pierre 
Questions as to the Origin and Extinction of Species. 411 
to any given geological period, and may hold that for each such 
period there are specific types, which, for the time at east, are 
invariable. : 
condition and various states of preservation, it is still more 
lave greater significance than if they appeared in the Middle 
a Wh sara iSyned di ithout known suc 
. en specific types disappear without any kno - 
api teams rer: ae ei it seems unlikely that 
We should have failed to discover their continuance, we may 
fairly assume that they have become extinct, at least locally ; 
and where the field of observation is very extensive, as in the 
great coal fields of Europe and America, we may esteem 
a es such 
-€xtinction as practically general, at least for the northern hem1- 
