There are more than 2 million described species living on Earth today, but many, many more are yet to be discovered. Each species is different, and each reproduces itself faithfully, yielding descendants that are unique to that species. Thus, the parent organism hands down information specifying, in extraordinary detail, the characteristics that the offspring will have. This phenomenon of heredity is central to the definition of life: it distinguishes life from other processes, such as the growth of a crystal, or the burning of a candle, or the formation of waves on water, in which structures are generated without the same type of link between the peculiarities of parents and offspring.
