ers. It is especially noteworthy that in these long-horned phyla the main incidence of selection seems to be diverted to the horns from the teeth which appear to be dwarfed or arrested in evolution. In the short-horned phyla, on the other hand, including one series at least, protected by more slender limbs and more rapid movements, the teeth are constantly sharpened and improved; this may be interpreted as caused by the selection of changes of proportion in the teeth.

The teeth, however, of all these phyla of titanotheres are of a mechanical type which does not admit of further evolution; they have reached a stage which is a cul-de-sac,' beyond which no progress is possible. The change of environment and of flora, therefore, finds these animals incapable of further mechanical betterment either through heredity or through the selection of variations of proportion. All the titanotheres become suddenly extinct, and it is noteworthy that all other herbivorous quadrupeds having this culde-sac type of grinding tooth also became extinct in North America and in Europe either during the Oligocene or Miocene periods.

This is an outline of the only theoretical interpretation which can be offered at present. In it heredity, ontogeny, environment, and selection are supposed to be in continuous interaction or

1 Osborn, H. F.: "Rise of the Mammalia in North America." Vice-Presidential Address before the American Association for the Advancement of Science. Section of Zoology. Madison, Wis., August 7, 1893.

interplay. One feature has been omitted: that is, that all the branches of all the phyla, with one exception, show a continuous and progressive increase in size. This increase in size is, however, itself interpreted not only as a response to favorable environment, but also to the selection of hereditary variations in size due to the fact that the larger quadrupeds are better able to stand off the attacks of their carnivorous enemies.

CONCLUSION

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This interpretation, finally, is seen to include the coöperation of factors recognized by Buffon, by Lamarck, and by Darwin, except as to the transmission of acquired characters, which is left in doubt. There is, however, a new principle in the "mutation of Waagen" or rectigradations of Osborn," unknown to Darwin and due to causes entirely unknown to us at the present time, and perhaps, as already intimated, unknowable. In this connection it is interesting to recall the comment of Aristotle on the survival-of-thefittest theory (the bracketed insertions [ ] and italics are our own) :

1

"What, then, hinders but that the parts in Nature may also thus arise [namely, according to law]. For instance, that the teeth should arise from necessity, the front teeth sharp and adapted to divide food, the grinders broad and adapted to breaking the food into pieces.

1 Osborn, H. F.: From the Greeks to Darwin, 8vo, Macmillan & Co., 1894, p. 55.

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[Another explanation may be offered.] Yet, it may be said, that they were not made for this purpose [i.e. for this adaptation], but that this [adaptive] purposive arrangement came about by chance; and the same reasoning is applied to other parts of the body in which existence for some purpose is apparent. And, it is argued, that where all things happened as if they were made for some purpose, being aptly [adaptively] united by chance, these were preserved, but such as were not aptly [adaptively] made, these were lost and still perish, according to what Empedocles says concerning the bull species with human heads. This, therefore, and similar reasoning, may lead some to doubt on this subject.

"It is, however, impossible that these [adaptive] parts should subsist [arise] in this manner; for these parts, and everything which is produced in Nature, are either always, or, for the most part, thus [i. e., adaptively] produced; and this is not the case with anything which is produced by fortune or chance, even as it does not appear to be fortune or chance that it frequently rains in winter. . . If these things appear to be either by chance, or to be for some purpose, and we have shown that they can not be by chance, then it follows that they must be for some purpose. There is, therefore, a purpose in things which are produced by, and exist from, Nature."

Paleontology at present seems to support the philosophical contention of Aristotle, that when we come to the minute slowly progressing internal changes, the fittest originates in law.

EVOLUTION AND PSYCHOLOGY

BY

G. STANLEY HALL

DARWIN'S CONTRIBUTION TO PSYCHOLOGY

THE contributions of Darwin to psychology have not been adequately recognized. Not only in his famous seventh chapter on “Instinct” in the Origin of Species; in the second and third in the Descent of Man, comparing the psychic powers of men and animals; in his Expressions of Emotions, and in Domestication, but sometimes in other works, he not only showed a depth of insight into, but laid anew the foundations of, genetic as well as comparative psychology. These should, and I believe will, eventually make him regarded as hardly less the founder of a new departure in this field than in that of classification, form, and structure. For him the soul of man is no whit less the offspring of that of animals than is his body. Our psychic powers are new dispensations of theirs. The ascending series of gradations is no more broken for the psyche than for the soma. The gaps are no wider or more numerous from the lowest to the highest in the one than in the other. The affinities and analogies are as close, and the soul in

herits as much from our venerable, brute forbears as does the body. The rudiments are as numerous and, to those who can rightly interpret them, as significant. From the higher anthropoids, we need to go down the evolutionary stage but a little way to span an interval quite as great as that separating even the existing great apes from the lowest savages.

But Darwin's method is always and everywhere objective and observational, never subjective or introspective. Few who have ever written about the mind of man know or say so little about consciousness, which has spun its Merlin spell of enchantment about our craft and all its works and ways. His language is the concrete facts of life and mind, and not the categories and intuitions that an ingrowing intellect loves to manipulate. The brute soul explains that of man, rather more than man explains the brute; the unconscious explains the conscious and not conversely. He posits a natural history rather than a philosophy of mind. As Steinthal said language could be studied only historically— "Sie ist was sie geworden "--so for Darwin the true, ultimate knowledge of our psyche is the description of all developmental stages from the amoeba up; and those move most surely among the altitudes who have most carefully explored the depths in which the highest human powers originate. Emotions are best studied in their outward expressions in gesture, will is investi