504 The American Naturalist. [June, 
the outer cusps often became crescentic, while the inner cusp 
remained rounded. And it was therefore during all the lower 
Eocene, that the external cusps surpassed the internal cusps in 
progressive development. 
The bearing of these facts is this: in Embryogenesis, we 
are dealing with repetition of ancestral history; we should 
not expect, however, that this repetition would invariably 
extend back of the form characteristic of the Eocene period. 
As a matter of fact, the upper molars repeat their Eocene but 
not their Mésozoic form. The lower molars also repeat the 
Eocene form, and this, as explained above, owing to the con- 
servative proportions of the cusps, is also the Mesozoic form, the 
protocone still being the most prominent cusp in the crown. 
While, perhaps, not thoroughly satisfactory, there is a great 
deal of probability that the discrepancy between the embryonic 
and phyletic order in the upper molars, is due to these differ- 
ences in their phyletic history. 
In the accompanying table I havesummed up the phylo- 
genetic order observed by Cope and myself, and the ontogene- 
tic order observed by Röse and Tacker. 
THE Fusion THEORY or Cusp ORIGIN. 
As we have seen, the fusion theory was first proposed by 
Kükenthal; it was afterward independently advanced by 
Rose. We find in addition to the grounds given above, one 
very strong argument against this theory derived from 
Paleontology, is in the law of molar evolution, namely, that 
the cusps appear at or near the apex of the crown, and 
development takes place from above downward. Thus, so far 
as we can judge from Dromotherium and Microconodon, the 
lateral cusps first appear on the sides of the protocone, and 
much later, the fang subdivides; the lateral cusps are at the 
outset very much smaller than the medium cusp, and it is 
only after a long course of evolution that they attain the same 
size. i 
Now, if the fusion theory were correct, and the triconodont 
crown, for example, were constituted by the fusion of three — 
