SOME FUNDAMENTAL CONCEFTS OF EARTH HISTORY 169 
ling and distortion were attendant upon these events and the 
irregularities of the continents were continually aggravated. 
Lines of weakness developed and here, as we might expect, vol- 
canic and earthquake activity are in evidence. 
Conditions favorable for the maintenance of life no doubt 
ensued long before the earth attained its full growth, but we 
have no means of knowing when or whence or how or where that 
life was initiated, except that doubtless it was in the water, and 
the first forms were plantlike in nature. By the time the first 
available legible record was made in the oldest exposed sedi- 
mentary rocks, both animal and plant life were highly developed 
and widely deployed. A great lapse of time must be represented 
by this development, a period, it may be, equal to or greater than 
all subsequent time. 
By way of summary, then, it may be stated that the Planetesi- 
mal Hypothesis provides for the beginning of the solar system 
by a spiral nebula, from the arms of which have developed the 
planetary bodies, while the central part has become, or remained, 
the sun. Limiting our attention to the earth we may trace first 
the growth of the lithosphere, the solid part, by accretions of 
planetesimals, then the development of the atmosphere, and a 
little later of the hydrosphere, by release and closer in drawing 
and capture of their component elements. The oceans have 
always occupied essentially their present basins and have merely 
overlapped more or less the continental margins and from time 
to time have transgressed the interiors of the great land masses. 
Unlike the Laplacian Hypothesis this one does not demand 
symmetry and uniformity either in the spacing and masses and 
motions of the planetary bodies or in the progress of their de- 
velopment and history, but provides latitude for all observed 
and probable variations. The occurrence of arid and glacial con- 
ditions on the earth is thus not only allowable, but is a probable, 
an almost necessary feature of actual reactions and interactions 
between lithosphere, hydrosphere and atmosphere. The hy- 
pothesis seems to meet the necessities of the solar system and so 
far no critical objections have been advanced against it, although 
it has been abundantly discussed before the learned societies of 
the United States. 
