10 ANIMALS OF THE PAST 
pletely and minutely may this change occur 
that under the microscope the very cellular 
structure of the wood is visible, and as this 
varies according to the species, it is possible, 
by microscopical. examination, to determine 
the relationship of trees in cases where noth- 
ing but fragments of the trunk remain. 
The process of fossilization is at best a slow 
one, and soft substances such as flesh, or even 
horn, decay too rapidly for it. to take place, so 
that all accounts of petrified bodies, human or 
otherwise, are either based on deliberate frauds © 
or are the result ofa very erroneous misinter- 
pretation of facts. That the impression or 
cast of a body might be formed in nature, 
somewhat as casts have been made of those 
who perished at Pompeii, is true ; but, so far, no 
authentic case of the kind has come to light, 
and the reader is quite justified in disbelieving 
any report of “a petrified man.” 
Natural casts of such hard bodies as shells 
are common, formed by the dissolving away of 
the original shell after it had become enclosed 
in mud, or even after this had changed to 
stone, and the filling up of this space by the 
