On the Tendency of Varieties to Depart

Indefinitely From the Original Type

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    On the day of 1858 Mar 09 Alfred Russel Wallace (1823 Jan 08 -- 1913 Nov 07) sent from the island of Ternate in Indonesia a paper with the above title. That little paper (for full text see the Appendix) has gained fame, not so much because of its content, but because it goaded Charles Darwin into publishing "On the Origin of Species". Wallace had sent the paper to Darwin with a request that he pass it on to Charles Lyell for presentation to the Linnean Society. Lyell and Joseph Hooker had that paper read to the meeting of the society on the evening of 1858 July 01, along with some excerpts from two of Darwin's unpublished writings. It was subsequently published in Volume 3 of the Proceedings of the Linnean Society (pages 53 - 62). Thus Wallace kicked off a major revolution in scientific thought.

    Although born in Wales, Wallace was born of English parents and always referred to himself as an Englishman. Biologists regard him as the founder of evolutionary biogeography, the study of what species live where and why, the study that led Wallace to his version of the theory of evolution by means of natural selection. Born poor, he worked from age 14 and was largely self-taught. Like Darwin, he had read Malthus' "Essay on the Principle of Population". He also read Darwin's "Journal" from his voyage on the Beagle and "Vestiges of the Natural History of Creation" (1844), which offered an evolutionary view of nature. With his friend Henry Walter Bates (1825 Feb 08 B 1892 Feb 16), he went to the Amazon (1848 - 1852), paying his expenses by capturing and selling natural history specimens to museums and private collectors; those specimens consisted mostly of butterflies, beetles, and birds. Samuel Stevens of Bloomsbury Street in London served the men as their agent, thereby giving Wallace a connection to both buyers and scientists. Wallace then went to the Malay Archipelago in 1854, arriving in Singapore in April and spending 8 years in Indonesia. Because of his collecting activities, Wallace knew that each species displays considerable variation among its individuals. That observation provided the primary insight into the mechanism of evolution. Darwin only became aware of the degree of variation in the wild when he classified barnacles, but Wallace discerned that variation in his role as a commercial collector. He also noticed that similar species exist as neighbors separated by natural features, such as the rivers that divide the different species of monkeys of the Amazon rain forest.

    Where Charles Darwin had begun his career as a naturalist believing in the Creationism that most people believed in at the time, Wallace already believed in the transmutation of species when he began his career as a traveling naturalist. He also drew considerable inspiration from "Vestiges of the Natural History of Creation", a book of popular science published anonymously by Robert Chambers (1802 Jul 10 -- 1871 Mar 17) in 1844: in that book Chambers advocated an evolutionary origin for the solar system, Earth, and all living things. Indeed, in 1845 Wallace wrote a letter to Henry Bates, in which he said,

    "I have a rather more favourable opinion of the 'Vestiges' than you appear to have. I do not consider it a hasty generalization, but rather as an ingenious hypothesis strongly supported by some striking facts and analogies, but which remains to be proven by more facts and the additional light which more research may throw upon the problem. It furnishes a subject for every student of nature to attend to; every fact he observes will make either for or against it, and it thus serves both as an incitement to the collection of facts, and an object to which they can be applied when collected."

    Thus incited, Wallace went to South America in 1848, to the basin of the Amazon River, and remained there four years to collect facts as well as specimens to sell back to England. He had devised the hypothesis that in a natural world shaped by evolution he would find closely related species inhabiting neighboring territories and had gone seeking evidence to support or refute that hypothesis. In 1853 he published his paper "On the Monkeys of the Amazon", in which paper he revealed his finding that barriers to movement across the land, such as wide rivers like the Amazon and its major tributaries, often divide the geographical ranges of closely related species from each other.

    After returning to England from South America he went to Indonesia to continue his collecting and observations. In September of 1855 the Annals and Magazine of Natural History published "On the Law Which has Regulated the Introduction of New Species", which Wallace wrote in February of that year. In that paper he presented the observations that he had made of the distribution of species over various geographic and geological provinces and concluded it with his Sarawak Law -- "Every species has come into existence coincident both in space and time with a closely allied species." He thus extended the discovery that he had made in Amazonia and prepared the field of his imagination for the crop it would bear three years later.

    By February 1858 his biogeographical research in the Malay Archipelago had convinced Wallace that life does actually evolve. As he later wrote in his autobiography:

    "The problem then was not only how and why do species change, but how and why do they change into new and well defined species, distinguished from each other in so many ways; why and how they become so exactly adapted to distinct modes of life; and why do all the intermediate grades die out (as geology shows they have died out) and leave only clearly defined and well marked species, genera, and higher groups of animals?"

    Wallace wrote in his autobiography that, while lying in bed with a fever, he thought about Thomas Malthus's idea of positive checks on human population growth and devised the idea of natural selection when he applied Malthus's idea to animals. Wallace described the discovery as follows:

    "It then occurred to me that these causes or their equivalents are continually acting in the case of animals also; and as animals usually breed much more quickly than does mankind, the destruction every year from these causes must be enormous in order to keep down the numbers of each species, since evidently they do not increase regularly from year to year, as otherwise the world would long ago have been crowded with those that breed most quickly. Vaguely thinking over the enormous and constant destruction which this implied, it occurred to me to ask the question, why do some die and some live? And the answer was clearly, on the whole the best fitted live... and considering the amount of individual variation that my experience as a collector had shown me to exist, then it followed that all the changes necessary for the adaptation of the species to the changing conditions would be brought about... In this way every part of an animals organization could be modified exactly as required, and in the very process of this modification the unmodified would die out, and thus the definite characters and the clear isolation of each new species would be explained."

    Wallace had conducted a correspondence with Darwin since at least May 1857. Like many others, he provided Darwin with observations that Darwin used to support his own theory of evolution through descent with modification (i.e. natural selection). In his replies Darwin commented on Wallace's work on the distribution of species and some of the ideas that Wallace had expressed on the subject, commenting in one letter (dated 1857 Dec 22) that "without speculation there is no good and original observation". Darwin added a statement to the effect that he believed that he had gone much further than Wallace had in his theorizing.

    On the day of 1858 Jun 18 Darwin received the manuscript of Wallace's essay "On the Tendency of Varieties to Depart Indefinitely From the Original Type", which Wallace had written in February, along with a request that Darwin read it and then pass it on to Charles Lyell if he believed it worthy of consideration. Though Wallace did not use the term "natural selection", his essay nonetheless described how environmental pressure can promote the evolutionary divergence of a species from a similar species. Darwin did, indeed, send the manuscript to Lyell along with comments to the effect that Wallace had produced an excellent short abstract of Darwin's own ideas. He added that, even though Wallace had not said whether he wanted the essay published, he would write to Wallace and offer to send the manuscript to any journal Wallace named.

    At that point the illness of Darwin's baby son intervened and Darwin gave the problem of publication to Lyell and Joseph Hooker. Both men decided to publish Wallace's paper in a joint presentation before the Linnean Society of London with some unpublished writings of Darwin's to establish Darwin's priority (he had been working on "descent with modification" for twenty years at that time). On 1858 Jul 01 the assembled members of the Linnean Society heard Wallace's paper read along with a draft essay that Darwin had sent to Hooker in 1847 and a letter that Darwin had sent to Asa Gray in 1857.

    Born and raised as a commoner in the class-based British society of the time, Wallace could only accept the arrangement after the fact (and fairness requires that we note the length of time it took letters to get to and from Indonesia in those days, which may have been a factor in Lyell and Hooker's decision to proceed with the presentation). Indeed, without that association with Darwin, Wallace's hypothesis would not likely have made any impact at all. As it turned out, Wallace gained access to the highest levels of the scientific community and he accepted the arrangement good-naturedly. When Wallace returned to England, he met Darwin and the two men became good friends, with Wallace becoming one of the strongest and most effective defenders of "On the Origin of Species" after Darwin published it.

    Like Darwin, most people discern little difference between the ideas in Wallace's paper and Darwin's own theory of evolution by means of natural selection. But we can actually see subtle differences between the two men's approach to the theory. Darwin emphasized competition between individuals of the same species in the struggle to survive and reproduce. Wallace put his emphasis on the pressures that the environment exerts on varieties, thereby shifting the composition of a species as the less fit members of the population die out. He conceived the process of evolution as reflecting a negative feedback mechanism, one whose action kept species and varieties adapted to their environment. We can see Wallace describe that mechanism in one passage of his famous 1858 paper:

    "The action of this principle is exactly like that of the centrifugal governor of the steam engine, which checks and corrects any irregularities almost before they become evident; and in like manner no unbalanced deficiency in the animal kingdom can ever reach any conspicuous magnitude, because it would make itself felt at the very first step, by rendering existence difficult and extinction almost sure soon to follow."

    Further, Wallace avoided the teleological implications of Darwin's version of the theory by conceiving the engine of evolution as a process (which does not necessarily have a final cause), rather than as a force (which implies a definite direction). To guide Darwin to that same avoidance, Wallace wrote in an 1866 letter that Darwin should substitute Spencer's term "survival of the fittest" for the term "natural selection":

    "This term is the plain expression of the fact; Natural Selection is a metaphorical expression of it, and to a certain degree indirect and incorrect, since even personifying Nature, she does not so much select special variations as exterminate the most unfavourable ones . . . Natural Selection is, when properly understood, so necessary and self evident a principle that it is a pity that it should be in any way obscured . . . ."

    That natural selection (survival of the fittest) can work in bizarre, even counter-intuitive ways, came clear when Darwin and Wallace considered sexual selection and warning coloration. In 1867, Darwin wrote in a letter to Wallace that he had run into some difficulty in understanding how some caterpillars could have evolved conspicuous color schemes. How could any such helpless creature so easy for a predator to see survive? Darwin had already hypothesized that many color schemes that give animals a conspicuous appearance came about through sexual selection, a factor that Wallace believed had little importance. However, Darwin knew that sexual selection could not apply to caterpillars, because caterpillars don't engage in sexual behavior. So what, Darwin asked Wallace, makes the caterpillar stand out so clearly? Wallace wrote back and said that he and Henry Bates had observed that many of the most spectacular butterflies that they collected had a peculiar odor and taste. He added that John Jenner Weir (1822 Aug 09 -- 1894 Mar 23) had told him that birds would not eat a certain kind of common white moth because they found it unpalatable. "Now, as the white moth is as conspicuous at dusk as a coloured caterpillar in the daylight", Wallace wrote, it seemed likely that the conspicuous color scheme served as a warning to predators and thus could have evolved through natural selection: the predators themselves had evolved an aversion to the bright color schemes, so the caterpillars that had color schemes similar would survive predation better than those that didn't display such schemes. At a subsequent meeting of the Entomological Society, Wallace took this idea the next necessary step and asked for any evidence anyone might have on the topic. In 1869, Weir published data from experiments and observations involving brightly colored caterpillars that supported Wallace's idea. We can thus count warning coloration as one of a number of contributions Wallace made in the area of the evolution of animal coloration in general and the concept of protective coloration in particular. It also played an important part in a life-long disagreement Wallace had with Darwin over the importance of sexual selection. In 1878 Wallace published his book "Tropical Nature and Other Essays", in which he wrote extensively on the coloration of animals and plants and proposed alternative explanations for a number of cases Darwin had attributed to sexual selection and he revisited the topic at length in his 1889 book "Darwinism".

    In spite of such disagreements, Wallace explained and defended the theory of evolution by natural selection in the book "Darwinism". In that book he offered an elaboration of his original hypothesis, proposing that natural selection could bring about the development of barriers against the production of hybrids and thereby drive the reproductive separation of two varieties. That process, in the right circumstances, would lead to the evolution of new species. He suggested that when two populations of a given species had diverged beyond a certain stage, each variety adapted to particular conditions in the environment, hybrid offspring would be less well-adapted to that environment than either parent form. Those hybrids would not survive the action of natural selection. Under such conditions, natural selection would also promote the development of barriers to hybridization: individuals that avoided hybrid matings would tend to have more fit offspring and those that didn't avoid such matings would have less fit offspring. Thus the hybrid-avoiding varieties would prevail in the population and thus contribute to the growing reproductive isolation of the two incipient species. Biologists came to know this idea as the Wallace effect. Wallace had suggested to Darwin, in private correspondence as early as 1868, that natural selection could play a role in preventing hybridization but that he had not worked it out to this level of detail. Biologists today continue to make discoveries that confirm its validity.

    Many historical studies of the development of the theory of evolution mention Wallace only in passing, presenting him as merely the man who goaded Darwin into publishing his own theory. In fact, Wallace conceived his own distinct theory of evolution, which differed noticeably from Darwin's. Even Darwin regarded Wallace as a leading thinker on the ideas of evolution. Indeed, through both their private correspondence and their published works, as noted above, Darwin and Wallace exchanged knowledge and stimulated each other's thinking about evolution for many years after 1859.

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Appendix:

On the Tendency of Varieties todepart indefinitely from theOriginal Type

by

Alfred Russel Wallace

    One of the strongest arguments which have been adduced to prove the original and permanent distinctness of species is, that varieties produced in a state of domesticity are more or less unstable, and often have a tendency, if left to themselves, to return to the normal form of the parent species; and this instability is considered to be a distinctive peculiarity of all varieties, even of those occurring among wild animals in a state of nature, and to constitute a provision for preserving unchanged the originally created distinct species.

    In the absence or scarcity of facts and observations as to varieties occurring among wild animals, this argument has had great weight with naturalists, and has led to a very general and somewhat prejudiced belief in the stability of species. Equally general, however, is the belief in what are called "permanent or true varieties,"--races of animals which continually propagate their like, but which differ so slightly (although constantly) from some other race, that the one is considered to be a variety of the other. Which is the variety and which the original species, there is generally no means of determining, except in those rare cases in which the one race has been known to produce an offspring unlike itself and resembling the other. This, however, would seem quite incompatible with the "permanent invariability of species," but the difficulty is overcome by assuming that such varieties have strict limits, and can never again vary further from the original type, although they may return to it, which, from the analogy of the domesticated animals, is considered to be highly probable, if not certainly proved.

    It will be observed that this argument rests entirely on the assumption, that varieties occurring in a state of nature are in all respects analogous to or even identical with those of domestic animals, and are governed by the same laws as regards their permanence or further variation. But it is the object of the present paper to show that this assumption is altogether false, that there is a general principle in nature which will cause many varieties to survive the parent species, and to give rise to successive variations departing further and further from the original type, and which also produces, in domesticated animals, the tendency of varieties to return to the parent form.

    The life of wild animals is a struggle for existence. The full exertion of all their faculties and all their energies is required to preserve their own existence and provide for that of their infant offspring. The possibility of procuring food during the least favourable seasons, and of escaping the attacks of their most dangerous enemies, are the primary conditions which determine the existence both of individuals and of entire species. These conditions will also determine the population of a species; and by a careful consideration of all the circumstances we may be enabled to comprehend, and in some degree to explain, what at first sight appears so inexplicable--the excessive abundance of some species, while others closely allied to them are very rare.

    The general proportion that must obtain between certain groups of animals is readily seen. Large animals cannot be so abundant as small ones; the carnivora must be less numerous than the herbivora; eagles and lions can never be so plentiful as pigeons and antelopes; the wild asses of the Tartarian deserts cannot equal in numbers the horses of the more luxuriant prairies and pampas of America. The greater or less fecundity of an animal is often considered to be one of the chief causes of its abundance or scarcity; but a consideration of the facts will show us that it really has little or nothing to do with the matter. Even the least prolific of animals would increase rapidly if unchecked, whereas it is evident that the animal population of the globe must be stationary, or perhaps, through the influence of man, decreasing. Fluctuations there may be; but permanent increase, except in restricted localities, is almost impossible. For example, our own observation must convince us that birds do not go on increasing every year in a geometrical ratio, as they would do, were there not some powerful check to their natural increase. Very few birds produce less than two young ones each year, while many have six, eight, or ten; four will certainly be below the average; and if we suppose that each pair produce young only four times in their life, that will also be below the average, supposing them not to die either by violence or want of food. Yet at this rate how tremendous would be the increase in a few years from a single pair! A simple calculation will show that in fifteen years each pair of birds would have increased to nearly ten millions! whereas we have no reason to believe that the number of the birds of any country increases at all in fifteen or in one hundred and fifty years. With such powers of increase the population must have reached its limits, and have become stationary, in a very few years after the origin of each species. It is evident, therefore, that each year an immense number of birds must perish--as many in fact as are born; and as on the lowest calculation the progeny are each year twice as numerous as their parents, it follows that, whatever be the average number of individuals existing in any given country, twice that number must perish annually,--a striking result, but one which seems at least highly probable, and is perhaps under rather than over the truth. It would therefore appear that, as far as the continuance of the species and the keeping up the average number of individuals are concerned, large broods are superfluous. On the average all above one become food for hawks and kites, wild cats and weasels, or perish of cold and hunger as winter comes on. This is strikingly proved by the case of particular species; for we find that their abundance in individuals bears no relation whatever to their fertility in producing offspring. Perhaps the most remarkable instance of an immense bird population is that of the passenger pigeon of the United States, which lays only one, or at most two eggs, and is said to rear generally but one young one. Why is this bird so extraordinarily abundant, while others producing two or three times as many young are much less plentiful? The explanation is not difficult. The food most congenial to this species, and on which it thrives best, is abundantly distributed over a very extensive region, offering such differences of soil and climate, that in one part or another of the area the supply never fails. The bird is capable of a very rapid and long-continued flight, so that it can pass without fatigue over the whole of the district it inhabits, and as soon as the supply of food begins to fail in one place is able to discover a fresh feeding-ground. This example strikingly shows us that the procuring a constant supply of wholesome food is almost the sole condition requisite for ensuring the rapid increase of a given species, since neither the limited fecundity, nor the unrestrained attacks of birds of prey and of man are here sufficient to check it. In no other birds are these peculiar circumstances so strikingly combined. Either their food is more liable to failure, or they have not sufficient power of wing to search for it over an extensive area, or during some season of the year it becomes very scarce, and less wholesome substitutes have to be found; and thus, though more fertile in offspring, they can never increase beyond the supply of food in the least favourable seasons. Many birds can only exist by migrating, when their food becomes scarce, to regions possessing a milder, or at least a different climate, though, as these migrating birds are seldom excessively abundant, it is evident that the countries they visit are still deficient in a constant and abundant supply of wholesome food. Those whose organization does not permit them to migrate when their food becomes periodically scarce, can never attain a large population. This is probably the reason why woodpeckers are scarce with us, while in the tropics they are among the most abundant of solitary birds. Thus the house sparrow is more abundant than the redbreast, because its food is more constant and plentiful,--seeds of grasses being preserved during the winter, and our farm-yards and stubble-fields furnishing an almost inexhaustible supply. Why, as a general rule, are aquatic, and especially sea birds, very numerous in individuals? Not because they are more prolific than others, generally the contrary; but because their food never fails, the sea-shores and river-banks daily swarming with a fresh supply of small mollusca and crustacea. Exactly the same laws will apply to mammals. Wild cats are prolific and have few enemies; why then are they never as abundant as rabbits? The only intelligible answer is, that their supply of food is more precarious. It appears evident, therefore, that so long as a country remains physically unchanged, the numbers of its animal population cannot materially increase. If one species does so, some others requiring the same kind of food must diminish in proportion. The numbers that die annually must be immense; and as the individual existence of each animal depends upon itself, those that die must be the weakest--the very young, the aged, and the diseased,--while those that prolong their existence can only be the most perfect in health and vigour--those who are best able to obtain food regularly, and avoid their numerous enemies. It is, as we commenced by remarking, "a struggle for existence," in which the weakest and least perfectly organized must always succumb.

    Now it is clear that what takes place among the individuals of a species must also occur among the several allied species of a group,--viz. that those which are best adapted to obtain a regular supply of food, and to defend themselves against the attacks of their enemies and the vicissitudes of the seasons, must necessarily obtain and preserve a superiority in population; while those species which from some defect of power or organization are the least capable of counteracting the vicissitudes of food, supply, &c., must diminish in numbers, and, in extreme cases, become altogether extinct. Between these extremes the species will present various degrees of capacity for ensuring the means of preserving life; and it is thus we account for the abundance or rarity of species. Our ignorance will generally prevent us from accurately tracing the effects to their causes; but could we become perfectly acquainted with the organization and habits of the various species of animals, and could we measure the capacity of each for performing the different acts necessary to its safety and existence under all the varying circumstances by which it is surrounded, we might be able even to calculate the proportionate abundance of individuals which is the necessary result.

    If now we have succeeded in establishing these two points--1st, that the animal population of a country is generally stationary, being kept down by a periodical deficiency of food, and other checks; and, 2nd, that the comparative abundance or scarcity of the individuals of the several species is entirely due to their organization and resulting habits, which, rendering it more difficult to procure a regular supply of food and to provide for their personal safety in some cases than in others, can only be balanced by a difference in the population which have to exist in a given area--we shall be in a condition to proceed to the consideration of varieties, to which the preceding remarks have a direct and very important application.

    Most or perhaps all the variations from the typical form of a species must have some definite effect, however slight, on the habits or capacities of the individuals. Even a change of colour might, by rendering them more or less distinguishable, affect their safety; a greater or less development of hair might modify their habits. More important changes, such as an increase in the power or dimensions of the limbs or any of the external organs, would more or less affect their mode of procuring food or the range of country which they inhabit. It is also evident that most changes would affect, either favourably or adversely, the powers of prolonging existence. An antelope with shorter or weaker legs must necessarily suffer more from the attacks of the feline carnivora; the passenger pigeon with less powerful wings would sooner or later be affected in its powers of procuring a regular supply of food; and in both cases the result must necessarily be a diminution of the population of the modified species. If, on the other hand, any species should produce a variety having slightly increased powers of preserving existence, that variety must inevitably in time acquire a superiority in numbers. These results must follow as surely as old age, intemperance, or scarcity of food produce an increased mortality. In both cases there may be many individual exceptions; but on the average the rule will invariably be found to hold good. All varieties will therefore fall into two classes--those which under the same conditions would never reach the population of the parent species, and those which would in time obtain and keep a numerical superiority. Now, let some alteration of physical conditions occur in the district--a long period of drought, a destruction of vegetation by locusts, the irruption of some new carnivorous animal seeking "pastures new"--any change in fact tending to render existence more difficult to the species in question, and tasking its utmost powers to avoid complete extermination; it is evident that, of all the individuals composing the species, those forming the least numerous and most feebly organized variety would suffer first, and, were the pressure severe, must soon become extinct. The same causes continuing in action, the parent species would next suffer, would gradually diminish in numbers, and with a recurrence of similar unfavourable conditions might also become extinct. The superior variety would then alone remain, and on a return to favourable circumstances would rapidly increase in numbers and occupy the place of the extinct species and variety.

    The variety would now have replaced the species, of which it would be a more perfectly developed and more highly organized form. It would be in all respects better adapted to secure its safety, and to prolong its individual existence and that of the race. Such a variety could not return to the original form; for that form is an inferior one, and could never compete with it for existence. Granted, therefore, a "tendency" to reproduce the original type of the species, still the variety must ever remain preponderant in numbers, and under adverse physical conditions again alone survive. But this new, improved, and populous race might itself, in course of time, give rise to new varieties, exhibiting several diverging modifications of form, any of which, tending to increase the facilities for preserving existence, must, by the same general law, in their turn become predominant. Here, then, we have progression and continued divergence deduced from the general laws which regulate the existence of animals in a state of nature, and from the undisputed fact that varieties do frequently occur. It is not, however, contended that this result would be invariable; a change of physical conditions in the district might at times materially modify it, rendering the race which had been the most capable of supporting existence under the former conditions now the least so, and even causing the extinction of the newer and, for a time, superior race, while the old or parent species and its first inferior varieties continued to flourish. Variations in unimportant parts might also occur, having no perceptible effect on the life-preserving powers; and the varieties so furnished might run a course parallel with the parent species, either giving rise to further variations or returning to the former type. All we argue for is, that certain varieties have a tendency to maintain their existence longer than the original species, and this tendency must make itself felt; for though the doctrine of chances or averages can never be trusted to on a limited scale, yet, if applied to high numbers, the results come nearer to what theory demands, and, as we approach to an infinity of examples, become strictly accurate. Now the scale on which nature works is so vast--the numbers of individuals and periods of time with which she deals approach so near to infinity, that any cause, however slight, and however liable to be veiled and counteracted by accidental circumstances, must in the end produce its full legitimate results.

    Let us now turn to domesticated animals, and inquire how varieties produced among them are affected by the principles here enunciated. The essential difference in the condition of wild and domestic animals is this,--that among the former, their well-being and very existence depend upon the full exercise and healthy condition of all their senses and physical powers, whereas, among the latter, these are only partially exercised, and in some cases are absolutely unused. A wild animal has to search, and often to labour, for every mouthful of food--to exercise sight, hearing, and smell in seeking it, and in avoiding dangers, in procuring shelter from the inclemency of the seasons, and in providing for the subsistence and safety of its offspring. There is no muscle of its body that is not called into daily and hourly activity; there is no sense or faculty that is not strengthened by continual exercise. The domestic animal, on the other hand, has food provided for it, is sheltered, and often confined, to guard it against the vicissitudes of the seasons, is carefully secured from the attacks of its natural enemies, and seldom even rears its young without human assistance. Half of its senses and faculties are quite useless; and the other half are but occasionally called into feeble exercise, while even its muscular system is only irregularly called into action.

    Now when a variety of such an animal occurs, having increased power or capacity in any organ or sense, such increase is totally useless, is never called into action, and may even exist without the animal ever becoming aware of it. In the wild animal, on the contrary, all its faculties and powers being brought into full action for the necessities of existence, any increase becomes immediately available, is strengthened by exercise, and must even slightly modify the food, the habits, and the whole economy of the race. It creates as it were a new animal, one of superior powers, and which will necessarily increase in numbers and outlive those inferior to it.

    Again, in the domesticated animal all variations have an equal chance of continuance; and those which would decidedly render a wild animal unable to compete with its fellows and continue its existence are no disadvantage whatever in a state of domesticity. Our quickly fattening pigs, short-legged sheep, pouter pigeons, and poodle dogs could never have come into existence in a state of nature, because the very first step towards such inferior forms would have led to the rapid extinction of the race; still less could they now exist in competition with their wild allies. The great speed but slight endurance of the race horse, the unwieldy strength of the ploughman's team, would both be useless in a state of nature. If turned wild on the pampas, such animals would probably soon become extinct, or under favourable circumstances might each lose those extreme qualities which would never be called into action, and in a few generations would revert to a common type, which must be that in which the various powers and faculties are so proportioned to each other as to be best adapted to procure food and secure safety,--that in which by the full exercise of every part of his organization the animal can alone continue to live. Domestic varieties, when turned wild, must return to something near the type of the original wild stock, or become altogether extinct.

    We see, then, that no inferences as to varieties in a state of nature can be deduced from the observation of those occurring among domestic animals. The two are so much opposed to each other in every circumstance of their existence, that what applies to the one is almost sure not to apply to the other. Domestic animals are abnormal, irregular, artificial; they are subject to varieties which never occur and never can occur in a state of nature: their very existence depends altogether on human care; so far are many of them removed from that just proportion of faculties, that true balance of organization, by means of which alone an animal left to its own resources can preserve its existence and continue its race.

    The hypothesis of Lamarck--that progressive changes in species have been produced by the attempts of animals to increase the development of their own organs, and thus modify their structure and habits--has been repeatedly and easily refuted by all writers on the subject of varieties and species, and it seems to have been considered that when this was done the whole question has been finally settled; but the view here developed renders such an hypothesis quite unnecessary, by showing that similar results must be produced by the action of principles constantly at work in nature. The powerful retractile talons of the falcon- and the cat-tribes have not been produced or increased by the volition of those animals; but among the different varieties which occurred in the earlier and less highly organized forms of these groups, those always survived longest which had the greatest facilities for seizing their prey. Neither did the giraffe acquire its long neck by desiring to reach the foliage of the more lofty shrubs, and constantly stretching its neck for the purpose, but because any varieties which occurred among its antitypes with a longer neck than usual at once secured a fresh range of pasture over the same ground as their shorter-necked companions, and on the first scarcity of food were thereby enabled to outlive them. Even the peculiar colours of many animals, especially insects, so closely resembling the soil or the leaves or the trunks on which they habitually reside, are explained on the same principle; for though in the course of ages varieties of many tints may have occurred, yet those races having colours best adapted to concealment from their enemies would inevitably survive the longest. We have also here an acting cause to account for that balance so often observed in nature,--a deficiency in one set of organs always being compensated by an increased development of some others--powerful wings accompanying weak feet, or great velocity making up for the absence of defensive weapons; for it has been shown that all varieties in which an unbalanced deficiency occurred could not long continue their existence. The action of this principle is exactly like that of the centrifugal governor of the steam engine, which checks and corrects any irregularities almost before they become evident; and in like manner no unbalanced deficiency in the animal kingdom can ever reach any conspicuous magnitude, because it would make itself felt at the very first step, by rendering existence difficult and extinction almost sure soon to follow. An origin such as is here advocated will also agree with the peculiar character of the modifications of form and structure which obtain in organized beings--the many lines of divergence from a central type, the increasing efficiency and power of a particular organ through a succession of allied species, and the remarkable persistence of unimportant parts such as colour, texture of plumage and hair, form of horns or crests, through a series of species differing considerably in more essential characters. It also furnishes us with a reason for that "more specialized structure" which Professor Owen states to be a characteristic of recent compared with extinct forms, and which would evidently be the result of the progressive modification of any organ applied to a special purpose in the animal economy.

    We believe we have now shown that there is a tendency in nature to the continued progression of certain classes of varieties further and further from the original type--a progression to which there appears no reason to assign any definite limits--and that the same principle which produces this result in a state of nature will also explain why domestic varieties have a tendency to revert to the original type. This progression, by minute steps, in various directions, but always checked and balanced by the necessary conditions, subject to which alone existence can be preserved, may, it is believed, be followed out so as to agree with all the phenomena presented by organized beings, their extinction and succession in past ages, and all the extraordinary modifications of form, instinct, and habits which they exhibit.

Ternate, February, 1858.

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