 The brief account of some of the early work on unicellular organisms given above illustrates some of the distinctions that are elaborated in the rest of this entry. One group of questions is concerned with the properties of hereditary relations, the sources of variations (in particular, whether they can be environmentally induced), the stability of variations and their regulation, and so on. A second class of questions is concerned with the way hereditary information is stored and transmitted. It is here that we can locate the debates about nuclear versus cytoplasmatic inheritance and about the primacy of DNA as the information store of the cell. Although the distinction between these two groups of questions is useful, many of the topics and controversies to be discussed in this entry involve both.