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The Conjecture of Vaunt Mandarast is mentioned in passing by Arsibalt when Barb mentions that

"Fraa Jesry believes that advanced life forms must be extremely rare in the universe."

Arsibault responds with the following summary of the Conjecture:

"Billions of plants infested with unicellular glop. Almost none with multicellular organisms - to say nothing of civilizations."

The Earth equivalent would be the Rare Earth Hypothesis. In their book "Rare Earth - Why Complex Life is Uncommon in the Universe," Peter D. Ward and Donald Brownlee discuss reasons why life may be common, but multicellular organisms would only gain a foothold in a few areas.