Sentence:  Now that all mammals are led back to a distant diphyodont stem, it is also true that the further we go back both in <WSD>palingenesis</WSD> and embryogenesis, the more widespread heterodontism is—all modern homodontism proving to be secondary. 
Meaning of the word marked with <WSD> tags:  The apparent repetition, during the development of a single embryo, of changes that occurred previously in the evolution of its species.