376 DARWIXISJI chap. 



througli Avhich animals hare passed, it will naturally be ex- 

 pected that ^ye should find important e\'idence of evolution. 

 We should hope to learn the steps l^y which some isolated 

 forms have been connected with their nearest allies, and in 

 many cases to have the gaps filled up which now separate 

 genus from genus, or species from species. In some cases these 

 expectations are fulfilled, but in many other cases we seek 

 in vain for e\adence of the kind we desire ; and this absence 

 of evidence with such an apparent wealth of material is held 

 by many persons to throw doubt on the theory of evolution 

 itself. They urge, with much appearance of reason, that all 

 the arguments we have hitherto adduced fall short of demon- 

 stration, and that the crucial test consists in being able to 

 show, in a great number of cases, those connecting links which 

 Ave say must have existed. Many of the gaps that still remain 

 are so vast that it seems incredible to these Avidters that they 

 could ever have been filled up by a close succession of species, 

 since these must have spread over so many ages, and have 

 existed in such numbers, that it seems impossil^le to account 

 for their total absence from deposits in which great numbers 

 of species belonging to other gi'oups are preserved and have 

 been discovered. In order to appreciate the force, or weakness, 

 of these objections, we must inquire into the character and 

 completeness of that record of the past life of the earth which 

 geology has unfolded, and ascertain the natiu'e and amount 

 of the evidence which, under actual conditions, we may expect 

 to find. 



The Number of hioim Species of Extinct Animals. 



"\Mien we state that the known fossil mollusca are consider- 

 ably more numerous than those which now live on the earth, 

 it appears at first sight that our knowledge is very complete, 

 but this is far from being the case. The species have been 

 continually changing throughout geological time, and at each 

 period have probably been as numerous as they are noAv. If 

 we diAade the fossiliferous strata into twelve great divisions 

 — the Pliocene, INIiocene, Eocene, Cretaceous, Oolite, Lias, 

 Trias, Permian, Carboniferous, Devonian, Sihman, and Cam- 

 brian, — we find not only that each has a very distinct and 

 characteristic molluscan fauna, but that the different sub- 



