I've got a data frame with far too many rows to be able to do a spatial correlogram. Instead, I want to grab 40 rows for each species and run my correlogram on that subset.

I wrote a function to subset a data frame as follows:

    samp <- function(dataf)
{
    dataf[sample(1:dim(dataf)[1], size=40, replace=FALSE),]
}

Now I want to apply this function to each species in a larger data frame.

When I try something like

culled_data = ddply (larger_data, .(species), subset, samp)

I get this error:

Error in subset.data.frame(piece, ...) : 
  'subset' must evaluate to logical

Anyone got ideas on how to do this?