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the Drepanidæ, consisting of nine genera and about forty species. Some species have a short, massive bill like that of a parrot; in some the bill is short and slender, and in several others it is long and curved downward. The striking modifications of form and habit occurring in this family recall the diversities of the marsupial fauna of Australia. The group is probably an ancient one which, in the absence of much competition from other kinds of birds, has branched out and become adapted to live in a number of different situations.

In a very high percentage of cases, the members of a species have a continuous range. Exceptional cases occur where a once widely ranging species has become extinct in parts of its former area of distribution. The Torrey pine of California presents such a case, since it is found only in a small grove on the mainland near San Diego and on some of the small islands off the coast of southern California. Often, but less frequently, the species of a genus have a continuous range, or a range broken by relatively narrow barriers. The same principle applies in decreasing degrees to the genera of a family, the families of an order, and the orders of a class. It may be regarded as the expression of the tendency of groups of organisms to remain somewhere near their place of origin.

There are, however, striking cases of discontinuity and a consideration of these is instructive in affording further insight into the agencies which affect the distribution of species. On the top of the Alps, Pyrenees, and Caucasus Mountains there occur several identical or closely related species which are not found in the intervening areas. In some cases these are the same as, or closely related to species inhabiting the Arctic Zone. The explanation of these cases of discontinuity is afforded by the fact that during the last glacial period an immense sheet of ice gradually extended from the polar regions, driving ahead of it the flora and fauna of the continent. With the subsequent retreat of the ice sheet, species adapted to live in a cold climate followed it back and also ascended the mountains, so that they now survive only in these widely separated localities.

Other discontinuities of distribution are accounted for by the extinction of ancestral forms in intermediate regions. Marsupials are now found only in the Australian region and in the New World, but remains of marsupials occur in the early Tertiary deposits of Europe and Asia, as well as in North and South America. Similarly the tapirs, which are now represented in South America and southeastern Asia, were formerly found in Europe, China, and North America. The camel family, which at present is native only in Asia and South America, is found in the Tertiary and later formations of North America, Asia, and Europe. Also the monkeys, to come nearer home, are found only in the Ethiopian region, the Neotropical region, and in Asia, but, as we have seen in the previous section, monkeys were found during the Tertiary in North America and Europe as well as in the continents they now inhabit.

REFERENCES

CRAMPTON, H. E., The Doctrine of Evolution: Its Basis and Scope. N. Y., Columbia University Press, 1911.

DARWIN, C. R., The Origin of Species. London, Murray, 1859; 6th ed., 1880.

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The Descent of Man and Selection in Relation to Sex. London, Murray, 1871.

GEDDES, P., AND THOMSON, J. A., Evolution. N. Y., Holt, 1911. GRABAU, A. W., A Textbook of Geology, 2 vols. Boston, Heath, 1920

21.

HAECKEL, E., The Natural History of Creation, 2 vols. N. Y., Appleton, 1876.

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The Evolution of Man, 2 vols. N. Y., Putnam, 1910.

HUXLEY, T. H., Man's Place in Nature. N. Y., Appleton, 1863. , American Addresses. N. Y., Appleton, 1877.

, Darwiniana. N. Y., Appleton, 1896.

JORDAN, D. S., AND KELLOGG, V. L., Evolution and Animal Life. N. Y., Appleton, 1907.

KELLOGG, V. L., Darwinism To-day. N. Y., Holt, 1907.

LE CONTE, J., Evolution and Its Relation to Religious Thought. N. Y., Appleton, 1892.

LULL, R. S., Organic Evolution. N. Y., Macmillan, 1917.

LYELL, SIR CHARLES, Principles of Geology (11th ed.). Appleton, 1872. METCALF, M. M., Organic Evolution. N. Y., Macmillan, 1906. NEWMAN, H. H., Readings in Evolution, Genetics, and Eugenics (2nd ed.). University of Chicago Press, 1925.

NUTTALL, G. H. F., Blood Immunity and Blood Relationships. Cambridge University Press, 1904.

OSBORN, H. F., From the Greeks to Darwin. N. Y., Macmillan, 1894.
Men of the Old Stone Age. N. Y., Scribner, 1915.
ROMANES, G. J., Darwin and After Darwin, 3 vols. Chicago, Open
Court Co., 1892-97.

SCOTT, W. D., The Theory of Evolution. N. Y., Macmillan, 1911.
SMITH, G. E., The Evolution of Man. Oxford University Press, 1924.
SPENCER, H., Synthetic Philosophy, 10 vols. N. Y., Appleton, 1878-98.
WALLACE, A. R., The Geographical Distribution of Animals. 2 vols.
London, Macmillan, 1876.

London, Macmillan, 1892.
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Island Life (2nd ed.). -, Darwinism (3rd ed.). WEISMANN, A., The Evolution Theory, 2 vols. London, Arnold, 1904. WIEDERSHEIM, R., The Structure of Man an Index to His Past History. London, Macmillan, 1895.

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CHAPTER XVII

HOW IS ORGANIC EVOLUTION CAUSED?

The present chapter deals with several topics of a more or less controversial nature. There is practically no difference of opinion among informed scientific men as to whether or not evolution has taken place. That is no longer a live issue. But there is a good deal of difference of opinion as to how and why evolution may have been brought about. The evidences for evolution are perfectly cogent, even though all theories as to how the process of evolution might have been caused should be swept away. No one doubts the fact of embryonic development, although if a biologist were asked to explain why a frog or a chick develops from a fertilized egg, I fear that he could not give a very satisfactory reply. Although embryonic development is a familiar phenomenon, we seem to be actually much farther from explaining why it occurs than we are from accounting for the evolution of species.

Lamarck, as we have seen, attributed organic evolution mainly to the transmission of characters acquired by organisms in adjusting themselves to their environment; but, for the reasons that have been stated, most biologists regard the Lamarckian factor alone as incapable of affording a satisfactory explanation of evolution, and a great many biologists reject it entirely. The theory which has received the widest recognition as offering a probable explanation of evolution is that of natural selection, which was advanced independently by Charles Darwin and Alfred Russel Wallace.

The idea of natural selection was suggested to both Darwin and Wallace after reading Malthus' celebrated Essay on Population. The chief thesis of that able essay is that human population tends to increase faster than its means of support, and that

it is consequently kept within bounds by various checks, among which are war, pestilence, and famine. The pressure upon the means of subsistence that results from this overmultiplication inevitably leads to a struggle for existence which, for a large part of mankind, has been more or less severe as far back as human history has been traced. Darwin and Wallace carried over the doctrine of Malthus into the world of plant and animal life. All organisms, it was pointed out, tend to multiply at a geometrical rate of increase. There must inevitably be a struggle for the necessities of life. Many organisms must perish for lack of adequate means of support. A female codfish lays nearly 10,000,000 eggs, but only two on the average produce fishes which reach adult age. It has been estimated that if all the progeny of an oyster could be saved for only four generations the shells would make a heap eight times the size of the earth! Under such conditions competition must indeed be keen.

The ideas which Darwin and Wallace added to the doctrine of Malthus were those of variability and selective survival. It is a familiar fact that organisms vary. Among the variations there will naturally be some which are better adapted to meet the conditions of life than others, and these on the average will be preserved. Given hereditary variations subjected to the struggle for existence, there will result what Spencer has termed in his famous phrase, "the survival of the fittest." This whole process of selective survival of the fittest hereditary variations is called natural selection.

The term natural selection was chosen to desiginate a certain similarity between selection, as it was conceived to take place in nature, and the artificial selection carried on by the breeder of plants and animals in order to effect the desired improvements of his stock. If a breeder is endeavoring to produce a superior variety of race horses he breeds from those animals having the highest record for speed. Our modern race horse is a specialized product of many generations of selective breeding. Similarly, the strong and heavy draft horse represents a product of selective breeding for very different qualities. Through selection. the

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