Two anomalies confront Marxism as its refutation: the durability of capitalism and the passivity of its working class. Successive encounters with these anomalies — encounters stimulated by different political and economic circumstances — have shaped many incarnations of Western Marxism. Classical Marxism, for example, which included such disparate thinkers as Kautsky, Luxemburg, Plekhanov, Jaurès, Adler, Bauer and Hilferding, emerged out of Marxism's golden age. Europe's historical circumstances between 1890 and 1920 could be interpreted as vindicating Marx's scientific investigations. During this period economic forces did appear to be propelling Europe toward a major international crisis and class struggle did appear to be escalating. The events warranted optimism and anomalies could be passed off as temporary aberrations.