Among animals which unite for each birth, and which are highly locomotive, doubtful forms, ranked by one zoologist as a species and by another as a variety, can rarely be found within the same country, but are common in separated areas.
Kumaldi bakoitza egiteko batu eta lekuz sarri aldatu ohi diren animalien artean, nekez aurki daiteke herrialde berean zoologo batek espezietzat eta beste batek aldakitzat sailkaturiko dudazko formarik, baina arruntak dira horrelakoak eskualde bereizietan.