THE ASTRONOMICAL DEDUCTIONS. 19 



hypothesis is not hampered by this Hmitation. If the joint mass was 

 larger, their initial distances may have been greater; if smaller, they 

 must have been less. Within the limits thus imposed by the mechanics 

 of the case, the nuclei may have been separated by any distance, abstractly 

 considered, from the maximum permitted down to surface contact. 



In fact, however, the degree of nearness consistent with the present 

 state of things, was hmited by the consequences of growth, for the increase 

 of the masses of the nuclei by the ingathering of the planetesimals may 

 have drawn the nuclei toward one another, or even together. This was 

 conditioned by the moment of momentum which the accessions carried 

 into the nuclei, which varied widely. Separation later by centrifugal 

 action would be theoretically possible, under assignable conditions, but not 

 at all inevitable, perhaps not at all probable. If the initial distance of the 

 nuclei were sufficient, however, the nuclei might approach one another so 

 long as growth was a ruling influence. Tidal action would run concur- 

 rently with this and would oppose approach, under most conditions, but 

 during the more rapid stages of growth, the tidal effect may possibly have 

 been less than the effect of increasing mass. But the tidal effect would 

 increase as the bodies were drawn toward one another, while in the later 

 stages of growth the increase of the mass would decline in rate. At a certain 

 stage the two effects may be presumed to have balanced one another, after 

 which recession would begin through the preponderance of the frictional 

 effect of the tides. From that stage, the history would proceed along the 

 lines determined by the mutual interaction of the matured bodies.* 



It will be seen that the range of specific assignments under this phase 

 of the planetesimal hypothesis has a wide amplitude, embracing the per- 

 missible assignments as to the original distances between the nuclei, as 

 to the original masses of the nuclei, and hence as to the amount of their 

 growth, as to their planes of revolution, their eccentricities of orbit, etc. 

 Under this amplitude, it is possible to suppose that the two bodies at the 

 cHmax of their approach reached precisely the relations which were indi- 

 cated by Darwin in his backward tracing of their history. On the other 

 hand, so far as the hypothesis itself is concerned, it is equally possible 

 that the approach of the bodies was much less close, and hence that their 

 recession under tidal influence was correspondingly less. It will be seen, 

 therefore, that this hypothesis has very much greater adaptabiHty than 

 the hypothesis of centrifugal separation, and does not equally hamper us 

 respecting subordinate hypotheses, such as a molten state, a viscous 

 interior, or a particular amount or a particular distribution of the hydro- 

 sphere. We are quite free to follow backward from the present observa- 

 tional data, when it shall be possible to do this on firm ground, with the 

 utmost complacency as to the results, and to accept these as indicating the 

 original relations, whether they imply a former state of coalescence, or of 

 close approach, or of more distant approach. It is possible that the earth 

 and moon were drawn together by their growth into just those relations 

 which Darwin assigned to them when, in his backward tracings of their 



'■ The effects of contraction are here neglected. 



