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then such peculiarities are maintained; and if the process be harmony with external nature, physical and organic; and continued, they are even exaggerated by successive accumula- that equilibrium being attained and maintained, the species, tions of change, so that at length there is formed a new race, we may believe, will continue unaltered. But suppose that which may be perpetuated by the avoidance of intermixture the surrounding circumstances should during a long epoch with the parent stock. Now many varieties may arise from undergo change,-such as a change from heat to cold, or the one specific or parent stock, and these varieties may them-reverse, or a change in the relative dryness or moisture of the selves give origin to others, any or all of which may be per- air, or a slow elevation or subsidence of the level of the petuated by being continually set apart for separate breeding land, or, most important of all, some change in the relaor cultivation. Indeed, the tendency to vary, once begun, tions of organism to organism, as the extinction or excessive seems rather to increase, and so greater and greater diver- multiplication of any one or more species of plants or gences from the parent stock arise. animals, or the emigration or immigration of the latter in great numbers, then, the equilibrium being destroyed amongst them, any other species may either multiply so as to occupy any vacant space in "nature's polity," or it may be actually extinguished itself; or we may conceive that it can only be preserved by becoming gradually, in a succession of generations, adapted by slight or important modifications to the changed conditions around it. Sometimes, as in the case of changes of climate, its general constitution alone may undergo the required alteration; but if, on the other hand, any correlated or accidental structural variation should pre sent itself in any individuals of the species, which will render those individuals better suited to contend for existence under the altered condition of things,-and Mr. Darwin says: "I think it would be a most extraordinary fact if no variation ever had occurred useful to each being's welfare, in the same manner as so many varieties have occurred useful to man," then the individuals so characterized will have a better chance of survival; their offspring, possessing, on the strong principle of inheritance, the advantageous peculiarity, will be fostered and maintained, whilst those who do not possess it will decline; and at last, after many, perhaps countless, generations, each the subjects of a continued selective process, a new race or breed, or an "incipient species," will be established, which will not only multiply faster than its unmodi with surrounding circumstances, and therefore better suited to survive and be perpetuated, will share a place with the parent species, or shut it out in the "struggle for existence,' and extirpating it and all the less adapted intermediate varieties, will take their place entirely, and henceforth represent the species. In its turn, however, if the surrounding conditions undergo further changes, some new variety may arise within its own ranks, and under the same slow but everwatchful and continually-operating process of selection, may be reared side by side with it, or ultimately be substituted for it.

The process by which such varieties are preserved, and which Mr. Darwin aptly terms "artificial selection," is exemplified in the mode in which all our edible varieties of cultivated plants and most of our useful breeds of domestic animals have originated, and are now maintained, or continually improved. The example most carefully studied, and most particularly adduced by the author, is that of the various breeds of domestic pigeons, which are all derived from the rock-pigeon (Columbia livia). The extreme divergence of these from the parent type, and the great differences amongst themselves, are so striking,-the fantail, with its erect tail containing an additional number of feathers; the carrier, with its fleshy appendages about the head; the pouter, with its enormous crop; and the tumbler, with its short face and its curious propensity to turn over in the air,-that, by a naturalist ignorant of their common origin, they might be mistaken for distinct species. So, too, in regard to many of the breeds of dogs and cattle, there exist inextricable doubts whether they are varieties or species. These and all similar varieties or breeds, it is shown, are not produced by man directly, but by his conscious or unconscious selection, for the purposes of cultivation or breeding, of such varieties as nature herself has evolved, and are either useful or pleasing to himself.

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But in the wild state, the same tendency to variation amongst certain species of both plants and animals undoubt-fied relatives, but being ultimately completely harmonized edly exists, manifesting itself not merely in downright monstrosities, but in those comparatively slight modifications which characterize the different individuals of a species, in casual alterations of a more marked degree, or in still more important and persistent changes, thus giving rise to accidental varieties, to permanent varieties, and to doubtful or sub-species. Such gradations of change are met with in many plants, as, for example, in the roses and brambles, and also amongst animals, especially in certain invertebrate creatures. They have occasioned, and do now occasion, the greatest difficulty to naturalists, causing a confusion between varieties and species; and they are held by Mr. Darwin to at least suggest However slow the operation of this "natural selection" the two-fold belief that there is "an actual passage" from one may be, for it can only operate for the good of the individual to another, and that a a well-marked variety may be called by successive minute modifications in descent, we have only an incipient species." Of this belief he finds confirmation in (says Mr. Darwin) to call in the aid of sufficient time,-and a comparison of the behaviour of the varieties of a species the mind is perfectly overwhelmed by the pictures which our with that of the species of a genus,-a comparison which author draws in the attempt to convey a feeble notion of the shows that they present the same laws as to frequency, quite incomprehensible periods that have elapsed since orgeographical distribution, and tendency to become dominant, ganic beings were first introduced upon our planet, more and other analogies easily understood, "if species have once especially as he maintains that, to arrive at that epoch, we existed as varieties, and have thus originated," but " utterly must go vastly further back than the period when the earliest inexplicable if each species has been independently created." known fossiliferous rocks, the Silurian series, were being Our author next proceeds to show how it is conceivable formed at the bottom of certain seas,-to imagine that any that a process of selection is constantly going on in nature, conceivable amount of change in those forms may have taken comparable, but not identical, with the artificial selection of place, through the agency of a natural selection continually human agency, and adequate to transform occasional slight adding up, "increment by increment," the successive varieties varieties into permanent ones, and permanent varieties or that may have occurred. On these grounds, indeed, he mainincipient species into good and true species. Both plants and tains not only that all species, both of plants and animals, animals, even the least fruitful or fertile, multiply so fast, have sprung from previous naturally-selected varieties, but that if left without check, the whole earth would speedily be he asserts his belief that, as seen in the case of the varieties choked by the progeny of a single pair. In 500 years one of cultivated vegetables and domesticated animals, wider and pair of elephants, the slowest breeding animals known, would wider divergences from a parent species have occurred, and produce fifteen millions of descendants. An annual plant and that thus all the species of one genus have sprung from its progeny, yielding only two seeds a year each, would in a pair of common progenitors, and all the genera of one twenty years have produced a million plants. But all organic family from an ancestral couple still further removed in beings mutually invade each other's ground, and tend to ex-time; and more than this, he believes that, as a consequence terminate one another. Life is a continual effort to live. of successively antecedent divergences having still more and Competition is a constant law of existence, not merely of more remote origins, all the families of an order, and all the plants and animals against any unfavourable physical condi- orders of a class, are but the offspring from single primordial tions, but of one living thing against another, obvious enough forms. "I believe," he says, "that animals have descended in the case of remote species, but often even more close in from at most only four or five progenitors, and plants from an that of allied species or of the individuals of one species, be- equal or lesser number. Analogy would lead me one step cause their wants are similar. In this "struggle for exist- further, namely, to the belief that all animals and plants ence," as Darwin calls it, weakly individuals and their weakly have descended from some one prototype."-p. 484. offspring are destroyed, while the stronger ones and their strong offspring are preserved. Hence an ever-operating process of "natural selection" is continually going on in the organic world, of which death itself is in one sense the agent. So long as the circumstances surrounding any species continue uniform, the effect of this kind of selection is to cast out the individuals less fit, and to foster those which are more fit to live, and thus to bring the entire species into perfect

So far back as the year 1839, Mr. Darwin had sketched the outline of this theory of the origin of species by natural selection. In 1844, a more developed account of it was submitted in manuscript to Dr. Hooker, and its purport communicated to Sir C. Lvell. In 1857, Mr. Darwin gave an outline of them in a letter to Dr. Asa Grav, the American botanist. In 1858, in consequence of Mr. Arthur Wallace, a brother naturalist, then residing at Ternate, one of the Molucca Islands,

AUG. 1860.

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having quite independently arrived at simular conclusions, and embodied them in a paper entitled "On the Tendency of Varieties to Depart indefinitely from the Original Type," Mr. Darwin's views were first published, by being read with Mr. Wallace's paper before the Linnæan Society, and printed in the third volume of their Journal. For twenty years Mr. Darwin has continued his researches and reflections, amassing facts, making observations, pursuing the subject in all its ramifications, and wrestling with many and obvious difficulties. His present work, which is to be followed by one still more abounding in facts, is, as he deservedly entitles it, long argument" in support of his theory, which is so strategically defended, that no weak point has been entirely overlooked. The key to his whole position is the doctrine of natural selection, operating on varieties produced by modification in descent. This, indeed, is the vera causa to which he would attribute the successive origination of new species from old ones, both of plants and animals. The nature and scope of this power is traced in many ways. Unlike artificial selection, which, as exercised by man, acts only partially, that is on a few individuals of a species, it continually hangs over an entire species. Unlike artificial selection, which only culls out varieties useful or pleasing to man, but not necessarily useful, nay, even injurious to the individual, it avails itself only of such variations as are for the good of the individual, and thus ultimately for the improvement or advancement of the species. Again, artificial selection acts ostensibly in variations in the size, proportions, local form, and colour of individuals, whilst natural selection scans deeply their very inmost structure, acts even on the embryo in the seed or ovum, operates at all periods of life and on both sexes, and, in the case of animals, under the form of sexual selection, avails itself of all individual improvements in masculine prowess or attractiveness: in a word, it slowly but steadily assimilates successive advantages of all kinds and degrees of the most interior character.

Lastly, artificial selection acts only in comparatively brief periods of time, yet what curious marvels it may educe! whilst natural selection works on for ages, during practically unbounded periods, and by summing up slowly accumulated results may exercise an indefinable amount of change. "How fleeting," says our author, "are the wishes and efforts of man! how short his time! and consequently how poor will his products be compared with those accumulated by nature during whole geological periods. Can we wonder, then, that nature's productions should be far truer in character than man's productions; that they should be infinitely better adapted to the most complex conditions of life, and should plainly bear the stamp of far higher workmanship?" Again, "I can see no limit to the amount of change, to the beauty and infinite complexity of the co-adaptations between all organie beings, one with another, and with their physical conditions of life, which may be effected in the long course of time by nature's power of selection." And Mr. Wallace concludes his memoir by stating that "there appears no reason to assign any definite limits to the progression of certain classes of variations further and further from the original type." In fine, artificial selection evolves only varieties, leaving the species untouched, whilst natural selection evolves new species, with all their peculiar characteristics of colour, form, internal structure, instincts, and habits, and has thus, during the long eras of an unknown and unfathomable past, given birth to the genera, and families, and orders, and even to the classes, of plants and animals. The process of natural selection, Mr. Darwin admits, might, under certain circumstances, be interfered with by intercrossing, "which plays a very important part in nature in keeping the individuals of the same species, or of the same variety, true and uniform in character;" but intercrossing would not always be thus unfavourable. Again, a large number of individuals, spread over a large and continuous area and isolation, under certain conditions, may operate favourably. As natural selection implies the alternate extermination of the less modified and less adapted races, it explains the extinction as well as the origin of races in geologic time. Involving the principle of all plants and animals arising from a few common progenitors by continuous "descent with modification," going on, like the known variations within species, to produce wider and wider divergences from the primitive stock, it would account for these three general facts: viz., the groupings and subgroupings of organized beings, the gradational series which they form, and their exquisite adaptation to their respective conditions of existence; for it harmonizes with the first-mentioned fact, and explains it on strictly genealogical grounds; it shows by its slow operation the way in which "nature makes no leans," and it affords an apparently rational exposition of the doctrine of "final causes," involved in the third fact. The two great laws of unity of type" and "the conditions of existice," laws which are acknowledged by all naturalists, are

likewise embraced by this theory of natural selection; unity
of type being, on this hypothesis, really a unity of descent,
and adaptation to external conditions being the result of slowly
Such being the mode of action and the powers claimed by
selected modifications imparting a special fitness to live.
Mr. Darwin for the process of natural selection, he frankly
admits that whether it "has really thus acted in nature must
be judged of by the general tenor and balance of the evi-
dence given in the following chapters," of which we proceed
to present a close analysis.

The "laws of variation" in plants and animals first occupy
Ex-
attention. Varieties, it is urged, cannot be due to chance, but
unfortunately we are yet very ignorant of their causes.
ternal conditions, such as climate and food, affect plants much,
animals less; and the same varieties may occur under different
Variations may commence in the seed or egg,
conditions, and different varieties under the same conditions.
The reproductive system must often be acted on in some un-
known way.
Habit, use or disuse, and changes in correlated parts, may
and show themselves early or late in life.
all, however, though in a less degree, determine change.
Parts repeated in series are very prone to variation; so are
rudimentary parts, and so are parts which are unusually
highly developed. Lowly organized animals, consisting of
parts repeated in series, are more liable to vary than others,
whose parts are more specialized. Parts not wanted are
wasted, on the principle of economy, to aid in the nourishment
of parts requiring or taking a higher development. Specific
characters are more variable than generic; in other words, on
the theory of descent with modifications, parts which have
already varied are still more liable to variation; for species
having descended from one genus, their specific differences
are the evidences of variation from their common generic
progenitor. The species of one genus differ much in their
female birds, and these very characters are peculiarly liable
secondary sexual characters, as the plumage of male and
species; whilst the reverse
to vary in individuals of the same species. Parts which are
uniformly developed in all the species of a genus are little
liable to vary in individuals of one
is the case with parts inordinately developed in certain
species. The different species of one genus often vary analo-
gously; and so do different breeds of one species. Many
same laws appear to have acted in
variations both in species and in breeds exhibit a tendency to
reversion towards some ancestral type. In fine, on reviewing
the whole subject, the "
families,
producing the lesser differences between varieties of the same
species, and the greater differences between species of the
same genus."

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But the theory of the origin of species, genera,, orders, and classes, by "descent with modification," through natural selection, is not without its difficulties, and in encountering these all the resources of Mr. Darwin's well-stored and ingenious mind are most happily employed in well-matured symmetry and order.

The ab

The first difficulty met with is the absence of confusion amongst existing species, by reason of numerous transitional forms between them, or between the supposed parent species and the new or offspring-species. If, it may be asked, the latter had been derived from the former by slowly-accumulated changes, why are not some of the steps of such changes apparent either in contemporary beings or in the that in any one country and at any one time, we observe a past records of geological formations? The fact is, however, remarkable definition of species unblended by intermediate forms. This very obvious and important objection Mr. Darwin meets by urging the extreme slowness of the supposed changes, and their probable limitation to a few only of contemporary species, and maintains that we do see, especially in the case of plants, "a few species presenting slight modifications of structure in some degree permanent." sence of intermediate living forms is attributed to this,--that under all circumstances the ultimated new species being necessarily and alone perfectly adapted to external conditions will therefore outrun and exterminate the less-perfectly by its origin through natural selection, can alone survive, and we have no right to expect (exceptlapted intermediate forms, and often even the parent form also; so that in reality ing in rare cases) to discover directly connecting links between them, but only between each and some extinct and supplanted form." So, too, where two closely-allied species occupy neighbouring territories, we should not "expect often to find intermediate species in intermediate zones, for these will be supplanted by the two allied forms on either hand." The absence of intermediate extinct forms is subsequently more particularly considered.

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The second difficulty consists in having to admit that animals of extremely divergent characters, for example, terrestrial, aquatic, and aerial species, allied or not allied,—

have been formed by modification from one another, or from tains, affords demonstrative proof against the Lamarckian a common stock; and that organs of such extreme perfection theory of direct transmutation, he asserts his belief in the as the eye, or of such aberrant character as the electric operation of that principle; conceiving that it may act not organs of fishes, or of such trivial service as many parts of merely for the good of the individual, but also for that of the plants and animals are, have arisen in the like manner. In family; and that the presence of such well-distinguished reply to this, Mr. Darwin points to the well-known finely- castes being beneficial to the entire community, the parents regulated gradational forms presented by the series of genera which produce the most desirably characterized neuters, are and orders, and contends that "it is conceivable that flying in the course of successive generations continually being infish might,"-due time being given,-"have been modified directly preserved through the advantages possessed by their into perfectly-winged animals." Again: equally well-known more and more specially endowed neuter offspring; and thus structural and functional gradations exist in homologous or natural selection comes indirectly into play upon these latter corresponding parts in a series of plants or animals,-as, for in their turn. Moreover, he supposes, that in the still more example, the swim-bladder of fishes, which is the correspond- complex cases where two or three distinct kinds of neuters ing or homologous part with the simple lung of the frogs, exist, intermediate forms (comparable to those which occur and the gradually more and more developed lungs of reptiles, in the origin of species themselves) have been produced, but birds, and mammals. As to the production of an eye, he have been eliminated by a twofold or threefold natural selecsays:-"Even in the case of any organ, if we know of a long tion, acting in this instance also indirectly through the parents. series of gradations in complexity, each good for its pos- After this exhaustive pursuit of his adopted theory in so comsessor, then, under changing conditions of life, there is no plicated a mode of action, Mr. Darwin says, "I am bound logical impossibility in the acquirement of any conceivable to confess that with all my faith in this principle, I should degree of perfection through 'natural selection." In regard never have anticipated that natural selection could have been to such extremely divergent forms as the bats, and such ex-efficient in so high a degree, had not the case of these neuter ceptional parts as the electric organs of fishes, he suggests insects convinced me of the fact." And one need not wonder the caution that our data may be imperfect, our present at his afterwards saying, "I do not pretend that the facts knowledge not yet yielding us the transitional forms and given in this chapter strengthen in any great degree my organs. If the frequent existence of unimportant parts, in theory; but none of the cases of difficulty, to the best of my plants and animals, be difficult to account for on the hypo- judgment, annihilate it." thesis of natural selection, which can only directly preserve and perpetuate what is good for the individual and the coming or perfected species, it is observed that we do not always readily know what is and what is not important; that those considered by us unimportant may be either parts on their way to development for the good of future species, or they may represent parts that have been important, and are yet retained though on their way to be aborted or lost; or they may have some necessary co-existence with parts that are important, by virtue of some obscure law of correlation of growth; or, lastly, they may be parts re-appearing by a law of reversion to some long-extinct ancestral type.

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The fourth difficulty relates to the grand general distinction between species and varieties, dependent on the usual sterility of the former when intercrossed, and the almost universal sterility of their occasional hybrid offspring, inter se, as compared with the absolute fertility of all kinds of varieties, even the most permanent, with one another, and with their parent stock, as well as of their mongrel offspring, inter se. This question, so important for an hypothesis which maintains that species spring from varieties, and that permanent varieties are but incipient species, is closely and ably scrutinized by Mr. Darwin, who, by drawing a comparison between the phenomena and laws of hybridism and of mongrelism, The third difficulty, which refers to animals only, and not maintains, contrary to the general opinion, that the facts of to their structure, but to their actions, is put in the form of the case, instead of indicating a real, pre-ordained, and imthis question: "Can instincts be acquired and modified through portant physiological discreetness, seem "rather to support natural selection ?" For instincts undoubtedly vary as much the view that there is no fundamental distinction between as form and structure. In treating this very interesting part species and varieties." Just as hybrids occur between species of the subject, it is remarked that, though instincts differ closely allied though different, so mongrels occur between from habits, in having a special origin and in being uncon- varieties; and excluding the question of their relative fertitrolled by will, yet that if a habit can be shown to become lity, inter se, hybrids and mongrels agree in the rules which permanent after some generations, as is seen in the case of govern their resemblances to one or other parent, and in the pointers, retrievers, shepherds' dogs, and tumbler-pigeons, fact that they can by proper intercrossing be reduced to either it is difficult to distinguish it from an instinct. New instincts parent form." Mongrels, however, are rather more prone to may therefore possibly arise from habits. Again, in a state vary in themselves than hybrids. From experiments made of nature, instincts, like structures, may arise or be lost, on plants, by attempting to fertilize one species with the owing to changes in surrounding conditions, as in the neces- pollen of another, it appears that the results graduate from sity for searching after new food, or for seeking protection perfect fertility to absolute sterility. Amongst animals, from new enemies; but the chief cause of variety in instincts, hybridity certainly occurs, but only between closely-allied Mr. Darwin thinks, lies in an inherent tendency to vary species; a fact which might be expected by the theory which slightly, just as forms and structures do, and so, like them, to regards such species, on the principle of their origin by descent become the subjects of natural selection, for the good of the with modification, as really more closely related genetically individual, the vast importance of most instincts in this than species wider apart. The ordinary rule of sterility of respect bringing them under the powerful operation of such species when intercrossed is not wholly without parallel in a law. The author states that he could give many instances the case of varieties in plants, some of which are also inferof a natural tendency of instincts to vary, mentioning particu- tile together: perhaps they are really frequently so, only larly the migratory instinct, and that displayed by birds in the then they are set down to be species not varieties. On the construction of nests. The instinctive fear of man grows upon graver question of the sterility of hybrids inter se, as comthe animals of an island or district newly taken into human pared with the fertility of mongrels under the same circumpossession. Even such remarkable instincts as that "which stances, Mr. Darwin admits that he "doubts whether any leads the cuckoo to lay her eggs in other birds' nests," as the case of a perfectly fertile hybrid animal can be considered as slave-making instincts of certain ants," and the "comb- thoroughly authenticated." But he remarks that their inmaking power of the hive bee," may be conceived to be the fertility is very different from that of intercrossed species, result of gradual modification and natural selection, and to being dependent on defective organization in the reproductivo have been perfected to their present wonderful condition, by organs; that certainly the hybrids experimented on have the preservation and ultimate preponderance of lines or suc- most commonly been the offspring of the same parents, which cessions of individual creatures, in which some rudimentary would inevitably tend to increase the probability of their instinct occasionally appeared, and then became strengthened being sterile, and that the effects of confinement have not and rendered permanent, by inheritance and natural selection, been taken into account. Furthermore, he believes, on cerbecause it was good for them. "By far the most serious tain grounds, that the rule of the sterility of hybrids, inter difficulty which my theory has encountered," says Mr. Dar- se, is not an absolute one, and that domestication of the two win, "are the remarkable cases of diversity both of form and parent and allied species may at last render their hybrid proinstincts in the neuters or sterile females of certain insect duce fertile, but of this he admits there is no positive evidence. communities, as, for example, of certain ants, in whose nests, Finally, he concludes on this most important subject, that besides the males and fertile females, not only one, but two, neither the sterility of first crosses, nor that of hybrids, is or even three kinds or castes of neuters, having totally different absolutely universal; that it is not a special endowment, but forms and instincts, exist under the name of working castes rather a different phenomenon in the two cases, and in both, and soldier castes; for these, being by nature invariably variable in degree, highly susceptible of being favoured or sterile, could not possibly directly inherit from predecessors, or interfered with, and governed by common yet singularly comhand down to posterity, any dawning or perfected peculiarity plex laws and conditions, not yet at all well understood. nder the influence of natural selection. Nevertheless, éven The fifth difficulty is the absence of all traces of those the case of these nenter insects, which, Mr. Darwin main-intermediate varieties which, on the natural selection theor.

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of all organized forms in groups under groups, fossil as well as living, for they are all kindred by descent; the facts, that fossil species fill up apparent gaps in the ranks of existing plants and animals, and that "the more ancient a form is the more it generally differs from the living," and therefore the more likely it is to connect generic or even ordinal groups, than to stand merely between any two species; that the organic remains of consecutive formations are more closely allied than those of recent ones; that the later species are more highly organized, or have more specialized structures than the early ones,-for specialization of parts implies advantages to the being, which are seized upon by natural selection and inheritance, and so are continually being preserved, improved. and perpetuated; and lastly, the fact of the well-known per sistence of the same types of animals and plants, modified though they be, in the same geographical areas at successive geological epochs, as of the armadillo kind in South America and the marsupials in Australia,-circumstances which no climatic or other physical theory is able to explain.

operating as it does very slowly, must formerly have existed | characters of the species of certain epochs; the great "unity" on the earth, and the number of which must have been 22 enormous." Geology," it is admitted, "assuredly does not reveal any such finely-graduated organic chain;" and, says Mr. Darwin, "this perhaps is the most obvious and gravest objection which can be urged against my theory." Again, he remarks, "the number of intermediate and transitional links between all living and extinct species must have been inconceivably great. But assuredly if this theory be true, such have lived upon this earth." The explanation of the absence of such intermediate links in geological strata rests on the imperfection of the geological record. The time that has elapsed since the dawn of organic life upon this planet is immeasurably great; the geographical areas occupied by many of the intermediate varieties may have been very limited or quite local; the physical conditions necessary for their preservation in a sedimentary deposit might not have concurred for their entombment and preservation,-for geological formations, if of any depth, require a regulated and continuous subsidence at the bottom of the sea or other water. Again: it is marine and fresh-water animals which The geographical distribution of plants and animals in have the best chance of being entombed, but many of these space is next considered, and shown to be equally in accordwould have an organization necessarily very perishable; geo-ance with the theory, as is their geological succession in time. logical formations have evidently been often intermittent,-do Natural areas at present undoubtedly exist, to which allied not follow in regular sequence, and probably have generally organic forms are evidently more or less confined by barriers, taken longer to deposit than the average duration of specific such as the ocean, mountains, deserts, rivers, or isthmuses forms; besides, who is to say whether any particular fossil between neighbouring seas. The flora and fama of the Old plant or animal is a variety or a species? The apparently and New Worlds, of South America, South Africa, and Ausabrupt coming in of a species in any one formation without tralia, of the Gulf of Mexico and the Pacific Oceans, respectransition-forms, may be sometimes due to its sudden immi- tively differ, yet the climate and other physical conditions in gration into the region in which the deposit took place. certain parts are alike. They are natural areas, within each Lastly, the most powerful argument of all,-our geological of which, though the species differ at different parts, they collections, on the evidence of which we at present only can manifest decided affinities, so that representative species are rely, are poor and paltry,-incalculably so, indeed,-in propor- found all over,-indicative, Mr. Darwin believes, of a "deep tion to the number of generations which must have passed organic bond, penetrating through space and time, over the away, even during the formation of any particular series of same areas of land and water, and independent of their physedimentary deposits: hence any conclusions from them alone sical conditions." This bond is inheritance, depending on the must be considered only provisional. common descent of all the individuals of one species from a single parent variety, and of all the allied species, genera, families, and orders from common progenitors of still more and more ancient and quite inconceivable date. It remains to show how if any species, and even the allied species of the same genus, are descended from the same parents, they are now so widely spread,-a difficulty, however, which also occurs in the case of the doctrine of the direct creation of species by single pairs. In explanation of this sometimes remarkable dispersion it is observed, that the existing natural areas are mostly continuous, and that when they are not, they have probably been broken up by subsidences or elevations; that the means of dispersal of all kinds of seed and of certain ova, especially of fresh-water productions (which are very widely diffused), are far more numerous than at first sight would appear, viz., by currents of air and water; by whirlwinds, whirlpools, and floods; by the natural continuity of seas and rivers; by icebergs; by crevices in drifting timber; by the agency of animals, as of fishes, which may swallow seeds, and in turn be eaten by birds; by birds themselves, especially by water-fowl of long flight, on whose backs and feet small plants and the seed-loaded mud of ponds (as shown by direct and curious observation) might readily be transported to great distances; and lastly by seed-eating birds, which might or might not be slain, after considerable flights. The larger animals are proverbially limited at present to very definite areas; but in regard to the wider distribution of allied species in the past, and to the general distribution of many of them now, it must be borne in mind that migration of all kinds would take place in time, especially under the influence of changes of temperature, acting on their supply of food. This is exemplified by an analysis of what appears to have occurred in that extraordinary and comparatively recent great glacial epoch, of the occurrence of which abundant evidence exists, and during which an exceedingly low temperature extended from the poles much further than it does at present, when all living creatures must have been driven for a time towards the equator, and again withdrawn as the cold receded north and south. Such migrations, occurring over long periods, and specially favoured or interrupted, at various times and in various localities, by elevations or subsidences of tracts of land, and by the rising or falling of isthmuses and islands in the ocean, would affect the movements of both marine and terrestrial animals, and it can be shown that they account in a remarkable manner for the existing distribution of animals on the earth. Looking to this generally, it is stated by Mr. Darwin, as also in accord with the theory of descent with modification, that lowly-organized species, which on that view must have had earlier progenitors, are more widely dispersed than more highly-organized ones; and that the species of widely-diffused genera, being the supposed descendants of

The abrupt appearance of whole groups of allied species in particular geological formations, which has so often been held to be fatal to the Lamarckian theory of the transmutation of species, is believed by Mr. Darwin to be owing to the occur rence of long intervals,-" enormous blanks,"-between the periods of formation of any two successive strata, much longer perhaps than the time required for their deposit; certain facts are quoted by him to show that previous conclusions as to the sudden appearance of particular classes of animals at particular epochs have undergone alteration in consequence of later discoveries revealing their earlier existence; and our present ignorance of the geology of other parts of the world than Europe and the United States should teach us not to dogmatize on this point.

Finally, the abrupt appearance of groups of allied species in the lowest known fossiliferous strata, the lowest Silurian deposits, is a still graver difficulty; for, "if my theory be true," says Mr. Darwin, "it is indisputable that before the lowest Silurian stratum was deposited long periods elapsed, as long as, or probably longer than, the whole interval from the Silurian age to the present day; and that during these vast yet quite unknown periods of time, the world swarmed with living creatures." Yet their remains are not found, and their absence is not satisfactorily accounted for. "The case at present must remain inexplicable; and may be truly urged as a valid argument against the views here entertained." This is, indeed, the strongest argument in favour of the immutability of species; but yet it may break down under a better knowledge of those earliest strata and their subjacent rocks. It is not impossible that antecedent formations may lie buried at the bottom of the sea, or may have been metamorphosed by telluric heat. In some of the azoic rocks, traces of phosphates and bitumen have been found.

Having thus considered the difficulties which beset his theory, Mr. Darwin occupies the remainder of his volume by showing how many of the admitted facts of the organic world harmonize with or are most easily explained by it.

In the first place, most of the great facts of paleontology accord better with the theory of descent by modification through natural selection, than with any other; such as, for example, the usually gradual appearance of species in succession one after another; the divergence of certain betteradapted and therefore dominant species into wider and wider groups of descendant species, so as to make it appear as if the forms of life had changed simultaneously over the world; the opposite condition of restriction of the less adapted species to fewer or very few descendant representatives; the grand process of extinction of the less-adapted species continually going on; the fact that species do never, because they can never, re-appear when once lost; the consequential change in the

their own generic ancestral type, are themselves also widely been announced by Owen, are numerous and but little varied diffused. The distribution, proportionate number, and chain earlier as well as in lower species, and must have been racters of the plants and animals aboriginal to oceanic islands simple and numerous, according to Mr. Darwin, in the unare also shown to harmonize with Mr. Darwin's theory. The known progenitors of the vertebrate and articulate animals, number of species, as one would expect in such isolated spots, and in the unknown progenitors of the flowering plants. is small for the area of ground they occupy; nevertheless the The leading facts in embryology are also adduced as capable proportion of peculiar or endemic species is unusually large. of full explanation by a theory which is founded on the prinThough different from, yet they are always allied to, the ciple of descent. The resemblances between the embryos of neighbouring continental forms, as if they had been colonized allied animals and allied plants reveal, according to Mr. Darfrom thence at some far distant period, and as if both had win, their community of descent, "for the embryo is the undergone mutual separation by descent with modification. animal in its less modified form.' As it develops, its parts Even the species on adjacent islands, as in the Galapagos become specialized by modification, and thence it is that in its group, are sometimes different, though allied. The two last- early stages it better exhibits its genealogical relations to named classes of facts are both quoted to show that physical other species, through their remote and common progenitors. conditions, which must here be very much alike, are not the Agassiz has remarked, too, that ancient species are more only determining causes of species through natural selection, embryonic in their characters than existing ones. The exbut that the organic conditions resulting from the mutual tremest amount of specialization of parts,-such, for example, relations of plants and animals amongst each other, in their as determine the characters of the individuals of a species,so-called "struggle for existence," are even more important occur late in life or growth; that is to say, when natural agents, here as well as elsewhere, in determining the ultimate selection begins to operate, which it cannot do in the embrycharacter of the dominant kinds. It appears that on oceanic onic state. A tendency to late specialization, as in the unusual islands certain orders are not represented at all: the batra- growth of horns in cattle, may become an inherited and chians (frogs, toads, and newts), for example, whose spawn is selected characteristic; and on this principle may be explained killed in salt water. Terrestrial mammals are not found the postponed specialization illustrated in the stages of cateraboriginal in islands more than 300 miles from a continental pillar, chrysalis, and perfect insect. area, nor in islands situated at much shorter distances, if separated by very deep channels; but those winged mammals, the bats, are found on most oceanic islands, however remote from larger areas. Continental trees are not found on tant islands, but certain orders whose species are herbaceous on continents, are represented by tree-forms of the same order, as if they there had had room to be gradually modified into trees. The land-shells on islands are mostly endemic; the few continental forms may have been transported thither, clinging in their young state to water-birds' feet, or lodged, when hibernating in the adult state, in the crevices of drifting timber,-for, when hibernating, the mouth of their shell is closed by a membrane, and, as Mr. Darwin has proved, they will survive a considerable time in salt water.

Lastly, the frequent presence, in both plants and animals, of rudimentary organs having no utility-a puzzle to the teleological biologist or advocate for "final causes" in everything, dis-explained by others on the insufficient ground of their being outlets of excretion, or safety valves of nutritive effort, -or accounted for on what Mr. Darwin characterizes as the unmeaning principle of "a common plan" or "unity of design," or, as it is said, "for the sake of symmetry," or "to complete the scheme of nature,"-affords another illustration of the applicability of the theory of descent with modification. For on that hypothesis, rudimentary organs may either be inherited parts retained but no longer needed, disused and therefore atrophied,-as the eyes of the mole, the wings of certain birds, and the wings of certain beetles; or parts, then called nascent organs, on their way to development in future species, by descent with modification; or the results of a tendency to revert to some character of an old progenitor. Sometimes, in certain species of a genus, such parts are entirely absent or aborted. Rudimentary organs are usually proportionally larger in the embryo than in the adult: sometimes, though present in embryos, they afterwards disappear, in accordance with the general fact that natural selection, to whatever end it works, operates more and more as growth proceeds.

To the subject of classification, Mr. Darwin's theory has many evident and interesting applications. The affinities of all organic beings, as brought out in all natural systems of classification, exhibit themselves in descending series of groups. In bringing together individuals, as males and females, and sometimes (as we have seen in the case of ants) neuters also, the young and the old, varieties of all shades, and even monstrosities,-into one specific group, the principle of descent is always and of necessity employed. Varieties themselves are also sub-classified on similar genetic relations. This same principle, as indicated by inherited qualities, Mr. Darwin believes has been "unconsciously used" by systematists "in grouping species under genera, and genera into higher groups," which in fact, when really naturally defined, are strictly genealogical. Descent is the hidden bond which naturalists have been seeking in their attempted systems. In such systems, the parts of plants or animals taken advantage of for classifying are not those which have a merely functional or physiological importance, but those which are common to successively larger and larger groups, specific, generic, ordinal, or classic,-that is, which have been inherited from more and more remote progenitors,-just those, in fact, which indicate true affinity and not mere utility or analogy. Those remarkable "aberrant" forms which, like the lepidosiren, might fancifully be called "living fossils," are explained by their being "failing groups" of very ancient descent, conquered by more successful competitors; hence their remote connexion with existing forms.

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The "unity" of type manifested both in plants and animals by the few common patterns on which all their almost endless "variety" of forms is founded,--the general correspondences, or homologies as they are called, between certain parts, say between the limbs of the different vertebrate animals, the serial correspondences or homologies seen in the successively repeated parts or segments of the internal skeleton and other organs of those animals,--the like facts observable in the articulate and most other of the invertebrate animals, and lastly, the obvious morphological relations of the parts of plants,-facts which are confessedly inexplicable on the hypothesis of "uses " or "final causes" merely, and of which Mr. Darwin declares he sees no sufficient explanation in such abstract phrases as "unity of design," or "common plan of creation,"-are at once intelligible, however varied and involved they may be, on the theory of descent from a few parent stocks. And just as it is by natural selection that the whole individual is modified, so by the same agency are the component serial segments of its frame developed and adapted to their special uses; for these same segments, as stated under the law of "vegetative repetition," which has

In finally reviewing the facts and considerations relating to classification, morphology, embryology, and the study of rudimentary organs, Mr. Darwin says they "proclaim so plainly that the innumerable species, genera, and families of organic beings with which this world is peopled, have all descended, each within its own class or group, from common parents, and have all been modified in the course of descent, that I should without hesitation adopt this view, even if it were unsupported by other facts and arguments."

In a concluding chapter, Mr. Darwin gives an able and glowing recapitulation of his whole case. On the primary question of the mutability of species, he re-affirms that, on a review of the facts of variation as they affect varieties and species, and of the laws of hybridity and mongrelism, "no one can draw any clear distinction between individual differences and slight varieties, or between more plainly marked varieties and sub-species and species." "It cannot be asserted that organic beings in a state of nature are not subject to variation; it cannot be proved that the amount of variation, in the course of long ages, is a limited quantity, . and we are too apt to assume, without proof, that the geological record is so perfect, that it would have afforded us plain evidence of the mutation of species if they had undergone mutation." On the secondary question of the mode in which, or the laws by which, species have originated, he says:-" These laws, taken in the largest sense, being Growth with Reproduction; Inheritance, which is almost implied by reproduction; Variability from the indirect and direct action of the external conditions of life, and from use and disuse; a Ratio of Increase so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing Divergence of Character and the Extinction of less improved forms.'

From this recapitulatory chapter, we shall make but two more sets of extracts,-the one relating to the extent to which the author would carry out his views, and the other referring to the very obvious objections that may be made to his theory on theological grounds.

After having stated, what we have already quoted, that analogy would lead him "to the belief that all animals and

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