Some account of those who advocated or discussed the possibility of evolution having occurred - many writers in the decades just before the publication of the "Origin of Species".
Introduction
His experiences on the H.M.S. Beagle led him to the accumulation of data and study on the problem of the origin of species. In 1858 his work was nearly finished, when the parallel arrival at similar conclusions by Wallace impelled him to publish the present "brief extracts" from his memoir.
Chapter I Variation Under Domestication
Cultivated animals and plants vary greatly, perhaps due to their being raised under varied conditions of life. These conditions appear to act directly on the whole organism or on certain parts alone, or act indirectly by affecting the reproductive system. Habits, the use or disuse of parts, appear to be influential in variation; but the laws of variation are unknown or but dimly understood. Only inherited variations are of importance for us; many variations are inherited, but the laws governing inheritance are for the most part unknown. It is often stated that domestic varieties run wild and revert to their aboriginal stocks; this is far from certain. Domestic varieties generally vary more than wild species. In a domesticated animal or plant the amount of variation may be such that it is frequently claimed that there has been descent from several wild species; this is uncertain. The author studied intensely domestic pigeon races, which vary greatly, and concludes that all come from a single ancestor, the rock pigeon. Some varieties of animals or plants may have arisen suddenly, but most appear to have originated gradually by continued selection on the part of breeders, ancient as well as modern. Sometimes this selection has been essentially unconscious, the breeders not intentionally aiming at the variation eventually produced. Natural variability, the presence of large numbers of individuals, the value of the form to man, facility in preventing of making cross-breeds are all of advantage in selection.
Chapter II. Variation Under Nature
Although less than under domestication, variation does occur abundantly in nature. Individual differences are present and in some instances there is inordinate individual variation. Further, there are frequently well marked varieties and sub-species and forms which are a doubtful species - these appear to be varieties on the way toward becoming good new species. The greatest variation is found among wide-ranging and common species. Species of the larger genera in each country vary more frequently than the species of the smaller genera. Many of the species included within the larger genera resemble varieties in being very closely but unequally related to each other, and in having restricted ranges. Chapter III. Struggle for existence
There is in every species a struggle for existence, due to the fact that even the slowest breeding plant or animal produces far more offspring than can, in the long run, possibly survive (Malthusian doctrine). There are numerous checks to increase, such as food supply, climatic vicissitudes, epidemics, etc. There are complex relationships between various animals and plants in this struggle for existence. The struggle for life is most severe between individuals and varieties of the same species.
Chapter IV. Natural Selection; or the Survival of the Fittest
The principle of selection acting in regard to variation applies in nature as in domestication, here due to the struggle for existence, in the course of which the less favorable variations tend to be eliminated, the more favorable ones tend to survive and produce progeny. The process differs from human selection, which is for visible characters; in nature, invisible, constitutional characters which make for survival and propagation may be acted upon; over long periods of time seemingly trifling characters may be acted on; selection works at all ages and on both sexes. A choice of mates and hence of their representation by offspring may affect the characters of one sex or the other. It seems that close inbreeding lessens vigor and fertility and hence we find in nature that in hermaphroditic animals or plants with both sexual elements there are frequently adaptations allowing and favoring cross-fertilization. Such crossing favors preservation of variability and increases the possibilities for natural selection. Large continental areas are most favorable for the production of new forms of animal life; isolation (as on an island) will tend to "fix" new species. Natural selection will tend to the extinction of species unable to compete with related forms; on the other hand it will tend to increasingly diversify the members of a successful species into forms - varieties and eventually separate species - which can enter into modes of life different from one another. The result of this diversification (and, parallel to this, extinction) is the "tree of life", also expressed in systems of classification. Natural selection tends to "improve" each creature in relation to its conditions of existence (although it is difficult to define "improvement" or "advance" in nature satisfactorily). However many simple organisms, already well fitted for survival, persist little changed. Two forms from distinct genera each becoming adapted to an identical new mode of life may tend to resemble each other in many features (convergence) but never become identical.
Chapter V. Laws of Variation
Variations are not due to chance; but we are in great measure ignorant of the laws under which they occur. Variations are seen under changed conditions of life; but it is difficult to say whether they are directly produced by these changed conditions or by the work of natural selection. The use or disuse of organs (Lamarck!) may be a factor, and climate may be effective; but again it is difficult to say whether the effect is direct or related to natural selection. The occurrence and accumulation of varieties in one organ may be accompanied by variation in other organs; the reasons are often obsc~re. It sometimes appears that an excess growth of one part is "compensated" by reduction of another region or organ. Multiple, rudimentary and lowlyorganized structures are variable; so also is any part developed (as compared with allied species) in an extraordinary manner. Characters peculiar to a species are more variable than those common to all members of a genus. Secondary sexual characters are variable. Distinct species of a genus present analogous variations (parallelism). A variation may be a reversion to a long-lost character.
Chapter VI. Difficulties of the Theory
Some difficulties are serious, but not fatal; the greater number are only apparent. They may be classed under four heads, the first two of which are treated in this chapter.
One: why, if species have descended from other species by fine gradations, do we not everywhere see innumerable transitional forms?
In a well-stocked country a new form will tend to replace and exterminate its own less improved parent form and other less favored forms with which it competes. Closely allied species now living on a continuous area without transitional types may have originated in originally separate regions; or if originated in one area, any transitional types early present in an intermediate zone would tend to be crowded out by their more successful neighbors.
Objection two: is it possible that an animal having, for instance, the structure and habits of a bat, could have been formed by the gradual modification of some other animal with widely different habits and structure?
This implies that it is impossible to conceive of a series of viable and successful forms with structures intermediate between those of a "normal" ancestor and the specialized end type. But structural gradations of this sort are often seen today; for example, various squirrels and the "flying lemur" among mammals bridge much of the gap toward the bats. It seems hard to imagine the gradual development of such a complex structure as the human eye, but in nature are found light-perceiving organs of all grades of structure from a simple sensitive pigment spot up. Living examples also show the possibilities of a gradual evolutionary transition in function of a structure without loss of viability. There are numerous cases in which a gradual transition appears difficult; organs which seem to us to be of too little importance to be affected by natural selection show changes and many of the beauties of nature lie in features which appear to be non-utilitarian and hence not susceptible to natural selection. Most such cases, however, do not appear to offer strong arguments against evolution by selection. It may be noted that natural selection cannot produce any modification in a species exclusively for the good of another species, although there may be mutually advantageous adaptations; it can, however, produce modifications harmful to other species.
Chapter VII. Miscellaneous Objections to the Theory of Natural Selection
It is argued that longevity is advantageous so that if the theory of natural selection is correct, all descendants should live longer than their progenitors. But this is a false premise; for example, annuals may have advantages over perennials in cold climates. It is argued that distinct species differ in many characters, not single ones; how could it be that many parts could be modified by selection at once? But a long time may have been taken to evolve a species, and the changes may have taken place seriatim. It is argued that many characters of no obvious use have been modified and hence some other force than natural selection operates. But it is difficult to be sure that any character is non-utile; and one part may be modified in correlation with other (useful) characters.
Mivart has brought forward many objections, which are discussed. He believes that natural selection is incompetent to account for the incipient stages of useful structures. Various cases cited by him - such as the giraffe's neck, whale's whalebone, the beak of a shoveller duck, the flatfish, prehensile tails, mammary glands, etc. - are discussed, and it is concluded that, in general, incipient stages can be utilized and the objection invalid. Mivart is inclined to believe that new species appear suddenly. But although marked structural changes are occasionally seen to crop out suddenly in an individual in domestic animals, the chances in nature of such individuals surviving and transmitting such modifications in unaltered form are small.
Chapter VIII. Instinct
Returning to the major objections. Three: how can complex instincts be acquired and modified through natural selection?
It can be shown that instincts vary under a state of nature and hence natural selection might act by the cumulation of slight variations in instincts just as on slight variations in structure. Under domestication it is clear that instincts may be acquired or lost, or modified by selection. On examination of special instincts - such as the nesting habits of the cuckoos, the slave-making habits of certain ants, and the cell-making of the bee - they are found not merely in a perfected state, but in less developed form in related types, indicating here (as in the case of structural features) the utility of intermediate stages and the possibility of the gradual development of complicated instincts through natural selection. It is objected that for instincts requiring associated structural changes, it is difficult to imagine that both appeared simultaneously. But if, as seems reasonable, the development were gradual, the argument loses force. It is further pointed out that selection cannot act on neuters (as in bees, ants) since they have no progeny. But just as characters of one sex or the other are carried in both (for example, a bull as a producer of good milk cows or fat beef cattle), so the fertile members of a colony may, to their advantage, carry the potentialities of producing successive generations of neuters.
Chapter IX. Hybridism
Four: how can we account for species, when crossed, being sterile or producing sterile offspring, whereas, when varieties are crossed, their fertility is unimpaired?
This question implies that species were originally endowed, on their creation, with sterility, in order to prevent their "confusion". Note that two subjects are involved: sterility of first crossing, so that no hybrid is produced, and sterility of the hybrids if they are produced. It is far from universally true that species are sterile when crossed; there are various degrees of sterility, and arguments for absolute sterility between species are sometimes rendered effective only by 5. considering two forms thought to be distinct species as mere varieties if they do cross. In cases where hybrids are obtained between species they range from perfectly sterile to perfectly fertile. Sterility appears to indicate unknown differences in the reproductive systems or elements of the forms concerned. It has not been acquired through natural selection, and in the case of hybrid sterility, it at least appears to depend on their whole organization having been disturbed by being compounded from two distinct forms. The curious fact is noted (but with the explanation obscure) that in a number of plants, two or three races are found which differ in their reproductive organs and are sterile unless fertilized by another race. Although it is generally assumed that varieties when crossed are fertile, and their offspring as well, this is far from universal. There is no notable difference in the nature of hybrids between distinct species, and hybrids ("mongrels") between varieties. In sum, sterility is not opposed to a belief that distinct species originated as varieties of a single species.
Chapter X. On the Imperfection of the Geologic Record
Varieties intermediate between species are not ordinarily found at the present day. This is to be expected, since in general one living form is not descended from another similar living type, but rather the two have descended from a common ancestor now extinct. It might be objected that geologic time was too short for the necessary changes to take place. But observation will impress upon one the fact that geologic changes take place very slowly and that hence geologic time must have been very long to effect the notable changes that have occurred. In England, for example, there has accumulated a thickness of sediments of nearly fourteen miles, and the process of accumulation was not a continuous one. Our paleontological collections are very imperfect and do not show all connecting forms. But the chances of preservation of many types are relatively poor; many areas of the earth are unexplored. Further, many forms presumably lived at times and in areas where no subsidence of land occurred and hence no sediments were formed. In various fossiliferous formations which have been studied we fail to find gradual evolutionary changes between forms in their upper and lower parts. But in such cases the actual time of deposition of the formation may have been too short for marked change to have occurred, or the species studied may have been stable during this period of time. Where it is found that species in lower and upper parts are different but without gradations, there may have been a time gap in the deposition of the formation.
It is urged that the sudden, full-fledged appearance of groups in a given formation is a decisive argument against evolution. But such seemingly sudden appearance may be due to gaps in the preserved record or to imperfect knowledge of available fossils; in recent years several supposed examples of sudden appearance have been nullified by the discovery of earlier transitional forms. The best negative argument that remains is the sudden appearance of abundant organisms in the Cambrian and the almost complete absence of fossils in older strata.
Chapter XI. On the Geologic Succession of Organic Bein~s
Our knowledge of this succession is in general in agreement with the theory of evolution; related species succeed each other in successive formations, although at variable rates of change; 6.
species once lost do not reappear. This holds true for larger groups as well as for species. Extinction appears in general to have been gradual and due to failure to succeed in competition with other forms. In marine deposits the animals found change almost simultaneously the world over from formation to formation, this due presumably to the development through natural selection of new forms which migrate widely and exterminate old-fashioned types. Despite imperfections in the record, many fossil animals are seen to be intermediate between living types, and the fauna of any given period is intermediate in nature between those which precede and follow. In general "higher" (i.e. more specialized, better adapted) animals are present in later periods as compared with earlier ones. In history from the later Tertiary on to Recent times, we find in each major area of the world a uniformity of animal types throughout, the forms of each area evolving in a fashion parallel to those of the other regions.
The last few pages of the chapter review the content of chapters X - XI.
Chapter XII. Geographical Distribution
The faunas of the continental areas differ from one another; within each, members of the types present are found in a variety of climatic zones. This distribution is due to the descent with variation of the modern forms in each area from stocks earlier present; barriers prevent the migration of newly evolved forms from one region to another. Presumably each new species came into existence in a single area and spread only as far as physical barriers permitted.
There are, however, examples of discontinuous distribution which require study and explanation. It is improbable that the topography of the earth has undergone major changes in the way of major elevations and submergences in recent geologic periods; nevertheless some connections between continents and islands not now present may have existed in the past and allowed migration. Many land plants may cross ocean barriers in the form of seeds; birds may transport small animals and plant materials on muddy feet, etc. The glacial period just behind us may account for many examples of modern discontinuous distributions. Two temperate or mountainous areas now separated by warmer regions with different floras or faunas may have been connected by a continuous band of colder climate in the Pleistocene. In the warmer interglacial phases of the Pleistocene, temperate climates may have extended farther north than now and thus permitted, for example, freer exchange between Eurasia and North Rmerica. Differences in faunas and floras of regions now broken in continuity may be due to different conditions and corresponding selection since their separation. The presence of similar temperate climate plants in northern and southern hemispheres suggests that at some time the now-tropical intervening areas were temperate, allowing free spread of such forms. Possibly an alternation of northern and southern glaciations may have occurred and aided exchange.
Chapter XIII. Geographic Distribution Continued
Since river systems are separated from one another by land, exchange or migration of fresh-water life would appear impossible. But various "accidents" may occur; transportation by birds, whirlwinds, capture of a tributary by another river system, etc. It is difficult to account for the faunas and floras of oceanic islands. The faunas are peculiar in that there are few forms and most of them endemic (i.e., peculiar to the island) in many groups. Certain groups - notably amphibians - are never present, related to the fact that salt water is fatal to frogs, toads and salamanders. These peculiarities argue against special creation. Terrestrial mammals are never found on islands more than 300 miles from a continent. Islands separated by shallow water reasonably resemble the continental area more than islands more deeply separated. Oceanic islands (as the Galapagos) have faunas most closely related to those of the nearest continent - from which the forms present have presumably migrated. In a group of oceanic islands (as the Galapagos) the barriers between islands tend to the development of a number of distinct species of the few stocks which reached the group as a whole.
The last few pages give a summary of chapters XII - XIII.
Chapter XIV. Mutual Affinities of Organic Beings : Morphology - Embryology - Rudimentary Organs
The classification of animals - into phyla, classes, orders, families, genera, species - is not an arbitrary scheme for pigeonholing animals or plants but in general agrees well with the supposition of descent with modification: the various species of a genus are presumably derived from a common ancestor, the genera of a family ditto. Of body structures, similarities in those less susceptible to natural selection give the best clues as to actual relationship - this in contrast to analogous structures, adaptive in nature which may develop independently in two or more unrelated groups. Evolutionary relationship has been widely used even if unconsciously, in much scientific work on plant and animal classification. The study of structure (morphology) is based on the fact that each major group of animals has a "type" structural plan, and despite variations, the structures present can be recognized as the same - i.e., homologous. Thus the human arm, bat wing, etc., are homologous, but have developed variations fitting varied needs.
The study of development - embryology - gives much insight into the mode of evolution of different animal types. Members of various animal groups are much more similar in early embryonic stages than in later stages or the adult. But the features of resemblance are those which have no adaptive value, and embryos as well as adults may differ in adaptive features.
Organs well developed and useful in some forms are frequently found in a rudimentary, atrophied or aborted condition in other, related types. In some instances these may represent a nascent condition, in progress toward further development; but in general they appear clearly to be reduced and useless remnants, modified in relation to natural selection operating in negative fashion.
Chapter XV. Recapitulation and Conclusion
There are many and serious objections which may be advanced against the theory of descent with modification, but these appear overbalanced by the positive evidence. It is objected that although varieties of animals and plants are fertile when crossed and thus demonstrate relationship, species are generally infertile, and hence argument from the one situation does not apply to the other. But the distinction is not clean-cut, and there are many complex and unsolved problems concerning breeding. Problems are also met in geographical distribution, but the influence of glacial conditions, etc. explains much here. The absence of existing intermediates between species is presumably due to their failure to survive in competition with their descendants; the paucity of our knowledge of fossil intermediates can be attributed to our paucity of knowledge of extinct forms.
On the opposite side, the variability seen and selected in domesticated forms can have been utilized similarly by nature in the case of wild forms for selection and the survival of the fittest.
There is no clear line of demarcation today in many cases between varieties and species, and this agrees with the supposition that species now distinct originated as simple varieties of a single species. Dominant species tend presumably, to spread and differentiate into a series of species of a dominant genus and on to higher categories; this reasonably explaining how it is that a satisfactory classification of animals and plants is possible. Numerous seemingly anomalous features in the structure, distribution, etc. of animals and plants are difficult to understand under a theory of special creation, but fit logically into a theory of evolution. Although there are many problems regarding geographical and geological distribution, the facts, on the whole, much more strongly support the idea of evolution than special creation. Classification, morphology and embryology all furnish more support to evolutionary theory than problems opposing it.
To conclude, the facts and considerations set forth have convinced the author that species have been modified, chiefly but not entirely, by natural selection, through a long course of descent. Such news need not shock religious feelings. Disbelief in the mutability of species by scientists in the past was partly due to lack of realization of the length of geologic time, but chiefly because we are slow in admitting great changes of which we do not see the steps and because of the difficulty of changing basic concepts which one has long accepted. Some naturalists have recently advocated the view that many reputed species within genera are not "real" species and have originated by modification from a common parent, but that others are true species independently created. This intermediate stand is inconsistent.
How far can the doctrine of the origin of species through evolution be applied to the origin of higher categories of animals and plants? Have all living things descended from some one prototype? The answer is difficult to arrive at, although the evidence tends toward a belief in the ultimate natural relationship of all plants and animals.
Acceptance of the theory of evolution will revolutionize many fields of natural history. Students of classification, for example, will look at problems of species, etc. in a new light; impetus will be given to a study of the causes of variation. The study of geology and especially of fossils will be seen in new aspects. The evolutionary view of life and its history seems to the author one of more grandeur than the one based on special creation.