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 Infusoria and rotatory animals. Their propagation can be observed and obtained ad libitum in a room.

The equivocal generation has only reason in its favour and a very limited sphere of observations, owing to the excessive difficulty of observing such infinitely small creatures.

Propagation by sexual generation and by the formation of offsprings, has in its favour the greater mass of observations: which observations are positive in this case, and seconded by the form, appearance and size of these beings. The necessary conditions to these two modes of generation can be expressed by the following formule: