Sentence:  It was not that Powell and many other Britons did not fear for the quality of the race and complain on occasions about “<WSD>bastardization</WSD>” and “mongrelization,” but more often than not, their eugenic presuppositions were (and perhaps still are) implicit in their analyses of the birthrate rather than specifically articulated. 
Meaning of the word marked with <WSD> tags:  The combining of separate races in marriage or breeding; miscegenation.