The answer is:
By showing the degradation of the descendants of both classes.
Social Darwinism asserted that the winners of the class struggle would pass on their "successful" genes to the next generation, and humankind would evolve into a stronger species.
In "The Time Machine," Wells subtly criticizes the adaptation of Darwin's theory when the Time Traveller discovers that the Eloi are not a superior capitalist society that has developed. Instead, the descendants of the upper class (who are lazy, weak and unintelligent from lack of work or struggle) have subjugated the Morlocks, who are the descendants of the "weak" working class and have become strong, fierce and cruel met-eating creatures to survive.