ORIGIN OF LIFE, 9 
for our own convenience direct our attention espe- 
cially to any particular stage of its hypothetical 
- existence. At the same time we must be equally 
free to admit that, in concentrating our attention 
upon the nebular stage, or upon any other, we arbi- 
trarily break into a mysterious cycle of existence 
whose Cause is to us unfathomable. It is needless 
for my purpose, however, to attempt to concentrate 
the reader’s attention upon a period so remote in the 
history of our Universe. We are led to believe that 
the primordial nebula. as it cooled and condensed 
acquired a more rapid axial rotation; that masses 
were. gradually thrown off from its circumference ; 
and that these in their turn condensed into rotating 
spheroids, which continued to circulate round the 
parent mass in elliptical orbits. Assuming, then, 
with the Evolutionist, that our own planet had a past 
history of this kind, we must also assume that it 
gradually changed from a gaseous to a fluid state 
before beginning to solidify by the formation of a 
superficial crust—a crust which gradually thickened 
as the fervent heat of it and of the fluid nucleus 
abated by heat radiations into space. Until this 
stage of the Earth’s history had been far advanced, 
no Living Things could have existed upon its surface. 
“Hence,” as Sir William Thomson said,* “when 
* Inaugural Address at Meeting of British Association, /Vadure, 
Aug. 3, 1871, p. 269. 
