Esperanto. "Formal arithmetic was begun at six and in less than a year they were solving mentally problems in fractions and percentage. At the age of nine both were doing Junior High School work. They speak French fluently, and have made progress in Italian and have embarked upon Russian. They are

FIG. 180-Two remarkably similar twins. Over the left eye of each twin there is a small mole. The two men are very much alike in manner and other qualities, and their voices are almost indistinguishable. (After Wiggam.)

much alike in their tastes and dispositions. Their mental tests and their vocabulary tests give almost the same scores."

In their physical measurements the two girls were remarkably alike. They exhibited the same reactions to blood tests and to vaccination. The palm prints and the prints of the soles of the feet were so similar that the same descriptive formula applied to both. At eight years of age the right upper incisor tooth was in the same stage of retarded development, and on the

upper lip of each, a little above the outer corner of the mouth, was a small, pigmented mole.

One of Galton's correspondents writing of a pair of twin boys says: "They have had exactly the same nurture from their birth up to the present time; they are both perfectly healthy and strong, yet they are otherwise as dissimilar as two boys could be, physically, mentally, and in their emotional nature."

FIG. 181-Hands of the twins represented in the preceding figure, showing an unusual fold behind the third finger. (After Wiggam.)

When we compare such cases with the remarkable similarities presented by the two girls described by Dr. Gesell, we cannot help being strongly inclined to agree with Galton when he says: "There is no escape from the conclusion that nature prevails enormously over nurture when the differences of nurture do not exceed what is commonly to be found among persons of the same rank of society and in the same country."

REFERENCES

BALL, W. P., Are the Effects of Use and Disuse Inherited? London, Macmillan, 1890.

CONKLIN, E. G., Heredity and Environment (5th ed.). Princeton University Press, 1922.

GALTON, F., Inquiries into Human Faculty (2nd ed.). N. Y., Dutton, 1907.

KAMMERER, P., The Inheritance of Acquired Characters. N. Y., Boni

and Liveright, 1924.

PACKARD, A. S., Lamarck. N. Y., Longmans, Green, 1901.

PEARL, R., Modes of Research in Genetics. N. Y., Macmillan, 1915. RIGNANO, E., The Inheritance of Acquired Characters.

Open Court Co., 1911.

THOMSON, J. A., Heredity. N. Y., Putnam, 1913.

Chicago,

WEISMANN, A., Essays on Heredity. 2 vols. Oxford, Clarendon Press, 1891-92.

The Evolution Theory. 2 vols. London, Arnold, 1904.

CHAPTER XVI

ORGANIC EVOLUTION

A. HISTORICAL ORIENTATION

The world of life presents an almost infinite variety of different forms. The species of animals that have been described number several hundred thousands, and the total number of living species must certainly be over a million, and may be several millions. When we add to these the numerous species of plants, and reflect that the existing species of plants and animals constitute but a small fraction as compared with the number which have lived in previous geological ages, we may be able to form some conception of the vast multitudes of different forms which have come into being upon our earth. How did all these diverse creatures arise?

Less than a century ago it was the prevalent belief that each species was created much as we find it today. The doctrine of creation applied to species is a manifestation of a general tendency of mankind to attribute to supernatural causes the occurrence of phenomena which cannot be readily explained. Ideas of natural law and the uniformity of nature have but slowly supplanted the conceptions of a primitive cosmogony, the belief in the supernatural origin of species being one of the last to hold out in the light of advancing knowledge.

Although it is only recently that the theory of evolution has come to prevail, we nevertheless find suggestions of it even in ancient times. Theories of the naturalistic origin of living beings were advanced by Democritus, Anaximenes, Heraclitus, Diogenes of Apollonia, Empedocles, and other Greek philosophers, but it is doubtful if any of these speculators had the modern conception of evolution as the derivation of higher from lower forms of life. They believed instead that all kinds of living

beings had an independent natural origin from the materials of the earth. Aristotle, however, who was a naturalist as well as a philosopher, apparently held to the gradual succession of living beings from the very simplest up to man, but it is not certain that he believed that species actually arose from others by a process of descent.

For the first sixteen centuries of the Christian era the doctrine of evolution was practically in abeyance. The teachings of the first chapters of the book of Genesis were held to establish the doctrine of creation beyond question, and any active opposition to the prevalent conviction would have been very unpopular, if not dangerous. During the seventeenth and eighteenth centuries thought gradually became more emancipated from theological dogma. Among speculative philosophers the theory of evolution had been sympathetically discussed by Bacon, Leibnitz, Hume, Spinoza, and especially Kant. Near the middle of the eighteenth century the great French naturalist Buffon (1707-88) expressed a belief in the mutability of species as a result of the influence of climate and other physical surroundings. While he pointed out many of the structural indications of a common origin for the members of different groups of animals, he vacillated between the advocacy of evolution and the expression of a pious adherence to the doctrine of special creation, so that his real opinions have proven a source of much perplexity to subsequent interpreters. Apparently he was very desirous of presenting the facts which favor the doctrine of evolution, but his deference to the teachings of the Church (for he had some unpleasant encounters with ecclesiastical authorities on account of his heretical opinions) led him to profess belief in the accepted doctrine of creation at the same time.

In England, Dr. Erasmus Darwin (1731-1802), the grandfather of Charles Darwin whose name is now so closely linked with the doctrine of evolution, set forth with much force and in considerable detail various evidence that all organisms are descended from a common primitive ancestral form, or, as he expressed it, "from a similar living filament." In Germany,