chord and eventually forced a trans- 
position in thought. 
Early in the Prodromus , Steno 
stated the problem that he wished to 
solve with his second criterion: “Given 
a substance possessed of a certain fig- 
ure, and produced according to the 
laws of nature, to find in the substance 
itself evidences disclosing the place 
and manner of its production.” His 
solution, the basic principle for any 
historical reconstruction, holds: 
If a solid substance is in every way like 
another solid substance, not only as re- 
gards the conditions of surface, but also 
as regards the inner arrangement of parts 
and particles, it will also be like it as 
regards the manner and place of pro- 
duction. 
Past processes cannot be observed 
in principle; only their results remain. 
If we wish to infer the processes that 
formed any geological object, we must 
find clues in the object itself. The 
surest clue is detailed similarity — part 
by internal part — with modern objects 
formed by processes we can observe 
directly. Similarity can be mislead- 
ing — and great mistakes have been 
made in applying Steno’s principle — 
but our confidence in common origin 
mounts as we catalog more and more 
detailed similarities involving internal 
structure and chemical composition as 
well as external form. 
Thus, Steno argues, sedimentary 
rocks must be the deposits of rivers, 
lakes, and oceans because they “agree 
with those strata which turbid water 
deposits.” Fossil shells once belonged 
to animals, and crystals precipitated 
from fluids as we make salt or rock 
candy today. 
With these two principles — molding 
and sufficient similarity — Steno es- 
tablished both prerequisites for geo- 
logical, or any historical, reconstruc- 
tion: he could determine how and 
where objects formed, and he could 
order events in time. Steno’s genius, 
to say it one more time, lay in es- 
tablishing this new conceptual frame- 
work for observation, not in the acuity 
of the subsequent observations them- 
selves. Steno’s break with older tra- 
ditions stands out most clearly in his 
complete failure, save in one sheepish 
passage, to consider the primary sub- 
ject that obsessed his colleagues: the 
identification of goals and purposes 
for all things, including what we now 
regard as purely physical processes 
of uplift, erosion, and crystallization. 
In one fleeting passage, Steno cites 
dissimilarity of function as a reason 
subservient to his usual argument 
about internal resemblance for stating 
that rocks and bones form differently. 
But he quickly adds the disclaimer, 
“if one may be permitted to affirm 
aught about a subject otherwise so 
little known as are the functions of 
things.” As Foucault also argues, the 
subjects you leave out of your tax- 
onomies are as significant as the ones 
you put in. 
The four-part organization of the 
Prodromus has generally been viewed 
as disjointed or even incoherent— 
thrown together by a man itching to 
leave Florence but forced to justify the 
grand duke’s patronage. I view it in- 
stead as a comprehensive and tightly 
reasoned brief for a science of geology 
based upon the two principles of mold- 
ing and sufficient similarity. 
Part one is a teaser, a specific ex- 
ample to demonstrate the power of 
the general method. The glossipetrae, 
Steno argues, must be sharks’ teeth 
because they are identical in form and 
internal arrangement with the objects 
he had plucked from the mouth of 
his quarry from Leghorn. They so- 
lidified before the rocks that enclose 
them because they impress their form 
upon the surrounding sediment. 
Therefore — and now the argument be- 
gins to move toward revolutionary gen- 
erality — sedimentary rocks were not 
created with the earth, but have 
formed as the deposits of turbid waters 
in rivers, lakes, or oceans. Moreover, 
similar marine fossils are often found 
high in mountains and far from the 
sea; these fossils also solidified before 
the strata enclosing them. Thus, the 
earth has an extensive history: seas and 
lands have changed places, and moun- 
tains have emerged from the waters. 
In the second part, Steno argues 
that glossipetrae are but one example 
of the general problem of solids within 
solids, and that the principles of mold- 
ing and sufficient similarity can es- 
tablish proper taxonomic subdivisions 
based on common modes of origin. 
The third part treats the major classes 
of solids within solids and establishes 
two basic categories for objects within 
rocks: fossils that harden before the 
enclosing strata, and crystals and veins 
that form within solid rocks. 
The fourth part, a reconstruction 
of the geological history of Tuscany, 
has been problematic or even embar- 
rassing to geologists who wish to view 
Steno as their founding saint. (The 
Catholic Church, by the way, is also 
considering Steno for sainthood, and 
he may eventually attain an unprece- 
dented double distinction.) Steno con- 
structs his history to match biblical 
chronology, with two cycles of dep- 
osition — from the original void and 
from Noah’s universal ocean. The es- 
sence of this part, however, is not his 
continued loyalty to Moses — Steno 
was, after all, not a man of our cen- 
tury — but rather his demonstration 
that the principles of molding and suf- 
ficient similarity can be used not only 
to classify objects (part three), but 
also to reconstruct the history of the 
earth from these objects (part four). 
The last part of the Prodromus dem- 
onstrates by specific example, drawn 
from the local terrain, that the proper 
classification of solids within solids 
can establish a science of geology. 
In 1678, Athanasius Kircher pub- 
lished a figure showing all letters of 
the alphabet, including the contrac- 
tion Ai, etched in veins of calcite. 
Today, we chuckle and dismiss the 
well-formed letters as accidents. But 
to Kircher they were no less significant 
than the shells of clams also found 
in rocks. One might argue that clams 
are more complex than letters, but 
a Venetian work of 1708 depicted an 
agate that seemed to show, in its bands 
of color, Christ on the cross with all 
proper accouterments, including a sun 
on the favored right side and a moon 
on the despised left. The caption pro- 
claimed in German doggerel: Solche 
wunderbarliche Gestalt, hat die Natur 
in ein Agat gemahlt — “Nature herself 
has painted this wonderful figure in 
an agate.” Why was a clam in a rock 
different from a letter or a crucifixion? 
Since alphabets and religious scenes 
cannot be preexisting objects buried 
in strata, they must be made by a 
plastic power in the rocks themselves. 
As long as “odd things in rocks” 
formed a single category, clams and 
sharks’ teeth would also be manifes- 
tations of the plastic force and no sci- 
ence of paleontology or of historical 
geology would be possible. But Steno’s 
classification recognized the basic dis- 
tinction between fossils that hardened 
before the rocks that enclosed them 
and intruding veins that might by ac- 
cident resemble some abstract form 
or design. 
Steno changed the world in the sim- 
plest and yet most profound way. He 
classified its objects differently. 
Stephen Jay Gould teaches biology, 
geology, and the history of science 
at Harvard University. 
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