Sentence:  And thus the hypothesis that living matter always arises by the agency of pre-existing living matter, took definite shape; ... It will be necessary for me to refer to this hypothesis so frequently, that, to save circumlocution, I shall call it the hypothesis of <WSD>"Biogenesis</WSD>"; and I shall term the contrary doctrine—that living matter may be produced by not living matter—the hypothesis of "Abiogenesis". 
Meaning of the word marked with <WSD> tags:  The principle that living organisms are produced only from other living organisms.