214 The American Geologist. Apiii, 1905 
equilibrium with the oceanic segments to-day, as seems highly 
probable, the selective weathering process brought about a differ- 
ence in depression of only 1 mile in 500 or GOO miles, or about 
one-fifth of 1 per cent. * * * 
Not only is the evolution of the great abysmal basins and of 
the continental platforms thus assigned to a very simple and ine- 
vitable process, but there is 'therein laid the foundation for sub- 
sequent deformation of the abysmal and continental type. 
* * _ * A theoretical scantiness of time for a prolonged 
evolution previous to the Cambrian period has been deduced from 
a molten earth, but this does not apply to the planetesimal hypo- 
thesis. The supposed limitation of the sun's thermal endurance 
would apply if the arguments could be trusted, but their founda- 
tion has been cut away by recent discoveries. It is not the least 
of the virtues of the planetesimal hypothesis that it opens the 
way to a study of the problem of the genesis and early evolution 
of life free from the duress of excessive time limits and of other 
theoretical hamperings, and leaves the solution to be sought untram- 
meled, except by the conditions inherent in the problem itself, which 
are surely grave enotigh. 
It is assumed that the conditions on which life is now de- 
pendent were prerequisites to its introduction. As already indi- 
cated, an atmosphere and hydrosphere sufficient to sustain life 
may have been acquired when the earth was about the size of 
Mars, or one-tenth grown. If, to be conservative, a preliminary 
growth of twice this amount be allowed, there still remains be- 
tween this and the Cambrian record the growth of four-fifths of 
the mass of the earth. So far, therefore, as atmosphere and 
hydrosphere are concerned, life may have been introduced early 
in the history of the earth, and may have had a vast interval 
for development previous to the earliest legible record. There is 
another essential condition — a sufficiency, but not an excess, of 
heat and light. If the formation of the parent nebula involved 
only the outshooting of a small fraction of the ancestral sun, the 
solar supply of heat and light may not have been so seriously 
disturbed as to have fatally affected its availability to furnish what 
was necessary for life at any stage of the earth's growth. * * * 
* * * There is little ground for apprehension that the in- 
falling planetesimals would be seriously dangerous to the early 
forms of life, for in the first place the atmosphere must have 
been then, as now. an effective cushion, checking the speed of 
the planetesimals and partially dissipating them, and, in the second 
place, the early organisms were probably all aquatic and were 
further protected by their water covering * * * 
So soon as plants and animals had come into action, all the 
great factors potential in the earth's physical evolution were in 
play. 
By hypothesis, volcanic action only began some time after 
the beginning of the earth's growth, for it was delayed (1) by ine 
