Select random sample by group, with additional condition in R

Based on this post, I'm trying to make a sample of rows. Using the same R iris data example:

head(iris)

I've correctly create a sample of 15 rows for each species

Selec_ir<-iris[ with(iris, unlist(tapply(seq_len(nrow(iris)),
                          Species, FUN = sample, 15,replace=FALSE))), ]

But now how to create a sample based on the condition that the new selected row must be at least after 20 rows from the last selected one?