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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 Cope 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 ofa 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 
