-6 The American Geologist. i->bniaiy. i<>oi. 
palaeozoic life. From these, and the published results of the 
survey the following conclusions have been drawn. It is true 
that the eye cast over a page of expressive drawings meets 
more saliently relieved the physical characteristics, size, orna- 
mentation, shape and sculpture of fossil shells and organisms 
than it does in a line of selected fossils where the number, the 
surficial dullness and imperfection perhaps fail to produce "at 
sight" the impression made by luminous and exactingly ex- 
ecuted drawings. Still there can be no question that the "orig- 
inal specimen" can never be evaded, and for scientific pur- 
poses it is of incomparable value. In a field of general obser- 
vation, devoid of the extreme precision of "species making," 
or taxonomic study, both figure and specimen can be usefully 
studied, and in this paper, conceived as a contribution to evo- 
lutional studies, they have been made to supplement each other, 
the lack of opportunity and material forcing me to rely on 
drawings perhaps more confidently than I should. 
Dr. Sacco in his examination of the Tertiary mollusca of 
Piedmont* has been impressed with those gradations of form, 
sculpture, color, size, etc., which insensibly merge one species 
into another, and he deduces some general conclusions, which 
are of interest in the examination of any group or succession 
of groups of fossils at any horizon. He finds a species, in ;i 
typical condition, has grouped about it more or less aberrant 
forms, referable, however, to it ; he thinks, by reason of prior- 
ity a variety of species is given the name which should be as- 
signed to the essential form, which through the custom of 
terminology receives another name. He finds some species 
mutable and evanescent, others less changeable and dirigible 
only between certain fixed limits of form ; he finds the ex- 
tremely variable the most vitalized, and they are those which 
continue for a long series of geological periods, as -"their great 
oscillatory polymorphism around a typical form permits them 
to adapt tliemselves to the diverse conditions which succeed 
each other in i\\c same region, in different geological periods," 
while more conservative forms disappear or become so fully 
modified, as to form new and serviceable species. He notes 
the fact that upon ornamented species the progress of change 
• I^<> Varinzioni der MoUuschi, Dott. Fbderico Sacco; Bulletino <trllii 
l^orii tti Miiltiiiildilirii ftilliiiliii. 1S!»M. 
