I think he was actually less biased. I was actually just reading John McPhee's Looking For a Ship, and McPhee quotes Darwin discussing the geology of the Valparaíso region, and notes that Darwin divines the processes that created the formations of that terrain essentially correctly ... before plate tectonics was even a theory with a name in the scientific literature. It is of course impossible to separate out to what extent his results are improved by his hesitance in publishing data without overwhelming evidence, but I would guess that his rationality was significantly above par for his era.