DIASTROPHISM AND THE FORMATIVE PROCESSES 9 



the solar system and probably not more than a thousandth part of 

 the radius of the sun's sphere of control. This last is the more 

 significant standard, for the sun's sphere of control is the dynamic 

 field within which the planets had their origin and have 'ever since 

 had their being. It is therefore a reasonable inference that the 

 members of the little group shared much the same evolutionary 

 conditions, were formed in much the same way, and of much the 

 same material. But even if there was some gradation in the nature 

 , of the material due to position in the system — which we will con- 

 sider later — its effects are measurably equated in the comparisons, 

 because Mars lies outside the earth and Venus inside, while the 

 moon, as a member of the earth system, presumably partook of 

 the common material from which both earth and moon were 

 derived, though perhaps not in precisely the same way. At any 

 rate, though some differences of original material must be presumed 

 to have entered into the formation of these four bodies, such 

 differences could scarcely have been at all radical. Besides, it will 

 be seen later to be possible to deduce the more important differences 

 which affected the selection of material in the formation of these 

 bodies. This will be considered in an article following this one, as it 

 goes too far afield to be introduced here. 



All members of this little group of bodies are small compared 

 with the four giant planets outside them, ♦and yet they are large 

 relative to the majority of satelHtes and planetoids. They form 

 an intermediate group, and deductions respecting them may be 

 checked by the extremes on either hand. Among themselves 

 they form a graded series well suited to our purpose. The moon 

 is distinctly small and has no appreciable atmosphere or hydro- 

 sphere; it may be taken to represent such bodies as are formed of 

 molecules heavy enough and sluggish enough to be controlled by a 

 limited attractive force, a force too feeble to hold the lighter and 

 swifter order of molecules. Mars represents a stage of growth at 

 which sufficient gravitative power has been reached to maintain a 

 limited atmosphere and apparently the beginnings of a hydro- 

 sphere. Venus represents a much more advanced stage at which 

 the gravitative power is sufficient to hold a very notable atmosphere 

 and probably a rather massive hydrosphere. The earth, as we 



