ia, 
30 THE EVOLUTION OF LIFE 
knowledge, from higher to lower forms of life, till at 
the last it is maintained as a mode of origin only for 
the very lowest and most minute of living things, 
has been regarded by many as one of the most 
weighty arguments against the de novo origin of 
living matter. 
But this objection is robbed of all its seeming 
strength when it is said that the modern Evolu- 
tionist would only expect to obtain evidence con- 
cerning the de zovo origin of the minutest specks of 
“living matter ’—gradually emerging, in fluids, into 
the region of the visible, as opened up by a powerful 
microscope, and subsequently developing into the 
most elementary living things. 
(2) The second general consideration is this. 
The formula omne vivum ex vivo is supposed to 
derive its authority from the fact that the experience 
of mankind generally—both skilled and unskilled— 
testifies to its truth. But it is almost unnecessary 
to say that observation is of no avail in regions 
where it becomes impossible, and consequently that 
observation cannot tell us whether previously 
invisible specks of living matter have arisen from 
invisible living germs, or by an independent process 
of origination. As John Stuart Mill said, ‘‘ Though 
we have always a propensity to generalise from un- 
varying experience, we are not always warranted in 
doing so. Before we can be at liberty to conclude 
that something is universally true because we have 
never known an instance to the contrary, we must 
have reason to believe that if there were in nature 
1 “System of Logic,” 6th ed., vol. i. p. 349. 
