EVIDENCE OF FOSSIL REMAINS 95 



Turning now to the second division of palseontological 

 evidence, we come to those groups where abundant 

 materials make it possible to arrange the animals of 

 successive epochs in series that may be remarkably 

 complete. For the reasons specified, the backboned 

 animals provide the richest arrays of these series, and 

 such histories as those of horses and elephants have 

 taken their places in zoological science as classics. 

 But even among the invertebrates significant cases 

 may be found. For example, in one restricted locality 

 in Germany the shells of snails belonging to the genus 

 Paludina have been found in superimposed strata in the 

 order of their geological sequence. The ample material 

 shows how the several species altered from age to age 

 by the addition of knobs and ridges to the surface of the 

 shell, until the fossils in the latest rocks are far different 

 from then* ancestors in the lowermost levels. Yet the 

 intervening shells fill in the gaps in such a way as to 

 show almost perfectly how the animals worked out 

 their evolutionary history. This example illustrates 

 the nature of many other known series of mollusks and 

 of brachiopods, extending over longer intervals and 

 connecting more widely separated ages like the Second- 

 ary and the present period. 



Since the doctrine of evolution and its evidences 

 began to occupy the thoughts of the intellectual world 

 at large, no fossil forms have received more attention 

 than the ancient members of the horse tribe. As we 

 have learned, a modern horse is described by com- 

 parative anatomy as a one-toed descendant of remote 

 five-toed ancestors. When the hoofed animals of 

 modern times were reviewed as subjects for compara- 



