described in Kendall (1948). Speciation (birth) and extinction (death)
rates may be constant or vary through time according to an \R function
specified by the user. In the latter case, \code{BIRTH} and/or
described in Kendall (1948). Speciation (birth) and extinction (death)
rates may be constant or vary through time according to an \R function
specified by the user. In the latter case, \code{BIRTH} and/or