PALEONTOLOGY 75 



That every extinct group had a limited duration in 

 time is explained by competition and the struggle for 

 existence. That life has been continuous on the 

 earth is a necessary deduction from the theory of 

 descent. That the generalised types were followed 

 by the specialised types is also a necessary result of 

 descent with modification, as also is the general and 

 uniform advance of life on the earth. 



There is only one objection to the theory and that 

 is that some groups of animals and plants appear to- 

 have come in suddenly. There are not many in- 

 stances, the chief of them being the Graptolites in 

 the early Ordovician, the dicotyledonous plants in 

 the middle of the Cretaceous, and the eutherian- 

 Mammals at the commencement of the Eocene. 

 The explanation of this difficulty is, no doubt, the 

 imperfection of the record ; and it will be noticed 

 that two out of the three groups live upon the land 

 where their remains are not easily preserved. The 

 sudden appearance of the dicotyledonous plants and 

 eutherian Mammals is probably due to migration ; 

 the early forms having been developed on land, 

 which is now under the Pacific or Atlantic Oceans. 

 That of the Graptolites is due either to the .same 

 cause , or to there being an unrepresented period of 

 time between the Cambrian and the Ordovician. 

 Indeed the fewness of these abrupt changes is good 

 evidence that our palaeontological knowledge is more 

 complete than we might expect it to be. For the 

 more imperfect it is the more there would be of these 

 sudden appearances. 



That the palceontological succession is really due 



