Individuals of the same species often present, as is known to every one, great differences of structure, independently of variation, as in the two sexes of various animals, in the two or three castes of sterile females or workers among insects, and in the immature and larval states of many of the lower animals.
Mundu guztiak dakien bezala, espezie bereko banakoek egiturako ezberdintasun handiak erakusten dituzte, aldakuntzatik aparte, hainbat animaliaren bi sexuetan, intsektuen eme antzu edo langileetan, eta behe-animalietariko askoren gaztaro eta larbaroan jazotzen den moduan.