When was a control used in the investigation of the amphipods and sea butterflies? Why was a control necessary?

pteropods or sea snails, also called sea angels, produce chemical deterrents to ward off predators, and some species of amphipods take advantage of this by carrying pteropods piggyback to gain protection from their voracious predators.

Why was progress in science interrupted during the dark ages? The Roman Empire fell in the 5th century thus advancements in science came to a halt. As science halted in Europe, where else did it advance? Science advanced in other areas of the world like China, Polynesia, and Arab nations.