230 The American Naturalist. [March, 
The order of the Vertebrates, as classified by zoologists, accord 
_ with the order of succession as found by geology, and are divided 
into five great sub-divisions, viz: Fishes, Amphibians, Reptiles, 
Birds, and Mammals, 
Only in the vertebrates do we find calcified teeth. In the 
fishes, some of the lower orders have teeth only partially calcified ; 
which, in fact, appear simply as horny excrescences upon the 
mucous covering of the mouth. Even teeth such as these did 
not, like Minerva from the brow of Jove, spring into being, full- 
formed. If it were found that the teeth, or any other organ, ap- 
peared fully-formed, and suddenly, serious objections might then 
be made against evolution. Nature takes no leaps, but every- 
where, in everything, takes some tissue, already existing, and by 
slight modifications transforms it into some complex organ. So 
we find that even the simple teeth of the fishes can be traced 
back to simpler structures in the Invertebrates. Upon the 
approximating folds of the radiate-shaped mouth of the sea-stars 
are hardened papillz and spicules which function as masticating 
organs. 
Coming higher in the scale we find the mouths of the Crus- 
tacea provided with a firm chitinous framework, with ridges, spi- 
cules or sete forming tooth-like processes opposite each other 
and moved by muscles. These serve as an efficient apparatus 
for the comminution of foodand are called the masticatory stomach. 
Among the Mollusca, the mouths of several varieties are pro- 
vided with jaws, covered with a cuticular membrane, on which 
are small teeth or hooks directed backward. When this mem- 
brane is protruded these teeth are erected, and when drawn back 
they come together like pincers and hold the prey. 
The comparative anatomy and gradation of teéth may be seen 
by the table on page 232. 
Evidence from Paleontology—tThe diversity of size, structure 
and specialized function of the teeth of the Mammalian family are 
directly traceable to the mode of obtaining food. 
The constant use of one variety of food has so developed one. 
portion of the teeth, and atrophied, through disuse, the other 
portion, that the mouths of the existing families of the Mammalia 
