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suffer the loss of their own lives in protecting their off spring from enemies. Many mother birds have the instinct to flutter about a discovered nest crying and apparently broken-winged, thus leading the predatory fox or weasel to

FIG. 165.-Nest and run-way of the pocket-gopher.

fix his attention on the mother and to leave the nest unharmed. This development of parental care and protection of the young reaches its highest degree in the case of the human species. The existence of the family, which is the unit of human society, rests on this high development of care for the young.

CHAPTER XVI

GEOGRAPHICAL DISTRIBUTION OF ANIMALS

148. Geographical distribution.-Under the head of 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 zoogeography. 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 cotton-tail 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 black rabbits of the islands of Lower Califonia, and the marsh-hare of the South and the water-hare of the canebrakes, and that of all their relatives. Such a

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FIG. 166.-Map showing the distribution of the clouded Skipper butterfly (Lerema accius) in the United States. The butterfly is found in that part of the country shaded in the map, a warm and moist region.-After SCUDDER.

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FIG. 167.-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 shaded 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.

map is very instructive, and 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.

149. Laws of distribution.-The laws governing the distribution of animals are reducible to three very simple propositions. Every species of animal is found in every part of the earth having conditions suitable for its maintenance, unless

(a) Its individuals have been unable to reach this region, through barriers of some sort; or

(b) Having reached it, the species is unable to maintain itself, through lack of capacity for adaptation, through severity of competition with other forms, or through destructive conditions of environment; or

(c) Having entered and maintained itself, it has become so altered in the process of adaptation as to become a species distinct from the original type.

150. Species debarred by barriers.-As examples of the first class we may take the absence of kingbirds or meadowlarks or coyotes in Europe, the absence of the lion and tiger in South America, the absence of the civet-cat in New York, and that of the bobolink or the Chinese flying-fox in California. In each of these cases there is no evident reason why the species in question should not maintain itself if once introduced. The fact that it does not exist is, in general, an evidence that it has never passed the barriers which separate the region in question from its original

home.

Local illustrations of the same kind may be found in most mountainous regions. In the Yosemite Valley in California, for example, the trout ascend the Merced River to the base of a vertical fall. They can not rise above this, and so the streams and lakes above this fall are destitute of fish.

151. Species debarred by inability to maintain their ground. -Examples of the second class are seen in animals which man has introduced from one country to another. The nightingale, the starling, and the skylark of Europe have been repeatedly set free in the United States. But none of these colonies has long endured, perhaps from lack of adaptation to the climate, more likely from severity of competition with other birds. In other cases the introduced species has been better fitted for the conditions of life than the native forms themselves, and so has gradually crowded out the latter. Both these cases are illustrated among the rats. The black rat, first introduced into America from Europe about 1544, helped crowd out the native rats, while the brown rat, brought in still later, about 1775, in turn practically exterminated the black rat, its fitness for the conditions of life here being still greater than that of the other European species.

Certain animals have followed man from land to land, having been introduced by him against his will and to the detriment of his domestic animals or crops. To many of these the term vermin has been applied. Among the vermin or 66 animal weeds " are certain of the rodents (rats, mice, rabbits, etc.), the mongoose of India, the English sparrow, and many kinds of noxious insects. Of all the vermin of this class few have caused such widespread destruction of property as the common European rabbit introduced into Australia. The annual loss through its presence is estimated at $3,500,000.

It often happens that man himself so changes the environment of a species that it can no longer maintain itself. Checking the increase of a species, either by actually killing off its members or by adverse change in its surroundings, is to begin the process of its destruction. Circumstances become unfavorable to the growth or reproduction of an animal. Its numbers are reduced, fewer are born each year, and fewer reach maturity, it grows rare,

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