Of course, the ideas and theories of Karl Marx were essentially what made socialism into a widespread and fairly radical political movement. There were socialists before Marx, but Marx made the movement more relevant to the modern and industrialized world of the late 1800s.
By the late 1800s (Marx died in 1883), however, Marx's ideas were coming to have somewhat less of an impact. There were other ideas that were pulling at socialism and leading it away from Marx's revolutionary vision. This can be seen most clearly in the move towards "Social Democracy" by people who believed in "evolutionary socialism." To these socialists, revolution as not likely to happen. Socialists should, instead of working for revolution, work within democratic systems to help improve the lives of workers. With the rise of this sort of socialism, Marx's ideas were coming to have less and less impact on socialism and its development.