Because they frequently interbreed, marked differences in appearances result, which makes them difficult to classify. It's even more complicated with the numerous species of insects.
以軟體動物為例，由於貝類常出現不同種的配對，產生外型奇特的差異，造成分類上的困難；對種類龐大的昆蟲，分類學者更是越分越累，問題顯然更複雜。