Feudalism declined during the crusades because

they undermined feudalism, they lost a lot of money in the wars

Explanation:

The fall of the Roman Empire begin in the medieval age. The medieval age brought the plagues, famines, diseases and in the middle of it all is feudalism. Feudalism or the feudal system was a dominant social system used in western mainly from the 5th to 11th century. In feudal system land are rewarded to lower classes for services, military services and loyalty.

darwinism is survival of the fittest there for social darwinism is the survival of the wealthiest