d73 
first one alone being concave, the divided ventral is introduced 
earlier in the ontogony and, finally, the division of the outlines by 
digitations occurs in the earliest neanic substage, replacing the sim- 
pler sinuous outlines of the preceding suborders. 
In the evolution of a series heredity therefore acts according toa 
definite law of replacement. Zhe ancestral characters are brought 
into contact with new adaptive characteristics, which are being con- 
tinually introduced tnto 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 resorbed and disappear 
during this process. 
It is a fact, as shown by the writer and especially by Barrande 
and Dr. Branco, that the embryonic shell has varied comparatively 
little throughout time in the Ammonoidea, Nautiloidea, Belem- 
noidea and Sepioidea. But these statements do not apply to the 
earliest times in evolution of these types, when they branched off 
from the common stock. The embryos of the Ammonoidea and 
Nautiloidea become quite different from each other, the embryos of 
the Belemnoids remained like those of the Ammonoids, almost 
exactly similar to those of the Nautilinide as shown by Chalmas 
and Branco, and finally in the Sepioidea, the protoconch or em- 
bryonic shells changed more completely and soon disappeared. 
Attention has been already ,called to this remarkable fact in the 
history of the evolution of these forms, that the separation of the 
orders took place rapidly, and in the embryos as well as in the 
adults near the origin of the orders, and that the comparative 
invariability of the embryo was confined to the subsequent history 
of these types after separation. There is also considerable ground 
for the conclusion that the young, not the earliest stages of shell, 
are more variable among the degraded types than among progres- 
sive forms. The facts already stated with regard to the young of 
Baculites and some crioceran forms show this. 
This paper cannot be devoted to the discussion of the apparent 
reasons for these changes, but we have been able to explain the 
mode in which they take place. Zhe mode in each case is the ear- 
lier or accelerated development of ancestral characters, which as we 
have said follow the same law, whether progressive and tending té 
preserve the characters of the type, or retrogressive and tending to 
destroy the characters of the type. 
