INTRODUCTION. 



It is many years since a sustained attempt has been made to unite the various 

 branches of the animal kingdom into a natural, coherent system, or genealogical 

 tree, that would indicate the rise and decline of the important functions and 

 organs, map out the highways of organic evolution, and assign in geological terms 

 the approximate dates and surroundings for the critical events in structural in- 

 novation and readjustment that have marked its progress. 



The most important problem involved in such an undertaking is to discover 

 which one, if any, of the many existing invertebrate phyla forms the trunk line of 

 descent from the lowest vertebrates to the ccelenterates, and through them to the 

 protozoa. 



The vertebrates abruptly make their appearance as fully formed fishes, at 

 the close of the Silurian or the beginning of the Devonian. They were evidently 

 more highly organized than any of the invertebrate types that had appeared up to 

 that time, and they must have arisen, either by a marked transformation of some 

 of the known, preexisting types, or from some extinct and totally unknown ones. 

 On either supposition, the apparent absence of transitional forms is surprising, 

 since the relatively large size, distinctive form, and well developed skeleton of 

 primitive vertebrates, under the known conditions, should leave behind some 

 recognizable traces of their predecessors in the form of fossils. 



The real missing links in the graded series of animal forms that most concern 

 the morphologist belong, therefore, to the Silurian period. There the main 

 trunk of the animal kingdom, upon which the whole vertebrate stock rests, is 

 lost, leaving without reason or warning a vast unknown abyss beside which the 

 gap between man and his immediate predecessors sinks into microscopic in- 

 significance. 



On the one side are the vertebrates, including a long series of animals, from 

 the lowest fishes to man. All of them agree in their fundamental plan of struc- 

 ture and mode of development; the principal organs of any member of the series 

 may be surely identified in the others, and the general trend of evolution in the 

 phylum is clearly indicated. The fishes, for example, are the lowest members of 

 the series, and they are followed by the amphibia, reptiles, and mammals. Com- 

 parative anatomy shows the gradual evolution of form and structure in this 

 series as a whole, and its evidence is corroborated, in the main, by the embryonic 

 development of any member of the series, while the geologic, or historic record 

 harmonizes with, and confirms the testimony of the other two. In fact, the 

 vertebrates clearly constitute a common stock, a single phylum of the animal 



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