THE ORIGIN OF THE EARTH. 3 



thought and of critical scrutiny. Especially is this true of the hypo- 

 theses of the origin and early states of the earth, to which we now 

 turn. 



HYPOTHESES OF THE EARTH'S ORIGIN. 



It is the nearly unanimous conviction of astronomers that the 

 solar system was evolved in some way from a nebula of some form. 

 Until recent years, the majority of astronomers accepted the special 

 theory of Laplace, presently to be sketched. So general has been 

 this acceptance that the theory of Laplace has come to be known as 

 "The Nebular Hypothesis," and when this phrase is used without 

 qualification, this particular hypothesis is usually meant. The advance 

 of inquiry, however, makes it necessary now to consider at least two 

 other hypotheses, each of which postulates that the solar system arose 

 from a nebula, but a nebula whose constitution and mode of evolu- 

 tion differed from that postulated by Laplace. In a broad sense, all 

 these are nebular hypotheses. Each of them embraces sub-hypotheses 

 or variations, but in their basal features . they are distinct and form 

 three definite classes. 



I. The gaseous hypothesis. — In this, the parent nebula is assumed 

 to have been formed of gas aggregated by gravity in accordance with 

 the laws of gases, and to have been evolved into the present state by 

 a gradual passage from the original system of gaseous dynamics into 

 the present system of planetary dynamics. The type of the class is 

 the Laplacian hypothesis. 



II. The meteoritic hypothesis. — In this, the parent nebula is assumed 

 to have been a swarm of meteorites, the individual members of which 

 moved in diverse directions and suffered frequent collisions, attended 

 by heat, light, and vaporization. The type is the conception worked 

 out by G. H. Darwin, in which the swarm of meteorites is thought 

 to have behaved essentially as a coarse gas, the evolution of the system 

 being dynamically like that of a gaseous system: indeed, the initial 

 meteoritic aggregation may have actually passed, at a later stage, 

 into a gaseous one by the vaporization of the constituent meteorites. 



III. The planetesimal hypothesis.— In this, the constituents may 

 be molecules or small masses of any kind moving in orbits about a 

 common center. They are not primarily controlled by collision and 

 rebound as in the preceding cases, but by revolution about their com- 



