258 The American Geologist. October, i895' 
logeny and cuvild not be used to illustrate the evolution of the class. 
These objections are now met, as indicated above, by studying devel- 
opment within the families and genera. 
It may be accepted as a law that animals near their points of origin 
in early geologic time evince marked tendencies toward rapid evoKition. 
They found the earth comparatively unoccvipied or occujjied by inferior 
animals, and through such conditions favoraVjle to numerical increase 
were forced to migrate in every direction and thus to come in contact with 
andadjvist themselves to many different physical surroundings. Similar 
cases of quick evolution of new series and many species occur in later 
times as well as to-day, whenever a stock finds an unoccupied field. In 
this connection the author instances the well-known Planorbis develop- 
ment in the Steinheim basin and the evokition of the extensive family 
of the Arietidce, consisting of eleven distinct series, arising, culminat- 
ing and ending within the limits of the Lower Lias. Another marked 
case illustrating the same point, which might be mentioned, is the ex- 
treme differentiation of the genus Gaminarns, an amphifjod from lake 
Baikal, which probably since glacial times has developed from a single 
form one hundred aud fifteen species, many of them highly ornamented 
and specialized, and together constituting more species than are known 
elsewhere, althovigh the genus is world wide. 
Before taking up a detailed description and discussion of the various 
genera and species of cephalopods which furnish professor Hyatt with 
the main points of his argument, he introduces a chapter on the princi- 
ples of bioplastology. or the characteristics of development and decline 
in the life of an individual. This has already been published in part 
elsewhere and reviewed in the Geologist. The terms adopted will be 
found usefvil in accurately indicating any stage of ontogeny or any kind 
of development and in correlating them with periods of phylogeny. 
A comple'te application of the ontogenetic stages is then made to the 
shell covered Cephalopoda, together withdefinitionsof descriptive terms 
for various features and structures. Of these the iiupfesHed zone is the 
one to which special attention is called, as it forms the basis for the in- 
vestigation of the inheritance of an acquired characteristic. The im- 
pressed zone is primarily the area on the dorsvim which is flattened, 
convex or indented by the contact of the growing whorl with the venter 
of the already formed whorl of the next inner volution. 
It is shown that the impressed zone is invariably consequent upon 
close coiling, never appearing in ancestral forms in the early stages un- 
less through this agency. It is, therefore, a mechanical and necessary 
result of the pressure of one whorl upon another and must be accepted 
as strictly an acquired character. The influence of tachygenesis, or the 
progressive earlier inheritance of characters, results in the development 
of the impressed zone (then called the dorsal furrow) in very young 
shells belonging to later genera, before the whorls come in contact with 
each other. In other words, the genera geologically later than the an- 
cestral forms of the different groups show an impressed zone during 
growth stages before the appearance of the mechanical conditions which 
