376 DARWINISM chap. 



through which animals have passed, it will naturally be ex- 

 pected that we should find important evidence of evolution. 

 We should hope to learn the steps by which some isolated 

 forms have been connected with their nearest allies, and in 

 many cases to have the gnps filled up which now separate 

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

 expectations are fulfilled, but in many other cases we seek 

 in vain for evidence of the kind we desire ; and this absence 

 of evidence w4th such an aj^parent wealth of mateiial 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 

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

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

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

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

 existed in such numbers, that it seems impossible to account 

 for their total absence from deposits in which groat numbers 

 of species belonging to other groups 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 nature and amount 

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

 to find. 



The Number of known Species of EoMnct Animals. 



When we state that the knoAvn 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 now. If 

 we divide the fossilifei'uus strata into t^velve great divisions 

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

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

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

 characteristic molluscan fauna, but that the different sub- 



