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least, with respect to their account of these substances, little more 
than mere copyists of Pliny ; who, although he had affixed names to 
these substances, descriptive of the forms they possessed, meant not 
thereby to give any opinion as to their origin. 
In the thirteenth century, Albert le Grand * speaks of -the branch 
of a tree being found, on which was a bird’s nest, with birds 
contained in it, the whole being a mass of stone. The vis formativa 
of Aristotle was, however, sufficient to account, in the opinion of 
Albert, for this extraordinary phenomenon ; for the opinions, which 
were entertained at this period, respecting these substances, were 
exceedingly incorrect. The doctrine of equivocal generation, which 
had been adopted by the disciples of Aristotle, contributed very 
much to mislead those who made these substances the subject of 
their inquiries; since, by also adopting the aid of certain occult 
qualities, their origin was supposed to be thus satisfactorily ac- 
counted for. Certain plastic powers were supposed to employ their 
influence in the earth, in creating substances, which bore the figure 
and resemblance of various vegetable and animal substances. To 
account for their formation, therefore, it was thought sufficient to 
refer to the hidden powers of the vis plastica, the vis formativa, and 
the vis lapidijicativa. 
In the sixteenth century, about the year 151/, the workmen em- 
ployed in rebuilding the citadel of St. Felix, at Verona, discovered, 
that the rock, on which it was built, was full of petrified shells. 
This discovery excited the attention of the learned, to a very con- 
siderable degree ; some attributing them to the active influence of 
the vis formativa; whilst others, perceiving their exact resemblance 
to real shells, declared, that they must be actual marine bodies thus 
enveloped in stone, by some accident. We learn, that when Fra- 
castorius was asked his opinion respecting this phenomenon, he 
* Mineral, tract. I. lib. i. 
