BIOLOGICAL CHANGE IN GEOLOGICAL TIME. 107 



Yet the Pecopteris of the Coal Measures is very similar to the 

 bracken of the upland slopes of England and Wales, from which 

 we may conclude that the ferns, at least, of the Carboniferous 

 flora flourished under very varied conditions of moisture and 

 temperature all through the Secondary and Tertiary epochs. 

 These remarkable and instructive facts doubtless present 

 great difficulties, but they cannot be ignored and must be 

 taken into account in any adequate consideration of this 

 subject. 



Extinctions. 



The term " exterminations " applied to the extinctions or 

 dying out of species or genera during geological time seems to 

 imply a sudden termination of the existence of the whole of 

 the individuals ; but such sudden extinctions, as was well 

 said by Mr. Hudleston, are more apparent than real.* An 

 apparent extinction may only have been occasioned by the 

 migration of a species to another area the rocks of which have 

 not been examined or possibly have been destroyed. Extended 

 and more careful research has over and over again given a 

 greater stratigraphical range to species and genera than had 

 before been regarded as established. Species thought to be 

 limited to a particular formation have been subsequently found 

 in newer and, in some cases, much newer rocks. I have myself 

 found species that were thought to be confined to certain 

 formations in other beds sometimes much higher in the strati- 

 graphical scale. This result of extended examination of 

 fossiliferous rocks was well exemplified by tlie extension of 

 the known stratigraphical range of the trilobite, Arethusina 

 KonincH, which up to a certain time had not been found 

 higher than in a zone of the Upper Silurians of Bohenda, 

 althou,^h in that and lower zones it was most abundant, and 

 accordingly the species was considered to be quite chaiacteristic 

 of these rocks. But at length the A. Konincki was discovered 

 in the much newer Upper Devonian rocks of Westphalia. 



Such facts as these render it certain that future research will 

 give similar results, and this forbids the conclusion that a 

 species or a genus has become extinct at the time of the forma- 

 tion of the newest bed in which it has hitherto been found. 

 Even those species of Ammonites which are usually regarded as 

 marking certain zones in the Jurassic rocks may not have had 



* Journal of the Victoria Institute, vol. xxxvii, p. 184. 



