Summarizing the thought of Spencer and Sumner:
In a free society people are able to pursue their own interests as they see fit, provided they respect the equal rights of others. As cooperation in a regime of contract replaces exploitation in a regime of status, the fittest prosper not by coercing others but by assisting them through voluntary exchanges. (Adam Smith had previously dubbed this process the “invisible hand.”) Here as elsewhere survival of the fittest is an iron law of social existence, but this standard of fitness is far removed from that invoked by the specter of social Darwinism. Voluntary cooperation, not coercive exploitation, is the standard of fitness in a free society.
Spencer and Sumner emphasized that market competition differs dramatically from biological competition. Market competition, unlike biological competition, produces immense wealth, thereby making it possible for many people to survive and prosper who otherwise could not. Moreover, the sophisticated division of labor that develops in a market economy generates specialization, and this specialization generates social interdependence — a condition in which every person must rely on the cooperation and assistance of others for necessary goods and services. The solitary individual cannot produce everything he needs or wants in a market economy, so he must persuade many others to assist him. This condition of survival cultivates the character traits (or virtues) necessary for peaceful interaction – those civilizingmores, as Sumner called them, that make social interaction not only productive and mutually beneficial but pleasant as well.Smith doesn't defend Darwin against the charge of promoting bloody conflict between the classes (we know who really does that - typical case of psychological projection). This guy does, at the same time showing how idiotic it is to call someone a "social Darwinist" in the sense its coiner intended.
