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and the evidence afforded by its fossil remains is very important 

 and convincing. The series made in the case of the horses found 

 by Marsh and Gope and those described by Gaudry are universally 

 quoted as the strongest proofs of evolution. This evidence is 

 considered complete, because naturalists understand and have 

 thoroughly studied the skeleton, and because it is internal and has 

 been assumed to be more invariable than the shell. All of these 

 arguments have their due weight, but there are no examples 

 of greater invariability than exist between the shells of the Nauti- 

 lus now existing and those of Barrandeoceras (Nautilus) of the 

 Cambrian, or the Triassic and Silurian Orthoceras, or of the 

 Prodissoconch stage in the young of Pelecypoda as demonstrated 

 by Jackson, or of the Protegulum among Brachiopoda as shown by 

 Beecher. The Prodissoconch and Protegulum are embryonic 

 shells that have persisted from the earliest horizons of geologic 

 time and are still to be found in living shells attached to their 

 apices. 



The conclusions arrived at by the study of the vertebrate 

 skeleton are reliable, but they are neither more conclusive nor 

 important in theoretical meaning than any other series of equally 

 well-understood hard parts in any other branch of the animal 

 kingdom found as fossils when traced out in the same thorough 

 and careful manner. 



How unreasonable it would seem to a student of fossil 

 Mammalia, if he were requested to do what it would be appropriate 

 to require from a student of the fossil Cephalopoda, viz., to 

 describe from the investigation of a single perfect fossil skeleton of 

 an adult, not only the characteristics of the skeleton at the 

 stage of growth at which the animal died, but the develop- 

 mental stages of this same skeleton, and in case it were the 

 remains of an old, outgrown animal, also, the retrograde metamor- 

 phoses through which it had passed during its last stages of decline. 

 It might require a life time to make out the stages of a single 

 species of mammal satisfactorily from the isolated specimens which 

 would be found and the attempt would be hopeless for all the 

 youngest stages of growth, while the bones were still cartilaginous. 



This kind of evidence, however, is readily obtainable among 

 fossil Cephalopods with relation to the shell and other hard parts 

 as among living animals, and it can be obtained in good col- 

 lections everywhere, whether "in situ" or in museums. Thus it 



