196 THE POPULAR SCIENCE MONTHLY. 



are below-ground are those organisms which are buried in the past, 

 and which we can study only through their fossil remains. 



Most naturalists not only believe that, if we could trace back the 

 history of life, we should find each group bearing evidence of wider 

 and wider relationship as we receded from the present time, but 

 they also believe that we should ultimately find that every form of 

 life is related to every other in such a way as to show that, in the 

 remote past, they all met in a single starting-point the common an- 

 cestor of all living things. 



When we come to examine the evidence for this theory of the com- 

 mon origin of all life, we find that it is almost entirely general in its 

 character. There is a nearly complete absence of specific and definite 

 proof. We find an abundance of fossil forms, which we may regard 

 as connecting links between one great group of animals and another ; 

 but even in the mo it favorable cases the attempt to follow the history 

 of any particular species back for a considerable length of time soon 

 ends in a total failure, for we lose track of its line of descent en- 

 tirely, and can go on only by substituting, for the species with which 

 we started, the genus, family, or more comprehensive group to which 

 we have traced it. 



Once in a while we find, in the later geological formations, a fossil 

 animal which exhibits such affinity to several closely related recent 

 species, that there is a strong presumption that it is the common an- 

 cestor from which they have descended. We have enough evidence 

 to enable us to trace the horse and its allies through several geological 

 periods with considerable accuracy, and to reach a form which is widely 

 different from the horse, and which shows relationship to quite differ- 

 ent groups of recent mammals. There are a few other cases where 

 the evidence is equally abundant ; but more usually it fails completely, 

 and, although the fossils of the later formations show a very close re- 

 lationship to their living allies, the resemblance is not exact enough to 

 prove that the fossil forms are the direct ancestors of the recent ones, 

 rather than more distant relations, connected by some unknown fossil 

 form. 



In place of the exact evidence which would be necessary to prove 

 that the nearest fossil allies of recent species are in the direct line of 

 descent, we have only the vague general evidence which is furnished 

 by those fossils which unite in themselves the characteristics of widely 

 separated families, or classes, or orders of animals. While the attempt 

 to trace any particular species of bird and any given species of reptile 

 to a common ancestor would be hopeless, we do find fossil organisms 

 in whose structure certain general characteristics of the class Birds are 

 united to certain general characteristics of the class Reptiles, in the 

 way which we might expect if those animals are the descendants of 

 true reptiles and the ancestors of the true birds of the present day. 

 There is no proof that this actually is the case, and it is perfectly 



