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SCIENCE 



[N. S. Vol. XXX. No. 760 



it described took the place of the heroic deeds 

 celebrated in their epics. But its greatest 

 value was in making, in the first half of the 

 nineteenth century, a foundation for the de- 

 velopment of geological theories respecting 

 the age and evolution of the earth, and these 

 theories, in turn, were important factors in 

 Darwin's elaboration of his " Origin of 

 Species." 



The next important step in cosmogony was 

 Helmholtz's contraction theory of the heat of 

 the sun, published in 1854, which not only was 

 not contradictory to the Laplacian theory, but 

 was generally supposed to be a proof of its 

 correctness. 



In the latter half of the nineteenth cen- 

 tury the Laplacian theory was supplemented 

 by the consideration of some factors originally 

 omitted, chiefly by Roche and Sir George Dar- 

 win, and some objections were urged against 

 it, chiefly by Babinet and Faye. But the 

 writings of practically all astronomers show 

 that it was generally accepted without funda- 

 mental modifications. For example. Sir 

 George Darwin in his classical researches on 

 tidal evolution frankly stated that he ac- 

 cepted it in its main outlines; and in 1886 C. 

 Wolf, of the Paris Observatory, reprinted in 

 book form a series of articles appearing earlier 

 in Bulletin Astronomique, Vols. I. and II., 

 which clearly supported this theory. In the 

 preface to this volume we read: 



Men principal but, en gcrivant ces articles, 6tait 

 de montrer que la thgorie de Laplace rSpond 

 encore aujourd'hui le mieux possible aux condi- 

 tions que Ton est en droit d'exiger d'une hy- 

 pothfese cosmogonique. 



In the late nineties Professor Chamberlin 

 in studying the earth's atmosphere, and par- 

 ticularly its origin and history, became skep- 

 tical of the soundness of the Laplacian theory ; 

 and simultaneously some of its weaknesses 

 were forced on me while considering it in my 

 classes in descriptive astronomy. Toward 

 the end of 1899 we had several conferences 

 on the question of its correctness, and as a 

 result of these discussions we decided to test 

 it, first as to its agreement with the facts es- 



tablished by observations, and secondly as to 

 its self-consistency. The results of these in- 

 quiries are contained in a paper published by 

 Professor Chamberlin in the Journal of 

 Geology, February-March, 1900, and in one 

 by myself in the Astrophysical Journal, 

 March, 1900. It is well known that the con- 

 clusions reached in these papers seemed to us 

 so adverse to the theory as to compel us to re- 

 ject it as being no longer a satisfactory hypoth- 

 esis; and since that time many astronomers 

 have placed themselves on record as being in 

 agreement with us. 



Immediately after the publication of these 

 papers constructive work was begun, chiefly 

 by Professor Chamberlin. The first account 

 of the new hypothesis which was developed 

 was published by Professor Chamberlin in 

 Year Book No. 3 of the Carnegie Institution, 

 pp. 208-253 (1904), and another was pub- 

 lished by myself in the Astrophysical Jour- 

 nal, Vol. 22, pp. 165-181 (1905). In Cham- 

 berlin and Salisbury's " Geology," Vol. 2, pp. 

 38-81 (1906), under the title of The Planet- 

 esimal Hypothesis, Professor Chamberlin 

 gives an extensive account of the proposed 

 theory. Some of the subheadings are: Sub- 

 varieties of the Hypothesis, The Hypothetical 

 Origin of the Solar Nebula, The Contingencies 

 of Stellar Collision, The Contingencies of 

 Close Approach, The Special Consequences of 

 Close Approach, The Acquisition of Rotatory 

 Motion, The Result a Spiral Nebula, The 

 Assigned Nebular Origin not Vital, The Evo- 

 lution of the Nebula into Planets, The Part 

 Played by Ellipticity of Orbit, The Evolution 

 of Circularity, The Time Involved, The Bear- 

 ing of the Mode of Accretion on the Direction 

 of Planetary Rotation, The Spacing-out of 

 the Planets, . . . He closes the chapter with 

 the following summary: 



The planetesimal hypothesis thus assumes that 

 the solar system was derived from a nebula of 

 the most common type, the spiral, and that the 

 matter of this parent nebula was in a finely 

 divided solid or liquid state before aggregation, 

 in harmony with the continuous spectra of spiral 

 nebulae. It regards the knots of the nebula as the 

 nuclei of the future planets, and the nebulous 



