1881. ] Recent Literature. 795 
Planorbis and more like a Paludina in its general shape. It should 
be borne in mind that the forms belonging to the first series were 
connected by hybrids, and the whole series “ presents to the or- 
dinary observer a chaos of similarities and differences.” 
Referring our readers to the paper itself for details and illus- 
trations, we may, without further delay and without criticising 
the author’s mode of research, except to say that the time and 
patience spent upon the work, and the author’s evident candor 
and accuracy, lead us to accept his results as sound inductions, 
now quote the conclusions of this elaborate memoir. 
(1) The extraordinary modifications and series of shells found 
at Steinheim are in one way exceptional, and owe their existence 
to exceptional conditions. 
2) These conditions appear to be the isolation of the modified 
descendants of Planorbis Zevis, due to the absence of competing 
types, and the character of the environment. 
his environment was suitable for the propagation and per- 
petuation of the distinctive peculiarities of their series, and unfavor- 
able in various degrees to those of the sub-series of the first 
Series, 
(4) That while the perpetuation and survival of the differential 
characteristics can be thus accounted for, we must look to other 
Causes for the production of the parallel forms and the regularity 
of succession of these forms, as shown in the arrangement in the 
different series, and in the development of the individual. 
lanorbis levis. 
(6) That while the influence of the environment must be ad- 
mitted as paramount in exceptional instances, it for the most part 
Produces these exceptions in extreme cases of parasitism, and the 
Steinheim shells are not parasites 
Under the same laws as other forms occurring in other localities. 
(7) That this appears to be the case except in so far as they are 
that and other reasons. 
Professor Hyatt next inquires, “what is this law of heredity 
and growth which maintains the type, causes parallelisms and 
constrains variation under ordinary conditions, but still, in certain 
fases, is forced to give way to physical influences. 
