The current H1N1 swine pandemic raises the question why the cross-species infection and cross-subtype mutation are so easy. In this study, we use ANOVA to analyze all influenza A virus hemagglutinins available in database in order to answer this question. The results show that there are differences between species and between subtypes in most cases, but the intra-subtype/species variation is generally larger than the inter-subtype/species variation, which provides the rational for cross-species infection and cross-subtype mutation.