Final answer:
Sir Arthur Conan Doyle refers to the brutal conquest and inhumane treatment of South American indigenous populations by Spanish conquerors, as detailed by historical figures like Bartolomé de las Casas, resulting in mass killings, oppression, and demographic changes.
Step-by-step explanation:
Sir Arthur Conan Doyle, in referencing the massacres of populations such as that of the South Americans by the Spaniards, is referring to the tragic and brutal conquest of the Americas by Spanish conquerors. This conquest involved inhumane treatment and enslavement of the indigenous people, which was chronicled by observers like Bartolomé de las Casas, who documented the Spaniards' cruel and barbarous actions. These actions included unjust wars, mass killings, and the imposition of severe oppression on the native peoples leading to significant population declines due to violence and diseases like smallpox. Spaniards' conquest has left a lasting legacy in the Americas, including changing demographics with the rise of mestizos and the spread of Catholicism.