitive stock apelike, manlike, but not exactly the one nor the other. No apes nor monkeys now extant could apparently have been ancestors of primitive man. None can ever 'develop' into man. As man changes and diverges, race from race, so do they. The influence of effort, the influence of surroundings, the influence of the sifting process of natural selection, each acts upon them as it acts upon man.
"The process of evolution is not progress, but better adaptation to conditions of life. As man becomes fitted for social and civic life, so does the ape become fitted for life in the tree-tops. The movement of monkeys is toward simianity, not humanity. The movement of cat life is toward felinity, that of the dog races toward caninity. Each step in evolution upward or downward, whatever it may be, carries each species or type farther from the primitive stock. These steps are never retraced. For an ape to become a man he must go back to the simple characters of the simple common type from which both have sprung. These characters are shown in the ape baby and in the human embryo in its corresponding stages, for ancestral traits lost in the adult are evident in the young. This persistence comes through the operation of the great force of cell memory which we call heredity.
"The evidence of biology points to the descent of all mammals, of all vertebrates, of all animals, of all organic beings, from a common stock. Of all the races of animals the anthropoid apes are nearest man. Their divergence from the same stock must be comparatively recent. Man is the nomadic, the apes are the arboreal, branch of the same great family.
"Evolution does not teach that all or any living forms are tending toward humanity. It does not teach, as in Bishop Wilberforce's burlesque, 'that every favorable variety of the turnip is tending to become man.' It is not true that evolutionists expect to find, as Dr. Seelye has affirmed, 'the growth of the highest alga into a zoophyte, a phenomenon for which sharp eyes have sought, and which is not only natural but inevitable on the Darwinian hypothesis, and whose discovery would make the fame of any observer.'
"It is no wonder that a clear thinker should have rejected 'the Darwinian hypothesis' when stated in such terms as this. The line of junction in evolution is always at the bottom. It is the lowest mammals which approach the lowest reptiles; it is the lower types of plants which approach the lower types of animals ; it would be the lowest Alga, to use Dr. Seelye's illustration, which would be transmutable into the lowest zoophyte ; it is the unspecialized, undifferentiated type from which branches diverge in different ways. Humanity is not the 'goal of evolution,' not even that of human evolution. There will be no second creation of man, except from man's own loins. There will not be a second Anglo-Saxon race unless it has the old Anglo-Saxon blood in its veins.
"Adaptation by divergence for the most part of slow stages is the movement of evolution. While occasional leaps or sudden changes occur in the process, they are by no means the rule. In most cases of 'saltatory evolution' the suddenness is in appearance only. It comes from our inability to trace the intermediate stages. When an epoch-making character is acquired, as the wings of a bird or the brain of man, the process of readjustment of other characters goes on with greatly increased rapidity. But this rapidity of evolution is along the same lines as the slower processes. Radical changes from generation to generation never occur.
"We do not expect to find birds arising from a 'flying fish in the air, whose scales are disparting into feathers.' A flying-fish is no more of the nature of a bird than any other fish is. A cow will never give birth to a horse, nor a horse to a cow. The slow operation of existing causes is the central fact of organic evolution, as it is of the evolution of mountains and valleys. Seasons change as the relations which produce them change. But midsummer never gives way to midwinter in an instant. Nor does the child in an instant become a man, tho in some periods of growth epoch marking causes may make development more rapid. Life is conservative. The law of heredity is the expression of its conservatism. Life changes slowly, but it must constantly change, and all change is, by necessity, divergence.
"There is in Nature no single 'law of progress,' nor is progress in any group a necessity regardless of conditions. That which we call progress rests simply on the survival of the better adapted, their survival being accompanied by their reproduction. Those that live repeat themselves. The 'innate tendency toward progression' of the early evolutionists is a philosophic myth. Progress and degeneration are alike the resultants of the various forces at work from generation to generation on and within a race or species. The same forces which bring progress to a group under one set of conditions will bring degradation under another. In their essence the factors of evolution are no more laws of progress than the attraction of gravitation is. Cosmic order comes from gravitation. Organic order comes from the factors of evolution. Evolution is simply orderly change.
"Nor is evolution identical with the notion of spontaneous generation. There is no necessary connection between the one theory and the other. If there is now spontaneous generation of protoplasm, it cannot take the form of any creature we know. An organism fresh from the mint of creation would be too small for us to see with any micro-scope. It would be too simple for us to trace by any instrumentality now in our possession. It would contain but a few molecules, and a molecule in a drop of water is as small as an orange beside the sun. Such a race of creatures, spontaneously generated, without concessions to environment, would grow hoary with the centuries before it came to our notice. Its descendants would have belonged for ages to the unnumbered hosts of microbes before we should be aware of its creation.
"Evolution is not a creed or a body of doctrine to be believed on authority. There is no saving grace in being an evolutionist. There are many who take this name and have no interest in finding out what it means or in making any application of its principles to the affairs of life. For one who cares not to master its ideas there is no power in the word. Evolution is not a panacea or a medicine to be applied to social or personal ills. It is simply an expression of the teaching of enlightened common sense as to the order of changes in life. If its principles are mastered, a knowledge of evolution is an aid in the conduct of life, as knowledge of gravitation is essential in the building of machinery.
"There is nothing 'occult' in the science of evolution. It is not the product of philosophic meditation or of speculative philosophy. It is based on hard facts, and with hard facts it must deal. It seems to me that it is not true that 'Evolution is a new religion, the religion of the future.' There are many definitions of religion, but evolution does not fit any of them. It is no more a religion than gravitation is.
"One may imagine that some enthusiastic follower of Newton may, for the first time, have seen the majestic order of the solar system, may have felt how futile was the old notion of guiding angels, one for each planet, to hold it up in space. He may have received his first clear vision of the simple relations of the planets, each forever falling toward the sun and toward one another, each one by the same force forever preserved from collision. Such a man might have exclaimed, 'Great is gravitation; it is the new religion, the religion of the future!' In such manner, men trained in dead traditions, once brought to a clear insight of the noble simplicity and adequacy of the theory of evolution, may have exclaimed, 'Great is evolution ; it is the new religion, the religion of the future!' But evolution is religion in the same sense that every truth of the physical universe must be religion. That which is true is the truest thing in the world, and the recognition of the infinite soundness at the heart of the universe is an inseparable part of any worthy religion."
The evidences which have convinced men of science that the origin of the various species of plants and animals through descent is a fact are many and are drawn from various sources ; however, no more than an outline of those facts which help best to an understanding of the doctrine can be given here. The same facts were once used in debate concerning the probabilities of the truth or untruth of the doctrine of descent, but today the biologist feels it unnecessary to stand as an advocate arguing for belief in evolution. As the days have long passed when the shape of the earth, or the behavior of the members of the solar system, was a fit subject for debate, so the days are now closed when the truth or falsity of the law of organic descent is a debatable thesis. That the earth is subspherical, that the planets revolve about the sun, and that species of organisms descend from other species, are now to be considered matters uncontrovertible.
Darwin wrote for a generation which had not accepted evolution, and which poured contempt on those who upheld the derivation of species from species by any natural law of descent. He did his work so well that "descent with modification" is now universally accepted as the order of nature in the organic world; and the rising generation of naturalists can hardly realize the novelty of this idea, or that their fathers considered it a scientific heresy to be condemned rather than seriously discussed.