ORGANIC DIFFERENTIATION 75 



only cite them here in order to give a clear idea of the nature of 

 these classes. 



The comparatively abrupt appearance of so many organic 

 forms has sometimes been regarded as evidence against the 

 evolutionary theory. Again and again it has been proved that 

 a new flora and fauna have suddenly appeared in some 

 geological stratum after the complete disappearance of older 

 ones preserved in the strata immediately antecedent, and the 

 most ardent disciples of Cuvier considered this an unanswerable 

 argument in favour of the hypothesis of independent creations. 

 Alcide d'Orbigny succeeded in computing twenty-seven 

 catastrophic phenomena of such a kind. However, it has been 

 possible to show that in many cases layers now directly super- 

 posed in certain localities were at one time separated by inter- 

 mediate strata containing transitional forms, or else to establish 

 the fact that the higher layer was laid down only after 

 a long period of subsidence, during which the lower layer had 

 been subjected to considerable erosion. Hence the argument is 

 destroyed. On the other hand, we have been able to follow, 

 through long periods of time, continuous successions of forms 

 which are manifestly derived from one another, but which 

 might have been regarded as distinct species had they been 

 considered by themselves. This, for instance, is the case with 

 the spiral Ammonites of the Secondary Period, so thoroughly 

 studied by Neumayer, Mosjisowic, Douville, Haug, and 

 others ; with Planorbidse of the Miocene lake at Steinheim 

 in Wurtember^, of Paludinidae of the great Pliocene lakes of 

 Slavonia and many others. We are consequently justified in 

 assuming that every apparently sudden displacement of one fauna 

 and flora by another is really the result of some interference 

 in the deposits, whether of short or long duration, often due to 

 a sudden subsidence which occurred between two apparently 

 consecutive periods, corresponding to the two faunas and floras 

 which appear to succeed each other. In connexion with the 

 differences we observe between them, we do not know what part 

 may have been played by transformations in situ and well- 

 authenticated migrations of animals and plants from one region 

 to another. 



Palaeontology teaches us as little about the origin of life as 

 it does about the origin of organic types or the causes that have 

 produced them. Its sadly defective data can be of use only in 



