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CHAPTER XV

GEOGRAPHICAL DISTRIBUTION

Is not the biological laboratory which leaves out the ocean and the mountains and meadows a inonstrous absurdity? Was not the greatest scientific generalization of your times reached independently by two men who were eminent in their familiarity with living beings in their homes?-HUXLEY.

UNDER the head of “Geographical Distribution ” we consider the facts of the diffusion of organisms over the surface of the earth, and the laws by which this diffusion is governed.

The geographical distribution of animals is often known as “zoögeography.” In physical geography we may prepare maps of the earth which shall bring into prominence the physical features of its surface. Such maps would show here a sea, here a plateau, here a range of mountains, there a desert, a prairie, a peninsula, or an island. In political geography the maps show the physical features of the earth, as related to the states or powers which claim the allegiance of the people. In zoogeography the realms of the earth are considered in relation to the types or species of animals which inhabit them.

Thus a series of maps of the United States could be drawn which would show the gradual disappearance of the buffalo before the attacks of man. Another might be drawn which would show the present or past distribution of the polar bear, black bear, and grizzly. Still another might show the original range of the wild hares or rabbits of the United States, the white rabbit of the Northeast, the cottontail of the East and South, the jack rabbit of the plains, the snowshoe rabbit of the Columbia River, the tall jack rabbit of California, the marshhare of the South and the waterhare of the canebrakes, and that of all their relatives. Such a map is very instructive, and 21

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Fig. 183.- Map showing the distribution of the Canadian Skipper butterfly, Erynnis

manitoba, in the United States. The butterfly is found in that part of the country shown in the map. This butterfly is subarctic and subalpine in distribution being found only far north or on high mountains, the two southern projecting parts of its range being in the Rocky Mountains and in the Sierra Nevada Mountains. (After Scudder.)

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15 Fig. 184.-Map showing the distribution of the Clouded Skipper butterfly, Lerema

accius, in the United States. The butterfly is found in those parts of the country shown in the map by the shading marks--the warm, moist Southern and Eastern parts. (After Scudder.)

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it at once raises a series of questions as to the reasons for each of the facts in geographical distribution, for it is the duty of science to suppose that none of these facts is arbitrary or meaningless. Each fact has some good cause behind it.

It was this phase of the subject, the relation of species to geography, which first attracted the attention of both Darwin and Wallace. Both these observers noticed that island life is neither strictly like nor unlike the life of the nearest land, and that the degree of difference varies with the degree of isolation. Both were led from this fact to the theory of derivation, and to lay the greatest stress on the progressive modification resulting from the struggle for existence.

In the voyage of the Beagle Darwin was brought in contact with the singular fauna of the Galapagos Islands, that cluster of volcanic rocks which lies in the open sea about six hundred miles west of the coasts of Ecuador and Peru. The sea birds of these islands are essentially the same as those of the coast of Peru. So with most of the fishes. We can see how this might well be, for both sea birds and fishes can readily pass from the one region to the other. But the land birds, as well as the reptiles, insects, and plants, are largely peculiar to the islands. Many of these species are found nowhere else. But other species very much like them in all respects are found, and these live along the coast of Peru. In the Galapagos Islands, according to Darwin's notes,

“there are twenty-six land birds; of these, twenty-one, or perhaps twenty-three, are ranked as distinct species, and would be commonly assumed to have been here created; yet the close affinity of the most of these birds to American species is manifest in every character, in their habits, gestures, and tones of voice. So it is with the other animals and with a large proportion of the plants. “. . . . The naturalist, looking at the inhabitants of these volcanic islands in the Pacific, feels that he is standing on American land.”

This question naturally arises: If these species have been created as we find them on the Galapagos, why is it that they should all be very similar in type to other animals, living under wholly different conditions, but on the coast not far away? And, again, why are the animals and plants of another cluster of volcanic islands—the Cape Verde Islands—similarly related to those of the neighboring coast of Africa, and wholly unlike those of the Galapagos? If the animals were created to match their conditions of life, then those of the Galapagos should be like those of Cape Verde, the two archipelagoes being extremely alike in soil, climate, and physical surroundings. If the species on the islands are products of separate acts of creation, what is there in the nearness of the coasts of Africa or Peru to influence the act of creation so as to cause the island species to be, as it were, echoes of those on shore? If, on the other hand, we should adopt the obvious suggestion that both these clusters of islands have been colonized by immigrants from the mainland, the fact of uniformity of type is accounted for, but what of the difference of species? If the change of conditions from continent to island causes such great and permanent changes as to form new species from the old, why may not like changes take place on the mainlands as well as on the islands? And if possible on the mainland of South America, what evidence have we that species are permanent anywhere? May they not be constantly changing? May what we now consider as distinct species be only the present phase in the changing history of the series of forms which constitute the species? The studies of island life can lead but to one conclusion: These volcanic islands rose from the sea destitute of land life. They were settled by the waifs of wind and of storm, birds blown from the shore by trade winds, lizards and insects carried on drift logs and floating vegetation. Of these waifs few came perhaps in any one year, and few, perhaps, of those who came made the islands their home; yet, as the centuries passed on, suitable inhabitants were found. That this is not fancy we know, for we have the knowledge of specific examples of the very same sort. We know how many animals are carried from their natural homes. One example of this may be seen by those who have approached our eastern shores by sea in the face of a storm. Many land birds—sparrows, warblers, chickadees, and even woodpeckers—are carried out by the wind, a few falling exhausted on the decks of ships, a few others falling on offshore islands, like the Bermudas, the remainder drowning in the sea. Of the immigrants to the Galapagos the majority doubtless die and leave no sign. A few remain, multiply, and take

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Fig. 185.-Locusts, Schistocerca, taken on Galapagos Islands, Pacific Ocean, all descended from a common ancestor but now scattered over

the various islands and varying in size and markings: a, Schistocerca melanocera Stol (Charles Island); 6, S. intermedia borealis Snod-
grass (Abingdon and Bindlow Islands); c, S. intermedia Snodgrass (Duncan Island); d, S. literosa Walker (Chatham Island); e, S. melano-
cera lineata Snodgrass (Albemarle Island); f, S. melanocera immaculata Snodgrass (Indefatigable Island). The species intermedia is
probably a hybrid between the other two species.

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