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.
Any substance of medicinal value contained in plant species indigenous to tropical rain forests will eventually be discovered if those species are studied by scientists is an assumption required by the argument.