xiii EVOLUTION 223 



finally a period was reached at which the differences were 

 so considerable as to necessitate the placing of the ancestral 

 forms in a different species from their descendants at the 

 present day. And in the same way, if we could trace back 

 the species of any one genus, we should find them gradually 

 approach one another in structure until they finally con- 

 verged in a single species, differing from those now existing, 

 but standing to all in a true parental relation. 



It will be seen that, on this hypothesis, the relative like- 

 ness and unlikeness of the various species of frogs are 

 explained as the result of their descent with greater or less 

 modification or divergence of character from the ancestral 

 form : and that we get an arrangement or classification in 

 the form of a genealogical tree, which, on this hypothesis, 

 is a strictly natural one, since it shows accurately the 

 relationship of the various species to one another and to 

 the parent stock. So that on the theory of evolution, a 

 natural classification of any given group of allied organisms 

 is simply a genealogical tree. 



Now it is evident that the only way in which we could 

 be perfectly sure of an absolutely natural classification of 

 the species of any kind of animal the frog, for example 

 would be by obtaining specimens as far back as the distant 

 period when the genus first came into existence. 



Forming part of the solid crust of the earth are a series of 

 sedimentary or stratified rocks, and the researches of geolo- 

 gists have shown that these present a general order of 

 succession, the lowest, when undisturbed, being in every 

 case older than the more superficial layers. Imbedded 

 in these rocks are found the remains of various extinct 

 animals in the form of what are called fossils and it might 

 perhaps, on first considering the subject, be supposed that, 



