How did Ivanovsky's primary contribution differ conceptually from Beijerinck's?

Answer

Ivanovsky provided the empirical proof that a filterable agent existed; Beijerinck provided the theoretical classification.

Ivanovsky documented the data showing something smaller than bacteria caused disease by passing the filter, while Beijerinck provided the theoretical classification, recognizing it as a unique biological entity capable of reproduction.

How did Ivanovsky's primary contribution differ conceptually from Beijerinck's?
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