THE ORIGIN AND DEVELOPMENT OF THE EARTH 125 
4.—Effects of the Planetary Hypothesis on Scientific Ideas. 
Such a radical change of thought as that involved in the 
giving up of the Laplacian hypothesis and the acceptance, 
either provisionally or otherwise, of the newer ideas of Cham- 
berlin and his co-workers could hardlly fail to affect scientific 
thought. We used to be taught that the earth was originally 
intensely hot; that its atmosphere was at the same time very 
heavy. We were told that the oceans were once composed of 
hot water, and that life could not exist until they had had 
time to cool. The atmosphere was said to be steadily decreas- 
ing in amount, and the atmosphere moon was held up to us 
as a horrible example of what the earth would some day 
come to. 
Under the new hypothesis, conditions were very different, 
and these conditions coincide with the evidences of geology 
and bilogy. The earth was at one time, during the stage when 
it was just developing as a planet, too small to hold an atmos- 
phere, just as is the case with the moon at the present time. 
Gradually the planet increased in size until it became large 
enough to hold an atmosphere—that is, about as large as the 
planet Mars. From that time on the earth has been growing, 
and its atmosphere increasing. When the oceans first formed 
they were probably no warmer than those of today, and the 
first life began in conditions essentially the same as those 
which now exist. Hundreds of millions of years age there 
were great glaciers that reached far down into the torrid belt 
—to within 18 degrees of. the equator, and hundreds of 
millions of years ago there were deserts, just as there are 
today. The interior of*the earth is not inherited from a 
molten mass, nor is the center of the earth molten at the 
present time. Volcanoes, instead of springing from a great 
internal reservoir of molten material are comparatively super- 
ficial in their origin. These facts we know from geology and 
physics. They exist, yet they fit no known hypothesis but the 
Planetesimal. In closing, however, it will be well to bear in 
mind that the planetesimal hypothesis is not proved. The 
difference between a hypothesis and a theory is essentially 
the difference between perhaps and probably. A hypothesis 
