R. rattus frequents ships much more commonly than R. norwegicus. Statistics show a considerable decline in number of rats per ship fumigated at British ports during the period 1925 to 1937, and many precautions are taken to prevent the passage of rats between ship and shore. Despite this, several ports show an undiminished population of R. rattus, or if R. rattus has declined so also has R. norwegicus. R. rattus is apparently capable of maintaining itself in some numbers independent of renewed influxes from incoming ships. R. r. rattus is the form found most commonly on the west and northeast coasts, but R. r. alexandrinus is most numerous at London and Plymouth.