Richard Dawkins initially defined meme as a noun which “conveys the idea of a unit of cultural transmission, or a unit of imitation”. John S. Wilkins retained the notion of meme as a kernel of cultural imitation while emphasizing the meme’s evolutionary aspect. The meme as a unit provides a convenient means of discussing “a piece of thought copied from person to person”, regardless if that thought contains others inside it, or forms part of a larger meme. A meme could consist of a single word, or a meme could consist of the entire speech in which that word first occurred. This forms an analogy to the idea of a gene as a single unit of self-replicating information found on the self-replicating chromosome.
While the identification of memes as “units” conveys their nature to replicate as discrete, indivisible entities, it does not imply that thoughts somehow become quantized or that “atomic” ideas exist which cannot be dissected into smaller pieces. A meme has no given size.
Some critics have seen the inability to pin an idea or cultural feature to its key units as an insurmountable problem for memetics. Blackmore meets such criticism by stating that memes compare with genes in this respect; that while a gene has no particular size, nor can we ascribe every phenotypic feature directly to a particular gene: it has value because it encapsulates that key unit of inherited expression subject to evolutionary pressures.
The 1981 book “Genes, Mind, and Culture: The Co-evolutionary Process” by Charles J. Lumsden and E. O. Wilson proposed the theory that genes and culture co-evolve, and that the fundamental biological units of culture must correspond to neuronal networks that function as nodes of semantic memory. Coauthor Wilson later acknowledged the term meme as the best label for the fundamental unit of cultural inheritance in his 1998 book “Consilence: The Unity of Knowledge”, which elaborates upon the fundamental role of memes in unifying the natural and social sciences.