THE DEVELOPMENT OF VERTEBRATA. 283 



ancestors of existing animals. The geologist has shown us 

 that many of the rocks that form the surface of the earth 

 beneath the soil were in immeasurably remote times formed 

 as sediments on the floor of the sea or lakes or elsewhere ; 

 and he has shown how we can determine the relative ago 

 of these sedimentary rocks and recognize a series of periods 

 in the Earth's history. Now embedded in these rocks aro 

 the fossil remains of extinct animals, and though the know- 

 ledge so obtained of the life of these successive periods is of 

 necessity very incomplete, it goes far to confirm the con- 

 clusions of Comparative Anatomy and Embryology. Al- 

 though at all periods there were abundant forms of life 

 which were, as it were, only leaves on the lower branches 

 of the tree of life, and in no way ancestral to the forms of 

 later periods and the present day, yet many ancestral or 

 approximately ancestral forms can be recognized. 



In the Vertebrate series, palaeontology (the study of 

 fossils) begins only with the fishes, for only organisms with 

 hard, durable skeletons can as a rule be fossilized. In the 

 Silurian and Devonian periods, there were abundant fishes 

 many of them specialized in ways of their own, but some 

 of which may very well have been ancestral both to the 

 dogfish and to the land-Vertebrates. In the succeeding 

 Carboniferous period, the earliest Amphibia are found, far 

 more primitive and fish-like than any existing Amphibia. 

 In the Permian period come forms which link the Amphibia 

 to the Reptiles ; in the Triassic period, Reptiles which have 

 many striking resemblances to Mammals ; and at the end 

 of that period true Mammals were in existence. Birds, 

 which are nothing but highly specialized Reptiles, did not 

 appear till a later stage. Later still we can trace the 

 evolution of most of the great orders of Mammalia from 

 simpler members of that group. 



6. Investigation of the Causes of Evolution. When 

 this great fact of the descent of all Vertebrates and, by 

 extension of the idea, though on less complete evidence, 

 of all other animals from some common ancestor is 

 admitted, the question arises : How have these wonderful 

 changes of form, structure, habit, and instinct been 



