A drawing made by etching lines in the sand is likely to be erased before anybody could have photographed or otherwise reproduced it. In these general respects, memes are similar to genes and to other replicators, like computer viruses or crystals. The genetic metaphor for cultural transmission is limited, though. Genes can only be transmitted from parent to child. For genes to be transmitted, you need a generation.
Memes only take minutes to replicate.
On the other hand, the copying-fidelity of memes is in general much lower. If a story is spread by being told from person to person, the final version will be very different from the original one. That makes it difficult to define or delimit memes. There are several selection criteria which determine in how far a particular meme will be successful. Objective criteria determine whether the knowledge conveyed by a meme can reliably predict events in the outside world.