498 T. C. CHAMBERLIN 



sifting process that preceded must have removed all constituents 

 whose motions had any notable dispersing component; nor can any 

 such component arise from mutual interaction without jeopardy. 

 Almost the only line along which a body so small as a lo-mile plan- 

 etoid could organize itself by the granular method seems to have 

 lain in acquiring very early a higher central density and a less out- 

 ward extension than that assigned. This might perhaps have been 

 done by the reaction above noted. The peril of dispersion and the 

 narrow margin of control in such cases lead to the conclusion that 

 the smallest order of planetoids and satellites lie near — or perhaps 

 quite on — the limit of possible genesis by even this divergent phase 

 of the gaseous line of descent. 



This conclusion tallies with the fact that no planetoids or 

 satellites of the smaller order are known at the earth's distance 

 from the sun, or within it. Bodies of this type appear only at the 

 distance of Mars and beyond. The dynamic conditions of this 

 inner region are perhaps too adverse for this type of formation. 

 In the outer region conditions are notably less restrictive, but 

 even there they undoubtedly put lower limits on the size of bodies 

 formed by the gaseo-granular method of assemblage. 



In the light of these considerations there seems little warrant 

 for supposing that such bodies were ever formed in sufficiently 

 great multitudes to have built up the earth or to have pitted the 

 surface of the moon by their impacts. The number of lunar 

 craters is estimated at 30,000. If each of these is the grave of an 

 extinct planetoid, one might expect that a few living ones would 

 have lingered to tell the story. The negative testimony of the 

 heavens as to their existence in this region seems rather to favor the 

 view that their restriction to the outer region implies that they are 

 themselves witnesses to the limitation of this line of genesis in both 

 place and frequency. 



With the general lines and limits of nuclear evolution thus 

 defined, our remaining task is to find the median places between 

 the two extremes that fit the earth, Venus, Mars, and the moon. 

 It is clear from the present state of these bodies that much sifting of 

 the original solar gases was required, for while the earth, Venus, 

 and Mars hold envelopes of gases of moderate molecular weight, 



