Dulcian

The Dulcians were a race of humanoids, characterised by relative physical weakness, higher-order brains, and zealous pacifism.

Biology
Dulcians looked like humans, but they had larger brains, a binary vascular system, no unnecessary organs, and weaker frames. (TV: The Dominators)

Technology
The Dulcians were at a relatively high level of technology. They used vision screens in order to communicate with people far away. They had walls which could change colours and would disappear to function as doors.

In order to travel, Dulcians used travel capsules. These travel capsules were small capsules which would fit two people, were completely automated and allowed Dulcians to quickly travel across the planet. (TV: The Dominators)

Culture
The Dulcians were led by the Council, which was turn led by a director. These councils were made of around six Dulcians who discussed the matters of the government. Sub-groups of the Council included the Scientific and Emergency Committees.

Dulcians were a very peaceful race, and outlawed war. They were known for accepting fact and debating actions before doing them. As such, Dulcians were thought not to be curious or adventuresome. They also believed it was undignified to struggle against one's fate.

Dulcian measurements of time included annos (years) and lunars. (PROSE: The Dominators)

History
The second council led by Director Olvin banned atomic weapons many years before the Dominator invasion. The seventh council, led by Director Manus, began more tests with atomic weapons, using an island as a test site. The tests were eventually stopped, and the island, now known as the Island of Death, was kept as a warning for future generations. Except for occasional student visits, the island was left uninhabited.

The Doctor visited Dulkis at some point before the Dominators invaded. He didn't seem to know about the Island of Death.

172 years later, Dulkis was invaded by the Dominators, who landed on the Island of Death to absorb the radiation. They planned to take the strongest Dulcians as slaves and then destroy the planet for fuel. The Dulcians, being a peaceful people, were unwilling to fight back. Luckily, the Second Doctor had also arrived on the Island of Death and was able to stop the Dominators' attempts. (TV: The Dominators)

Other references
During his trial at the hands of the Time Lords, the Second Doctor cited the Quarks and the Dominators' attempt to enslave "a peace loving race" as an example of the evils which he had fought against. (TV: The War Games)