PUBLICATIONS. 863 
that instead of a single such series there are several, all originating from 
a primitive stock and each one passing progressively from the straight 
form through arcurate, loosely coiled to closely coiled forms. 
The order Ammonoidea is in like manner a branch from the prim- 
itive stock. The author says: ‘Thus although both are orders and 
taxonomically equal, we cannot compare the whole of the Ammonoidea 
with the whole of the Nautiloidea, but only with a more or less perfect 
single series of that order.” The Ammonoidea have primitive straight 
radicals, but they are few in number and become extinct in Devonian 
time, Jeaving several branches of closely coiled forms, the Goniatitine,. 
from which the latter Ammonoids spring. 
During the evolution of the Nautiloidea and Ammonoidea, the dif- 
ferentiation takes place most rapidly during the earlier periods of their 
phylogeny, and their differentiating parts are of more structural impor- 
tance than later in the life-history. Professor Hyatt formulates these 
facts in the following general law of evolution: ‘“ Zypes are evolved 
more quickly and there are greater structural differences between genetic 
groups of the same stock while still near the point of origin than appear 
subsequently. The variations or differences take place quickly in funda- 
mental structural characteristics, and even the embryos may become different 
when in the earliest period of evolution, but subsequently only more super- 
ficial structures become subject to great variations.” 
This law however does not apply to the degenerate forms which 
appear during the stages of decline of the several genetic series. 
In the phylogenetic series, characteristic modifications appear first 
in the later stages of growth of individuals. These modifications are: 
transmitted to succeeding generations, but are progressively accelerated,. 
appearing in earlier and earlier ontogenetic stages. The law of accelera- 
tion is formulated as follows: ‘“ Zhe ancestral characters are brought into 
contact with new adaptive characteristics, which are being continually intro- 
duced into the adult and adolescent stages of ontogeny, and these eventually 
replace the former which are crowded back to make room for them into: 
earlier stages than those at which they first appeared, and in many cases 
the latter are reabsorbed and disappear during this process.” 
In the interpretation of the more or less uncoiled degenerate forms 
of the later phylogenetic stages of the Ammonoidea, the law of accelera- 
tion still governs the changes. During Jurassic time degenerate forms 
are local and the degeneration is exhibited only in the adult stages of 
the individual. In later Cretaceous time, the majority of the forms. 
show degeneration wherever they occur, and this degeneration sets in. 
