IV 
ON THE PERSISTENT TYPES OF ANIMAL LIFE 
Proceedings of the Royal Institution of Great Britain, vol. tit, 1858-62, 
pp. 151-153. (friday, June 3, 1859.) 
THE successive modifications which the views of physical geolo- 
gists have undergone since the infancy of their science, with regard 
to the amount and the nature of the changes which the crust of the 
globe has suffered, have all tended in one direction, viz., towards the 
establishment of the belief, that throughout that vast series of ages. 
which was occupied by the deposition of the stratified rocks, and 
which may be called “geological time” (to distinguish it from the 
“historical time” which followed, and the “ pre-geological time,” 
which preceded it), the intensity and the character of the physical 
forces which have been in operation, have varied within but narrow 
limits; so that, even in Silurian or Cambrian times, the aspect of 
physical nature must have been much what it is now. 
This uniformitarian view of telluric conditions, so far as geological 
time is concerned, is, however, perfectly consistent with the notion of 
a totally different state of things in antecedent epochs, and the 
strongest advocate of such “physical uniformity” during the time 
of which we have a record might, with perfect consistency, hold 
the so-called “nebular hypothesis,” or any other view involving 
the conception of a long series of states very different from that 
which we now know, and whose succession occupied pre-geological 
time. 
The doctrine of physical uniformity and that of physical pro- 
gression are therefore perfectly consistent, if we regard geological 
time as having the same relation to pre-geological time as historical 
time has to it. 
