114 The American Geologist. Febmary, i904. 
and sea have been in former ages radically unlike the present. 
This latter fact does not entirely contradict the principle of 
relative permanence of the continental and oceanic areas, but 
suggests that the principle must not be applied too rigidly. 
Investigations seem to prove the unequal density of the 
earth, and the same conclusion is reached by physical deduc- 
tion. The fact of epeirogenic oscillations and orogenic uplifts 
would seem to imply that under slowly applied and long con- 
tinued forces the deeper rocks of the globe are viscous. This 
seems to require isostatic equilibrium ; which in turn implies 
that the sub-oceanic portions of the lithosphere must be denser 
than the continental areas ; and that the lithosphere of the 
southern or oceanic hemisphere must be denser than that of the 
northern or continental hemisphere. These conclusions as to 
the varying density of the earth may not be wholly contra- 
dictory to the nebular hypothesis, but they are certainly more 
favorable to the new hypothesis. Long duration of gaseous 
and liquid stages in the life of the globe would favor diffusion, 
convection and resulting homogeneity. The new hypothesis, 
forming the globe from cold solid materials, favors hetero- 
geneity, and is in better harmony with the geologic facts and 
the philosophic deductions relating to the earth's structure. 
Life on the Earth. 
Under the new hypothesis the problem of the origin and 
duration of life may be quite a different matter from the same 
problem under the old hypothesis. With the high temperatures 
required by the nebular hypothesis, life was at first impossible, 
and the low temperature of space has seemed to prohibit the 
importation of life germs from abroad. This placed a limit 
to both the origin and the duration of life on the planet. Re- 
cently the announcement has been made that the extremely low 
temperature of liquid hydrogen is not fatal to life germs. This 
being true it not inconceivable that germs might endure the 
cold of celestial spaces, and might reach the earth by way of 
meteorites ; or even that they might have existed on the plan- 
etesimals of the earth-moon zone or ring. 
A much less speculative thought is this, — that under the 
planetesimal hypothesis the ordinary conditions of temperature 
and moisture requisite for life as we know it were probably 
fulfilled long before the planet attained full size. The duration 
