I A CHAPTER IN DARWINISM 23 



same time here and there along the line of march, 

 certain forms have been supposed to have " fallen 

 out," to have ceased to improve, and being happily 

 fitted to the conditions of life in which they were 

 long ago existing, have continued down to the 

 present day to exist in the same low, imperfect 

 condition. It is in this way that the lowest forms 

 of animal life at present existing are usually ex- 

 plained, such as the microscopic animalcules, Amoebae 

 and Infusoria. It is in this way that the lower or 

 more simply -made families of higher groups have 

 been generally regarded. The simpler living Mollusca 

 or shellfish have been supposed necessarily to repre- 

 sent the original forms of the great race of Mollusca. 

 The simpler Vertebrates have been supposed neces- 

 sarily to represent the original Vertebrates. The 

 simpler Worms have been supposed necessarily to 

 be the stereotyped representatives of very ancient 

 Worms. 



That this is, to a certain extent, a true explana- 

 tion of the existence at the present day of low forms 

 of animals is proved by the fact that we find in very 

 ancient strata fossil remains of animals which differ, 

 ever so little, from particular animals existing at the 

 present day; for instance, the Brachiopods (lamp- 

 shells), Lingula and Terebratula, the King-crabs, 

 and the Pearly Nautilus are found living at the 

 present day, and are also found with no appreciable 

 difference in very ancient strata of the earth's crust ; 

 strata deposited so long ago that most of the forms 

 of life at present inhabiting the earth's surface had 



