Many important types of medicine have been developed from substances discovered in plants that grow only in tropical rain forests. There are thousands of plant species in these rain forests that have not yet been studied by scientists, and it is very likely that many such plants also contain substances of medicinal value. Thus, if the tropical rain forests are not preserved, important types of medicine will never be developed.
There are substances of medicinal value contained in tropical rain forest plants not yet studied by scientists that differ from those substances already discovered in tropical rain forest plants is an assumption required by the argument.