THE ORIGIN OF THE EARTH. 61 



warrant the assumptions that (1) the central mass was overwhelmingly 

 large, (2) the knots were quite irregular in size and very unequal in 

 distance from the center, and (3) the nebulous part was small relative 

 to the central mass, and probably large relative to the knots. Reasons 

 for the last conclusion will be given later. 



The attractive powers of the knots and their limitations. — It is 

 assumed that the masses of matter in the knots were sufficiently large 

 to hold themselves together in spite of the differential attraction of the 

 central mass; otherwise they would soon have been scattered. They 

 seem to have successfully maintained themselves in nebulae that appear 

 to have undergone some notable degree of evolution, as for example 

 the knots shown near the large central nucleus in Fig. 13. 



On the other hand, it is presumed that the gravity of the more tenu- 

 ous nebular matter was insufficient to aggregate it directly, in the 

 presence of the central attraction, for this portion seems to show a 

 progressive tendency to a more general diffusion, as may be seen by 

 comparing the cloudy flocculent distribution of the nebulous matter 

 in Fig. 12, whose form implies that it is young in evolution, with the 

 more diffuse distribution in Figs. 15, 17, 18, and 21, whose central 

 coilings imply more advanced states of evolution. The planetesimals 

 of this diffuse nebulous portion are, therefore, assumed to be controlled 

 essentially by the gravitation of the main mass, and to revolve in indi- 

 vidual orbits about it. 



The irregularity of the forms of the knots seems to imply that 

 their organization is also planetesimal, though the larger ones may be 

 able to hold gases also. The direction of revolution of these knots is 

 supposed to be usually the same as that of the rotation of the nebula 

 as a whole, but subject to local and special influences that might lead 

 to important variations. 



The knots the planetary nuclei. — The knots of the solar nebula 

 are regarded as the nuclei about which gathered the planetesimals to 

 form the future planets; not that all such nuclei necessarily retained 

 their independence and grew to planets, but that no planet probably 

 developed except from such a nucleus. The figures already presented 

 (Figs. 12-21) show clusters of knots and aggregates of irregu'ar form 

 susceptible of development into complex planetary systems, such as the 

 large planets and their families of satellites. The earth-moon system 

 is assigned to a couplet of companion nuclei of quite unequal sizes. 



