Sentence:  Again the student of evolutionary biology will find a fascinating , if to our minds grotesque , anticipation of the <WSD>theory</WSD> of chance variations and the natural elimination of the unfit in Lucretius , who in turn seems to have borrowed the concept from the philosopher Empedocles . 
Meaning of the word marked with <WSD> tags:  a tentative insight into the natural world; a concept that is not yet verified but that if true would explain certain facts or phenomena