The Origin of Species: By Means of Natural Selection of the Preservation of Favoured Races in the Struggle for Life. –

Summary of Chapters 12-13 – Geographical Distribution: Summary by Susan Fleck

1. Neither the similarity nor the dissimilarity of the inhabitants of various regions can be wholly accounted for by climatic and other physical conditions. In the U.S. we meet with the most diversified conditions—there is hardly a climate or condition in the Old World which cannot be paralleled in the New—at least as closely as the same species generally require. Nevertheless, how widely different are their living productions! If we compare large tracts of land in Australia, S. Africa, and western S. America (between latitudes 25-35 degrees), we shall find parts extremely similar in all their conditions, yet it would not be possible to point out three faunas and floras more utterly dissimilar. However, if we compare the productions of South America south at lat. 35 degrees with those north of 25 degrees, exposed to considerably different conditions; yet they are incomparably more closely related to each other than they are to the productions of Australia or Africa under nearly the same climate. Analogous facts could be given with respect to the inhabitants of the sea.

2. Barriers of any kind, or obstacles to free migration, are related in a close and important manner to the differences between the productions of various regions. We see this in the great difference in nearly all the terrestrial productions of the New and Old Worlds, excepting in the northern parts, where the land almost joins—where there might have been free migration for the northern temperate forms. The same for Australia, Africa, and S. America under the same latitude—totally isolated from each other. On each continent, we find different production on the opposite sides of lofty and continuous mountain ranges, of great deserts and even of large rivers. However, as these continental features are not as impassable, or likely to have endured so long as the oceans separating continents, the differences are very inferior in degree to those characterized of distinct continents.

3. The same law applies to the sea. E.g., The marine inhabitants of the eastern and western shores of S. America are very distinct, with extremely few shells, crustacean, or Echinodermata in common. But about thirty percent of the fishes are the same on opposite sides of the Isthmus of Panama, leading naturalists to believe that the isthmus was formerly open.

4. There is affinity of the productions of the same continent or of the same sea, though the species themselves are distinct at different points and stations. It is a law of the widest generality, and every continent offers innumerable instances. Nevertheless, the naturalist, in travelling, e.g., from north to south, is struck by the manner in which successive groups of beings, specifically distinct, though nearly related, replace each other. We see in these facts some deep organic bond, throughout space and time, over the same areas of land and water, independently of physical conditions. The naturalist is led to inquire what is this bond.

5. The bond is simply inheritance, that cause which alone produces organisms quite like each other, or, as we see in the case of varieties, nearly alike. The dissimilarity of the inhabitants of different regions may be attributed to modification through variation and natural selection, and probably in a subordinate degree to the definite influence of different physical conditions.

6. The degrees of dissimilarity will depend on the migration of the more dominant forms of life from one region into another having been more or less effectually prevented, at periods more or less remote; on the nature and number of the former immigrants; and on the action of the inhabitants on each other in leading to the preservation of different modifications; the relation of organism to organism in the struggle for life being the most important of all relations. The high importance of barriers comes into play by checking migration; as does time for the slow process of modification through natural selection.

7. Widely-ranging species, abounding in individuals, which have already triumphed over many competitors in their own widely-extended homes, will have the best chance of seizing on new places, when they spread into new countries: they will be exposed to new conditions, and will frequently undergo further modification and improvement; and thus they will become still further victorious, and will produce groups of modified descendants.

8. According to these views, it is obvious that the several species of the same genus, though inhabiting the most distant quarters of the world, must originally have proceeded from the same source, as they are descended from the same progenitor. In the case of those species which have undergone during the whole geological periods little modification, there is not much difficulty in believing that they have migrated from the same region; for during the vast geographical and climatic changes which have supervened since ancient times, almost any amount of migration is possible.

9. Single Centers of Supposed Creation: However, regarding species which have been produced within comparatively recent times: We are thus brought to the question, largely discussed by naturalists, namely, whether species have been created at one or more points of the earth’s surface. There are many cases of extreme difficulty in understanding how the same species could possibly have migrated from one point to the several distant and isolated points where now found. Nevertheless the simplicity of my view that each species was first produced within a single region captivates the mind. [Remember, individuals identically the same cannot have been produced from parents specifically distinct.] He who rejects it, rejects the vera causa [true cause] of ordinary generation with subsequent migration, and calls in the agency of a miracle.

10.The area inhabited by a species is continuous. When a species inhabits two points so distant from each other, that the space could not have been easily passed over by migration, an explanation is called for. The incapacity of migrating across a wide sea is more clear in the case of terrestrial mammals than perhaps with any other organic beings; accordingly, we find no inexplicable instances of the same mammals inhabiting distant points of the world. If the same species can be produced at two separate points, why do we not find a single mammal common to Europe and Australia or South America? The conditions of life are nearly the same so that a multitude of European animals and plants have become naturalized in America and Austrailia [through man’s agency]. Some plants, from their varied means of dispersal can be found over wide and broken interspaces.

11.In many cases where we cannot explain how the same species could have passed from one point to another, might be explained by geographical changes which have certainly occurred within recent geological times which must have rendered discontinuous the formerly continuous range of many species.

12.Can the several species of a genus which must on our theory all be descended from a common progenitor—can they have migrated, while undergoing modification during their migration from some one area? If, when most of the species inhabiting one region are different from those of another region, though closely allied to them, it can be shown that migration from the one region to the other has probably occurred at some former period, our general view will be much strengthened; for the explanation is obvious on the principle of descent with modification. A volcanic island, e.g., upheaved and formed at the distance of a few hundred miles from a continent, would receive from it in the course of time a few colonists, and their descendants, though modified, would still be related by inheritance to the inhabitants of that continent. [Darwin presents cases of this, inexplicable on the theory of independent creation.]

13.Means of Dispersal: [Darwin gives credit to Lyell and others who have ably treated this subject.] Changes of level in the land must have been highly influential: a narrow isthmus now separates two marine faunas; submerge it, or let it formerly have been submerged, and the two faunas will now blend together. Where the sea now extends, land may at a former period have connected islands or possibly even continents together, and thus have allowed terrestrial productions to pass from one to the other. No geologist disputes that great mutations of level have occurred within the period of existing organisms. [Now, we have evidence of the supercontinent theory upon which Darwin speculated.]

a. But I do not believe that within the recent period most of our continents which now stand quite separate have been continuously, or almost continuously united . . . Several facts in distribution . . . are opposed to the admission of such prodigious geographical revolutions within the recent period.

14.Accidental means: [Darwin describes several ways in which plant seeds can be distributed widely within continents or to relatively nearby islands; e.g., by surviving floating in salt-water (he performed experiments about survival times); several ways through living birds, including eating fish which have devoured seeds; and icebergs carrying seeds long distances during the ice age.] Considering these several means of transport, and others to be discovered, it is explainable how during the course of geological time, plants have thus become widely transported. But due to ocean currents and other factors, the floras of distant continents would not by such means become mingled.

15.Dispersal during the Glacial Periods: Darwin provides many examples and facts to support his claim that during ice ages, during the advance of cold weather and ice stemming from the polar regions, plants (via dispersal means) and animals steadily migrated widely away from the polar regions toward the more temperate temperatures (i.e., toward the equator) over those long periods of time; then, as the ice and subsequent cold temperatures receded to the polar regions, those plants and animals suited for the colder habitats migrated back toward the polar regions.

a. In the case of alpine species that live in upper elevations of mountain ranges: those species which could not tolerate the more extreme climate had to migrate ‘downward’ to a more suitable climate, and then back up the mountain as the climate changed back again.

b. During the long time that they were in the temperate zones, northern forms would be mingling with southern forms. When the warmth returned, some of the northern more temperate forms, but which needed a colder climate, would have ascended any adjoining high land. The change of temperature must have been very slow, and plants possess a certain capacity for acclimatization. This explains why we have some few species [of plants] identically the same in the northern and southern temperate zones and on the mountains of the intermediate tropical regions.

16.Fresh-Water Productions: Darwin explains how it is possible that fresh-water species belonging to different classes have an enormous range of distribution throughout the world, in spite of the barriers of land and oceans: [Through natural selection] they have become fitted, in a manner useful to them, for short and frequent migrations from pond to pond; from stream to stream, within their own countries. From different continents in southern continents: this probably indicates dispersal from an Antarctic center during a former warm period.

a. It is probable that they are occasionally transported by what may be called accidental means: Darwin gives us possible means: Fishes still alive not very rarely dropped at distant points by whirlwinds; Ova retain their vitality for a considerable time after removal from the water – these eggs are transported by birds that have captured the fish. Dispersal is mainly attributed to changes in the level of the land within the recent period – causing rivers to flow into each other -- or this having occurred during floods, without any change in level. Darwin provides other means of dispersal for plants and fresh-water shell animals; e.g., from being in mud stuck to the feet of ducks and other shore birds.

On the Inhabitants of Oceanic Islands

1. The species of all kinds which inhabit oceanic islands are few in number compared with those on equal continental areas – including plants and insects. [many facts and examples given]. He who adheres to the creation of each separate species, will have to admit that a sufficient number of the best adapted plants and animals were not created for oceanic islands; for man has [in recent history] unintentionally stocked them far more fully and perfectly than did nature.

2. Although the species are few in number, the proportion of endemic kinds [i.e., those found nowhere else in the world] is often extremely large. E.g., the number of endemic birds in the Galapagos Archipelago.

3. Oceanic islands are sometimes deficient in animals of certain whole classes, and their places are occupied by other classes; thus in the Galapagos Islands, reptiles, and in New Zealand gigantic wingless birds, take, or recently took, the place of mammals. [It is doubtful whether New Zealand should be considered an island.] Regarding plants, Dr. Hooker has shown that in the Galapagos, the proportional members of the different orders are very different from what they are elsewhere.

4. Facility of immigration seems to have been fully as important as the nature of the conditions in order to account for all of these large differences when compared to continents.

5. Absence of Batrachians and Terrestrial Mammals on Oceanic Islands: Batrachians (frogs, toads, newts) are never found on most oceanic islands. It seems that islands are peculiarly fitted for these animals—some have been introduced and have since become a nuisance. But these animals and their spawn (with exception of one known species) are immediately killed by sea-water, so we see the great difficulty in their being transported there. But why they should not have been created there, it would be very difficult to explain. Except for domestic animals kept by natives, Darwin did not know of a single mammal inhabiting an island more than 300 miles from a continent. [Galapagos has the rice rat.] Small mammals do occur in many parts of world on very small islands, when lying close to a continent. Hardly an island can be named on which our smallest quadrupeds have not become naturalized and greatly multiplied.

6. There is a relation between the depth of the sea separating islands from each other or from the nearest continent, and the degree of affinity of their mammalian inhabitants (of those close-in islands). One example is Britain, separated by a shallow channel from Europe, and the mammals are the same on both sides; and so it is with all the islands near the shores of Australia.

7. [Facts in #5 and #6] accord better with the belief in the efficiency of occasional means of transport, carried on during a long course of time, than with the belief in the former connection of all oceanic islands with the nearest continent; for on this latter view . . . the various classes would have immigrated more uniformly, and from the species having entered in a body, their mutual relations would not have been much disturbed, and consequently they would either have not been modified, or all the species in a more equable manner.

8. On the Relations of the Inhabitants of Islands to Those of the Nearest Mainland: The most striking and important fact for us is the affinity of the species which inhabit islands to those of the nearest mainland, without being actually the same. In the Galapagos Archipelago almost every product of the land and of the water bears the unmistakable stamp of the S. American continent. E.g., of 26 land birds, 21-23 are ranked as distinct (endemic) species; and would commonly be assumed to have been here created; yet the close affinity of most of these birds to American species is manifest in every character. So it is with the other animals and many plants. Why should this be so, several hundred miles from the continent?

a. There is nothing in the conditions of life, in the geological nature of the islands, in their height or climate, or in the proportions in which the several classes are associated together, which closely resembles the conditions of the South American coast: in fact, there is a considerable dissimilarity in all these respects. On the other hand, there is a considerable degree of resemblance in the volcanic nature of the soil, in the climate, height, and size of the islands, between the Galapagos and Cape Verde Islands; but what an entire and absolute difference in their inhabitants! The inhabitants of the Cape Verde Islands are related to those of Africa, like those of the Galapagos to S. America. Facts such as these admit of no sort of explanation on the ordinary view of independent creation; whereas on our view, it is obvious that the Galapagos Islands would be likely to receive colonists from S. America by occasional means of transport. And such colonists would be liable to modification—the principle of inheritance still betraying their original birthplace.

b. On a smaller scale, the same law which has determined the relationship between the inhabitants of islands and the nearest mainland, applies to islands within the same archipelago. In the Galapagos, the endemic species of the various islands are related to the same class of species on the other islands in a very much closer manner than to the inhabitants of the S. American continent. These islands, relatively near to each other would almost necessarily receive immigrants from the same original source, and from each other—but probably at different time intervals. Also, keep in mind that each island is of a different age and stage of subsidence – thus exposed to different conditions on the different islands, and compete with a different set of organisms. The Galapagos islands are separated from each other by deep arms of the sea and many by large distances.

9. Overall Summary of Geographical Distribution chapters 12 & 13

a. Given all these facts, and knowing there are probably more means of occasional transport than thus discussed; knowing how often a species may have ranged continuously over a wide area, and then have become extinct in the intermediate tracts—the difficulty is not insuperable in believing that all the individuals of the same species, wherever found, are descended from common parents.

b. The same holds with respect to distinct species belonging to the same genus, which on our theory have spread from one parent-source—remembering that some forms of life have changed very slowly, enormous periods of time having been thus granted for their migration; and considering the important part the last Glacial period has played.

c. All the grand leading facts of geographical distribution are explicable on the theory of migration, together with subsequent modification and the multiplication of new forms. We thus understand the high importance of barriers, whether of land or water, in not only separating, but in apparently forming the several zoological and botanical provinces—and the concentration of related species within the same areas, and the linkages under different latitudes.

d. Knowing that the mutual relation of organism to organism is of the highest importance, we see why two areas having nearly the same physical conditions should often be inhabited by very different forms of life—due to—length of time elapsed since colonists entered one of the regions.

According to our theory, these several relations throughout time and space are intelligible; for whether we look to the allied forms of life which have changed during successive ages, or to those which have changed after having migrated into distant quarters, in both cases they are connected by the same bond of ordinary generation; in both cases the laws of variation have been the same, and modifications have been accumulated by the same means of natural selection.